p2infohouse.org · FIELD MANUAL PERFORMANCE EVALUATION AND TROUBLESHOOTING AT METAL-FINISHING WASTEWATER TREATMENT FACILITIES T. N. Sargent, P.E. G. C. Patrick, P.E. E. H. Snider,

* - .

PB 8 5 - 1 0 7 2 7 4

Field Manual - Performance Evaluation and Troubleshooting at Metal-Finishing Wastewater Treatment Facilities

Engineering-Science, Inc., Atlanta, GA

Prepared for

Industrial Environmental Research Lab. Cincinnati, OH

Sep 84

U.S. DEPARTMENT OF COMMERCE National Technical Information Service

P- 7s

FIELD MANUAL

PERFORMANCE EVALUATION AND TROUBLESHOOTING AT

METAL-FINISHING WASTEWATER TREATMENT FACILITIES

T. N. Sargent, P.E. G. C. P a t r i c k , P.E.

E. H. Snider, Ph.D., P.E. Engi n e e r i ng-Science , I n c . At1 anta, Georgia 30329

Con t rac t No. 69-03-3040

P r o j e c t O f f i c e r

Thomas J . Powers

I n d u s t r i a l P o l l u t i o n Cont ro l D i v i s i o n I n d u s t r i a1 Envi romental Research Labora tory

C i n c i n n a t i , Ohio 45268

EPA-600/2-84-152 September 1984

OFFICE OF RESEARCH AND DEVELOPMENT U.S. ENVIROMENTAL PROTECTION AGENCY

C I N C I N N A T I , O H I O 45268

N AT1 ONA L TECHNIC A L INFORMATION SERVICE

REPRODUCED BY

U.S DEPARTMENT OF COMMERCE SPRINGFIELD. VA. 22161

I. REPORT NO.

EPA-600/2-84-152

2. SPONSORING AGENCY N A M E A N D ADD.RESS

U. S. Environmental Pro tec t1 on Agency I n d u s t r i a l Environmental Research Laboratory I n d u s t r i a l P o l l u t i o n Contro l D i v i s i o n C i n c i n n a t i , OH 45268 5. SUPPLEMENTARY NOTES

2.

'erinr/

19. SECURITY CLASS (This Reporr) UNCLASSIFIED

20. SECURITY CLASS (This page)

I I,

3 , RECIPIENT'S ACCESSION N O . 5 107274 5 , REPORT DATE

21. N O . O F PAGES

282 22. PRICE

September 1984 5. PERFORMING O R G A N I Z A T I O N CODE

$. PERFORMING O R G A N I Z A T I O N REPORT N O

-- 10. PROGRAM ELEMENT NO.

1 1 . CONTRACTlGRANT N O .

68-03-3040 (SBE02) 13. TYPE OF REPORT A N D PERIOD COVERED F i n a l 1983

14. SPONSORING AGENCY CODE

EPA/600/ 12

6. ABSTRACT

This manual p rov ides a t e c h n i c a l f i e l d guide o r re fe rence document f o r use i n improv- i n g t h e performance o f f a c i l i t i e s f o r t h e t reatment o f metal f i n i s h i n g wastes. main purpose o f t h e manual i s t o p r o v i d e a t r o u b l e s h o o t i n g guide f o r i d e n t i f y i n g p r o b l ems, ana lyz ing problems , and so l v i ng p r o b l ems. The manual descr ibes general procedures f o r e v a l u a t i n g t h e performance o f t rea tment processes and equipment common- l y used i n metal f i n i s h i n g waste t reatment. .The procedures a l s o cover o t h e r i tems r e l a t e d t o t h e e f f e c t i v e o p e r a t i o n o f t rea tment f a c i l i t i e s . eva lua te compliance problems and develop Operat ion and Maintena'nce (O&M) s p e c i f i c s are descr ibed i n a rev iew o f t h e l i t e r a t u r e , f o l l o w e d by an assessment o f t h e causes o f p e r m i t v i o l a t i o n s and t h e recommended measures f o r improv i ng compl iance. I The u n i t processes descr ibed i n t h i s manual a r e those commonly used i n t h e t rea tment o f metal f i n i s h i n g wastes. They a r e t h e f o l l o w i n g : e q u a l i z a t i o n , o i l removal, cyanide o x i d a t - ion , chromium reduc t ion , pH c o n t r o l , metal p r e c i p i t a t i o n , f l o c c u l a t i o n , sedimentat ion, f i l t r a t i o n , g r a v i t y th icken ing , b e l t f i l t e r presses, vacuum f i l t r a t i o n , pressure f i l t r a t i o n f o r dewater ing, and c e n t r i f u g a t i o n . manual c o n t a i n s i n f o r m a t i o n on theory o f opera t ion , d e s c r i p t i o n o f equipment, opera- ti onal procedures , t y p i c a l performance v a l ues , and a t r o u b l eshoot i ng guide.

The

The methodology used t o

For each o f these u n i t processes, the

7 . KEY WORDS A N D DOCUMENT ANALYSIS

DESCRIPTORS Ib. lDENTIFIERS/OPEN E N D E D TERMS IC. COSATI FieldiGroup ~ ~

Metal F i n i s h i ng E l e c t r o p l a t i n n g Wastewatter Treatment

8. D ISTRIBUTION STATEMENT

RELEASE UNLIMITED

E P A Form 2220-1 (Rev . 4-77) PREVIOUS EDITION I S O B S O L E T E

i

NOTICE

This document has been reviewed i n accordance w i t h U.S. Enviromental Mention o f t rade P r o t e c t i o n Agency p o l i c y and approved f o r p u b l i c a t i o n .

names o r commercial products does n o t c o n s t i t u t e endorsement o r recommenda- t i o n f o r use.

ii

FOREWORD

When energy and material resources are e x t r a c t e d , processed, conver ted , and used , t h e r e l a t e d p o l l u t i o n a l impacts on our environment and even on our hea l th o f t e n r e q u i r e t h a t new and i n c r e a s i n g l y more e f f i c i e n t p o l l u t i o n con- t r o l methods be used. The I n d u s t r i a l Environmental Research Laboratory- C inc inna t i ( I E R L - C i ) a s s i s t s i n developing and demonstrating new and improved methodologies t h a t w i l l meet these needs both e f f i c i e n t l y and economically.

For t h e f i r s t t i m e , an ope ra t ing and t roub leshoo t ing manual has been p u t t oge the r t o assist p l a n t ope ra to r s and managers of wastewater t r ea tmen t p l a n t s f o r the metal f i n i s h i n g indus t ry . This performance eva lua t ion and t roubleshoot ing guide has been reviewed by t r u e e x p e r t s of p l a t i n g wastewater t r ea tmen t - t h e p l a n t ope ra to r s . Without their day t o day i n s i g h t i n t o o p e r a t i o n a l and maintenance problems, the i n i t i a l f i e l d manual e f f o r t would n o t have been p o s s i b l e .

A more d e t a i l e d development of t h i s guide i s d e s t i n e d f o r t h e f u t u r e . Only by widespread use and eva lua t ion can t h i s manual be r ev i sed and upgraded t o r e f l e c t t he c u r r e n t and f u t u r e t r ends of complex t r ea tmen t and c o n t r o l technology systems r equ i r ed €or the metal f i n i s h i n g i n d u s t r y , It is hoped t h a t t h i s f i e l d manual w i l l guide p l a n t o p e r a t o r s i n achiev ing c o n s i s t e n t l y h ighe r performance l e v e l s .

Requests f o r f u r t h e r in format ion regard ing t h i s f i e l d manual on performance and t roub leshoo t ing a t metal f i n i s h i n g wastewater t rea tment f a c i l i t i e s should be d i r e c t e d t o t h e I n d u s t r i a l P o l l u t i o n Cont ro l Div is ion , IERL, C inc inna t i .

...

David G. Stephan Di rec to r

I n d u s t r i a l Environmental Research Laboratory 1 . . - I t 1

iii

AB ST RAC T

Th is manual p rov ides a techn ica l f i e l d guide o r re fe rence document f o r use i n improving t h e performance o f f a c i l i t i e s f o r the t reatment o f m e t a l - f i n i s h i n g wastes. The main purpose o f t h e manual i s t o p rov ide a t r o u b l e s h o o t i n g guide f o r i d e n t i f y i ng , a n a l y z i ng , and so l v i ng prob l ems.

The manual descr ibes general procedures f o r e v a l u a t i n g t h e performance o f

The procedures a l s o cover o t h e r i tems r e l a t e d t o t h e e f f e c t i v e opera- t rea tment processes and equipment commonly used i n metal f i n i s h i n g waste t r e a t - ment. t i o n o f t rea tment f a c i l i t i e s .

The methodology used t o evaluate compliance problems and develop Operat ion and Maintanence (O&M) s p e c i f i c s a r e descr ibed i n a rev iew o f t h e l i t e r a t u r e , f o l l o w e d by an assesment o f t h e causes o f p e r m i t v i o l a t i o n s and t h e recommended measures f o r improv ing compliance.

The u n i t processes descr ibed i n t h i s manual a re those commonly used i n t h e They a r e t h e f o l l o w i n g : e q u a l i z a t i o n , o i l t rea tment of metal f i n i s h i n g wastes.

removal , cyanide o x i d a t i o n ; chromium reduct ion, pH c o n t r o l , metal p r e c i p i t a t i o n ; f l o c c u l a t i o n , sedimentat ion, f i l t r a t i o n ; g r a v i t y t h i c k n i n g , b e l t f i l t e r presses, vaccum f i l t r a t i o n ; b e l t f i l t r a t i o n f o r dewatering, and c e n t r i f u g a t i o n . For each o f these u n i t processes, t h e manual con ta ins i n f o r m a t i o n on theory o f opera t ion , d e s c r i p t i o n o f equipment, opera t iona l procedures, t y p i c a l performance values, and t r o u b l e s h o o t i n g guide.

i v

CONTENTS ~

Foreword Abstract Figures Tab1 es Abbreviations and Symbols

1. Introduction

2. Conclusions and Recommendations

3 . Problem Assessment and Recommendations f o r

Purpose and Scope of Manual Manual Format

Conclusions Recommendations

Improving Permit Compliance Conventional Wastewater Treatment Problem Assessment Recommendations f o r Improvi ng Permit Compl i ance Resource Recovery

Introduction Theory of Operation Description of Equipment Operational Procedures Typical Performance Values Troubl eshooti ng Guide

Introduction Theory of Operation Description of Equipment

4. Equalization

5. Oil Removal

6. Cyan

Operational Procedures Typical Performance Va Troubl eshooti ng Guide de Oxidation Introduction Theory of Operation Description of Equipme Operational Procedures

ues

t

Typical Performance Values Troubleshooting Guide

Theory of Operation Description o f Equipment Operational Procedure Typical Performance Values Troubleshooting Guide

7. Chromium Reduction - TRt-

i i i i v

v i i i i x

X

1 1 1 3 3 5 8 8

10 14 16 19 19 19 2 1 25 29 29 29 32 33 35 42 51 52 57 57 57 62 64 70 70 74

74 74 75 77 85 85

V

Contents ( c o n t . )

8. pH Control In t roduct ion Theory of Operation Descr ip t ion of Equipment Operational Procedures Typical Performance Values Troubleshooting Guide

In t rod u c t i on Theory of Operation Descr ip t ion of Equipment Ope r a t i ona 1 Procedures Typical Performance Values Troubleshooting Guide

I n t roduc ti on Theory of Operation Descr ip t ion of Equipment Operational Procedures Typical Performance Values Troubleshooting Guide

In t roduct ion Theory of Operation Descr ip t ion of Equipment Operational Procedures Typical Performance Values Troubleshooting Guide

In t roduc t ion Theory of Operation Descr ip t ion of Equipment Operational Procedures Typica l Performance Values Troubleshooting Guide

9. Metal P r e c i p i t a t i o n

10. Floccula t ion

11. Sedimentation

12. F i l t r a t i o n

13. Gravity Thickening

14. B e l t

I n t roduc t ion Theory of Operation Descr ip t ion of Equipment Opera t iona l Procedures Typical Performance Values Troubleshooting Guide F i l t e r Presses In t roduc t ion Theory of Operation

Operational Procedures Typical Performance Values Troubleshooting Guide

& PCr,JipRtc..t

88 88 88 91 98

109 109 113 113 113 115 118 128 132 135 135 135 136 137 144 144 147 147 147 148 149 159 159 163 163 163 164 167 171 171 179 179 179 183 185 191 191 196 196 196 'I 97

197 204 204

vi

Contents ( c o n t . )

15. Vacuum F i l t r a t i o n In t roduct ion Theory of Operation Descr ip t ion of Equipment ope ra t iona l Procedures Typica l Performance Values Troubleshooting Guide

16. Pressu re F i l t r a t i o n f o r Dewatering In t roduct ion Theory of Operation Descr ip t ion of Equipment Operational Procedures Typical Performance Values Troubleshooting Guide

In t roduct ion Theory of Operation Descr ip t ion of Equipment ope ra t iona l Procedures Typical Performance Values Troubleshooting Guide

17. Cent r i fuga t ion

References

. * i

207 207 207 208 21 4 221 222 229 229 229 231 235 24 1 241 245 245 245 246 252 262 263

270

v i i

FIGURES

Number

1 2 3 4 5 6 7 8 9

10 11 12 13 14 15 16 17 18 19 20 21

22 23 24 25 26 27 28 29 30 31 32 33

Elec t rop la t ing Indus t ry Conventional Wastewater Treatment Methods of Equal iza t ion A P I Separator Line Diagrams of P r i n c i p a l DAF Process Var i a t ions Des t ruc t ion T i m e Required f o r Cyanate Vs p H D i s t r i b u t i o n of HOCL and OCL Two-Stage Cyanide Des t ruc t ion Hexavalent Chromium Reduction w i t h Su l fu r Dioxide Rela t ionship Between Hexavalent Chromium, pH, and Retention T ime f o r Su l fu r Dioxide Rela t ionship Between ORP, Sodium B i s u l f i t e Required, and pH A Sample T i t r a t i o n Curve Feedback Mode of p H Control Feedforward Mode of pH Cont ro l Feedback-Feedf orward Mode of pH Control Lead S o l u b i l i t y f o r a Typical Wastewater Lead S o l u b i l i t y as a Function of Carbonate Concentration Metal Concentration Versus Treatment Process Metal Concentration A f t e r Treatment Process Metal Concentration versus Treatment Process S o l u b i l i t y of Selected Heavy Metal Hydroxides Theore t i ca l S o l u b i l i t i e s of Metal Hydroxides and S u l f i d e s as a Function of pH Wastewater Treatment Processes f o r Removing Heavy Metals E f f e c t of Gravity Thickening upon So l ids Concentration E f f e c t of Time on Sludge Compaction Liquid Zones i n a Continuously Operated Thickener A Simple B e l t F i l t e r P res s Operating Zones of a Vacuum F i l t e r Typical Equipment Layout of Rotary Vacuum F i l t e r System Cutaway V i e w of a F i l t e r Press Side V i e w of a F i l t e r Press Cross Sec t ion of Concurrent Flow Solid-Bowl Cent r i fuge Schematic Diagram of a Basket Cent r i fuge D i s c Type Cent r i fuge

Page

9 22 37 39 59 61 63 76

81 83 90 93 94 96

114 116 117 119 122 124

126 127 180 182 184 198 21 0 21 3 232 233 247 249 25 1

v i i i

TABLES

Number

1 2 3 4 5 6 7 8 9

10 11 12 1 3 14 15 16 17 18 19 20 21 22 23 24 25

26 27 28

29

30

Equal iza t ion Troubleshooting Guide O i l Removal Process Monitoring Requirements O i l Removal Troubleshooting Guide Cyanide Oxidation Process Monitoring Requirements Horsepower Requirements f o r Medium Agi t a t ion Cyanide Oxidation Troubleshooting Guide Chromium Reduction Process Monitoring Requirements Chromium Reduction Troubleshooting Guide pH Ad] ustment Troubleshooting Guide Metal P r e c i p i t a t i o n Process Monitoring Requirements Metal P r e c i p i t a t i o n Troubleshooting Guide F loccula t ion Process Monitoring Requirements F loccula t ion Troubleshooting Guide Sedimentation Process Monitoring Requirements Sedimentation Troubleshooting Guide F i l t r a t i o n Process Monitoring Requirements F i l t r a t i o n Troubleshooting Guide Gravity Thickening Process Monitoring Requirements Gravity Thickening Troubleshooting Guide B e l t F i l t e r Presses Process Monitoring Requirements B e l t F i l t e r Press Troubleshooting Guide Vacuum F i l t r a t i o n Process Monitoring Requirements Typical Performance Values f o r Vacuum F i l t r a t i o n Dewatering Vacuum F i l t r a t i o n Troubleshooting Guide Pressure F i l t r a t i o n f o r Dewatering Process Monitoring Requirements Pressure F i l t r a t i o n f o r Dewatering Troubleshooting Guide Cen t r i fuga t ion Process Monitoring Requirements Summary of Opera t iona l Var iab les Affec t ing Cent r i fuge

Typical Performance Values f o r Cen t r i fuga l Thickening

Cen t r i fuga t ion Troubleshooting Guide

Performance

and Dewatering

Page

30 43 53 65 69 71 78 86

110 121 133 138 145 151 160 169 172 187 192 200 205 21 6 223 224

238 242 254

257

2 64 265

Px

LIST OF ABBREVIATIONS AND SYMBOLS

Abbreviations

A:S - A i r t o So l ids Ratio

API - American Petroleum I n s t i t u t e

DAF - Dissolved A i r F lo ta t ion

HRT - Hydraulic Retention Time

IAF - Induced A i r F lo ta t ion

ORP - Oxidation-Reduction Po ten t i a l

P I - Proport ional Plus I n t e g r a l

P I D - Proportional-Integral-Derivative

SOR - Surface Overflow Rate

SS - Suspended Sol ids Concentration

SVR - Sludge Volume Rat io

TSS - Tota l Suspended Sol ids Concentration

Symbols

C - Concentration

Ce - Eff luent TSS Concentration

C - I n f l u e n t TSS Concentration

'i CR - Sludge TSS Concentration

MVG - Mean Veloci ty Gradient f o r Flocculat ion

M - Dynamic Viscos i ty

P - Power Input f o r Flocculat ion

- In f luen t TSS Concentration t o ' E l a r i f i e r

PD - g -

Qw - 0

Q -

-

Qi - QR - SL - v -

Polymer Dosage

Polymer Flowrate

Flowrate of Wastewater

Ff f l u e n t Flowra t e

In f luen t Flowra t e

I n f l u e n t Flowrate t o C l a r i f i e r

Sludge Removal Flowrate

So l ids Loading

Volume

X

SECTION 1

INTRODUCTION

PURPOSE AND SCOPE OF MANUAL

The purpose of t h i s manual is t o provide a technica l f i e l d guide o r

reference document f o r use i n improving the performance of f a c i l i t i e s

f o r the treatment of metal f in i sh ing wastes. The main purpose of the

manual i s t o provide a troubleshooting guide f o r the following: 1 )

i den t i fy ing problems, 2 ) analyzing problems, and 3 ) solving problems.

p l a n t p e r s o n n e l r e s p o n s i b l e f o r was te t r e a t m e n t p r o c e s s e s and

achieving permit compliance must be knowledgeable about not only the

problem areas , bu t a l s o w i t h e l ec t rop la t ing and r e l a t ed metal f i n i s h i n g

concepts and in-p lan t process modifications and changes as they r e l a t e

t o the waste treatment processes. This manual descr ibes general proce-

dures f o r evaluat ing the performance of treatment processes and equip-

ment commonly used i n metal f i n i s h i n g waste treatment. The procedures

also cover other items r e l a t e d t o the e f f e c t i v e operation of treatment

f a c i l i t i e s . Troubleshooting guides, operat ing s t r a t e q i e s , and process

monitoring mater ia l a r e discussed i n d e t a i l f o r each u n i t process com-

monly used i n metal f i n i sh ing waste treatment.

MANUAL FORMAT

I t is assumed t h a t the manual user has a general understanding of

treatment f a c i l i t i e s and t h e i r operation. The s t y l e , language, and for -

m a t of the manual are directed to the level and technical knowledge of a

technician with some experience with in-plant operat ion, design, inspec-

t i on , o r performance evaluat ion.

1

The manual

Sec t ion 1

Sec t ion 2

Sect ion 3

Sec t ions 4

is organized i n t o the following major s e c t i o n s :

INTRODUCTION. The purposel scopel and format of t h e

manual a r e descr ibed i n t h i s section.

CONCLUSIONS AND RECOMMENDATIONS. _The major conclusions and recommendations regarding compliance - problems and

develoDment of OGM snecifics are described in this section.

PROBLEM ASSESSMENT AND RECOMMENDATIONS FOR IMPROVING

PERMIT COMPLIANCE. The causes of permit v i o l a t i o n s and

t h e recommended measures f o r improving compliance a r e

descr ibed i n t h i s sec t ion .

through 17. UNIT PROCESS EVALUATION AND TROUBLESHOOTING

INFORMATION. For each u n i t process commonly used i n t h e

treatment of metal f i n i s h i n g wastes, t h i s s e c t i o n con-

t a ins t h e fol lowing information:

o In t roduct ion

o Theory of Operation

0 Descript ion of Equipment

o Operat ional Procedures

o Typical Performance Values

o Troubleshooting Guide

2

SECTION 2

CONCLUSIONS AND RECOMMENDATIONS

Conclusions

This manual d e t a i l s performance eva lua t ion of wastewater t rea tment

f a c i l i t i e s f o r meta l - f in i sh ing wastes. Troubleshooting guides on u n i t

p rocess ope ra t ions are inc luded t o h e l p p inpo in t causes of t rea tment

malfunctions.

It i s concluded t h a t ope ra to r s and owners of me ta l - f in i sh ing p l a n t s can

u t i l i z e t h i s manual t o h e l p b r ing t h e i r wastewater t r ea tmen t f a c i l i t i e s i n t o

compliance. This can be accomplished b e s t by a j o i n t e f f o r t on t h e p a r t of

management and t h e ope ra to r s .

Monitoring of t r ea tmen t parameters i s t h e key f a c t o r i n determining

performance of f a c i l i t i e s and an e a r l y warning f o r non-compliance t r ends .

The purpose o f t h i s manual i s t o provide a t e c h n i c a l f i e l d guide o r

r e f e r e n c e document f o r u s e i n improving t h e performance of f a c i l i t i e s f o r

t h e t r ea tmen t of metal f i n i s h i n g wastes. The main purpose of t h e manual i s

t o provide a t roub le shoo t ing guide f o r t h e following: 1) i d e n t i f y i n g problems,

2 ) ana lyz ing problems, and 3 ) so lv ing problems.

P l a n t personnel r e spons ib l e f o r waste t rea tment processes and achiev ing /

permi t compliance must be knowledgeable about n o t on ly the problem areas, b u t

a l s o with e l e c t r o p l a t i n g and r e l a t e d meta l f i n i s h i n g concepts and in -p lan t

process mod i f i ca t ions and changes as they r e l a t e t o t h e waste treatment pro-

cesses. This manual d e s c r i b e s g e n e r a l procedures f o r eva lua t ing t h e performance

of t r ea tmen t processes and equipment commonly used i n metal f i n i s h i n g waste

The l i t e r a t u r e search a l s o inc luded equipment manufacturers ' in format ion ,

d a t a provided by p r o f e s s i o n a l o rgan iza t ions , and communications with personnel

who were f a m i l i a r wi th t h e t r ea tmen t and d i s p o s a l of metal f i n i s h i n g wastes.

3

The o b j e c t i v e s of t h e l i t e r a t u r e review were t o c o l l e c t d a t a which would a i d

i n i d e n t i f y i n g t h e major causes of permit v i o l a t i o n s , and t o c o l l e c t informa-

t i o n which could be used t o develop opera t ion and maintenance (O&M) s p e c i f i c s

q u a l i f y i n g t h i s d a t a and informat ion , methods and techniques f o r improving

compliance of f a c i l i t i e s could be developed.

An a n a l y s i s of permit v i o l a t i o n s w a s conducted t o understand t h e problems

associated with t rea tment of metal f i n i s h i n g wastes. The t roubleshoot ing

manual was then prepared t o address t h e problems. An a n a l y s i s of permit

v i o l a t i o n s w a s performed u t i l i z i n g t h e Quar te r ly Noncompliance Report publ ished

by t h e Off ice of Water of t h e United S t a t e s Environmental P r o t e c t i o n Agency.

The r e p o r t l i s t e d t h e major i n d u s t r i e s t h a t w e r e o u t of compliance and t h e

parameters t h a t w e r e o u t of compliance. The v i o l a t i o n s i n t h e Noncompliance

Report were l i s t e d by S I C code. The SIC numbers used f o r i d e n t i f y i n g i n d u s t r i e s

wi th metal wastes were 3471 ( e l e c t r o p l a t i n g ) , 3631, 3632, 3633, 3639, 3714,

3721, and 3731.

Operation and Maintenance (O&M) s p e c i f i c s f o r t h e t rea tment of metal

f i n i s h i n g wastes were obtained from a s e a r c h of t e c h n i c a l p u b l i c a t i o n s , a

computerized l i t e r a t u r e sea rch , and c o n t a c t s with equipment manufacturers

and o p e r a t o r s . The information c o l l e c t e d from these sources w a s then

i n t e r p r e t e d and compiled. This f i e l d manual was developed from information

o b t a i n e d from t h e s e sources .

I t is assumed t h a t t h e manual u s e r has a g e n e r a l understanding of treat-

ment f a c i l i t i e s and t h e i r opera t ion . The s t y l e , language, and format of t h e

manual are d i r e c t e d t o t h e l e v e l and t e c h n i c a l knowledge of a technic ian wi th

some experience with i n - p l a n t opera t ion , des ign , i n s p e c t i o n , or performance

eva lua t ion .

Recommendations

The l e v e l of p o l l u t a n t s discharged t o p u b l i c l y owned t rea tment works

by p l a n t s fol lowing pre t rea tment must meet new r e g u l a t i o n s . The e l e c t r o -

p l a t i n g and meta l - f in i sh ing p o i n t source c a t e g o r i e s i s regula ted by t h e

USEPA ( F e d e r a l R e g i s t e r , 48(137), 32462-324885 J u l y 15, 1983).

4

Improving the l e v e l of permit compliance f o r t r ea tmen t of metal-

f i n i s h i n g wastes i s a two-step process . The f i r s t s t e p i s t o i d e n t i f y

t h e problem and t h e second s t e p i s t o t ake t h e necessary c o r r e c t i v e a c t i o n s .

P e r m i t compliance problems are g e n e r a l l y i n t he fo l lowing fou r c a t e g o r i e s :

1) d e s i g n , 2 ) opera t ion , 3 ) a d m i n i s t r a t i o n , and 4 ) maintenance.

The importance of proper des ign cannot be o v e r s t a t e d . Each u n i t

' process , along with the i n t e g r a t e d waste t r ea tmen t system, must be designed

with numerous f a c t o r s accounted f o r . An improperly designed system seldom

o p e r a t e s w e l l . Design improvements t o enhance permi t compliance is a

long-term process ; near-term improvements a r e seldom a t t a i n a b l e through

des ign changes.

Performance of a well-designed system may be a f f e c t e d by improper

o p e r a t i n g procedures. Thus, an o p e r a t o r ' s f a m i l i a r i t y with c o r r e c t O&M

procedure can d i r e c t l y improve ope ra t ion of a system. I t i s the goa l of

t h i s p u b l i c a t i o n t o provide adequate O&M procedures and t roubleshoot ing

guides t o produce improved l e v e l s of permit compliance i n me ta l - f in i sh ing

waste t r ea tmen t p l a n t s . 1

Adminis t ra t ion a f f e c t s permit compliance, although o f t e n i n an i n d i r e c t

manner. Such i tems as s t a f f supe rv i s ion , mot iva t ion , funding, and planning

a f f e c t t h e ope ra t ion of a f a c i l i t y , which, i n t u r n , a f f e c t s a l l a s p e c t s of

t h e t r ea tmen t p l a n t .

F i n a l l y , maintenance a f f e c t s permit compliance d i r e c t l y . I n numerous

i n s t a n c e s throughout t he d e s c r i p t i o n s t h a t fo l low, r o u t i n e in spec t ion and

maintenance are c i t e d as t h e ch ie f d e t e r r e n t s t o ope ra t ing problems, and

hence t o permi t v i o l a t i o n s . A competent, we l l - t r a ined maintenance group

i s ind i spensab le i n t h e smooth and s u c c e s s f u l ope ra t ion of a t rea tment

p l a n t . Although t h e tendency i s t o c a t e g o r i z e permi t compliance problems a s

belonging s t r i c t l y t o one of t h e fou r a r e a s d iscussed above, t h e f a c t i s

t h a t most problems have a s p e c t s of two o r more areas. P l a n t s owners and

o p e r a t o r s t h a t hope t o improve permi t compliance must s t r i v e t o achieve

improvement i n a l l fou r c a t e g o r i e s .

5

RESOURCE RECOVERY RECOMMENDATIONS

P o l l u t i o n c o n t r o l l e g i s l a t i o n has a f f e c t e d i n d u s t r y by i n c r e a s i n g t h e

economic p e n a l t y a s soc ia t ed with i n e f f i c i e n t use of r e sources . I n t h e

p l a t i n g i n d u s t r y , €or example, loss of a r a w material i n t h e wastewater can

r e s u l t i n t h r e e d i s t i n c t c o s t i tems: replacement of t h e m a t e r i a l , removal

of t h e material from t h e wastewater before d i scha rge , and d i s p o s a l of t h e

r e s i d u e . S imi l a r c o s t items e x i s t f o r process water: replacement of water

( n o longer inexpensive t o purchase) used i n process ing , process ing t h e water

i n t h e wastewater t r ea tmen t system, and process ing i n t h e water by the

t r ea tmen t p l a n t a f t e r d i scharge i n t o a pub l i c sewer sys t em.

I n response t o t h e increased c o s t . o f raw material l o s s e s , p l a t i n g shop

processes are being modified t o reduce t h e s e losses as w e l l as water

consumption. Recent years a l s o have seen t h e c o s t - e f f e c t i v e a p p l i c a t i o n

of va r ious s e p a r a t i o n processes t h a t reclaim p l a t i n g chemicals from r i n s e

waters, enab l ing both t h e r a w material and t h e water t o be reused.

The impact of resource recovery and p o l l u t a n t load r educ t ion modi f ica t ions

on waste t r ea tmen t and s o l i d waste d i s p o s a l c o s t s must be measured, i f t h e s e

mod i f i ca t ions a r e t o be eva lua ted . Cos t of s o p h i s t i c a t e d t r ea tmen t necessary

f o r e l e c t r o p l a t i n g wastewater and of r e s idue d i s p o s a l o f t e n provides a

s i g n i f i c a n t economic i n c e n t i v e f o r r e source recovery.

REDUCED LOADING RECOMMENDATIONS

Modif ica t ions tha t w i l l reduce t h e p o l l u t a n t s o r wastewater loadings on

a t rea tment f a c i l i t y range from us ing flow r e s t r i c t o r s t o e l imina te excess

d i l u t i o n i n r i n s e tanks t o i n s t a l l i n g recovery u n i t s , such as reve r se osmosis

and evapora t ion , t o s e p a r a t i n g p l a t i n g chemicals from r i n s e water f o r r e c y c l e

t o t h e p l a t i n g ba th . Actions t h a t can minimize wastewater volume inc lude t h e

1) Implementing r igo rous housekeeping p r a c t i c e s t o l o c a t e and r e p a i r

water l eaks qu ick ly ,

2 ) Employing mul t ip l e counter f low r i n s e tanks t o reduce r i n s e water

use s u b s t a n t i a l l y ,

6

3) Employing spray r i n s e s t o minimize r i n s e w a t e r u s e ,

4 ) Using conduc t iv i ty c e l l s t o avoid excess d i l u t i o n . i n t h e r i n s e t anks ,

5) I n s t a l l i n g flow r e g u l a t o r s t o minimize water u s e , and

6 ) Reusing contaminated r i n s e water and t r e a t e d wastewater where f e a s i b l e .

S teps t o minimize p o l l u t a n t loadings inc lude :

0

0

0

0

0

0

0

0

0

Implementing a r igorous housekeeping program t o l o c a t e and r e p a i r l eaks

around process ba ths ,

Replacing f a u l t y i n s u l a t i o n o r p l a t i n g racks t o prevent excess ive

s o l u t i o n drag-out,

I n s t a l l i n g d r i p t r a y s where needed,

Using sp ray r i n s e s o r a i r knives t o minimize s o l u t i o n drag-out from

p l a t i n g ba ths , Recycling r i n s e waters t o p l a t i n g ba th t o compensate f o r s u r f a c e

evapora t ion losses,

Using s p e n t process s o l u t i o n s as wastewater treatment reagents ( a c i d

and a l k a l i n e c l ean ing ba ths are obvious examples),

Using minimum process ba th chemical concen t r a t ions ,

I n s t a l l i n g recovery processes t o rec la im p l a t i n g chemicals from r i n s e

waters f o r r ecyc le t o t h e p l a t i n g ba th , and

Using process ba th p u r i f i c a t i o n t o c o n t r o l t h e l e v e l of i m p u r i t i e s

and prolong t h e b a t h ' s s e r v i c e l i f e .

Closed-loop chemical recovery from a r i n s e stream can o f t e n provide t h e

s o l u t i o n t o treat . Applying a closed-loop recovery sys t em t o a p l a t i n g

ope ra t ion e l i m i n a t e s t h e need t o t r ea t the r i n s e water normally a s soc ia t ed

w i t h t h a t s t e p . In t h e case of r i n s e streams r e q u i r i n g p re t r ea tmen t ( f o r examples,

P 1 va nr -1 at- m e s c o n t a * ning p o l l u t a n t s n o t e f f e c t i v e l y removed

by convent iona l end-of-pipe technology ( f o r example, some types of complexed

metals), i n s t a l l i n g a closed-loop s y s t e m t o r e c y c l e the r i n s e may reduce

t h e investment needed t o comply wi th t h e e f f l u e n t q u a l i t y l i m i t a t i o n s .

7

SECTION 3

PROBLEM ASSESSMENT AND RECOMMENDATIONS

FOR IMPROVING PERMIT COMPLIANCE

CONVENTIONAL WASTEWATER TREATMENT

Conventional wastewater treatment i n the e l ec t rop la t ing indus t ry

cons i s t s of the following u n i t processes (see Figure 1 ) :

o Chromium reduction ( i f needed) of segregated chromium waste

streams t o reduce the chromium from i t s hexavalent form t o the

t r i v a l e n t s t a t e , which then can be p rec ip i t a t ed as chromium

hydroxide by a l k a l i neu t r a l i za t ion ,

o Cyanide oxidat ion ( i f needed) of segregated cyanide-bearing

waste streams t o oxidize the tox ic cyanides t o harmless carbon

and nitrogen compounds,

o Neut ra l iza t ion of the combined metal-bearing wastewaters,

a c i d / a l k a l i wastewaters, s t rong chemical dumps, and the e f f l u e n t

from the cyanide and chromium treatment systems t o a d j u s t the pH

within acceptable discharge l i m i t s and t o p r e c i p i t a t e the d i s -

solved heavy metals as metal hydroxides,

o C l a r i f i c a t i o n , i n which flocculating/coagulating chemicals a r e

added t o promote the i n i t i a l s e t t l i n g of the p rec ip i t a t ed metal

hydroxides,

o Gravity thickening over extended t i m e t o increase s o l i d s content ~

of sludge before d isposa l . ~

These u n i t processes provide e f f e c t i v e , r e l i a b l e treatment f o r many

e l ec t rop la t ing waste streams. That i s not t o say, however, t h a t such

treatment is s u i t a b l e f o r a l l appl ica t ions or t h a t the "normal" design

8

I

Figure 1.

Electroplating Industry Conventional Wastewater Treatment

& Wastewater

* Solid waste

Legend: S = sulfonator C = chlorinator

ORP = oxidation reduction potential

disposal

SOURCE: USEPA ENVIRONMENTAL REGULATlONS AND TECHNOLOGY: THE ELECTROPLATING INOUSTRY, €PA 825110-60-001, AUGUST 1980, P. 13.

9

parameters ( r e t en t ion t ime, , reagent dosage, and so f o r t h ) w i l l provide

e f f e c t i v e po l lu t an t removal from every ind iv idua l p l a t e r ' s wastewater

discharge.

PROBLEM ASSESSMENT

A p l a n t assessment i s the i n i t i a l s t e p i n a po l lu t ion con t ro l pro-

gram. A p l a n t assessment involves a thorough ana lys i s of the operat ions

of a metal f i n i s h i n g p l a n t t h a t r e l a t e t o po l lu t an t sources and water

use. The information generated during a p l an t assessment is used i n

eva lua t ing the appl ica t ion of in-plant changes f o r reducing chemical

loss and water use.

A p l a n t assessment includes the following s teps: 1 ) inspec t ion of

the p l a t i n g room layout , 2 ) review of p l a n t opera t ing p rac t i ces , 3 )

examination of process water use, 4 ) performance of sampling and labora-

to ry ana lys i s t o charac te r ize waste streams and t o determine dragout

r a t e s , and 5 ) i d e n t i f i c a t i o n of the frequency, volume, and character-

i s t ics of batch dumps.

L a b o r a t o r y a n a l y s e s of was tewater samples a r e performed u s i n g

s tandard EPA-approved techniques. Throughout t h i s manual, var ious

a n a l y t i c a l parameters and t h e i r concentrat ions a r e discussed. For a l l

tests the a n a l y t i c a l methodology presented i n the EPA document "Methods

f o r Chemical Analysis of Water and Wastes" ( * ) o r "Standard Methods f o r

the Examination of Water and Wastewater"(9) should be followed.

successful operaticm and ma ~

intenance (O&M) of a waste treatment ~~

plant requires consistent performance that exceeds regulatory compliance

leve ls . Fa i lu re t o meet these compliance l eve l s can r e s u l t i n cos t ly

d isposa l a l t e r n a t i v e s , f i n e s , damage t o the environment, and adverse

10

. publ ic react ion. If a treatment f a c i l i t y f a i l s t o meet compliance

s tandards , the problem usua l ly f a l l s under one of the following causes:

1 ) Shock loadings (hydraul ic or contaminants) t o t h e waste

t reatment p l an t ,

Poor understanding of O&M procedures, 2 )

3 ) Poor process cont ro l ,

4 ) Equipment f a i l u r e , and

5 ) Treatment p l an t design inadequacies.

The p o t e n t i a l effect of good 0 6 M on each of the f i v e ca tegor ies of

non-compliance reasons is discussed below. I f a p l an t f i nds i t s e l f i n

non-compliance it should determine which ca tegor ies of causes a r e appl i -

cab le and take appropriate ac t ion t o see i f improved OfM could be e f fec-

t i v e i n improving its performance.

Shock Loadings

Shock loadings of flow o r contaminants f requent ly cause treatment

process upsets which can r e s u l t i n non-compliance. Sources of these

shock loadings can be e i t h e r s p i l l s o r r e l eases from production batch

operat ions o r c leaning operations. Their impact on the treatment pro-

cess can be mitigated by i n s t a l l i n g s u f f i c i e n t equal izat ion. Their

impact can a l s o of ten be cont ro l led by changes i n opera t ing procedures

i n the production f a c i l i t y o r i n t h e treatment p lan t . Of forepos t i m -

portance is communication between prcductian and waste-treatment person-

nel . I f waste-treatment personnel are no t i f i ed of p o t e n t i a l shock loads

i n s u f f i c i e n t t ime, m i t i g a t i n g a c t i o n o f t e n can be t a k e n , such a s

d i v e r t i n g the shock load t o sidestream equal iza t ion t o temporarily

bypass s n s i t i v e processes , o r manually modifying process operat ing

parameters t o a d j u s t f o r the shock loading. t

Modification of production procedures w i t h respec t t o s p i l l con t ro l

and opera t ina procedures f o r batch processes and cleaning operat ions can

reduce the magnitude of shock loadings i n seve ra l ways. Wastes from

batch o r c leaning operat ions can be re leased slowly o r during times of

11

low flow. S p i l l s can be cleaned up usinq dry chemicals and not be hosed

down the drain. Chemical handling procedures can be modified t o reduce

the l ikel ihood of s p i l l s and chemicals can be s to red i n diked a r e a s t o

contain s p i l l s t h a t do occur.

In a l l cases implementation of the above procedures requi res good

t r a in ing of a l l personnel i n proper operat ing procedures t o con t ro l

shock loading. P a r t of t h i s t r a in ing must include making production

personnel aware t h a t t h e i r procedures impact waste treatment. This

f a c t o r is becoming increas ingly important a s some f a c i l i t i e s have had t o

c u r t a i l production i n order t o achieve discharge compliance l eve l s .

Treatment p lan t personnel a l s o must be t ra ined i n the proper operat ing

procedures t o mi t iga te the impact of a shock load. T h i s ac t ion may be

d ive r t ing flow t o sidestream equal iza t ion , bypassing a n o i l water separ-

a t o r while a non-oily hydraul ic shock load is occurr ing, no t i fy ing

production t o s t o p o r slow an excessive discharge, o r o ther appropriate

procedures.

Poor Understanding of O t M Procedures

A good understanding of O&M procedures is e s s e n t i a l t o success-

fully operate a facility for the treatment of metal finishing wastes. An

opera tor who is w e l l versed i n the proper O&M procedures can usua l ly

operate the treatment f a c i l i t y t o meet permit compliance even though one

o r more of the above causes of permit v io l a t ion e x i s t s a t the treatment

f a c i l i t y . This manual was developed t o a s s i s t operators i n implementing

the proper O&M procedures a t the treatment f a c i l i t y . While no manual

can be general enough f o r a l l p l an t s and y e t s p e c i f i c enough f o r one

p l a n t , the in t en t ion of t h i s manual i s t o a id i n the understanding of

the cause /e f fec t r e l a t ionsh ip f o r s eve ra l treatment processes. Once an

opera tor has developed a cause /e f fec t r e l a t ionsh ip f o r the cont ro l var i -

ab les a t the treatment f a c i l i t y , then s p e c i f i c adjustments and/or set-

poin ts can be establ ished. A successfu l O&M program w i l l be a t t a ined

when the operator can i d e n t i f y a p o t e n t i a l problem, understand the cause

of t h e problem and what e f f e c t it w i l l have on compliance, and t h e n ad-

j u s t the con t ro l var iab les so a s t o mi t iga te the problem.

1 2

Poor Process Control

One of the most common causes of continuous poor performance and

f requent non-compliance is poor process control . Poor process c o n t r o l

r e s u l t s i n the treatment p l an t not achieving its f u l l capaci ty and

ef f ic iency . when the f u l l or design e f f i c i ency is not achieved, the

blame is put f requent ly on poor design, bu t it must be remembered t h a t

the design is based upon the assumption of good process con t ro l w h i c h

may o r may not be occurring. Good process cont ro l can only be achieved

by w e l l t ra ined operators who know and understand t h e i r equipment and

the impact of a l l operat ing var iab les under t h e i r control . T h i s in-

c ludes understanding the i n t e r a c t i o n between operat ing var iab les and the

t rade-offs of ten involved. As an example, increas ing the b e l t tension

i n a b e l t f i l t e r p ress can r e s u l t i n a d r i e r cake but w i l l a l s o r e s u l t

i n more s o l i d s i n the f i l t r a t e and a sho r t e r b e l t l i f e . However, the

s o l i d s i n the f i l t r a t e might adversely a f f e c t the performance of o ther

t reatment processes such a s an o i l coalescer .

Process con t ro l through good operat ions i s p a r t i c u l a r l y important

i n the metal f i n i s h i n g indus t ry where seve ra l waste t reatment processes

requi re c r i t i c a l con t ro l of operat ing var iab les t o achieve good t r e a t -

ment performance. Examples include pH con t ro l €or metal p r e c i p i t a t i o n ,

and pH and oxida t ion- reduct ion p o t e n t i a l (OW) c o n t r o l for chromium

reduct ion and cyanide oxidation. A r e l a t i v e l y s l i g h t change i n these

operat ing var iab les can r e s u l t i n s i g n i f i c a n t degradation i n per f or-

mance, non-compliance, and i n the case of cyanide reduct ion, t h e poten-

t i a l f o r r e l ease of t ox ic gases.

Equipment Fa i lu re

Equipment f a i l u r e can r ead i ly cause a treatment

meet regula tory compliance leve ls . The impact of the

can be minimal when r epa i r s a r e implemented quickly or

p l an t t o f a i l t o

equipment f a i l u r e

the impact may be

major with p a r t s and r e p a i r s taking days t o obtain and i n s t a l l . It i s

the re fo re e s s e n t i a l t o minimize equipment f a i l u r e and downtime. This

1 3

minimization of downtime can be achieved p a r t i a l l y by a s u f f i c i e n t p a r t s

inventory and overdesign; it a l s o requi res qood operat ion and mainte-

nance of e x i s t i n g treatment p l an t equipment. Mechanical equipment h a s a

se t of design operat ing condi t ions and any time these condi t ions a r e

exceeded, premature equipment f a i l u r e can occur. Trea tment p l a n t per-

sonnel should be aware of these design condi t ions and i n t e g r a t e them

with p l an t operat ing procedures t o insure t h a t mechanical equipment is

not unduly s t ressed . I t should be noted t h a t t h i s stress does n o t always come from mechanical forces . Improper pH l e v e l s can corrode

equipment and excessively high temperatures can cause mater ia l s of

cons t ruc t ion t o f a i l . Once equipment f a i l u r e has occurred, prompt

r e p a i r of equipment by w e l l t ra ined maintenance personnel is e s s e n t i a l

t o minimize the impact and prevent recurrence.

Good operat ions and maintenance can a l s o prevent equipment f a i l u r e

by a regular and order ly inspec t ion of equipment f o r wear o r o ther e a r l y

s igns of equipment f a i l u r e such as v ibra t ion . A good prevent ive main-

tenance program is another e s s e n t i a l f a c e t of prevent ing premature

equipment f a i l u r e . Treatment P l a n t Desisn Inadequacies

N o amount of good operat ions and maintenance can make a poorly o r

improperly designed treatment p l a n t achieve cons i s t en t compliance w i t h

regula tory s tandards; conversely, poor operat ions and maintenance can

make even the b e s t designed treatment p l an t f a l l i n t o noncompliance.

Before any major design modifications a r e implemented, the p o t e n t i a l f o r

t reatment p l a n t performance improvement through improved O&M should be

inves t iga ted thoroughly.

RECOMMENDATIONS FOR IMPROVING PERMIT COMPLIANCE

The l e v e l of po l lu t an t s t h a t may be discharqed

treatment works (POW) is regulated by the USEPA.

~

~~~~

t o publ ic ly owned

Spec i f i ca l ly , the

14

l e v e l of po l lu t an t s dischargable by a p l an t f a l l i n g within the e l ec t ro -

p l a t i n g and metal f i n i sh ing poin t source ca tegor ies is l imi ted by regu-

( l o ) A copy of l a t i o n s issued i n the Federal Register on Ju ly 1 5 , 1983.

these regula t ions is included a s Appendix B of t h i s manual.

Improving the l e v e l of permit compliance f o r t reatment of metal

f i n i s h i n g wastes is a two-step process. The f i r s t s t e p i s t o i d e n t i f y

the problem and the second s t e p i s t o take the necessary co r rec t ive

ac t ions . P e r m i t compliance problems genera l ly w i l l f a l l i n t o the f o l -

lowing four ca tegor ies : 1 ) design, 2) operat ion, 3 ) adminis t ra t ion , and

4 1 maintenance.

A s mentioned elsewhere i n t h i s repor t , the importance of proper

design cannot be overstated. Each u n i t process along with t h e i n t e -

gra ted waste treatment system must be designed with numerous f a c t o r s

accounted for . A system which is improperly designed w i l l seldom be a

system which can be operated w e l l . Improvement of permit compliance by

improving design is a long-term process; near-term improvements a r e

seldom a t t a i n a b l e through design changes.

A well-designed system may not perform a s expected because of i m -

proper operat ing procedures. A number of ins tances of improper opera-

t i on have ex is ted ; improvement of operat ion i s a d i r e c t funct ion of t he

opera tors ' f a m i l i a r i t y with c o r r e c t OLM 'procedures. It is the goal of

t h i s publ ica t ion t o provide adequate O&M procedures and t roubleshoot ing

guides t o produce improved l eve l s of permit compliance i n metal f i n i s h -

ing waste treatment p lan ts .

Administration a l s o a f f e c t s permit compliance, although o f t en i n an

i n d i r e c t manner. Such items a s s t a f f supervis ion, motivation, funding,

and planning a f f e c t the operat ion of a f a c i l i t y , which i n turn a f f e c t s

a l l aspec ts of the t reatment p lan t .

F ina l ly , maintenance a f f e c t s permit compliance d i r e c t l y . I n num-

erous ins tances throughout the descr ip t ions w h i c h follow, rout ine i n -

spect ion and maintenance a r e c i t ed a s the chief de t e r r en t s t o operat ing

15

problems, and hence t o permit v io l a t ions . A competent, wel l - t ra ined

maintenance group is indispensable i n the smooth and success fu l opera-

t i o n of a t reatment p l a n t .

Although the tendency is t o ca tegor ize permit compliance problems

as belonging s t r i c t l y t o one of the four areas d iscussed above, i n f a c t

most problems have aspec ts of two or more a reas . A p l a n t which hopes t o

improve its permit compliance must s t r i v e t o achieve improvement i n a l l

four areas.

( 1 1 RESOURCE RECOVERY

Pol lu t ion c o n t r o l l e g i s l a t i o n has a f f e c t e d indus t ry by inc reas ing

the economic penal ty a s soc ia t ed with i n e f f i c i e n t use of raw ma te r i a l s .

In the p l a t i n g indus t ry , f o r example, loss of a raw ma te r i a l i n t h e

wastewater can r e s u l t i n th ree d i s t i n c t c o s t items: replacement of t he

ma te r i a l , removal of the mate r i a l from the wastewater before d ischarge ,

and d i sposa l of t he res idue . Similar c o s t items e x i s t f o r process

water: replacement of water (no longer inexpensive t o purchase) used i n

processing, prepara t ion of t he water i n a t rea tment system, and proces-

s ing of the water by the waste t reatment p l a n t a f t e r discharge i n t o a

sewer system.

I n response t o the increased c o s t of r a w material lo s ses , p l a t i n g

shops are modifying t h e i r processes t o reduce these lo s ses a s w e l l as

water consumption. Recent years a l s o have seen t h e c o s t - e f f e c t i v e

app l i ca t ion of var ious sepa ra t ion processes t h a t reclaim p l a t i n g chemi-

cals from r i n s e waters, enabl ing both the raw ma te r i a l and the water t o

be reused.

The impac t of r e s o u r c e r e c o v e r y and p o l l u t a n t l o a d r e d u c t i o n

modif icat ions on waste t rea tment and s o l i d waste d i sposa l . cos t s must be

measured i f these modif icat ions a r e t o be evaluated. Cost of s o p h i s t i -

cated t reatment necessary f o r e l e c t r o p l a t i n g wastewater and of res idue

d i s p o s a l o f t en provides a s i g n i f i c a n t economic incen t ive f o r resource

recovery .

16

Modifications f o r reducing the p o l l u t a n t o r wastewater loading on a

t rea tment f a c i l i t y range from using flow r e s t r i c t o r s t o e l imina te excess

d i l u t i o n i n r i n s e tanks t o i n s t a l l i n g recovery u n i t s , such a s r eve r se

osmosis and evapora t ion , t o s epa ra t ing p l a t i n g chemicals from r i n s e

water f o r r ecyc le t o the p l a t i n g bath. Actions t h a t can minimize waste-

water volume inc lude the following:

Implementing r igorous housekeeping p r a c t i c e s t o l o c a t e and

r e p a i r water leaks quick ly ,

Employing mul t ip l e counterflow r i n s e tanks t o reduce r i n s e water

use s u b s t a n t i a l l y ,

Employing spray r i n s e s t o minimize r i n s e water use ,

Using conduct iv i ty c e l l s t o avoid excess d i l u t i o n i n the r i n s e

tanks , I n s t a l l i n g flow r e g u l a t o r s t o minimize water u s e ,

Reusing contaminated r i n s e water and treated wastewater where

feasible.

s t e p s to minimize p o l l u t a n t loading include:

0

0

0

0

0

0

0

Implement ing a r i g o r o u s housekeep ing program t o l o c a t e and

r e p a i r l eaks around process ba ths , r ep lac ing f a u l t y equipment

on p l a t i n g racks t o prevent excessive s o l u t i o n drag-out, i n s t a l -

l i n g dr ip t r a y s where needed, and so f o r t h ,

Using spray r i n s e s o r a i r knives t o minimize s o l u t i o n drag-out

from p l a t i n g baths , Recycling r i n s e water t o p l a t i n g bath t o compensate f o r su r face

evaporation lo s ses , Using spen t process s o l u t i o n s as wastewater t rea tment reagents

( ac id and a l k a l i n e c leaning baths are obvious examples),

Using minimum process bath chemical concent ra t ions , I n s t a l l i n q recovery processes t o reclaim p l a t i n g chemicals from

r i n s e waters f o r r ecyc le t o t h e p l a t i n g bath,

Using process bath p u r i f i c a t i o n t o c o n t r o l t h e l e v e l of impur-

i t i e s and prolong the bath 's s e r v i c e l i f e .

17

Closed-loop chemical recovery from a r i n s e stream can o f t e n provide

the s o l u t i o n t o handling wastes t h a t a r e d i f f i c u l t o r expensive t o

t r e a t . Applying a closed-loop recovery system t o a p l a t i n g ope ra t ion

e l imina te s the need t o t r e a t t h e r i n s e water normally a s soc ia t ed w i t h

that s t ep .

In t h e case of r i n s e streams requ i r ing pre t rea tment ( f o r example,

cyanide o r chromium) o r r i n s e s conta in ing p o l l u t a n t s n o t e f f e c t i v e l y

removed by conventional end-of -pipe technology ( f o r example, some types

of complexed m e t a l s ) , i n s t a l l i n g a closed-loop system t o r ecyc le the

r i n s e may reduce the investment needed t o comply w i t h t h e e f f l u e n t

q u a l i t y l i m i t a t i o n s .

i

18

SECTION 4

EQUALIZATION

INTRODUCTION

One of t he most f r equen t ly encountered problems i n metal f i n i s h i n g

wastewater t rea tment is process upse ts r e l a t e d t o i n t e r m i t t e n t high

f lowra tes and h ighly v a r i a b l e contaminant l e v e l s . Batch d ischarge of

high s t r e n g t h p l a t i n g s o l u t i o n s and p l a t i n g wastes o f t e n can cause l a r g e

fluctuations i n flowrates, pi, m e t a l s concentrations, cyanide concentra-

t i o n , and o the r c r i t i ca l contaminant l e v e l s . These f l u c t u a t i o n s a r e

o f t e n so severe that process upse t and subsequent permit v i o l a t i o n s can

occur before the system is ab le t o compensate. For t h i s reason, waste-

water equa l i za t ion o f t e n i s implemented t o c o n t r o l t h e extreme f l u c -

t u a t i o n s i n flow and contaminant concent ra t ions , Likewise, whenever

t rea tment process upsets occur, e s p e c i a l l y on a p e r i o d i c b a s i s , proper

equa l i za t ion should be one of t h e f i r s t ope ra t ing condi t ions t o be

checked.

THEORY OF OPERATION

Wastewater equa l i za t ion can accomplish t h r e e b a s i c t a sks . The

f i r s t i s hydraul ic o r f low equa l i za t ion which dampens hydraul ic surges

t o downstream t rea tment processes. Flow equa l i za t ion can only be accom-

p l i s h e d by p l ac ing some type of v a r i a b l e l i q u i d volume s to rage tank i n

t h e wastewater flow path upstream of s e n s i t i v e process equipment. T he

l i q u i d is s t o r e d i n the v a r i a b l e volume tank dur ing high flow periods

and then i s r e l eased dur ing low flow pe’riods. Therefore, t he hydrau l i c

e q u a l i z a t i o n c a p a b i l i t y i s d i r e c t l y r e l a t e d t o t h e u t i l i z e d v a r i a b l e

volume of the tank.

19

The second equa l i za t ion task i s contaminant o r concent ra t ion equal-

i z a t i o n . Contaminant equa l i za t ion i s accomplished by mixing low and

high contaminant concent ra t ion wastewater , which r e s u l t s i n an average

concent ra t ion w i t h dampened f l u c t u a t i o n . As w i t h f low e q u a l i z a t i o n ,

some type of s to rage must be provided which s t o r e s h igh ly ( o r s l i g h t l y )

contaminated flow u n t i l such t i m e as t h e contaminant concent ra t ion

cyc le s t o t h e o the r extreme. Contaminant equa l i za t ion can be accom-

p l i s h e d by p l ac ing i n the flow pa th a s to rage tank and mixer with s u f f i -

c i e n t volume t o mix the low and high concent ra t ion wastes o r by co l l ec -

t i n g only the high concent ra t ion wastes i n a s to rage tank and bleeding

them back dur ing per iods of low contaminant concent ra t ion . The l a t t e r

system has the disadvantage t h a t t h e contaminant concent ra t ion must be

monitored cont inuous ly and r ap id a c t i o n taken based on ins tan taneous

contaminant concent ra t ion .

The t h i r d equa l i za t ion task, an o f f shoo t of the ear l ier d iscussed

two, is t o allow means t o c o n t r o l t h e load t h a t reaches the process

wi th in l i m i t s se t by the opera tor . T h i s is accomplished by p o s i t i o n i n g

the o u t l e t l i n e near the bottom of the e q u a l i z a t i o n bas in and provid ing

va lv ing that allows the opera tor the opt ion of i nc reas ing o r decreas ing

flows t o h i s process from the e q u a l i z a t i o n bas in . The opera tor would

have t h e opt ion of lowering flow dur ing period of high concent ra t ions

and r a i s i n g flows dur ing per iods of low concent ra t ions and could c o n t r o l

the load t o t h e process t o des i r ed l e v e l s . This system has the disad-

vantage t h a t it requ i r e s more a n a l y t i c a l work than the o the r two systems

and r equ i r e s a l a r g e r equa l i za t ion tank t h a t w i l l al low the opera tor t o

vary the tank l e v e l and has the advantage that it w i l l reduce the poss i -

b i l i t y of organic shock and allows much f i n e r c o n t r o l of t h e process.

Mixing i s an i n t e g r a l p a r t of a l l equa l i za t ion techniques. Flow

equa l i za t ion r equ i r e s mixing t o keep suspended material i n suspension.

Without it, s o l i d s w i l l accumulate i n the s to rage tank thereby reducing

the e f f e c t i v e volume of t he tank. Contaminant equa l i za t ion r equ i r e s

mixing t o blend the low and high s t r e n g t h wastes toge ther . Typica l ly ,

t h e mixers a r e v e r t i c a l shafts w i t h t u rb ines o r marine p rope l l e r s . If

20

t anks a r e deep, d r a f t tubes a r e i n s t a l l e d o r mul t ip l e impe l l e r s a r e

p laced on elongated s h a f t s . Horsepower requirements f o r e q u a l i z a t i o n

mixing t y p i c a l l y run between 0.02 and 0.04 horsepower/1000 ga l lons i f

g r i t t y m a t e r i a l i s no t p re sen t i n the waste.* I f , however, g r i t t y

material is p resen t , horsepower requirements f o r mixing can exceed 2.5

horsepower/1000 ga l lons . I n a l l cases, i f mixing i s occurr ing i n cir-

c u l a r tanks, then b a f f l e s must be i n s t a l l e d on the s idewal l s .

DESCRIPTION OF EQUIPMENT

Wastewater equa l i za t ion can be accomplished by a number of d i f -

f e r e n t methods. Some equa l i za t ion occurs i n sewer l i n e s , P o l l u t i o n

c o n t r o l equipment a l s o has some surge capac i ty a v a i l a b l e t o dampen o u t

f l u c t u a t i o n s . However, due t o the na ture of t he v a r i a t i o n s i n t h e metal

f i n i s h i n g i n d u s t r i e s , t hese methods provide l i t t l e i f any e f f e c t i v e

equa l i za t ion . Severa l t y p i c a l equa l i za t ion methods o the r than inhe ren t

system equa l i za t ion a r e described below.

Batch Processing

Equal iza t ion by batch process ing involves s t o r a g e and batch pro-

ces s ing of t he wastewater i n t h e same tank. To accommodate continuous

systems , mul t ip l e r e a c t o r s are operated i n p a r a l l e l . When one r e a c t o r

is f u l l , t he flow is switched t o another r e a c t o r while t he wastewater i n

t h e f i r s t r e a c t o r is being t r e a t e d . Reactor s to rage capac i ty must be

s u f f i c i e n t t o handle t h e maximum volume of wastewater t h a t could be

generated dur ing the t i m e it t akes t o t rea t t h e wastewater and empty

wastewater from the o ther r e a c t o r (see Figure 2 ( a ) ) . A l t e rna t ive ly , t he

*This and the following s e c t i o n s p re sen t information i n a mix of English

Enqineerinq and metric u n i t s . This mix of u n i t s occurs because, a l -

though metric u n i t s are now p re fe r r ed f o r d a t a r epor t ing , many of t he

q u a n t i t i e s are repor ted s t i l l almost exc lus ive ly i n English u n i t s , and

" r u l e s of thumb" have .been c a s t i n t hese u n i t s . A t a b l e of metric-

English conversions is provided with t h i s r e p o r t as Appendix C.

21

system can be operated i n batch mode with one tank when the system i s

used t o t r e a t wastewater flows from batch type p l a t i n g ope ra t ions . This

technique is employed o f t e n when spec ia l i zed t rea tment i s requi red f o r

i n t e r m i t t e n t waste flow.

The batch processing procedure i s a very e f f e c t i v e equa l i za t ion

technique s i n c e wastewater i n t h e t rea tment r e a c t o r i s c o n s i s t e n t f o r

each t rea tment cyc le . The system, however, r e q u i r e s t h a t e i t h e r t h e

flow must be i n t e r m i t t e n t with s u f f i c i e n t t i m e f o r wastewater t rea tment

t o occur between flows, o r mul t ip le r e a c t o r s must be provided. The

la t te r is usua l ly p r o h i b i t i v e l y expensive. Theref o re , t h i s process i s

found p r imar i ly on small i n t e r m i t t e n t flows from batch metal f i n i s h i n g

ope ra t ions whose wastewater r e q u i r e s spec ia l i zed t rea tment , such as

p l a t i n g s o l u t i o n s which c o n t a i n h i g h c o n c e n t r a t i o n s of c y a n i d e o r

chromium.

Continuous Processing from Batch Storage

c o n t i n u o u s p r o c e s s i n g from b a t c h s t o r a g e u s u a l l y i n v o l v e s two

s to rage tanks ope ra t ing on a fill-and-draw cyc le . While one tank i s

being f i l l e d , t h e o the r tank i s being discharged t o a waste t rea tment

f a c i l i t y (see Figure ' 2 ( b ) ) . This procedure i s un l ike batch process ing ,

i n which t h e equa l i za t ion and waste t rea tment s t e p s occur i n t h e same

vesse l . Using t h i s technique the waste t rea tment f a c i l i t y rece ives a

waste stream wi th uniform c h a r a c t e r i s t i c s while process ing t h e conten ts

of an ind iv idua l tank. The stream c h a r a c t e r i s t i c s , however, o f t e n vary

s i g n i f i c a n t l y from tank t o tank.

This equa l i za t ion technique r equ i r e s , e i t he r automated i n f l u e n t and

e f f l u e n t valves con t ro l l ed by leve l i n d i c a t o r s i n the batch equa l i za t ion

tanks , o r i n t e n s i v e opera tor superv is ion . The equa l i za t ion tanks m u s t

be s i zed such t h a t t h e volume from a t l e a s t one flow cyc le o r contami-

nant concent ra t ion cyc le can be c o l l e c t e d i n one tank. If contaminant

equa l i za t ion is des i r ed , t h e tanks must also be mixed. This system i s

bes t s u i t e d f o r r e l a t i v e l y small flows and/or r ap id ly varying ( c y c l i n g )

flows t h a t w i l l permit reasonably s i z e d tanks. Its primary de f i c i ency

22

(a) Batch Processing

- -

n Reactor

W t

-1

- ,-x-(s to rag+-

- - L Reactor m

W (b) Continuous Processing from Batch Storage

a * * m Reactor %

(c) Side-Stream Equalization

Figure 2, Methods of equalization. SOURCE: EPA REPORT NUMeER 60042-81-148

23

is t h a t some degree of automation o r labor i s requi red t o opera te it.

Also, t h e s t e p change i n r eac to r feed c h a r a c t e r i s t i c s t h a t occurs when

the flow is switched from one s to rage tank t o another can be a disadvan-

tage unless t he opera tor keeps h i s knowledge of tank concent ra t ions

updated.

S i de -S tr eam Equa l i z a ti on

The side-stream equa l i za t ion method involves t h e c o l l e c t i o n and

s to rage of flow o r contaminant concent ra t ion surges t h a t r e s u l t from

scheduled dumping opera t ions i n t h e production p l a n t . The surge i s

d i v e r t e d t o a s to rage v e s s e l f o r subsequent process ing dur ing s l a c k

per iods . This procedure i s sometimes used t o s t o r e an a l k a l i waste i n a

p l a n t f o r f u t u r e combining with a waste stream t h a t is normally a c i d i c .

The c o l l e c t e d a l k a l i could be dispensed i n t o t h e normally a c i d i c waste

stream t o reduce consumption and c o s t of n e u t r a l i z i n g reagents . The

r eve r se is o f t e n p rac t i ced t o a c i d i f y hexavalent chromium wastes p r i o r

t o reduct ion (see Figure 2 ( c ) ) .

1 Side-stream equa l i za t ion f o r contaminant surges i s u s u a l l y imple-

mented by manually opening d ive r s ion va lves o r d i r e c t l y pumping t o t h e

s to rage tank. The major problem wi th the system i s t h a t e i t h e r advanced

knowledge of t he event must be a v a i l a b l e o r on l ine monitoring equipment

must be i n s t a l l e d . wi th t h e poss ib l e exception of monitors f o r flow,

conduct iv i ty , and p H , on l i n e monitoring is very d i f f i c u l t and expensive

t o implement and maintain. Probably t h e b e s t implementation of t h i s

type of equa l i za t ion is as a s p i l l c o l l e c t i o n tank which can be used t o

c o l l e c t and hold sp i l l s i f and when waste t rea tment personnel are no t i -

f i e d of t he s p i l l f a s t enough t o implement t h e system.

I f only flow equa l i za t ion is requi red , side-stream equa l i za t ion can

be very e f f e c t i v e . It can be implemented e i t h e r using flow meters

connected t o pumps and c o n t r o l valves o r by a pass ive system of weirs

t h a t d i v e r t s excess ive flow t o a s to rage tank. The s t o r e d water can

then be pumped back i n t o t h e system dur ing low flow. The major advan-

tage of t he system is t h a t it r e q u i r e s minimal s to rage volume and i s

24

r e l i a b l e and r e l a t i v e l y simple t o implement.

t h a t it provides l i t t l e contaminant equal iza t ion .

The major disadvantage i s

Flow-Through Equal iza t ion

The system of flow-through equa l i za t ion dampens and g r e a t l y reduces

flow and concent ra t ion v a r i a t i o n s , b u t does n o t e l imina te them. Typi-

c a l l y t h e system opera tes on a continuous b a s i s and r e q u i r e s a l a r g e

tank, mixers, tank b a f f l e s , and e f f l u e n t c o n t r o l devices such a s weirs,

o r i f i c e s , c o n t r o l valves, o r pumps (see Figure 2 ( d ) ) . For flow equa l i -

za t ion , t he tank must be designed t o f l u c t u a t e i n volume i n response t o

i n f l u e n t flow r a t e s . For contaminant e q u a l i z a t i o n , t h e tank must be

l a rge enough t o hold the volume which corresponds t o the d ischarge cyc le

of high o r low concent ra t ion wastes. The more cyc le s t h e tank can hold,

t he b e t t e r t h e equa l i za t ion achieved. The mixers f o r contaminant equa-

l i z a t i o n must be s u f f i c i e n t t o keep t h e tank completely mixed and a l l

suspended s o l i d s i n suspension. I f t he tank i s round, b a f f l e s must be

i n s t a l l e d along t h e s i d e w a l l s t o assure good mixing. The e f f l u e n t

c o n t r o l devices s e l e c t e d w i l l depend on t h e o b j e c t i v e of t h e equaliza-

t i o n . For contaminant e q u a l i z a t i o n , broad c r e s t e d weirs a r e f r equen t ly

used. For simple and r e l i a b l e flow and contaminant equa l i za t ion , narrow

w e i r s , p ropor t iona l w e i r s , o r o r i f i c e s a r e used. Pumps o r flow c o n t r o l

valves a r e used f o r maximum flow equa l i za t ion .

The major advantage of a flow-through equa l i za t ion system is i ts s i m -

p l i c i t y of ope ra t ion and the continuous na tu re of i t s discharge. Flow

and contaminant concent ra t ion changes do occur, b u t very gradual ly . This

gradual change usua l ly provides downstream processes such a s p~ ad jus t -

ment wi th adequate time

system is the l a rge tank

t o respond. The major disadvantage of t h e

s i z e gene ra l ly required.

The o b j e c t i v e of equa l i za t ion i s t o l i m i t f l u c t u a t i o n s i n waste-

water flow and/or contaminant concent ra t ion t o l e v e l s t h a t w i l l not

adverse ly a f f e c t downstream t rea tment processes. Equal iza t ion can occur

25

on a continuous basis such a s i n a flow-through system o r can be i n i t i -

a ted upon demand a s w i t h s i d e stream equa l i za t ion .

Process Monitoring and Operational C r i t e r i a

Equal iza t ion monitoring takes two forms. F i r s t , w i t h the exception

of a continuous f low-through system, the wastewater must be continuously

monitored f o r flow and/or contaminant concent ra t ion . Monitoring i s

requi red t o i n d i c a t e when flow should be d i v e r t e d t o s i d e stream equa l i -

z a t i o n or t o determine the f i l l and draw cyc le s i n batch equa l i za t ion .

Monitoring can be accomplished using either automated on-line equipment

o r by p e r i o d i c measurements by t h e opera tor . The former i s s u b j e c t t o

breakdown and the l a t t e r is s u b j e c t t o slow response tha t may completely

m i s s flow o r contaminant surges . Therefore, it is necessary t o imple-

ment both monitoring techniques whenever poss ib l e .

It is a l s o e s s e n t i a l that a form of monitoring be conducted by

production personnel. Whenever production personnel a n t i c i p a t e gener-

a t i n g an abnormal flow o r ,contaminant surge , the production personnel

must n o t i f y waste t rea tment personnel p r i o r t o the even t and i n d i c a t e

t h e t i m e , magn i tude , and d u r a t i o n of t h e t h e abnormal waste f l o w .

S imi l a r ly , i f a s p i l l o r acc iden ta l r e l e a s e of a l a r g e q u a n t i t y of

contaminants o r water i s de tec t ed i n t h e production area, then pro-

duc t ion personnel must n o t i f y wastewater t rea tment personnel immedi-

a t e l y . This information is a c r i t i ca l p a r t of wastewater monitoring and

is abso lu te ly e s s e n t i a l f o r e f f e c t i v e ope ra t ion of equa l i za t ion equip-

men t .

The second part of wastewater equa l i za t ion monitoring is a pe r iod ic

program t o eva lua te the e f f e c t i v e n e s s of t he equa l i za t ion process. As a

minimum, this eva lua t ion should occur monthly with batch systems o r

a f t e r any downstream equipment experiences ope ra t iona l problems which

could be a t t r i b u t e d t o poor equa l i za t ion . To be e f f e c t i v e , t he monitor-

i ng .must be s e t up i n a r igorous t e s t program i n which i n f l u e n t and

e f f l u e n t flow and contaminant concent ra t ions are measured simultaneous-

l y . The frequency of sample c o l l e c t i o n dur ing t h e t e s t programs and the

26

dura t ion of the program w i l l depend upon the r a t e of flow and contamina-

t i o n v a r i a b i l i t y . Typica l ly , these v a r i a t i o n s fo l low some p a t t e r n and

e x h i b i t times of peak and minimum flow and contaminant concent ra t ion .

TO be e f f e c t i v e , the monitoring program must c o l l e c t s u f f i c i e n t grab

samples t o detect these v a r i a t i o n s , u sua l ly sampling every hour f o r 24

t o 48 hours. I n some cases, however, where the v a r i a t i o n s occur rapid-

l y , more samples w i l l be required. I f poss ib l e? the days f o r equal iza-

t i o n monitoring should be s e l e c t e d such t h a t t h e maximum v a r i a t i o n i n

flow and contaminants t h a t has been experienced under normal ope ra t ing

condi t ions is occurring.

Upon completion of the tes t , t h e i n f l u e n t and e f f l u e n t flow and

contaminant concent ra t icn should be compared g raph ica l ly by p l o t t i n g

i n f l u e n t and e f f l u e n t parameters versus t i m e on t h e same graph. The

peak e f f l u e n t va lues should be s u b s t a n t i a l l y less than t h e peak i n f l u e n t

values and wi th in the design ope ra t ing values f o r downstream equipment.

The maximum r a t e of change f o r e f f l u e n t contaminant concent ra t ion must

a l s o be evaluated when the contaminant concent ra t ion can a f f e c t chemical

feed rates i n downstream equipment. Too r ap id a change can r e s u l t i n

poor performance of equipment such as pH c o n t r o l l e r s .

Process Control S t r a t e g i e s

For the most p a r t , t h e opera tor has l i t t l e c o n t r o l over t h e equal-

i z a t i o n process. For continuous f low-through equa l i za t ion , c o n t r o l can

be exerc ised by a d j u s t i n g t h e pumping rate o r o r i f i c e openings. For

batch equa l i za t ion processes , l i t t l e c o n t r o l can be exerc ised over t he

e q u a l i z a t i o n p r o c e s s , e x c e p t f o r t h e f i l l - a n d - d r a w c y c l e which is

usua l ly dictated by flow p a t t e r n s . Side-stream e q u a l i z a t i o n , however , is d i r e c t l y c o n t r o l l a b l e by ope ra to r s by e i t h e r a d j u s t i n g set po in t s on

automatic monitoring equipment o r by manually d i v e r t i n g t h e flow based M . t o = + rnuiltr -- TV -- inf- -*.- fr a i d cri-

t e r i a f o r i n i t i a t i n g s i d e stream e q u a l i z a t i o n can n o t be developed;

however, the following facts should be evaluated:

27

o S e n s i t i v i t y of downstream equipment t o t h e surge .

o P o t e n t i a l f o r v i o l a t i n g permit condi t ions i f a t rea tment process

upse t occurs.

o Reserve capac i ty i n s i d e stream equa l i za t ion tank f o r a second

more severe surge.

o Likelihood of another event r equ i r ing s i d e stream equa l i za t ion

p r i o r t o r e l e a s e of c u r r e n t equa l i za t ion volume.

o Compatability of l i q u i d c u r r e n t l y i n e q u a l i z a t i o n tank wi th

c u r r e n t waste stream (i .e, , a c u r r e n t a c i d i c wastestream should

not be d ive r t ed t o s i d e stream equa l i za t ion i f a cyanide waste

i s c u r r e n t l y s to red there . Hydrogen cyanide gas could be pro-

duced).

I n add i t ion t o t h e monitoring of process ope ra t ing v a r i a b l e s spec i -

f i c t o s i d e stream equa l i za t ion , s e v e r a l gene ra l ope ra t ing and mainte-

nance procedures should be performed r o u t i n e l y on a l l e q u a l i z a t i o n

equipment. These include the following :

0

Equal iza t ion tanks should be checked monthly f o r accumulation of

s o l i d s on the bottom of t h e tank. When t h e s o l i d s accumulation

s i g n i f i c a n t l y reduces tank volume, t h e tank must be cleaned.

Equal iza t ion tanks should be checked weekly f o r accumulation of

f l o a t i n g mater ia l . I f t h e f l o a t i n g m a t e r i a l i n t e r f e r e s with

w e i r s , pump in t akes , l e v e l i n d i c a t o r s , o r mixers it must be

removed.

Tanks , b a f f l e s , mixers , and p ipes should be inspec ted annually

f o r condi t ion of materials of cons t ruc t ion . Corroded or worn

p a r t s should be r epa i r ed or replaced.

Monitoring programs t o eva lua te t h e performance of equa l i za t ion

should be conducted approximately twice per year o r a f t e r a

t r e a t m e n t p r o c e s s u p s e t t h a t c o u l d be l i n k e d t o i n a d e q u a t e

equa l i za t ion .

Flow meters, l e v e l i n d i c a t o r s , and on-line monitors should be

c a l i b r a t e d annually o r a t t he manufac turer ' s s p e c i f i e d i n t e r v a l ,

whichever i s less.

28

o All pumps, mixers, and automated valves should be included i n a

regular prevent ive maintenance program.

TYPICAL PERFORMANCE VALUES

Almost any des i r ed l e v e l of equa l i za t ion can be achieved by varying

the s i z e and type of equa l i za t ion equipment. Therefore , performance

c r i t e r i a must be e s t ab l i shed f o r i nd iv idua l f a c i l i t i e s based on the

s e n s i t i v i t y of downstream treatment processes .

TROUBLESHOOTING G U I D E

A t roubleshoot ing guide f o r opera t ion of t he equa l i za t ion tanks o r

basin is presented i n Table 1 . Two major opera t ing problems a r e con-

s idered; these are inadequate contaminant equa l i za t ion and inadequate

flow equal iza t ion . For each problem, s e v e r a l probable causes , checks

and monitors, and c o r r e c t i v e ac t ions are given.

29

w 0

OPERl

l a . I i

l b . 1

I 1

1

IC. P 1

Id . !

I

le.

1 f . I

TABLE 1 EQUALIZATION TROUBLESIIWrING GUIDE

PROBABLE CAUSE CHECK OR MONITOR REASON CORRECTIVE ACTION __

____I_ _ _ _ _ _ _ _ _ _ _ _ _ _ ~ _-_-I-- ___ __

NG PROBLEM: 1. Contaminant e q u a l i z a t i o n inadequate . ____ _______ --

#ken or worn mixer - eller. .

,dequate bas in volume - [sed by sol ids de- ,its on t h e b o t t o m tank.

; s i n g mixing baf - - s on c i r c u l a r tank.

m-operly located - rer.

idequate bas in - :e.

hdecliiate mixing - rsepower.

Check c o n d i t i o n o f mixer - i m p e l l e r .

Check f o r solids accumula- - t i o n s by d r a i n i n g t ank or checking tank side w a l l dep th w i t h a pole or s i m i l a r device .

Check f o r c o n d i t i o n of - b a f f l e s on o u t e r w a l l of c i r c u l a r tanks.

Check l o c a t i o n o f mixers; - s t a t i o n a r y mixers may n o t have t h e p rope r impeller s u h e r g e n c e and mixers i n deep t anks may be miss ing d r a f t t ubes or m u l t i p l e i m p e l l e r s . S u h e r g e d mixers too c l o v e to t h e s u r f a c e may vor t ex and lose e f f i c i e n c y .

Conduct moni tor ing pro- - gram to de te rmine i f bas in is l a r g e enough to c o n t a i n wastewater f low between l o w and h i g h f low and/or contaminant c y c l e s . (Note: f o r good e q u a l i - z a t i o n more than one c y c l e volume may be r e q u i r e d ) .

Check mixing horsepower; check for s o l i d s d e p o s i t i o n i n e q u a l i z a t i o n tank.

Broken or worn i m p e l l e r s - can d r a m a t i c a l l y reduce t h e e f f i c i e n c y o f mixing.

Reduced b a s i n volume - l i m i t s s t o r a g e c a p a b i l - i t ies and hence e q u a l i - z a t i o n capac i ty .

B a f f l e s must be p r e s e n t - to break circular f low p a t t e r n s which d o n o t c o n t r l b u t e to mixing.

The e f f i c i e n c y o f a mixer - can be d r a m a t i c a l l y a f f e c t e d by- improper placement.

Inadequate b a s i n volume - l i m i t s s t o r a g e c a p a c i t y and hence e q u a l i z a t i o n c a p a c i t y .

Mixing is d i r e c t l y r e l a t e d - to i n p u t horsep&er.

Replace i f worn.

Clean t ank and remove s o l i d s .

Repai r or r e p l a c e damaged or miss ing b a f f l e s .

Hove and a d j u s t as r equ i r ed . Reference manufac tu re r ' s recommendations and d e s i g n s p e c i f i c a t i o n s .

Modify or add t ank volume a s r equ i r ed .

Add mixing horsepower a s r equ i r ed . Reference manufac tu re r ' s recommen- d a t i o n s . T y p i c a l va lues range between 0.02 and 0 . 0 4 lrp/1000 g a l l o n s . I f g r i t t y ma te rka l is p r e s e n t , much l a r y e r mixir i i j horsepower will be r equ i r ed .

w P

PF

OPERATIN

2a. Flow i n d i c t rol a r e J c a l i l

2b. Imprc f l u e i

2c. Leakc conti

2d. Inadi VOlUl

TABLF 1 ICon t inued)

EQUALIZATION TR0UBLESHC"ING GUIDE

BABLE CAUSE CHECK OR mlNITfJR REASON CORRFCfIVE ACTION

PROBLEM 21 Flow equalization inadequa te .

e t e r s or l e v e l tore t h a t con- ank d i s c h a r g e p r o p e r l y set or ated.

e r c h o i c e of e f - control dev ice .

around e f f l u e n t 1 d e v i c e s .

- Check for proper s e t t i n g - and d a t e of l a s t c a l i b r a - t ion .

- Check t y p e of e f f l u e n t con- - trol dev ice .

- Check f o r e r o s i o n and wear - a round e f f l u e n t weire and o r i f i c e p l a t e s .

u a t e s t o r a g e - Monitor t ank l e v e l d u r i n g o p e r a t i o n .

These d e v i c e s , h e n i n c o r p o r a t e d i n t o t h e system, d i r e c t l y c o n t r o l t h e f l o w to and from t h e e q u a l i z a t i o n b a s i n .

E f f l u e n t c o n t r o l d e v i c e s d e t e r m i n e the degree of control t h a t can be e x e r c i s e d on e f f l u e n t f low. Pumps can p r o v i d e best e q u a l i z a t i o n fo l lowed by v a l v e s , o r i f i c e s , p ropor - t i o n a l w e i r s , and narrow r e c t a n g u l a r weirs.

The amount o f f low e q u a l - i z a t i o n is d i r e c t l y p r o p o r t i o n a l t o t h e changes i n t a n k volume. Also, i n most e q u a l i z a t i o n t e c h n i q u e s , as t h e w a t e r level approaches t h e top o f t h e t a n k , t h e e f f l u e n t f l o w is d r a m a t i c a l l y i n c r e a s e d to prevent o v e r f i l l i n g .

- C a l i h r a t e and a d j u s t as r e q u i r e d . Re fe rence m a n u f a c t u r e r s c a l i b r a t i o n p r o c e d u r e and d e s i g n speci- f i c a t i o n s f o r p r o p e r s e t t i n g .

- Uodify d i s c h a r g e d e v i c e a s r e q u i r e d .

- Repair or r e p l a c e as r e q u i r e d .

- Modify t ank d e k i g n or e f f l u e n t control d e v i c e s as r w u i r e d .

SECTION 5

OIL REMOVAL

INTRODUCTION

The s o u r c e s of o i l found i n me ta l was tewater a r e from c o o l i n g

opera t ions , cleaning operat ions, and lubr ica t ion . O i l is t y p i c a l l y

present i n wastewater i n three forms: f r e e o i l s , emulsif ied o i l s , and

so luble o i l s . Free o i l s i n wastewater a r e those which rise t o the

sur face of the wastewater due t o buoyant forces i n a s h o r t qu iescent

period. Emulsified o i l is o i l dispersed i n the wastewater as small

d rop le t s which can range i n s i z e from 0.1 t o 100 microns. Soluble o i l

is discussed i n the next paragraph. A wide v a r i e t y of o i l removal pro-

cesses is ava i l ab le f o r removing the f r e e and emulsif ied forms of o i l .

The processes include s k i m i n g , coalescing, emulsion breaking, f l o t a -

t i o n , cen t r i fuga t ion , u l t r a f i l t r a t i o n , reverse osmosis, carbon adsorp-

t i on , and b io log ica l oxidation. The most widely used and conventional

treatment processes

t i on and sk i 'ming ,

other processes a r e

not covered i n t h i s

i n tfie metal f in i sh ing indus t ry a r e g rav i ty separa-

coalescing, emulsion breaking, and f l o t a t i o n . The

not widely u t i l i z e d a t t h i s t i m e and, t he re fo re , a r e

manual.

Soluble o i l s a r e those organic o i l s which a r e dissolved i n the

wastewater. Soluble o i l s a r e genera l ly the l i g h t e r , more polar f r a c t i o n

of an o i l mixture. Soluble o i l s a r e not removable by the techniques

described i n the preceding and i n t h i s chapter. Techniques such a s

g rav i ty separa t ion , f l o t a t i o n , and emulsion breaking r e s u l t i n removal

of f r e e and emulsif ied o i l , bu t so luble o i l s remain dissolved i n the

wastewater. Soluble o i l s a r e , i n genera l , biodegradable, so t h a t the

32

most success fu l t reatment f o r removal of so lub le o i l s i s o f t e n biode-

gradat ion i n a b io log ica l wastewater t reatment system.

Along with the top ic of o i l removal, t he s u b j e c t of t o t a l t o x i c

organics (TTO) must be addressed. The ma te r i a l s which make up the TTO

a r e l i s t e d i n Appendix B; they a r e t y p i c a l l y degreasing so lven t s used i n

the c leaning of metal p a r t s , The waste so lven t s w i l l conta in o i l s , and

w i l l e x i s t a s f r e e , emulsif ied, and so lub le o i l s i n t h e wastewater. The

concent ra t ion of TTO i n metal f i n i s h i n g wastewaters can be g r e a t l y

reduced by proper handling and d i sposa l of so lven t s and o the r organics ,

e s p e c i a l l y by segregat ion of these wastes f o r c o n t r a c t d i s p o s a l o r

reclamation f o r recyc le .

C o n t r o l and management p r a c t i c e s € o r w a s t e s o l v e n t s and o t h e r

components which make up the TTO should be devised t o fol low resource

recovery guide l ines . S p e c i f i c a l l y , development of segrega t ion f a c i l i -

ties f o r these ma te r i a l s should be implemented, and d i s p o s a l should be

by a reputab le cont rac tor . The p o t e n t i a l f o r reclamation of the organ-

ics should be explored; i f reclamation is no t f e a s i b l e then d i s p o s a l

must be performed by a reputab le d i sposa l con t r ac to r . The importance of

proper d i sposa l f o r ma te r i a l s containing the compounds on the TTO l ist

must be emphasized. Those ma te r i a l s a r e known t o e x h i b i t s i g n i f i c a n t

t o x i c i t y ; severe environmental and human h e a l t h problems can r e s u l t from

t h e i r improper handling o r d i sposa l .

THEORY OF OPERATION

The theory of opera t ion f o r t h e equipment used t o remove f r e e and

emulsif ied o i l from wastewater i s discussed below.

Gravi ty Separator

Separator design is based on the s p e c i f i c g r a v i t y d i f f e rence be-

tween the d ispersed o i l globules and the wastewater. The l i g h t e r o i l

globules rise t o the wastewater su r face and a r e removed. The o i l r i s e

r a t e is descr ibed by Stokes ' Law which s t a t e s the r i s e r a t e i s d i r e c t l y

33

proport ional t o the d i f fe rence i n the weight of t he o i l globule and the

wastewater it displaces .

Emulsion Breaking

Emulsified o i l is dispersed i n wastewater as small d rop le t s . These

drople t s can range i n s i z e from 0.1 t o 100 microns. With pure o i l and

pure water, an o i l emulsion is highly unstable s i n c e both phases quickly

sepa ra t e and assume a shape having the minimum i n t e r f a c i a l area. With

o i l y wastewater, however, an o i l emulsion is o f t e n formed which is much

m o r e stable due to emulsifiers which a l t e r the oil-wastewater interface.

Three common emuls i f ie rs a r e e l e c t r i c a l charge, chemical a c t i o n ,

and mechanical ass is tance. E l e c t r i c a l charge emuls i f ie rs e s t a b l i s h a

repuls ive e lectr ical f i e l d a t the oil-wastewater i n t e r f a c e . This type

of emuls i f ie r i s normally a negl ig ib le cause of s t a b i l i t y except f o r

those wastewaters t h a t have a low s a l t content o r d i l u t e o i l concentra-

t ions . Chemical emuls i f ie rs a r e mater ia l s such a s soaps o r detergents

which cause chemical react ions a t t he oil-wastewater i n t e r f a c e . Mech-

a n i c a l emuls i f ie rs a r e f i n e l y divided c o l l o i d s such as s i l t , c l ay , o r

metal f i n e s which a t t a c h t o the o i l o r are coated by t h e o i l .

Emulsion s t a b i l i t y is af fec ted by d r o p l e t s i z e , emulsion age, and

w a s t e w a t e r v iscosi ty . Smaller drople t s are t y p i c a l l y more s t a b l e .

Emulsions general ly become more stable with age because the emuls i f ie r

is t r a n s p o r t e d t o t h e o i l t h r o u g h t h e was tewater by d i f f u s i o n . A

higher wastewater v i s c o s i t y general ly r e s u l t s i n a more s t a b l e emulsion

s ince there is more r e s i s t a n c e t o the movement of small drople t s thus

reducing t h e p o s s i b i l i t y of coalescence.

Coalescing Separator

Coalescing sepa ra to r s

a t o r s with skimmers except

work i n a similar manner t o grav i ty separ-

t h a t coalescing separators remove smaller and

less bouyant p a r t i c l e s . The bas i c p r i n c i p l e of coalescing involves the

p r e f e r e n t i a l wet t ing of a coalescing medium by o i l d rop le t s which accu-

mulate on the medium, and then rise t o the surface of t he solut ion. The

34

most important requirements f o r csalescing media a r e w e t t a b i l i t y f o r

o i l , large surface a rea , and contact between the o i l p a r t i c l e s and the

coalescing medium.

A i r F lo ta t ion

I n a f l o t a t i o n process, f i n e gas bubbles are introduced i n t o t h e

wastewater. These gas bubbles a t t a c h t o o i l globules r e s u l t i n g i n an

aggregate with a s p e c i f i c g rav i ty much less than t h a t of t h e surrounding

wastewater. The attachment of the f i n e , micron s i z e a i r bubbles t o the

o i l globules occurs by seve ra l mechanisms. The f i r s t mechanism i s t h e

adhesion of an a i r bubble t o the o i l globule o r by t h e d i r e c t c o l l i s i o n

of t h e a i r b u b b 1 e globule . The second mechanism is the t rapping of the r i s i n g a i r bubble under an o i l globule f l o c

s t r u c t u r e . The t h i r d mechanism is the adsorpt ion of t h e a i r bubble by

an o i l globule f l o c . The rise r a t e of t he aggregate i s described by

Stokes ' Law which s t a t e s t h a t the ' r a t e of rise is d i r e c t l y proport ional

t o the d i f fe rence i n weight of the aggregate and the wastewater i t

d isp laces . The quant i ty of gas which can be dissolved i n t h e wastewater

is described by Henry's Law, which states t h a t f o r gases of low so lub i l -

i t y t he gas mass dissolved i n water is d i r e c t l y proport ional t o i t s

p a r t i a l pressure.


Gravity Separators and Skimmers

Gravity separators normally remove f r e e o i l and l i t t l e o r no emul-

s i f i e d o r soluble o i l . The separa tors are used as primary treatment

processes and are p a r t i c u l a r l y appl icable f o r t he separa t ion of l a rge

quant i t i es of f r e e o i l .

Gravity separa tors range from lagoons with o i l r e t e n t i o n booms and

o i l removal devices t o tanks with automatic o i l skimmers. The American

Petroleum I n s t i t u t e ( A P I 1 has developed design c r i t e r i a f o r grav i ty oil

35

sepa ra to r s . Separators based on these design p r i n c i p l e s a r e t h e most

common type of o i l separator and a r e r e fe r r ed t o a s API separators .

A l i n e diagram of an API separator is presented i n Figure 3. The

separator includes a t r a s h rack, a forebay with o i l skimmer and b a f f l e ,

an i n l e t d i f f u s i o n device, and o i l separa t ion channels each with a

scraper and o i l skimmer. The t r a s h rack removes s t i c k s , rags , s tones ,

and o ther deb r i s . The forebay d i s t r i b u t e s the i n f l u e n t . The o i l which

rises t o the surface i n the forebay is re ta ined by the b a f f l e and re-

moved by the s k i m e r . The i n l e t d i f f u s i o n b a f f l e reduces flow turbu-

lence and d i s t r i b u t e s the flow equal ly over the channel c ros s s e c t i o n a l

area. The f r e e o i l rises i n t h e c h a n n e l s , i s c o l l e c t e d w i t h t h e

scraper , and is removed with the skimmer.

A v a r i a t i o n of the grav i ty separator c a l l e d t h e p a r a l l e l p l a t e

separator is a l s o commonly used. I t c o n s i s t s of a grav i ty separator

with p a r a l l e l p l a t e s i n s t a l l e d i n the tank t o assure laminar flow condi-

t ions. The small d i s tance between the p l a t e s a l s o reduces the d is tance

the o i l globule must rise, which i n tu rn r e s u l t s i n a decrease i n the

required a c t u a l surface overflow r a t e and h y d r a u l i c d e t e n t i o n t i m e .

Emulsion Breaking

Chemical emulsion breaking can be accomplished a s e i t h e r a batch

process or a continuous process. The emulsion breaking process

resembles gravi ty separat ion except that concurrent chemical treatment

occurs t o break up the emulsified o i l . The mixture of emulsified o i l s

and water is i n i t i a l l y t r e a t e d by the addi t ion of chemicals t o the

wastewater. A means of a g i t a t i o n ( e i t h e r mechanical o r by increasing

the turbulence of t he wastewater stream) is provided t o ensure t h a t t he

chemical added and the emulsified o i l s a r e adequately mixed t o break the

oi l /water emulsion bond. F ina l ly , the o i l y res idue (commonly c a l l e d

scum) t h a t r e s u l t s rises t o t h e su r face and is separated from the re-

maining wastewater by a skimming o r decanting process. The skimming

process can be accomplished by any of the many types of mechanical

sur face skimmers t h a t a r e present ly i n use. Decanting methods include

36

Figure 3. API separator.

37

removal of the o i l y surface residue v ia a technique such as cont ro l led

tank overflow or by removal of the demulsified wastewater from the

bottom of the tank. Decanting can be accomplished with a s e r i e s of

tap-off l i n e s a t various l eve l s which allows the separated o i l s t o be

drawn off t h e top or allows the wastewater t o be drawn of f the bottom

u n t i l o i l appears i n the wastewater l i ne . With any of these arrange-

ments, the o i l is usual ly d iver ted t o s torage tanks f o r f u r t h e r proces-

s ing or hauling by a l icensed cont rac tor .

Coalescing

Coalescing a l s o resembles g rav i ty separa t ion , . except t h a t the tank

is f i l l e d with various configurat ions of coalescing media. The coales-

cing m e d i a can be p l a t e s , f ib rous media, r e t i c u l a t e d polyurethane foams,

or loose media so long as they provide a tor tuous path f o r the waste-

water. Most media a r e i n s t a l l e d i n the form of r e l a t i v e l y s m a l l dia-

meter ca r t r idges of a standard s i z e , stacked i n a p a r a l l e l pos i t ion .

Orientat ion may be v e r t i c a l or hor izonta l ; however, hor izonta l designs

can be more d i f f i c u l t t o se rv ice s ince the e n t i r e separa tor ves se l must

f i r s t be drained.

A i r F lo t a t ion

A va r i e ty of f l o t a t i o n methods a r e commonly used f o r oil-wastewater

separat ion. The fundamental d i f f e rence between these methods i s the

mechanism by which the a i r is introduced i n t o the wastewater. The two

most common methods a re the dissolved a i r f l o t a t i o n (DAF) process and

the induced a i r f l o t a t i o n (1-1 process.

Many v a r i a t i o n s of t h e D A F p r o c e s s a r e used. Three p r i n c i p a l

va r i a t ions a r e f u l l , partial , and recycle operation. Simplified l i n e

diagrams of the three p r inc ipa l D A F process va r i a t ions a r e presented i n

Figure 4, These va r i a t ions d i f f e r i n the por t ion of the wastestream

which is pressurized and sa tura ted with gas. The f u l l p ressur iza t ion

operat ion t r e a t s the e n t i r e i n f l u e n t wastestream. The p a r t i a l operat ion

t r e a t s a por t ion of the i n f l u e n t wastestream. The recycle operat ion

38

COMPIESSED A I R

I

I -1

EFFLUENT INFLUENT

PRESSURIZATION SYSTEM

ROTATION TANK - OIL DISCHARGE

SLUOGE OISC3ARGE

A- TOTAL P9ESSURIZATION

1 EFFLUENT INFLUENT I I PRESSURIZATION

SYSTEM 1

t I

COMPRESSED AIR

E- PARTIAL '9ESSURIZATION

ROTATION TANK

SLUOGE DISCHARGE

EFFLUENT INFLUENT 1

ROTAT ION

I I SLUOGE DISCHARGE 1 PRESSURIZATION

SYSTEM

c- RECYCLE PRESSURIZATION

Figure 4. Line diagrams of principal DAF process variations.

39

t r e a t s a por t ion of the e f f l u e n t wastestream. A t y p i c a l recycle System

includes a compressed a i r source, an aera t ion tank, a feed pump, and a

f l o t a t i o n tank. The compressed a i r i s dissolved i n t o the r e tu rn waste-

water a t an elevated pressure and s tored i n the ae ra t ion tank. The

pressurized wastewater is subsequently mixed with the main wastestream

and released to atmospheric pressure i n the f l o t a t i o n tank where the

dissolved a i r i s released from so lu t ion as f i n e a i r bubbles, approxi-

mately 30 t o 120 microns i n diameter. The gas bubbles a t t a c h t o the o i l

globules and the aggregate r i s e s t o the surface and i s collected.

The induced air f l o t a t i o n process cons i s t s of a mul t ice l led tank , f r o t h launders, a recycle pump, and s t a t i c a i r inductors or a sp i r a to r s .

The flow throuqh the c e l l s is i n s e r i e s . The wastewater en te r s the

f i r s t c e l l where a i r is introduced through the ac t ion of an a s p i r a t o r .

The a s p i r a t o r u t i l i z e s recycle process e f f luen t . The a i r is released i n

the c e l l i n the form of a i r bubbles approximately 1000 microns i n d i a -

meter, somewhat coarser than those i n a DAF process. The a i r bubbles

a t t ach to the o i l globules and the aggregate rises t o the surface and i s

co l lec ted . The remaining wastewater flows through a b a f f l e i n t o the

subsequent c e l l s and u l t imate ly ou t of the system. Alternate I A F sep-

a r a t o r s use d i f f e r e n t methods t o induce a i r i n t o the wastewater. One

system forces a i r under pressure i n t o the l iquid. Another common system

induces air in to , the wastewater through high speed mixers.

DAF separa tors commonly include a chemical feed system t o introduce

coagulant a ids such as polymers and metal sa l ts , including f e r r i c chlo-

r ide , alum, and l i m e . IAF separa tors almost without exception include a

chemical feed system. The chemicals a r e introduced a t a var ie ty of

po in ts i n the system. Some systems include a separa te rapid mix and

f loccu la t ion chamber.

Skimming Devices

Skimming devices to remove f l o a t i n g o i l a r e an i n t e g r a l p a r t of API

g rav i ty , p a r a l l e l p l a t e , and coalescing separators . Four common

40

skimming devices a re the ro ta ry drum, the r o t a t a b l e s l o t t e d pipe, the

f l o a t i n g w e i r , and the b e l t ' ( o r rope) type.

With the ro t a ry drum device, a hor izonta l drum approximately 0.3 t o

0.6 meters ( 1 t o 2 f e e t ) i n diameter is p a r t i a l l y submerged i n the

wastestream. Skimming is accomplished as the drum r o t a t e s with the

flow, picking up a t h i n o i l f i lm which is l a t e r scraped off t he drum and

removed. The depth of submergence is not c r i t i ca l a s long as the drum

is i n contac t with the o i l layer. A submergence depth of approximately

3 cm ( 1 inch) is typica l . The r o t a t i o n a l speed is t y p i c a l l y 0.15 t o

0.45 m/sec (0.5 t o 1.5 f t / s e c ) , but the optimum speed depends upon the

amount of o i l t o be removed and o i l v i scos i ty . The device t y p i c a l l y

r e s u l t s i n a recovered o i l t h a t is low i n water content.

With the r o t a t a b l e s l o t device, a ho r i zon ta l s l o t t e d pipe is sub-

merged i n the wastestream with the s l o t s normally above the sur face .

Skimming is accomplished by r o t a t i n g the pipe such t h a t the s l o t s a r e

submerged, allowing the o i l t o flow i n t o the pipe and t o be removed.

This device can r e s u l t i n a recovered o i l t h a t may be low o r high i n

water content depending on the amount of ca re exercised i n submerging

the s l o t and ad jus t ing it during the skimming operation.

,

With a f l o a t i n g w e i r device, a f l o a t i n g device suspends a horizon-

t a l w e i r i n the wastewater channel. The device is weighted such t h a t

t h e w e i r c r e s t i s j u s t above t h e w a t e r s u r f a c e b u t below t h e o i l

surface. This pos i t ion ing r e s u l t s i n the o i l flowing continuously i n t o

the w e i r channel. The recovered o i l e x i t s the channel t o a discharge

po in t v i a a f l e x i b l e hose. The device t y p i c a l l y r e s u l t s i n a recovered

o i l high i n water content.

With the b e l t ( o r rope) device an endless b e l t is pul led through

the wastewater. Skimming is accomplished a s f l o a t i n g o i l s adhere t o the

b e l t . After passing through the wastewater, the b e l t continues on a

d e v i c e l o c a t e d above t h e l i q u i d l e v e l where t h e r ecove red o i l i s

squeezed or scraped from the belt and removed. The o i l v i scos i ty

41

and a t t r a c t i o n to the b e l t mater ia l slows t h e b e l t speed and perform-

ance. Baff les a re commonly.used t o d i r e c t the f l o a t i n g o i l t o the b e l t .

B e l t s a r e t y p i c a l l y 0.3 t o 0.6 meters ( 1 t o 2 f e e t ) i n width. The

device typ ica l ly r e s u l t s i n a recovered o i l with low water content .

OPERATIONAL PROCEDURES

The objec t ive of o i l removal is t o reduce f r e e , emulsif ied, and

so luble o i l t o a l e v e l t h a t is acceptable f o r discharge. This o f t e n

requi res mult iple s t eps t h a t f i r s t remove f r e e o i l and then emulsif ied

o i l by emulsion breaking or a i r f l o t a t i o n .

Process Monitoring

The performance c r i t e r i o n f o r evaluat ing O i l removal equipment 1s

e f f l u e n t t o t a l o i l concentration. While t h i s parameter provides the

ove ra l l performance of the system, seve ra l o ther parameters must be

measured and incorporated i n t o a regular monitoring program t o success-

f u l l y and cons i s t en t ly operate an o i l removal system. These parameters

a re l i s t e d i n Table 2.

Of t h e pa rame te r s l i s t e d i n Table 2 , f low should be moni tored

continuously. Total i n f luen t and e f f l u e n t o i l along with pH, suspended

s o l i d s , and temperature should be measured a t regular i n t e r v a l s a s

spec i f ied or a f t e r any o i l v io l a t ion o r process upset caused by exces-

s i v e o i l . Sampling may be e i t h e r composite or grab with grab sampling

normally prefer red f o r o i l measurements. Samples always should be col-

l ec ted i n a reas of s u f f i c i e n t l y high turbulence t o e n t r a i n f r e e o i l .

I n f luen t and e f f l u e n t o i l s a l so should be analyzed f o r f r e e , emulsif ied,

and soluble o i l f r a c t i o n s on a monthly bas i s or a f t e r poor performance

of o i l removal equipment i s noted o r suspected. More frequent a n a l y s e s

will be required i f a p l an t upset or p e r m i t v io l a t ion is suspected.

Determination of o i l content of water is a c r i t i c a l procedure.

Procedures fo r sample co l l ec t ion , s torage , and ana lys i s should be per-

formed i n s t r i c t accordance with es tab l i shed standards. Q u a l i t y

42

TABLE 2

OIL REMOVAL PROCESS MONITORING REQUIREMENTS

Parameter Frequency Comment

1 . Flow Continuously To determine overflow rate and hydraul ic r e t e n t i o n t i m e (HRT) .

2. I n f l u e n t t o t a l o i l Weekly To determine mass loading of o i 1.

3 . I n f l u e n t f r e e , so lub le , Monthly To determine mass

u l a r form of o i l . o r emuls i f ied o i l loading f o r a p a r t i c -

4. E f f l u e n t t o t a l o i l

5 .

6.

7 .

8.

9.

10.

E f f l u e n t f r e e , s o l u b l e , o r emuls i f ied o i l

Suspended s o l i d s

Waste o i l volume

Waste sludge volume

Temperature

PH

Weekly

Monthly

Weekly

Daily

Daily

To determine per for - mance and determine loading t o downstream processes .

To determine per for - mance of t he system.

To determine of 0i.l t o be

To determine of s ludge t o disposed.

q u a n t i t y disposed.

q u a n t i t y be

cont inuous ly Temperature w i l l a f f e c t o r per s h i f t o i l removal per for -

mance.

continuously p~ w i l l a f f e c t o i l o r per s h i f t removal performance.

43

assurance including dup l i ca t e analyses and blanks i s indispensable. I f

a v io l a t ion of permit conditions is de tec ted , sample c o l l e c t i o n , s t o r -

age , and a n a l y s i s p rocedures should be i n v e s t i g a t e d as p a r t of an

assessment of the p l a n t program.

The ana lys i s of samples fo r t o t a l o i l conten t should be conducted

according t o one of t he procedures given i n t h e c u r r e n t e d i t i o n of (9) "Standard Methods f o r the Examination of Water and Wastewater".

While other techniques a r e commonly used, they may no t be accepted by

l o c a l regulatory agencies. "Standard Methods" , however, does not pro-

v i d e a t e c h n i q u e f o r d i f f e r e n t i a t i n g between f r e e , e m u s i f i e d , and

so luble o i l . This d i f f e r e n t i a t i o n is p a r t i c u l a r l y important when evalu-

a t i n g equipment t h a t is e f f e c t i v e i n removing one o i l c l a s s i f i c a t i o n ,

b u t no t another. As an example, a g rav i ty separa tor may appear t o be

malfunctioning because it is removing only a small f r a c t i o n of the t o t a l

o i l . I f the major i ty of the o i l i s f r e e o i l , t h i s w i l l be the case.

I f , however , the majority of the o i l is so lub le o r emulsified o i l , t he

grav i ty separa tor may be doing an exce l l en t job removing the f r e e o i l

t h a t it was designed t o remove. A procedure f o r determining the f r e e ,

so luble , and emulsified f r a c t i o n s of o i l s is provided below.

Free O i l - -

P l a c e a measured amount of t h e r a w was tewa te r i n a s e p a r a t o r y

funnel. Shake the sample vigorously and l e t stand quiescent ly f o r

approximately two hours. Draw off a por t ion of the subnatant and de-

termine its o i l concentration. The o i l measured is the emulsified and

so lub le f r a c t i o n . The d i f f e rence between the t o t a l o i l concentration

and t h a t measured i s t h e f r e e o i l c o n c e n t r a t i o n of t h e was tewa te r

sample.

Soluble O i l - -

a w e d amtint nf the yaw w a t e - m i n a q p w ~ r ~ A.

funnel and ac id i fy with 10 m l / l of concentrated hydrochloric ac id .

Next, add 200 mg/lsodium chlor ide and 200 mg/l diatomaceous e a r t h , Shake

vigorously and l e t stand quiescent ly f o r a minimum of e i g h t hours.

F i l t e r t h e mix tu re through a w e t f i l t e r pape r and measure t h e o i l

44

concentration of the f i l t r a t e . The o i l concentrat ion measured i s the

soluble f r a c t i o n of the wastewater sample.

Emulsified O i l - -

The d i f fe rence between the o i l concentrat ion of t h e emulsified and

so lub le f r a c t i o n measured i n the Free O i l determination and t h e o i l

concentration of the soluble f r a c t i o n measured i n the Soluble deter-

mination is the emulsified o i l concentrat ion of t h e wastewater sample.

Example Calculat ions

Several example ca lcu la t ions a r e provided below f o r c a l c u l a t i n g

operat ing parameters e s s e n t i a l t o operat ion of an o i l removal system.

Hydraulic Retention Time (HRT)--

HRT i s the average length of t i m e wastewater spends i n a process

tank. Since the removal of o i l does not occur instantaneously, it i s

e s s e n t i a l t h a t the wastewater spend a minimum t i m e i n t h e o i l separator .

The HRT ca lcu la t ion is:

VOL HRT = - FLOW

where VOL is the l i q u i d capaci ty of t h e tank and FLOW is the flow t o the

separator . Note t h a t the VOL and FLOW must be expressed i n s i m i l a r

u n i t s such as cubic meters o r gal lons. As an example, i f an o i l water separa tor had a volume of 10,000 ut3 and the flow t o t h e tank w a s 2500 m 3

per hour the c a l c u l a t i o n would be:

2500 m’

h r -

Surf ace Overflow R a t e - -

S u r f a c e o v e r f l o w r a t e i s t h e p r o c e s s f low r a t e d i v i d e d by

a c t i v e surface of t he a rea of a c l a r i f i e r o r o i l water separator .

t h e

The

45

r e s u l t is u s u a l l y expressed as gal lons per square f o o t per minute and

represents the loading on the treatment process with respec t t o i t s

surface area. The loading can a l s o be expressed a s a ve loc i ty by con-

ver t ing gal lons t o cubic meters. This parameter is usua l ly expressed as

meters/minute and is used pr imari ly f o r c i r c u l a r c l a r i f i e r s and a i r

f l o t a t i o n devices where it represents the average v e r t i c a l ve loc i ty of

the water from the submerged center feed w e l l t o the sur face overflow

weirs.

The surface overflow r a t e i s ca lcu la ted as follows:

FLOW SURFACE OVERFLOW RATE = SURFACE AREA

where sur face a rea is the quiesant a rea of t he o i l separa tor excluding

i n f l u e n t and e f f l u e n t weirs and b a f f l e s , and flow includes any recycle

flow. A s an example, i f a 10-foot diameter dissolved a i r f l o t a t i o n o i l

water separa tor with a +foot diameter center feed w e l l had a flow r a t e

of 80 GPM and a recycle flow r a t e of 40%, the ca l cu la t ion would be:

80 + (0.4 X 80) 2 SURFACE OVERFLOW RATE = (,o 2 - 2 ) = 1.6 G a l / f t /min

4

A i r t o Sol ids Ratios (A:S)- -

ti on

mass

The a i r to s o l i d s r a t i o is used i n conjuct ion with the a i r f l o t a -

process and represents the r a t i o of the mass of appl ied a i r t o the

of o i l p l u s SS. T h i s ratio is c a l c u l a t e d as follows:

8,990 SCFM O I L CONC X FLOW

A:S r a t i o =

where OIL CONC is o i l concentrat ion i n milligrams per l i t e r (mg/L), FLOW

is wastewater flow i n ga l lons per minute (gpm) , and SCFM is a i r flow i n

standard cubic f e e t per minute (scfm).

A s an example the a i r requirement f o r a dissolved a i r f l o t a t i o n

tank with an in f luen t o i l concentrat ion of 60 mg/L, a flow r a t e of 50

GPM, and an A:S r a t i o of 0.03 would be ca lcu la ted as follows:

46


Gravity Separation & Skimmers--

Gravity separa tors t y p i c a l l y o f f e r minimal opportunity f o r opera tor

cont ro l . Per iodica l ly , the tank must be checked f o r s o l i d s depos i t s on

the bottom of the tank or on the p a r a l l e l p l a t e s i f a p l a t e separa tor i s

used. S imi la r ly , i f any i n f l u e n t and e f f l u e n t w e i r s a r e used they must

be pe r iod ica l ly checked f o r t r a s h and debr i s accumulations which might

impede even flow d i s t r i b u t i o n . The weirs must a l s o be checked t o assure

they a r e l e v e l and t o maintain proper l i q u i d depth. The l a t t e r i s

extremely important when s l o t t e d pipes a r e used t o remove the co l l ec t ed

o i l .

The hydraulic r e t en t ion t i m e and sur face overflow r a t e should a l s o

be ca lcu la ted and compared t o equipment design values. Flow t o an o i l

water separa tor is usua l ly not con t ro l l ab le , but the ca l cu la t ions can

i n d i c a t e i f the system is overloaded o r requi res more equal iza t ion .

S imi l a r ly the temperature and suspended s o l i d s should be monitored.

Cold temperatures can reduce the rise ve loc i ty of o i l globules by as

much as 50% and high suspended s o l i d s can lead t o rapid s o l i d s accu-

mulation i n the tank. -6

The o i l removing or skimming equipment should be checked d a i l y f o r

proper opera t ion and depth of f l o a t i n g o i l blanket. Excessive b lanket

depth, i .e. , more than a few inches f o r most equipment, can rap id ly lead

t o a system f a i l u r e . Rotary drum and b e l t skimming devices should be

checked a t l e a s t weekly f o r drum o r b e l t sur face i n t e g r i t y , sc raper

blade i n t e g r i t y , and tension aga ins t drums. Rotating s l o t skimming

devices should be checked as of t en as required t o keep o i l blanket an

the s l o t s t o prevent excessive water entrainment. The s l o t s must a l s o

be checked pe r iod ica l ly f o r leve lness and accumulation of t r a s h o r

deb r i s . Floating w e i r s must be checked d a i l y t o make sure t r a s h i s not

. . w - t h - a - 5 4 - e r a t i w ; r-". Carz rwzt 5e tzkcn ix spzra t rng

47

co l l ec t ing on the weirs and t o make sure the weirs a re f r e e f l o a t i n g and

properly balanced. Per iodica l ly , the f l e x i b l e hose must be checked f o r

i n t e g r i t y .

Coalescers--

The operat ion of coalescers is e s s e n t i a l l y the same as t h a t f o r

grav i ty separators . The frequency of inspect ion f o r s o l i d s accumulation

and plugging of the coalescers must, however, be on a t least a monthly

bas is .

Emulsion Breaking--

Chemical addi t ion is not a separa te u n i t process f o r oi l -water

separat ion. Rather, it is used p r i o r t o o r i n conjunction with other

separa t ion processes. Chemical addi t ion i s f i r s t used t o break o r

demulsify emulsified o i l s i n the wastewater. The treatment i s u s u a l l y

d i rec ted toward des t ab i l i z ing the dispersed o i l d rop le t s or chemically

binding or destroying any emulsifying agents present .

As previously s t a t e d , chemical demulsifying processes include

a c i d i f i c a t i o n , coagulation, s a l t i n g out , and demulgation with organic

c leaving agents. Acids genera l ly c leave emulsions more e f f e c t i v e l y than

do coagulant salts, bu t a r e more expensive. I n addi t ion , the r e s u l t a n t

wastewater must be neut ra l ized a f t e r separat ion. Coagulation with

aluminum or i ron sal ts may be e f f e c t i v e and is commonly used. However,

the r e s u l t a n t hydroxide sludges may be voluminous and a r e d i f f i c u l t t o

dewater. Coagulation with polymer may be e f f e c t i v e and i s sometimes

used. The r e s u l t a n t sludge q u a n t i t i e s a r e smaller than those f o r metal

salts but a r e sometimes more d i f f i c u l t t o dewater. The s a l t i n g ou t of

an emuls i f ie r may be achieved with the addi t ion of l a rge q u a n t i t i e s of

an inorganic s a l t , thereby increas ing the dissolved s o l i d s content of

the wastewater. Organic demulgators may be extremely e f f e c t i v e demul-

Sifying agents bu t a r e genera l ly very expensive and special ized. Demul-

ga tors a r e normally considered only i f they a r e found t o be e f f e c t i v e i n

extremely low concentrat ion due t o cos t and, i n many cases , t h e i r toxi-

c i t y a t higher concentrat ions.

48

The ef fec t iveness .of chemical pretreatment i s normally determined

with bench sca l e j a r tes t ing . A representa t ive wastewater sample is

s p l i t i n t o severa l a l iquo t s . Each a l iquo t is reacted with a d i f f e r e n t

quan t i ty of the chemical under consideration. After each addi t ion , the

a l iquo t s may be f l a s h mixed, f loccula ted , and quiescent ly s e t t l e d and

then examined t o note any reac t ion and emulsion breaking and the r a t e

and ef fec t iveness of separat ion.

One t y p i c a l p rocedure f o r j a r t e s t s i s t h a t s u g g e s t e d by t h e

T r e t o l i t e Division of P e t r o l i t e Corporation , i n t h e i r I n d u s t r i a l Bench

Test ing

1 .

2 .

3 .

4.

5.

6.

7.

8 .

9.

10.

Procedures Manual. ( ’ The procedure i s as follows.

Secure appropriate sample f o r t e s t ing .

F i l l one l i t e r beakers t o e i t h e r 500 o r 1000 m l mark. Test

volume is dependent on volume of sample ava i l ab le and volume

required f o r water q u a l i t y ana lys i s .

P l a c e beake r s on padd le mixer and beg in mixing a t maximum

speed.

Dose beakers with appropr ia te chemical(s1 and appropr ia te

volumes.

Mix on high speed fo r 1 minute.

When chemical addi t ions and 1 ninute rapid mix i n t e r v a l s a r e

complete, reduce s t i r r e r speed t o slow mix f o r 1 minute mini-

mum. (Note: Mix times can be var ied t o more c lose ly dupl ica te

f u l l scale system).

Turn mixer off and remove b a k e r s f o r s e t t l i n g . S e t t l i n g t i m e

is dependent on system and r a t e of contaminant removal (10-30

minutes should prove adequate).

W i p e paddles clean with a paper towel o r rag i n preparat ion f o r

the next test.

Repeat s t eps 2-8 u n t i l the desired treatment program has been

After the chosen s e t t l i n g t i m e has e lapsed, sample beakers f o r

appropriate water q u a l i t y ana lys i s .

49

The j a r t e s t procedure is usefu l f o r determining the optimum pH f o r

separa t ion , the most e f f ec t ive coagulant and coagulant a i d , the optimum

chemical dosage and order of chemical addi t ion , the required rapid and

slow mix times , the required s e t t l i n g , and the estimated separa t ion

e f fec t iveness .

The pretreatment chemicals may be added d i r e c t l y t o a process l i n e

or may require separa te rapid mix and f loccu la t ion chambers. The opera-

t i on of these chambers is discussed elsewhere i n t h i s document under

"Chemical Addition Equipment" and "pH Control".

A i r F lo t a t ion

A i r f l o t a t i o n , as compared t o g rav i ty separa t ion , has seve ra l con-

t r o l mechanisms t h a t a r e d i r e c t l y under the con t ro l of the operator .

These include the system pressure , recycle r a t e , a i r t o s o l i d s r a t i o ,

and chemical addi t ion r a t e . The opera tor a l s o has some con t ro l of

sur face overflow r a t e and hydraulic residence t i m e . Typical ly increas-

ing the system opera t ing pressure and a i r t o s o l i d s r a t i o w i l l improve

performance. However, care should be taken i n increas ing these because

pas t a c e r t a i n a i r t o s o l i d s r a t i o very l i t t l e improvement i s obtained

and a considerable amount of energy is expended. A s the a i r t o s o l i d s

r a t i o is increased it may a l s o be necessary t o increase the recycle r a t e

i n order t o provide s u f f i c i e n t water f o r a i r t o i n i t i a l l y dissolve.

However, the increase i n recycle r a t e s w i l l increase the sur face over-

flow r a t e , a s i t u a t i o n which can lead t o performance degradation i f the

overflow rate exceeds the system design spec i f i ca t ions .

The DAF pocess is t y p i c a l l y operated with a 40 t o 60 ps ig pressure,

20 t o 50 percent recyc le , 1 t o 4 gpm/sq f t sur face overflow r a t e , 1 t o 2

minute ae ra t ion tank de ten t ion t i m e , 0.01 t o 0.06 a i r t o s o l i d s r a t i o

and 20 t o 40 minute f l o t a t i o n tank de ten t ion t i m e . Typical ly with the

DAF process , compressed a i r is used as the feed gas because i t is re la -

t i v e l y inexpensive. However, i n e r t gases a r e sometimes used instead.

For example, ni t rogen gas is used t o minimize corrosion by keeping i ron

i n so lu t ion . The choice of gas can depend a l s o upon the nature of the

50

wastewater and any regulat ions concerning off-gas removal. The volume

of f r o t h layer produced i n the DAF process is approximately equal t o 0 .2

t o 2.5 per cen t of the i n f l u e n t waste flow.

IAF separa tors t y p i c a l l y use a recycle flow rate of 100 t o 400

percent w i t h 5 t o 10 minute detent ion t i m e and r e s u l t i n a f r o t h pro-

duction r a t e equal t o 1 t o 15 percent of t he i n f l u e n t flow r a t e .

When operat ing a i r f l o t a t i o n u n i t s , pressure gauges and flowmeters

should be checked d a i l y f o r proper readings and c a l i b r a t e d on a t least a

year ly basis . In add i t ion , any w e i r s should be checked monthly f o r

t r a s h or deb r i s accumulation and levelness . I f sc raper arms a r e used t o

s k i m the f l o a t , they should be checked monthly f o r proper submergence,

usua l ly around 1/2 t o 3/4 inches when water l e v e l i s a t the base of t he

weirs. They must a l s o be checked f o r scraper blade i n t e g r i t y and proper

seal when crossing the sludge hopper.


The performance of o i l removal equipment is extremely dependent

upon the d i s t r i b u t i o n of o i l between the f r e e , emulsif ied, and so lub le

forms. Gravity o i l s epa ra to r s t y p i c a l l y have produced an e f f l u e n t with

a t o t a l o i l concentration between 50 and 100 mg/L. However, g rav i ty

separa tors with an emulsion breaking pretreatment s t e p give t y p i c a l

performance ranges from less than de tec tab le t o 40 mg/L of e f f l u e n t o i l .

Coalescers should provide s l i g h t l y b e t t e r performance than g r a v i t y

separators . Limited information regarding t h e i r performance i n d i c a t e s

t h a t an e f f l u e n t of 1 t o 50 mg/L of oil can be achieved ( includes u n i t s

with and without emulsion breaking).

DAF and IAF separa tors t y p i c a l l y produce e f f l u e n t s of equal qual-

i t v . A f l n t

s epa ra to r , t y p i c a l l y produces an e f f l u e n t with an o i l concentration l e s s

than 20 mg/L. When used without pretreatment, the f l o t a t i o n separa tor

can produce an e f f l u e n t with an o i l concentrat ion of 25 t o 100 mg/L.

51

TROUBLES HOOTING GUIDE

A guide fo r troubleshooting the o i l removal process i s presented i n

Table 3 . Six problem areas a re noted; a l l the problem areas dea l w i t h

either excessive o i l i n the aqueous e f f l u e n t or excessive water i n the

co l l ec t ed o i l phase. In many cases a de t a i l ed ana lys i s of the o i l

emulsion breaking treatment w i l l be required t o c o r r e c t the problem.

Careful sampling, j a r t e s t evaluat ions, and o i l analyses a r e required t o

def ine the problem and inves t iga t e co r rec t ive ac t ions .

52

Ln W

OPERAT'

l a . Flc

lb . Em1 so

IC. Sh4 tal

Id. Sei f i

l e . Ex oi se

I f . Ch ch

TABLE 3 OIL REMOVAL TROUBLESHOOTING GUIDE

. (OBABLE CAUSE CHECK OR MONITOR REASON CORRECTIVE ACTION

--_____ _______ -~ __ _____ __ - __ ; PROBLEM 1: Grav i ty s e p a r a t o r h a s h igh e f f l u e n t o i l concen t r a t ion .

__-- _____ . a t e too high. -

i i f i e d and/or - ,le o i l p re sen t .

I c i r c u i t i n g of - occur r ing .

rator t ank - Id wi th solids.

i s i v e f l o a t i n g - i l a n k e t i n r a t o r tank.

3e i n i n f l u e n t - %c ter i za t ion .

Check peak and average f low - rate, h y d r a u l i c r e t e n t i o n time (HRT), and s u r f a c e over - f low rate and compare manu- f a c t u r e r ' s recommendation and des ign criteria.

Monitor the f r e e , e m u l s i f i e d , - and s o l u b l e o i l c o n c e n t r a t i o n e n t e r i n g and l eav ing t h e separator.

Check i n f l u e n t and e f f l u e n t - w e i r s and b a f f l e s f o r plugged, broken, or mis s ing u n i t s . Check weirs f o r f o r l eve lness .

Dra in t ank and i n s p e c t or - rod w i t h a pole to d e t e c t t h e p re sence of s o l i d s or s l u d g e s on t h e bottom of t h e tank.

Monitor e f f l u e n t f o r f r e e - o i l and v i s u a l l y i n s p e c t f o r f l o a t i n g oil i n e f f l u e n t .

Check v i s c o s i t y , t empera ture , - and d e n s i t y of cail-water s o l u t i o n and c o i l e c t e d oil. Compare to o r i g i n a l des ign c o n d i t i o n s and manufac tu re r ' s recommendations.

S e p a r a t o r s a r e des igned - to ach ieve q u i e s c e n t c o n d i t i o n s i n t h e oil- water s e p a r a t i o n zone. Excess ive f low promotes tu rbu lence .

G r a v i t y s e p a r a t o r is - e f f e c t i v e f o r f r e e oil only .

When s h o r t c i r c u i t i n g - occur s the o i l does n o t have t i m e to s e p a r a t e from t h e water.

A s t h e s o l i d s collect i n - t h e tank t h e HRT is dec reased and t h e o i l does n o t have s u f f i c i e n t t i m e to s e p a r a t e from t h e water.

I f f l o a t i n g o i l l a y e r - becones too deep i t can ex tend down to t r e a t e d water e f f l u e n t port.

I f p r o c e s s changes r e s u l t - i n changes i n t h e d e n s i t y and v i s c o s i t y o f t h e o i l and water, t h e d e s i g n requi rement f o r g r a v i t y s e p a r a t o r s can change.

Decrease f low r a t e ; add more oil-water separator c a p a c i t y i i n c r e a s e e q u a l i z - a t i o n i f peak f low is excess ive .

Troubleshoot S t e p 6 ; i n s t a l l emuls ion b reak ing equipment i f n o t p re sen t ; c o n t r o l s o u r c e s o f s o l u b l e oi l .

Clean, repair, r e p l a c e , or a d j u s t as needed.

C lean and remove a l l s o l i d s and s ludges .

Adjus t o i l skimming d e v i c e s as r equ i r ed .

Modify separator d e s i g n as requ i r ed .

TABLF 3 (Con t i n u e d 1

OIL REMOVAL TROUPLFSIIOOTING GUIDF

PROBABLE CAUSE CHFCK OR MONITOR RFASON CORRECTIVE ACTION

2d. S l o t located water

OPERATING

G r a v i t y separator h a s a very h i g h water c o n t e n t i n collected oi l .

- I n s p e c t and monitor f o r water - T r e a t e d water can contam- - Repair a s r e q u i r e d . f low when oil is n o t b e i n g i n a te collected oi 1. removed.

o i l collectors are - Check d i s t a n c e between slot - I f water too close to t h e - Adjus t as r e q u i r e d (may too n e a r t h e oil and oil water i n t e r f a c e d u r i n g b o t t o m o f t h e slot, wave r e q u i r e a d j u s t i n g of an

i n t e r f a c e . nonskimning o p e r a t i o n . a c t i o n caused by wind or e f f l u e n t w e i r opposed to h y d r a u l i c s u r g e s c a n a d j u s t i n g s l o t ) . c a u s e water to e n t e r t h e slot.

PROBLEM 3: C o a l e s c e r h a s h i g h e f f l u e n t o i l c o n c e n t r a t i o n .

-

Monitor o p e r a t i o n o f collec- - T r e a t e d water can contam- - Adjus t equipment or i n s t r u c t t i o n slot. i n s p e c t f l o a t i n g i n a t e collected oi l . operators a s requi red . w e i r f o r l e v e l n e s s and p r e s e n c e o f t r a s h i n w e i r s .

Check d i s t a n c e between - I f water too close to t h e - C l e a r w e i r to remove heavy bottom o f w e i r and oil-water bottom of t h e weir, wave d e p o s i t s # a d j u s t w e i r a s i n t e r f a c e . a c t i o n caused by wind or s p e c i f i e d by manufacturer.

h y d r a u l i c s u r g e s can c a u s e water to e n t e r t h e w e i r .

3a. C o a l e s c e r s c logged or - Drain tank J n d i n s p e c t . - For proper performance - Clean and repair a s r e q u i r e d . o i l must come i n c o n t a c t w i t h c o a l e s c i n g p l a t e s and proper v e l o c i t i e s must be mainta ined .

brok n. . - Monitor pH. - pfl can a f f e c t s u r f a c e

properties of oil d r o p l e t s and t h e i r a b i l i t y to coalesce.

3c. See S t e p 2. - See "'I 'rouhlesl~ooting - See "Troubleshoot ing G r a v i t y S e p a r a t o r s " G r a v i t y S e p a r a t o r s "

- Adjus t pl4 to d e s i g n or normal o p e r a t i n g c o n d i t i o n s .

- Sec S t e p 2 .

-

___

OPERA?

4a. I1 a:

4b. I1 f

4c. I1 PI

4d. 11 rl

4e. E 0'

4f . SI D<

49. SI SI

4h. Ii a< a

TAeLF 3 (Continued)

011. REMOVAL TROUPLFSHOOTING GUIDE

'ROBABLE CAUSE CHFCK OR MONITOR RFASON CORRECTIVE. ACTION .

IG PROBLEM 4: A i r f l o t a t i o n u n i t has h i g h e f f l u e n t o i l c o n c e n t r a t i o n .

lequate d i s s o l v e d be ing s u p p l i e d .

lequate r e c y c l e 1.

lequate d e l i v e r y isure.

l equa te h y d r a u l i c m t i o n t i m e (HRT).

s s i v e s u r f a c e - f low rate.

-t c i r c u i t i n g o f occur r ing .

nmer n o t removing ids f a s t enough.

I ope r che m i ca 1 i t i o n (where I i c a b l e ) .

Monitor oil c o n c e n t r a t i o n ; - monitor a i r flow3 de termine a i r : s o l i d s ( o i l ) ratio.

Check r e c y c l e f low rate. -

Check d e l i v e r y p r e s s u r e and - check c a l i b r a t i o n of p r e s s u r e gauge . Monitor f low rate; check tank - f o r accumula t ion of solidsi c a l c u l a t e HRT.

Moni tor f low rate and - c a l c u l a t e s u r f a c e over f low r a t e .

Check e f f l u e n t w e i r s f o r - plugging and l e v e l n e s s . Check i n f l u e n t b a f f l e s .

I n s p e c t skimmers f o r broken scapers, proper submergence i n t h e w a t e r , poor seal a l o n g t h e edges or s l u d g e hopper; check speed., '

Check cliemical f e e d purps f o r proper o p e r a t i o n , s e t t i n g , and c a l i b r a t i o n .

I n s u f f i c i e n t a i r be ing s u p p l i e d to f l o a t t h e mass of oil p r e s e n t .

Recycle f low is o f t e n e s s e n t i a l to p r o v i d e enough water to d i s s o l v e t h e required a i r .

I n s u f f i c i e n t p r e s s u r e can r e s u l t i n t h e a i r n o t d i s s o l v i n g i n t h e water.

S h o r t HRT's do n o t 4 l h w s u f f i c i e n t time f o r solids to s e p a r a t e .

Large over f low rates c a n c a r r y solids o u t of t h e tank and break foam.

S h o r t c i r c u i t i n g c a n r e s u l t i n i n a d e q u a t e d e t e n t i o n t i m e s .

A i r f l o t a t i o n foam b r e a k s w i t h t i m e and can c a u s e r e e n t r a i n m e n t of o i l par t i c 1 e6 .

Inadequate chemical c o n d i t i o n i n g can r e s u l t i n s i g n i f i c a n t l y reduced perfoimance of a i r f l o t a t i o n equipment.

Adjus t as r e q u i r e d ; t y p i c a l range is 0.01 to 0.06 pounds of a i r per pound of oil.

a d j u s t a s r e q u i r e d ; see d e s i g n and m a n u f a c t u r e r ' s recommen- d a t i o n s . Typica l v a l u e s range from 20 to 50% r e c y c l e .

Adjus t a s requi red1 see d e s i g n and manufac turer ' s reconmen- d a t i o n s . Typica l v a l u e s range from 40 to 60 psig.

Decrease f low rate; t r o u b l e - s h o o t "Fqual iza t ion" ; c l e a n tank. T y p i c a l v a l u e s a r e 20 t o 40 minutes.

Decrease f l o w rate; t r o u b l e - s h o o t "Equalization". T y p i c a l v a l u e s are 1 to 4 gpm/sg f t .

Clean and a d j u s t weirs a s requi red .

Repair and replace damaged s c a p e r s a s r e q u i r e d . Adjus t suhmergence and rota- t i o n a l speed a s r e q u i r e d .

Clean , repair, and a d j u s t as r e q u i r e d .

TABLP 3

OIL RlWOVAL 'RMURLFSHCKWING GUIDP (Continued)

PROBABLE CAUSE CHECK OR WONIlOR RFASON C O R R K T I V I A C T I O N

- Perform jar test to optimize type dosage and pn of chemical conditioning agent. The nature of thr waste MY have changed.

If polymer is being used check initial dilution and placement of polymer addition.

-

- lnadequatr chemical conditioning can result in significantly reduced performance of air flotation equipment.

- Proprr mixing of polymer and vente essential. Polymrrs muat be initially diluted to 0.1 to 0.5% for proper mixing to occur. Polywr csn be sheared by injecting it upstream of a centrifugal pump.

- adjust chemical addition equipment as required.

- Adjust initial polymer dilution procedures sa required. Change polymer addition point to aaaure adequate mixing prior to dovnstream releasr in air flotation tank.

~ ~

LTING PROELR4 58 Float from air flotation unit contains cxcesmin vater.

Leak in float - Check hopper vith skimmer - Hater contaminating - Repeir or adjust as required. collection hopper. a r ~ turned off. If vater concentrated oil.

enters hopper, lrak is present in hopper or top of hopper located below vater surface.

hTING PRDBLEW 61 hulaion breaking is not converting ell emulsified oil to free oil.

Poor mixing betveen - Check mixlng equipment for - Proper mixing of condi- - Repair and replace a8 oil and chemicals. worn mixers, missing tioning agent and waste necessary.

bafflen, and adequate horse- water Is essential for pwer (See Table 1). conditioning agents to be

effective.

Improper conditioning - Perform jar test to determine - Waste characteristics - Adjust chemlcal addition agent or dosage.c optimum conditioning agent, deterrine optimum equipment DS required.

dosage, and plr. conditioning agent, dosage, and pH.

Fluctuating or - See 'pH Trouhleehooting - M e t emulsion - See -pH Troubleahoatlng improper pn. Guide. ' breaking techniques are Guide."

pH-semi tive.

SECTION 6

CYANIDE OXIDATION

INTRODUCTION

Cyanide (CN-1 is used i n the metal f in i sh ing indus t ry t o keep metal

i ons ( z inc , copper, etc. ) i n the e l ec t rop la t ing so lu t ions from p rec ip i -

t a t i ng . Cyanide is t y p i c a l l y added as sodium cyanide (NaCN) o r hydro-

cyanic ac id (HCN). Added e i t h e r way, the cyanide ion is both h ighly

tox ic and soluble . Furthermore, i f the p H of cyanide containing waste

drops below 7, hydrogen cyanide gas ( H C N ) which is extremely poisonous

w i l l be evolved.

THEORY OF OPERATION

The most f requent ly employed method f o r t r e a t i n g cyanide waste is

cyanide des t ruc t ion by chlor inat ion. Other oxidants such a s ozone a r e

used also. During the oxidat ion process, cyanide may be e i t h e r par-

t i a l l y oxidized t o a less tox ic form, cyanate (CNO-), o r completely

oxidized t o carbon dioxide (CO ) and ni t rogen (NZ). This corresponds t o

what i s commonly r e fe r r ed t o a s "one-stage" and "two-stage" treatment

respect ively. Both stages can be accomplished using various forms of

ch lo r ine including ch lor ine gas dissolved i n water, sodium hypochlorite

( N a O C 1 ) , or calcium hypochlor i te ( C a ( O C l ) 2 ) . Acid hydrolysis is a l s o

used f o r destroying cyanate.

2

Chlorine Oxidation

The f i r s t s t age of the cyanide des t ruc t ion i s ch lor ine oxidat ion of

cyanide t o cyanate. The equation f o r t h i s reac t ion is:

57

C 1 + NaCN + 2 N a O H -> NaCNO + 2NaC1 + H 0 2 2

This reac t ion has an intermediate s t e p t h a t f i r s t converts ch lor ine

gas t o the hypochlor i te ion (OCL-) which then r eac t s with the cyanide.

Because the oxidat ion react ion depends upon the presence of the hypo-

c h l o r i t e ion, it is necessary t o perform this reac t ion a t a pH g r e a t e r

than 8.5. This requi res t h a t c a u s t i c (NaOH) be added t o ensure t h a t a

pH g r e a t e r than 8.5 is maintained a t a l l times, and t o maintain i t

during the reac t ion which consumes two c a u s t i c molecules f o r each cya-

nide ion oxidized. I f the pH i s allowed t o decrease below 7 . 0 , the

reac t ion w i l l s t o p and the highly tox ic gas, cyanogen ch lor ide , w i l l be

evolved. The o v e r a l l reac t ion of cyanide t o cyanate i s both time and pH

dependent. A t a p H of 10.0 o r higher , oxidat ion of cyanide t o cyanate

is accomplished rap id ly and completely i f oxidat ion ( 1 3 ) minutes a r e maintained.

The second s t age of the cyanide des t ruc t ion

per iods of 30 t o 120

prccess i s ch lor ine

oxidat ion of cyanate t o ni t rogen and carbon dioxide which i s then con-

ver ted t o bicarbonate and carbonate ions. This reac t ion depends upon

the presence of hypochlorous ac id (HOC11 f o r an oxidant , a s opposed t o

one s t age oxidat ion which requi res the hypochlorite ion. Therefore the

pH must be less than 8.5, normally i n the range 8.0 t o 8.5. The r e l a -

t i o n s h i p between t h e c y a n a t e o x i d a t i o n r e a c t i o n , pH, I p d r e q u i r e d

de ten t ion t i m e i s displayed i n Figure 5. However, f o r a c t u a l p l a n t

operat ions a de ten t ion t i m e of 60 minutes i s recommended. ( 1 3 ) The

chemical formula f o r cyanate oxidat ion is:

3C12 + 6NaOH + 2 N a C N O e 2 NaHCO + N 2 + 6NaC1 + 2H 0 3 2

As previously s t a t e d , the ch lor ine required f o r cyanide oxidat ion

can be added t o the system i n severa l forms including ch lor ine gas, sodium nypocnior i te s o i u t i on, ana calcium n y p ocn io r i t e s o i u t i on. cn io - r i n e gas- i s q u i t e so luble i n water and when dissolved i n water, ch lor ine

hydrolyzes rap id ly t o form hypochlorous ac id (HOC11 and/or hypochlor i te

ion (OC1-) depending upon the pH. The e f f e c t of p H on the d i s t r i b u t i o n

d S

58

n 10c W

W I- a z a >. 0 a 0 U n a

a

W

3 0 W

-

W E I- - 2 0 I- Z W I- W a

ac

6C

40

20

0 f .u

Figure 5.

&U 3.U

PH

1 U.U'

Destruction time required for Cyanate vs pH.

59

of hypochlorous acid and hypochlorite ion i n water a t two d i f f e r e n t

temperatures is shown i n Figure 6. When dissolved i n c a u s t i c and water (pH>8.5) , chlor ine w i l l form t h e hypochlorite ion (OCl -1 . The d isso lu-

t i o n reac t ion f o r the hypochlorite ion is

+ C12 + 2NaOH -> 2Na + OC1- + C1- + H20.

I t can be seen from the equation t h a t one molecule ( o r one kilogram

mole) of ch lor ine gas w i l l r equi re two molecules ( o r t w o kilogram moles)

of caus t i c and w i l l y i e ld one molecule ( o r one kilogram mole) of hypo-

c h l o r i t e ion (OC1-) . The r a t i o of ch lor ine gas t o c a u s t i c i s important

because caus t i c and ch lor ine a r e genera l ly added a t a cont ro l led r a t i o .

Note that we genera l ly add mater ia l s t o be reacted by weight (kg o r lb),

although reac t ions occur by the molecule o r mole bas i s . The mass i n

kilograms divided by the molecular weight gives the number of kilogram

moles.

The main advantage of using ch lor ine gas is t h a t the chemical cos t

i s the lowest of the three oxidizing agents. However, t he re i s a g r e a t

disadvantage because ch lor ine gas is d i f f i c u l t t o handle and is toxic .

Therefore, the choice of using ch lor ine gas becomes a trade-off between

mater ia l s handling-safety and cost .

Sodium hypochlor i te (NaOCL) requi res no preparat ion and can be used

d i r e c t l y a s an oxidizing agent. The main advantage i n using sodium

hypochlor i te i s t h a t i t i s r e l a t i v e l y easy and s a f e t o handle and use.

The use of calcium hypochlor i te [Ca(OCl),I requires t h a t , f o r b e s t

r e s u l t s , it be dissolved i n t o so lu t ion before in t roduct ion i n t o the

oxidizing react ion. When calcium hypochlor i te is not dissolved i n water

f i r s t , increased q u a n t i t i e s of sludge w i l l be produced.

Acid Hydrolysis

A second method f o r des t ruc t ion of cyanate is ac id hydrolysis.

Acid hydrolysis i s accomplished by lowering the pH t o a value of 2 o r 3

by adding s u l f u r i c acid. Five minutes reac t ion t i m e is required t o

des t roy the cyanates. ( 1 3 ) Two disadvantages of t h i s procedure a r e t h a t

60

0

30

Figure 6 .

40

50

60

70

80

90

i 00

PH Distribution of HOCL and OCL.

61

the pH must be lowered t o 2 t o 3 with acid; then a f t e r hydrolysis , t h e

pH has t o be . ra i sed again t o pH 6 t o 9 f o r discharge. T h i s requi res

s i g n i f i c a n t volumes of neu t r a l i z ing agents lboth ac id and c a u s t i c ) which

increases the t o t a l dissolved s o l i d s of the wastewater, Furthermore, i f

incomplete des t ruc t ion of cyanides occurs i n the f i r s t s tage f o r any

reason, hydrogen cyanide gas w i l l be re leased when the acid is applied.


Typical equipment f o r cyanide treatment cons i s t s of reac t ion ves-

sels, mixers , pH c o n t r o l l e r s , and ch lor ina tors . T h e s e can be operated

i n a batch o r continuous flow mode. Ei ther mode can be used t o achieve

s i n g l e stage cyanide oxidat ion ( t h e conversion of cyanide t o cyanate) o r

t w b s t a g e o x i d a t i o n ( t h e conve r s ion of cyan ide t o CO and NZ). A

t y p i c a l system f o r a continuous two-stage oxidat ion u n i t is shown i n

F igu re 7. I t includes two reac t ion vesse ls , two automatic pH control-

lers, two mixers, and a ch lo r ina to r with dual c o n t r o l ' c i r c u i t s t o con-

t r o l independently the ch lor ine dose f o r each reac t ion vesse l based on

r eac to r oxidat ion reduct ion p o t e n t i a l (see Sect ion 7, Chromium Reduc-

t i o n 1. when only s ingle-s tage cyanide reac t ion i s required, the second

reac t ion vesse l is de le ted from the system. The advantage of s ingle-

stage operat ion is reduced chemical and c a p i t a l cos t s as compared t o

two-stage operation. The disadvantage of s ing le s t age oxidat ion of

cyanide t o cyanates is t h a t cyanates are somewhat toxic. However, they

2

a r e much less t o x i c than c y a n i d e s , b u t i n some a r e a s d i s c h a r g e of

cyanates may no t be allowed.

Batch cyanide oxidat ion equipment b a s i c a l l y resembles the s ingle-

s tage continuous equipment except t h a t i t is operated i n a batch mode.

This mode of operat ion requi res e i t h e r an i n t e r m i t t e n t flow o r mu1 t i p l e

tanks. In e i t h e r case, increased opera tor involvement is required t o

f i l l and draw the tanks and t o r ead jus t the pH c o n t r o l l e r between the

f i r s t and second s t age when two-stage oxidat ion i s pract iced. For these

reasons , batch t reatment is seldom prac t i ced when average flows exceed

15 GPM.

62

Figure 7. Two-stage cyanide de8truCtlOn.


To con t ro l the operat ion of the cyanide des t ruc t ion process , t h e

opera tor should conduct the necessary process monitoring, perform any

con t ro l ca l cu la t ions needed , and understand the process cont ro l s t r a t e -

gies .

Process Monitorinq

The main performance c r i t e r i o n used t o evaluate the cyanide reduc-

t i on process i s the r eac to r e f f l u e n t concentration of cyanide i n one

s t a g e oxidat ion o r the second r eac to r e f f l u e n t concentrat ion of cyanide

and cyanate i n t w o s tage oxidation. While these parameters provide the

o v e r a l l performance, many o ther parameters should be measured and incor-

porated i n t o a regular monitoring program t o successfu l ly and consis-

t e n t l y operate a cyanide reduction process. These parameters a r e l i s t e d

i n Table 4. For continuous t reatment systems, flow, pH, and oxidat ion

reduct ion p o t e n t i a l (ORP) a r e normally recorded on a continuous basis .

The probes f o r these u n i t s should be ca l ib ra t ed weekly with d a i l y cal i -

b ra t ion preferable . The cyanide i n the process i n f l u e n t and the cyanide

and f r e e ch lor ine i n each r eac to r vesse l e f f l u e n t should be measured

d a i l y using a composite sample f o r cyanide and a grab sample f o r chlo-

r ine. These measurements a r e required t o ve r i fy adequate treatment and

t o e s t a b l i s h a working r e l a t ionsh ip between f r e e ch lor ine , ORP, and

cyanide des t ruc t ion . The e f f l u e n t temperature and suspended s o l i d s

should a l s o be measured on a d a i l y bas i s t o provide an understanding of

how these parameters a f f e c t e f f i c i ency of des t ruc t ion .

In addi t ion t o the above s tandard, monthly o r qua r t e r ly analyses of

the t reatment p l a n t i n f l u e n t f o r i n t e r f e r i n g agents should be performed.

These agents e i t h e r cause an excessive use of ch lor ine o r form cyanide

complexes t h a t cannot be oxidized by chlor ine. These agents include

cuprous , nickelous , fe r rous , and f e r r i c ions. The presence of cuprous

ion i n cyanide wastes produces an apparent in te r fe rence t o the destruc-

t i o n of cyanides. The cuprous ion exe r t s a ch lor ine demand of 0.56 l b s

of ava i l ab le C12 per lb of cuprous copper present . Therefore, a t l e a s t

64

TABLE 4

CYANIDE REMOVAL PROCESS MONITORING REQUIREMENTS


1.

2.

3 .

4.

5 .

6.

7.

8.

9.

Reactor Temperature

Flow

In f luen t Cyanide Concentration

Eff luent Cyanide Concentration

Ef f luen t Cyanate Concentration

Chlorine Concentration

Reactor p H

Reactor ORP

Ef f luent Suspended Sol ids Concentration

10.Reactor Sludge Volume

Daily

Continuously

Da i l y

Da i l y

Daily

Daily

Continuously

Continuously

Daily

Daily

TO measure temperature.

To determine HRT and t o a d j u s t chemical dosages.

To determine des t ruc- t i o n e f f ic iency .

To measure any res idua l .

To determine loading t o downstream processes.

To measure excess chlor ine.

TO con t ro l c a u s t i c addi t ion.

To con t ro l ch lor ine addi t ion.

To determine loading t o downstream processes.

To determine t reatment chemical requirements 'and mixing ef fec t iveness .

65

an a d d i t i o n a l 0.56 lb s of ava i lab le C 1 2 has t o be used i n the oxidat ion

of cyanides t o account f o r t h i s chlor ine demand.

The above discussion of cuprous ions a l s o holds t rue f o r nickelous

ion which e x e r t s 2.24 lbs of ava i lab le chlor ine demand per l b of nicke-

l o u s ion . T h i s 2 . 2 4 l b s / l b r e p r e s e n t s an e x c e s s of 3.7 times t h e

t h e o r e t i c a l amount required f o r the oxidation of nickelous ion t o nicke-

l i c ion. This excess has been found experimentally t o be necessary t o

e l iminate the nickelous in t e r f e rence .

Ferrous and f e r r i c ions present i n wastewaters t h a t contain cyanide

can react withcyanide = t o form a complex t h a t is only s l i g h t l y oxidiz-

a b l e by ch lor ine .


Hydraulic residence time (HRT) represents t he average length of

t i m e wastewater spends i n the chemical reac t ion tank assuming complete

,mixing of the reactor . Since the oxidation of cyanide is not an i n -

stantaneous r eac t ion , i t is e s s e n t i a l t h a t t he wastewater remain i n the

react ion tank f o r a t l e a s t the minimum recommended HRT.

The HRT ca lcu la t ion is:

VOL HRT = - FLOW

where :

VOL = the volume of l i q u i d i n the tank FLOW = . t h e flow t o the reactor .

Note t h a t VOL and FLOW must be expressed i n s imi l a r u n i t s such a s cubic

meters o r gal lons. As an example, i f a cyanide oxidat ion vessel had a

volume of 6000 gal lons and the flow t o the tank was 8000 gallons/hour

the c a l c u l a t i o n would be:

r- * m *

= 0.75 HRS 8000 GAL/HR HRT = w"" "-

66


The p r i n c i p a l operatinq s t r a t e g i e s f o r process cont ro l of both one

and two s t age cyanide oxidation a r e pH, ORP (which con t ro l s the ch lor ine

a d d i t i o n ) , mixing, and HRT. Of these var iab les the operator has ready

con t ro l of pH and ORP. The HRT, while having a major impact on the

process, is es tab l i shed by the wastewater flow r a t e and the r eac to r

design, and the re fo re the operator has l i t t l e cont ro l f o r continuous-

flow systems. However, f o r batch systems the operator has con t ro l of

the HRT.

For f i r s t s t age oxidation, t he ORP should be maintained between 350

and 400 mV ( 1 2 ) f o r a pH between 9.5 and 10. Typically, t h i s w i l l r e s u l t ( 1 3 ) In i n near ly complete oxidation of cyanide i n 30 t o 120 minutes.

general , increasing ORP w i l l decrease the required reac t ion time (HRT) . Simi la r ly , increasing pH t o a value of 10 w i l l decrease required reac-

t i o n t i m e (HRT). Above a pH of 10, no improvement i n performance 1s

achieved. Therefore, operat ing a t a pH above 10 w i l l only r e s u l t i n

increased c a u s t i c consumption and a high s t rength wastewater t h a t must

be neutral ized.

For two s t a g e treatment the f i r s t s t a g e i s opesated a s described

above. However, t he second s tage is t y p i c a l l y operated a t a pH between

8 and 8.5 and the ORP is maintained a t approximately 600 mV. Increasing

the ORP w i l l decrease the react ion t i m e but unlike f i r s t s t age

oxidat ion a s t he pH i s decreased react ion time is a l s o decreased,

In p r a c t i c e , however, t h e pH is seldom allowed t o decrease below 8.0 f o r

t w o reasons. F i r s t , i f incomplete oxidation of cyanides occurred i n the

f i r s t s t age f o r any reason, extremely t o x i c hydrogen cyanide gas could

be released when the pH i s dropped t o 7 o r below. Secondly, i f the pH

were t o drop t o 5 o r below, ni t rogen t r i c h l o r i d e , an explosive compound,

- 0

In addi t ion t o the above parameters s eve ra l other items should be

checked by the operator on a rou t ine basis . These include reac tor

mixing, r e a c t o r bottom f o r s o l i d s deposi t ion, the reac tor e f f l u e n t f o r

67

suspended s o l i d s , s o l i d s production/ temperature , and the condi t ion of

chemical feed equipment.

Mixing is important f o r four major reasons. F i r s t l y , good mixing

is e s s e n t i a l t o assure good contac t between cyanide and the oxidizing

agent, chlor ine. Secondly, good mixing is required t o minimize s h o r t

c i r c u i t i n g which can cause the a c t u a l HRT t o be much shor t e r than the

ca lcu la ted HRT. Thirdly, i f inadequate mixing is present i n the f i r s t

s tage oxidat ion, cyanide p r e c i p i t a t e s which resist oxidat ion can form.

Las t ly , inadequate mixing can r e s u l t i n s o l i d s s e t t l i n g ou t i n the

reac t ion vesse l , thereby reducing its e f f e c t i v e volume and HRT.

To assure good mixing the opera tor should pe r iod ica l ly in spec t the

basin. The operator should a l s o check i n f l u e n t and e f f l u e n t s t r u c t u r e s

f o r i n t e g r i t y . The i n f l u e n t and e f f l u e n t structures should be located

a t .opposite ends of the reac t ion vessel . C i r cu la r tanks should a l s o be

equipped with mixing b a f f l e s along the s i d e walls. The operator e i t h e r

should occasional ly dra in the tank o r rod the s i d e w a l l s t o make s u r e

solids are not being deposited i n the tank. The operator a l s o should

in spec t the mixer. The impel ler should be checked f o r proper submer-

gence and wear. The mixer should a l s o be checked f o r proper horsepower

(see Table 5) espec ia l ly i f poor mixing occurs and no o ther cause can be

found . The r eac to r water temperature a l s o should .be pe r iod ica l ly checked.

While wastewater temperature cannot usua l ly be cont ro l led i t can a f f e c t

reac t ion r a t e s and general performance of the system. By monitoring

temperature, va r i a t ions i n performance. can sometimes be understood.

S imi la r ly , suspended solkds concentrat ions should be monitored. A

s i g n i f i c a n t increase i n s o l i d s across a first s t age reac tor can ind ica t e

inadequate mixing and can r e s u l t i n cyanides being car r ied through as

68

TABLE 5

HORSEPOWER REQUIREMENTS FOR MEDIUM AGITATION

V e s s e l V o l u m e (gal) Horsepower

100 0.27

1000 2.3

10,000 12

100,000 80

. .

69


The oxidation of cyanide by chlor ine can achieve e f f i c i e n c i e s of

99% o r g r e a t e r with respec t t o cyanides not complexed w i t h i r o n ( f e r r o u s

and f e r r i c i o n s ) , giving e f f l u e n t cyanide concentrat ions i o r cyanide no t

complexed with i ron t y p i c a l l y ranging from non-detectable t o 10 ppb.

For total cyanide, the concentrations t y p i c a l l y run from non-detectable

t o 100 ppb.

TROUBLESHOOTING GUIDE

A troubleshooting guide f o r operat-sn of the cyanide oxidation pro-

cess is presented i n Table 6. The operat ing problem a reas a r e threefo ld

-- high cyanide i n e f f l u e n t s , high cyanates i n e f f l u e n t s , and s o l i d s

accumula t ion i n t h e r e a c t o r s . The h igh cyanide-cyanates problems

usua l ly involve chemical phenomena o r equal izat ion; the problem of

solids accumulation is pr imar i ly physical i n nature and is resolved by

adequate reac tor mixing.

70

TARLF 6 CYANIDE OXIDATION TROURLESHOOTIEIG GUIDF

P

OPERATING

la. Improper f

‘PH.

lOBABLE CAUSE CHECK OR MO#)NITOR RFASON CORRFCTIVF ACTIOH

PROBLEM 1: High cyanide in the effluent.

or - See “pH Control Trouble- - Cyanide oxidation is - See ”pH Control Trouble- luctuatlng reactor shooting Guide.” pH sensitive. p~ shooting Guide..

levels less than 9 dramtically reduce the reaction rate.

lb. Ina equate equali- zat f on.

Id. I n dequate mixing. 9

le. In dequate resi- deice t time.

- See ”Equalization Trouble- - F l o w surges can - See “Fqualization Trouble- shooting Guide.” cause erratic per- shooting Guide.”

’ formance of pH control equipment and wash the cyanide through the treatment process before it has had tire to react.

- Check ORP set point. - Low chlorine dos- - Check calibration and ages can prevent

condition of ORP probe. - Check chemical feed cyanides (ORP should

equipment for proper be between +350 to + opera t ion.

the destruction of

400 mV in first stage destruction).

- Check condition of mixer - i mpe 1 ler .

- Check all mixing baffles including influent, effluent, and sidewall baffles on circular tanks.

horsepower. (See Table 5). - Check motor RPH and

Inadequate mixing can result in for- matlon o€ solids containing cyani de which resist cyanide destruction. It can also result in reactor short circuiting, poor pii control, and p o r contact between chlorine and cyanides.

- Clean, calibrate, and adjust chlorine addition equipment and ORP controller as required.

- Repair and adjust as required.

- Check reaction vessel - Arcumulation of - Clean reactor. for solids accumula- solids in the re- - Redure flow. tion. action vessel and

- Monitor reactor flow excessi VP flow can and calculate IIHT. reduce hydraulic

residence tire, thus l~aviny inadequate time for thr reaction to 01 rur.

___

- I f . s

t f

-

OPERA

2a. I f r

2b. I 2

2c. I f 0

Zd. I

TAHLF 6 (Continued 1 i.

CYANIDE OXIDATION T R O I J B L F S I I ~ I N G GUIDF

~~

PROBABLE CAUSE CHECK OR NONITOR RFASON CORRFCTIVF ACTION

n i f l c a n t quan- - Moni tor wastewater f o r - I r o n cyanide com- - I d e n t i f y s o u r c e of ies of f e r r o u s or presence of i r o n i o n s . p l e x e s are formed i n f e r r o u s and f e r r i c 1011

r i c i o n s p r e s e n t . p r e s e n c e of f e r r o u s and e l i m i n a t e . and cyanide i o n s . They are very s t ab le and r e s i s t a n t to m o s t t r e a t m e n t techniques .

NG PROBLM 2: High c y a n a t e s i n t h e e f f l u e n t .

roper or c t u a t i n g c t o r p H .

dequate equal i - - - ion.

roper or c t u a t i n g

dequate mixing. -

See "pH C o n t r o l Trouble- s h o o t i n g Guide."

-

S e e " E q u a l i z a t i o n Trouble- - s h o o t i n g Guide."

Check ORP set p o i n t . - Check c a l i b r a t i o n and c o n d i t i o n o f ORP probe. Check chemical f e e d equipment f u r p r o p e r o p e r a t i o n .

Check c o n d i t i o n of m i x e r - 1 m p e 1 ler . Check a l l n i x l n g b a f f l e s i n c l u d i n g i n f l u e n t , e f f l u e n t , and sidewall b a f f l e s 0 1 1 c l I c U l d K tdnks. Check motor HPM dnd t i o r s c ~ p a w ~ ~ r . (See Table 5)

Cyanate o x i d a t i o n is - See "pH C o n t r o l Trouhle- p H s e n s i t i v e . pH l e v e l s b e t w e e m 8.0 and 8.5 s h o u l d be main- t a i n e d .

s h o o t i n g Guide."

Flow s u r g e s c a n - S e e " F q u a l i x a t i o n Trouhle- cause e r ra t ic per- s h o o t i n g Guide." formance o f pH control equlpment and wash t h e c y a n a t e through t h e treatment process b e f o r e i t h a s had t i m e to react.

Low c h l o r i n e do- s a g e s c a n p r e v e n t t h e d e s t r u c t i o n o f c y a n i d e s (ORP s h o u l d be around +600 mV f o r second s t a g e cyanide d e s t r u c t i o n ) .

Inadequate mixing cdn r e s u l t i n reactor s h o r t ci r c u i t i n g , poor pll c o n t r o l , and poor c o n t a c t between c h l o r i n e dnd cydndttes.

- Clean , cal ibrate , and a d j u s t c h l o r i n e a d d i t i o n equipment and ORP c o n t r o l l e r a s requi red .

- Repai r and a d j u s t a s requi red .

TABLE 6 (Cont inued)

CYANIDE OXIDATION TROUBLESHOOTING GUIDE

2e.

OPE1

3a.

3b.

-

PROBABLE CAUSE CllECK OR MONITOR REASON CORRECTIVE ACTION

nadequate resi- - Check r e a c t i o n v e s s e l - Accumulation of - Clean reactor' ence t i m e . f o r sol ids accumula- solids i n t h e re-

t i o n . a c t i o n v e s s e l and - Monitor reactor f low e x c e s s i v e f low can

and calculate HRT. reduce h y d r a u l i c

reduce flow.

r e s i d e n c e t i m e , t h u s i n a d e q u a t e t i m e f o r t h e r e a c t i o n to occur.

PING PROBLEM 3: S o l i d s accumula te i n reactors.

nadequate mix- - Check c o n d i t i o n of ng. mixers , mixer horse-

power, RPH o f mixers i f variable speed mixers are used and c o n d i t i o n of s i d e wal l b a f f l e s i f c i r c u l a r t a n k s are used.

- Inadequate mixing - c a n r e s u l t i n t h e format ion of pre- c i p i t a t a n t s t h a t c o n t a i n cyanides . P r e c i p i t a t e d cya- n i d e s resist oxi- d a t i o n .

Repair and r e p l a c e equipment a s requi red .

r i t or meta l f i l - - I n s p e c t s o l i d s i n t a n k s - G r i t t y solids such ngs p r e s e n t i n to determine t h e i r na- a s metal f i l i n g s a s t e w a ter. t u r e . c a n n o t be k e p t i n

s u s p e n s i o n u n l e s s very h i g h mixing horsepower is provided.

- I n s t a l l g r i t chamber or s imi l a r g r i t c o l l e c t i o n sys tem ups t ream of t h e process.

I ' I 1

I

SECTION 7

CHROMIUM REDUCTION

INTRODUCTION

Chromium is used i n the metal f i n i s h i n g indus t ry a s a corrosion

i n h i b i t o r e i t h e r i n the chromate form, C r 0 4 ion, o r a s t he dichromate

form, C r 2 0 7 ion. As e i t h e r ion, chromium e x i s t s with a valence of +6

(hexavalent) . A t t h i s valence chromium is both t o x i c and soluble .

Reduction of chromium from the +6 valence s ta te t o t h e +3 valence s t a t e

and subsequent p r e c i p i t a t i o n of the t r i v a l e n t chromium is the most

common method of chromium removal from wastewater. o ther methods i n -

clude ion exchange and carbon adsorption.

-2

-2

THEORY OF OPERATION

Reduction is a chemical react ion i n which one o r more e lec t rons a r e

t r a n s f e r r e d t o the chemical being reduced (e.g., +6 chromium) from the

chemical i n i t i a t i n g the t r a n s f e r (i. e., reducing agent ) . There a r e

s e v e r a l reducing agents t h a t can be used t o supply e lec t rons t o the

hexavalent chromium t o reduce it t o t r i v a l e n t chromium. These reagents

include metallic i r o n , f e r rous s u l f a t e , s u l f u r dioxide, and various

forms of s u l f i t e s such a s sodium s u l f i t e , sodium b i s u l f i t e , and sodium

metabisu l f i te . The chemical r eac t ions f o r chromium reduction by the

most commonly used reagents are shown below.

Sul fur Dioxide:

> C r 2 ( S 0 1 + N a S O + H 2 0 N a C r o + 3SO + H so - 2 4 2 2 7 2 2 4 .4 3

74

Sodium Sulf i t e :

> C r (SO4I3 + 4Na-SO. + 4 H - 0 N a C r o + 3Na SO + 4H2S04 - 2 2 2 7 2 3 L Y L

Sodium B i s u l f i t e :

N a C r 0 + 3NaHSO + 2.5 H2S04 - > cr2 2 2 7 3

Sodium Metabisulf i te

N a C r 0 + 1.5 N a S 0 + 2.5 H2S04 - > 2 2 7 2 2 5

s04

2 C r

The choice of reducing agent depends on

+ 4H 0 + 2.5 N a SO 3 2 2 4

S 0 4 ) 3 + 2.5H 0 + 2.5 Na2S04 2

t he o v e r a l l process opt ions

and loca l ized c o s t conditions. Sodium metabisu l f i te o f f e r s s t a b i l i t y

and d r y chemical feed c a p a b i l i t i e s as advantages over t he use of b i s u l -

f i t e . A disadvantage of using s u l f u r dioxide is the p o t e n t i a l l y hazard-

ous s i t u a t i o n t h a t e x i s t s when s u l f u r dioxide i s s to red and handled.

Advantages of s u l f u r dioxide include ease of automatic cont ro l and its

lower chemical cost . Ferrous s u l f a t e reduction has been reported t o

have the advantage of e f fec t iveness which is independent of pH. How-

ever , i t has a l s o been pointed o u t t h a t f e r rous s u l f a t e produces four

times t h e q u a n t i t i e s of s l u d g e produced through t h e u s e of s u l f u r

dioxide o r b i s u l f i t e .


Chromium reduction can be achieved i n e i t h e r t he batch o r con-

tinuous-flow mode. Both modes require a reac t ion vesse l , mixer, pH

c o n t r o l l e r , ORP c o n t r o l l e r , and chemical addi t ion equipment (See Figure

8 ) . Batch systems a r e usua l ly l imited t o small flows and a r e operated

manually by employees who monitor the pH and ORP and add chemicals as

required. The advantages of batch treatment a r e t h a t the l e v e l of

hexavalent chromium discharge and complexity of automation a r e both low.

Continuous-flow systems, however, must have automated chemical feed

systems t h a t a r e capable of maintaining proper pH and ORP. The advan-

tage of continuous-flow treatment i s t h a t t he manpower needs a r e low.

75

SULFURIC ACID SULFUR DIOXIDE

3 RAW WASTE (HEXAVALENT CHROMIUM)

I I I I I I 1

7- II

-- I -- --aoRp CONTROLLER

r I I (TRIVALENT CHROMWM)

REACTION TANK

Figure 8. Hexavalent chromlum .reduction with sulfur dioxide.

OPERATIONAL PROCEDURE

The o b j e c t i v e of chromium reduct ion is t o conver t hexavalen t chrom-

ium t o t r i v a l e n t chromium. This procedure i s c r u c i a l because hexavalen t

chromium is h ighly t o x i c t o a q u a t i c organisms and is a known carcinogen.

It is a l s o very so lub le i n water i n its hexavalen t s t a t e and is very

d i f f i c u l t t o remove wi thout f i r s t conver t ing it t o the t r i v a l e n t s t a t e .

T r i v a l e n t chromium i s less t o x i c than hexavalent chromium and can be re -

moved r e a d i l y from wastewater by s tandard chemical p r e c i p i t a t i o n between

p H l e v e l s of 7 and 9.

To c o n t r o l t he ope ra t ion of hexavalent chromium reduc t ion proces-

ses, t h e ope ra to r should conduct t he necessary process monitoring,

perform any c o n t r o l c a l c u l a t i o n s needed, and understand the process

c o n t r o l strategies o r v a r i a b l e s .

Process Monitoring

The ma jo r p a r a m e t e r u s e d t o a s s e s s t h e e f f i c i e n c y of chromium

reduc t ion processes is t h e concen t r a t ion of hexavalen t chromium

(chromium +6). The main performance parameters used t o monitor t h e

ope ra t ion of t h e chromium (+6) reduct ion process a r e the flow and t h e

concen t r a t ion of hexavalent chromium i n t h e r e a c t o r e f f l u e n t . W h i l e

t hese parameters provide the o v e r a l l performance, many o t h e r parameters

must be measured and incorpora ted i n t o a regular monitoring program t o

s u c c e s s f u l l y and c o n s i s t e n t l y ope ra t e a chromium reduct ion process. The

parameters which should be monitored f o r chromium reduct ion a r e l i s t e d

i n Table 7 .

For continuous-flow t rea tment , flow, p H , temperature, and ORP a r e ~ ~~

normally recorded whenever t h e process i s i n opera t ion . E f f l u e n t t r i v a - ~~ ~

~~ ~

l e n t chromium and hexavalent chromium concen t r a t ions a r e t y p i c a l l y

measured us ing 24 h r composite samples. The remaining ana lyses should

be performed on a r e g u l a r bu t less f r equen t b a s i s , t y p i c a l l y weekly.

77

TABLE 7

CHROMIUM REDUCTION PROCESS MONITORING REQUIREMENTS


1 . Reactor Temperature

2. Flow

Continuously To understand changes i n repigj.on r a t e f o r C r t o C r . +3

Continuously To determine HRT and t o a d j u s t chemical dosages.

3 . I n f l u e n t Hexavalent Weekly Chromium

4. E f f l u e n t Hexavalent Daily Chromium

5 . E f f l u e n t T r i v a l e n t Daily Chromium

To determine remova 1 e f f i c i e n c y .

To determinf6any r e s i d u a l C r . To determine loading to downstream t rea tment .

6. Reactor pH Continuously To c o n t r o l a c i d add i t ion .

7 . Reactor ORP Continuously To c o n t r o l s u l f i t e a d d i t i o n .

78

Example Ca lcu la t ions

One example c a l c u l a t i o n i s provided below f o r c a l c u l a t i n g

o p e r a t i n g parameters e s s e n t i a l t o ope ra t ion of a chromium reduc t ion

process.

Hydraulic Residence Time (HRT) i s the average l eng th of t i m e waste-

water spends i n t h e chemical r e a c t i o n tank. S i n c e the reduct ion of

chromium is n o t an ins tan taneous r eac t ion , i t is e s s e n t i a l t h a t t he

wastewater spend a minimum t i m e i n the r e a c t i o n tank. The HRT ca lcu-

l a t i o n is:

where

VOL = t h e volume of l i q u i d i n t h e tank and

FLOW = the flow t o t h e r e a c t o r .

Note t h a t VOL and FLOW must be expressed i n s i m i l a r u n i t s such a s cub ic

meters o r ga l lons . As an example i f a chromium reduc t ion tank had a

volume of 5000 ga l lons and t h e flow t o the tank was 2500 ga l lons /hour

the c a l c u l a t i o n would be:

= 2 h r 5000 g a l 2500 gal HRT =

h r

Process Cont ro l S t r a t e g i e s

The primary o p e r a t i n g s t r a t e g i e s which c o n t r o l t h e performance of a

chromium reduc t ion process a r e t h e reducing agen t used, t he r e a c t o r HRT,

and the r e a c t o r pH and ORP. The s e l e c t i o n of reducing agent i s t y p i -

c a l l y made dur ing the des ign of t h e f a c i l i t y , and t h e HRT is a func t ion

of t he r e a c t i o n volume and wastewater flow, both of which a r e u s u a l l y

beyond the c o n t r o l of t h e opera tor . T h i s l eaves pH and ORP a s t h e

primary p rocess c o n t r o l v a r i a b l e s over which the ope ra to r has con t ro l .

79

Operator c o n t r o l of the chromium reduct ion process is u s u a l l y exer-

c i sed by f i r s t a d j u s t i n g the PH of the r e a c t o r based on t h e HRT. The

r e l a t i o n s h i p between the r e a c t o r HRT and the requi red p H f o r chromium

reduct ion using s u l f u r d ioxide i s given i n Figure 9. I t i s important

t o understand the s i g n i f i c a n c e of Figure 9. From Figure 9 it can be

s e e n t h a t f o r a 20-minute r e t e n t i o n time, a pH of 3.0 o r l e s s i s

requ i r ed t o achieve complete reduct ion of chromium. If the r e t e n t i o n

t i m e of the system w e r e t o decrease from 20 minutes t o 10 minutes be-

cause of an inc rease i n the f lowra te , complete reduct ion of chromium

could s t i l l be achieved by decreas ing the ope ra t ing pH from 3.0 t o 2 . 5

o r less. However, i f the pH remained a t 3.0, incomplete chromium re-

duc t ion would occur. From Figure 9 it can. be observed t h a t i f t h e

r e t e n t i o n t i m e is 50 minutes, a PH of 4.0 o r less i s s u f f i c i e n t .

The remaining primary c o n t r o l parameter i s ORP. ORP is a measure

of the preponderance of e a s i l y ox id i zab le o r reduceable subs tances i n a

wastewater sample. The importance t o the ope ra to r is knowing whether

t h e r e is l a r g e q u a n t i t y of reducing subs tances t h a t may have an immedi-

a t e and very high demand f o r oxygen. S u l f i d e and s u l f i t e a r e examples

of such substances. This measurement, a l though n o t s p e c i f i c , is ins t an -

taneous. ( 1 4 ) The oxidat ion-reduct ion p o t e n t i a l i s measured i n a gal-

vanic ce l l c o n s i s t i n g of a re ference e l e c t r o d e (e.g. , calomel) and a n

i n d i c a t i n g e l e c t r o d e of a h ighly noble metal (e.g., platinum o r g o l d ) .

The calomel e l e c t r o d e i s the cathode and t h e i n e r t platinum o r gold

e l e c t r o d e is the anode. The anode is made of a h ighly noble metal so

t h a t t h e p o t e n t i a l f o r i t s oxida t ion i s less than t h a t of any ox id izab le

s o l u t i o n components. The anode thus is a s i te of the oxida t ion of

s o l u t i o n c o n s t i t u e n t s b u t i d e a l l y is not a f f e c t e d i tself .

Oxidat ion-reduct ion p o t e n t i a l measurements i n n a t u r a l waters and

wastewaters a r e d i f f i c u l t t o i n t e r p r e t . The only p o t e n t i a l s t h a t w i l l

r e g i s t e r i n the ORP c e l l a r e those from spec ie s t h a t can r e a c t a t the

i n d i c a t o r e l e c t r o d e s u r f a c e - t hese a r e c a l l e d e l e c t r o a c t i v e spec ies .

I n n a t u r a l waters only a few r e a c t i o n s proceed a t the e l e c t r o d e sur face .

80

50

40

30

20

10

0.1

Figure 9.

0.5 1 2 5 10 20 50

RETENTION IN MINUTES

Relationship between hexavalent chromium, pH, and retention t ime for sulfur dioxide.

31

A l l t he important redox r eac t ions involved i n the n i t rogen cyc le ,

t h e s u l f u r cyc le , and the carbon cyc le a r e no t completed a t the i n d i -

c a t o r e l e c t r o d e i n an ORP c e l l . A t b e s t , too , the vol tage reading

produced by an ORP c e l l is a r e f l e c t i o n of many r e a c t i o n s - i t is a

"mixed p o t e n t i a l " and i ts value i s d i f f i c u l t i f n o t impossible t o i n t e r -

p r e t i n any fundamental chemical terms. Moreover, when an ORP e l e c t r o d e

combination is immersed i n a water t he vol tage reading w i l l vary with

t i m e , u s u a l l y f a l l i n g from the i n i t i a l read ing obtained. This behavior

is due t o the genera l process of p o l a r i z a t i o n and of "poisoning" of t h e

i n d i c a t o r e l e c t r o d e su r face by the accumulation of ox ida t ion products on

t h e s u r f a c e of the e l ec t rode . (IS)

Despi te a l l of these l i m i t a t i o n s ORP measurements have been used

widely i n metal f i n i s h i n g t rea tment systems where, i f they a r e t r e a t e d

as i n d i c e s o r "black box measurements'' r a t h e r than fundamental i nd i -

c a t o r s of a s p e c i f i c chemical environment, they can be of q u a l i t a t i v e

use. ( 1 5 )

The ORP of a chromium reduct ion r e a c t o r is c o n t r o l l e d by the amount

of reducing agent added. The ORP set p o i n t f o r t he chromium reduct ion

is somewhat dependent upon the pH and the ions p r e s e n t i n so lu t ion . An

ORP value of +300 mv a t a pH of 2 . 3 w i l l normally be s a t i s f a c t o r y f o r

chromium (+6) reduct ion. The r e l a t i o n s h i p between pH

b i s u l f i t e is presented i n Figure 10. If inadequate

occurs under these circumstances the ope ra to r should

pH and/or i n c r e a s i n g the ORP ope ra t ing l e v e l .

and ORP f o r sodium

chromium reduct ion

t r y decreas ing the

Seve ra l o t h e r f a c t o r s a f f e c t the ope ra t ion of the chromium reduc-

t i o n process t o a lesser e x t e n t and should be checked any time poor

performance o r excess ive chemical use is encountered. Among these fac-

t o r s a r e mixing, temperature , and mass loading. Rapid and good mix ing

or th e wastewater , ac id , and reducing agent is important t o ensure

e f f i c i e n t use of chemicals. Overfeeding of chemicals i s u s u a l l y re -

qu i r ed i f mixing is inadequate. Furthermore, s o l i d s can s e t t l e ou t i n

the r e a c t i o n tank i f mixing 'is inadequate , a s i t u a t i o n which reduces t h e

82

8oo ' f 600 ;z w

> E

0 1 .o 2.0

Relative Quantlty of NeHSO, AddedlRequired per Time

Figure 10. Relationship between O W , sodium bisulfite required, and pH.

8 3

e f f e c t i v e volume of t he tank which i n t u r n reduces t h e HRT. When t h e

HRT is decreased , lower ope ra t ing pH l e v e l s and hence increased chemical

usage a r e requi red .

To ensure t h a t good mixing is maintained s e v e r a l items must be

checked p e r i o d i c a l l y by the opera tor . Among t h e i tems a r e t h e

following:

o Check mixer impe l l e r f o r wear and proper l o c a t i o n with r e s p e c t

t o the d r i v e s h a f t and t h e tank.

o Check b a f f l e s f o r wear and proper placement. All c i r c u l a r tanks

should have b a f f l e s a long the o u t e r v e r t i c a l w a l l and most o t h e r

tank wal l s should have some form of i n f l u e n t o r e f f l u e n t b a f f l e

t o p reven t s h o r t c i r c u i t i n g .

o Check tank bottom t o determine i f s o l i d s are accumulating. This

i s u s u a l l y a c c o m p l i s h e d e i t h e r by d r a i n i n g t h e t a n k o r by

rodding the sides t o determine water depth versus tank depth.

o Check mixer horsepower. Typica l mixer horsepower r a t i n g s a r e

g iven i n Table 5.

The e f f e c t of temperature' on t h e performance of chromium reduct ion

sometimes can be very s i g n i f i c a n t . Temperature p r i m a r i l y a f f e c t s reac-

t i o n r a t e s and, t o a lesser e x t e n t , mixing. The ope ra to r has l i t t l e

c o n t r o l over tempera ture , b u t p e r i o d i c a l l y monitoring t h e temperature

may enhance unders tanding of s l i g h t l y poorer performance dur ing pe r iods

of co ld weather. If l a r g e temperature swings occur i t may be necessary

t o decrease r e a c t o r pH. o r i n c r e a s e ORP l e v e l s . I t a l s o should be noted

tha t temperature changes can have dramat ic e f f e c t s on ins t rument Cal i -

b ra t ion . Any t i m e s i g n i f i c a n t temperature changes occur, pH and ORP

meters should be r e c a l i b r a t e d .

S i g n i f i c a n t i n c r e a s e s i n the mass loading of chromium t o a t r e a t -

ment process can r e s u l t i n temporary u p s e t s and increased chemical

usage. Hexavalent chromium loading i n the r e a c t o r e f f l u e n t a l s o direct-

l y r e l a t e s t o the p l a n t d i scha rge of chromium which o f t e n i s expressed

84

as a mass pe r t i m e ( i . e . , lb/day o r kg/day). For these reasons the

i n f l u e n t and e f f l u e n t loadings should be c a l c u l a t e d p e r i o d i c a l l y t o ga in

i n s i g h t i n t o process performance.


Hexavalent chromium concen t r a t ions i n the e f f l u e n t from chromium

reduct ion processes range from 0.009 t o 0.045 mg/L. Most va lues , how-

ever , a r e less than 0.02 mg/L i n a well-operated system.

TROUBLES HOOTING GUIDE

A t r o u b l e s h o o t i n g g u i d e f o r t h e chromium r e d u c t i o n p r o c e s s i s

presented i n Table 8. The major o p e r a t i o n a l problem i n chromium reduc-

t i o n processes is incomplete reduct ion of hexavalen t chromium which

resu l t s i n a s i g n i f i c a n t concen t r a t ion of Cr+6 i n t h e r e a c t o r e f f l u e n t .

The causes of incomplete reduct ion may be p h y s i c a l ( s h o r t - c i r c u i t i n g ,

sampling and a n a l y s i s e r r o r s , etc.) o r chemical ( i n c o r r e c t pH, ORP,

etc. 1.

85

PR(

OPERATIUG

la. Sampl: a n a l y i

lb . Retenl s h o r t d e s t r i

IC. Shor tc react.

Id. Improl

1

TABLE 8 CHROEIUM RFDUCTION TRWBLFSHOOTIUG GUIDF

\BLE CAUSE CHFCK OR MONITOR RFLSON CORRJXTIVE ACTION

tOBLEw 1 t Cr+6 c o n c e n t r a t i o n too h igh .

+6 m time too > complete C r t i on .

c c u i t i n g in i v e s s e l .

Check collection proce- - d u r e w i t h operator and c h e c k w i t h l a b to e n s u r e proper reporting o f r e s u l t s .

C a l c u l a t e re t e n t i o n time - v e r s u s t i m e shown i n r e t e n t i o n t i m e / p H g r a p h ( s e e F i g u r e 9 ) .

I n s p e c t m i x i n g r e g i m e of ' - t ank . Look f o r d e a d spots, s l u d g e a c c u m u l a t i o n , and i n l e t and o u t l e t s h o r t - circui t ing.

Check d e s i r e d p H s e t t i n g - o f meter v e r s u s r e t e n - t i o n t i m e .

Check c a l i b r a t i o n o f p H meter.

Check operation o f a c i d - a d d i t i o n e q u i p m e n t (pumps, v a l v e s , and s t o r a g e t a n k ) .

E r r o r c o u l d b e w i t h d a t a .

R e t e n t i o n t i n e may b e i n s u f f i c i e n t f o r c o m - plete chromium d e m t r u c t i o n .

S h o r t c i r c u i t i n g w i l l d e c r e a s e t h e e f f e c t i v e r e t e n t i o n time.

S e t p o i n t p H may be too h i g h f o r g i v e n r e t e n t i o n .

Meter may be o u t o f c a l i b r a t i o n .

F a i l u r e to a c h i e v e d e s i r e d p H c o u l d b e c a u s e d by i n s u f f i c i e n t a c i d , i n o p e r a b l e pumps or v a l v e s , or m a l f u n c t i o n i n c o n t r o l eys tem.

R e c a l c u l a t e new c o p c e n t r a t i o n a n d make s u r e h e x a v a l e n t chromium is to be measured , n o t to ta l chromium.

I n c r e a s e r e t e n t i o n t i m e by de- c r e a s i n g t h e f l o w r a t e ; or decrease pH.

S h o r t c i r c u i t i n g may b e r e d u c e d by i n s t a l l i n g b a f f l e s , i n - creaeing pumping a c t i o n o f mixe r , or i n s t a l l i n g i n l e t and o u t l e t b a f f l e s .

S e l e c t lower s e t p o i n t pH.'

Reca libra te.

C o r r e c t as n e c e s s a r y .

PF

le. I m p r c

I f . Ilydrc to re

TABLE B ( C o n t i n u e d )

CHROMIUM REDUCTION TROUBLESHOOTING GUIDE

CORRECTIVE ACTION IABLE CAUSE CHECK OR MONITOR REASON

'r ORP. - Check d e s i r e d ORP s e t t i n g .

- Check c a l i b r a t j o n o f ORP meter.

- Check l o c a t i o n o f probe. -

- Check o p e r a t i o n o f ~ - c h e m i c a l a d d i t i o n system.

ic s u r g e s - Moni tor f low rate o v e r - , t i o n v e s s e l . t i m e and check p l a n t

p r o d u c t i o n p e r s o n n e l .

S e t t i n g s r a n g e - E s t a b l i s h new s e t p o i n t from +200 nv to ORP . +400 mV w i t h a n a v e r a g e o f +300 mv.

Meter r e a d i n g may - R e c a l i b r a t e meter. n o t r e p r e s e n t a c t u a l ORP reading .

I f probe is n o t l o c a t e d a t e f f l u e n t , t h e n probe is n o t r e a d i n g e f f l u e n t ORP.

F a i l u r e to a c h i e v e d e s i r e d ORP c o u l d be c a u s e d by i n s u f f i - c i e n t c h e m i c a l s u p p l y , i n o p e r a b l e pumps or v a l v e s , or m a l f u n c t i o n i n c o n t r o l system.

- L o c a t e probe a t e f f l u e n t .

- C o r r e c t as n e c e s s a r y .

- E q u a l i z e f l o w by E v a l u a t e whether f l o w rates r e d u c e d e c r e a s i n g rate of r e t e n t i o n t i m e chromium w a s t e s u c h t h a t complete d i s c h a r g e from chromium r e d u c t i o n process t a n k s . is n o t accompl ished .

SECTION 8

pH CONTROL

INTROD UCTI ON

Cont ro l of p H i s a fundamental p r o c e s s f o r v i r t u a l l y every metal

f i n i s h i n g wastewater t r ea tmen t system. I t is t h e c rux of wastewater

meta l p r e c i p i t a t i o n t h a t p H be maintained w i t h i n a narrow range t o

ach ieve optimum metal removal. Close pH c o n t r o l i s r equ i r ed a l s o f o r

chromium reduc t ion , cyanide o x i d a t i o n , and p o s s i b l y f o r emulsion break-

i n g of o i l s . Furthermore, it may be r e q u i r e d t o p o s t - n e u t r a l i z e t h e

t r e a t e d wastewater t o a s s u r e t h a t t he d i scha rge i s w i t h i n r e g u l a t o r y

l i m i t s . Because of t h e number of t r ea tmen t p rocesses t h a t r e q u i r e pH

adjus tment , a me ta l f i n i s h i n g wastewater t r ea tmen t system may c o n t a i n

s e v e r a l pH c o n t r o l s teps , and may inc lude pH c o n t r o l w i t h i n selected

p l a n t p rocess ing u n i t s .

THEORY OF OPERATION

+ The parameter p H is a measure of the hydrogen i o n ( H ) concent ra -

t i o n i n a water o r wastewater s o l u t i o n . I t is c a l c u l a t e d as t h e nega-

t i v e loga r i thm (base 1 0 ) of t h e hydrogen i o n c o n c e n t r a t i o n , which means

t h a t f o r each d rop i n p H of one u n i t a t e n f o l d i n c r e a s e i n t h e H i o n

c o n c e n t r a t i o n t a k e s place. This means t h a t when t h e hydrogen i o n con-

c e n t r a t i o n i n c r e a s e s , t h e p H d e c r e a s e s and t h e water becomes more

+

"acidic.. Likewise , when t h e hydrogen i o n c o n c e n t r a t i o n dec reases , t h e

pH i n c r e a s e s and t h e waste becomes more " b a s i c " o r " a l k a l i n e . " When the

pH i s 7 t h e waste is n e i t h e r a c i d i c o r b a s i c and i s cons ide red n e u t r a l .

88

The ad jus tment of pH r e q u i r e s t h a t , t o lower t he pH of t h e waste-

water, some chemicals which r e l e a s e H i o n s must be d i s s o l v e d i n t h e +

water . The most common chemicals used i n waste t r ea tmen t a r e s u l f u r i c

a c i d ( H SO 1, hydroch lo r i c a c i d ( H C l ) , and n i t r i c a c i d (HNO 1. S i i n i -

l a r l y , t o raise t h e p H , some chemical t h a t reacts wi th t h e H i o n from : 2 4

s o l u t i o n must be added. The most common chemicals f o r r a i s i n g pH a r e

c a u s t i c ( N a O H ) , l i m e ( C a O H ) , and soda ash ( N a 2 C 0 3 ) . The l i m e and caus-

t i c work by r e l e a s i n g a hydroxide i o n ( O H - ) which reacts wi th an H i on +

+ t o form water, H20 .

form t h e b i ca rbona te i o n (HCO-) o r wi th two H

The soda a s h works by r e a c t i n g wi th one H i o n t o + i o n s t o form H 0 + C02. 3 2

Whenever p H l e v e l s a r e changed, t h e concept of b u f f e r i n g must be

d i scussed . Buf fe r ing is t h e r e s i s t a n c e of a s o l u t i o n t o pH changes.

This r e s i s t a n c e i s t y p i c a l l y low a t pH l e v e l s c l o s e t o 7 and very h igh

f o r pH l e v e l s less than 4 o r g r e a t e r than 10. The r e s i s t a n c e , however,

i s very dependent on t h e chemicals p r e s e n t i n t h e wastewater.

The..best way t o de te rmine b u f f e r i n g i s w i t h a t i t r a t i o n ’ curve . A

t i t r a t i o n curve i s gene ra t ed by t a k i n g a sample of wastewater and grad-

u a l l y adding t h e a c i d o r base and r eco rd ing t h e pH a f t e r a smal l amount

of t h e r e a g e n t is added and w e l l mixed. An example t i t r a t i o n curve i s

p l o c t e d i n F igure 1 1 . I t is c o n s t r u c t e d by p l o t t i n g t h e volume of

pH-adjusting chemical added ve r sus t h e pH of t h e waste sample a f t e r

mixing w i t h t h e a d j u s t i n g chemical. I n t h e example d i s p l a y e d i n F igure

1 1 , it can be seen t h a t t h e f i r s t 40 m l of t he c a u s t i c ( N a O H ) s o l u t i o n

r a i s e d t h e p H from approximate ly 1 t o 2. The n e x t 20 m l of t he c a u s t i c

s o l u t i o n , however, raised t h e pH t o approximate ly 11.5. In terms of

b u f f e r i n g the f i r s t segment of t h e curve i s r e f e r r e d t o as h i g h l y buf-

f e red . A l a r g e q u a n t i t y of c a u s t i c s o l u t i o n w a s r e q u i r e d t o make a

smal l c h a n g e i n p H . The s e c o n d segmen t i s r e f e r r e d t o as p o o r l y

b u f f e r e d . Only a small a d d i t i o n of c a u s t i c s o l u t i o n w a s r equ i r ed t o

make a l a r g e change i n pH.

The p r a c t i c a l e f f e c t s of b u f f e r i n g on pH adjus tment w i l l be d i s -

cussed i n l a t e r s e c t i o n s of t h i s c h a p t e r , b u t from t h i s example i t i s

89

13

7 7

9

PH 7

5

3

f I 0 70 20 30 40 50 60 70 80

CAUSTlC (NaOtf), MILLlLlTERS NORMALITY = 0.10 N -

Figure 11. A sample titration curve. . s

%;

90

e a s y t o see t h a t a pH of 2 could e a s i l y be maintained. A pH of 7, how-

e v e r , i s d i f f i c u l t t o main ta in because a very small a d d i t i o n of c a u s t i c

, s o l u t i o n can make a very s i g n i f i c a n t change i n p H .


The pH adjus tment p rocess t y p i c a l l y c o n s i s t s of a r e a c t i o n t ank ,

mixer, pH c o n t r o l system, and r eagen t f eed system. The pH adjus tment

p rocess may e i t h e r be a batch o r a continuous flow system. In a ba t ch

system, wastewater i s added t o the r e a c t i o n t ank ; then chemical r e a g e n t s

a r e added t o a d j u s t t he pH; and f i n a l l y t h e s o l u t i o n is d i scha rged t o

t h e subsequent t r e a t m e n t u n i t s . Con t ro l of t h e s t e p s i n t h e ba t ch pH

adjus tment p r o c e s s can e i t h e r be au tomat i c o r manual.

I n cont inuous flow systems, wastewater is con t inuous ly e n t e r i n g t h e

r e a c t i o n v e s s e l . The f l o w r a t e can e i t h e r be c o n s t a n t , i f e q u a l i z a t i o n

i s provided , o r v a r i a b l e . Of ten p H ad jus tment is accomplished i n more

than one r e a c t i o n v e s s e l . This i s e s p e c i a l l y t r u e when t h e se t

p o i n t i s l o c a t e d on a poor ly b u f f e r e d segment of t h e t i t r a t i o n curve.

The f i r s t ta3-k w i l l a d j u s t t h e pH c l o s e t o t h e se t p o i n t u s ing a re la-

t i v e l y l a r g e chemical f e e d system. The second tank w i l l t hen use a

small meter ing pump t o f i n e tune and a d j u s t t h e p H t o the s e t p o i n t

l e v e l . A l t e r n a t e l y , a s i n g l e tank can be used i n con junc t ion wi th a low

and h igh ra te chemical f e e d system. When t h e d e v i a t i o n between t h e s e t

p o i n t and t h e a c t u a l pH is l a r g e , t h e h igh r a t e system i s used. When

t h e d i f f e r e n c e i s small , t h e l o w r a t e system i s used t o f i n e tune t h e

ad jus tment . This system i s p r i m a r i l y used f o r ba t ch o p e r a t i o n s .

Automatic c o n t r o l of chemical r eagen t a d d i t i o n i s mandatory €o r

cont inuous systems. There are va r ious methods f o r p rov id ing c o n t r o l of

t h e pH ad jus tmen t p rocess . These methods i n c l u d e feedback, feedforward ,

and feedforward-feedback c o n t r o l t echn iques and s e v e r a l t ypes of c o n t r o l

s i g n a l g e n e r a t o r s . The method best s u i t e d f o r a p a r t i c u l a r a p p l i c a t i o n

i s dependent upon c h a r a c t e r i s t i c s of t h e w a s t e , h y d r a u l i c r e t e n t i o n time

of t h e r e a c t i o n v e s s e l , mixing, r e a c t i o n l a g s , and o t h e r f a c t o r s .

91

The common pH c o n t r o l techniques employed today a r e t h e feedback , feedforward , and feedforward-feedback c o n t r o l concepts . Feedback con-

t r o l bases t h e chemical feed r a t e on t h e d i f f e r e n c e or e r r o r between t h e

s e t p o i n t pH and t h e measured pH. Thus, a feedback system s o l v e s t h e pH

c o n t r o l problem by t r i a l and e r r o r . An example of a feedback c o n t r o l

system i s shown i n F igure 1 2 . The major advantage of t h i s s y s t s m i s

t h a t , because it o p e r a t e s by t r i a l and e r r o r , t h e system does n o t need

t o know t h e flow and pH of t h e i n f l u e n t . I t is a l s o r e l a t i v e l y in sen -

s i t i v e t o changes i n t h e t i t r a t i o n curve. A d i sadvan tage t o feedback

c o n t r o l i s i ts s u s c e p t i b i l i t y t o o s c i l l a t o r y r e sponse , which r e s u l t s

from an i n h e r e n t t i m e l a g i n the response. T h i s t i m e lag i s r e f e r r e d t o

a s "dead" t i m e and i s t h e t i m e r e q u i r e d t o d e l i v e r t h e chemica ls t o t he

wastewater, mix t h e chemical w i th t h e wastewater, and f o r t h e a p p r o p r i -

a t e chemical r e a c t i o n s t o occur . T h i s t i m e l a g means t h a t a f t e r a

c o n t r o l l e r i n i t i a t e s a c o n t r o l a c t i o n , it does n o t see the r e s u l t s of

t h a t a c t i o n u n t i l t h e "dead" t i m e has e lapsed . I f t h e c o n t r o l a c t i o n

w e r e e x c e s s i v e because of poor b u f f e r i n g , a r a p i d change i n f l o w r a t e , o r

e x c e s s i v e "dead" t i m e , t h e s e t p o i n t can be ove r shoo t b e f o r e t h e con-

t r o l l e r r e a l i z e s it. I t can then o v e r r e a c t t o t h e ove r shoo t and t h e pH

can begin t o o s c i l l a t e around t h e set p o i n t . This o s c i l l a t i o n can

r e s u l t i n unacceptab le performance and/or e x c e s s i v e chemical use.

A feedforward c o n t r o l system does n o t exper ience t h e o s c i l l a t i o n

problem a s s o c i a t e d w i t h feedback c o n t r o l . A feedforward system o p e r a t e s

by measuring t h e flow and pH of t h e wastewater be fo re it reaches t h e pH

adjus tment tank. It t a k e s t h i s in format ion and computes t h e necessa ry

chemical a d d i t i o n based on t i t r a t i o n informat ion t h a t has been pro-

grammed i n t o the c o n t r o l l e r . A schemat ic of a feedforward system i s

shown i n F igure 13. The primary advantage of t h e system i s t h a t it is

n o t s u s c e p t i b l e t o t h e problems encountered by feedback c o n t r o l . The

system, however, i s dependent upon having t h e a b i l i t y t o p r e d i c t t h e

b u f f e r i n g of t h e incoming wastewater. This means t h a t t h e s y s t e m can

on ly be used on wastewater streams which have b u f f e r i n g c a p a c i t y and

t i t r a t i o n curves t h a t do n o t vary wi th t i m e . This i s u s u a l l y a s e v e r e

r e s t r a i n t when c o n t r o l l i n g pH. The s y s t e m i s a l s o s u s c e p t i b l e t o

g radua l d r i f t s i n e f f l u e n t pH because of cumulative c o n t r o l l e r e r r o r

92

REAGENT

INFLUENT W A S T E W A E R

/

Figure 12. Feedback mode of ptl cmtrol.

93

REAGENT

pH CONTROLLER

I

TRANSMITTER TRANSMIT ER q - 6 INFLUENT

WASTE W AT

I

Figure 13. Feedtorward mode of pH

EFFLUENT +WA~TEW ATER

control.

which t h e c o n t r o l l e r has no way of d e t e c t i n g because it does n o t s ense

t h e pH of t h e wastewater a f t e r adjustment .

The most common u t i l i z a t i o n of feedforward c o n t r o l i s i n con juc t ion

w i t h feedback c o n t r o l . This is r e f e r r e d t o a s feedback-feedforward

c o n t r o l . This conf igu ra t ion i s shown i n Figure 14. The i n t e n t i s t h a t

t he feedforward c o u n t e r a c t s most of t h e problems caused by r a p i d changes

i n b u f f e r i n g and f lowra te and the feedback provides r e s i d u a l c o n t r o l and

s e t p o i n t t r ack ing . The advantage of t h i s c o n f i g u r a t i o n over feedf orward

c o n t r o l i s b e t t e r pH c o n t r o l f o r v a r i a b l e s t r e n g t h and b u f f e r i n g of t h e

i n f l u e n t waste. The above c o n t r o l systems can i n c o r p o r a t e s e v e r a l

d i f f e r e n t t ypes of c o n t r o l s i g n a l gene ra to r s t h a t a r e r e s p o n s i b l e f o r

d i r e c t l y c o n t r o l l i n g chemical a d d i t i o n equipment. These inc lude on-off

s i g n a l c o n t r o l l e r s , p r o p o r t i o n a l s i g n a l c o n t r o l l e r s , i n t e g r a l s i g n a l

c o n t r o l l e r s , d i f f e r e n t i a l s i g n a l c o n t r o l l e r s , and programmed s i g n a l

c o n t r o l l e r s . These c o n t r o l l e r s can be used i n d i v i d u a l l y o r , as i s more

o f t e n t h e case, i n combination. A b r i e f d e s c i p t i o n of t h e c o n t r o l l e r s

and t h e i r a p p l i c a b i l i t y i s provided below.

On-off C o n t r o l l e r s

On-off c o n t r o l l e r s simply gene ra t e c o n t r o l s i g n a l s t h a t t u r n c’iemi-

ca l f eed equipment on and o f f based upon the r e a c t o r pH. This type of

c o n t r o l l e r i s t y p i c a l l y slow and unable t o achieve f i n e pH c o n t r o l . Use

of on-off c o n t r o l l e r s is normally conf ined t o ba tch o p e r a t i o n where

speed i s n o t c r i t i c a l , and t o m u l t i p l e s t a g e cont inuous systems where

on-off c o n t r o l i s used i n the f i r s t s t a g e which i s designed f o r g ross pH

adjus tment , n o t se t p o i n t t r ack ing .

P ropor t iona l C o n t r o l l e r s

P ropor t iona l c o n t r o l l e r s gene ra t e c o n t r o l s i g n a l s p r o p o r t i o n a l t o

t h e d i f f e r e n c e between t h e s e t p o i n t pH and the a c t u a l pH ( e r r o r ) . This

means t h a t as the e r r o r grows t h e chemical a d d i t i o n i n c r e a s e s t o compen-

sate. T h i s technique is quick and e f f e c t i v e f o r l i n e a r reg ions of a

t i t r a t i o n curve. However, i t does have one drawback i n cont inuous flow

95

r----- ---- -l I I PH

r--- I ’ 1 1 I I I I I 1 I I 1 I

PH TRANSMITTER

\

INFLUENT 4 WASTEWATER

1 -

c c

7 +EFFLUENT

WA8TEWATER

Figure 14. Feedback-feedforward mode of pH control.

systems ; namely, t he c o n t r o l l e r i s con t inuous ly approaching t h e d e s i r e d

pH b u t does n o t reach i t before some of t h e wastewater f lows through t h e

r e a c t o r . The r e a c t o r pH i s always d i f f e r e n t from the d e s i r e d pH. T h i s

f a c t means t h a t the se t p o i n t pH w i l l have t o be o f f s e t t o compensate

f o r t h i s d i f f e r e n c e . The o f f s e t w i l l vary wi th chemical demand and

should be s e t based on average wastewater cond i t ions . The o f f s e t can

on ly be se t by t r i a l and e r r o r . This system i s very s u i t a b l e f o r ba t ch

o p e r a t i o n s which do n o t exper ience the o f f s e t problem and f o r cont inuous

flow systems wi th a r e l a t i v e l y c o n s i s t e n t wastewater which w i l l a l low

es t ab l i shmen t of an e f f e c t i v e o f f s e t .

I n t e g r a 1 C o n t r o l l e r s

I n t e g r a l c o n t r o l s i g n a l g e n e r a t o r s ( rese t c o n t r o l l e r s ) a r e used 12

con junc t ion wi th p r o p o r t i o n a l c o n t r o l l e r s t o e l i m i n a t e the o f f s e t prob-

l e m . This i s accomplished by g e n e r a t i n g a c o n t r o l s i g n a l t h a t is pro-

p o r t i o n a l t o t h e t i m e s i n c e t h e pH w a s l as t a t t h e set p o i n t . This

s i g n a l i s added t o t h e p r o p o r t i o n a l e r r o r s i g n a l . As t h e p r o p o r t i o n a l

s i g n a l d r i f t s away from t h e s e t p o i n t f o r t h e reasons desc r ibed i n t h e

prev ious d i s c u s s i o n , t h e i n t e g r a t o r gene ra t e s a p r o g r e s s i v e l y l a r g e r

c o n t r o l s i g n a l u n t i l t h e pH is fo rced back t o t h e se t p o i n t . Once t h e

s e t p o i n t i s reached, t h e i n t e g r a l s i g n a l drops back t o z e r o and t h e

whole p rocess begins aga in . This type of c o n t r o l l e r i s u s e f u l when

c l o s e pH c o n t r o l is requ i r ed and where t h e waste flow i s n o t c o n s i s t e n t

e i t h e r i n volume o r b u f f e r i n g c a p a c i t i e s . I n t e g r a l c o n t r o l i s never

used i n conjunct ion wi th ba t ch o p e r a t i o n s .

Der iva t ive C o n t r o l l e r s

Der iva t ive c o n t r o l s i g n a l g e n e r a t o r s a r e used p r i m a r i l y when the pH

must pas s through a r eg ion of l o w b u f f e r i n t e n s i t y as i n d i c a t e d i n a

t i t r a t i o n curve by a l i n e segment wi th a very l a r g e s lope . The d e r i -

v a t i v e c o n t r o l l e r g e n e r a t e s a s i g n a l p r o p o r t i o n a l t o t h e r a t e of change

of pH wi th r e s p e c t t o t i m e . As an example, i f t h e pH i s c o n s t a n t the

p r o p o r t i o n a l s i g n a l is ze ro . I f the pH is r a p i d l y d i v e r g i n g from t h e

set p o i n t , a l a r g e c o n t r o l s i g n a l is genera ted t o c o r r e c t the pH. I f

97

t he pH i s r a p i d l y approaching t h e set p o i n t , a l a r g e nega t ive c o n t r o l

s i g n a l is genera ted t o slow t h e approach t o t h e s e t p o i n t . Once gen-

e r a t e d , t h e d e r i v a t i v e s i g n a l is added t o t h e e r r o r s i g n a l and t h e

i n t e g r a l s i g n a l ( i f p r e s e n t ) .

The d e r i v a t i v e c o n t r o l technique is o f t e n used i n conjunct ion w i t h

p r o p o r t i o n a l c o n t r o l f o r ba t ch n e u t r a l i z a t i o n when t h e se t p o i n t i s i n a

poor ly bu f fe red l o c a t i o n on t h e t i t r a t i o n curve. I t is a l s o used i n

conjunct ion wi th p r o p o r t i o n a l and i n t e g r a l c o n t r o l ( a PID dev ice 1 f o r

cont inuous flow systems when t h e pH must pass through a poor ly bu f fe red

range on t h e t i t r a t i o n curve. Care must be e x e r c i s e d when us ing d e r i v a -

t i v e c o n t r o l f o r cont inuous systems. I f t he d e r i v a t i v e c o n t r o l i s made

t o o s e n s i t i v e , t h e system may o s c i l l a t e between high and low pH because

of the l a g o r dead t i m e i n t he system.

Programmable C o n t r o l l e r s

Programmable c o n t r o l l e r s are s o p h i s t i c a t e d c o n t r o l l e r s t h a t gen-

erate c o n t r o l s i g n a l s based on a l o g i c sequence t h a t i s programmed i n t o

the c o n t r o l l e r . For example, t h e c o n t r o l l e r can check the r e a c t o r pH

and choose an a p p r o p r i a t e p r o p o r t i o n a l ga in depending upon whether the

system is o p e r a t i n g i n a h igh o r low b u f f e r reg ion . S i m i l a r l y , an

i n t e g r a l and p r o p o r t i o n a l ' response r a t e could be s e l e c t e d a l s o . There

i s v i r t u a l l y . no l i m i t t o t h e complexi ty and d i v e r s i t y of c o n t r o l s t r a t e -

g i e s t h a t can be b u i l t i n t o these sys t ems . Therefore , i t is necessary

t h a t t he p rocess des ign manual and manufac tu re r ' s l i t e r a t u r e be r e f e r -

enced t o understand i n d i v i d u a l programmable c o n t r o l l e r s .


The o p e r a t i o n a l o b j e c t i v e of pH adjus tment i s t o c o n t r o l t he addi -

t i o n of a c i d i c and b a s i c chemicals t o t h e wastewater such t h a t a d e s i r e d

pH can be achieved and maintained i n t h e wastewater. This o b j e c t i v e

should be accomplished wi th t h e use of minimal amounts of chemicals i n

an e f f o r t t o keep c o s t s down and e f f l u e n t t o t a l d i s so lved s o l i d s from

98

becoming t o o high.

jus tment s t e p s a r e

The l a t t e r i s e s p e c i a l l y t r u e when s e v e r a l pH ad-

performed on t h e wastewater p r i o r t o d i scha rge .

P rocess Monitorina

Process moni tor ing of t he pil adjustmenc p rocess should c o n s i s t of

con t inuous ly monitoring and r eco rd ing e f f l u e n t pH from the pH c o n t r o l

p rocess . This in format ion i s e s s e n t i a l t o a s s u r e t h a t t he pH i s be ing

maintained w i t h i n t h e c r i t i c a l l i m i t s r e q u i r e d f o r metal p r e c i p i t a t i o n ,

cyanide o x i d a t i o n , chromium r e d u c t i o n , o r o t h e r pH-sens i t ive t r ea tmen t

p rocesses . Flow and i n f l u e n t pH must a l s o be measured on a cont inuous

b a s i s f o r feedforward c o n t r o l , and as f r e q u e n t l y as p r a c t i c a l f o r feed-

back c o n t r o l where such in fo rma t ion can be used t o unders tand d e v i a t i o n s

i n t h e e f f l u e n t p H . A d d i t i o n a l l y , r e a c t o r t e m p e r a t u r e s h o u l d be

measured p e r i o d i c a l l y o r when poor pH adjus tment occurs . Temperature

can change t h e speed a t which p H ad jus tment chemicals react and can

a f f e c t t h e c a l i b r a t i o n of p H moni tor ing equipment. A l l pH probes should

be c a l i b r a t e d d a i l y w i t h f r e s h b u f f e r s o l u t i o n s and flow meters monthly.

C a l i b r a t i o n of pH meters invo lves t h e use of s t a n d a r d s of a t l ea s t two

pH va lues ; s t a n d a r d s of pH 4.0, 7.0, and 10.0 are recommended.

In a d d i t i o n t o t h e s t anda rd t e s t s j u s t d e s c r i b e d , t i t r a t i o n cu rves

should be prepared f o r the i n f l u e n t wastewater f o r each p H ad jus tment

p rocess . These curves should be prepared from both composite and g r a b

samples u s i n g t h e same chemicals t h a t w i l l be used i n t h e a c t u a l pH

adjus tment . The f requency f o r p repa r ing t h e s e curves w i l l vary g r e a t l y

from one pH adjus tment p r o c e s s t o ano the r depending p r i m a r i l y on t h e

v a r i a b i l i t y of t h e waste. As a minimum, a s u f f i c i e n t number of t i t r a -

t i o n curves should be prepared t o d e f i n e t h e normal c h a r a c t e r i s t i c s of

t h e wastewater and then a t l e a s t on a q u a r t e r l y b a s i s t o a s s u r e t h a t t h e ~~~

wastewater i s n o t g r a d u a l l y changing i t s c h a r a c t e r i s t i c s . Whenever poor _ _ _ _ ~

pH adjus tment o r p rocess changes occur , new t i t r a t i o n curves should be

prepared and compared t o t h e h i s t o r i c a l t i t r a t i o n curves . I f t h e curves

d i f f e r s i g n i f i c a n t l y , t h e u p s e t w a s p robably caused by t h e non-charac-

ter is t ic w a s t e . I f t h e change r e p r e s e n t s a permanent change i n t h e

99

c h a r a c t e r i s t i c s of t h e wastewater, the pH c o n t r o l l e r s w i l l p robably need

r ead jus tmen t .

Process Con t ro l S t r a t e g i e s

The f o u r primary o p e r a t i n g v a r i a b l e s which enab le t h e o p e r a t o r t3

c o n t r o l t he pH adjus tment p rocess a r e p H c o n t r o l l e r ad jus tmen t s , t r e a t -

ment chemica ls , e q u a l i z a t i o n , and mixing.

pH C o n t r o l l e r Adjustments--

A s p r e v i o u s l y d i s c u s s e d , pH c o n t r o l l e r s use a v a r i e t y of c o n t r o l

s t ra tegies t o g e n e r a t e c o n t r o l s i g n a l s . The common adjus tments t h a t can

be made t o t h e c o n t r o l l e r s w i l l be desc r ibed below. The a p p l i c a b l e

manufac tu re r ' s l i t e r a t u r e and any a v a i l a b l e des ign manuals should be

r e fe renced f o r s p e c i f i c systems when a v a i l a b l e . This is p a r t i c u l a r l y

impor t an t when manufac turers use terminology d i f f e r e n t from t h a t used i n

t h i s manual. It should be s t a t e d a l s o t h a t t h e ad jus tment procedures

p re sen ted h e r e are s i m p l i f i e d . Whenever p o s s i b l e , f u l l y - t r a i n e d c o n t r o l

and in s t rumen ta t ion personnel should be used t o perform comprehensive

ad jus tment procedures .

On-Off Control--On-off c o n t r o l has two primary ad jus tmen t s ; t hey

are t h e s e t p o i n t s f o r t u r n i n g chemical f eed equipment on and o f f .

These s e t p o i n t s should b racke t t h e d e s i r e d p H such t h a t when t h e pH

s tar ts t o move away from the d e s i r e d pH t h e chemical f eed pumps a r e

tu rned on. Then, when t h e pH p a s s e s t h e d e s i r e d pH i n t h e r e v e r s e

d i r e c t i o n , t hey a r e tu rned o f f . The s e t p o i n t s , of cour se , have t o be

selected such t h a t the p H is main ta ined wi th in a c c e p t a b l e l i m i t s . They

a l s o must be a d j u s t e d such t h a t t h e s e t p o i n t f o r t u r n i n g t h e equipment

o f f i s n o t too c l o s e t o t h e s e t p o i n t f o r t u r n i n g t h e equipment on. If

t h e two s e t p o i n t s are t o o c l o s e , t h e equipment w i l l cyc l e on and o f f a t

an e x c e s s i v e r a t e . This can cause mechanical equipment t o w e a r and

sometimes can cause e l ec t r i ca l equipment t o overhea t . The set p o i n t f o r

t u r n i n g on t h e equipment is u s u a l l y c o n t r o l l e d by a d j u s t i n g , t he "set

p o i n t " s e t t i n g whi le t h e s e t p o i n t f o r t u r n i n g o f f t h e equipment i s

100

c o n t r o l l e d by t h e "dead band" s e t t i n g . The "dead band" s e t t i n g can be

p r e s e t by the manufacturer ( i n t e r n a l ) o r can be f i e l d a d j u s t e d

( e x t e r n a l ) .

P r o p o r t i o n a l Control--The primary c o n t r o l ad jus tments on a propor-

t i o n a l c o n t r o l l e r a r e t h e s e t p o i n t ana the g a i n ad jus tment . The g a i n

ad jus tment de te rmines the magnitude of t h e c o n t r o l s i g n a l r e l a t i v e t o

t h e pH e r r o r ( a c t u a l minus set p o i n t pH). That i s , as the ga in i s

i n c r e a s e d t h e magnitude of t h e c o n t r o l s i g n a l r e l a t i v e t o the e r r o r i s

inc reased . The g a i n should t y p i c a l l y be s e t as h igh as p o s s i b l e wi thou t

sending t h e system i n t o o s c i l l a t i o n . O s c i l l a t i o n can occur when t h e

g a i n is t o o h igh and e i t h e r flow, pH, o r b u f f e r i n g c a p a c i t y of t h e

wastestream changes. The c o n t r o l l e r d e t e c t s t h e change and responds

wi th a c o r r e c t i v e s i g n a l . The e f f e c t s of the c o r r e c t i v e s i g n a l a r e n o t

sensed by t h e c o n t r o l l e r , however, f o r a pe r iod e q u i v a l e n t t o the system

dead t i m e . Hence, t h e system can over-respond when t h e ga in is t o o h igh

and can cause t h e pH t o pass t h e set p o i n t i n t h e o t h e r d i r e c t i o n . This

c o n d i t i o n is n o t i n i t s e l f bad, u n l e s s t h e pH e n t e r s an unaccep tab le

range o r con t inues o s c i l l a t i n g f o r an extended pe r iod of t i m e . I f

e i t h e r c o n d i t i o n occur s , t h e ga in should be decreased . I f n a t u r a l

o s c i l l a t i o n s are n o t p r e s e n t a t t h e t i m e of ad jus tment , they can be

gene ra t ed by adding d i r e c t l y a small q u a n t i t y of base t o a pH r e d u c t i o n

u n i t o r a c i d t o a u n i t des igned t o i n c r e a s e p H . The a c i d o r base should

be added such t h a t it does n o t cause more than a 1 o r 2 pH u n i t change

i n t h e p rocess e f f l u e n t . I t should be noted t h a t some manufac turers use

a " p r o p o r t i o n a l band" c o n t r o l i n s t e a d of a g a i n c o n t r o l . This c o n t r o l

f u n c t i o n s i d e n t i c a l l y t o a g a i n c o n t r o l excep t i n r eve r se . I n c r e a s i n g

t h e p r o p o r t i o n a l band d e c r e a s e s t h e ga in .

The set p o i n t f o r p r o p o r t i o n a l on ly c o n t r o l l e r s may have t o be s e t

a t a p H v a l u e s l i g h t l y d i f f e r e n t from t h e d e s i r e d p H va lue . T h i s i s f o r

t h e reason d i s c u s s e d under "Desc r ip t ion of Equipment". The o f f s e t w i l l

be s t r i c t l y a matter of t r i a l and e r r o r . Typ ica l ly as the ga in i s

inc reased t h e o f f s e t can be decreased . Likewise, as b u f f e r i n g i n t e n s i t y

101

i n c r e a s e s the o f f s e t must be increased . In h i g h l y v a r i a b l e f low sys-

tems, t h e r e may be no need f o r an o f f s e t because of t h e f r e q u e n t swings

i n pH which w i l l negate the advantage of an o f f s e t .

P r o p o r t i o n a l Plus I n t e g r a l Cont ro l ( P I C o n t r o l ) --The f u n c t i o n of

t h e i n t e g r a l mode i s t o e l i m i n a t e the need f o r p r o p o r t i o n a l offset. I f

t o o much i n t e g r a l a c t i o n i s used, t h e r e s u l t w i l l be an o s c i l l a t i o n of

t h e measurement as t h e c o n t r o l l e r d r i v e s the va lve from one extreme t o

t h e o t h e r . I f t o o l i t t l e i n t e g r a l a c t i o n is used, t h e measurement w i l l

r e t u r n t o t h e s e t p o i n t t oo s lowly. The i n t e g r a l a c t i o n ad jus tment

c o n t r o l s how r a p i d l y t h e i n t e g r a l c o n t r o l s i g n a l changes as a f u n c t i o n

of t ime. Among t h e va r ious c o n t r o l l e r s manufactured , t h e i n t e g r a l

a c t i o n ad jus tment i s c a l i b r a t e d i n one of two ways--either i n minutes

p e r r e p e a t , o r t h e number of r e p e a t s p e r m i n u t e . For c o n t r o l l e r s

measuring i n t e g r a l a c t i o n i n minutes pe r r e p e a t , t h e smaller t h e i n t e -

g r a l number, t h e g r e a t e r the a c t i o n of t h e i n t e g r a l mode. On c o n t r o l -

lers t h a t measure i n t e g r a l a c t i o n i n r e p e a t s p e r minute, t h e ad jus tment

i n d i c a t e s how many r e p e a t s of t h e p r o p o r t i o n a l a c t i o n are genera ted by

the i n t e g r a l mode i n one minute. Thus, - f o r t h e s e c o n t r o l l e r s , thg

h ighe r t h e i n t e g r a l number, t he g r e a t e r t h e i n t e g r a l a c t i o n . The proper

amount of i n t e g r a l a c t i o n depends upon the system dead t i m e . The longer

t h e system dead t i m e , t h e less the i n t e g r a l a c t i o n must be.

A s imple way of a d j u s t i n g t h e i n t e g r a l a c t i o n i s t o se t i t t o i ts

minimum a c t i o n p o s i t i o n and a d j u s t the p r o p o r t i o n a l ga in t o i t s optimum

p o s i t i o n as desc r ibed i n the prev ious s e c t i o n . Then the system i s

allowed t o come t o s t e a d y s ta te . Achievement of s t eady s t a t e may r e -

q u i r e some wa i t ing i f p roper e q u a l i z a t i o n i s n o t p r a c t i c e d upstream and

t h e system con t inuous ly f l u c t u a t e s about t h e set p o i n t . When s t eady

s t a t e i s reached, a very slow p a t t e r n of pH g r a d u a l l y i n c r e a s i n g t o the ~~

set p o i n t and then moving away i n t h e same d i r e c t i o n should be ev iden t . ~~

The i n t e g r a l a c t i o n ad jus tment should then be inc reased g radua l ly . The

p a t t e r n should occur a t an i n c r e a s i n g ra te wi th the pH d e v i a t i o n s from

the set p o i n t becoming less and less. If too much i n t e g r a l a c t i o n i s

a p p l i e d , t h e pH w i l l s t a r t f l u c t u a t i n g s i g n i f i c a n t l y t o both sides of

t h e s e t p o i n t . I t must be remembered when performing the ad jus tment

102

t h a t i f the i n t e g r a l ad jus tment i s i n minutes p e r r e p e a t , t h e i n t e g r a l

a c t i o n i n c r e a s e s w i t h decreased s e t t i n g s on t h e ad jus tment . Conversely,

f o r ad jus tmen t s marked i n r e p e a t s - p e r minute, t h e l a r g e r t h e s e t t i n g ,

t h e l a r g e r t he i n t e g r a l a c t i o n .

The above procedure is u s u a l l y s a t i s f a c t o r y f o r f i e l d ad jus tments

when the manufacturer's recommended procedures are not available or when

q u a l i f i e d i n s t r u m e n t a t i o n and c o n t r o l pe r sonne l a r e n o t p r e s e n t . When

t h e manufac tu re r ' s p rocedures o r q u a l i f i e d pe r sonne l a r e a v a i l a b l e , they

should be u t i l i z e d .

Propor ti ona 1 - In teg ra 1 -Der iva t ive ( P I D C o n t r o l 1 --The d e r i v a t i v e

mode opposes t h e r a p i d changes i n p H t h a t o f t e n occur when t h e pH ad-

ju s tmen t procedure e n t e r s a p o o r l y bu f fe red s e c t i o n of a t i t r a t i o n

curve. The d e r i v a t i v e response ad jus tment i s measured t y p i c a l l y i n

minutes. The h ighe r t h e r ead ing on t h e ad jus tment c o n t r o l , t h e more

d e r i v a t i v e a c t i o n p r e s e n t . Too much d e r i v a t i v e a c t i o n causes excess ive

response of t h e c o n t r o l l e r and c y c l i n g i n t h e measurement. Too l i t t l e

d e r i v a t i v e a c t i o n has no s i g n i f i c a n t e f f e c t . As wi th the i n t e g r a l

c o n t r o l , t h e system dead t i m e is fundamental w i th r e s p e c t t o t h e maximum

a l lowab le d e r i v a t i v e response. I f t h e d e r i v a t i v e response exceeds t h e

system dead t i m e , t h e system w i l l become u n s t a b l e and w i l l o s c i l l a t e .

A f i e l d method f o r a d j u s t i n g t h e g a i n when manufac tu re r ' s recom-

mended procedures o r q u a l i f i e d i n s t r u m e n t a t i o n pe r sonne l a r e n o t a v a i l -

able i s provided below:

1 . S e t t h e i n t e g r a l t o maximum t i m e (minimum i n t e g r a l a c t i o n ) .

2. S e t t h e d e r i v a t i v e t o minimum t i m e (minimum d e r i v a t i v e a c t i o n ) .

3. Adjust t h e g a i n t o an optimum s e t t i n g as b e f o r e , excep t t h a t a

s l i g h t c y c l e should remain i n t h e measurement.

4. I n c r e a s e t h e d e r i v a t i v e t i m e u n t i l t h e c y c l e s t o p s .

5. I n c r e a s e t h e g a i n u n t i l t h e c y c l e s tar ts aga in .

6. Repeat S t e p s 4 and 5 u n t i l f u r t h e r i n c r e a s e s i n the d e r i v a t i v e

t i m e f a i l t o s t o p t h e c y c l e .

7. Decrease the g a i n t o s t o p t h e cyc le .

103

8. S e t t h e in ' t eg ra l t i m e equa l t o d e r i v a t i v e t i m e . NOTE: I f t h e

i n t e g r a l t i m e i s expressed a s b e a t s pe r minute, t h e t i m e i n t e -

g r a l c o n t r o l should be s e t t o ( l / d e r i v a t i v e s e t t i n g ) .

pH Adjustment Chemicals--

The o p e r a t o r o f t e n has some c o n t r o l over t he chemicals used f o r pH

adjus tment . Fac to r s which should be cons idered i n s e l e c t i n g a s u i t a b l e

chemical f o r a wastewater pH adjus tment p rocess i n c l u d e speed of r e a c -

t i o n , b u f f e r i n g q u a l i t i e s , p roduc t s o l u b i l i t y , r eagen t c o s t , and a v a i l a -

b i l i t y . A d i s c u s s i o n of some common n e u t r a l i z i n g a g e n t s fo l lows .

L ime Materials--Lime is a term used t o d e s i g n a t e c a l c i n e d o r burned

l imes tone (quick l ime o r CaO) and i ts hydra ted d e r i v a t i v e [hydra ted l i m e

o r C a ( O H ) 2 ] . The two b a s i c types of l imes tone used a r e h igh calcium and

do lomi t i c . High ca lc ium l imes tones c o n s i s t c h i e f l y of calcium ca rbona te

wi th a small 'amount of magnesium carbonate . Dolomitic l imes tones con-

t a i n n e a r l y e q u a l molar q u a n t i t i e s of calcium and magnesium ca rbona te .

Typ ica l i m p u r i t i e s p r e s e n t i n t h e l imes tone i n amounts of less than f i v e

p e r c e n t i n c l u d e s i l i c a , i r o n , and alumina.

The r e a c t i v i t y of limeistones d i f f e r s w i th p h y s i c a l c h a r a c t e r i s t i c s

(e.g., s i z e and shape ) and chemica l composition. As a r u l e , h igh c a l -

cium l imes tones are more r e a c t i v e than d o l o m i t i c l imes tones ; however,

pronnounced p h y s i c a l c h a r a c t e r i s t i c s may produce excep t ions t o the r u l e .

T h e o r e t i c a l l y , d o l o m i t i c l imes tone has g r e a t e r b a s i c i t y , b u t a c t u a l o r

a v a i l a b l e b a s i c i t y depends upon t h e c o n d i t i o n s of a p p l i c a t i o n . Pul-

v e r i z e d l imes tone i s a s t a b l e noncorros ive p roduc t t h a t i s amenable t o

dFy f eed ing . I t i s a v a i l a b l e i n bulk

mechanical conveyors are s u i t a b l e f o r

o r i n 36-kg (80 - lb ) bags. Various

unloading t h e bulk material.

Q u i c k l i m e - Q u i c k l i m e has a n a f f i n i t y f o r c a r b o n d i o x i d e and

water. Under normal hand l ing o r s t o r a g e c o n d i t i o n s , qu ick l ime w i l l

a i r - s l a k e , t h a t i s , abso rb mois ture and carbon d iox ide from t h e atmos-

phe re , c a u s i n g p h y s i c a l s w e l l i n g and a marked loss of chemical a c t i v i t y .

Consequently, qu ick l ime must be s t o r e d i n moisture-proof a r e a s t h a t a r e

104

f r e e from carbon d iox ide . Because of t h i s p o t e n t i a l loss of e f f e c t i v e -

n e s s , qu ick l ime i s u s u a l l y consumed wi th in a few weeks a f t e r manufac-

t u r e .

Quicklime i s a v a i l a b l e ' i n va r ious forms, r ang ing from 20-cm ( 8 - i n )

limps t o p u l v e r i z e d , and i s supp l i ed i n bulk o r i n 36-kg (80-lb) bags.

Dust from pu lve r i zed quick l ime can i r r i t a t e eyes and s k i n . Although i t

can be f e d d r y , f o r op t ima l e f f i c i e n c y it is s l a k e d ( h y d r a t e d ) and

s l u r r i e d be fo re use under c o n d i t i o n s t h a t w i l l y i e l d maximal r e a c t i v i t y .

Improper s l a k i n g w i l l adve r se ly a f f e c t r e a c t i v i t y . S l a k i n g u s u a l l y i s

c a r r i e d o u t a t t e m p e r a t u r e s of 8 2 O t o 9 9 O C ( 1 8 0 ' t o 210OF). The

s l a k i n g r e a c t i o n may r each completion i n 10 minu tes w i th h i g h l y r e a c t i v e

l i m e s o r i n more than 30 minutes f o r limes of lower r e a c t i v i t y .

Following s l a k i n g , t h e l i m e p u t t y u s u a l l y i s s l u r r i e d wi th water t o

a c o n c e n t r a t i o n of 10 t o 35 percent (based on d r y s o l i d s ) f o r f eed ing

purposes. Because t h e s l u r r y i s s u b j e c t t o d e t e r i o r a t i o n from carbona-

t i o n d u r i n g s t o r a g e , it is customary t o use it soon a f t e r i t is made.

The a p p l i c a b i l i t y of l i m e t o s p e c i f i c s i t u a t i o n s may be expec ted t o

vary s i g n i f i c a n t l y from s u p p l i e r t o s u p p l i e r . T e s t i n g under a c t u a l o r

s imula t ed p rocess c o n d i t i o n s i s t h e only sound b a s i s f o r de t e rmina t ion

of r e l a t i v e a p p l i c a b i l i t y ; e m p i r i c a l b a s i c i t y tes ts normally a r e of

va lue on ly when t h e a p p l i c a t i o n i s ana lagous t o t h e test .

Hydrated Lime--Hydrated l i m e is s u i t a b l e f o r d r y f eed ing o r f o r

s l u r r y i n g . Dust from hydra ted l i m e , a f i n e powder, can cause eye and

s k i n i r r i t a t i o n . The s t o r a g e c h a r a c t e r i s t i c s of d r y hydra ted l i m e a r e

s u p e r i o r t o quick l ime, b u t , as wi th any s t r o n g a l k a l i , c a rbona t ion can

cause d e t e r i o r a t i o n . Hydrated l i m e is supp l i ed i n bulk o r i n 23-kg

(50-lb) bags. Bulk unloading i s u s u a l l y accomplished by pneumatic

conveyor.

Sodium Hydroxide--Sodium h y d r o x i d e ( c a u s t i c s o d a ) i s a h i g h l y

r e a c t i v e a l k a l i t h a t i s marketed i n s o l i d o r s o l u t i o n form. The s o l u -

t i o n form i s t h e most convenient f o r handl ing because burn hazards t o

105

per sonne l a r e minimized. The s o l i d form is hygroscopic , and both the

s o l i d and t h e s o l u t i o n are s u b j e c t t o d e t e r i o r a t i o n from c a r b o n a t i o n

d u r i n g prolonged s t o r a g e . S o l i d and l i q u i d sodium hydroxide are sup-

p l i e d i n drums, bu t only the l i q u i d form i s a v a i l a b l e i n bulk ( t a n k c a r

o r t r u c k ) . Heated t anks should be used f o r s t o r a g e of 50-percent s o l u -

t i o n i n s i t u a t i o n s where the ambient temperature i s l i k e l y t o f a l l below

12OC (54OF).

S u l f u r i c Acid--Sulfuric a c i d is a h i g h l y r e a c t i v e acid t h a t is

supp l i ed i n l i q u i d form, u s u a l l y i n c o n c e n t r a t i o n s of 98 p e r c e n t . The

concen t r a t ed acid is s t r o n g l y hygroscopic and p r e s e n t s a burn hazard t o

personnel . D i l u t e s o l u t i o n s are h igh ly c o r r o s i v e t o i r o n and s t e e l ,

whereas concen t r a t ed s o l u t i o n s ( >93 p e r c e n t ) are n o t c o r r o s i v e t o i r o n

and s tee l . A maximum f r e e z i n g p o i n t of 8OC (46OF) is e x h i b i t e d a t a

c o n c e n t r a t i o n of 85 p e r c e n t . Depending on t h e c o n c e n t r a t i o n , f r e e z i n g

p r o t e c t i o n may be r e q u i r e d d u r i n g storage and t r a n s p o r t . S u l f u r i c a c i d

i s shipped i n carboys , b a r r e l s , t ank cars, or t r u c k s .

E q u a l i z a t i o n -- E q u a l i z a t i o n i s e s s e n t i a l t o s u c c e s s f u l pH adjus tment p a r t i c u l a r l y

when t h e s e t p o i n t p H is i n a p o o r l y bu f fe red r eg ion of t h e t i t r a t i o n

curve. The importance of e q u a l i z a t i o n can best be understood by exam-

i n i n g t h e "dead" t i m e concept. I f t h e pH, flow, o r b u f f e r c a p a c i t y of

t h e wastewater s t r eam changes s i g n i f i c a n t l y i n a t i m e frame s h o r t e r than

t h e d e a d t i m e o f t h e c o n t r o l s y s t e m , p r o p e r p H c o n t r o l c a n n o t be

achieved. S i m i l a r l y , as t h e rate of change approaches t h e dead t i m e pH

c o n t r o l becomes more d i f f i c u l t and more s o p h i s t i c a t e d c o n t r o l l e r s t h a t

i n c o r p o r a t e i n t e g r a l and d e r i v a t i v e a c t i o n are r e q u i r e d .

Whenever poor pH c o n t r o l occu r s , t h e o p e r a t o r should conduct a

moni tor ing program immediately t o de te rmine the v a r i a b i l i t y of t h e

i n f l u e n t t o t h e pH adjus tment s tep and t o de te rmine i f t h e poor p e r -

formance c o i n c i d e s wi th the v a r i a b i l i t y i n t h e i n f l u e n t . I f it does ,

t h e o p e r a t o r should t r o u b l e s h a t t h e e q u a l i z a t i o n process . A s a g e n e r a l

r u l e , i n c r e a s e d e q u a l i z a t i o n makes pH c o n t r o l easier.

106

Mixing--

Adequate m i x i n g i s e s s e n t i a l f o r good pH a d j u s t m e n t f o r t h r e e

c l o s e l y r e l a t e d reasons . F i r s t , mixing is r e q u i r e d t o blend p H a d j u s t -

ment chemicals w i t h the wastewater. Second, mixing d e c r e a s e s r e a c t o r

dead t i m e by d i s p e r s i n g t h e chemicals r a p i d l y throughout t he r e a c t o r .

L a s t l y , i t p rov ides e q u a l i z a t i o n and homogeneity throughour: t he r e a c t o r .

The l a t t e r i s impor t an t because t h e feedback p H c o n t r o l probe w i l l

respond t o t h e non-homogeneous c o n t e n t s of t h e r e a c t o r j u s t as r e a d i l y

a s t o f l u c t u a t i o n s i n t h e i n f l u e n t c h a r a c t e r i s t i c s .

To ache ive good mixing, t h e minimum horsepower requi rements p re -

s e n t e d i n Table 5 should be provided f o r mixing. The i n f l u e n t and

e f f l u e n t d e v i c e s should a l s o be l o c a t e d a t o p p o s i t e sides of t h e r eac -

t i o n t a n k , i d e a l l y w i t h one a t t h e t o p of t h e t a n k and one a t t h e

bottom. I f c i r c u l a r t anks are used, s i d e w a l l mixing b a f f l e s must be

provided. The tank must a l s o be checked p e r i o d i c a l l y f o r s o l i d s depo-

s i t i o n i n t h e bottom of t h e tank which can adve r se ly a f f e c t mixing and

dec rease t h e h y d r a u l i c r e s idence t i m e of t h e system. S i m i l a r l y , t h e

mixer i m p e l l e r should be checked f o r wear o r breakage.

Proper l o c a t i o n of the chemical feed and t h e feedback pH probe i s

a l s o e s s e n t i a l and should be v e r i f i e d by t h e o p e r a t o r . The chemica ls

e i t h e r should be added d i r e c t l y a t t h e p o i n t of i n f low t o the system o r

d i r e c t l y i n t o t h e mixing vo r t ex . Care should be e x e r c i s e d i n t h e l a t t e r

case t o a s s u r e t h a t t h e chemicals w i l l n o t cor rode t h e i m p e l l e r o r

i m p e l l e r s h a f t . The pH probe f o r feedback c o n t r o l should be l o c a t e d i n

t h e out f low of t h e r e a c t o r j u s t o u t s i d e t h e r e a c t o r . T h i s p rovides t h e

most r e p r e s e n t a t i v e sample f o r p rocess c o n t r o l .

I n f l u e n t and pH-adjusting r eagen t should e n t e r a t the same p o i n t ,

premixed f o r b e s t r e s u l t s . The d i r e c t i o n of in f low should oppose t h e (16) d i r e c t i o n of a g i t a t i o n f o r most e f f e c t i v e backmixing.

The e x i t should be d i a m e t r i c a l l y oppos i t e t o t h e p o i n t of e n t r y , t o

minimize s h o r t - c i r c u i t i n g . Otherwise a s i g n i f i c a n t p a r t of t h e i n f l u e n t

107

could pass by t h e v e s s e l wi thout t r e a t m e n t , and u n c o n t r o l l a b l e v a r i a -

t i o n s i n e f f l u e n t q u a l i t y could r e s u l t . If the i n f l u e n t e n t e r s a t t he . bottom on one s i d e , e f f l u e n t should leave a t t he t o p from the o t h e r

s i d e , and v i c e ve r sa . ( 1 6 )

In n e u t r a l i z a t i o n of i n d u s t r i a l was t e s , many s t reams wi th widely

d i f f e r i n g p r o p e r t i e s are u s u a l l y combined f o r t r e a t m e n t i n a s i n g l e

v e s s e l . Often both a c i d and b a s i c r eagen t s are r e q u i r e d when t h e i n -

f l u e n t pH may be on e i t h e r s i d e of 7. In t h e s e i n s t a n c e s , r eagen t can

be saved by p rov id ing a r e t e n t i o n v e s s e l upstream of t h e n e u t r a l i z a t i o n

tank. This smoothing v e s s e l should have enough r e s i d e n c e t i m e t o accom-

modate t h e l a r g e s t expected t r a n s i e n t i n f l u e n t v a r i a t i o n s . ( 1 6 )

P r o p e r l y l o c a t i n g t h e p o i n t of measurement i s a s i m p o r t a n t a s

l o c a t i n g t h e v e s s e l e x i t , and i s , i n f a c t , t i ed t o it. A r e p r e s e n t a t i v e

measurement of e f f l u e n t q u a l i t y is e s s e n t i a l and s o e l e c t r o d e s should be

p laced i n t h e e f f l u e n t stream. However, they should n o t be p laced o u t

of t h e nixed zone. Dynamic response i s a l s o impor t an t , so eve ry e f f o r t

should be made t o main ta in a r easonab le flow p a s t t h e e l e c t r o d e s . One

tries t o avoid t h e use of s t i l l i n g w e l l s o r o t h e r p r o t e c t l v e d e v i c e s

which restrict flow, and t o avoid l o c a t i o n s where flow i s v a r i a b l e . ( 1 6 )

Submersible e l e c t r o d e assembl ies w i l l always be more r e spons ive

than flow-through assembl ies s i n c e they a r e d i r e c t l y i n s e r t e d wi th in t h e

p rocess stream. Flow-through a s sembl i e s were des igned f o r p r e s s u r i z e d

s e r v i c e wi th a sample withdrawn con t inuous ly f o r measurement. Although

d e s i r a b l e f o r e a s y maintenance , the a d d i t i o n a l d e l a y caused by t h e

sample l i n e impedes c o n t r o l a c t i o n . (16)

Submersible e l e c t r o d e s are used p r i m a r i l y i n open v e s s e l s . They

should ex tend on ly s l i g h t l y below t h e s u r f a c e of t h e l i q u i d t o minimize

t h e p o s s i b i l i t y of leakage . Occas iona l ly , however , deeper submergence

is necessary . The a s sembl i e s may be air-purged t o p r o t e c t a g a i n s t

l eakage even under s e v e r e c o n d i t i o n s . (16 )

108

Flow-through e l e c t r o d e assembl ies are r equ i r ed f o r p r e s s u r i z e d

s e r v i c e , o r whenever a sample must be t r e a t e d p r i o r t o measurement.

Sample l i n e s should be a s s h o r t as p r a c t i c a l and v e l o c i t y high t o mini-

mize dead time. High v e l o c i t y a l s o he lps keep t h e e l e c t r o d e s c l e a n ,

a l though excess ive v e l o c i t y can cause e ros ion and even cleavage of t h e

electrodes. A v e l o c i t y of 2 f t / s e c p a s t the e l e c t r o d e s i s probably a s

h igh as t o l e r a b l e f o r long l i f e . ( 1 6 )


With adequate e q u a l i z a t i o n , t he proper s e l e c t i o n of c o n t r o l s t r a t e -

g i e s , and the proper ad jus tment of a l l c o n t r o l equipment, t h e wastewater

e x i t i n g a pH adjus tment s t e p should be c o n t r o l l a b l e w i t h i n - + 0.25 pH

u n i t s .


A gu ide f o r t roub le shoo t ing t h e pH adjus tment p rocess i s p resen ted

i n Table 9. The pH parameter is one of t h e f i r s t i n d i c a t o r s of po ten-

t i a l t r ea tmen t p l a n t u p s e t cond i t ions . Problem areas i n c l u d e , i n add i -

t i o n t o shock loads , pH i n s t a b i l i t y r e l a t e d to c o n t r o l l e r s e t t i n g s o r

breakdown, chemical f eed and mixing equipment f a i l u r e , and chemical

system used.

109

PH

OPEHATI WE

l a . Shock or [ I

lb. Buffe of ua cllang

1c. pll PI

I d . Poor

le. c:lleml equlfi

tAB1.E CAUSE CllCCK OR MGNITOR RGASON CORRECTIVE ACTION

- ___

vtoa:.m 1 8 O b c 1 J l a t l n g o r ' u n s t a b l e ~ I I . Previous p r f o r m a n c e e a t l s f a c t o r y .

loadlng of flou

l n t e n e l t y t e u a t e r 1.

be f a l l u r e .

l r l n g .

% I feed ?l i t € a l l u r e .

Check I n f l u e n t f l o u and @I - for abnormal l e v e l e or f l u c t u a t l o n e .

P r e p a r e new t l t r a t l o n - c u r v e and compare to p r e v i o u s curves . Check to see if c h e d c a l f e e d system a t maxlvum capa- cl ty.

Check c o n d l t l o n of probe and a l l e l e c t r l c a l connec- t l o n e . Clean And c a l l b r a t e probe. Check c o n d l t l a n of e l e c t r o l y t e f l u l d In. p robe If probe usee I t .

Check mixer, m l x e r Impel le r , - a l l b a f f l e e , and the tank fur ~ o l l d ~ ~ deposl t l o n .

Check f e e d equlpuent for smooth opera t 1 on, leaks, and rny b l a c k s y e I n t h e chemlcal d e l l v e r y patli. Check c a l l b r d t l o n of a l l c h e r l c a l u e t e r l ny bystems.

pll c o n t r o l l e r s a r e aptl- r l o e d f o r s p c l f l c f l w , ptl. and b u f f e r l n t e n s l t y c o n d l t l o n s . Cliangee In t h e s e parameters can c a u s e c o n t r o l l e r In- s t a b 1 11 t y .

Decreases I n b u f € e r 6n- t e n s l t y can c a u s e controllers to o s c l J l a t e . Large l n c r e a s e a I n buf- f e r l n t e n s l t y r a y c a m e chemlcal demand to exceed t h e clremlcal f e e d s y s t e m c a p a c l t y .

D i r t y probes respond s l o u l y and I r r e g u l a r l y . P o u t l n e r a l n t e n a n c e a€ prohes Is e s e e n t l a l .

Poor r l x l n g can c a u s e s h o r t c l r c u l t l n g and p a r c l ieu lca l dlsper- e l a n .

If t h e @I a d j u s t r e n t chemical Is n o t p r o p e r l y d e l l v e r e d to t h e r e a c t o r , no pl: ad j e s t r e n t can occur .

- P r e v e n t I n f l u e n t @I and f l o w I n v a r l a t l o n s e i t h e r throuyh I n c r e a s e d equal Izat lon or changes I n o p e r a t l n g procedures. Tune c o n t r o l l e r to m d k e I t less s e n e l t l v e to f l u c t u a t l a n s .

- I n c r r a s e e q u a l l z a t l o n , redd- j u s t controller to r e f l e c t n e w condl t l o n s J r o d l f y clierlcal a d d l t l o n e q u l p e n t I € I t s c a p a c l t y 1s exceeded. I f b u f f e r l n t e n s l t y decreased , c o n e l d e r u s l n g c h e r l c a l that p r o v l d e s h f f e r l n g such a8 soda ash.

- Clean. c a l l b r a t e , repalr, and a d j u s t a s requl red .

- Repai r as requl red . C l e h n Lank.

- Repal r and a d j u s t as reqiilred.

PRO1

If. Defect

lg. Contro down-

OYERATING I

p . 2a. Imprap' P bib pro1

yaln CI P

2b. Dirty I

2c. Excess tlue.

BLE CAUSE ClleCK OR W ) N I " L REASON COHHECTIVB ACTION

e chemlcals. - Check chemlcal feed con- - luproper or defectlve - Replace defectlve clieolcalti. talners for cliemlcal con- clberlcals can r d v e r s e l y tent, chemlcal strength, atfect pll adjustment. and tormrtlon of solld lumps Iupropar clberlcal con-

' when dry chemlcals a t e used. centratloci can be placed In feed tanku by accldent or Ignorance.

er break- - Check a11 other posslble - Controllers can break - Call quallfled repalruan. causes. down, eapeclally from

llghtnlng or other elec- . trlcal eurgrs.

OBLEWS 21 S l w controller reeponse. -

eettlng - Check galn eettlng and date - L w galn wlll cause slug- - Adjust controller propmtIonr1 rtLonal of last adjustment. glsh response. yaln. trol.

probe. - Check condltlon of probe. - Dirty probes ulll re- - Clean probe. spond slouly, partlcu- larly If olly materiel present In wastewater.

e 'dead. - Check mlrlng# check feed- - increaulng rlxlng 4 1 1 - Increase mlrlng~ move prohi back probe locrtlon decrease dead tlme. use probe that Is InuerLed and type. Probe should be located dlrectly Into outflar.

I n reactor outflow just outslde of reactor. probes that Insert directly Into o u t f l w are qulckest.

cLiny - Clirck type and foim of - chemacals. I.1ue reacts ulowly.

1.lquIdti react faster iban uolldu or slurrles.

- clldnye cheml ca 18.

TABLB 9 (Continued)

pll MJUS'I'MWr Y'ROUBLESIIOMING GUIDE

PR

3b. Contr

iliteg and 1 t I on too 1

3c. Cherl srntr quate

wwa

OPERATING

4a. lmpro Irr a

4b. Syste tlur

IABLE CAUSE CHECK OR HONIMR REASON CORRETrlVE ACTION

ller Is - Check type and Integral - Integral control does - Adjust 88 requlred. :lonal- control settlng. away with the offset 1 1 type requlreaent of propor- .eyral ac- tlonal controllers If !ttlog Is adjusted properly. t.

11 feed 1 nade-

- Check chemlcal feed system - Chemlcal feed system MY - Modify feed system, use m a r e for proper operatlon and be unable to dellver concentrated chemlcale, delivery rate. chemicals at a suffl- decreaee.wasteuater flow.

clent rate to adjust pll.

-_

ROBLE^ 4 1 p t~ osclllates around set polnt.

'r control- - Check type controller and - Improper adjustment of ustent. date of last adjustment. propor tlonal, Integral,

or der I va t I ve control I can cause osclllatlon.

- Adjust as requlred.

'dead" m long.

- Check rlalng, type of - Excesalve dead tlme cherlcals used, and llmlts a controller's locatlon of feedback ablllty to respond to control. pU changes. If repld

changes occur, the nys- tea wlll osclllate about set polnt.

- Improve rlxlngi uee llquid cherlcalo that react qulckera place pH probe dlrectly In outflow just outslde of tank. Use propor tlonal-Integral- . drrlvatlve controller, une cherlcaIn wl th Inherent butferlny capacty such as soda aeh.

-

SECTION 9

METAL PRECIPITATION

INTRODUCTION

P r e c i p i t a t i o n is a t r ea tmen t technique which i s commonly used t o

remove t h e fo l lowing metals from wastewater: cadmium, t r i v a l e n t

chromium, copper , l ead , n i c k e l , and z i n c . P r e c i p i t a t i o n i s t h e p rocess

of a d j u s t i n g the pH of a wastewater t o t h e minimum s o l u b i l i t y of t h e

metal o r metals . The i n s o l u b l e meta l p r e c i p i t a t e which i s formed can

then be e a s i l y removed from the wastewater by sed imenta t ion and/or

f i l t r a t i o n . Soluble metals a r e very s t a b l e i n s o l u t i o n and w i l l n o t

se t t le o u t even a t , l o n g d e t e n t i o n times.

THEORY OF OPERATION

P r e c i p i t a t i o n of me ta l s from s o l u t i o n involves a d j u s t i n g t h e pH t o

t h e p o i n t of minimum s o l u b i l i t y . An example of the s o l u b i l i t y of a

meta l ( l e a d ) versus t h e pH of t h e wastewater is presented by t h e curve

i n F igure 15. The optimum t r ea tmen t f o r a wastewater wi th c h a r a c t e r -

ist ics shown i n F igure 15 would 'be t o a d j u s t the pH of t h e wastewater t o

9.0 and then s e p a r a t e the i n s o l u b l e p r e c i p i t a t e from s o l u t i o n by c l a r i -

f i c a t i o n o r f i l t r a t i o n . Removing 100 pe rcen t of t h e i n s o l u b l e p r e c i p i -

t a t e o r f l o c would r e s u l t i n an e f f l u e n t l ead concen t r a t ion of 0.20

mg/L. I f t h e pH of t h e wastewater were a d j u s t e d t o 7.5, then t h e e f f l u -

e n t l e a d c o n c e n t r a t i o n would be 0.8 mg/L f o r 100 p e r c e n t removal of t h e

i n s o l u b l e p r e c i p i t a t e .

-

Heavy me ta l s can e x i s t i n e i t h e r t h e p a r t i c u l a t e form o r the s o l u -

b l e o r d i s s o l v e d form. The p a r t i c u l a t e metal concen t r a t ion r e p r e s e n t s

113

100.

10.0

1.0

0.10 I I I I I I I 5.0 6.0 7.0 8.0 9.0 10.0 11.0 1:

PH

.o

Figure 15. Lead solubility for a typicai wastewater.

114

. -

t h e d i f f e r e n c e between the t o t a l metal c o n c e n t r a t i o n and t h e s o l u b l e

metal concen t r a t ion . A t each pH va lue , t h e s o l u b l e meta l c o n c e n t r a t i o n

can be determined t h e o r e t i c a l l y from pH s o l u b i l i t y diagrams o r e x p e r i -

menta l ly by ana lyz ing f o r the d i s so lved metal concen t r a t ion . The d i s -

so lved metal c o n c e n t r a t i o n ' o f a s o l u t i o n can be measured by f i l t e r i n g

t h e sample wi th a 0.45 micron f i l t e r and then ana lyz ing t h e f i l t r a t e f o r

t h e metal concen t r a t ion . Analyses of aqueous s o l u t i o n s f o r metal con-

c e n t r a t i o n i s performed us ing an atomic abso rp t ion spectrophotometer

(AAS), an in s t rumen t which measures the r a d i a n t energy absorbed as the

sample passes through a flame. Procedures f o r t h e a n a l y s i s of most

common meta ls by AAS a r e conta ined i n r e f e r e n c e 8.

The s o l u b l e meta l concen t r a t ion of a sample should n o t be based on

t h e o r e t i c a l c o n s i d e r a t i o n s because of t h e e f f e c t t h a t o t h e r i o n s have on

the s o l u b i l i t y of a metal . An example of t h e e f f e c t t h a t carbonate has

on l e a d s o l u b i l i t y can be observed i n Figure 16. This f i g u r e shows t h a t

t h e l e a d s o l u b i l i t y a t a pH of 9.0 f o r carbonate c o n c e n t r a t i o n s of 16,

50, and 10,000 mg/L a s CaC03 is 0.07, 0.35 and 2.5 mg/L, r e s p e c t i v e l y .

Other meta ls can be s i m i l a r l y a f f e c t e d by i o n s i n s o l u t i o n . Because of

t h i s e f f e c t , the optimum pH value should be based on t e s t d a t a r a t h e r

than t h e o r e t i c a l va lues ,


The p rocesses g e n e r a l l y used i n metal p r e c i p i t a t i o n inc lude pH ad-

jus tment , s ed imen ta t ion , and sometimes f i l t r a t i o n . The f q n c t i o n s of pH

adjus tment and sed imenta t ion p rocesses must n o t be confused. The pH

adjus tment s t e p simply conve r t s a metal from t h e s o l u b l e ( i o n i c ) form t o

t h e i n s o l u b l e (hydroxide p r e c i p i t a t e d ) form. However, t h e t o t a l concen-

t r a t i o n of t h e meta l i n t h e wastestream does not change d u r i n g t h i s

process . The sed imenta t ion p rocess removes p a r t i c u l a t e metal from t h e

was tes t ream. However, t h e sed imenta t ion p rocess does not decrease t h e

s o l u b l e me ta l concen t r a t ion .

The r e l a t i o n s h i p between s o l u b l e and t o t a l metal concen t r a t ion and

t h e t r ea tmen t p rocesses involved i s presented i n Figure 17. Reading

115

' - L

8.0 9.0 10.0 11.0 1 0.01' 5.0 6.0 7.0

pn

.O

Figure 16. Lead solubility a s a function at carbonate concentration. (c,)

116

117

from l e f t t o r i g h t on Figure 17, it can be seen t h a t t h e pH adjustment

s t e p changes t h e r e l a t i v e proport ions of so lub le and in so lub le metal

con ten t , bu t does not i n i t s e l f reduce the t o t a l metal concent ra t ion .

However, c l a r i f i c a t i o n i n genera l removes the major i ty of the p a r t i c u -

l a t e mat te r , and f i l t r a t i o n removes even more, while leav ing t h e s o l u b l e

metal concent ra t ion unchanged. Because the optimum pH value f o r p rec i -

p i t a t i o n of a metal may occur a t a pH g r e a t e r than t h a t allowed by

regula tory a u t h o r i t i e s , p o s t pH adjustment o r f i n a l n e u t r a l i z a t i o n i s

sometimes necessary. The e f f e c t of f i n a l pH adjustment w i l l be t o

change the concent ra t ion of the p a r t i c u l a t e and so lub le metal concentra-

t i on . E i ther form may inc rease or decrease depending upon t h e metal

s o l u b i l i t y , and t h e i n i t i a l pH and f i n a l pH i n t h e p o s t pH adjustment

process . However, the t o t a l concent ra t ion of t h e metal w i l l no t change

dur ing the process. The two poss ib l e e f f e c t s t h a t f i n a l n e u t r a l i z a t i o n

can have on so lub le and p a r t i c u l a t e metal concent ra t ion a r e shown i n

F i g u r e 18. From F i g u r e 18 a s w i t h F i g u r e 1 7 , i t i s a p p a r e n t t h a t

changes i n p~ w i l l change t h e concent ra t ion of t h e forms of t h e metal

bu t w i l l no t i nc rease or decrease the t o t a l metal concentrat ion.

Changes i n the pH of wastewater can occur a s t h e r e s u l t of ac id or

a l k a l i add i t ion such a s i n t h e f i n a l n e u t r a l i z a t i o n basin. However,

changes i n t h e pH of a wastewater can a l s o occur because of i n t roduc t ion

of C02 i n t o the wastewater, e s p e c i a l l y f o r poorly buf fered waters . In- t roduct ion of CO i n t o the water can occur slowly i n a tank open t o t h e

atmosphere or more r ap id ly by a e r a t i o n or turbulence. 2


The ob jec t ive of p r e c i p i t a t i o n i s t o cause the metal ion t o form an

in so lub le p r e c i p i t a t e which can be removed by sepa ra t ion techniques such

a s sedimentation o r f i l t r a t i o n . To c o n t r o l t h e metal p r e c i p i t a t i o n pro-

ces s , t h e opera tor should perform the necessary process monitoring and

should apply the necessary c o n t r o l s t r a t e g i e s t o t h e system va r i ab le s . ___..__...

118

2 . - a 1.0 z 0 z < 5 0.5 8 a'

3

0

2 0.0 CUREICATlON flLTRATlON flNAL

NEWUAWTION

2 . 0 z '4

,.. .-. CUREICATlON flLTRATlON flNAL

NEWUAWTION 3

Figure 18. Metal concentration attar treatment process.

119

Process Monitorinq

Since p r e c i p i t a t i o n involves a number of processes such a s pH ad-

justment, sedimentation, and f i l t r a t ion , process monitoring w i l l involve

a l l these processes. The main parameters involved i n monitoring each of

t h e processes a r e t o t a l and d isso lved metal , pH, and t o t a l suspended

s o l i d s . The r egu la r or continuous monitoring requirements a r e summar-

ized i n Table 10.

p~ Adjustment -- The i n f l u e n t t o t a l metal concent ra t ion , e f f l u e n t so lub le metal con-

c e n t r a t i o n , and r eac to r pH should be monitored. Ions such a s carbonates

or phosphates may be analyzed t o determine i f these ions have an e f f e c t

on metal s o l u b i l i t y .

Sedimentation -- The e f f l u e n t t o t a l ’ and s o l u b l e m e t a l c o n c e n t r a t i o n s h o u l d be

measured i n order t o determine the p a r t i c u l a t e metal concent ra t ion t h a t

is not removed by sedimentation. I n f l u e n t and e f f l u e n t suspended s o l i d s

should be measured t o determine the amount of s ludge t h a t must be r e -

moved. A c o r r e l a t i o n may e x i s t between e f f l u e n t suspended s o l i d s and

e f f l u e n t t o t a l metal concentrat ion.

F i l t r a t i o n -- s ince f i l t r a t i o n is t y p i c a l l y t h e l a s t t reatment process employed,

e f f l u e n t t o t a l metal concent ra t ion w i l l i n d i c a t e permit compliance. The

i n f l u e n t and e f f l u e n t p a r t i c u l a t e metal concent ra t ion should be de t e r -

mined t o eva lua te f i l t e r performance. I n f l u e n t suspended s o l i d s should

be measured t o a l low con t ro l of the f i l t r a t i o n process .

The key t o success fu l ly opera t ing a metal p r e c i p i t a t i o n system i s

understanding the r e l a t i o n s h i p between the p a r t i c u l a t e and so lub le metal

concent ra t ion and the t reatment processes . An example of the d i f f e r e n c e

between p a r t i c u l a t e and so lub le metal concent ra t ion and how it a f f e c t s

permit compliance i s shown i n Figure 19. case No. 1 p re sen t s d a t a f o r a

._____-

120

O . l l Y

0.ilY

0.iLY

O.flY

*a,. toadrno

121

e E -10.0 7- 0

Figure 19.

C A S N0.3 Bamit Vlolanonl

Metal concantration versus treatment process. 1 2 2

t reatment system which has good opera t ion of pH adjustment and sedimen-

t a t i o n . Case No. 2 presents a system which has poor opera t ion o f . t h e pH

adjustment process . T h i s is evidenced by a high so lub le metal concen-

t r a t i o n . so lub le metal can be measured by f i l t e r i n g a sample immedi-

a t e l y a f t e r c o l l e c t i o n . Care must be taken t h a t the pH does not change.

A discuss ion of t h e va r i ab le s which a f f e c t t h e s o l u b i l i t y of each metal

a re presented i n the PERFORMANCE por t ion of t h i s Sect ion.

The t h i r d case , Case No. 3 , presen t s a t rea tment system which h a s

good opera t ion of the pH adjustment system b u t poor opera t ion of the

sedimentation process. Poor sedimentation performance w i l l be ind ica t ed

by high e f f l u e n t suspended s o l i d s concent ra t ion and high p a r t i c u l a t e

metal concent ra t ion . P a r t i c u l a t e metal concent ra t ion i s t h e d i f f e r e n c e

between t o t a l and so lub le metal concentrat ion. The causes ana remedies

f o r t roubleshoot ing the sedimentation process a r e presented r n t h e

s e c t i o n on sedimentation. The opera tor should not a t tempt t o c o r r e c t

t h e sedimentation problem by i n s t a l l i n g a f i l t r a t i o n u n i t because the

u n i t w i l l f o u l w i t h s o l i d s . Most f i l t r a t i o n systems should be i n s t a l l e d

f o r wastewaters t h a t a r e low i n suspended s o l i d s .


The c o n t r o l s t r a t e g i e s a s soc ia t ed with opera t ing a metal p rec ip i -

t a t i o n system involve the chemical reagent used i n pH neut raLiza t ion and

the opera t ing pH. o t h e r opera t ing va r i ab le s a s soc ia t ed with ope ra t ing

t h e u n i t p rocesses (pH adjustment, f l o c c u l a t i o n , c l a r i f i c a t i o n , and

f i l t r a t i o n ) a r e l i s t e d and discussed f o r each u n i t process .

Hydroxide P rec ip i t a t ion - -

The most common anion used t o p r e c i p i t a t e metals from wastewater i s

hydroxide. Hydroxide is gene ra l ly added t o t h e wastewater as calcium

hydroxide ( l i m e ) but sometimes added a s sodium hydroxide ( c a u s t i c ) . The

primary advantage f o r using lime is t h a t a good s e t t l i n g p r e c i p i t a t e i s

usua l ly formed. The advantage f o r using c a u s t i c i s less s ludge genera-

t i on . The s o l u b i l i t y of metal hydroxides i s presented i n Figure 20 .

123

SOLUT10N pH

. figure 20. Soh&iHty oi aelecied heavy metal hydroxides.

SOURCE: USeirh SULFIDE PFlECIPTATION OF HMW METALS, EPA-00042-6C-139. JUNe. 1980. D. 104.

\

124

Because of the d i f f e r e n t p~ ranges f o r minimum s o l u b i l i t y of var ious

metals , staged p r e c i p i t a t i o n i s sometimes needed.

Staged p r e c i p i t a t i o n is a s e r i e s of two or th ree s t e p s of a pH ad-

justment process with each s t e p followed by a separa t ion process such a s

sedimentation and/or f i l t r a t i o n . An example of the n e c e s s i t y of s taged

p r e c i p i t a t i o n is f o r a wastewater which requi res removal of both n i c k e l

and t r i v a l e n t chromium. The f i r s t s t age would c o n s i s t of pH adjustment

t o a pH of 8 . 5 followed by sedimentation and, poss ib ly , f i l t r a t i o n . The

second s t a g e would c o n s i s t of pH adjustment of t he wastewater from 8 . 5

t o 10.5 followed by sedimentation and/or f i l t r a t i o n .

s u l f i d e Prec ip i ta t ion- -

S u l f i d e p r e c i p i t a t i o n of heavy metals is gaining acceptance because

most metal s u l f i d e s a re even less so luble than metal hydroxides a t a lka-

l i n e pH values. Therefore, lower e f f l u e n t metal concentrat ions can be

accomplished through the use of s u l f i d e r a t h e r than hydroxide. A s w i t h

hydroxide p r e c i p i t a t i o n , the s o l u b i l i t i e s of metal s u l f i d e s a r e pH de-

pendent. The pH dependence f o r var ious metal hydroxides a r e presented

i n Figure 21. Figure 21 a l s o shows the metal s o l u b i l i t i e s of metal

s u l f i d e s . Sul f ide can be added a s hydrogen s u l f i d e , sodium s u l f i d e , or f e r rous s u l f i d e . If the s u l f i d e i s added a s a water-soluble compound

(e.g., H S or NaS), the process is r e f e r r e d t o a s so luble s u l f i d e pre-

c i p i t a t i o n (SSP). If the s u l f i d e is added a s a s l i g h t l y s o l u b l e s a l t

( f e r rous s u l f i d e ) , the process i s c a l l e d inso luble s u l f i d e p r e c i p i t a t i o n

( I S P ) . Figure 22 shows t y p i c a l p r e c i p i t a t i o n processes f o r hydroxide

p r e c i p i t a t i o n , SSP, and ISP.

2

( 4 )

I n a d d i t i o n t o a c h i e v i n g e x t r e m e l y low s o l u b i l i t i e s f o r m e t a l

s u l f i d e s , t he s u l f i d e process has the a b i l i t y t o remove chromates and

dichromates without prel iminary reduct ion of t he chromium t o the t r i v a -

l e n t s t a t e . Furthermore, the s u l f i d e process w i l l p r e c i p i t a t e metals

complexed with c e r t a i n complexing agents . I ron s u l f i d e has been demon-

s t r a t e d more e f f e c t i v e f o r metal p r e c i p i t a t i o n when c h e l a t i n g agents

____._.___

125

126

I Ib'

Fgm 2 2 Waaitswataf treatment procseses for removing heavy metals.

SQCRE: W A " Q 1 3 A B Z W ~

127

such a s EDTA or Rochelle s a l t a r e present . ( ” ) These t e s t s were per-

formed f o r copper, cadmium, chromium ( I I I ) , n icke l , and z inc p r e c i p i t a -

t ion . One disadvantage of the s u l f i d e process is t h e evolu t ion of t ox ic

hydrogen s u l f i d e gas f o r c e r t a i n s u l f i d e reagents i f t h e pH should

decrease below 8.0. However, the usage of i r o n s u l f i d e v i r t u a l l y elim-

i n a t e s the p o s s i b i l i t y of evolu t ion o f , hydrogen s u l f i d e . Another d i s -

advantage of t h e process i s that high l e v e l s of excess s u l f i d e sometimes

must be oxidized t o s u l f a t e before discharge.

A key t o good opera t ion of the s u l f i d e p r e c i p i t a t i o n i s applying

the c o r r e c t amount of s u l f i d e . A s wi th any p r e c i p i t a t i o n r e a c t i o n ,

excess r e a c t a n t ( s u l f i d e ) must be p re sen t t o d r i v e the p r e c i p i t a t i o n

r eac t ion t o completion. However, because s u l f i d e i s t o x i c , s u l f i d e

add i t ion must be c a r e f u l l y con t ro l l ed with only a minimum of excess

s u l f i d e used. High l e v e l s of unreacted s u l f i d e w i l l r equ i r e pos t -

t reatment such a s a e r a t i o n t o oxid ize s u l f i d e t o s u l f a t e .


Typical performance values f o r removal of metals commonly p resen t

i n p l a t i n g and f i n i s h i n g wastes a r e discussed i n t h e following. Also

discussed a r e the e f f e c t s t h a t var ious compunds or ions may have upon

the shape of t h e pH-solubi l i ty curve.

Chromium

A p~ range of 8 t o 9 is recommended t o achieve t h e minimum s o l u b i l -

i t y of t r i v a l e n t chromium. However, t h i s range i s very dependent upon

the concent ra t ion of anions i n s o l u t i o n , p a r t i c u l a r l y carbonates and

phosphates. Thomas and Theis have demonstrated t h a t a bicarbonate

a l k a l i n i t y and pyrophosphate a t concent ra t ions as low a s 250 mg/L a s

CaC03 and 30 mg/L a s P , r e s p e c t i v e l y , toge ther cause apprec iab le com-

p lexa t ion and may make a l t e r n a t i v e s o the r than lime imprac t ica l . Thomas

and Theis showed t h a t i f c a u s t i c was used f o r pH adjustment of a waste-

water conta in ing pyrophosphate and carbonates n o t only would t h e pH

range of minimum s o l u b i l i t y be a l t e r e d , b u t t h e superna tan t chromium

128

concent ra t ions would be g r e a t e r than 1.0 mg/L. They recommended t h a t

lime should be s u b s t i t u t e d f o r c a u s t i c soda. The use of lime w i l l cause

p r e c i p i t a t i o n and removal of most of the carbonate and pyrophosphate

spec ies from s o l u t i o n while providing doubly charged counter ions t o a i d

i n coagula t ion of the nega t ive ly charged C r ( O H ) 3 c o l l o i d which e x i s t s a t

pH values above 8. An a l t e r n a t i v e t o s u b s t i t u t i o n of l i m e f o r c a u s t i c

f o r s o l u t i o n s which contain chromium ( I I I ) , carbonates , and pyrophos-

p h a t e s i s t o s e g r e g a t e t h e c r ( I I 1 ) w a s t e s from waste r i n s e s which

conta in carbonates and phosphates. The advantage f o r t h i s a l t e r n a t i v e

is t h a t the c a u s t i c s ludge volume i s much lower than the l i m e s ludge

volume.

The presence of anions can a f f e c t the s o l u b i l i t y of copper. The pH

range of minimum s o l u b i l i t y f o r copper hydroxide without the presence of

i n t e r f e r e n c e s i s 7 . 5 t o 11.0. The so lub le copper concent ra t ion a t t h i s

l e v e l is approximately 0.03 mg/L. However, t h e presence of carbonates

can inc rease t h e so lub le copper concent ra t ion . ( ” ) Pa t t e r son showed

t h a t the so lub le copper concent ra t ion f o r a s o l u t i o n wi th a carbonate

spec ies concent ra t ion of 500 mg/L a s CaCo3 was 4.5 mg/L a t a pH of 7.5,

0.25 mg/L a t a pH of 8.5, and 0.09 mg/L a t a pH of 9.5. Therefore t h e

presence of b i ca rbona te a t a concen t r a t ion of 500 mg/L as CaC03 w i l l in-

c r ease t h e copper s o l u b i l i t y concen t r a t ion 150-fold a t a pH of 7 . 5 d

The opera tor can reduce the effect of carbonate by seve ra l methods.

One s o l u t i o n is t o reduce or e l imina te t h e concent ra t ion of carbonates .

An a l t e r n a t i v e is t o use l i m e and opera te a t a pH of 10.0 or higher so

a s t o form t h e in so lub le calcium carbonate p r e c i p i t a t e .

Lead

The pH range of minimum s o l u b i l i t y f o r lead i s 7.5 t o 8.5 f o r lead

Pa t te rson (’O) noted t h a t hydroxide p r e c i p i t a t i o n a s seen i n Figure 16.

129

the e f f e c t of carbonate on l e a d ' s o l u b i l i t y i s a complex func t ion of car -

bonate concentrat ion. Furthermore, he reported t h a t a t a t rea tment pH

near 9.0, increased carbonate w i l l increase lead s o l u b i l i t y while t h e

reverse p a t t e r n w i l l occur a t a pH near 6. Bicarbonate concent ra t ions

as low a s 25 mg/L a s CaCO3 w i l l r e s u l t i n a so lub le l ead concent ra t ion

of 4.1 mg/L a t a pH of 9.3.

Carbonate can be introduced i n t o t h e system i n many ways. one

source of carbonate i s municipal water suppl ies . Municipal water sup-

p l i e s i n t h e West, sou thcen t r a l , and Midwest reg ions of t h e United

S t a t e s have a l k a l i n i t i e s g r e a t e r than 100 mg/L a s CaC03. A second

source of carbonate is t h e atmosphere. Depending upon t h e equi l ibr ium

pos i t i on (pH and concen t r a t ion ) , aqueous S O l U t i O n S n o t i n equi l ibr ium

may e i t h e r evolve o r take up carbon dioxide. The equi l ibr ium bicarbo-

na te concent ra t ion a t a pH of 8.0 is 25 mg/L as CaC03. The t h i r d source

of carbonate is carbonate s a l t s used i n p l a t i n g s o l u t i o n s . Examples of

these inc lude sodium carbonate , potassium carbonate , and n i cke l carbo-

nate . The dosages of these salts i n s o l u t i o n can be extremely high

(e.g., 50,000 mg/L).

The problem of excess carbonate or bicarbonate concent ra t ions can

be solved by reducing o r e l imina t ing the carbonate concent ra t ion by con-

t r o l l i n g the source o r by adding lime t o the wastewater and p r e c i p i t a -

t i n g calcium carbonate. Calcium carbonate p r e c i p i t a t i o n forms a heavier

f l o c which a i d s i n t h e s e t t l i n g of the l i g h t e r l ead hydroxide f l o c .

Mercurx

Su l f ide p r e c i p i t a t i o n f o r mercury removal i s commonly used.

Pa t te rson ( ' 3 , reported t h a t s u l f i d e p r e c i p i t a t i o n w i l l achieve 99+

percent removal f o r high i n i t i a l mercury l e v e l s , bu t the minimum e f -

f l u e n t mercury concent ra t ion t h a t i s achievable wi th p r e c i p i t a t i o n

followed with f i l t r a t i o n o r a c t i v a t e d carbon appears t o be 10-20 pg/L.

Best results f o r s u l f i d e p r e c i p i t a t i o n a r e oljtained a t p~ values l e s s

than 9.0. Pa t t e r son ( 1 3 ) l i s t e d t h r e e drawbacks of s u l f i d e p r e c i p i t a -

t ion . These a r e the formation of so lub le mercury s u l f i d e complexes a t

130

high l e v e l s of excess s u l f i d e , the d i f f i c u l t y of monitoring excess

s u l f i d e l e v e l s , and the problem of tox ic s u l f i d e r e s i d u a l i n t h e t r e a t e d

e f f l u e n t .

Nickel

The t h e o r e t i c a l s o l u b i l i t y curve f o r p r e c i p i t a t i o n of n i c k e l a s

n i c k e l hydroxide was presented i n Figure 20. A s tudy conducted by

Pa t te rson has shown t h a t experimental r e s u l t s f o r n i c k e l hydroxide pre-

c i p i t a t i o n w i l l be s i m i l a r t o t h e t h e o r e t i c a l r e s u l t s . However, t h e

a c t u a l s o l u b i l i t y curve f o r carbonate p r e c i p i t a t i o n w i l l probably be

g r e a t e r than t h e t h e o r e t i c a l carbonate s o l u b i l i t y . This anomaly occurs

because the k i n e t i c s of p r e c i p i t a t i o n f o r n i cke l carbonate a r e very slow

and t h e p r e c i p i t a t e w i l l no t form wi th in t y p i c a l t rea tment p l a n t deten-

t i o n times. (20)

The t h e o r e t i c a l s o l u b i l i t y f o r z inc hydroxide p r e c i p i t a t i o n i s

presented i n Figure 20. This s o l u b i l i t y curve is f o r a wastewater which

is low i n carbonate a l k a l i n i t y . The e f f e c t of carbonate e i t h e r p re sen t

i n the wastewater o r induced by the uptake of atmospheric co i s t o

lower z inc s o l u b i l i t y with increased carbonate l e v e l s . ( 2 0 ) This e f f e c t

is e s p e c i a l l y s i g n i f i c a n t a t a pH value less than 9.0 where t h e presence

of carbonate a l k a l i n i t y can s i g n i f i c a n t l y enhance the z inc p r e c i p i t a t i o n

process .

2

o the r Metals

Metals o t h e r than those discussed i n t h e preceding can be removed

from metal f i n i s h i n g wastewates by p r e c i p i t a t i o n . For example, s i l v e r

may be recovered a s the s u l f i d e , a l though s i l v e r hydroxide is r e l a t i v e l y

so luble . (6’ Aluminum may be removed by p r e c i p i t a t i o n a s t h e hydroxide

or by phosphate p r e c i p i t a t i o n a t pH 5 . ( ” ) I n p o i n t of f a c t , metal

f i n i s h i n g wastewaters may be t r e a t e d f o r phosphate removal by add i t ion

of aluminum salts and p r e c i p i t a t i o n of AlPO

- .- .____

4’

131


The t roubleshoot ing guide f o r t h e metal p r e c i p i t a t i o n process is

presented i n Table 1 1 . The problem areas i n t h e metal p r e c i p i t a t i o n

p r o c e s s i n v o l v e t o o h i g h a c o n c e n t r a t i o n of s o l u b l e m e t a l i n t h e

e f f l u e n t (which o f t en t r a c e s t o inadequate pH adjustment or c o n t r o l ) and

too high a concent ra t ion of p a r t i c u l a t e metal i n t h e e f f l u e n t (which

usua l ly traces t o the s o l i d s sepa ra t ion sys tem) . In t h e case of s u l f i d e

p r e c i p i t a t i o n processes , . s u l f i d e odors may a r i s e due t o improper concen-

t r a t i o n s of t h e s u l f i d e sa l t or too s h o r t a r eac t ion t i m e .

132

P w w

OPFRATINC PROBLEM l a t o t a l metal colvzentmtlon In flnal d l r c h a r g e Is tm Dlyh.

l a . Soluble - t a l concen- - Corpare eoluble = t a l - S h l f t n in pll of wastewater tratlun lo dlechorge co.CeIltr.tIU1 tor pil from prr a d j u s t u n t to flnal Is 100 high. adjustment procese to d i s c h a r g e v l l l a f f e c t metal

soluble C o m e n t r n t l o n s o l u b l l l t y . f o r plant dlrchpyye.

Ib. Soluble u t a 1 CMC~D- - Chack operatlng tratlon In pl a d j w t - prob1e.n for t h e pl m e r i t discharge Is t- ad ju8 tment process. hlyh.

me proh1a.s I I a t e d for pU s d j u s t u i t (sea Table 91 El" result I" soluble u t a 1 c o n c e n t r a t l a , that le tm hlgb.

- check operatlng pll of - me opecat1.g en u y Dot pu a d j u e t r u t process. be a t o p t b y pll for

preclpltltlon.

PROBA8I.L' CAUSE

Poor perfor.a"ce Of f 11 t'atlo" syste. ( I f prouldrd).

- Perfog. COrreCtIvC actlo,, I', - Plltratlon unlts Should poll=L tLr dlsclmrqe f r a Table 11. e e d h e n t a t l o n system.

- T m blyb a concentratlon - Hodlfy s u l f l d e prr~lpl1~lIuw e' tm fL0.t re.ct1on procedures. Decrease bu l f idr t h e w i l l cause e u l f l d e d o r m .

appllc.tlon rate.

I

SECTION 10

FLOC-.XIATION

INTRODUCTION

F l o c c u l a t i o n is t h e process whereby p a r t i c l e s a r e g e n t l y a g i t a t e d

t o enhance p a r t i c l e agglomeration. The p u q o s r of f l o c c u l a t i o n is t o

i n c r e a s e the size of the f l o c because larger f loc p a r t i c l e s se t t le

f a s t e r than sma l l e r f l o c p a r t i c l e s . The f l o c c u l a t i o n process g e n e r a l l y

fo l lows t h e pH adjus tment p rocess which gene ra t e s t h e f l o c c u l a n t par -

t i c l e s and p r e c e d e s the s e d i m e n t a t i o n o r f i l t r a t i o n p r o c e s s which

zemoves the f l o c c u l a n t p a r t i c l e s from the wastewater. F loccu la t ion of

p a r t i c l e s can be accomplished i n tanks, b a s i n s , o r in pipes . Polymers,

which are used e x t e n s i v e l y t o enhance the particle agglomerat ion f o r

metal f i n i s h i n g wastewaters , a r e t y p i c a l l y added i n t h e f l o c c u l a t i o n

tank o r basin.

TBEORY OF OPERATION

The key t o f l o c c u l a t i o n is t o promote p a r t i c l e c o n t a c t and agglome-

r a t i o n without p rov id ing excess a g i t a t i o n tha t will s h e a r t h e f l o c i n t o

sma l l e r p a r t i c l e s . Ag i t a t ion should be c a r e f u l l y monitored and c o n t r o l -

led so t h a t t h e f l o c p a r t i c l e s w i l l n o t be sheered. The amount of

a g i t a t i o n h a f l o c c u l a t i o n tank can be determined by c a l c u l a t i n g the

mean v e l o c i t y g rad ien t . The mean v e l o c i t y g r a d i e n t is a f u n c t i o n of the

- power i n p u t , the bas in volume, and v i s c o s i t y of t h e water .

In a d d i t i o n t o depending upon proper a g i t a t i o n , p a r t i c l e c o n t a c t

and agglomerat ion will a l s o depend upon the s i z e of the p a r t i c l e s and

135

their charge in s o l u t i o n . coagulants a r e added because they d e s t a b i l i z e

. the p a r t i c l e s i n s o l u t i o n by a l t e r i n g the charge of the p a r t i c l e s and

b r i d g i n g small p a r t i c l e s i n t o masses or f l o c s . Genera l ly the l a r g e r t t e

size of the f l o c , the f a s t e r the p a r t i c l e s w i l l s e t t l e . Both i n o r g a n i c

metal sal- and o rgan ic polymers are used t o a i d in p a r t i c l e s e t t l i n g .

Some of the i no rgan ic chemicals used a r e aluminum s u l f a t e ( a lum) , f e r r i c

c h l o r i d e , f e r r o u s s u l f a t e , and l i m e .

Determinat ion of the b e s t coagulant f o r a given system should be

based upon j a r tests and scale-up f a c t o r s . J a r tests a r e a series of

bench scale tests which can bc employed t o s imula t e chemical a d d i t i o n ,

f l occua t ion . and c l a r i f i c a t i o n . A "gang s t i r r e r " is commonly used t o

perform f o u r t o s ix j a r t e s t s a t one t i m e . Each j a r test i s sub jec t ed

t o one d i f f e r e n t cond i t ion from the p r i o r t e s t , such a s polymer dosage.

when performing a j a r test, t h e ope ra t ing cond i t ion of the t e s t should

s imula t e t h e condi . t ions in the f i e l d . The cond i t ions of M e f u l l - s c a l e

f loccula t ion /c la r i f ica t ion system which should be dup l i ca t ed by the j a r

tes t inc lude s i m i l a r d i l u t i o n s of the concent ra ted polymer, s i m i l a r IIRT.

and s i m i l a r mixing i n t e n s i t y of the f loccula t ion /c la r i f ica t ion system.

The procedure f o r jar tes t ing f o r emulsion breaking presented i n Sec t ion

5 , O i l Removal, a r e a p p l i c a b l e w i t h on ly minor medi f ica t ion t o coaqula-

t i o n processes .

Chemical coagulants a r e used e x t e n s i v e l y in the t r e a t r e n t of metal

f i n i h s i n g wastes. However, t h e type and dosage of t h e coagulant com-

monly a r e n o t optimized. This f a i l u r e t o opt imize the system can cause

increased chemical c o s t and/or poor performance. The ope ra to r should

perform a series of j a r t e s t s t o determine t h e optimum dosage whenever

the wastewater c h a r a c t e r i s t i c s change. Every s i x t o twelve months, the

o p e r a t o r should e v a l u a t e d i f f e r e n t coagulants in o rde r t o select the

most c o s t - e f f e c t i v e coagulant .


Floccu la t ion of p a r t i c l e s is promoted by g e n t l y s t i r r i n g the waste-

water. There a r e s e v e r a l methods which range from simple, inexpens ive

136

devices to l a rge p ieces of equipment f o r flOCCUlatiOn. Three types of

f l o c c u l a t i o n equipment commonly used a r e i n - l i n e mixers, r e a c t o r - c l a r i -

f i e r s , and f l o c c u l a t i o n bas ins . I n - l i n e o r s t a t i c mixers a r e simple

devices mounted i n the pipe. A bypass which w i l l al low f o r the i n - l i n e

mixer t o be taken out of s e rv i ce should a l s o be provided.

FloCCUlatiOn can be an i n t e g r a l p a r t of a c l a r i f i e r ( r e a c t o r -

c l a r i f i e r s ) . This system is accomplished by providing an i n l e t w e l l

wi th a mechanical mixer. Variable speed mixers allow t h e ope ra to r

f l e x i b i l i t y i n c o n t r o l l i n g the mixing i n t e n s i t y .

Flocculat ion bas ins a r e used f o r systems which r equ i r e long mixing

per iods or f o r systems i n which i t i s necessary t o vary t h e mixing in-

t e n s i t y wi th in the basin. Long mixing per iods a r e des i r ed f o r slow re-

a c t i n g reagents such a s l imestone. F loccula t ion i n t h e tank i s usua l ly

provided by mechanical mixers b u t can be provided by e i t h e r s t a t i o n a r y

b a f f l e s o r d i f fused a i r .


To con t ro l the opera t ion of t h e f l o c c u l a t i o n process , t h e opera tor

should conduct the necessary process monitoring, perfdrm any c o n t r o l

c a l c u l a t i o n s needed, and understand the process c o n t r o l s t r a t e g i e s o r

v a r i a b l e s .

Process Monitorin$

The primary parameters which should be monitored t o c o n t r o l t h e

f l o c c u l a t i o n process a r e power inpu t and polymer dosage. o ther para-

meters which should be measured f o r t h e polymer system include optimum

dosage ( e . g . , by j a r t e s t i n g ) and polymer t y p e . These m o n i t o r i n g

r e q u i r e m e n t s are shown i n Table 12.

In add i t ion t o heeding these requirements, t h e opera tor should

in spec t the f l o c c u l a t i o n system regu la r ly . The opera tor should i n s p e c t

137

TABLE 1 2

FLOCCULATION PROCESS MONITORING REQUIREMENTS

Parameter Frequency comment

I . Polymer Dosage Regular ly - Dosage should be d e t e r - mined a t l e a s t hour ly .

2. Polymer J a r Tes t s Dai ly - Jar tests should be per - formed d a i l y t o determine optimum dosage.

3. Polymer Type

4. Polymer Feed Concentrat ion

5 . Floccu la t ion Hors@pOWer

Occas iona l ly - Jar t e s t s should be performed occas iona l ly t o s e l e c t b e s t polymer when p rocess problems d i c t a t e .

For each - Concent ra t ion should be batch c a l c u l a t e d and must n o t

exceed manufac turer ' s recommendations.

Occas iona l ly - Veloc i ty g r a d i e n t should be c a l c u l a t e d occas iona l ly so t h a t c o r r e c t power is appl ied .

mixing b a f f l e s and look f o r hydraul ic s h o r t - c i r c u i t i n g , to rn impe l l e r s ,

o r s o l i d s accumulation. Any d e f i c i e n c i e s should be cor rec ted immedi-

a t e l y .

Example Calcu la t ions

Example c a l c u l a t i o n s a r e shown below f o r d e t e r m i n i n g t h e mean

ve loc i ty g rad ien t and the polymer dosage.

Mean Veloci ty Gradient -- The mean ve loc i ty g rad ien t (MVG) i s a func t ion of t h e mixing in t en -

s i t y o r mixing horsepower. The mathematical formula f o r MVG i s a s

fol lows:

P X 550 1 / 2 MVG = ( mv )

where MVG = mean v e l o c i t y g rad ien t (sec-’ )

P = power inpu t (HP)

m = dynamic v i s c o s i t y ( l b - s e c / f t )

v = f l o c c u l a t o r volume ( f t

2

3

Consider a f l o c c u l a t i o n system with the fol lowing c h a r a c t e r i s t i c s :

Horsepower = 1 2 ~ p , v i s c o s i t y = 2.05 x lb - sec / f t , and volume =

75,000 ft3.

2

1 2 x 550 ) 1 / 2 -5

2.05 x 10 x 15,000 MVG = (

MVG = 66/sec

Polymer Dosage -- The polymer dosage can be determined from t h e following formula:

6 x 1 0 q x c

6000 X Q PD =

139

where: PD = polymer dosage (ppm)

q = polymer f lowra te ( g a l / h r )

c = polymer concentrat ion i n so lu t ion feed ( % )

Q = wastewater f lowrate (gpm)

Consider the following example: Polymer f lowra te = 2.0 g a l / h r , Polymer

concentrat ion = 0 . 5 % , and Wastewater f lowra te = 10 gpm. The polymer

dosage w i l l be

2.0 ga l /h r x 0.5 6 PD = x 10

6000 x 10

= 16.7 ppm


The two primary adjustments c o n t r o l l i n g t h e f l o c c u l a t i o n process

a r e mixing i n t e n s i t y and polymer addi t ion . The opera tor should deter,-

mine the optimum set po in t s f o r these adjustments based on the s e t t l i n g

a b i l i t y of the f locculan t p a r t i c l e s .

Mixing In tens i ty- -

The operator should con t ro l the mixing i n t e n s i t y such t h a t the de-

s i r e d l e v e l of a g i t a t i o n between metal p a r t i c l e s and polymer i s

achieved. The c o r r e c t amount of a g i t a t i o n w i l l r e s u l t i n p a r t i c l e s t h a t

s e t t l e rap id ly and do not r equ i r e high dosages of polymer. Under-agita-

t i o n between p a r t i c l e s and polymer w i l l r e s u l t i n the formation of very

small f l o c p a r t i c l e s t h a t do not s e t t l e r ead i ly . Under-agitation of the

f loccu la t ion process may be caused by not providing s u f f i c i e n t power o r

not allowing enough t i m e f o r par t ic le /polymer contac t .

The power inpu t t o a f l o c c u l a t i o n bas in should be con t ro l l ed t o ob-

t a i n a mean v e l o c i t y g rad ien t between 20 and 75 sec-l f o r a 15 t o 30 _____.._

minute de t en t ion t i m e . High ve loc i ty g rad ien t s should be used f o r

s h o r t e r de t en t ion times and low v e l o c i t y g rad ien t s should be used f o r

140

l o n g e r d e t e n t i o n t i m e s . polymers shou ld be added t o t h e i n l e t or

f u r t h e r upstream of the f l o c u l a t i o n bas in t o achieve maximum con tac t .

Under-agitation f o r i n - l i ne mixers i s a r e s u l t of no t providing

s u f f i c i e n t head loss through the mixer. The minimum pressure drop f o r

most i n - l i n e mixers should be approximately one f o o t of head f o r a f l u i d

ve loc i ty between 1 and 2 f e e t per second ( f p s ) . For most systems, a

four or s i x element i n - l i n e mixer w i l l be adequate. Lower element

mixers should be used f o r systems where t h e r e a r e changes i n the d i r ec -

t i o n of flow from the mixer t o t h e s e t t l i n g device.

over -ag i ta t ion of t h e wastewater may shear the f l o c i n t o smal le r

p a r t i c l e s which a l s o w i l l no t s e t t l e . Too much a g i t a t i o n may be caused

by opera t ing the f l o c c u l a t i o n paddles a t too high a speed or by using a

c e n t r i f u g a l pump t o pump f loccu la t ed wastewater. Also over-ag i ta t ion

may be caused by high v e l o c i t i e s i n t h e p ipe or t h e i n l e t t o t h e c l a r i -

f i e r .

(24,35,26,27) Polymer Addition--

Polymers used a s coagulants i n wastewater t reatment can be e i t h e r

s y n t h e t i c or na tu ra l . They a r e composed of many small compounds (mono-

mers) , which can be i d e n t i c a l t o each o t h e r , or of d i f f e r e n t ma te r i a l s .

polymers a r e chains of monomers whose number is var ied by r eac t ion t o

produce polymers with d i f f e r e n t molecular weights; those used i n waste-

water t reatment normally range from 100,000 t o 1,000,000. Their prime

func t ion i s t o a c t a s a "coagulant a i d , " a term appl ied t o chemicals

such as l i m e t h a t a r e added t o wastewater, i n add i t ion t o convent ional

coagulants , t o improve c o l l o i d a l d e s t a b i l i z a t i o n and f loccu la t ion .

The term "po lye lec t ro ly t e" is used t o descr ibe chemicals of charged

groups i n t h e form of a polymer. The po lye lec t ro ly t e s a r e f u r t h e r

c l a s s i f i e d according t o t h e type of charge they w i l l have i n so lu t ion ,

a s fol lows: ~

141

Anionic polye lec t ro ly tes -negat ive charges

Cat ionic Po lye lec t ro ly t e s -pos i t i ve charges

Nonionic Polymers (Nonionic P o l y e l e c t r o l y t e s ) - n e u t r a l charges

The v a r i e t y of monomers ava i l ab le a s b u i l d l i n g ma te r i a l s f o r poly-

mers is endless , meaning t h a t t he re is a l a rge assortment of polyelec-

t r o l y t e s a v a i l a b l e f o r use i n coagulat ion.

Polymers can be purchased dry or i n l i q u i d form. They a r e e a s i l y

handled a t the p l a n t s i t e and a re gene ra l ly nonhazardous. The usual

p ro tec t ion from d u s t is requi red , and the s to rage f a c i l i t i e s f o r t h e dry

powders must be moisture-free. The ma te r i a l s of cons t ruc t ion f o r s t o r -

age and handling equipment a r e normally s t a i n l e s s steel 316 and

p l a s t i c s ; s e l e c t i o n i s made on the b a s i s of the p o l y e l e c t r o l y t e chosen.

The coagulat ion mechanisms of p o l y e l e c t r o l y t e s a r e a s soc ia t ed w i t h

charge reduct ion of the c o l l o i d s , which r e s u l t s i n absorp t ion or enmesh-

ment of the ind iv idua l p a r t i c l e s t o form a s e t t l e a b l e mass, Due t o t h e

s i z e of t h e polymeric compound, t h e polymers can a t t a c h themselves to

the sur faces of the suspension a t one o r more sites. The excess p a r t of

the polymer chain extends i n t o the s o l u t i o n and absorbs onto o the r

p a r t i c l e s . The s i z e of the f l o c being formed i s gene ra l ly r e s t r i c t e d by

the s t r e n g t h of t h e a t t r a c t i v e fo rces between t h e p a r t i c l e s and those

between the s o l i d s and the stream.

Po lye lec t ro ly t e s a r e used ex tens ive ly i n coagulat ion processes

because of t h e i r v e r s a t i l i t y , range of p r o p e r t i e s , handling ease , and

e f f e c t on coagulat ion r a t e s . General ly , c a t i o n i c po lye lec t ro ly t e s a r e

used alone or as a i d s t o o the r coagulants i n forming m e t a l l i c s a l t s .

Anionic polymers a r e used a s coagulant a i d s i n c o l l o i d a l d e s t a b i l i z a t i o n

where negat ive charges a r e required t o br idge the pos i tve ly charged

c o l l o i d s . Nonionic po lye lec t ro ly t e s a r e added t o inc rease f l o c s i z e by

a t t ach ing themselves t o agglomerated co l lo ids .

142

po lye lec t ro ly t e s w i l l :

o Increase the s i z e and s t a b i l i t y of t h e f l o c s ;

o Decrease dosages of conventional chemical coagulants , such a s

alum;

o Decrease f l o c formation t i m e ;

o Extend t h e e f f e c t i v e range of coagulant dosage1

o Extend range of PH over which convent ional coagulants a r e e f f e c -

t i v e ; and

o Increase suspended-solids removal e f f i c i e n c i e s .

In a c t u a l p l a n t use , l i q u i d polymers a r e usua l ly d i l u t e d

to approximately a 1 % s o l u t i o n ( v o l / v o l ) . This i s done by adding one

p a r t (by volume) of polymer t o 100 p a r t s (by volume) of water . Dry

polymers a r e usua l ly d i l u t e d t o approximately a 0.1% s o l u t i o n , ( w t / w t ) .

This is done by adding one p a r t of dry polymer (by weight) t o 1000 p a r t s

of water (by weight ) . An example of a 1 % s o l u t i o n of l i q u i d polymer is

to add one ga l lon raw polymer t o 100 ga l lons of water . To prepare t h e

same s o l u t i o n f o r a j a r t e s t one adds one m i l l i l i t e r of l i q u i d polymer

t o 100 m i l l i l i t e r s of water . An example of a 0.1 % s o l u t i o n of d ry

polymer would be t o add .a34 l b s of polymer t o 100 ga l lons of water (834

l b s ) . To prepare 'a 0.1 % s o l u t i o n f o r a j a r t e s t one adds one gram of

polymer t o one l i t e r of water (1000 grams). Polymer so lu t ions i n excess

of these concent ra t ions may be d i f f i c u l t t o mix o r t o pump.

polymers vary widely a s f a r a s t h e i r s h e l f l i f e is concerned and

mixed polymers o f t e n lose t h e i r e f f e c t i v e n e s s i n as s h o r t a time a s one

day. It i s recommended t h a t only enough polymer t o l a s t f o r one d a y ' s

run t i m e be mixed. The method used t o add polymer t o water and mixing

times used a r e c r i t i c a l . A recommended procedure i s t o f i l l t h e mixing

tank t o about 3/4 of des i red l e v e l and s t a r t mixers and add polymer very

slowly. After a l l t h e polymer is added, t h e tank i s f i l l e d t o t h e

des i red l e v e l and mixed f o r a minimum of 30 minutes. Mixing times vary

widely for d i f f e r e n t polymers so one i s advised t o check w i t h the poly-

mer manufacturer f o r d e t a i l e d information.

_. ____

143

Dif fe ren t polymers a re o f t e n not compatible. I f polymers being

used a re changed, a l l l i n e s and pumps should be thoroughly f lushed o u t ,

as should mix tanks and day tanks, before the new polymer is added.

Polymers should be pro tec ted from extreme changes i n weather and so

should be s to red in s ide .

Handling, s to rage , and feed-flow c o n t r o l f a c i l i t i e s a r e a s impor-

t a n t as proper s e l e c t i o n of coagulants and determinat ion of t h e

coagulant dosage. The wrong coagulant dosage can adverse ly a f f e c t t h e

s e t t l i n g and f l o c c u l a t i o n r a t e s of suspended s o l i d s by causing t h e

c o l l o i d a l suspension t o r e s t a b i l i z e . Low coagulant concent ra t ions have

s i m i l a r e f f e c t s .

A d a t a shee t should be completed when performing j a r tests. The

da ta shee t conta ins the var ious parameters and observat ions which a r e

important when performing a j a r t e s t .


The performance of the f l o c c u l a t i o n process can be measured by how

w e l l the f l o c c u l a n t p a r t i c l e s s e t t l e o r a r e f i l t e r e d . A wel l operated

f l o c c u l a t i o n process w i l l g ene ra l ly requi re low dosages of coagulants

and w i l l produce low concent ra t ions of suspended s o l i d s i n t h e e f f l u e n t .


The t roubleshoot ing guide f o r t h e f l o c c u l a t i o n process i s presented

i n Table 13. The chief problem of t h e f l o c c u l a t i o n process i s poor

s e t t l i n g f l o c , which may be caused by under-ag i ta t ion , ove rag i t a t ion , o r

improper polymer use. Improper polymer u s e may include the use of an

i n e f f e c t i v e polymer o r the improper a p p l i c a t i o n of an otherwise e f f e c -

t i v e polymer.

144

TAPLF 1 3 FUX'CIILITJON TROIIPLFSHWTIW GllIDF

PRUBAPLF CAUSE CHPCK OR WNITOR RFESON WRPCTIVF ACTION

OPFR&TING PRUBLW 1: Poor s e t t l i n g f l o c . , la. U n d e r a g l t a t i w - C a l c u l a t e mix ing i n t e n s i t y - Inadequate p w - r w i l l n o t - ~ncreese horsepower.

(Tanks or Basins) O f b a s i n or tank. provide Correct particlP/ po1yrpr cmtrct.

- Check tank h a f f l e s and - Worn or broken h s f f l e a - Repair h a f f l e s and impeller m i x e r impeller. and i m p l l e r will cause as needed.

u n d e r a d t a t i o n .

- C a l c u l a t e t h e o r e t i c a l - S u f f i c i e n t d e t e n t i o n t i n e - Increase d e t e n t i o n t i m e d e t e n t i o n t i m e Of b a s i n r e q u i r e d for p a r t i c l e / by redueins f l o w r a t e .at or tank. pD1ymrr contact. th- p q ~ a l i z a t i o n hasin

l i f p o s s i h l e l .

- Ohserve hasin f o r s h o r t - c i r c u i t i n q .

Ib. Underagi ta t ion - Check q w r a t i o n of inl ine ( I n - l i n e rixing1 .ixer*.

- Determine nuiher o f chanaes i n d i r e c t i o n of flow due to numher o f element* for

o f hends I n pipinq. i n - l i n e .iXPT PIUS numher

1 C . uveraqi ta t i o n - C a l c u l a t P r1xin.a i n t e n s i t y (Tanks or Pasins) i n h s s i n or tank.

- Determine i r p e l l r r speed or we*.

- mok f o r excessive s h e a r forces act ina on f l o r c u l a n t p a r t i c l e s .

- S h o r t - d r c u i t i m due to accunulstion o f s o l i d s or

and o u t l e t s t r u c t u r e s may reduce d e t e n t i o n t i m e .

- lor0 or torn e1ewnt . may reduce LIXina c a p a c t i y .

improper placement of i n l e t

- Four t o six or more chanqes i n d i r e c t i o n of f low (velocity = 1 t o 2 f p s ) are recDlnpnded ni"1.W.

- h c e a s i v e p e r i n p u t may shear f l o r c u l a n t p a r t i c l e s .

- Fxcensive impeller " F e d may s h e a r f l o r c u l s n t p a r t i c l e s .

- FXCeBs~VP eheac can hp caused hy centr i fugal pumps or very hioh pipe velori ties.

- Remove s o l i d s or relorrte i n l e t and o u t l e t structures i f possible.

- arpir/rep1ace as IICEP*Sar-y.

- Add mre i n - l i n ? nixing hy addinq .ore chapaes i n . d i r e c t i o n o f flow (add in-live mixer or k n d a in piping. I

- Reduce horsepower.

- c o n t a c t agitator Da",,fa'-tYrrr t o de termine rorrert i n p l l ~ r Speed f o r p a r t i c u l a r app1 Ira t 10".

- Rpduc~ shear forCeB hy rep1acino rentrif"g.31 p w p s with positive disp lacement p""pF or replace p i p W i t h

irrqrr airnrter p i F .

I

TABLP 13 (Continuedl

FUTCIILATION TROUBLESBNY3TIW GUIDE

PROBABLE CAUSE CHECK OR HONIlOR REASON CORROCTIVE A C T I O l l

ld. Incorrect Polymer - observe locatio" of polymer - Improper 1mation of - Relocate polymer addition Addition addition. polymer addition will not 1oca tion.

provide sufficient time far po1ymer/partic1e CODtaCt.

- Determine concentration - Incorrect plymer solution - Hake-up new p 1 y . e ~ solution and shelf life of polymer concentration or too 1o*g at correct concentration or solution. oi so1vtion storage t i e be ="re storage t i m e of pre-

can reduce effect of pared solution Is short . plymer addition.

- Perform jar tests to - Conpare doeage determined - Adjust polyner solution flou- determine optimum polymer f r a jar tests to dosage rate to achieve optimum dosage. of full-scale system. dosage.

- Perform j a r tests to - optirm. coegvlanf Will - Select Dew coagulant. determine optimum depend " p n jar test re- coagulant type. Bults, cost, and applica-

bility to full scale system.

I , I

SECTION 1 1

SEDIMENTATION

INTRODUCTION

P a r t i c l e s or suspended s o l i d s which a r e generated i n pH adjustment

and f loccu la t ion processes can usua l ly be separated e f f e c t i v e l y from the

wastewater by g rav i ty separat ion or sedimentation. Since the s e t t l i n g

c h a r a c t e r i s t i c s of f l occu lan t p a r t i c l e s w i l l vary with the system, the

operat ion of a sedimentation u n i t should be based on an understanding of

the theory of the sedimentation process and the var iab les which a f f e c t

its e f f i c i ency .

THEORY OF OPERATION

S e t t l i n g basins handling wastewater must separa te a v a r i e t y of

types of mater ia l s i n the c l a r i f i c a t i o n zone. These mater ia l s vary from

d i s c r e t e p a r t i c l e s a t r e l a t i v e l y low concentrat ions t o suspensions of

h ighly f loccu len t s o l i d s a t e levated concentrat ions leve ls . For pur-

poses of discussion these mater ia l s a r e divided i n t o three general

c l a s ses : ( 2 2 )

1.

2.

Class I mater ia l s ;

( a ) d i s c r e t e particles, ( b ) s e t t l i n g r a t e independent of concentrat ion, and

( c ) s e t t l i n g r a t e equal t o overflow r a t e .

Class I1 mater ia l s ;

( a ) particle growth,

( b ) overflow r a t e and de ten t ion t i m e c r i t i c a l , and

( c ) r a t e of p a r t i c l e growth important.

147

3. c l a s s 111 mate r i a l s ;

( a ) high suspended s o l i d s concent ra t ion ,

(b l s e t t l i n g r a t e is a func t ion of the concent ra t ion , and

(c ) de ten t ion time and s o l i d s loading important.

sedimentat ion of Class I suspensions i s a t a cons tan t r a t e through-

o u t t h e sedimentation bas in and is , the re fo re , independent of depth.

These a r e d i s c r e t e p a r t i c l e s which w i l l no t f l o c c u l a t e , f o r example,

g r i t p a r t i c l e s .

Sedimentation f o r c l a s s 11 type suspensions occurs when l a r g e r

p a r t i c l e s s e t t l e a t f a s t e r r a t e s than smal le r p a r t i c l e s and overtake the

smal le r p a r t i c l e s i n t h e i r descent . I f the p a r t i c l e s have c h a r a c t e r i s -

t i c s which might cause agglomeration, then growth of t h e f i n e r p a r t i c l e s

t o l a r g e r ones w i l l occur. The g r e a t e r the tank depth, t h e g r e a t e r i s

the opportuni ty f o r con tac t among p a r t i c l e s . Therefore, removal of sus -

pended s o l i d s w i l l be dependent upon bas in depth a s w e l l a s p r o p e r t i e s

of the p a r t i c l e s and f l u i d . The growth of i nd iv idua l p a r t i c l e s enhances

removal r a t e s because l a r g e r p a r t i c l e s c r e a t e a reduced surface-area- to-

mass r a t i o and thus t h e drag fo rces opposing subsidence a r e reduced.

polymer addi t ion i s normally required t o improve p a r t i c l e agglomeration.

Class 111 suspensions a r e charac te r ized by r e l a t i v e l y high concen-

t r a t i o n s of mater ia l . The ma te r i a l may be f l o c c u l e n t , bu t no t necessar-

i l y so. "Hindered s e t t l i n g " is t h e t e r m genera l ly used t o desc r ibe

Class 111 s o l i d s separa t ion . An example of t h i s type of separa t ion may

be found i n a c t i v a t e d s ludge s e t t l i n g with high MLSS. Design of s e t -

t l i n g bas ins f o r Class I11 suspensions must consider hydraul ic a s we l l

a s mass sur face a rea loading. ( 2 2 )


The equipment used i n g r a v i t y sedimentation inc ludes c l a r i f i e r s ,

p a r a l l e l p l a t e or tube s e t t l e r s , and lagoons. The func t ion of t h i s

equipment is t o provide quiescent condi t ions t o permit the heavier-than-

water p a r t i c l e s t o settle.

148

There a r e many types of g r a v i t y c l a r i f i e r s i n s e r v i c e today. Con-

vent iona l c l a r i f i e r s vary i n complexity bu t gene ra l ly c o n s i s t of a c i r -

c u l a r o r rec tangular tank with a mechanical s ludge c o l l e c t i n g device or

with a s lop ing funnel-shaped bottom designed f o r s ludge c o l l e c t i o n .

Reactor c l a r i f i e r s combine f l o c c u l a t i o n and sedimentation i n one u n i t .

p a r a l l e l o r tube s e t t l e r s u t i l i z e a s e r i e s of ' inc l ined s e t t l i n g

p l a t e s i n c l o s e proximity t o each o ther . The inc l ined p l a t e concept

provides the e f f e c t i v e su r face a rea of each p l a t e a s pro jec ted onto t h e

ho r i zon ta l sur face . This arrangement provides a h igher e f f e c t i v e su r -

f ace a rea f o r a given ho r i zon ta l su r f ace . S ince s e t t l i n g of metal

f i n i s h i n g suspended s o l i d s is a func t ion of the e f f e c t i v e su r face a r e a ,

increas ing t h i s a r ea by adding inc l ined p l a t e s a l lows t h e h o r i z o n t a l

su r f ace t o be reduced. Therefore, t h e t o t a l ho r i zon ta l su r f ace a r e a

required f o r p a r a l l e l o r tube s e t t l e r s w i l l always be less than t h a t f o r

convent ional g rav i ty c l a r i f i e r s .

Lagoons a r e a l s o used f o r g r a v i t y s e t t l i n g . , The advantage of a

lagoon is t h a t very l i t t l e maintenance i s required o the r than pe r iod ic

sludge removal. However, appropr ia te d i sposa l of t h e s ludge i s re-

qu i red ; r ecen t s o l i d waste r egu la t ions can make e f f e c t i v e lagoon opera-

t i o n an expensive procedure.


TO c o n t r o l the opera t ion of the sedimentation process , t h e opera tor

should conduct t h e necessary process monitoring, perform any c o n t r o l

c a l c u l a t i o n s needed, and understand t h e process c o n t r o l s t r a t e g i e s o r

v a r i a b l e s . The opera tor a l s o should understand how t h e design of the

sedimentat ion equipment w i l l a f f e c t t rea tment performance.

The performance c r i t e r i o n f o r eva lua t ing sedimentation equipment i s

e f f l u e n t t o t a l suspended s o l i d s concent ra t ion . While t h i s parameter

a s ses ses the o v e r a l l performance of t h e system, seve ra l o the r parameters

149

must be measured and incorporated i n t o a regular monitoring program t o

operate a sedimentation system successful ly and cons i s t en t ly . The para-

meters of importance are l i s t e d i n Table 14.

The i n f l u e n t o r e f f l u e n t wastewater flowrate must be monitored t o

determine the s o l i d s loading and the hydraul ic loading t o the c l a r i f i e r .

If e i t h e r the s o l i d s loading o r the hydraul ic loading t o the c l a r i f i e r

is g r e a t e r than the design value, then the performance of the process

may decl ine s i g n i f i c a n t l y . Example ca l cu la t ions f o r determining t h e

s o l i d s loading and the hydraul ic loading (surface overflow r a t e ) a r e

discussed i n the following sect ion.

The i n f l u e n t TSS concentration should be measured t o determine the

i n f l u e n t s o l i d s loading and the e f f l u e n t TSS concentration should be

measured t o determine removal performance. In addi t ion t o these analy-

ses, the sludge depth and sludge suspended s o l i d s concentration should

be measured so t h a t a mass balance f o r suspended s o l i d s around t h e

sedimentation basin can be determined. A mass balance is important f o r

es t imat ing sludge wasting r a t e s and f o r ad jus t ing the timer w h i c h con-

t r o l s operat ion of the sludge wasting pumps. It is e s s e n t i a l t h a t t h e

samples co l lec ted f o r the mass balance analyses be flow proport ional .


Several computations should be made by the operator t o measure the

performance of a sedimentation un i t . These ca lcu la t ions a r e sur face

overflow r a t e , s o l i d s loading r a t e , and mass balance. These calcula-

t i o n s can be made with minimal e f f o r t and 'a re he lpfu l t o the operator i n

avoiding and a n t i c i p a t i n g problems.

Surface Overflow Rate--

The surface overflow r a t e of a sedimentation u n i t i s defined as t h e

__ flowrate divided by the e f f e c t i v e surface area.

2 Flowrate (GPD) 2

Surface Overflow Rate (GPD/Ft ) = Effec t ive Surface Area ( F t 1

150

TABLE 1 4

SEDIMENTATION PROCESS MONITORING REQUIREMENTS


1. Wastewater Flow

2. I n f l u e n t and Ef f luen t TSS

3. s ludge Level

4. sludge So l ids concent ra t ion

5. Temperature

6 . pH

Continuously To determine su r face overflow r a t e and TSS loading.

Dai ly To measure e f f i c i e n c y and TSS loading.

Daily To determine volume of s ludge t h a t must be wasted/disposed.

Daily To determine s o l i d s loading t o d i s p o s a l equipment.

occas iona l ly Temperature can a f f e c t performance.

Occasional ly pH a f f e c t s t h e coagula t ion a b i l i t y of many polymers and inorganic coagulants .

151

When ca l cu la t ing t h e surface overflow r a t e , only the e f f e c t i v e sur face

a rea should be included i n the ca lcu la t ion . The e f f e c t i v e surface area

is the area i n which s e t t l i n g occurs and does not include i n l e t or

o u t l e t zones o r dead space. Furthermore, the e f f e c t i v e sur face a rea f o r

tube settlers o r p a r a l l e l p l a t e separa tors should include the sum of the

sur face a rea of the p l a t e s projected on the hor izonta l surface.

The following ca lcu la t ion is offered a s a guide f o r determining

sur face overflow r a t e .

Given: Flow - 126,000 ga l lons per day N o . of Tanks = 2

Area of each Tank = 100 f t 2

Surface Overflow Rate (SORI is ca lcu la ted a s

2 In f luen t (GPD) 2

SOR (GPD/Ft ) - Tota l Tank Area ( F t )

126,000 GPD 2

2 x 100 F t

2 = 630 GPD/Ft

Sol ids Loading--

Even though the s e t t l i n g basin design may meet the hydraul ic load-

ing c r i t e r i o n , i t i s s t i l l poss ib le f o r overloading t o occur on a s o l i d s

basis . Sol ids overloading occurs when the s o l i d s loading t o the c l a r i -

f ier exceeds the a b i l i t y of the c l a r i f i e r t o t ranspor t s o l i d s t o the

bottom of the basin. The maximum s o l i d s loading f o r a s e t t l i n g basin

genera l ly w i l l be a funct ion of sidewater depth, overflow r a t e , and

feedwell s iz ing .

Sol ids loading i s determined by the following equation:

Q x C x 8.34

SA SL =

1 5 2

where

2 SL = S o l i d s loading ( lbs /day/ f t )

Q = I n f l u e n t f lowrate (mgd)

C = I n f l u e n t suspended s o l i d s concentrat ion (mg/L)

2 SA = E f f e c t i v e sur face a r e a ( f t 1

Material Balance--

A suspended s o l i d s mass balance should be c a l c u l a t e d around t h e

sedimentation process t o detemine i f s u f f i c i e n t s o l i d s a re being

removed. The amount of sludge t h a t should be removed from t h e sed i -

mentation tank i s t h e d i f f e r e n c e between the amount of s o l i d s e n t e r i n g

t h e c l a r i f i e r and t h e amount of s o l i d s leav ing t h e c l a r i f i e r .

This is shown mathematically as :

Mass t o remove = I n f l u e n t Mass - E f f l u e n t Mass

Q C = Q C . - QeCe R R i l

where Q. = I n f l u e n t Flow Rate t o C l a r i f i e r (gpm) 1

Qe = Effluent Flow Rate from C l a r i f i e r (gpm)

= sludge Removal Flow Rate (gpm) QR ci = I n f l u e n t wastewater TSS concentrat ion (mgjL)

ce = E f f l u e n t Wastewater TSS concentrat ion (mg/L)

cR = sludge TSS concentrat ion (mg/L)

Also, a mass balance on flow g i v e s t h e f a c t t h a t flow o u t is equal

t o flow i n minus t h e sludge removal rate, o r Qe = Qi - QR.

Since Q = Q . f o r systems where t h e i n f l u e n t f lowrate i s much g r e a t e r

than the sludge removal f lowra te , e 1

15 3

QRCR = Qe ( C i - . C e )

o r

An example problem which determines the sludge removal r a t e is:

Given: In f luen t TSS concentrat ion = 550 mg/L

Eff luent TSS concentrat ion = 30 mg/L

Sludge removal TSS concentrat ion = 20,000 mg/L

P lan t Flow Rate = 90 gpm

Find: Sludge Removal Flow Rate

Qe (Ci - C e ) Q, =

cR

90 gal/min ( 5 5 0 mg/L - 30 mg/L) QR = 20,000 mg/L

Q, - 2.3 gal/min

The amount of sludge t o be removed w i l l be a funct ion of the sludge

concentrat ion (C,) and the sludge removal flow r a t e (Q,). The g rea t e r

the sludge concentrat ion is, the lower the sludge removal r a t e w i l l be.

Conversely, the lower the s o l i d s concentrat ion is , t h e g rea t e r t h e

sludge removal r a t e w i l l be. The concentrat ion of the sludge w i l l

change with changes i n i n f l u e n t loading (Qi and C i ) , e f f l u e n t q u a l i t y

( C ) , and sludge removal r a t e s . The opera tor should f ind t h e sludge

removal flow r a t e which w i l l n o t hydraul ica l ly overload the downstream

dewatering u n i t s and which is high enough such t h a t the sludge does not

remain i n t h e c l a r i f i e r sedimentation tank f o r per iods longer than a few

hours. Allowing sludge t o remain i n the c l a r i f i e r f o r long per iods may

cause the sludge t o become cementous. Excess sludge i n a c l a r i f i e r

increases the torque on the c l a r i f i e r rake. High , torque i n t h e ~ c l a r i f e r

w i l l a c tua t e the torque l i m i t s w i t c h which w i l l t u rn o f f t h e c l a r i f i e r

rakes.

e

,

.. ~

154


Based on t h e monitoring da ta c o l l e c t e d and t h e values ca l cu la t ed

f o r sur face overflow r a t e , s o l i d s loading, or mass ( m a t e r i a l ) balance,

the operator can a d j u s t s eve ra l va r i ab le s of c l a r i f i e r opera t ion t o meet

permit requirements. The important v a r i a b l e s i n t h e opera t ion of a

sedimentation basin a r e sludge wasting, s o l i d s loading, hydraul ic load-

ing , and chemical add i t ion .

s ludge Wasting--

The s ludge wasting r a t e or sludge removal volume should be ad jus ted

so t h a t s ludge does not accumulate i n t h e c l a r i f i e r . The ope ra to r

should a d j u s t the sludge wasting r a t e so t h a t t h e mass of s ludge removed

is equal t o t h e d i f f e r e n c e between the i n f l u e n t mass of suspended s o l i d s

and the e f f l u e n t mass of suspended s o l i d s . An example of t h i s ca lcu la-

t i o n is presented i n the "Mater ia l Balance" s e c t i o n discussed previous-

ly . The two types of sedimentation systems i n which s o l i d s overloading

a r e most l i k e l y t o occur a r e p a r a l l e l p l a t e or tube sepa ra to r s and

s ludge r e c i r c u l a t i o n systems. So l ids overloading can occur i n tube

s e t t l e r s due t o plugging of the tubes. I f t h i s happens f r equen t ly , t h e

opera tor should backwash the tube s e t t l e r s more o f t en i n order t o e l i m i -

na t e plugging.

So l ids overloading can a l s o be caused by not wasting enough s ludge

from the c l a r i f i c a t i o n system. This condi t ion usua l ly occurs i n systems

t h a t employ s ludge r e c i r c u l a t i o n , bu t can occur i f sludge i s allowed t o

b u i l d up i n t h e c l a r i f i e r . The opera tor should c a l c u l a t e s o l i d s loading

from the equat ion presented above. I f t h e s o l i d s loading exceeds t h e

maximum value, t h e opera tor should waste s ludge from t h e c l a r i f i e r o r

should decrease t h e f lowra te temporarily.

Hydraulic Loading Capacity-- ...

In add i t ion t o a d j u s t i n g t h e flow r a t e t o c o n t r o l t h e s o l i d s load-

ing to the sedimentat ion u n i t , the opera tor should c o n t r o l the flow r a t e

t o the sedimentat ion process so a s n o t t o exceed i t s hydraul ic loading

capac i ty , i f poss ib le . The hydraul ic loading of a sedimentation bas in

155

is gene ra l ly determined by the su r face Overflow r a t e c a l c u l a t i o n ( s e e

preceding d i s c u s s i o n ) . The maximum hydraul ic capac i ty of a sedimenta-

t i o n u n i t or c l a r i f i e r w i l l be dependent upon t h e type of c l a r i f i e r , t h e

s e t t l i n g c h a r a c t e r i s t i c s of the p a r t i c l e s , and the chemical coagulants

used.

Surface overflow r a t e s of 600 t o 2880 gpd / f t2 a r e recommended f o r

calcium carbonate p r e c i p i t a t e . Lower r a t e s (600-1800 g p d / f t 1 a r e

recommended f o r aluminum and i ron f lot(" ) . Generally higher overflow

r a t e s a r e used f o r l i m e p r e c i p i t a t i o n because of favorable s e t t l i n g

c h a r a c t e r i s t i c s . Lower overflow r a t e s a r e employed f o r small systems

and f o r systems which use c a u s t i c o r soda ash f o r pH adjustment. The

hydraul ic loading capac i ty f o r a p a r t i c u l a r c l a r i f i e r should be based on

a ’ r e l a t i o n s h i p between flow r a t e and e f f l u e n t suspended s o l i d s t h a t has

been ca l cu la t ed from a c t u a l opera t ing da ta . I f the opera tor determines

t h a t flows in excess of a given flow r a t e w i l l cause t rea tment problems,

then t h e opera tor should take s t e p s necessary t o reduce the flow r a t e .

2

The most p r a c t i c a l way t o c o n t r o l the flow r a t e i s with a v a r i a b l e

l e v e l equa l i za t ion tank or surge tank. However, peak f low r a t e s can be

minimized by schedul ing tank washdowns and clean-ups during per iods of

low hydraul ic loading.

Coagulant Addition--

The opera tor can c o n t r o l t h e s e t t l i n g r a t e of p a r t i c l e s by a d j u s t -

i ng t h e coagulant o r polymer add i t ion system. The polymer add i t ion

va r i ab le s which can a f f e c t s e t t l i n g a r e polymer type and dosage, p o i n t

of i n j e c t i o n , and f l o c c u l a t i o n . The o p e r a t o r s h o u l d r e f e r t o t h e

t roubleshoot ing t a b l e f o r f l o c c u l a t i o n (see Table 1 3 ) f o r ad jus t ing t h e

polymer add i t ion system.

- Design C h a r a c t e r i s t i c s

The performance of sedimentation equipment w i l l depend upon opera t -

i n g v a r i a b l e s such a s i n f l u e n t f l o w , s l u d g e w a s t i n g , and polymer

156

add i t ion , but w i l l a l s o depend upon f a c t o r s beyond t h e cont ro l of t h e

operator. T h e s e f a c t o r s can be a function of e i t h e r the wastewater

c h a r a c t e r i s t i c s or the d e s i g n of the system. Generally, proper sed i -

mentation of the wastewater w i l l r e s u l t i n suspended s o l i d s concentra-

t i o n s of less than 50 mgfi.

The f a c t o r s which a r e associated with the design of a sedimentation

basin a r e discussed below.

Flow Variations--

Flow var ia t ions can be caused by extremes i n instantaneous flows or

by a constant speed pump c y d i n g on and o f f . High instantanous flows

can be reduced with equal izat ion, e l iminat ion of storm water inflow/in-

f i l t r a t i o n , or reduction of t h e r a t e a t which high volume wastes a r e

discharged.

Cycling on and off of a constant speed pump can cause pulsa t ing

turbulence i n the c l a r i f i c a t i o n zone and can damage the pump and motor.

A number of so lu t ions a re ava i lab le t o correct t h i s problem. P a r t of

the discharge from the pump can be recycled back t o the pump sump o r t h e

pump can be t h r o t t l e d with a valve, Other s o l u t i o n s include replacing

the constant speed pump with a var iab le speed pump and adding a s n a l l e r

capaci ty constant speed pump.

I n l e t Design--

The Manual of P r a c t i c e f o r Wastewater Trea tment P l a n t Design

r e p o r t s t h a t next t o t h e importance of proper s i z i n g of a s e t t l i n g tank

is proper i n l e t design. The i n l e t t o a sedimentation basin must be an

e f f e c t i v e arrangement to achieve hor izonta l and v e r t i c a l d i s t r i b u t i o n of

t h e flow across the e n t i r e cross-sect ional flow-through area. The i n l e t

design must a l s o d i s s i p a t e the i n l e t energy. The maximum i n l e t v e l o c i t y

t o the center i n l e t w e l l should no t exceed 3 f p s and t h e outflow veloc-

i t y should no t exceed 1 5 f e e t per minute. ( 2 2 ) __._

I n o rder t o d i s s i p a t e energy r e a d i l y , c l a r i f i e r i n l e t s a r e normally

equipped w i t h b a f f l e s o r small ports . I n l e t p o r t s should be s i zed f o r

157

v e l o c i t i e s i n the range of 1 5 t o 30 fpm. Baff les should extend from a

poin t s eve ra l inches below the water surface t o a po in t 6-12 inches

below the i n l e t point . ( 2 2 )

Density Currents--

Shor t -c i rcu i t ing can occur i n sedimentation basins i f the incoming

flow has a higher temperature o r dens i ty than the mass of l i q u i d i n t h e

basin. This condi t ion is usual ly evidenced by v i o l e n t r o l l i n g of masses of l i q u i d with entrained s o l i d s t o the surface. Shor t -c i rcu i t ing can

occur when the temperature suddenly increases 1 t o Z0C with l i g h t f l oc - culant-type s o l i d s . ( 2 2 )

Temperature increases can be e s p e c i a l l y s i g n i f i c a n t during start-

ups. Control over dens i ty cu r ren t s i s achieved by l i h i t i n g the tempera-

t u r e change t o l-Z0C over a period of a t l e a s t an hour. One method f o r

e l imina t ing temperature gradients i n the sedimentation basin i s t o

recycle the c l a r i f i e r underflow back t o the treatment processes. (22)

Out le t Design--

The overflow o r e f f l u e n t takeoff weir should be designed and opera-

ted such t h a t the c l a r i f i e r l i q u i d can be removed from the basin without

causing loca l ized high ve loc i ty up-drafts. These updraf ts cause pr rv i -

ously settled s o l i d s t o be c a r r i e d over the w e i r . For t h i s reason most

sedimentation basins have a sidewater depth between 8 and 1 2 f e e t .

In order t o e l iminate s h o r t c i r c u i t i n g caused by loca l ized ve loc i ty

u p d r a f t s , a m u l t i p l i c i t y of weirs is recommended. The Ten S t a t e s

Standards suggested a l i m i t of 15,000 ypd/ft2 f o r secondary s e t t l i n g

basins. ( 2 3 )

Tank Depth--

The depth of a tank w i l l a f f e c t performance by a f f e c t i n g p a r t i c l e

contact. s i n c e s e t t l i n g of f locculan t p a r t i c l e s is af fec ted by tank

depth, a g r e a t e r tank depth w i l l produce higher removal e f f i c i e n c i e s .

158


Sedimentation of f locculan t p a r t i c l e s from metal f i n i s h i n g waste-

water treatment should r e s u l t i n suspended s o l i d s concentrat ions less

than 50 mg/L.


The troubleshooting t a b l e f o r the sedimentation process is pre-

sented i n Table 15. Several types of problems a r e addressed; t hese

include high sludge blanket, high s o l i d s concentration i n the e f f l u e n t ,

f l o w shor t -c i rcu i t ing , mechanical torque problems i n the equipment,

def loccula t ion i n the c lar i f ier , and high metals concentrat ion i n the

e f f l u e n t . Many of the p o t e n t i a l so lu t ions involve cor rec t ion of mech-

a n i c a l problems.

159

TAALF 15 1 I

SEDIHPNTATION TROUBLESIYYMlNG GUIDE

PROBAPLE CAUSE CHECK OR H O N I m R RFASON CORRECTIVE ACTION

OPERlTING PROBLEM 1: Sludge blanket overflow weirs.

ta. Sludge blanket - Depth of s ludge blanket. - Sludge n o t being removed - Increase removal rate of too high. Operation of sludge p ~ p . a t f a s t enough r a t e . sludge f r o o clarifier.

OPERATING PROBLEM 2 : Sludge blanket too high. Sludge renova1 pumps operating properly.

2a. Defective scraper - B o t t o m sludge scraper - scraper blade out of - Drain c l a r i f i e r and a d j u s t b lades M) bottaa blade. a d j u s t w n t or broken. or repair B E C O ~ ~ T blade. sludge scrapers. Sludge not belng moved

t o c l a r i f i e r hopper.

OPFRATING PROBLEM 3: Excessive toique om rake Irecbanis..

3a. Sludge b lanket - Depth Of blanket and . Thick s ludge can cause - I”Ere*8e removal r a r e of tm hlgh. Sludge sludge concent,ation. torque overload. sludge. Blanket depth w i l l concent ra t ion tm high.

have to b decreased rap id ly to prevent damage to c l a r i f i e r .

3b. Foreign o b j e c t i n - Bottom of c l a r i f i e r - Foreign o b j e c t can wedge - Drain c l a r i f i e r and c l a r i f i e r . f o r f o r e i g n objects. scraper ana cause torque. r e t ” objec t .

3c. Bottom s ludge scraper - Bot tom scraper arm out of adjustment. adjustment.

3d. Top s k i m m e r out of - Top s k i m m e r . adjustment.

h. Hechanical problems. - Gear box, bearings. motor.

- s c r a p r blodee touching - Drain c l a r i f i e r and r e a d j u s t bottc. cauaing torque bottom =caper a r m and/or overload. blades.

- Skimmer Could h rub- bing d d e wal l s of c l a r i f i e r O K could have dropped causing

s k i o e r passes OWE

skimmings hopper.

Excessive drag due t o

e x C e B s i Y e torqye as

mechanical problems caii CaUSP torque prob1ers.

- Adjust b lade on end of skiomer

- r d j v n t b a f f l e of c l a r i f i e r i f

- Readjust position of t o p

arm.

mt of round.

skiwoe,.

- ~eplace or repoic d e f e c t i v e part.

TAPLF 15 (Continued)

SWIMEWI'ATION TROUBLESI!4KJTIUG GUIDF

PROBABLE CAUSE CHFCK OR wwmn REASON m n P m i v E ACTION

OPERITING PROBLeW 4: High TSS concent ra t ion in c l a r i f l e r e f f l u e n t .

- See S t e p s 1 and 2 . - see stepe 1 and 2 . 4a. Sludge b lanket tw - see steps 1 md 2. high.

4b. I n f l u e n t f l o v too - PIOW r a t e to c l a r i f i e r . - Prcessive fl0" r a t e s - Reduce f l o v rate. high. w i l l cause s o l i d s

carryover.

4c. Chemical dosage - Chemical dosage. "TO"9.

- L a , or high chemical - nun j a r feet. dosage can cauee s o l i d s neadjus t dosage. carryover.

4d. Sludge hopper an - E x i t lines of sludge - Plugged hopper w i l l - Unplug hopper. c l a r i f i e r f o r f l o a t i n g hopper. cause increased con- S o l i d s plugged. centration Of f l o a t -

i n 9 s o l i d s . S o l i d s "ill o w r f l o v veir.

4e. poor miring of waste- -. Chemical i n j e c t i o n p o i n t . - Poin t of entry of - I n j e c t chmicals a t proper water and chemicals. chemicals is critical poin t .

to proper mixing.

. Turbine d r i v e On r e a c t o r - Drive may be tw f a s t - Readjust speed of turbine clarifiers. or t w S l W . dr ive .

4 f . Short c i r c u i t i n g of - Leve l of c l a r i f i e r weirs. - uneven weirs CdlUBes - Level wire. c l a r i f i e r flov. s h o r t c i r c u i t i n g .

49. S o l i d s concent ra t ion P a r t i a l blocking of V notch - P a r t i a l blocking can - Clean weirs. to c l a r i f i e r tw high. veir*. cauee s h o r t c i r c u i t i n g .

- S o l i d s concentration. - Exceeding s o l i d s load- - Decrease s o l i d s loading. in9 r a t e W i l l cau*e s o l i d s carryover.

OPERATING PROeLEM 5 : s h o r t - c i r c u i t i n g of c la r i f ie r flous.

5a. Llllc-ve" v e i r s ,

5b. Plugged weirs

- see step 4f .

- see Stel, Of.

- see step 4f.

- see step 4 f .

- see s t e p 4f.

- see s t e p I f .

TABLE 15 (Con ti nued )

SEUIMMTATION IROUBLPSIIO(IZ1HG GUIDE

- Reduce f l a r a t e or place 5c. rcessive hydrau l i c - P l n , r a t e to c l a r i f i e r . - High flow r a t e w i l l add i t iona l u n i t s in service. i w d . cause s h o r t circuifing.

y t u e e n i n f l v e n r f l a r 5d. 7emperature d i f f e rence - Water temperatures. - Temperature d i f f e r e n c e s - Reduce hydraul lc load.

can cause s h o r t circuit- qnd c la r i f ie r water. ing .

OPMTING PROBLPl 6 ; DefloCNlat ion In c l a r i f i e r .

6a. Bxcess1ve shear. - I n j e c t i o n p i n t and agitar.on.

- Ercessive .iring or - Reduce mirlng or excessive turbulence con a g i t a t i o n . cause shear.

P

N m

OPEMTIWG PROBLM 7: High c o n c e n f r a f i ~ of metals i n e f f l u e n t .

-.

7a. 1.proper treat.ent - pH adjustment and - Improper chemical t r e a t - - correct pH steps. (See 'U, .eta1 p rec ip i t a t io" . rent " I l l fa"*= .e ta16 to A d j u e t e n t Troubleshooting prior to sedimentation.

remain I" S O l U t l O n . Guide and "eta1 P r e c i p i t a t i o n Trovbleshmfing Guide.)

7b. Incorrect polymer - Polymer dosage. - High or lw polymer - Run j a x teQt and r ead jus t dosage In c l a r i f i e r . dosage can a l l n , e a l i d s polymer dosage. See

CdTTYOVeZ. FlocCulatibn Troubleshooting Guide.

7c. Sludge blanket LOO high. - See steps 1 and 2. - See s t e F 1 and 2. - See Steps 1 and 2 .

7d. Short circuiting of - see step 5. - see step 5 . - see step 5 .

7e. see step 4. - see step 4. - see s t e p 4. - see s t e p 4.

c l a r i f i e r flows.

-

I

SECTION 12

FILTRATION

INTRODUCTION

F i l t r a t i o n is a separat ion process used t o remove suspended s o l i d s

generated by p r e c i p i t a t i o n and other suspended s o l i d s present i n t h e

was tewater . Generally, f i l t r a t i o n is used a f t e r c l a r i f i c a t i o n t o fur-

t h e r reduce the suspended s o l i d s concentration i n t h e f i n a l e f f l u e n t ,

thus enabling the treatment system t o meet more s t r i n g e n t e f f l u e n t re-

quirements o r t o permit the t r e a t e d wastewater t o be recycled. addi-

t i o n a l l y , f i l t e r s a r e sometimes used i n treatment systems i n which space

is severe ly l imi ted a s the s o l e suspended s o l i d s removal process.

I n the f i l t r a t i o n process, wastewater is passed through a bed of

porous mater ia l which separa tes the s o l i d s from the wastewater. Depend-

ing upon the na ture of the f i l t e r , one of two d i f f e r e n t separat ion pro-

cesses predominate. I n deep granular f i l t e r s the s o l i d s a r e removed

through an adsorpt ion/disposi t ion process a s the wastewater passes

through a deep bed of granular mater ia l . In pre-coat and c a r t r i d g e

f i l t e r s the s o l i d s a re removed through a mechanical s t r a i n i n g process a s

the wastewater passes through a t h i n l aye r of f i l t e r media.

THEORY OF OPERATION

F i l t r a t i o n involves the passage of water through porous media with

a r e s u l t i n g removal of suspended so l id s . A number of mechanisms, some

chemical and some phys ica l , a r e involved i n the s o l i d s removal process.

The predominant mechanisms a r e s t r a i n i n g , d i s p o s i t i o n , and adsorption.

163

The dominant mechanism f o r a given f i l t e r depends upon the physical and

chemical c h a r a c t e r i s t i c s of the wastewater and the f i l t e r .

S t ra in ing involves removal of p a r t i c l e s e i t h e r a t the f i l t e r sur-

face or within the i n t e r s t i c e s of the f i l t e r media and is af fec ted by

the f i l t e r media and p a r t i c l e s i z e and the' f i l t r a t i o n r a t e . For both

precoat f i l t e r s and c a r t r i d g e f i l t e r s , the predominant removal mechanism

is assumed t o be s t r a i n i n g which occurs a t the sur face of the f i l t e r

media. Although s t r a i n i n g a l s o occurs t o a l imi t ed degree i n deep

granular f i l t e r s , pr imari ly within the i n t e r s t i c e s of the media, i t s

importance is general ly minimized during design because it leads t o

rapid buildup of head loss which l i m i t s the length of f i l t e r runs.

Disposi t ion and adsorption within the f i l t e r bed a re the predom-

i n a n t s o l i d s removal processes i n deep granular f i l t e r s . Disposi t ion

involves g r a v i t a t i o n a l s e t t l i n g , d i f fus ion , and in te rcept ion and is

af fec ted by the physical c h a r a c t e r i s t i c s and s i z e of the media, t h e

f i l t r a t i o n r a t e , t h e f l u i d temperature, and the s i z e and dens i ty of the

suspended so l id s . Adsorption r e l i e s upon the attachment of the sus-

pended s o l i d s p a r t i c l e s t o the f i l t e r media. The amount of surface

ava i l ab le for adsorption is enormous, roughly 3,000 t o 5,000 square fee t

per cubic f o o t of media. Adsorption r e l i e s upon attachment and is

af fec ted by use of coagulants, the c h a r a c t e r i s t i c s of t h e wastewater

( e s p e c i a l l y p a r t i c l e s i z e ) , shear s t r eng th and dr iv ing force, adhesive-

n e s s of the suspended s o l i d s f l o c , and the c h a r a c t e r i s t i c s of t h e f i l t e r

media.

DESCRIPTION OF EQUIPMEXT

F i l t r a t i o n processes vary i n the type of equipment used, the manner

t h a t the d r i v i n g fo rce i s applied t o t h e f i l t e r , and t h e method of

backwashing.

164

Types of F i l t e r s

The four most common types of f i l t r a t i o n equipment used f o r t r e a t -

ment of metal wastes a r e granular deep-bed f i l t e r s , diatomaceous e a r t h

f i l t e r s , c a r t r i d g e f i l t e r s , and pressure f i l t e r s . Deep bed f i l t e r s

c o n s i s t of a basin or tank with an 18 t o 30 inch l a y e r of sand or other

f i n e granular material supported by an underdrain system and operated

under e i t h e r a g r a v i t y head or pressure head. Deep granular f i l t e r s a r e

genera l ly appl icable f o r the removal of suspended s o l i d s i n the 5-50

mg/L range, but can handle suspended s o l i d s concentrat ions up t o 1000

mg/L and provide about a 90 percent s o l i d s removal for short time periods.

Pre-coat f i l t e r s c o n s i s t of a number of porous septa support i rq a

t h i n l aye r of f i l t e r media. The f i l t e r medium i s genera l ly diatomaceous

e a r t h and pre-coat f i l t e rs general ly operate under a pressure head.

Pre-coat f i l t e r s can handle high suspended s o l i d s concentrat ions pro-

vided s o l i d s concentrat ions remain constant. Precoat f i l t e r s can pro-

vide up t o 98 percent s o l i d s removal.

The use of c a r t r i d g e f i l t e rs f o r wastewater treatment is l imited t o

e f f l u e n t po l i sh ing p r i o r t o wastewater recycling. Cartr idge f i l t e r s

c o n s i s t of c a r t r i d g e s of porous mater ia l t h a t a r e enclosed i n a housing

and operated under a pressure head. Cartr idge f i l t e r s a r e l imited t o

wastewaters with low suspended s o l i d s concentrat ions, bu t , depending up-

on the pore s i z e of the f i l t e r c a r t r i d g e , can provide e s s e n t i a l l y com-

p l e t e removal of suspended so l id s .

Pressure f i l t e r s a r e a l s o general ly used f o r e f f l u e n t pol ishing

a f t e r c l a r i f i c a t i o n . Pressure f i l t e r s c o n s i s t of sand media enclosed i n

a housing and operated under a pressure head. Pressure f i l t e r s can be

operated under a constant head or var iab le head. A var iab le head i s

used o f t en t o achieve a constant f lowrate.

165

Driving Force

T h e r e a r e th ree bas ic f i l t e r operat ing methods, and they d i f f e r i n

the way t h a t the d r i v i n g force is applied across t h e f i l t e r . These

methods a re re fer red t o a s constant-pressure f i l t r a t i o n , constant-rate

f i l t r a t i o n , and decl ining r a t e f i l t r a t i o n . For constant-pressure f i l -

t r a t i o n a constant d r i v i n g fo rce is applied aoross t h e f i l t e r for the

e n t i r e f i l t e r run. Because f i l t e r r e s i s t ance is lowest a t the s t a r t ,

the flow r a t e i s a t i t s peak. A s s o l i d s a r e captured, f i l t e r r e s i s t a n c e

increases and the r a t e of flow decreases. This operat ing method i s seldom used because a l a rge flow equal izat ion basin i s required t o d e a l

with the change i n flow during the f i l t e r run.

In constant-rate f i l t r a t i o n a constant pressure is appl ied t o the

f i l t e r , but f i l t e r res i s tance i s modulated through con t ro l of t h e flow

r a t e using a flow cont ro l valve. A t the Start of the f i l t e r run the

flow cont ro l valve is nearly closed, then a s s o l i d s accumulate, the flow

con t ro l valve is opened t o compensate for the increase i n f i l t e r resis-

tance. While s torage capaci ty is minimized t h e i n i t i a l aqd operat ing

c o s t s f o r the r a t e c o n t r o l l e r are high and water q u a l i t y is b e t t e r than

with dec l in ing r a t e f i l t r a t i o n . The constant r a t e system i s a l s o waste-

f u l of ava i l ab le head because excess head is l o s t i n the c o n t r o l l e r .

Additionally t h e con t ro l valve may set up high frequency surges i n the

f i l t e r bed.

Declining-rate f i l t r a t i o n u t i l i z e s a bank of f i l t e r s t o moderate

t h e e f f e c t of increases i n f i l t e r r e s i s t ance . As the f i l t e r s served by

a common header become d i r t y , the flow through the d i r t i e s t f i l t e r drops

r ap id ly , increasing the dr iv ing fo rce so t h a t t h e other f i l t e r s can

handle a d d i t i o n a l flow from the d i r t i e s t f i l t e r . This method provides a

more gradual decrease i n t h e r a t e of flow over t h e f i l t e r cycle and

provides a b e t t e r e f f l u e n t q u a l i t y than with constant r a t e operation.

A s with constant pressure f i l t r a t i o n , a l a rge upstream water s torage i s

needed.

166

Backwashinq

Once f i l t e r r e s i s t ance exceeds the ava i lab le d r i v i n g fo rce the

accumulated s o l i d s must be removed from the f i l t e r . In c a r t r i d g e fil-

t e r s t h i s process r equ i r e s dismantling of the f i l t e r housing and re-

placement of the f i l t e r ca r t r idges . For precoat f i l t e r s t h e f i l t e r

l a y e r is removed from t h e system by scraping the sep ta manually or by

washing e i t h e r d i r e c t l y or by a reverse f low. For deep granular f i l t e r s

and pressure f i l t e r s , the media a r e washed and returned t o service.

Backwashing of deep granular f i l t e rs involves reversing t h e flow

through the f i l t e r a t a r a t e s u f f i c i e n t t o expand t h e f i l t e r bed. The

deposited mater ia l i s then dislodged by hydraul ic shear ing a c t i o n of the

water and abrasion of the g ra ins of f i l t e r media. Where c leaning i s

inadequate, mud b a l l s , masses of f i l t e r e d s o l i d s , develop and over t i m e

w i l l grow and s ink deeper i n t o the f i l t e r bed, increas ing head loss and

decreasing e f f l u e n t q i a l i t y . Where backwash r a t e s a r e inadequate t o

thoroughly clean the f i l t e r , longer durat ion backwashes must be used ( 5

t o 10 o r a t an extreme, 15 minutes) t o provide the nqcessary cleaning.

A i r scour i s o f t e n used t o a s s i s t i n backwashing. Deep bed f i l t e r s can

e i t h e r be backwashed continuously or in te rmi t ten t ly .


The objec t ive of wastewater f i l t r a t i o n is t o achieve a high capture

of suspended s o l i d s and t o minimize operat ing and power requirements

without constraining wastewater f low. To achieve t h i s ob jec t ive , the

operator should know how t o perform the necessary c a l c u l a t i o n s , perform

the required process monitoring, and understand the process con t ro l

var iab les or s t r a t e g i e s .

Process Monitorinq

To maintain the desired l e v e l of f i l t r a t i o n performance, usual ly

measured by e f f l u e n t suspended s o l i d s or metal concentrat ion, the opera-

t o r should perform the necessary monitoring and understand how the

167

various parameters can be used to diagnose problems. The monitoring re-

quired t o troubleshoot the f i l t r a t i o n process i s l i s t e d i n Table 16.

Included i n t h i s t a b l e a re the frequency of ana lys i s and t h e reason for

monitoring t h e parameter.

The operat ion performance of the f i l t r a t i o n process is defined by

the i n f l u e n t and e f f l u e n t suspended s o l i d s , metal concentration or

t u r b i d i t y , and the f i l t r a t i o n r a t e . As a f i l t e r run progresses, t h e

captured s o l i d s increase the f i l t e r r e s i s t ance and thus e i t h e r increase

t h e head loss through the bed or decrease the f lowrate through t h e bed.

Decreasing the flowrate through t h e bed w i l l reduce the f i l t r a t i o n

capac i ty of the bed. By operat ing a t higher system pressures t h e r a t e

of flow can be maintained, but with m e t a l hydroxide f l o c s the re i s

considerable pene t ra t ion i n t o the f i l t e r and breakthrough can occur a t

r e l a t i v e l y low heads. While run lengths can be shortened t o maintain a

higher r a t e of flow, the effects of downtime and backwashing become

increas ingly important. Thus, f i l t e r operation focus is on obtaining

the longest poss ib le f i l t e r cycle t h a t is c o n s i s t e n t with minimal s o l i d s

breakthrough, p r a c t i c a l system head lo s ses , and acceptable f i l t r a t i o n

rates. Floc breakage a t h i g h e r pressures can be p a r t i a l l y compensated

f o r by use of alum or polye lec t ro ly tes t o condi t ion the f l o c p r i o r t o

f i l t r a t i o n .


The c a l c u l a t i o n used most of ten t o troubleshoot the f i l t r a t i o n

process i s the f i l t r a t i o n r a t e . The mathematical formula f o r f i l t r a t i o n

r a t e is

2 Flow r a t e (gpm) F i l t r a t i o n r a t e (gpm/ft 1 = 2 Surface area of f i l t e r ( f t )

Consider the following example ca l cu la t ion . Determine t h e f i l t r a -

t i o n r a t e f o r a system which has two f i l ters and each f i l t e r has a

%foot diameter. The flow r a t e is 60 gpm.

168

TABLE 16

FILTRATION PROCESS MONITORING REQUIREMENTS


1 . I n f l u e n t flow cont inuously - To determine f i l t r a t i o n r a t e .

2. In f luen t TSS weekly - To determine mass loading t o f i l t e r s .

- To t roubleshoot s h o r t f i l t e r run problem.

3 . Eff luen t TSS D a i l y

4. Headloss

- To determine f i l t e r performance.

- To determine whether t o backwash f i l t e r s .

COntinuously - To determine when t o backwash f i l t e r .

5. F i l t e r run time Each run - To i d e n t i f y s h o r t r u n per iods.

6 . Backwash f lowra te Continuously - To optimize backwash f lowra te and dura t ion .

7. Fi l te r a i d Daily - To optimize type and ( type and dosage) dosage.

8. I n f l u e n t and e f f l u e n t Weekly - To determine performance. metal concent ra t ion

169

60 F i l t r a t i o n r a t e =

2 x ( n x 5 2 , / 4

2 F i l t r a t i o n r a t e = 1.53 gpm/ft

The recommended f i l t r a t i o n r a t e s f o r granular f i l t e rs a r e shown

below.

ZYPE Slow Sand

Rapid Sand

High Rate Mixed Media

F i l t r a t i o n - 2 0.02-0.05 gpm/ft

2.0-6.0 gpm/ft2

6.0-8.0 gpm/ft2

Higher f i l t r a t i o n r a t e s can be obtained with c a r t r i d g e f i l t e r s or

pressure f i l t e r s . Generally these f i l t e r s a r e l imited by a maximum

pressure drop ac ross the f i l t e r of 60 ps i .

While most sygtems a r e designed t o operate a t the c o r r e c t f i l t r a -

t i o n r a t e , problems can occur when one o r more f i l t r a t i o n u n i t s a r e out

of s e rv i ce due t o backwashing, replacement of media, or broken equip-

ment.


F i l t e r operat ion is general ly automatic and r equ i r e s l imited oper-

a t o r control . Possible process va r i ab le s a r e thus l imited t o f i l t e r run

time, length and durat ion of backwash, and chemical conditioning of the

i n f l u e n t wastewater.

F i l t e r Run Time--

The length of the f i l t e r run i s general ly determined by the time

required t o reach a predetermined head loss or is set a t a predetermined

length of t i m e . Head loss con t ro l is the most commonly used because it

minimizes backwashing or f i l t e r renewal. Increasing t h e f i n a l head loss

p o i n t allows longer f i l t e r runs but r e s u l t s i n lower f i l t e r r a t e s and

170

poss ib l e s o l i d s breakthrough. The head loss p o i n t s e t t i n g i s a l s o

l imi ted by t h e maximum ava i l ab le system pressure .

Length and Duration of Backwash--

E f fec t ive c leaning of the f i l t e r media i n deep g ranu la r f i l t e r s i s

very important t o success fu l p l a n t opera t ion . I f the bed i s not cleaned

well, s o l i d s can accumulate, leading t o bed cracking and t h e formation

of mud b a l l s . Because a l l f i l t e r backwash water must be returned for

t reatment excess backwashing should be avoided. The a c t u a l requirements

f o r backwashing w i l l vary depending upon t h e na ture of the wastewater

and f i l t e r a i d s being used.

Chemical Conditioning of Inf luent--

F i l t e r performance can be improved by adding f i l t e r a i d s such a s

polymer and/or alum. These a c t t o s t rengthen t h e f l o c , c o n t r o l penetra-

t i o n of t h e f l o c i n t o t h e f i l t e r bed, and improve s o l i d s capture on

precoa t f i l t e r s . AS a r e s u l t f i n a l s o l i d s a r e reduced and al lowable

flow r a t e can be increased.

The amount of f i l t e r a i d required v a r i e s wi th i n f l u e n t wastewater

c h a r a c t e r i s t i c s and should be determined through t e s t i n g . The optimum

dose should be based upon des i red f i l t e r e f f l u e n t q u a l i t y a t the end of

the f i l t e r run.


The performance can be measured by t h e suspended s o l i d s l e v e l of

the l i q u i d e f f l u e n t from the f i l t r a t i o n process . A well-operated f i l -

t r a t i o n process produces an e f f l u e n t with low-suspended s o l i d s concen-

t r a t i o n s ; removal e f f i c i e n c e s of 90 t o 99 percent a r e common.


The guide shown i n Table 17 should be used t o t roubleshoot the fil-

t r a t i o n process . The t a b l e has been divided i n t o t h r e e sec t ions t o

171

TABLE I 1 TRATION TROUPLFSHODTING G U I E

IPRFSSURE FILIFRS-SAND1

PROBABLE CAUSE CHECK OR mlNIlOR REASON CORRECTIVE ACTION

OPERATING PROBLEM 1: High headlose through f i l t e r bed.

la. F i l t e r clogged with s o l i d s .

- Headloss thr-ough f i l t e r . - High headloss w i l l - Remove f i l t e r f r o @ S e r Y l C e

i n d i c a t e t h a t s o l i d s are and backwash. clogging f i l ter and f i l t e r requires bask- was hi ng .

- nuration of f i l t e r r"". - Duration of f i l t e r run - I f f i l t e r run is high CM-

as cmpared LO o t h e r pared to o t h e r s , then refer f i l t e r rune will i n d i - to 3e. 3e. and 3f. cate i f o t h e r problems e x i s t .

- s o l i d s concent ra t ion and - S o l i d s concent ra t ion and - Reduce s o l i d s concentration f l w r a t e s to f i l t e r . flow r a t e s r i l l i n d i c a t e (see C l a r i f i c a t i a n Trouble-

i f unusual c o n d i t i o n s nhaorinq Guide) or reduce exist i n p l a n t t h a t w i l l f l w - r e t e . shor ten ryn time of f 11 ters.

OPUIITlNG PROBLEM 2 : High headloss' thraugh f i l t e r jwt backwashed.

2a. I n s u f f i c i e n t backwash Of f i l t e r .

- Backwash f l w rate.

- Backwash pump. Check impeller clearance. check f o r partial blockage of Impeller.

- Packwash flow valve.

- Plugging in bottom spargers of f i l t e r s .

- I n s u f f i c i e n t rate of hack. wash water w i l l not a l l & s o l i d s to be removed from f i 1 ter.

- Defective pump is of ten the cause of loss of flow r a t e .

- Defec t ive flou valve ca" cause 105s of flou.

- Plugged spargerr; can

r e s t r i c t bckwash flow i"f" f i l t e r s .

- Increase backwaeh flow rate fo d e s i r e d value.

- If pump is d e f e c t i v e , i t w i l l have to be dismantled and repa i red .

TABLO I7 Icontlnued)

FILTRaTIDW TROURLSSHWTINC GUJDI' (PRESSUPF FILTRIS-SAND)

PROPABLB CAUSE C H ~ ow wimn RFaSON COrrRBeFIVF ACTIOII

- Defective valves in backwash system.

- Defectlve valves could - Check valves for; open and restrict backwash f l w closed positlo". valves Into filters. that are not functioning

properly ulll have to be dismantled and repaired,

- Inavfficient 1"Str"Dent - Deset Individual pcessure 1n*trument air pressure to backwash control alr pressure to backwash regulators to proper =yetem. COntrDlE O r VdlVes can netting.

l i m i t the operation of the system.

- II blockage of suctlon - Backflush suction lines of - Suctlo" plplng bf pumps or screens Inside back- plplng or dirty suction pump. Remove and clean

of backwash flar rates. wash water supply tank. screen* can CaYse a loss suction screens.

- Duratlon of backwash. - Complete backwash re- - Inc~ease backwash f i n e . quires specified flov

per1Cd Of time.

Defective timere may 1,ave rate for a specified t o be replaced.

Pilfer sklppinq backwash - If filter is colpletely - Vlsually ohserve a complete Cycle. automated a defective filter cycle. Have defective

clrcult can cause the circuits repaired. Packwash

backwash cycle. fllter to sklp the cella .mually.

I

, TA@lX 17 (Continued)

FILTRATIOII TROUPLFSHMTING GUIDE (GRANULAR DEEP P m FILTERS)

PROBABLE CAUSE CHFCK OR MONITOR REASMI CORRECTIVE ACTION

OPERATING PROBLEM I: High beadloes-through f i l t e r bed.

- Remove f i l t e r from service - High headloss w i l l i n d i -

i n f i l t e r and f i l t e r requires backwashing.

l a . F i l t e r needs - Headloss through f i l t e r . beckvaahing. c a t = e o l i d s accumulating and backwash it.

OPERATlLI: PROBLBI 2: High headloss through a f i l t e r j u s t beckwashed.

2a. I n s u f f i c i e n t backwash - Headloss greater than - Hlgh i n i t i a l headloss - Increase the s e t t i n g on t h e time to t h o r w g h l y normal and i n f l u e n t ( 1 - 2 f t ) even with IOU backwash t i m e r to provide

suspended s o l i d s con- clean the f i l t e r suspended s o l i d s . media. c e n t r a t i o n W i l l CsUSe

s h o r t f i l t e r rune.

longer lmckvaeh period.

2b. Inoperative s u r f a c e - v i s u a l l y inspect SYrfaCe - s u r f a c e wash arm must - Repair suface wash arm. wash arm ( i f wash arm. fYr" f r e e l y . a p p l i c a b l e ) .

Zc. Inoperable a i r ""ring system ( i f a p p l i c a b l e ) .

2d. Rapid accumulation of s o l i d s an t h e t a p sur face of t h e media due to:

1 ) Inadequate p r i o r c l a r i f i c a t i o n f o r sand f i l t e r e .

i i ) Excessive f i l t e r a i d dosages i n dual or nixrd- m-dia f i l t e r s .

- I n s p e c t pressure s e t t i n g - Preesure 8 e t t i n g could - Adjust pressure to Correct and aeration Dattern. be too hiah or la, and s e t t i n o .

a i r p r t h could be plugged.

~. - Unplug a i r ports.

. neaaure i n f l u e n t Tss - Hlgh TSS concent ra r in , - Improve c l a r i f i c a t l o " CO"Ce"trati0". w i l l reduce f i l t e r run perforwnce .

t i n e .

- Fualuste f l l t e r a i d - F i l t e r a i d dosage and - Reduce or eliminate f l l l r t dosage and type. type W i l l a f f e c t rate a i d dosage.

of headloss huild-up.

I

TI)PLF 17 (continued)

PILTltATION TROUPLESHO4Iffi GUIDE (GWNULAR DFFP RED FILTFRSI

CORRE€TIVE liCTION PROBABLE CAUSE enex OB IIONI~R REASON

OPErlATIK PROBLM 3: Shor t f i l t e r runs.

3a. See I t e m 2d. - see Item Zd. - See Item 2d. - See Item 2d.

3b. I n e u f f i c i e n t f i l t e r - Check backwash r a t e and - Lor backwash or s h o r t - see 1te. 2a. backwash . dura tion'. backwash d u r a t i m can

reduce f i l t e r runs.

- Check air scour or s u r f a c e - I n s u f f i c i e n t air scour or - See Items 2b and 2c. wash ( i f appl icable) . inoperable olr wash can

reduce filter mns.

OPERATING PROBLEM 4: Filter e f f l u e n t t u r b i d i t y increases suddenly b u t f i l t e r headloss 1s IOU.

l a . InadequBte dosage - nun jar t e s t s . of p 1 y r e r s as filter a i d .

- Jar tests rill i n d i c a t e - kdjusr polymer dosage. needed dosage of p I Y m P r .

4b. Coagulant feed eystem - Check chemical feeder.. - Excessive t u r b i d i t y can - Repair feeders . ualfuncfio". be caused by chemical

feeder malfunction.

4c. Change in i n f l u e n t - Check pn adjustment - Change i n pn can change - See pH Adjustment Trouble- c h a r a c t e r i s t i c s . and c l a r i f i c a t i o n . solubility and increase s h m t i n g Guide and

t u r b i d i t y . Sedi.entaLLon Trouble- e h m t i n g Guide.

OPERATING PROBLFII 5 : Wud b a l l formation.

5a. Inadequate backwash - Check backraeh f lowra te and - Packuasb f lowra te and - Provide Lackrash flow rate or s u r f a c e wash. duration. dura t ion may be too low. up to 20 g p / s q . f t . . and

Mi"t.31" proper auxi1ia.y scow ( s u r f a c e wash).

- C e ~ c n t a c c o u s media w i l l - Replace oedla. 5h. Media needs replacing. - I.ook f o r cementaceow media. reduce f i l t e r run time.

TABLE 17 (Continued)

FILTRATION TRWBLESllOOTING GUIDE (GRANULAR DEEP BEU FILTERS)

I I

CORRECTIVE ACTION PROBABLE CAUSE CHEK OR wniImR REASON

OPERATING PROBLEM 6: Loss of media during backwaahlng.

- Check backwash rate. - nigh backwash can cause - Reduce rate of backwash flow. 6a. El lCeBs lve f l w s used for backwashing. solids carryover.

- Excessive air e c a r can - Cut off the auxiliary scour 6b. A u x i l i a r y s c a r - Chedk hackrash prqram. excessive. cause Bollds carryover. 2 r l n . before t h e end of the

main backwash.

6c. Air bubbles a t t a c h - - Check for f l o a t i n g ing to anthracite anthraclte. cau*ing it to float.

- 1088 of a n t h r a c i t e can - Reduce alr s c a r . reduce effectiveness of filter syste..

I I 1 i I 1

TABLE I 7 (Continued)

FIL'IWATION TROUBLESHCWTING GUIDE (PRESSURE FILTERS-C~ETRlru;El

PROBABLE CAUSE CHECK OR MONITOR REISON CURRBCfIVE ACTION

OPERATING PROBLM 1: High differential pressure across filter.

- Replace cartridge. la. Cartridge plugged - Filter cartridge. - Plugged cartridge. with solids.

OPERATING PROBLEM 2: New cartridge but~salids breaking through. LDlr differential pressure.

2a. Defective -0- ring - Cartridge seal. , or cartridge eeal.

- salida could be byeassing - Replace seal. cartridge.

2b. Hole i n cartridge. - Cartridge. - Solide passing through ' - Replace cartridge. hole.

OPERRTINC PROBLEM 3: Low differential pressure-solids passing through cartridge - no holes In cartxidge - sea18 not defective.

3a. Wrong cartridge. - Cartridge type. - Particles t M *.a11 for - Change cartridge type. cartridge.

1 ; I

represent the d i f f e r e n t types of f i l t r a t i o n equipment. Each type of

equipment i s s u b j e c t t o its own kind of operat ing problems; most prob-

lems have to do w i t h high head loss, shor t c y c l e times, backwashing

problems, or inadequate s o l i d s capture.

. . .. . .. . . . -. . . -

178

SECTION 13

GRAVITY THICKENING

INTROO UCTION

Gravity thickening is a commonly used process f o r removing water

from sludges generated during wastewater treatment, thereby concentrat-

i n g them p r i o r t o d e w a t e r i n g o r d i s p o s a l . S ludges a r e t h i c k e n e d

pr imar i ly t o decrease the c a p i t a l and operat ing c o s t s of subsequent

sludge processing s t e p s by s u b s t a n t i a l l y reducing the volume. As shown

i n Figure 23, thickening from one t o two percent s o l i d s concentrat ion,

f o r example, decreases the sludge volume by f i f t y percent. h t r t h e r

concentrat ion t o f i v e percent s o l i d s reduces t h e volume t o one-f i f th of

its o r i g i n a l volume.

I n p l a n t s i n which thickening is the f i n a l treatment before sludge

d i sposa l , good operat ion of the g r a v i t y thickener can minimize f i n a l

d i sposa l c o s t s by reducing the volume of sludge t o be disposed. The

degree of thickening can a l s o a f f e c t the performance of downstream

processes, p a r t i c u l a r l y dewatering, since the e f f i c i e n c y of process

equipment is o f t en r e l a t e d t o the concentration of s o l i d s i n t h e feed

sludge.

THEORY OF OPERATION

Gravity thickeners work on the p r i n c i p l e t h a t sludge s o l i d s are

more dense than water and, under quiescent conditions, w i l l s e t t l e and

become concentrated over a period of time. As t h e sludge set t les , i t

forms an i n t e r f a c e w i t h the c l e a r water above. I n a thickener t h e

s o l i d s l a y e r below t h i s i n t e r f a c e is re fer red t o as the sludge blanket.

179

1009

w 50% 3

0 >

20% ------

F i e 23.

----- SOLIDS

\-

180

Thickening, concentrat ion, or compaction takes place i n the sludge

blanket as a r e s u l t of compression caused by the weight of s o l i d s above

and by the r e l ease of entrapped water. As the sludge becomes more

concentrated, the i n t e r f a c e moves downward. With t i m e , the i n t e r f a c e

moves more and more slowly s ince it becomes progressively more d i f f i c u l t

t o compact s o l i d s and release water entrapped i n the sludge. This

compaction is i l l u s t r a t e d i n Figure 24.

Sludge is removed by pumping from the bottom of the thickener once

the desired concentration, h a s been achieved. The supernatant , or c l e a r l i q u i d , is usual ly s e n t back t o an upstream treatment process since it

i s r a r e l y of good enough q u a l i t y t o be discharged d i r e c t l y .

Often i t is necessary t o add chemical coagulants or s e t t l i n g a i d s

t o the sludge t o improve its thickening c h a r a c t e r i s t i c s and t o maintain

a c l e a r supernatant. Polye lec t ro ly tes a re most commonly used f o r con-

d i t i on ing , although chemicals such as l i m e , alum, and f e r r i c ch lor ide

a r e a l s o used. The choices of conditioning a g e n t ( s ) and optimum dosage

r a t e s are made experimentally i n j a r t e s t s and through operat ing expe-

rience.

All g r a v i t y thickeners operate i n one of two ways: e i t h e r a s a

batch process or a s a continuous process. In batch thickeners , sludge

is pumped i n t o the thickener and then s u f f i c i e n t t i m e is provided f o r

s e t t l i n g . Once the des i red concentration is achieved, based upon e i t h e r

t h e sludge blanket ( i n t e r f a c e ) l e v e l or the amount of t i m e elapsed s i n c e

batch thickening began, c l e a r water i s decanted off the top u n t i l only

the sludge blanket remains. The sludge is then pumped t o the next

treatment process or t o f i n a l d i sposa l , making the thickener a v a i l a b l e

f o r a new batch of sludge.

Continous thickeners operate much l i k e c l a r i f i e r s . Sludge e n t e r s

a t mid-depth, usua l ly i n a c e n t e r well , and is removed by pumping from a

hopper o r co l lec t ion box a t t h e bottom. Sludge removal can be e i t h e r

continuous or i n t e r m i t t e n t w i t h sludge pumps operat ing on a t i m e d cycle.

The l a t t e r mode of operat ion is more common. The supernatant o r c l e a r

181

~~

TlME (minutes) Figure 24. Typical curve of etfect of time on sludge compaction.

18 2

l i q u i d e f f l u e n t continuously passes over weirs a t t h e top of t h e thick-

ener. The most bas i c difference between the two types of thickeners is

i n formation and behavior of the sludge blanket.

The concentration of s o l i d s within a continuously operated thick-

ener is shown i n Figure 25. There a r e three very d i s t i n c t zones which

develop i n such a thickener. The clear zone on top is composed of

l i q u i d t h a t eventual ly becomes the e f f l u e n t or supernatant escaping over

the w e i r s . This l i q u i d has a very low s o l i d s concentration. The n e x t

zone is c a l l e d the sedimentation zone; it is character ized by a f a i r l y

uniform s o l i d s concentration. Below the sedimentation zone is the

thickening o r compaction zone, character ized by an increasing s o l i d s

concentrat ion t o the poin t of sludge discharge.

The sludge blanket i n a continuous thickener is defined a s begin-

ning a t the top of the sedimentation zone. Looking i n t o an operat ing

thickener , one can o f t en see the sludge blanket below the c l e a r e f f l u -

ent. The sludge blanket depth is the main operat ional con t ro l t h a t a

treatment p l a n t operator has over a thickener. The blanket can be

lowered by increas ing the underflow sludge pumping r a t e (or reducing the

cycle time on i n t e r m i t t e n t pumping systems) and blanket depth can be

increased by decreasing the underflow r a t e .


Most g r a v i t y thickeners a r e c i r c u l a r tanks, e i t h e r concrete o r

s teel , with sloped bottoms t o he lp i n moving the thickened sludge t o a

hopper or c o l l e c t i o n box from which it is removed by pumping. Sludge

pumping is e i t h e r continuous or on an i n t e r m i t t e n t cycle , depending upon

the s i z e of the uni t . A r o t a t i n g rake-arm equipped with scrapers is usual ly provided t o push the sludge toward t h e cen te r and i n t o the

-. hopper. Sometimes p i cke t s a r e a t tached t o t h e rake arm t o gent ly agi-

t a t e t h e t h i c k e n e d s l u d g e and h e l p re lease w a t e r e n t r a p p e d i n t h e

sludge. Sludge e n t e r s t h e c l a r i f i e r i n a l oca l i zed area t h a t is en-

closed by b a f f l e s t o he lp maintain t h e s u i e s c e n t conditions required

f o r thickening. Relat ively c l ea r e f f l u e n t passes over w e i r s a t the top

183

184

and usual ly i s recycled back t o a preceding treatment process. Some

continuous thickeners a re rectangular o r even square basins , i n which

case they general ly have moving f l i g h t s t o push sludge toward one end

where it f a l l s i n t o a hopper o r co l l ec t ion box and is removed by pump-

ing.

Batch thickeners a r e s i m i l a r t o continuous thickeners except t h a t

they lack inf luent b a f f l e s and e f f l u e n t weirs. Since sludge i s added

batchwise r a t h e r than continuously, i n f l u e n t b a f f l e s a re no t necessary

i n batch thickeners. Likewise, e f f l u e n t weirs a re not needed. Batch

thickeners a l s o have sloped bottoms and a r o t a t i n g arm equipped with

p i c k e t s and scrapers t o he lp compact the sludge. The thickened sludge

is pumped d i r e c t l y o u t of the bottom a f t e r the supernatant is decanted.


The main objec t ive i n operat ing thickeners i s t o provide the thick-

e s t possible sludge without upse t t ing the q u a l i t y of the supernatant ,

This ob jec t ive usual ly is accomplished by providing the maximum t i m Q

possible f o r thickening t o occur, or i n other .words, the longest pOS-

s i b l e s o l i d s residence t i m e . In batch thickeners, one cont ro ls thQ

s e t t l i n g t i m e f o r each batch, taking i n t o account the number of batches

t h a t must be thickened i n a given period of t i m e .

I n continuous thickeners , cont ro l of t h e time ava i lab le f o r thick-

ening is usual ly achieved by ad jus t ing the underflow pumping r a t e or

cycle time t o maintain a cons tan t sludge blanket depth. Al te rna t ive ly ,

the underflow r a t e can be cont ro l led t o maintain a constant s o l i d s

concentrat ion i n the thickened sludge, which i n d i r e c t l y determines the

average s o l i d s residence time within the thickener.

Process Monitorinq

The main parameter used i n evaluat ing thickener performance i s the

s o l i d s concentration i n t h e thickened sludge.

185

Continuous gravi ty thickeners a re usual ly designed t o meet t w o condi t ions - maximum hydraul ic overflow r a t e and a maximum s o l i d s load-

ing. The overflow r a t e is general ly expressed a s a sur face loading r a t e

with u n i t s of gal lons per day per square f o o t of thickener surface area

( g a l / f t /day). A maximum overflow r a t e is establ ished t o in su re t h a t

adequate residence time is provided f o r a sludge blanket t o form and f o r

solids separat ion or c l a r i f i c a t i o n t o occur. Otherwise, t h e supernatant

would contain an unacceptably high s o l i d s concentration. The maximum

s o l i d s loading r a t e is determined by how w e l l the sludge compacts, and

is establ ished t o insure adequate time f o r s o l i d s thickening or compac-

t i o n t o occur i n the sludge blanket' layer. The s o l i d s loading i s a l s o

expressed a s a surface loading, with u n i t s of pounds of s o l i d s per

square f o o t of surface per day ( l b / f t /day).

2

2

The operat ion of grav i ty thickeners is f a i r l y simple, provided the

operator has s u f f i c i e n t data t o allow a determination of the cause of

problems when they occur. Even small changes i n sludge c h a r a c t e r i s t i c s ,

temperature, p H , flow rates, or other f a c t o r s can reduce thickener

performance. To determine the cause of a problem, a complete record of

operat ing da ta is necessary. with such information, the operator can

compare per iods of good and poor thickener performance and e a s i l y iden-

t i f y changes i n operat ing conditions which may be causing poor thicken-

ing. The data which should be monitored rout ine ly a re presented i n

Table 18.

In addi t ion t o the data co l lec ted above, s eve ra l calculated para-

meters can be used to charac te r ize and monitor thickener performance.

Among these a r e sludge volume r a t i o , s o l i d s loading r a t e , surface over-

flow r a t e , and sludge wasting r a t e .

The sludge volume r a t i o (SVR) is an est imate of the average s o l i d s

__._ .. -residence time or thickening time provided i n a continuous thickener.

I t is ca lcu la ted by dividing t h e sludge blanket volume by t h e underflow

pumping r a t e and i s usua l ly expressed i n u n i t s of hours or days.

186

TABLE 18

GRAVITY THICKENING PROCESS MONITORING REQUIREMENTS


1 . I n f l u e n t suspended s o l i d s Daily To determine s o l i d s concent ra t ion (mg/L). loading and removal

e f f i c i ency .

2. Underflow suspended s o l i d s Daily To c a l c u l a t e s o l i d s concent ra t ion (mg/L). loading t o downstream

t rea tment processes .

3 . Sludge blanket depth (€t ot Daily To determine whether me te r s ) . s ludge is accumulating

i n c l a r i f i e r .

4. Eff luen t or superna tan t Daily TO determine removal suspended s o l i d s concentra- e f f i c i ency . t i o n (mg/L).

5. Chemical dosage r a t e Daily To ad jus t /op t imize chemical dosage a s necessary.

(ppm or mg/L).

Continuously To c a l c u l a t e s o l i d s 3 . 6 . I n f l u e n t flow r a t e ( m /nun or gpd) . loading.

7. Underflow pumping r a t e Daily TO determine s ludge (gpm or m 3 /ruin). loading t o downstream

treatment processes .

187

The s o l i d s loading r a t e is the mass of suspended s o l i d s appl ied t o

the thickener each day divided b y ’ t h e thickener su r face a rea . It has

the u n i t s of l b / f t 2 day or kg/m 2 day.

The su r face overflow r a t e is the t o t a l volume of water passing over

the e f f l u e n t weirs i n a day divided by t h e th ickener su r face a rea . I t

is expressed i n u n i t s of y a l / f t day or L/m day. 2 2

The sludge wasting r a t e r e f e r s t o the mass of suspended s o l i d s re-

moved i n the underflow each day, expressed on a dry weight bas i s . It is

ca l cu la t ed by mult iplying the average underflow pumping r a t e by t h e

underflow suspended s o l i d s concent ra t ion and has u n i t s of lb/day o r

%/day.


The fol lowing examples show how the var ious opera t ing parameters

a r e ca lcu la ted . Consider a g rav i ty thickener f o r which the fol lowing

da ta have been co l l ec t ed .

( 1 ) Thickener diameter = 20 f e e t

( 2 ) Average sludge blanket depth = 8 f e e t

( 3 ) Underflow sludge pumping is i n t e r m i t t e n t . Pumps a r e on f o r 10

minutes every ha l f hour and pump a t a r a t e of 15 ga l lons per

minute.

(4) I n f l u e n t f low r a t e = 25 gpm

( 5 ) I n f l u e n t suspended s o l i d s (TSS) concent ra t ion = 15,000 mg/L

(6) Average underflow pumping r a t e = 5 gpm

(7) Underflow suspended s o l i d s concent ra t ion = 72,000 mg/L

188

Sludge volume Ratio-- 2 Thickener Surface Area = (Diameter/Z) (3.14) = 314 square feet

sludge Blanket volume = (Thickener surface Area)(Blanket Depth)

= (314 square feet)(8 feet) = 2,512 cubic feet in blanket

Conversion From Cubic Feet to Gallons = 7.48 gallons/cubic foot

(2,512 cubic feet)(7.48) = 18,792 gallons in blanket

Average underflow Pumping Rate = (Pumping Rate)(Pumping Time)/

(Cycle Time) = (15 gpm)(10 minutes)/(30 minutes) = 5 gpm

SVR = (Sludge Blanket volume)/(Average Underflow Pumping Rate)

= (18,792 gallons)/(5 gpm) = 3,758 minutes

(3,758 minutes)(l,440 minutes per day) = 2.61 days

solids Loading Rate-- 2 Thickener surface Area = (Diameter/Z) (3.14) = 314 square feet

Daily Flow = (25 gallons/minute)(l,440 minutes/day) = 36,000 gpd

(36,000 gpd)/(1,000,000) = 0.036 Mgd

Mass of Influent solids per Day = (Infl. Flow)(Infl. TSS)(8.34)

= (0.036 Mgd)(15,000 mg/L)(8.34) = 4,504 lb solids per day

Solids Loading Rate = (Mass of Influent Solids/Day)/(Thickener

Surface Area) = (4,504 lbs solids per day)/(314 sq ft)

= 14.3 lb/sq ft day

Surface Overflow Rate--

Daily Effluent Flow = (Influent Flow Rate)-(AVerage underflow

Pumping Rate) = (25 gpm) - ( 5 gpm) = 20 gpm

(20 gpm)(1,440 minutes per day) = 28,800 gpd

surface Overflow Rate = (Daily Effluent Flow)/(Thickener surface Area) = (28,800 gpd)/(314 square feet) = 92 gal/sq ft/day

Sludge Wasting Rate--

Daily Volume of sludge underflow = ( 5 gpm)(1,440) = 7,200 gpd

(7,200 gpd)/(1,000,000) = 0.0072 Mgd -. ___ __ .

Sludge Wasting Rate = (Daily Underflow Volume)(Underflow Solids

Concentration)(8.34) = (0.0072 Mgd)(72,000 mg/L)(8.34) = 4,323 lb solids per day

189

For the i n f l u e n t mass of s o l i d s = 4,504 lb/day,

(4,323)/(4,504) x 100 = 96.0 % of s o l i d s a r e i n underflow

Process Control. S t r a t e g i e s

The s implest type of thickener t o operate is a batch thickener

s ince the only opera t iona l cont ro l is the amount of time provided before

decanting off the c l e a r l iquid. The optimum time must be determined

from operat ing experience and w i l l depend upon the des i red underflow

s o l i d s concentration and the t o t a l amount of sludge t h a t must be thic-

kened i n a given period of t i m e with the ava i lab le equipment.

I n a continuous g r a v i t y thickener, the depth of the sludge blanket

(or pos i t ion of the sludge-liquid i n t e r f a c e ) is t h e main process v a r i -

ab l e used f o r control . The sludge blanket can be lowered by increasing

t h e underflow sludge pumping r a t e (or reducing the cycle t i m e on i n t e r -

m i t t e n t pumping systems) and can be r a i sed by decreasing the underflow

ra t e . Lowering t h e sludge blanket e s s e n t i a l l y reduces the average

s o l i d s residence time within the thickener and, a s is the case i n batch

thickeners, t h i s usual ly r e s u l t s i n a lower s o l i d s concentration i n the

underflow. Increasing the sludge blanket depth r e s u l t s i n a longer

s o l i d s residence t i m e which usua l ly r e s u l t s i n a higher underflow s o l l d s

concentration.

I

One advantage of maintaining a lower sludge blanket is t h a t some

e x t r a sludge s torage capaci ty i s provided within t h e thickener f o r per-

i ods of high sludge loadings or for occasions when downstream equipment

is being serviced or repaired.

Sometimes thickeners a r e operated t o maintain a minimum sludge

volume r a t i o (SVR). While t h e SVR is he lpfu l i n understanding t h e

operat ion of a thickener, i t s use as an operat ional control parameter is

l imited unless the underflow s o l i d s concentrat ion and other f a c t o r s a r e

a l s o considered. For example, the SVR could increase while a t the save

t i m e fhe underflow s o l i d s concentration drops, i f the i n f l u e n t s o l i d s

___

190

concentration decreases or if a problem occurs w i t h the operation of t h e

chemical addi t ion system.

Sol ids loading and s o l i d s wasting r a t e s are use fu l i n s eve ra l ways.

The s o l i d s wasted each day should be almost the same a s the s o l i d s fed

t o t h e t h i c k e n e r . I f n o t , one of t h r e e t h i n g s must be happening.

Ei ther s o l i d s a r e accumulating i n the sludge blanket , s o l i d s a r e being

l o s t from the sludge blanket, or s o l i d s a r e leaving the thickener with

the e f f luen t . On a short-term b a s i s , only the l a s t of these p o s s i b i l -

i t i e s is r e a l l y t o be avoided, s ince a high s o l i d s concentration i n t h e

supernatant d e f e a t s the purpose of thickening and can cause problems i n

o t h e r treatment processes. Over a longer period of t i m e changes i n the

sludge blanket can a l s o cause problems. A s discussed e a r l i e r , t he re a r e

l i m i t s t o the range i n which the sludge blanket can be maintained w i t h -

o u t reducing underflow s o l i d s concentration too much or increas ing

s o l i d s t o the p o i n t t h a t underflow pumping becomes d i f f i c u l t .


Performance of g r a v i t y thickeners can vary considerably depending

upon design f ea tu res such a s s o l i d s loading and surface overflow ra tes .

Another f a c t o r a f f e c t i n g performance is the nature of the sludge. Metal

hydroxide sludges a r e l i g h t e r and do not thicken a s w e l l a s lime sludges

t h a t a r e high i n calcium carbonate concentration. A gravity.,:thickener

i s designed t o achieve about 95 percent s o l i d s capture and w i l l produce

a thickened sludge ranging from 4 t o 10 percent so l id s . Sludges much

th icker than 10 percent may be achievable, but are o f t en d i f f i c u l t t o

pump and handle. A s a minimum, the thickener should about double t h e

s o l i d s concentrat ion of the sludge.

4-


The problems which a r e commonly associated with g r a v i t y thickener

operat ion have been summarized i n the form of a troubleshooting guide i n

Table 19. Speci f ic d e t a i l s on pumps, sludge rake d r ives , and other

191

TABLE 19 GRAVITY THICKFNING TROUBLFSHMTING GUIDF

i

PROBABLE CAUSE CHECK OR NONITOR REASON CORFFCTIVE ACTION

OPERATING PROBLUl I: lh ickened s l u d g e is tm t h i n .

l a . U n d e r f l w pumping r a t e is t W g r e a t .

- Blanket d e p t h and p v l p l n g r a t e .

lb. S u r f a c e o v e r f l w rate is too g r e a t i n cont inuous . thickener.

1.2. l n s v f f i c i e n t t i m e

ba tch t h i c k e n e r s . for s e t t l i n g I"

P W N

Id. S h o r t - c i r c u i t i n g of f l w through th ickener .

le . Improper chemical dosage or wrong chemical be ing used.

I f . Sludge c o l l e c t i o n or conveying equipmeot n o t func t ion i i ig proper ly .

- Check for high solids in e f f l u e n t and i n c r e a s e d f e e d flow rate.

- Check t h o time a l l w e d for e e t t 1 i n g a g a i n s t previous d a t a and perform ba tch s e t t l i n g tests.

- Check for h igh e f f l u e n t Bolida concentrattone.. I n s p e c t s u r f a c e Of c l a r i f i e r f o r ev idence of uneven f l a w and s o l i d s d ischarge . Check level Of w e i r s .

- Check chemical a d d i t i o n system for malfunct ion and check chemical flow.

- Check f o r "on-unifocm b l a n k e t and r a t - h o l i n g lconingl. Check to see t h a t d r i v e is working

check f o r broken rake or pickets .

p roper ly . use pro& to

- Low b l a n k e t I n d i c a t e s pY.pl"g r a t e is tm high , m y be confirmed I f pumping r a t e hae been i n c r e a s e d r e c e n t l y .

- Increased o v e r f l a r r a t e I e S U l t s f r o l h igher f l o w and u s u a l l y r e s u l t s !n g r e a t e r solids losses i n t h e e f f l u e n t .

- To determine i f s u f - f i c i e n t time is being provided f o r s e t t l i n g and I f s l u d g e c h a r a c t e r - istics have changed.

- S h o r t c i c c u i t i n g reduces the time a v a i l a b l e for s e t t l i n g and z e s u l t 6 in higher e f f l u e n t s o l i d s .

- Decrease pumping of th ickened s l u d g e to main t a i n a b l a n k e t volume between 1/4 and 1/2 o f , total t h i c k e n e r volume.

- Reduce flow to t h i c k e n e r I f p s s i b l e t o keep a maximum SUrface o v e r f l Y

r a t e of 600-800 gpd/ff . (See a l s o Items l e and 3 a ) .

9

- I f necessary and possible. a l l - more t i n e per bard, f o r S e L t l l n g and tlbickenlsq to OECUI.

. Level w e i r s and repair or a d j u s t influent baffles LO e l i m i n a t e s h o r t circui Ling.

- Problem may be e q u i p e n t

s e t t i n g . I f n e i t h e r , t h e dosage or chemical used must be changed.

mal func t ion or improper

- Sludge M Y be accumu- l a t i n g e l sewhere in t h i c k e n e r than t h e s ludge hopper or broken r a k e s and pickets may ccuee water to be entrappfd i n th ickened s ludge .

. Repair, r e p l a c e , or a d j u s t equ1p.e-t I f t h l s Is t h e problem. Otherwise,

new c h e d c a l and/or dosage r a t e s to be used.

perfor. j a r tests to select

~ mepair or r e p l a c e broken e q a I p m m t , d r o i n tank i f necessary .

TABLF 19 IContinuedI

GRRVITY THICIEllING TROUOLESHOWrILY: GUIDF

PRCl8ABLE CAUSF CHFCK CR IIOPImR RFASO11 M)RRMTIYF ACTION

OPERITING PROB'm 2: Torque overload OD sludge callectlon equipment.

P W w

~~~ ~

- Check sludge blanket depth - Sludge can get tw thick - Agifafe sludge in front 2a. neavy .cc"m"l.tion of sludge in M t t o m and underflow solids to -"e. especially i f Of ca11eerion equipent

sludge too thick. or air. of thickener - concentration. blanket is very thick. using rod. water jet.

2b. Foreign object - Inspect a11 vi%ible parts - To determine the presence - Remove foreign object. jamming equipment. for foreign obiects. and location of foreign Use grappling ho&, i f

Probe along front Of under- objects. possible, for objects water CollectiO" equipment. underwater. Drain tank

if nereeeary.

Zc. lechanlcal prob- - Check malntenabce records. - P w r maintenance, such as - Correct any problems lems such as lubricant reselvoirs. and improper lubrication, can and revise routine

lubrication. increases torqYe. to prevent future problems. cause friction that oaintenance procedures to insufficient free movement *f part*.

OPERATING PROBLEM 3: Plugging of sludge P m P S 9r pipelines.

3a. Underflow sludge - Check blanket level - Sludge can become tM,

thick to pump. concentration t m and underflow Bolids

Insufficient high due to concentration.

p"mpin9.

OPERATIWG PROBLEH 4; Poor solids capture.

- Flush mf plugged lines with water. making sure a l l valves are open. If this doesn*t work, try 2a above. Increase underflow pumping rate or decrease punp cycle time fo reduce blanket level and underflow solids concentration.

- See I t e m Ih. - See Item lb. 4a. Overflo" '=!e - See Item lb. too lsigh.

4tl. Shnrl circuiting - see Item Id. - See I t e m Id. - See I f e l Id. of flow through L hi ckener .

TABLE 19 (Continued)

GRAVITY THICKENING TROUBLESHWING GUIDE

PROBlBLE CAUSE CHECK OR W3K)NITOR R E S W CORRECTIVP *CrION

I C . Improper chemical - see Item le. dosage 01 wrong chemical.

- see Itel ,e. - see It-. le.

I d . Exceesively high - Check blanket level and - A high sludge blanket - Increase underfla underflow pumping rate. can =="Be carryover of pumping to reduce blanket

solids i n effluent. level to half or less of Sludge blanket due to Insufficient underflow pumping rate.

total thickener volume.

., OPERATING PROBLEW 5: Changing sludge blanket level at Constant underflaw pumping rate.

5a. Changing influent - Influent, effluent, and - To isolate the causes of - If underflow solids con- f l a ro tes or underflow rates and aolide the fluctuations and centration and effluent solids leadings. concentrations. determine how rapidly quality are acceptable. do

they occur. nothing. If either is inadequate. make adjustmnrs In underfla rate to achieve an acceptable average condition Or, If possible, even out flow rate to thickener.

equipment can be found i n the nanufacturer's l i t e r a t u r e . The two pre-

dominant problems considered in Table 19 are inadequate th ickening of

the s ludge and problems with s ludge blanket l e v e l .

- ... . - - - - - ...-. ~ ..... . . . .. . .. . . -. . . . . . . ~ ~

195

SECTION 14

BELT FILTER PRESSES

The treatment of metal p l a t i n g wastewater o f t en generates d i l u t e

sludges during treatment processes such a s metal p rec ip i ta t ion . These

sludges a r e d i f f i c u l t t o dispose of because of t h e i r volume and t h e i r

l i q u i d nature. It is t he re fo re advantageous t o dewater t h e sludges.

Dewatering reduces volume and weight of sludges and allows them t o be

handled a s s o l i d s .

A b e l t f i l t e r p ress is a device f o r accomplishing sludge dewater-

ing. The process produces a solids cake an'd a r e l a t i v e l y . c l e a n water

stream c a l l e d the f i l t r a t e . Often belt f i l t e r presses used f o r dewater-

i ng a r e preceded by a g r a v i t y thickener o r centr i fuge t o increase t h e

s o l i d s concentration of the sludge fed t o the f i l t e r . This combination

helps t o maximize t h e s o l i d s content of t h e cake produced.

THEORY OF OPERATION

F i l t r a t i o n f o r dewatering is a process whereby s o l i d s a re separated

from a l i q u i d by passing t h e l i q u i d through a porous medium on which t h e

s o l i d s remain, eventua l ly bui lding up t o form a cake t h a t is removed f o r

disposal. The formation of a s o l i d s cake and the f a c t t h a t s o l i d s a r e

removed pr imar i ly on the surface of the medium r a t h e r than throughout

i ts depth a r e t w o d i s t inguish ing f e a t u r e s t h a t charac te r ize t h i s type of

f i l t r a t i o n . Both f ea tu res a r e important s ince t h e ob jec t of dewatering

is t o maximize t h e recovery of s o l i d s i n a concentrated form.

____

196


The b e l t f i l t r a t i o n process includes three bas ic opera t iona l

stages: chemical conditioning of t h e feed s l u r r y , g r a v i t y drainage t o a nonfluid consistency, and compaction of the previously dewatered sludge.

These s t ages a r e shown i n the simple b e l t f i l t e r p res s shown i n i n

Figure 26. Although present-day b e l t presses a re more complex, they

follow the same p r i n c i p l e s indicated i n Figure 26.

Chemical conditioning is e s s e n t i a l f o r successful and c o n s i s t e n t

performance of the b e l t f i l t e r p re s s , a s f o r other dewatering processes.

A s shown i n Figure 26, a f locculan t (u sua l ly an organic polymer) i s

added t o the sludge p r i o r t o the sludge being fed t o the b e l t p re s s .

Free water then d ra ins from the conditioned sludge i n the g r a v i t y drain-

age s tage of the press.

Following g r a v i t y drainage the sludge e n t e r s a contac t zone usua l ly

cons is t ing of t w o b e l t s . The sludge is i n i t i a l l y g e n t l y pressed between

the t w o b e l t s and car r ied tpwards a safies of r o l l e r s with genera l ly de-

creasing diameters. This stage subjec ts the sludge t o continuously in -

c reas ing pressure and shear forces. Progressively, more and more water

is expel led throughout the r o l l e r s ec t ion t o the end where the cake i s

discharged. A sc raper blade is o f t e n employed f o r each b e l t a t the d i s -

charge poin t t o remove the cake from the b e l t s .

After scraping the cake from the b e l t s , the b e l t s a r e subjected t o

a s p r a y wash t o c l e a n a n y remain ing s o l i d s from t h e b e l t s u r f a c e .

Usually t r e a t e d wastewater is used as a water source. The spray water

is then combined with the f i l t r a t e and returned t o the i n f l u e n t of t h e

t reatment plant.

OPERATIONAL PROCEDLRES

The objec t ive of operat ing a b e l t f i l t e r p re s s is t o concentrate

sludges i n t o a dry cake s u i t a b l e f o r disposal . The f i l t e r p re s s a l s o

produces a f i l t r a t e t h a t must be maintained r e l a t i v e l y f r e e of so l id s .

197

ROTARY ORUM CONDITIONER

HORIZONTAL ORAINAGE SECTON

Figure 26. A simple belt filter press.

198

Excessive s o l i d s i n the f i l t r a t e w i l l g e t recycled through the t reatment

process and may adversely a f f e c t other treatment operations.

Process Moni t o r i n q

The p r i m a r y c r i t e r i a f o r e v a l u a t i n g t h e performance of a b e l t

f i l t e r p re s s a r e percent s o l i d s i n t h e f i l t e r cake and t h e percentage of

t o t a l s o l i d s en ter ing the f i l t e r p ress t h a t a r e captured i n the cake.

The c a l c u l a t i o n of these parameters and other parameters necessary f o r

successfu l ly operat ing a f i l t e r . p ress a re l i s t e d i n Table 20.

The above parameters, with the possible exception of f i l t r a t e flow

r a t e and t o t a l dissolved s o l i d s concentration, should be measured any

time the b e l t press i s used. F i l t r a t e flow r a t e and t o t a l dissolved

s o l i d s concentrat ion may be measured l e s s f requent ly b u t s t i l l should be

measured approximately monthly. In addi t ion the b e l t speed and the type

and appl ica t ion r a t e of chemical condi t ioner should be recorded.


Inf luent Suspended Solids--

I n f l u e n t suspended s o l i d s a r e of ten d i f f i c u l t t o measure d i r e c t l y

i n thick sludges. Therefore t h i s parameter is of ten calculated by

measuring the t o t a l s o l i d s i n the i n f l u e n t and subt rac t ing the f i l t r a t e

t o t a l dissolved so l id s . I f poss ib le the f i l t r a t e t o t a l dissolved s o l i d s

sample should be co l lec ted p r i o r t o the addi t ion of wash water.

Percent Capture--

P e r c e n t c a p t u r e i s t h e p e r c e n t a g e of i n f l u e n t s o l i d s t h a t a r e

captured i n the cake. Percent capture i s ca lcu la ted a s follows:

F i l t ra te SS x F i l t r a t e Flow) Inf luent SS x Inf luent Flow - .. __ PI Capture = 100 x ( 1 -

An a l t e r n a t e , but less s e n s i t i v e technique, f o r ca lcu la t ing percent

capture is a v a i l a b l e when d e t a i l s about the f i l t r a t e a r e unavailable:

199

TABLE 20

BELT FILTER PRESS PROCESS MONITORING REQUIREMENTS


1 .

2.

3 .

4.

5 .

6.

7.

8 .

I n f l u e n t flow r a t e (gpm o r L / m i n ) .

F i l t r a t e flow r a t e (gpm o r L / m i n ) .

I n f l u e n t t o t a l s o l i d s concent ra t ion (mg/L).

F i l t r a t e suspended s o l i d s concent ra t ion (mg/L).

Percent s o l i d s i n cake.

N e t cake production r a e ( l b / h r , kg/hr , yd /hr , o r m / h r ) .

PH

f 3

Chemical condi t ioner type and concent ra t ion .

Continuously - To s e t chemical dosages and t o set flow r a t e .

Monthly - To determine mass ( s o l i d s ) loading of f i l t ra te ,

Dai ly - To set chemical dosage and t o set flow r a t e .

Monthly - To determine mass ( s o l i d s ) loading of f i l t r a t e ,

- TO determine percent Weekly capture .

weekly - To s e t loading r a t e .

COntinUOUSly - High or low pH values can inc rease s o l u b i l i t y of p a r t i c l e s .

Weekly - To s e t chemical dosage.

200

Wet Cake Production Rate x Percent Cake Sol ids Inf luent SS x I n f l u e n t Flow % capture =

When using the above equations care must be taken t o convert all para-

meters t o s imi l a r un i t s .

A s an example, if a b e l t f i l t e r p ress received 50 l i t e r s /min of

20,000 mg/L sludge and produced 190 kgfiour of 30% cake the percent

capture would be:

= 95 190 kq/hr x (1,000,000 mg/kg) x (hr /60 min) x 30 20,000 mg/L x 50 l/min % Capture =

Conditioning Agent Application Rate--

As previously described, the pretreatment of the sludge with chemi-

c a l condi t ioning agents is e s s e n t i a l f o r good dewatering. Typically,

the condi t ioning agent is a polymer which is purchased i n a dry or

l i q u i d form. The polymer i s then d i l u t e d with water t o a concentrat ion

t h a t t y p i c a l l y ranges from 1000 t o 5000 mg/L (0.1 t o 0.5 pe rcen t ) . The

operator is advised t o see manufacturer 's recommendations f o r concentra-

t ions. T h i s d i l u t e polymer so lu t ion is then appl ied t o the sludge on a

bas i s of mass of polymer t o mass of sludge suspended so l id s . The appl i -

ca t ion r a t e of condi t ioner can be ca lcu la ted a s follows:

Polymer Dosage x In f luen t Flow x In f luen t SS Polymer Solut ion Concentration Feed Rate =

where polymer dosage is the r a t i o of the weight of polymer appl ied per

weight of i n f l u e n t s o l i d s and feed r a t e is the polymer pumping ra te .

A s an example, suppose a sludge stream conta in ing 20,000 mg/L (2 .0

Percent) s o l i d s i s being dewatered using a b e l t f i l t e r p re s s a t a r a t e

of 50 l i t e r s per minute. J a r t e s t i n g has ind ica ted t h a t a polymer

dosage r a t e of f i v e pounds of polymer per ton of dry s o l i d s i s the

optimum dosage r a t e and the manufacturer recommends t h a t t h e polymer be

d i l u t e d and premixed with water t o a concentrat ion of 5000 mg/L (0.5%

-- ---solution). The polymer feed rate could then be ca lcu la ted a s follows:

5 l b ton 50 20,000 mg - ton 2000 l b - min L

= 0.5 L/min 5000 Feed Rate =

mg/L

201

One must use extreme care t o make sure t h a t t h e proper u n i t s and

conversion f a c t o r s a re used.


The operator of a b e l t f i l t e r p ress has s e v e r a l operat ing var iab les

t h a t a r e under h i s d i r e c t control . These var iab les and t h e i r e f f e c t on

dewatering a r e l i s t e d below.

Sludge Feed Rate--

Increasing sludge feed r a t e w i l l increase machine throughput i f the

b e l t speed is high enough, but w i l l a l s o cause an increase i n cake

moisture . Conditioning Agent--

Proper s e l e c t i o n of conditioning agent can dramatical ly increase

s o l i d s capture and percentage of s o l i d s i n cake. Select ion of polymer

is usua l ly made based on ja r test r e s u l t s .

Conditioning Agent Application Rate--

A s polymer dosages increase, both cake s o l i d s and s o l i d s capture

increase u n t i l an upper l i m i t is reached. If sludge is undercondi-

t ioned, improper drainage occurs i n t h e g r a v i t y drainage sec t ion , and

e i t h e r ex t rus ion of inadequately drained s o l i d s from t h e compression

sec t ion or uncontrolled overflow of sludge from the drainage sec t ion may

occur. Most b e l t presses can be equipped with sensing devices which can

be s e t t o automatical ly shu t off the sludge feed flow i n case of under-

conditioning. Both underconditioned and overconditioned sludges can

b l ind the f i l t e r media. In add i t ion , overconditioned sludge d ra ins so

rap id ly t h a t s o l i d s cannot d i s t r i b u t e across the b e l t . Vanes and d i s -

t r i b u t i o n weirs included i n the g r a v i t y drainage sec t ion help a l l e v i a t e

the problem of d i s t r i b u t i o n of overconditioned sludge across the b e l t .

Inclusion of a sludge blending tank before the b e l t p re s s can a l s o

reduce t h i s problem.

.___

202

Bel t Speed--

Increasing b e l t speed can inc rease machine throughput bu t w i l l .

t y p i c a l l y r e s u l t i n lower cake s o l i d s conten t because both g r a v i t y

drainage t i m e and press time a r e decreased. Conversely, decreas ing b e l t

speed should r e s u l t i n a d r i e r cake.

Be l t Tension--

Increas ing b e l t t ens ion w i l l promote a d r i e r cake bu t s o l i d s cap-

t u r e t y p i c a l l y w i l l decrease and b e l t wear w i l l increase .

Washwater Flow Rate--

~n i nc rease i n washwater flow and/or pressure can inc rease cake

s o l i d s concent ra t ion i f t h e washwater was not adequately c leaning the

b e l t , bu t a l s o inc reases the flow requ i r ing r e t r ea tmen t back t o t h e head

end of the p l a n t . The flow r a t e required f o r b e l t washing i s usua l ly 50

t o 100 percent of t h e flow r a t e of sludge t o t h e machine and t h e pres-

sure is t y p i c a l l y 690 kPa (100 p s i ) or more. The combined f i l t r a t e and

b e l t washwater flow is normally about 1.5 times t h e incoming s ludge

flow.

B e l t Material--

B e l t s wi th d i f f e r e n t pore s i z e s a r e a v a i l a b l e . Increas ing t h e

po ros i ty w i l l increase t h e s o l i d s conten t of t h e cake, bu t w i l l decrease

the percent capture .

pH--

The pH of t h e s ludge usua l ly has l i t t l e e f f e c t on polymers. The p~

can, however, s i g n i f i c a n t l y a f f e c t t h e s ludge ma te r i a l . Most s ludges

a r e metal p r e c i p i t a t e s and changing the pH can r e s o l u b i l i z e a s i g n i f i -

c a n t f r a c t i o n of t h e sludge. This r e s o l u b i l i z a t i o n is most l i k e l y t o

happen if sludges from two sepa ra t e p r e c i p i t a t i o n s a t d i f f e r e n t pH

l e v e l s were mixed.

Polymer Addition Point--

It i s e s s e n t i a l t h a t the polymer and s ludge be proper ly mixed.

Mixing usua l ly occurs i n a small tank p r i o r t o the b e l t f i l t e r p ress .

203

However, i n some systems polymer is in j ec t ed t o the feed pipe t o the

f i l t e r press. Changing the in j ec t ion poin t can s i g n i f i c a n t l y change the

e f fec t iveness of the polymer. Typically, increasing the dis tance be-

tween the i n j e c t i o n poin t and the press w i l l improve performance.

I n f l u e n t Sol ids Concentration--

In general , a t h i cke r incoming sludge w i l l produce a d r i e r cake.

However, the i n i t i a l s o l i d s concentration is no t normally a process

cont ro l variable. I t is customary, un less spec ia l condi t ions apply, t o

d e l i v e r a s thick a sludge as p r a c t i c a l t o t h e b e l t f i l t r a t i o n un i t .


The performance of a properly operated b e l t f i l t e r p r e s s w i l l vary

s i g n i f i c a n t l y depending pr imar i ly upon the mater ia l being dewatered.

Metal carbonates, such a s those generated during lime p r e c i p i t a t i o n ,

dewater e a s i l y with cake s o l i d s concentration i n t h e range of 35 t o 45%

and c a p t u r e r a t i o s of o v e r 95%. Hydroxide s l u d g e s u s u a l l y do n o t

dewater a s w e l l a s carbonates. Typical cake s o l i d s concentrat ions w i l l

be 20 t o 30% with capture r a t i o s of 90 t o 95%.


The troubleshooting guide f o r the b e l t f i l t e r p re s s process is

presented i n Table 21. The major problem i n b e l t f i l t e r press operat ion

is too t h i n or watery a sludge cake. The cor rec t ive methods involve

e i t h e r mechanical cor rec t ions ( b e l t speed, appl ica t ion r a t e , e t c . ) or

chemical changes (adjustments t o the f loccula t ion process, e t c . ) . The

problem of excessive b e l t wear is a l s o addressed is Table 2 1 .

204

TABLE 2I BELT FlLTER PRESS

TROUBLESIIO(II1NG GUlD€

PRonant.6 CAUSE CHECK OR M0U)NIlOR AWSON CORAWTIVE ACTION

~~

OPERATING PROBLEM 1: Dewatered s ludge n o t th lck enough.

~~~~

la . sludge appllc?.tlon - Check s ludge Pumping r a t e . - Ercesslve pumping CaUQes - Adjust I n f l u e n t sludge rate tca hlgh. e r c e s s l v e l y th ick cake pumping r a t e .

t h a t does not have tlne to dewater.

Ib. Belt speed to0 hlgh.

15. I n c o r r e c t polymer. dose.

N 0 In

Id. Poor polymer-sludge mixing

le. &It tenelon h a d e - PUate.

I f . Improper belt type.

19. I n s u f f l c l e n t wash l a t e r or preesure.

- Check b e l t speed.

- Check polymer type. hi- t l a l d l l u t l o n . end flow rate. Determine polymer

pouhd polymer to ton of d r y s o l l d s bae ls .

a p p l l o a t l o n r a t e 0" a

- Check polymer-eludge .iring.

- Check belt tenslon.

- Check belt type and manu- f a c t u r e r ' s recomnendarlonn.

- check b e l t t o 8ee I f . If I* clean a f t e r

washing.

- sludge 1; c o r r l e d t h r m g h b e l t press before I t ha8 t h e to dry.

- proper p o l y r e r dosage I n e n s e n t l a 1 far e f f e c t i v e dewo t e r l n g .

- 1.proper .i"lW O f p o l y l e l and s ludge can completely negate t h e b e n e f l t s of galy.eT..

- 1"sYffIEIe"t t e n d o n *I11 not torce "ater o u t of cake.

- B e l t poroe1ty can d i r e c t l y a f f e c t cake a o l l d s content.

- A d i r t y belt can l n h l b l t t h e t r a n s p o r t of water through the belt dur lng deWater109.

- Adjus t belt epeed.

- ~f polymer dose le much less or much g r e a t e r than opt1.u. dose. performance u l l l decrease. Use j a r test procedure to d e t e r - mine opt1.u. dose.

- Enhance mlxlng. Change po1y.e~ a d d l t l o n p i n t .

- ,"crease belt tenslo".

- Change belt.

- Increase "dter preesure and/or f lov .

20

6

SECTION IS

VACUUM FILTRATION

INTRODUCTION

Vacuum f i l t r a t i o n is a process used for dewatering sludges generat-

ed during wastewater treatment. The two products of vacuum f i l t r a t i o n

a r e a r e l a t i v e l y c l e a r l i q u i d , the f i l t r a t e , and a s o l i d s cake which i s s u i t a b l e f o r f i n a l disposal . Depending upon the type of sludge being

dewatered, vacuum f i l t r a t i o n can produce a cake t h a t is t y p i c a l l y be-

tween 20 and 30 percent s o l i d s . Often, vacuum f i l t e r s used f o r dewater-

i ng a r e preceded by a g r a v i t y thickener o r a cent r i fuge t o increase t h e

s o l i d s concentrat ion of the sludge fed t o t h e f i l t e r , a combination

which helps t o maximize the s o l i d s content of the cake produced.

THEORY O F OPERATION

F i l t r a t i o n f o r dewatering is a process whereby s o l i d s a r e separated

from a l i q u i d by passing the l i q u i d through a porous medium on which the

s o l i d s remain and eventual ly bui ld up t o form a cake t h a t is removed f o r

disposal . The formation of a s o l i d s cake and the f a c t t h a t s o l i d s a r e

removed pr imar i ly on the surface of the medium r a t h e r than throughout

i t s depth are two d is t inguish ing f e a t u r e s t h a t charac te r ize t h i s type of

f i l t r a t i o n . Both f e a t u r e s are important s ince the objec t of dewatering

is t o maximize the recovery of s o l i d s i n a concentrated form.

Two s t e p s a re involved i n the vacuum f i l t r a t i o n process. I n the

f i r s t , a solids cake is formed when a port ion of the water i n the sludge

d ra ins through the f i l t e r medium leaving a more concentrated but s t i l l

very w e t sludge l a y e r o r cake behind. In t h e second s t e p , addi t iona l

207

water d ra ins from the s o l i d s , r e s u l t i n g i n a much d r i e r cake t h a t can be

handled a s a s o l i d mater ia l and is s u i t a b l e f o r disposal. Thus, f i l t r a -

t i o n t o form a dry s o l i d s cake works on the same bas i c p r i n i c i p l e a s do

sludge drying beds, which a re sometimes used f o r dewatering of metal

f i n i s h i n g sludges.

The r a t e a t which a s o l i d s cake is formed and i s dr ied depends upon

the r a t e a t which water d ra ins through the f i l t e r medium. In sludge

drying beds, g r a v i t y is the fo rce which causes the water t o dra in out of

the sludge s o l i d s ( t h e importance of evaporation i s neglected i n t h i s d i scuss ion) . To increase the r a t e a t which sludge can be dewatered and

thereby reduce the land area or s i z e of equipment required, vacuum

f i l t e r s were developed. Such machines apply a suc t ion o r vacuum t o one

s i d e of the f i l t e r medium while l i q u i d sludge is applied t o the other .

Water is removed by the fo rce of the pressure d i f f e r e n t i a l t h a t develops

across t h e medium, a pressure d i f f e r e n t i a l which i s much g r e a t e r than

the fo rce exerted by gravi ty .

Many types of vacuum f i l t e r s and f i l t e r media have been developed,

and s e v e r a l have been successfu l ly used f o r dewatering sludges generated

during treatment of metal f i n i s h i n g wastes. A l l operate on the same

bas ic p r i n c i p l e , d i f f e r i n g pr imari ly in t h e means by which the sludge i s

appl ied t o and the s o l i d s cake removed from the medium. Although the

process i t s e l f i s f a i r l y simple, the complexity of both machine-related

and process- o r sludge- r e l a t e d va r i ab le s makes the operation of vacuum

f i l t e r s as much of an a r t a s a science. For example, t o optimize f i l t e r

performance o f t en requi res t h a t sludge conditioning with polymers and

other chemicals be used. Although j a r t e s t s can be used t o select ap-

propr ia te polymers, f ind ing the bes t combinations and optimum dosages is

l a rge ly a matter of operat ing experience.

DESCRIPTION OF EQUIPMENT __

A l m o s t a l l of the vacuum f i l t e r s used i n wastewater treatment ap-

p l i c a t i o n s operate continuously and have a s t h e i r p r i n c i p a l component a

hor izonta l c y l i n d r i c a l drum which r o t a t e s p a r t i a l l y submerged i n a vat

or tank containing the sludge t o be dewatered, a s shown i n Fiqure 27.

The drum i s divided by p a r t i t i o n s or s e a l s t r i p s i n t o multiple Campart-

ments o r s ec t ions each of which is connected by pipe t o a r o t a r y valve

within the drum. Bridge blocks in t h i s valve d iv ide t h e drum compart-

ments i n t o th ree zones re fer red t o a s the cake formation zone, t h e cake

drying zone, and the cake discharge zone.

The port ion of the drum which i s submerged i n the sludge v a t is the

cake formation zone. Vacuum applied through the r o t a r y valve t o t h i s

por t ion of the drum causes l i q u i d ( f i l t r a t e ) t o pass through the medium

with the sludge being retained on its surface i n the form of a wet cake.

The sludge v a t is usual ly equipped with a rec iproca t ing a g i t a t o r t o keep

the s l u r r y mixed and s o l i d s i n suspension. As the drum r o t a t e s , each

sec t ion or compartment is passed through the cake formation zone and on t o t h e cake drying zone. Beginning where the r o t a t i n g drum leaves t h e

v a t of l i q u i d sludge, the cake drying zone usua l ly represents 40 t o 60

percent of the drum surface and ends a t the poin t where t h e i n t e r n a l

vacuum is shu t o f f by the r o t a r y valve. Here t h e sludge cake and.drum

sec t ion e n t e r the cake discharge zone where the cake i s removed from t h e

medium.

A few vacuum f i l t e r s have a c J r a v i t y f e e d system which a p p l i e s

sludge d i r e c t l y on the top of the drum f o r cake formation r a t h e r than

having the drum pass through a v a t of sludge. Otherwise, t h e operation

of t h i s type of f i l t e r is e s s e n t i a l l y the same as t h e others .

The cake discharge cycle depends upon the type of f i l t e r medium

used. The e a r l i e s t type developed employs a scraper mechanism and a

pressure blow-back system f o r cake removal. A p o s i t i v e a i r pressure is

continuously applied through the r o t a r y valve t o the drum segment j u s t

preceding the scraper o r "doctor" blade: the a i r a i d s i n loosening t h e

d r i ed cake f o r removal. Often a f i n e spray is used t o c lean the medium

a f t e r cake removal, with the washings being captured i n a trough f o r

recycle t o a preceding treatment process.

- .- .

209

Figure 27. Operating zones of a vacuum filter.

2 10

More recent ly the belt-type ro t a ry vacuum f i l t e r s have gained i n

popular i ty s ince they do not depend upon int imate contact between a

scraper and the r o t a t i n g drum f o r e f f e c t i v e removal of the s o l i d s cake.

Rather, the f i l t e r medium forms a continuous b e l t t h a t leaves t h e r o t a t -

ing drum when it en te r s the cake discharge zone and re turns j u s t before

the cake formation zone. This kind of f i l t e r i s shown i n Figure 27.

Three types of media or drum coverings have been used commonly: c o i l

SpZdIIgS, closely spaced strings, and either woven cloth o r metal fabr ic .

I n one type of c o i l spr ing, belt-type vacuum f i l t e r , two l aye r s of

s ta in less s t e e l spr ings wrap around the drum and a c t a s the f i l t e r

medium. After leaving the cake drying zone, the c o i l sp r ings leave t h e

drum and a r e separated i n such a manner t h a t the sludge cake is l i f t e d

o f f the lower l a y e r and discharged off the upper l a y e r with the a id of a

posi t ioned t i n e bar. The two c o i l sp r ing l aye r s a r e then washed with a

water spray before re turning t o t h e drum j u s t ahead of the cake forma-

t i o n zone.

A very s i m i l a r type of f i l t e r uses c l o s e l y spaced s t r i n g s which a r e

wrapped around the f i l t e r drum and serve a s t h e medium. After leaving

t h e drum, the s t r i n g s pass over a series of discharge and r e t u r n r o l l s

which f r e e s the cake from the medium. The s t r i n g s then pass through a

set of a l ign ing combs before re turning t o the drum.

The o ther type of r o t a r y vacuum b e l t f i l t e r has a f i b e r b e l t made

of woven c l o t h ( e i t h e r n a t u r a l o r s y n t h e t i c ) or metal. A wide range of

mater ia l s has been developed f o r use as f i l t e r media and the s e l e c t i o n

of media type and pore s i z e f o r use with a p a r t i c u l a r sludge is a c r i t - i c a l var iab le a f f e c t i n g f i l t e r performance. In t h i s type of f i l t e r , t h e

b e l t (or f i l t e r medium) leaves the cake drying zone and passes over a

small diameter discharge roll which f a c i l i t a t e s cake removal. Generally

t h i s type of f i l t e r has a small diameter curved bar between t h e p o i n t

where t h e b e l t leaves the r o t a t i n g drum and t h e discharge roll. The

p o s i t i o n of t h i s bar can be adjusted so t h a t i t pushes a g a i n s t the in-

s i d e of the b e l t t o con t ro l b e l t tension and maintain dimensional s t a -

b i l i t y . After the cake is discharged, t h e b e l t usual ly i s washed w i t h

21 1

water sprays from one o r both s ides before it re tu rns t o the drum j u s t

ahead of the cake formation zone.

There a r e seve ra l important a u x i l i a r y equipment items which a r e

p a r t of t h e vacuum f i l t r a t i o n system. These a r e vacuum pumps, vacuum

rece ive r , f i l t r a t e pump, sludge pump, chemical feed system, sludge

conditioning tank, s i l e n c e r , and, with some u n i t s , blowers. In l a r g e r

f a c i l i t i e s these a u x i l i a r y items a r e o f t en remote from the f i l t e r s

themselves and seve ra l f i l t e r s may be served by the same equipment

through common headers. A schematic i l l u s t r a t i o n of a t y p i c a l system is

presented i n Figure 28.

The most important a u x i l i a r y item is, of course, the vacuum pump

which provides the negative pressure f i l t e r i n g force. Several types of

vacuum pumps have been used including reciprocat ing p o s i t i v e displace-

ment types, c e n t r i f u g a l ( w e t ) , and r o t a r y o r lobe pumps (semi-wet).

Depending on s o l i d s content and chemical composition, the f i l t r a t e is

sometimes used a s s e a l w a t e r on these pumps. A s i l e n c e r usual ly i s

provided on the discharge of a vacuum pump t o reduce the noise produced

during its operation. The " w e t " operat ing pumps must a l s o have provi-

s ions f o r water separat ion and draining of t h e s e a l water. Vacuum pumps

a r e usual ly s i z e d t o provide between 1 and 2 scfm/sq f t of f i l t e r sur-

f ace a r e a a t negat ive operat ing pressures of 20 inches of mercury.

Each f i l t e r genera l ly is supplied with a vacuum receiver located

between the r o t a r y valve (which d i s t r i b u t e s the vacuum i n s i d e the drum)

and t h e vacuum pump. The p r i n c i p a l purpose of the vacuum rece iver i s t o

separa te the a i r from t h e f i l t r a t e , a s w e l l a s t o serve a s a reservoi r

f o r t h e f i l t r a t e pump suct ion. With dry-type vacuum pumps, a moisture

t r a p i s provided between the vacuum rece iver and the pump.

Usually a f i l t r a t e pump is provided t o car ry away t h e water sepa-

r a t ed i n the vacuum receiver . Spec ia l ly designed c e n t r i f u g a l pumps t h a t

operate a t very low n e t p o s i t i v e suc t ion heads a r e used f o r t h i s pur-

pose, s ince the rece iver is often under a negative pressure of about 20

i n c h e s of mercury. Check valves genera l ly a r e provided on the discharge

21 2

Figure 28. Typical equipment layout of rotary vacuum filter system.

213

s i d e of these pumps t o prevent a i r from leaking back through t h e pump

and i n t o the receiver . Sometimes the f i l t r a t e pump is omitted when a

barometric leg o r g r a v i t y discharge is provided on the bottom of t h e

receiver .

Pis ton, diaphragm, and progressive c a v i t y pumps a r e a l l commonly

used as sludge feed pumps. They a r e i n s t a l l e d so t h a t a constant capa-

c i t y can be maintained a t a given s e t t i n g . Usually a s t r o k e counter or

other t o t a l i z i n g device is provided f o r flow measurement.

A sludge conditioning tank usual ly is provided i n c lose proximity

t o t h e vacuum f i l t e r from which the conditioned sludge e i t h e r flows by

g r a v i t y o r is pumped t o the f i l t e r . The tank a c t s as both a mixing

vesse l and a f loccula t ion tank. Other aspects of the chemical addi t ion

system are common t o o ther sludge handling systems and a re described i n

a separa te sect ion.

In vacuum f i l t e rs t h a t have a p o s i t i v e pressure blow-back system t o

l i f t t h e medium from the f i l t e r drum ahead of a scraper or doctor blade,

low capaci ty lobe type a i r blowers a r e used. These a r e usua l ly s ized t o

provide about 0.25 scfm/sq f t of f i l t e r surface area and operate a t

pressures of about 2 psig.

OPEPATIONU PROCEDURES

The main o b j e c t i v e of vacuum f i l t e r o p e r a t i o n i s t o produce a

sludge cake with a high s o l i d s concentrat ion (low moisture con ten t ) t h a t

i s s u i t a b l e f o r f i n a l disposal.. In doing so, t h e f i l t r a t e t h a t is pro-

duced must be low i n suspended s o l i d s so t h a t problems a r e no t created

when it i s recycled back t o preceding treatment processes. Further,

these two objec t ives must be met without reducing the f i l t e r yield below

a l e v e l s u f f i c i e n t t o handle the sludge s o l i d s generated during waste-

water treatment. O t h e r w i s e , s o l i d s w i l l accumulate i n t h e system and

W i l l eventual ly a f f e c t other treatment processes. Maintaining a n accep-

t a b l e f i l t e r y i e ld o f t en requires the addi t ion of chemical coagulants

and f i l t e r a ids . Another process objec t ive is t o minimize t h e operat ing

___ __

21 4

cos t s associated with sludge conditioning. Vacuum f i l t e r performance

cannot be maximized w i t h respect t o a l l of t h e s e ob jec t ives a t t h e same

time, s ince some of them a r e conf l i c t ing . Therefore, good vacuum f i l t e r

operation represents a compromise between these objec t ives i n which

f i l t e r performance is optimized f o r a p a r t i c u l a r appl icat ion.

Process Monitorinq

The c r i t e r i a used t o evaluate vacuum f i l t e r performance a r e s o l i d s

concentration or water content of t h e cake, e f f i c i e n c y of s o l i d s removal, and f i l t e r yield. The e f f ic iency of s o l i d s removal usua l ly i s

expressed a s the percent s o l i d s recovery (or s o l i d s cap tu re ) wh ich i s

the mass r a t i o of dry cake s o l i d s produced by the f i l t e r t o the dry

sludge s o l i d s fed t o the f i l t e r over a given period of t i m e . A decrease

i n s o l i d s recovery means t h a t more s o l i d s a r e passing through the f i l t e r

medium. Since t h i s usual ly r e s u l t s i n higher s o l i d s concentrat ions i n

the f i l t r a t e , the e f f i c i e n c y of f i l t r a t i o n is sometimes expressed a s a

funct ion of the f i l t r a t e s o l i d s concentrat ion, although t h e s o l i d s

removed from the c l o t h by water sprays should a l s o be accounted f o r .

The f i l t e r y i e ld refers t o the amount of s o l i d s , on a dry weight b a s i s ,

removed a s part of the s o l i d s cake over a given period of time.

In order t o evaluate vacuum f i l t e r performance and diagnose pro-

blems, a complete record of s eve ra l operat ing parameters must be kept.

These are summarized i n Table 22.

This information should be recorded rou t ine ly , along with machine

operat ing condi t ions such a s drum speed, drum submergence, vacuum pres-

su re , etc. It is p a r t i c u l a r l y important t h a t any changes i n performance

or operat ing condi t ions be recorded along with t h e t i m e t h a t they a r e

made.

2 1 5

TABLE 22

VACUUM FILTRATION PROCESS MONITORING REQUIREMENTS


1 .

2.

3.

4 .

5.

6.

7.

8 .

9.

10.

11.

Feed s ludge f lowrate (GPM or L/min).

F i l t r a t e f lowra te (gpm o r L/min).

T o t a l and suspended s o l i d s concent ra t ion i n feed s ludge slurry (mg/L).

T o t a l s o l i d s concen- t r a t i o n i n s o l i d s cake ( p e r c e n t by weight) .

Tota l and suspended s o l i d s concent ra t ion i n f i l t r a t e (mg/L).

Concentration of chemica l condi t ioners added t o s ludge (ppm o r p e r c e n t ) .

Feed rate f o r chemical condi t ioners (L/min o r gpm).

Feed s ludge temperature ( F or Oc).

Feed sludge pH.

0

Wash water f lowra te (gpm o r L/min).

So l ids concent ra t ion i n wash water (mg/L).

Dai ly

Weekly

Dai ly

Dai ly

Dai ly

Weekly

Weekly

Dai ly

Dai ly

Weekly

Weekly

To determine f lowrate .

To determine f i l t e r performance.

To determine feed ra te t o f i l t e r .

To determine s o l i d s removal and f i l t e r e f f i c i e n c y .

To determine s o l i d s loading i n f i l t r a t e .

To set condi t ion ing requirements.

TO set condi t ioning requirements.

Temperature a f f e c t s f i l t e r a b i l i t y .

High or low pH values can change s o l u b i l i t y of p a r t i c l e s .

To determine volumes of l i q u i d i n mass balance.

To determine s o l i d s i n wash water, s o l i d s removal, and f i l t e r e f f i c i e n c y .

2 16


Example c a l c u l a t i o n s of opera t ing parameters a r e presented below.

Consider a r o t a r y b e l t vacuum f i l t e r f o r which t h e fol lowing d a t a have

been co l l ec t ed :

Feed s ludge flow r a t e = 20 gpm

F i l t r a t e flow r a t e = 18 gpm

Feed suspended s o l i d s = 40,000 mg/L (4.0 pe rcen t )

F i l t r a t e suspended s o l i d s = 600 mg/L

Cake s o l i d s concent ra t ion = 300,000 mg/L (30 pe rcen t )

Cloth wash water spray = 6 gpm

s o l i d s concent ra t ion i n wash water = 4,900 mg/L

The s o l i d s feed r a t e ( d r y s o l i d s fed t o the f i l t e r per hour) i s

ca l cu la t ed a s follows:

s o l i d s feed r a t e = ( feed sludge flow r a t e ) ( f e e d suspended s o l i d s ) 3

= ( 2 0 gpm)(60 min/hr)(3.785 L/gal)(4O g/L)(kg/lO g)

= 182 kg/h = 400 l b / h r

The s o l i d s removed ( d r y s o l i d s removed per hour) is ca l cu la t ed as

fol lows :

s o l i d s removal = ( f eed s o l i d s m a s s ) - [ ( f i l t r a t e s o l i d s mass)+

(wash water s o l i d s mass)]

= 400 lb /hr - [ ( 1 8 gpm)(O.6 g/L)+(6 gpmI(4.9 g /L) I x

[(3.785 L /ga l ) ( lb /454 g)(60 min/hr) l

= 380 lb /hr

The f i l t e r e f f i c i e n c y , expressed a s percent s o l i d s recovered, i s

ca l cu la t ed a s fol lows:

So l ids removal ,oo Sol ids feed r a t e

F i l t e r e f f i c i e n c y ( $ 1 =

21 7

F i l t e r e f f i c i ency = (380)/(400) x 100 = 95.0 percent

Process Control S t r a t eg ie s

The operat ion and performance of a vacuum f i l t e r is determined by

seve ra l var iab les t h a t a r e r e l a t ed e i t h e r t o the machine i t se l f (machine

va r i ab le s ) o r t o the s p e c i f i c sludge dewatering appl ica t ion (process

var iab les 1. Among these a r e the. following:

Machine Variables Process Variables

Vacuum Pressure Sludge Charac te r i s t i c s

D r u m Speed Chemical Addition

Drum Submergence Sol ids Concentration

F i l t e r Medium Sludge Feed Rate

Tank Agitat ion Temperature

Spray Water P res su re PH

Pos i t ion of Doctor Blades

Machine Variables--

One of the most important machine-related var iab les is t h e vacuum

pressure appl ied during formation and drying of the sludge cake. Within

l i m i t s , the f i l t e r y i e ld can be increased by increas ing the vacuum s ince

a higher vacuum genera l ly r e s u l t s i n g rea t e r cake thickness during the

formation process. P r a c t i c a l limits usua l ly a r e encountered a t vacuum

pressures of about 15 inches of mercury, s ince energy requirements

increase rap id ly with higher operat ing pressures . Further, many waste-

water sludges a r e h ighly compressible, though t h i s is not t rue f o r a l l

types of sludge, and under higher vacuums' these sludges form an imper-

vious cake t h a t is d i f f i c u l t t o dewater. In such a case, increased

vacuum pressures may reduce f i l t e r yield.

T h e drum s p e e d , o r t h e r a t e a t which t h e f i l t e r drum r o t a t e s ,

. a f f e c t s both cake formation and cake drying operat ions. A s t h e drum

speed is reduced, the cake has a longer period of time t o dewater,

r e s u l t i n g i n a d r i e r cake. However, reducing drum speed reduces f i l t e r

21 8

y i e l d so t h a t l e s s sludge can be dewatered i n any given period Of t i ne .

Conversely, an increase i n the drum speed r e s u l t s i n a higher f i l t e r y i e l d s ince more medium is passed through the cake formation zone during

any given period of t i m e . A wet ter cake a l s o r e su l t s , however, Since

the sludge cake spends less time i n the cake drying zone.

The degree t o which the f i l t e r drum is submerged i n the sludge v a t ,

re fe r red t o a s drum submergence, usual ly is expressed a s a percentage of

the drum circumference o r f i l t e r medium surface a rea ( t h e d i f fe rence

between these is a constant value). Drum submergence normally can be

varied between 15 and 25 percent. An increase i n submergence increases

the cake formation area (and the re fo re cake formation time) but reduces

the area (and time) ava i lab le f o r cake drying; t h i s usua l ly r e s u l t s i n a

wetter but t h i cke r cake. Decreasing the drum submergence produces a

thinner but d r i e r cake. A minimum submergence must be maintained t o

prevent vacuum on the f i l t e r from being broken.

Probably the most important machine var iab le is the f i l t e r medium

i t s e l f . Select ion of a f i l t e r medium s u i t a b l e f o r use i n a p a r t i c u l a r

appl ica t ion is one of t h e most c r i t i c a l design considerat ions since i t

can determine the operat ing and performance limits of the vacuum f i l t e r .

Materials which have been used a s f a b r i c media include co t ton , rayon,

a c r y l i c , polyolef in , po lyes t e r , polypropylene, and nylon. Metal wire or

mesh media a l s o have been used, as have s t a i n l e s s s t e e l c o i l spr ings.

A l l of these media can be categorized a s open or t i g h t , with open media

having l a r g e r s i z e pores through which small s o l i d p a r t i c l e s can pass.

Although a t i g h t f i l t e r medium w i l l remove a higher percentage of f i n e

particles, thereby increasing f i l t e r e f f i c i ency , i t can bl ind or s top up

completely making f i l t r a t i o n impossible. Spray washing of the media

a f t e r cake discharge helps t o prevent t h e build-up of f i n e p a r t i c l e s

which might otherwise b l ind the f i l t e r media. The c h a r a c t e r i s t i c s of

the f i l t e r medium usua l ly change w i t h u s e a s it becomes worn or p a r t i a l -

l y blinded with so l id s . While such changes usual ly occur gradual ly , i t

can eventua l ly become necessary t o replace t h e medium. Occasionally the

medium type has t o be changed, e i t h e r t o improve f i l t e r performance o r

because the previously used medium is no t ava i l ab le . Whenever the

__ ____

219

f i l t e r medium is changed, and p a r t i c u l a r l y when a d i f f e r e n t type of

medium is used , the operation and performance of the vacuum f i l t e r can

be expected t o change a l s o ; sometimes these changes are s i g n i f i c a n t .

Two machine va r i ab le s t h a t a f f e c t s o l i d s removal from the f i l t e r

media a re the pressure of the water sprays and the pos i t ion of the

doctor blades. Both a re of c r i t i c a l importance and very minor ad jus t -

ments can r e s u l t i n improved o r diminished performance. Optimization of

water spray pressure and doctor blade posi t ion is a matter of operat ing

experience.

The f i n a l machine var iable requir ing discussion is the degree of

a g i t a t i o n provided i n the sludge vat through which the f i l t e r drum

r o t a t e s . Agitat ion is required t o prevent s o l i d s from s e t t l i n g , and

some minimum l e v e l needed t o meet t h i s ob jec t ive must be determined from

operat ing experience. However, t oo much a g i t a t i o n can r e s u l t i n shear-

ing or break-up of t h e sludge f l o c r e s u l t i n g i n a lower f i l t e r e f f i c i e n -

cy and higher s o l i d s l e v e l s i n the f i l t r a t e .

Process Variables--

The phys ica l and chemical c h a r a c t e r i s t i c s of t h e sludge t o be de-

watered together represent t h e s i n g l e most important process var iab le

a f f e c t i n g vacuum f i l t r a t i o n . Different sludges e x h i b i t dramatical ly

d i f f e r e n t dewatering c h a r a c t e r i s t i c s . Among the f a c t o r s t h a t determine

f i l t e r a b i l i t y a r e the s i ze , shape, densi ty , and e l e c t r i c a l charge of

s o l i d p a r t i c l e s , v i s c o s i t y of the l i q u i d , and compress ib i l i ty of t h e

sludge so l id s . Generally these f a c t o r s a re no t d i r e c t l y under the

opera tor ' s cont ro l s ince they r e s u l t from optimization of upstream

treatment processes.

To compensate f o r poor f i l t e r i n g c h a r a c t e r i s t i c s o r t o increase

cake s o l i d s concentrat ion, polymers and other chemicals of ten a r e added

t o the sludge p r i o r t o f i l t r a t i o n . These may be required t o help i n

cake formation and/or cake drying. Polymers and other coagulant a i d s

promote f loccu la t ion , which increases p a r t i c l e s i z e and improves f i l t e r

e f f i c i ency . F i l t e r i n g a i d s such a s lime, diatomaceous e a r t h , c lay, f l y

2 20

ash , and other mater ia ls can be mixed with t h e sludge t o improve s o l i d s

recovery, f a c i l i t a t e good cake formation, and reduce sludge compressi-

b i l i t y (which helps i n cake drying) . Se lec t ing t h e proper coagulants

and f i l t e r a i d s f o r a p a r t i c u l a r appl icat ion is based upon j a r t e s t s ,

while optimizing t h e i r use is a matter of operat ing experience. Optimi-

za t ion of chemical usage is important s ince chemical c o s t usua l ly repre-

s e n t s the g r e a t e s t s ing le operat ing c o s t assoc ia ted with vacuum f i l t r a -

t ion.

Another process-related var iab le is the s o l i d s concentrat ion of t h e

feed sludge. Up t o a point , a higher s o l i d s concentration i n the feed

sludge usual ly r e s u l t s i n a d r i e r cake discharge and a higher f i l t e r

yield. Another advantage of higher feed s o l i d s l e v e l s is t h a t chemical

requirements f o r sludge conditioning o f t e n a r e reduced. For these rea-

sons, vacuum f i l t e r s a r e sometimes preceded by g r a v i t y thickeners o r

cent r i fuges t h a t increase the sludge s o l i d s concentration. Changes i n

the operat ion of upstream thickeners , cen t r i fuges , or even c l a r i f i e r s

can improve f i l t e r performance if the changes increase the concentrat ion

of s o l i d s i n t h e feed sludge. However, there a r e p r a c t i c a l l i m i t s a s t o

how hi9h the feed s o l i d s concentration can be, a s determined when pump-

ing, mixing, and o ther problems begin t o occur. The maximum s o l i d s

concentrat ion usua l ly fed t o vacuun f i l t e r s is from 8 t o 10 percent:

above these numbers cake formation becomes d i f f i c u l t .


The most important f a c t o r a f f e c t i n g the performance of a vacuum

f i l t e r i s t h e type of sludge being dewatered, I f sodium hydroxide i s

used f o r pH adjustment t o p r e c i p i t a t e metal hydroxides, a l i g h t , slimy,

and r e l a t i v e l y d i f f i c u l t t o dewater sludge w i l l r e s u l t . One of the main

problems caused by t h i s type of sludge i s t h a t i t can bl ind the f i l t e r

medium and form an impermeable cake t h a t dewaters poorly. These pro-

blems usual ly can be overcome, however, by using proper conditioning

agents, such a s polymers and f i l t e r a ids .

__

221

When lime is used f o r p r e c i p i t a t i o n of metals, the amount of excess

l i m e determines t h e dewatering c h a r a c t e r i s t i c s of the sludge. When very

l i t t l e o r no excess lime i s added, most .of the sludge formed w i l l be

metal hydroxides. I f considerable excess lime is used, a l a rge amount

of calcium carbonate w i l l be present i n the sludge, making i t heavier

and more e a s i l y dewatered, s ince calcium carbonate sludges tend t o form

much more permeable cakes.

Table 23 gives t y p i c a l performance values f o r ro t a ry vacuum f i l t e r s

appl ied t o both metal hydroxide and lime-carbonate sludges.


The problems which a re commonly associated with vacuum f i l t e r oper-

a t i o n s have been summarized i n the form of a troubleshooting guide i n

Table 24. This is a general guide and some p a r t s of the vacuum f i l t r a -

t i o n system may have unique operat ing problems; these a r e usual ly ad-

dressed i n the manufacturer 's l i t e r a t u r e .

Vacuum f i l t r a t i o n is one of t h e more d i f f i c u l t wastewater t reatment

processes t o operate , because of the number of va r i ab le s which a f f e c t

performance. Therefore, whether diagnosing operat ing problems o r op t i -

mizing f i l t e r performance, i t is important t o change o n l y a s i n g l e

var iab le a t a t i m e . Otherwise, the cause of a problem ( o r i t s s o l u t i o n )

cannot be determined with c e r t a i n t y . I t is e s p e c i a l l y important t h a t

a l l o ther va r i ab le s be h e l d constant when determining optimum dosages

f o r sludge conditioning.

222

TABLE 23 TYPICAL PERVORMANCE VALUES FOR VACUUM FILTRATIbN DEWATERING

Feed Solids Cake S o l i d s S o l i d s Typical TYpa of Concentration Concentration Recovery Conditioning Sludge ( 5 ) ( 0 ) ( 0 ) Chemicals

Metal Hydroxide

- without conditioning

- w i t h conditioning

High CaC03

- without conditioning

- with conditioning

L i m e ;

Earth

Polymers

2-6 15-25 85-95 Diatomaceous

4-a 20-30 90-95 Anionic

Anionic

Diatomaceous 2-a 20-30 90-95 Polymers

6-10 30-45 95+ Earth

2 2 3

N N P

TAH1.I. 24 VACUW FILTRATION


PROBABLE CAUSE CHECK OR HONITUR REASON "RECTIVF acploN

OPERATIffi PRORLW 1 1 % i n cake. p ~ r dewatering.

la. F i l t e r media blinding. - Digest media and check spray washes.

lb. Improper chemical - Check chemical dosage. a d d i t i o n system for

na1f""ction.

I C . Inadequate vacuum. - vacuum gauges and i n p u t for l eaks 01 broken seals.

ld. Drum speed tea hlgh. - Drum speed and opera t ing records.

- TO determine if bl inding i e occurring and i f i t is c m s e d by i n e u f f i c i e n t washing.

- I f no problems with equipment. the optimum dosage may have changed.

- To i s o l a t e the problem.

- hdjust spray wa6heb or, i f necessary, s h u t down f i l t e r and clean media.

- nepair or a d j u s t chemical a d d i t i o n

sary , determine new .optimum dosaoe rarer.

- ~epa ir or a d j u s t V~CUUI

syst.3. or, i f neces-

system to achieve d e s i r e d V~CUUI.

- Reduce drum epeed i f - To determine i f a change has occurred. neCe*Sary.

le. Drum submergence tao - Drum svhnergence and - To determine I f a - Increase drum submergence to get t h i c k e r cake, decrease I t to get d r i e r cake.

IO". opera t ing records. change has occurred.

- I t s o l i d s have - netermine i f t h e s o l i d s If. Feed sludge s o l i d s - S o l i d s feed rate and level can be increased by changes in upstream

may be requlred. processes. Otherwise. new operating condi t ions w i l l have to he d e f e n i n e d .

concent ra t ion has operating records. decreased, d i f f e r e n t decreased. opera t ing condi t ions

.-

OPERATING FROBLM 2: Hlgh s o l i d s i n f i l t r a t e - low efficiency.

2a. Improper chemical - check chemical dosage. addi t i v n s y s t e m

for malfunction.

~

- I t no problems w i t h e q u i p e n t , the opti- wvm dosage may have changed. .

~ e p i r or a d j u s t chemical a d d i t i o n

determine new dosage rates.

system or, I f ""P'Eary,

TA0I.F 24 (Continued)

VACUU PlLTBATIOH 11IWBl.fSfUXFTIffi G U l D E

N N m

PROBAPLE CAUSE CHfCK OR wDNl¶OR PFIS0)I COORBCIIVE ACTlOU

- Check chemical I 5c. Impfoper chemical dosbge - poor cake f o r b t i o n . for .a l f"nct lon.

l d d l t l p n e q v i p e n t

Sd. F i l p a t e p.p off - Check f i l t r a t e pump or clogged. and discharge.

- If 110 problems with - Eepalr or a d j u s t equ1pent. opt1- chemical odd l t lon

system and, I f

new d-oge rates.

6osages way have changed. necessary. deter.1ne

- If on, I t may be clogged.

- Turn M pwp O r

""Clog it.

5e. Drain l i n e clogged. - Draln l i n e for f l w . - oral" line may stOp - Clea r d ra in l i n e . f 10".

5f. Vacuum p m p atopped. - see I t en 3. - see It=. 3. - see I t e l 3.

59. Seal strips missing. - Drue interior. - LOSS Of VaC"". - Replace Beal s t r l p . would prewnt re.OYa1 of filtrate.

OPRIATIUG PROBLEW 6: Low vat l eve l .

6a. reed r a t e too l a . - Fee8 r a t e , s o l i d s y i e ld , Operating records.

cake for.= faster - Increase feed rate than ealida app l i ed fO f i l ter . speed.

or decrease drum

- Close Vat d r a l n valve. - open valve can dra in 6b. V a t dra in valve open. - vat d r a i n valve. "at.

OPWATING PROBLFU 1: F i l t r a t e receiver v ih ra t ing .

la. F i l t r a t e pump ie - Check piop o u t l e t clogged. f o r f l o w .

l b . Loose bolts or Other - cheek gaskets .

rouoting. etc. par t s . inpaction p l a t e s ,

7c. Check to see i f b a l l - C h e c k b a l l v a l v e . v a l v e in filtrate IX"' or surtion 1i,,e is w"r,>.

- P l o w Ind ica t e s posa ib l r Clog.

- Damage can r e s u l t .

- worn h a l l valves can CI1"SP "ibra tions.

Replace "or" p a r t s .

TABLO 2 1 I con t inued)

VACUW FIL¶IATIDN T'ROUBLFSIUZOTINO GUIDF

N N 4

PROBABLE CAUSE CHECK an N O N I ~ R RFASON CORRE73lVF A C r I O N

7d. A i r leaks in euc t ion - 1.pact Iine'fo. l ine . leaka.

7e. D i r t y drum face. Check drum face.

- seal leaks. - Leaks i n a i r l i n e can cause v i b r a t Ions.

- mew" "a" rcaulte i n p r e s s u r e eurgea.

- uneve. "am". rc.o1tm i n 8"rges.

- Clean off drum face d t h v i t h hose.

- Replace .ea1 a t r i p s .

ea. F i l t r a t e pump elqged.

ah, High v a t level.

8.2. Cooling v a t e c f lW t o "a" pump too high.

Bd. Bl ind ing of r d i s c a u d n g h i g h 'I.=- p*esB"re.

Be. Highly compre.dhle sludge forming i m p r r s h l e cake.

- F i l t r a t e P U P o u t p u t for fl-.

- see Item 6.

- Cooling vster f l a .

- Check media and s p r a y washer.

- Cake d ryness and cons i s t ency . seck chemical a d d i t i o n sYBteD.

- V ~ N U D p u p d r a v l n g wa te r from receiver.

- See I t e D 6.

-

- TO de te rDlne if r d i s is soiled.

- M y require a d d i t i o n of 11- or other f i l t e r aid to improve s l u d g e c h a r a e t e r i a t l c a .

- Turn p u p O f f sod clean.

- See I t e D 6.

- Decrease cooling rater flar.

- a d j u s t s p r a y vaehe r and clean Dedi., i f necemsary.

- Adjust cheric~l o d d i t i o n r a t e n to i.prove cake c h a r a c t e r i m t i c s .

9a. High doeage of lime. - pH a d j u e t e n t to deter- - High l i m e dosage0 - Fine t u n e 11- Dine if eXceadVe line o* preaence Of ""- dosage (see pH ad- l e added. r e a c t e d lire may EaUse justment and metal

*c*le. p r e c i p i t o t i o " . )

i'

I

I TABLE 24 IContinued)

VACUUM FIL'NIATION TROUBLESWOTING G U I D E

PRORABLE CAUSE CHECK OR WMIITOR REASON CORRmIVE ACTION

9b. cam is precipitating - Influent calcium con- - I f l n f l u e n l calcium OUt a, solutio". centrotion and calciu. concentration i*

solubility at operating greater than solubll- Pll. i t y , the" scale "ill

form.

N N m

Fine tune lime dosage, operate pH adjuet.ent aL dlf- ferent pH or decrease

In e q u i p e n f by washing thoroughly w i t h iurlaflc acid after operation.

retention tile of sludge

SECTION 16

PRESSURE FILTRATION FOR DEWATERING

INTRODUCTION

Pressure filtration is a process used f o r dewatering sludges gen-

e ra ted during wastewater treatment. Pressure f i l t r a t i o n results i n t he

generat ion of two products: a r e l a t i v e l y c l e a r l i qu id , ca l l ed the f i l -

t r a t e , and a s o l i d s cake t h a t i s s u i t a b l e f o r d i r e c t disposal . Compared

t o o ther mechanical dewatering processes, pressure f i l t r a t i o n genera l ly

achieves the dr ies t s o l i d s cake for any p a r t i c u l a r sludge. There i s a

c o s t assoc ia ted with improved performance, however, a s pressure f i l t e r s

a r e usua l ly more expensive t o operate than o the r types of dewatering

equipment. Generally, b u t no t always, p ressure f i l t e rs used f o r sludge

dewatering are preceded by a g rav i ty thickener or a cent r i fuge t o i n -

c rease the s o l i d s concentrat ion of the sludge fed t o the f i l t e r . Since

pressure f i l t e rs a re designed and operated a s batch processes , i t may be

important t o reduce sludge volume by thickening p r i o r t o f i l t r a t i o n .

THEORY OF OPERATION

The b a s i c p r i n c i p l e by which pressure f i l t r a t i o n works is the same

a s f o r vacuum f i l t r a t i o n : s o l i d s a r e separated from a l i qu id by passing

the l i q u i d through a porous medium on which the s o l i d s remain, eventual-

l y bu i ld ing up t o form a cake t h a t is removed f o r disposal . T h i s pro-

cess is much l i k e what happens when sludge is applied t o a sand drying

bed. Water d ra ins from the sludge i n t o t h e sand leaving behind a con-

cen t r a t ed s o l i d s layer . Given time, t h e force of g rav i ty w i l l cause

enough water t o dra in out so t h a t a sludge cake w i l l form'which can be

handled and disposed of a s a so l id . I n vacuum f i l t e r s , a negat ive

229

pressure d i f f e r e n t i a l t h a t i s equivalent t o about 1000 times t h a t of

sand drying is applied t o speed up the drying process. Pressure f i l t e r s

apply even g r e a t e r forces t o the sludge--up to' 20,000 times the pressure

differential of sand drying-whioh results i n an even d r i e r solids cake.

Almost a l l pressure f i l t e r s used i n sludge dewatering appl ica t ions

a r e designed t o operate i n a batch mode, a s opposed t o continuous opera-

t ion. The usual sequence of events is a s follows. F i r s t , t h e sludge

s l u r r y is applied t o the f i l t e r and a sludge cake is formed on the

f i l t e r medium a s the f i l t r a t e passes through i t and is removed. A s

addi t iona l sludge is pumped i n t o the f i l t e r , the cake grows th i cke r . and

the backpressure increases. Eventually a po in t i s reached when the

sludge feed pumps a re stopped. This p o i n t is based e i t h e r upon time,

decrease i n f eed ra t e , o r operat ing pressures. Depending upon the type

of f i l t e r , a cake compression s t e p may follow, i n which a d d i t i o n a l water

is removed from the cake. Any remaining sludge s l u r r y is then drained

from the f i l t e r , a f t e r which the f i l t e r is opened t o expose and d i s -

charge the s o l i d s cake. Once the cake has been removed and the media

cleaned ( i f necessary) , the f i l t e r is closed and the next cycle can

begin.

Some f i l t e r s and some types of sludge require t h a t a d d i t i o n a l s t eps

be performed. For example, t o prevent bl inding of the media, t o improve

s o l i d s capture, and t o allow easy cake removal, i t may be necessary t o

apply a precoat t o the f i l t e r media before introducing t h e sludge.

Precoating i s done o f t en when a greasy or s t i c k y sludge, e.g., m e t a l

hydroxide, is being dewatered.

Sludge c h a r a c t e r i s t i c s can a f f e c t dewaterab i l i ty by pressure f i l -

t r a t i o n j u s t a s they a f f e c t vacuum f i l t r a t i o n . Generally, t h e much

higher operat ing pressures compensate f o r poor sludge c h a r a c t e r i s t i c s ,

although there a r e exeptions t o t h i s rule. Chemical conditioning with

polymers o r inorganic chemicals may be necessary t o improve t h e sludge

compressibi l i ty ( r e s u l t i n g i n a d r i e r cake) and achieve a good u u a l i t y

f i l t r a t e . Sludge conditioning may a l s o be necesary t o ge t good cake

release.

230


AS with vacuum f i l t r a t i o n , the formation of a s o l i d s cake and the

f a c t t h a t s o l i d s a re removed pr imar i ly on the surface of the f i l t e r

medium r a t h e r than throughout i ts depth a re two f e a t u r e s t h a t d i s t i n -

guish pressure f i l t r a t i o n f o r sludge dewatering from other f i l t r a t i o n

processes. This d i s t i n c t i o n is important s ince pressure f i l t e r s a r e

used i n o ther app l i ca t ions , such a s i n c a r t r i d g e f i l t e r s and s t ra iners used onl ine f o r the removal of s o l i d s from l i q u i d streams. Since such

types of pressure f i l t e r s a r e no t used i n sludge handling and t reatment

operat ions, they w i l l not be discussed i n t h i s sec t ion of t h e manual.

Batch pressure f i l t e r s can be divided i n t o th ree major groups:

( 1 ) Filter Presses-including plate-and-frame presses and recessed-

p l a t e presses ,

( 2 ) Leaf, P l a t e , and Candle (Tubular) F i l t e r s - - f i l t e r s which take

t h e i r names from the shape o r o r i e n t a t i o n of the surfaces t h a t

c o l l e c t the f i l t e r cake,

( 3 ) Variable-Volume Fi l ters-- in which a cake is formed by pressure

f i l t r a t i o n and then compressed t o expel the excess water re-

maining i n it. . * i.

The d i s t i n c t i o n between these groups is somewhat a r b i t r a r y and d i f f e r e n t

people may consider the same kind of f i l t e r t o belong i n d i f f e r e n t

groups. T h e s e points a r e no t important, however, since a l l operate on

the same b a s i c p r inc ip l e s .

F i l t e r Presses

F i l t e r presses represent the most common type of pressure f i l t e r s

and c o n s i s t of numerous p l a t e s with corrugated surfaces over which t h e

f i l t e r medium is draped, a s shown i n Figures 29 and 30. The f i l t e r

medium is usua l ly i n t h e form of woven c l o t h and of ten i s made of syn-

t h e t i c monofilament f i b e r . The operat ing pressure is created by pumps

-. -. ____

231

FILTRATE DRAIN HOLES

Figure 29. Cutaway view of a filter press.

2 32

a Z

a Iy

X

Iy

23

3

which introduce the sludge s l u r r y between layers of t h e f i l t e r medium.

One end of the p re s s is f ixed w h i l e the other end can be moved t o a l low

separa t ion of the p l a t e s and discharge of t h e s o l i d s cake. There a r e

two main types of f i l t e r presses , t h e p l a t e and frame press and the

recessed p l a t e press.

In the plate-and-frame press , the p l a t e s a r e separated by hollow

frames which a r e covered with the f i l t e r medium. The a l t e r n a t i n g p l a t e s

and frames a re held t i g h t l y together during f i l t e r operation t o form a

continuous u n i t , Sludge is introduced i n t o the hollow frames and the s o l i d s cake t h e n forms on the f i l t e r medium (within the frames). The

f i l t r a t e passes through and dra ins along the corrugat ions i n t h e p l a t e

from which, depending upon t h e ind iv idua l f i l t e r , it may d r a i n d i r e c t l y

o u t the bottom of each p l a t e .or may flow through a channel t h a t runs the

length of the press , Sludge feed continues u n t i l t h e frames a r e f u l l ,

a s judged by t i m e , decrease i n feedra te , or increase i n backpressure.

A t the end of the batch, the p re s s usua l ly is drained of any remaining

l i q u i d a f t e r which i t is opened and the p l a t e s and frames separated t o

r e l ease the s o l i d s cake.

Another type of f i l t e r p ress i s the recessed-plate press . I n t h i s

type of press , the p l a t e s a re shaped so t h a t a hollow space e x i s t s

between t h e m i n which the s o l i d s cake forms. There a r e no separa te

hollow frames between adjacent p la tes . Sludge en te r s the press through

por t s i n the center of the p l a t e s . Otherwise, the recessed-plate p re s s

looks and is operated j u s t l i k e the p l a t e and frame press .

L e a f , P l a t e , and Candle F i l t e r s

P r e s s u r e f i l t e r s i n t h i s g roup have a number of ho l low f i l t e r

elements suspended e i t h e r v e r t i c a l l y or hor izonta l ly w i t h i n a closed

---..-vessel. The shape of these elements is what gives the various types of

f i l t e r s i n t h i s group t h e i r names. These f i l t e r elements a r e o f t e n

covered with a woven c l o t h f i l t e r medium, although there a re some appl i -

ca t ions i n which the sur face of the f i l t e r element a l s o serves a s t h e

f i l t e r medium. Sludge flows i n t o the vesse l under pressure and l i qu id

234

(filtrate) flows o u t through the hollow elements t o a common discharge

manifold. In the process, t h e s o l i d s l e f t behind on t h e f i l t e r medium

bui ld up t o form a cake which eventually causes the operat ing pressures

t o increase and sludge f eed ra t e s t o decrease.

A t the end of an operat ing cycle, the vesse l is depressurized and

any remaining s l u r r y i s drained. The f i l t e r is then opened and t h e

f i l t e r elements a r e removed (usua l ly t h i s process is automatic). The

cake is then removed from the f i l t e r elements by one or more of s e v e r a l

methods including vibrat ion, scraping, compressed a i r blow-back, cen-

t r i f u g a l force, and leaf ro t a t ion .

Variable-Volume F i l t e r s

The t h i r d type of pressure f i l t e r f i r s t forms a s o l i d s cake, usual-

l y by pressure f i l t r a t i o n , and then compresses i t t o expel a d d i t i o n a l

water contained i n the w e t cake. The purpose is t o produce a drier cake

than i n conventional f i l t e r presses . The variable-volume name r e f e r s t o

the f a c t t h a t i n these f i l t e r s the s i z e of t h e chamber i n which the cake

is formed changes during operat ion by t h e movement of a diaphragm. I n f a c t , these f i l t e r s a r e of ten r e fe r r ed t o as diaphram f i l t e r s . The most

common type of variable-volume diaphragm pressure f i l t e r somewhat resem-

b l e s a recessed p l a t e f i l t e r press. A l l of the f i l t e r s of t h i s type

- operate on the same bas ic p r i n c i p l e , although t h e r e a r e numerous var ia- . <

t i o n s i n the ind iv idua l s t e p s of an operat ing cycle.


The main objec t ive of using pressure f i l t e r s f o r dewatering is t o pro-

duce a s o l i d s cake f o r discharge t h a t has a very high s o l i d s concen-

t r a t i o n . High s o l i d s concentration is important s ince the moisture

content of t h e sludge d i r e c t l y a f f e c t s haul ing and d isposa l cos ts - - i t i s

c e r t a i n l y n o t des i rab le t o pay more f o r h a u l i n g water . i n the form of a

w e t cake, if a d r i e r cake can be achieved. To achieve a high concentra-

t i o n i n the cake, very high operat ing pressures a re required, a s is a

t i g h t f i l t e r medium (one with very small pore s i z e s ) . As a r e s u l t of

..

235

these f ea tu res , f i l t r a t e q u a l i t y is usual ly n o t considered t o be a

performance l imi t ing f a c t o r i n pressure f i l t r a t i o n . More important is

the f i l t e r y i e ld , o r mass of s o l i d s (dry weight) t h a t can be dewatered

i n a given amount of time. An increase i n the y i e l d usua l ly requi res

t h a t the batch cycle time be decreased, which i n tu rn r e s u l t s i n z

wetter cake. Therefore, the object ive of pressure f i l t e r operat ion is

t o maximize cake s o l i d s w h i l e s t i l l achieving a f i l t e r y i e ld t h a t is

s u f f i c i e n t t o process a l l t h e sludge generated i n upstream treatment

processes.

Process Monitorinq

F i l t e r press performance is measured by the s o l i d s content i n the

feed sludge, required chemical conditioning dosages, cake s o l i d s ccn- t e n t , t o t a l cycle t i m e , s o l i d s capture, and the f i l t e r yield. Of these,

the l a s t two r equ i r e some explanation.

Sol ids capture , a l s c c a l l e d s o l i d s recovery, is the mass r a t i o of

cake ( o r thickened sludge) s o l i d s t o the feed s o l i d s f o r a s i n g l e batch

run. A low s o l i d s capture means t h a t s o l i d s a r e l o s t i n the f i l t r a t e

and a r e no t p a r t of the s o l i d s discharge. The s o l i d s y i e ld (removal)

refers t o the amount of s o l i d s , on a dry weight b a s i s , t h a t can be re-

moved by a pressure f i l t e r during a s ing le batch run. The y i e ld multi-

p l i e d by t h e number of batches run must equal o r exceed the sludge gene-

r a t i o n r a t e if s o l i d s a re no t t o accumulate upstream i n the treatment

system.

The performance parameters discussed above a r e a l l i n t e r r e l a t e d ;

f o r example, a s the feed s o l i d s content increase's, the required chemical

dosages and t o t a l cycle t i m e usual ly decrease, while the f i l t e r y i e l d o r

throughput usual ly increases . As the chemical conditioning dosage i s

----.increased t o the optimum l e v e l , the cake s o l i d s content, s o l i d s capture ,

and y i e ld all increase, while the cycle t i m e decreases.

236

I n o r d e r t o e v a l u a t e p r e s s u r e f i l t e r per formance and d i a g n o s e

opera t ing problems, a complete record of s e v e r a l opera t ing parameters

must be kept . These a r e l i s t e d i n Table 25.

This information should be recorded r o u t i n e l y , p re fe rab ly f o r each

batch. I t is e s p e c i a l l y important t h a t changes i n p re s su re s e t t i n g s ,

f i l t e r cyc le t i m e s e t t i n g s , chemical condi t ion ing dosages, e t c . be

recorded along with the t i m e (ba tch number) t h a t they a r e made. In t h i s

way a u s e f u l record of t h e e f f e c t s of process v a r i a b l e s on performance

w i l l be developed.


Consider a pressure f i l t e r f o r which t h e fol lowing d a t a have been

co l l ec t ed :

volume of sludge fed per run = 10,000 ga l lons

volume of f i l t r a t e produced per run = 9,000 ga l lons

Feed suspended s o l i d s = 40,000 mg/L ( 4 percen t )

F i l t r a t e suspended s o l i d s = 0.6 g/L

Cake s o l i d s concent ra t ion = 400,000 mg/L (40 p e r c e n t )

Sludge feed volume and f i l t r a t e volume per run a r e func t ions of

cyc le time and maximum opera t ing pressures . The s o l i d s f ed per f i l t e r

run (d ry weight b a s i s ) is ca l cu la t ed a s fol lows:

So l ids fed per run = (volume of sludge f ed ) (Feed suspended s o l i d s )

= (10,000 ga l lons (3.785 ~ / g a l ) ( 4 O g/L)

= 1514 Kg = 3335 l b s .

The s o l i d s l o s t per f i l t e r run is ca lcu la t ed a s fol lows:

~

S o l i d s l o s t p e r r u n = (volume of f i l t r a t e ) ( F i l t r a t e suspended

s o l i d s )

= (9,000 g a l l o n s ) ( 3 . 7 8 5 ) ( 0 . 6 g/L)

= 20 Kg = 45 lbs .

237

TABLE 25

PRESSUF3 FILTRATION PROCESS MONITORING REQUIREMENTS


1 . Batch cyc le t i m e s (minutes ) . P e r batch To determine system performance.

2. volume of sludge fed per per batch To determine s o l i d s batch (ga l lons o r l i t e r s ) . feed r a t e , s o l i d s

capture , and s o l i d s y i e ld .

3. Total and suspended s o l i d s P e r batch see Comment 2 . i n feed s l u r r y (mg/L or per- c e n t by weight) .

4. Feed s l u r r y back-pressure Per batch To determine pressure ( p s i ) . h i s t o r y dur ing

operat ion.

5. Machine hydraul ic pressure Per batch TO a s ses s system opera- ( p s i ) . t i on .

6. F i l t r a t e f lowra te (gpm or Per batch TO c o r r e l a t e f i l t r a t e L/min). s o l i d s t o s o l i d s

capture . -

7. F i l t r a t e suspended s o l i d s Per batch see Comment 6. concent ra t ion .

8 . Tota l s o l i d s concent ra t ion Per batch To determine percent of cake (pe rcen t by weight ) . cap ture and s o l i d s

y ie ld .

9. Dosage r a t e for chemical Pe r batch To determine condi t ion- condi t ions and f i l t e r a i d s ing requirements. (mg/L, ppm, or percent by weight)

~- 10.Fged slEdge temperature Pe r batch Temperature a f f e c t s

( F or C ) . s o l u b i l i t y of p a r t i c l e s .

~- 10.Fged slEdge temperature Pe r batch Temperature a f f e c t s

( F or C ) . s o l u b i l i t y of p a r t i c l e s ,

11.Feed sludge pH. Per batch pH a f f e c t s s o l u b i l i t y of p a r t i c l e s .

2 38

The s o l i d s y i e ld f o r a s i n g l e run i s calculated a s follows:

Sol ids y i e ld = (Sol ids fed per run) - (Sol ids l o s t per run) - 3335 - 45 = 3,290 lbs.

The s o l i d s capture f o r a s i n g l e run is ca lcu la ted a s follows:

Sol ids capture (pe rcen t ) = (Sol ids y i e l d ) / ( S o l i d s f ed per run) x 100

(3,290/3,335) x 100 - 98.7 %


The operat ion and performance of pressure f i l t e r s f o r sludge dewa-

t e r i n g is determined by a r e l a t i v e l y small number of va r i ab le s , compared

t o other mechanical dewatering processes. Although con t ro l of the

process may be manual, semi-automatic, o r f u l l y automatic, t h e bas ic

operating cycle and machine r e l a t e d va r i ab le s a r e the same.

A typical cycle begins with the c los ing of the f i l t e r , a f t e r w h i c h

sludge is pumped i n t o the press u n t i l i t is e s s e n t i a l l y f u l l of cake.

Sludge pumping then continues with increasing back pressures and de-

creasing sludge flows. The end of t h e sludge feed s t e p i n t h e f i l t e r

cycle is determined when back-pressures reach the designed maximum

(usua l ly 100 p s i , but sometimes a s much a s 220-250 p s i ) . To reach t h i s

p o i n t t y p i c a l l y takes 20 t o 30 minutes . The high back-pressure i s then

maintained f o r a pe.ricd of usual ly one t o four hours, during which time

a d d i t i o n a l water is removed from t h e cake a s f i l t ra te . Based upon

e i t h e r time o r f i l t r a t e flow r a t e (which becomes near ly z e r o ) , t h e

f i l t e r cycle is ended by r e l i e v i n g the pressure, opening the f i l t e r , and

removing the s o l i d s cake. The only real var ia t ion on t h i s cycle is when

a var iab le volume f i l t e r is used. In t h i s case, t h e high-pressure

holding t i m e is much less, and it i s followed by an addi t iona l s t e p i n

239

which diaphragms behind t h e f i l t e r media are expanded hy high pressure

a i r o r water t o force out water and f u r t h e r concentrate s o l i d s i n t h e

cake.

T h e r e f o r e , t h e o n l y machine v a r i a b l e s t h a t a r e under o p e r a t o r

c o n t r o l are the time s e t t i n g s f o r each s t e p i n the f i l t e r cycle and, i n

the case of some machines, the operating back-pressures (which a r e

determined by the sludge feed pumps). Cycle t i m e is usua l ly the main

machine var iable a f f e c t i n g f i l t e r y i e ld . In r a r e cases, i t may be

necessary t o select a d i f f e r e n t type of f i l t e r medium t o improve per-

formance, p a r t i c u l a r l y i f the sludge cake is no t removed e a s i l y from the

medium o r high f i l t r a t e s o l i d s concentrations a re a problem.

O f the process (nqn-machine) var iables , the most obvious a r e those

r e l a t e d t o the sludge. The sludge c h a r a c t e r i s t i c s t h a t improve dewater-

i ng by vacuum f i l t r a t i o n a re a l s o advantageous f o r pressure f i l t r a t i o n .

Because of the g r e a t e r energy involved i n pressure f i l t r a t i o n , however,

the sludge va r i ab le s a r e considered t o be less of a f a c t o r i n dewatering

performance.

However, t h e var iab le over which t h e operator can exerc ise complete

con t ro l i s chemical conditioning. The wide range of chemicals used i n

pressure f i l t r a t i o n a l s o a r e s i m i l a r t o those f o r vacuum f i l t r a t i o n .

F i l t e r presses usua l ly require a f i l t e r a i d and/or a precoat mater ia l t o

reduce bl inding and t o f a c i l i t a t e release of the cake from the media

during the unloading stage. Inc ine ra to r ash o r diatomaceous e a r t h a r e

t y p i c a l preooat mater ia l s t h a t are added j u s t before sludge charging.

Precoat appl ica t ion is a c r i t i ca l s t e p i n the batch operat ing cy-

cle, and manufacturers' recommendations f o r mixing and applying precoat

must be followed e x p l i c i t l y t o achieve good r e s u l t s . A poor precoat can

cause s e v e r a l problems, one of which is high s o l i d s i n the f i l t r a t e .

However, the most important reason f o r achieving a good precoat i s t o

in su re good s o l i d s cake r e l ease from the media. Otherwise, i t w i l l be

necessary t o wash the medium with high pressure water sprays a f t e r each

batch t o prevent media bl inding and maintain performance. T h i s s t e p can

-

240

be extremely t i m e consuming, i n addi t ion t o defea t ing t h e whole purpose

of f i l t r a t i o n by d i l u t i n g w i t h water the cake s o l i d s t h a t remain s tuck

on the media.

Other operat ing procedures include rout ine water washing of t h e

f i l t e r media t o prevent bl inding and t o maintain f i l t e r yield. Occa-

s i o n a l l y ac id washing with d i l u t e muriat ic ac id is necessary t o remove

calcium carbonate scale t h a t forms on the media. It is a l s o OccaSiOnal-

ly necessary t o replace worn media, although t h i s is usual ly only done

a f t e r f i l t e r y i e lds drop t o the poin t t h a t water and acid cleaning a r e

no longer s u f f i c i e n t t o maintain acceptable y i e lds .


Most sludges from metals f i n i s h i n g wastewater treatment dewater

very well i n pressure f i l ters , provided proper precoat appl ica t ion and

chemical conditioning a r e used. Typically, a s o l i d s cake from 30 t o 50

percent by weight is achieved, with a s o l i d s recovery of 95 t o 99 per-

cent.


The troubleshooting guide for the pressure f i l t r a t i o n process is

presented i n Table 26. Major problem a reas include f i l t e r bl inding,

improper sludge conditioning, and high cake moisture content. Bo th

chemical and mechanical methods of solving these problems a r e con-

sidered.

241

TAWLF 26 PRFSSURF FILmATION MR DEUATFRING

TRouaLesH0on)x: GUIDE

C O R R W I V I liCTION PROWAPLE CAUSE CHECK m MONITOR BFAS(yI

N & N

OPFRATING PROBLEM 1: Lov c a k e solids Eoncentratlon lhlgh moist~re content i n cake).

la. F i l t e r cyc le t i ~ e too - opera t ing records to - To determine i f opti- - 1ncreaee f i l t e r cyc le tinme. shor t . correlate f l l t r a t e opera t ing condi t ions

f l w and cycle t i m e w i t h have changed. cake moisture content.

lb. Inproper sludge c a d i - - Chemical and f i l t e r a i d - To determine i f mech- - Repair ony ..lf"nctioni"g tioninq. feed systems and m i x e r s . anica1 problems exist 0, equip len t or change dosage

i f a dosage change is Of e i t h e r chemica l s or filter required. a i d end re-evaluate filter

performance.

OPERATING PRO" 21 Cake d ischarge l e d i f f i c u l t or i n c w p l e t e .

2a. Inadequate precDet on - Precoat s l u r r y .Iring and - To determine i f a l ech- - Repair a"y malfunctioning media. pvnping syste.. s n i c a l problem existe or e q u i p e n t or change e i t h e r

i f precoat mix or app1ice- precrmt Lii Or s p p l i c s t i o n procedUKe. Pol low manu-

for I n i t i a l changes.

t i o n procedure should be changed. f a c t u r e r e . recoomendations

- See I t e m lb. 2b. Improper sludge - See I t e m lb. - See i t e m lb. conditioning.

OPERATING PROBLEU 3: F i l t e r cyc le t i m e excess ive ly long.

3a. Feed s o l i d s concentra- - operation of upstream - Inadequate t h l c k e n l n g can - If p s e i b l e , Improve tion to2 IW. thickening process. check reevl f i n a" excessive perforneDce Of upstream

solids feed rate w i t h h i s - ti- to h i l d up 801168 torical data . cake. a c h i e v e h i g h e r feed 8011ds;

t h i c k e n i n g opera t ion to

o t h e r w i s e . see I t e m 36.

3h. Improper sludge cosdi- - See Item lb. - See Item lb. - See I t e m Ib. tioning.

N Ip w

TABLF 26

PRFSSURE F1LTR)lTION Wfl DRIlTEXING (Conllnued)

m o u n m " I w GUIDE

PROBABLE CAUSE CHECK OR HONITOR RPlSON CORflECTIVE lCTIOW

OPERATING PROBLEU 4: Frequent media blinding, failure to develop cake on media.

4a. Inadequate precmt OD - See Item Za. - see Ifem 2a. - see Iter 2a. -die.

4b. Imitial sludge feed - Feed sludge pumping rates - To establish whether - Keep Initial feed rate low to rate t m high (where and correlate with previous l w e r pumplng rates have develop cake slarlyr or no precmt is used). performance. previously resulted in conslder use of p~ecoar if

less blindlng and better problems persist. cake formation.

4c. Improper sludge - See i t e n lb. - See Item lb. - See Item lb. conditioning.

OPERATING PROBLEU 5: Pressures Increasing during precDat applicatlm.

5a. F i l t e r medla becoming - Filter media. blinded, possibly with carbonate scale.

- To establish If blinding - Y a r e r wash filter media. Acid has occurred. rash media to remove carbonate

buildup.

5b. Improper sludge con- - See Item lb. Also check for - See Item lb. dltioning causing media blinding. or increased f i l t e r cycle

high moisture content in cake

t i m e s .

- See Item lb.

5c. Improper precoat m i x - See Item 2.3. Also check - see Item 2a. - see 1te. 2a. or feed procedure. manufacturer's recommended

procedures for precwt applicalion.

OPER&TIP%' PROBLEM 6: Solids cake sticks to conveying e q u i p e n f after removal from filter.

ha. Improper sludge cundi- - See Item lb. tioni "9.

- See Item Ib. - See item Ib. Also may need to change to more inorganic chemicals (p.9. line) or different filter aid m f e r i a l .

i

i I

TABLE 26

PRPSSURE DILTRITIMI mR URIIITERING I (Continued)

mounL=n"c GUIDE

PROEIBLE CIUSE CHECK MI m N l ? O R R P I S M CORRECTIVE ACTION

~ ~~

OPERATING PROBLW 71 Failure to for. adequate Bedl between plates.

~~~~~~

7a. Plates O U t O f align- - Check alignment. - See Probable Cause. - Realign plates . ment.

7b. Inadequate skimming. - Check s t a y bo~ses. - See Probable Cause. - Adjust sklmmlng of stay

bosses.

lc. Foreign object - open ?*e** and inspect - See Probable Cause. - Remove forelgn object. pre"e"t1n.g a good sealing faces for foreign eea1. o b j e c t s .

-

' I I 1

SECTION 1 7

CENTRIFUGATION

INTRODUCTION

Centr i fugat ion is a process used f o r concentrating sludges generat-

ed during wastewater treatment. Often, cen t r i fuges a re used i n place of

g r a v i t y thickeners. Centrifuges have a l s o been I n s t a l l e d , usua l ly

following a thickener , a s dewatering devices. The d i f fe rence between

thickening and dewatering depends upon the f i n a l water content of the

sludge. The product of thickening is s t i l l a f l u i d t h a t can be pumped,

while the product of dewatering is a cake t h a t can be handled as a s o l i d

and i s t ransported by conveyers and dumpsters. Whether a sludge needs

t o be thickened only or dewatered p r i o r t o d isposa l depends upon many

f ac to r s . S imi l a r ly , the object ive of cent r i fuga t ion (whether f o r thick-

ening or dewatering) w i l l be d i f f e r e n t from one f a c i l i t y t o the next ,

although the bas ic p r i n c i p l e s and operat ing procedures are the same.

THEORY OF OPERATION

Centr i fugat ion works on the same p r i n c i p l e a s grav i ty s e t t l i n g and

thickening, except t h a t the c e n t r i f u g a l forces which cause t h e p a r t i c l e s

t o se t t le a r e much g r e a t e r than the force of gravi ty . I n a c l a r i f i e r or.

thickener, t h e suspended s o l i d s which a r e more dense than w a t e r s ink t o

the bottom of the tank leaving c l e a r l i q u i d above. In cen t r i fuges , a

bowl a c t s a s a highly e f f e c t i v e s e t t l i n g chamber. When sludge

i s introduced i n t o the bowl and is thrown aga ins t the i n s i d e wal l , it

forms a pool or l aye r of l iquid. The suspended s o l i d s i n t h i s pool move

toward the wall , a s a r e s u l t of t h e i r g rea t e r densi ty , and leave behind

a l a y e r of c l e a r l i q u i d ca l l ed the cen t r a t e . The c e n t r a t e then is

245

removed, leaving a thickened sludge o r s o l i d s cake f o r f u r t h e r t r e a t m e n t

o r disposal . Since the same p r inc ip l e is involved a s w i t h other thick-

ening devices, sludges t h a t thicken poorly by g r a v i t y a r e genera l ly more

d i f f i c u l t t o thicken and dewater by centr i fugat ion. As is the case i n

g r a v i t y thickening, both polymers and chemical coagulants o f t en a r e used

t o condition sludges p r i o r t o centr i fugat ion.


The method by which the cen t r a t e and s o l i a r e separate i s t h e

main d i f fe rence between t h e many types of cent r i fuges used i n wastewater

treatment. The bas ic operat ing p r i n c i p l e s a r e the same f o r a l l , how-

ever. Three bas ic types of centr i fuges have been used commonly f o r

wastewater treatment appl ica t ions i n the metal f i n i s h i n g industry; the

s o l i d bowl decanter , the imperforate basket , and the disk-nozzle cen t r i -

fuge. These w i l l be discussed separa te ly below.

S o l i d Bowl Decanter

The s o l i d bowl decanter centr i fuge, a l s o sometimes c a l l e d a s c r o l l

or conveyor centr i fuge, i s the most common type used f o r dewatering

wastewater sludges. This type of cent r i fuge operates i n a continuous or flow-through mode and c o n s i s t s of a r o t a t i n g hor izonta l c y l i n d r i c a l bowl

containing a screw type conveyor or s c r o l l w h i c h a l s o r o t a t e s , b u t a t a

s l i g h t l y lower o r higher speed than t h e bowl, a s shown i n Figure 3 1 .

The d i f f e r e n t i a l speed between the bowl and t h e conveyor is used t o move

the s o l i d s toward one end of the bowl f o r discharge. The bowl is usu-

a l l y a cyl inder made of mild o r stainless steel with a conica l s ec t ion

a t one end. The conica l sec t ion is the discharge end f o r t h e sludge and

serves a s a dewatering beach o r drying deck. Sludge e n t e r s t h e r o t a t i n g

bowl through a s t a t i o n a r y feed pipe extending i n t o the hollow s h a f t of

the r o t a t i n g conveyor and is d i s t r i b u t e d through p o r t s i n t h i s hollow

s h a f t i n t o a pool within the r o t a t i n g bowl.

. .._.____

There a r e two main types of s o l i d bowl centr i fuges. The most com-

mon is the counter-current type i n which the s o l i d s cake and c e n t r a t e

246

Figure 31. Cross sect ion of concurrent flow solid - bowl centrifuge.

247

move toward opposite ends of t h e bowl . f o r discharge. The c e n t r a t e i s

decanted off the top of the l i q u i d pool through a p o r t or over a weir, t h e locat ion of which can be adjusted t o change the depth of t h e l i q u i d

within the bowl. In the other type of cen t r i fuge , the concurrent flow

type (Figure 31 1, the sludge s l u r r y i s introduced a t the f a r end of t h e

bowl from the dewatering beach and the sludge s o l i d s and c e n t r a t e both

flow i n the same d i r ec t ion . The cen t r a t e is withdrawn by a skimming

device or r e tu rn tube located near the junct ion of the bowl and the

beach. C l a r i f i e d cen t r a t e then flows i n t o channels i n s i d e the s c r o l l

hub and r e t u r n s t o the feed end of the machine where i t i s discharged

over ad jus tab le w e i r p l a t e s through discharge p o r t s b u i l t i n t o t h e bowl

head"

Imperforate Basket Centrifuge

The imperforate basket centr i fuge, a l s o known as a knife-discharge

centr i fuge, is a semi-continuous feeding and s o l i d s discharging u n i t

t h a t r o t a t e s about a v e r t i c a l a x i s , a s shown i n Figure 32. Sludge is f ed i n t o t h e bottom of the basket and sludge s o l i d s form a cake on t h e

bowl walls as the u n i t ro t a t e s . The cen t r a t e is displaced over a b a f f l e

o r w e i r a t the top of the uni t . Sludge feeding is continued e i t h e r f o r

a s p e c i f i c period of time ot' u n t i l the suspended s o l i d s i n the c e n t r a t e

reach a predetermined concentration.

After sludge feeding i% stopped, the cent r i fuge slows t o about 70

rpm, and a plowing kni fe moves i n t o pos i t i on , e i t h e r automatical ly or

manually, t o cut the sludge away from the wal ls ; t h e sludge cake then

drops through the open bottom of the basket. After plowing t e r m i n a t e s ,

t h e cent r i fuge begins t o acce lera te and feed sludge again i s introduced.

A t no time does t h e cent r i fuge a c t u a l l y s t o p ro t a t ing . Many u n i t s a l s o

have a skimming s t e p which precedes plowing. A s p e c i a l skimmer nozzle

swings i n t o pos i t ion t o remove the low concentration s o l i d s which remain

i n the basket near the i n s i d e surface of the l i q u i d pool. These a r e

recycled back t o an upstream process along with the cen t r a t e .

-

248

FEED

POLYMER 1 I In SKIMMINGS

1 CAKE ' CAKE

Figure 32. Schematic diagram of a basket centrifuge.

.- - - - .

243

In smaller f a c i l i t i e s , where the volume of sludge t o be handled i s

less, cent r i fuga t ion may be s t r i c t l y a batch operation i n which r o t a t i o n

s tops completely a f t e r each batch is processed. I n t h i s case, operat ion

is usual ly no t automated and the equipment is somewhat simpler i n de-

sign.

Although the cake s o l i d s concentration and y i e l d a r e not a s high

f o r t h e basket cen t r i fuge a s f o r a s o l i d bowl cen t r i fuge , s o l i d s recov-

e r y is genera l ly b e t t e r and o f t en less polymer and o ther condi t ioning

chemicals a r e required. Obviously, the cake s o l i d s concentrat ion must be considered a s average s o l i d s content, since the s o l i d s content i s

maximum a t the bowl wall and decreases toward t h e center .

Disk-Nozzle Centrifuge

The disk-nozzle cent r i fuge is a continuous-flow v e r t i c a l bowl ma-

chine which contains a number of c lose ly spaced d i sks a g a i n s t which t h e

p a r t i c u l a t e so l ids set t le , a s shown i n Figure 33. The feed s l u r r y

normally e n t e r s through the top although bottom feed i s also poss ib l e ,

and passes down through a feedwell i n the cen te r of the ro to r . An

impel ler within the r o t o r acce lera tes and d i s t r i b u t e s t h e feed s l u r r y ,

f i l l i n g the r o t o r i n t e r i o r . The heavier s o l i d s se t t le outward toward

the circumference of the r o t o r under increasing c e n t r i f u g a l force. The

l i q u i d and the l i g h t e r particles flow inward through the cone-shaped

d i s c stack. These l i g h t e r s o l i d s a r e s e t t l e d out on the underside of

the d i s c s , where they agglomerate, s l i d e down the d i s c s , and miqrate o u t

t o the nozzle region, The c e n t r i f u g a l ac t ion causes t h e s o l i d s t o con-

c e n t r a t e a s they se t t le outward. A t the outer r i m of t h e r o t o r bowl,

the high energy imparted t o the f l u i d forces t h e concentrated mater ia l

through t h e r o t o r nozzles.

.

Often, a port ion of the thickened sludge discharge i s recycled back

i n t o the concentrat ing chamber near the base of the r o t o r t o increase

the f i n a l concentrat ion i n the sludge and t o help prevent plugging of

the nozzles. I n addi t ion t o providing a f lushing ac t ion , the recycle

a l s o permits the use of l a r g e r nozzles w h i c h a r e less l i k e l y t o plug.

- - -. __ ___

2 5 0

SLUDGE DISCHARGE

Figure 33. Disc type centrifuge.

251

Plugging o r clogging of the nozzles o r the space between d i s k s (which

a r e general ly separated by only 1 t o 2 m m ) i s the main reason t h a t disk-

nozzle cent r i fuges have not gained wider acceptance i n wastewater t r e a t -

ment. However, f o r thickening sludges from the treatment of metal

f i n i s h i n g wastes, which contain l i t t l e o r no f ib rous or g r i t t y ma te r i a l ,

they a r e gaining acceptance.

OPERATIGNAt PROCEDURES

The purpose of cent r i fuga t ion is t o minimize both the w a t e r content

of the thickened sludge or s o l i d s cake and the s o l i d s concentrat ion i n

the c e n t r a t e while maintaining s u f f i c i e n t machine throughput t o prevent

a buildup o r accumulation of sludge i n upstream treatment processes.

Generally, e f f o r t s t o increase the s o l i d s concentration r e s u l t i n a

c e n t r a t e of poorer q u a l i t y , while improving c e n t r a t e q u a l i t y usua l ly

reduces the sludge or cake s o l i d s concentration. Acceptable l i m i t s on

the s o l i d s content of the c e n t r a t e a r e determined when the recycled

c e n t r a t e begins t o i n t e r f e r e with the operat ion of o ther treatment pro-

cesses o r f i n e s o l i d s begin t o bui ld up t o unacceptable l e v e l s i n t h e

sludge handling system. Although a high s o l i d s concentration and good

c e n t r a t e q u a l i t y o f t en can be achieved a t the same time, t o improve both

may r equ i r e t h a t the machine throughput o r s o l i d s y i e ld be reduced and

t h a t the use of chemicals f o r conditioning be increased. There i s a

l i m i t on how much the y i e l d can be reduced, a s s o l i d s must be removed

f o r d i sposa l as f a s t a s they a r e generated o r they w i l l bu i ld up and

a f f e c t the performance of o ther treatment processes. S imi l a r ly , t he re

are l i m i t s on chemical usage s i n c e such usage represents a s i g n i f i c a n t

p a r t of t h e t o t a l o p e r a t i n g . c o s t s a s s o c i a t e d w i t h c e n t r i f u g a t i o n .

Therefore, g o d . cen t r i fuge operat ion is a compromise between f i n a l

s o l i d s concentrat ion, c e n t r a t e q u a l i t y , s o l i d s loading o r y i e l d , and

operat ing cos t s .

Process Monitoring

The main c r i t e r i a used i n evaluat ing centr i fuge pekformance a re

concentrat ion of the s o l i d s discharge, concentration of s o l i d s i n t h e

2 5 2

c e n t r a t e , s o l i d s capture o r recovery, and s o l i d s reinoval or y ie ld (or

machine throughput). of t h e s e the l a s t two require some explanation.

Sol ids capture , also ca l l ed s o l i d s recovery, i s t h e mass r a t i o of

cake ( o r thickened s ludge) s o l i d s t o the feed s o l i d s over equal sampling

periods. A lower s o l i d s capture means t h a t a g r e a t e r f r a c t i o n of t h e

feed s o l i d s a r e l o s t t o the cen t r a t e and a r e n o t p a r t of the s o l i d s d i s -

charge. For a constant s o l i d s feed r a t e , the r e s u l t is a higher concen-

t r a t i o n of s o l i d s i n the cen t r a t e .

The s o l i d s y i e l d (removal) r e f e r s t o the amount of s o l i d s , on a dry

weight bas i s , t h a t can be removed.by the cent r i fuge during a given per-

iod of t i m e . AS a long term average, the y i e ld must equal o r exceed the

s o l i d s feed r a t e i f sludge s o l i d s a r e not t o accumulate i n t h e treatment

system.

In order t o evaluate cent r i fuge performance and diagnose problems,

a complete record of s eve ra l operat ing parameters must be kept. These

a r e l i s t e d i n Table 27.

The information l i s t e d i n Table 27 should be recorded rou t ine ly ,

along with machine operat ing condi t ions such a s bowl speed, weir set-

t i n g s o r pool depth, sludge recycle r a t e , e t c . It i s p a r t i c u l a r l y

important t h a t any changes i n performance or operat ing condi t ions be

recorded along with the time t h a t they occur.


Example ca lcu la t ions of operat ing parameters a re presented below.

Consider a continuous s o l i d bowl cent r i fuge operation f o r which t h e

following data have been col lected:

Feed sludge flow r a t e = 40 gpm

Centrate flow r a t e = 36 gpm Feed suspended s o l i d s = 40,000 mg/L (4 .0 pe rcen t )

Centrate suspended s o l i d s = 2,250 my/L (0.23 percen t )

253

TABLE 27

CENTRIFUGATION PROCESS MONITORING REQUIREMENTS

Parameter Frequency Comment ~ ~~ ~~

1 . sludge f lowra te o r volume.

2. Sludge suspended s o l i d s concentrat ion.

3. Dewatered cake suspended s o l i d s concentrat ion.

4. Dewatered cake volume.

5. Cent ra te f lowrate.

6. Cent ra te suspended s o l i d s concentrat ion.

7. pH, Temperature

8. Polymer type and dosage.

Daily

Daily

Daily

Daily

Weekly

Weekly

Daily

Weekly

To determine s o l i d s feed r a t e .

To determine s o l i d s feed r a t e .

To determine cen t r i fu - ga t ion dewatering performance.

To determine volume of cake t o be disposed.

To determine s o l i d s capture .

To determine s o l i d s capture .

TO eva lua te e f f e c t s of pH and temperature.

To determine condi t ion- i n g requirements.

254

Solids cake concentration = 350,000 mg/L (35 percen t )

Polymer dosage r a t e = 2 lb/ ton

Concentration of polymer feed = 1.0 percent

The s o l i d s feed r a t e (dry s o l i d s fed t o cent r i fuge per hour) is

ca lcu la ted as follows:

So l ids Feed Rate - ( feed sludge flow r a t e ) x ( feed suspended s o l i d s )

= ( 4 0 gpm) ( 6 0 min/hr ) (3.785 L / g a l ) x ( 4 0 g/L) - 363 kg/hr = 800 l b f i r

Sol ids removed (dry s o l i d s p e r hour) is ca lcu la ted a s follows:

Sol ids removed = ( feed s o l i d s mass appl ied) - ( c e n t r a t e s o l i d s mass)

= (800 l b / h r ) - (2.25 g/L)(lb/454g)(36qpm)(3.7854 L/gal)

(60min/hr)

= 760 l b /h r

So l ids capture or s o l i d s recovery is ca lcu la ted a s follows:

So l ids Capture = (760)/(800) x 100 = 94.9 percent


The operat ion and performance of a cent r i fuge i s determined by a

number of var iab les t h a t can be r e l a t e d either t o the machine i t s e l f

(machine v a r i a b l e s ) or the s p e c i f i c sludge thickening or dewatering

appl ica t ion (process v a r i a b l e s ) . Although these va r i ab le s d i f f e r for

Various types of cen t r i fuges , many a r e of general importance. Among

them a r e the following.

255

Machine Variables

Centrifuge Design

Bowl Speed

Pool Depth

Conveyor Speed

Process Variables

Sludge C h a r a c t e r i s t i c s

Chemical Addition

Sol ids Concentration

Sol ids Feed Rate

Temperature

PH

Recycle of Sludge

These var iab les and the e f f e c t t h a t these va r i ab le s have on s o l i d s

recovery and cake s o l i d s concentration a re presented i n Table 28.

Machine Variables--

Some o f t h e most i m p o r t a n t machine v a r i a b l e s a r e f i x e d by t h e

cen t r i fuge design and a r e n o t under operator control. Among these a r e

the bowl length t o diameter ratio, bowl angle o r s lope of the dewatering

deck, s c r o l l or conveyor design, flow p a t t e r n s (concurrent o r counter-

c u r r e n t ) , d i s k spacing, and nozzle diameter. Although important a s

design choices, a d e t a i l e d discussion of these var iab les and t h e i r

effects on t h e process w i l l not be presented i n t h i s manual.

The r o t a t i o n a l speed of the cent r i fuge i s one of t h e most important

f a c t o r s a f f e c t i n g performance s i n c e c e n t r i f u g a l fo rce speeds up the

sedimentation process. An increase i n bowl speed provides more gravi ty-

s e t t l i n g fo rce , thus providing g r e a t e r c l a r i f i c a t i o n of the cen t r a t e and

compaction of the so l id s . In s o l i d bowl decanters , g r e a t e r c e n t r i f u g a l

fo rces a l s o can h e l p t o squeeze water o u t of the sludge on t h e drainage

deck, thereby producing a d r i e r s o l i d s cake. Sometimes, higher bowl

speeds can reduce polymer usage. I f these advantages outweigh the

increased power c o s t s , operat ion a t higher speeds can be bene f i c i a l .

. . There a r e problems with increasing bowl speeds, however. Higher

speeds sometimes r e s u l t i n shear ing of the sludge f loc . Since smaller

p a r t i c l e s s e t t l e ' m o r e slowly, higher bowl speeds can r e s u l t i n a higher

concentration of suspended s o l i d s i n t h e cen t r a t e . This problem can be

avoided l a r g e l y i n machines where coagulant a i d s a re added i n t e r n a l l y

256

TABLE 26 SUMMARY OF OPERATIONAL VARIABLES AFFECTING

CENTRIFUGE PERFORMANCE

Effect of increase i n variable on Variable % Solids Recovery Cake Solids Concentration

Machine Variables

Bowl Speed Increase

Pool Depth Increase

Scro l l ing Speed Decrease

Increase

Decrease

Decrease

Process Variables

Feed Rate Decrease Increase

Feed Concentration Decrease Increase

Temperature Increase Increase

Chemical Addition Decrease Increase

*Sludge Recycle Decrease Increase

* Disk-nozzle cent r i fuges only.

257

i n t o ' the bowl and f loccula t ion occurs a f t e r the s o l i d s a r e up t o t h e

bowl speed. Another problem with higher speeds, e s p e c i a l l y i n s o l i d

bowl centr i fuges with a conveyor o r s c r o l l , is t h a t tremendous pressures

a r e set up between the s o l i d s and the surfaces of s c r o l l and bowl,

t e n d i n g t o lock t h e s e two p a r t s t o g e t h e r . F u r t h e r , w i t h a b r a s i v e

sludges, high f r i c t i o n and c o s t l y wear resu l t . Some sludges, though no t

e s p e c i a l l y abrasive, have poor s l i d i n g c h a r a c t e r i s t i c s and resist con-

veying. This res i s tance can impose high loads on the s c r o l l and on the

gear u n i t which d r ives it. Therefore, f i n a l adjustment of bowl speed

must be a compromise between degree of c l a r i f i c a t i o n , degree of cake

dryness, chemicals used, and c o s t s of maintenance and power. Because of

these and o ther complicating f a c t o r s , bowl speed is not normally varied

on most cen t r i fuge models once a cent r i fuge i s i n s t a l l e d .

Pool depth a f f e c t s the performance of both conveyor and basket

centr i fuges, while disk-nozzle centr i fuges usual ly a r e designed t o

operate completely f i l l e d without a var iable pool depth. ' S e t t l i n g

theory says t h a t t h e b e s t c l a r i f i c a t i o n (or s o l i d s cap tu re ) occurs when

l i q u i d depths a r e shallow, s ince the s o l i d p a r t i c l e s have a s h o r t e r

dis tance t o t r a v e l t o be separated from the l iquid. However, experience

has shown the re t o be a l i m i t t o how shallow t h e pool can be and s t i l l

maintain good s o l i d s capture. Too shallow a pool r e s u l t s i n a high

l inear ve loc i ty through the centr i fuge, a condition which causes turbu-

lence t h a t resuspends s e t t l e d p a r t i c l e s and prevents s e t t l i n g of very

f i n e p a r t i c l e s . In conveyor type cen t r i fuges , the turbulence caused by

t h e s c r o l l a l s o has more adverse e f f e c t on s e t t l e a b i l i t y when the pool

depth is shallow. S ince the residence t i m e wi thin the cent r i fuge is

s h o r t e r when the pool depth i s shallower, less time is ava i l ab le f o r

flocculation to cccur. This also can resul t in poorer c e n t r a t e quality.

There a r e l i m i t s t o how deep the l i q u i d pool i n a cent r i fuge can

however. As s e t t l i n g theory p red ic t s , increasing t h e depth too much

can a l s o cause a low s o l i d s capture and poor cen t r a t e q u a l i t y s ince t h e

p a r t i c l e s must se t t le through a g r e a t e r dis tance t o be removed. There

a r e a l s o cases i n which a shallow pool i s desirable . In a s o l i d bowl

conveyor type cen t r i fuge , lowering the pool exposes more of the drainage

258

deck or beach a rea , and increases the time ava i lab le f o r dewatering.

T h i s condition r e s u l t s i n a d r i e r s o l i d s cake.

With most centr i fuges, pool depth i s more e a s i l y changed than bowl

speed, but s t i l l requires s eve ra l hours of labor and down-time. There-

fo re , o t h e r remedies usua l ly a r e t r i e d f i r s t when a problem exists.

In s o l i d bowl decanter cen t r i fuges , the conveyor or s c r o l l r o t a t e s

a t a s l i g h t l y d i f f e r e n t speed ( e i t h e r f a s t e r o r slower) than the bowl.

This speed d i f f e r e n t i a l can normally be varied by ad jus t ing gear r a t i o s

manually while the machine is not running. Some cent r i fuges a r e equip-

ped with an automatic backdrive, making the d i f f e r e n t i a l speed r e l a t i v e -

l y easy t o change. Usually the pool depth is f ixed i n such machines.

Conveyor speeds normally a re designed or adjusted t o t h e minimum t h a t

s t i l l provides s u f f i c i e n t conveying capacity.

Increasing the d i f f e r e n t i a l between the bowl speed and the s c r o l l

speed normally results i n a wetter sludge cake and poorer s o l i d s re-

covery because of increased turbulence and reduced s o l i d s residence

time. However, t h e machine throughput o r y i e l d is increased; t h a t is ,

more sludge can be processed i n a given period of time. A reduction i n

d i f f e r e n t i a l speed reduces turbulence in s ide the pool, y ie ld ing a b e t t e r

q u a l i t y cen t r a t e and a d r i e r cake. D i f f e r e n t i a l speed reduction a l s o

reduces the r a t e of wear on t h e conveyor blades when poorly d e g r i t t e d

sludges a re handled. The main disadvantage is t h a t machine throughput

i s reduced.

Operating a t too low a d i f f e r e n t i a l speed can cause the p i l e of

s o l i d s formed i n f r o n t of the s c r o l l conveyor blade t o increase i n over-

a l l he ight such t h a t it i n f r i n g e s on t h e c l a r i f i e d l i q u i d area. This

condi t ion may r e s u l t i n the skimming of some f i n e s o l i d s from the top of

the cake p i l e t o the c e n t r a t e , thereby lowering s o l i d s capture. A t very

low differentials, it is even possible t o plug a centrifuge w i t h solids.

259

Process Variables--

Among t h e process-related va r i ab le s , the most bas ic a re those asso-

c i a t e d w i t h the sludge i tself . The f a c t o r s t h a t inf luence s e t t l i n g and

g r a v i t y thickening a l s o a f f e c t centr i fugat ion. Among these a r e p a r t i c l e

size, shape, and dens i ty , v i s c o s i t y of the l i q u i d , and consis tency of

t h e sludge. These va r i ab le s a r e genera l ly n o t under d i r e c t cont ro l of

the operator , but simply r e s u l t from optimizing upstream treatment pro-

cesses. I n some cases, changes i n the operat ion of an upstream process

can a f f e c t the thickening and dewatering proper t ies of t h e sludge gene-

r a t ed without reducing performance.

Of ten , polymers or o t h e r chemica l c o n d i t i o n e r s can be used t o

improve the c h a r a c t e r i s t i c s of a sludge p r i o r t o cent r i fuga t ion . These

polymers genera l ly work by promoting f loccula t ion of the sludge s o l i d s ,

thereby increasing s o l i d s capture. Usually f loccula t ing agents a r e

required t o achieve an acceptable cen t r a t e q u a l i t y . Although polymers

can sometimes improve the compatabili ty of a sludge, t h e i r use i n cen-

t r i f u g e s genera l ly r e s u l t s i n a wetter s o l i d s cake. The wetter cake

r e s u l t s because the a d d i t i o n a l p a r t i c l e s t h a t a r e captured a r e very f i n e

and t h e i r presence i n the s o l i d s cake makes it more d i f f i c u l t f o r t h e

entrapped water t o be released. J a r tests a re used t o select the b e s t

polymers and t o determine approximate dosage l e v e l s , a f t e r which oper-

a t i n g experience is required t o optimize polymer usage. Very small

changes i n sludge proper t ies sometimes can r equ i r e t h a t polymer dosages

be changed and can occasional ly even n e c e s s i t a t e the use of d i f f e r e n t

types o r combinations of polymers. In addi t ion t o wasting money, adding

t o o much polymer (overdosing) can h u r t performance.

The s o l i d s concentration i n the sludge feed can a f f e c t both s o l i d s

capture and cake dryness. A higher feed concentration general ly r e s u l t s

i n lower s o l i d s capture and higher concentrations of suspended s o l i d s i n

t h e cen t r a t e . However, depending upon the Concentration range, t he re

a r e some sludges which show b e t t e r c l a r i f i c a t i o n a t higher s o l i d s con-

cent ra t ions . The concentration of s o l i d s i n the feed s l u r r y can a l s o

a f f e c t the optimum coagulant dose, and j a r tests should be performed

whenever the feed concentration changes s i g n i f i c a n t l y . The other e f f e c t

.- .

260

of higher s o l i d s concentrations i n the feed is t o produce a d r i e r sludge

cake. If a d r i e r cake i s an object ive of cent r i fuge operat ion, changes

i n the operation of upstream treatment s t eps which a f f e c t sludge concen-

t r a t i o n , such as c l a r i f i c a t i o n and thickening, can improve cent r i fuge

performance . The r a t e a t which sludge is fed t o a cent r i fuge is important. When

the volume of sludge centrifuged during a given period of time is in-

creased, the residence t i m e within the cent r i fuge i s decreased and more

s o l i d s a r e l o s t i n the centrate . However, a higher feed r a t e sometimes

can r e s u l t i n a d r i e r cake since the addi t iona l s o l i d s lest i n the

c e n t r a t e a re f i n e s which tend t o en t r ap water within the cake. When

cent r i fuga t ion i s n o t performed on a continuous bas i s , taking a longer

period of time t o cent r i fuge a batch of sludge can improve performance

and possibly can reduce chemical requirements f o r coagulation. In

f a c i l i t i e s i n which cent r i fuga t ion is a continuously operated process,

the feed rate can only be decreased by increasing t h e Concentration of

the sludge feed.

Recycle of concentrated s o l i d s usual ly is performed only i n disk-

nozzle type centr ieuges where seve ra l funct ions a r e served. F i r s t ,

recycle permits con t ro l over the s o l i d s residence time i n the cent r i fuge

and h e l p s t o optimize operation under equilibrium conditions. If t h e

feed r a t e changes, the recycle r a t e can be changed accordingly t o main-

t a i n the required s o l i d s residence t i m e t o achieve the desired degree of

s o l i d s concentration. Second, the u s e of recycle helps keep the velo-

c i t i es high through t h e concentrating sec t ion so t h a t the nozzles do no t

become plugged. In f a c t , some centr i fuges a re designed with l a r g e r

nozzles ( t o prevent plugging) which require t h a t recycle be used i n

order t o achieve the necessary s o l i d s residence times. The t h i r d reason

f o r recycle o r f o r changing the recycle r a t e is t o provide a longer

__._ -. residence t i m e and thereby achieve even g rea t e r concentration of t h e

sludge so l id s .

261

Another process var iable a f f e c t i n g cent r i fuga t ion i s temperature.

A s is the case i n g rav i ty s e t t l i n g and thickening, an increase i n sludge

temperature reduces the v i s c o s i t y of t h e l i qu id and r e s u l t s i n b e t t e r

cen t r i fuge performance s ince s o l i d p a r t i c l e s s e t t l e f a s t e r . Although

temperature i s n o t usual ly a cont ro l led va r i ab le , it i s of importance i f

seasonal changes a f f e c t the temperature of the sludge fed t o a cen t r i -

fuge. For example, paorer s o l i d s capture and a wetter cake can be

expected during winter months when sludge temperatures drop.

Changes i n e i t h e r o r machine o r process-related va r i ab le s genera l ly

r e s u l t i n changes i n centr i fuge performance. Some f a c t o r s which a f f e c t

performance a r e n o t subjec t t o change, such a s those r e l a t ed t o c e n t r i -

fuge design o r sludge thickening and dewatering c h a r a c t e r i s t i c s . Of t h e

var iables which can change, some a r e used i n c o n t r o l l i n g cent r i fuge

operat ion while o the r s (such as temperature and feed concentrat ion) may

depend upon the operat ion of upstream treatment processes. The t y p i c a l

effects of these va r i ab le s on performance a r e summarized i n Table 23.

I t should be noted t h a t these e f f e c t s only apply over normal operat ing

ranges when one var iable is changed a t a t i m e . Generally pred ic t ions

cannot be made a s t o the e f f e c t s of changing seve ra l va r i ab le s a t once.


TWO f a c t o r s must be considered when r e f e r r i n g t o t y p i c a l per for -

mance values f o r cent r i fuga t ion of sludges generated during treatment of

metals f i n i s h i n g wastewater. F i r s t is the type of sludge. I f sodium

hydroxide is used f o r p H adjustment t o p r e c i p i t a t e metal hydroxides, a

l i g h t , slimy, and r e l a t i v e l y d i f f i c u l t t o dewater sludge w i l l r e s u l t .

When l i m e is used, the amount of excess lime determines the dewatering

c h a r a c t e r i s t i c s of the sludge. When very l i t t l e or no excess lime is

added, most of t h e sludge formed w i l l be metal hydroxides. I f consider-

a b l e excess l i m e i a used, a l a r g e amount of calcium carbonate w i l l be

present i n t h e sludge, making it heavier and more e a s i l y dewatered.

The second f a c t o r t o consider is the objec t ive of cent r i fuga t ion .

Most of ten, cen t r i fuges a r e used f o r dewatering and a r e sized t o give a

262

high s o l i d s concentration cake. Sometimes, however, cen t r i fuges a r e

intended t o funct ion as thickeners p r i o r t o f i n a l dewatering by another

process. This is espec ia l ly common when disk-nozzle centr i fuges a r e

used. The product of c e n t r i f u g a l thickening is s t i l l a pumpable l i q u i d ,

r a t h e r than a s o l i d s cake.

Table 29 gives some typical appl ica t ions and performance values f o r

d i f f e r e n t types of cent i fuges applied t o both metal hydroxide and l i m e

carbonate type sludges.


The problems which a re commonly associated with cent r i fuge opera-

t i ons have been summarized i n the form of a troubleshooting guide i n

Table 30. This is a general guide and some s p e c i f i c types or brands of

cent r i fuges may be subjec t t o unique operat ing problems which usua l ly

a r e covered i n the manufacturer's l i t e r a t u r e .

The most important f a c t t o remember regarding centr i fuge operat ion

is t h a t a l l of t h e machine and process va r i ab le s which have been discus-

sed a r e i n t e r r e l a t e d i n some way. Therefore, when evaluat ing the e f f e c t

of any one va r i ab le , care must be taken t o in su re t h a t a l l o the r s remain

unchanged. Otherwise, t h e cause of a p a r t i c u l a r problem (or i ts solu-

t i o n ) cannot be determined with ce r t a in ty .

263

TABLE 29 TYPICAL P E R F O W C E VALUES FOR CENTRIFUGAL

THICKENING AND DEWATERING

Feed Solids Cake Solids S o l i d s Polymer Centrifuge Concentration Concentration Recovery Dosage

( lb. per Type/S ludge ( 0 ) ( % ) ( % ) Type dry ton)

Solid Bowl (Scrol l )

Metal Hydroxide 0.5-4 15-45 80-95 0-1 0

High CaCo3 2-8 90-99 20-50

Imperforate Basket

Metal Hydroxide 0.5-4 10-30

High CaC03 2-8

80-95

90-99

0-5

15-45

*Disk-Nozzle

Metal Hydroxide 0.2-1 1-5 85-90 0-1 0

High CaCo3 0.5-2 2-10 90-95 0-1 0

*Generally used only for thickening.

264

TABLE 30 CPNTRIWGITlON ~OUWLFSHOOTIW GUIDF

PROBABLE CAUSE lcentrifuge Type)

CHECK OR W I T O R RFASON

OPERATING PROBLFM I: P-r centrate quality - low solids recovery.

la. Impropr chemical - Chemical addition dosage. (a11 types1 system.

lb. Peed rate t- high. - Flow d a b records (A11 types) and performance.

s e t t i n g r a y be - Repair Or reset equ,p- wrang oc e q u i p e n t Dpnt O r perfor. jar .alf""ctio"ing. If tests to establish new neither. sludge plymer dosage. characteristics may have changed.

nigh f l a s can - Reduce flov to c e n t r i - reduce detention fuge or reduce sludge

with period of g o d

this is the case.

times. ccupare recycle.

operation to *ee if

- Dilute sludge feed or - Determine if g o d IC. Feed solids - Check feed solids concentration tM against previous operatian has been reduce flow rate. hlgh. (A11 types) values. achieved at the came

high feed solids concentration.

ld. Pool depth setting - Check weir or ef- '- Generally, pmr urong.(Solid b l , fluent port position solids capture

operating records. shall- a pml depth, but not always. conpace with previous performance.

basket) and conpare with results fron COO

le. Improper speed dif ferentia1 &tween bar1 and conveyor.(Solid bow1 I

If. Worn or damaged conveyor flights. ( S o l id bowl 1

- Differential speed s e t t i n g and operating records. Erceasire build-up of solids in bowl.

- Vibration or unusual noise; exressivr build-up of solids i n bowl.

- Probably' t m great a differential causing turbulence i n liquid -1. If solids build-up is great, speed differential may be too small.

Puild-up of solids raYSeS conveyor mtcainnent into re" tra te di s c l ~ d i yp.

- Adjust weir or port settings.

- Adjuat differential speed.

Repair or replace solids conveyor.

TAPLF 30 (Continued)

CENTRIRKiATTION TROUBLESHDVPlffi GUIDF

PROBABLE CAUSF C H K K OR HONImOR REISON CORRFCTIVE ACTION

N m m

19. 1.proper bml - Check howl speed and speed.lA11 types) operating records.

lh. Solids discharge - Check solids dir- line or "Ollie charge l i n e and

solid bowl) objects and low flow. plugged.lDiek-noirle, nozzle for foreign

OPERATING PROBLUl 2: Cake or sludge discharge too wet.

- Insufficient speed - Increase bml res"1tB i n p r speed or repair solids capture. or replace L 1 f Y . C .

tioning equipment.

- Lar sludge discharge - UnFlvg solid. discharge line or n a r 1 e . resv1ts in p r

solids capture.

2d. lvproper chemical dosage.(All types)

2h. Feed rate too high. 11111 types)

2c. Feed solids coscentration too IOW.IAII t y p e )

2d. Pool depth t m g r e a ~ f s o l i d bowl, Easket)

2e. Improper bowl sped. (A11 types)

2f. rentrate outlet partially Plugged. ( S o l i d howl. Disk- ,,'rrzlrl

- Chemical addition system and chenical flow rate.

- Flow and performance data.

- solids concentration'aed operating record*.

- weir or effluent PYt s e t t i n g .

- Check bml speed and Operating recorda.

- Check outlet for fore ign objects.

- seftlng may be wrong or eq"ip.e"t .alf""ctio"ing.

- High flows reduce retention the. Compare with period of g m a operation.

- Higher feed solids usually give e drier cake.

- Shallower -1 exposes more beach or drying deck.

- Insufficient s p e d results in poor sludge conpaction.

- LOW ccntrate flow results is thinner sludge discharge.

- Repair equiment

Run jar tes ta ta or adjust doaage.

determine dosage.

- Reduce flow to centrifuge or

recycle. increase sludge

- Deterelme If thickened sludge can be obtained f r a prev1ova treat-nt steps.

- Adjust weir or put SeLfi"gS.

- Increase speed or repair/rep1ace malfunctioning equi p e n t .

- ClPA" outlet port.

N

-4 m

PROBABLE CAUSE CHECK OR MONI?OR RFASW CORReCTIVE ACTION

OPERATING PROBLM 3: lligh torqe alarm.

3a. Feed r a t e too high. - Flow records. (A11 types)

3b. Feed solids too - Solids data records. high.(All types)

3c. Foreign Uterial in - Inspect interior. aachine.(All types)

3d. Gear unit mi=- - Vlbrarion. aligned.(All types)

le. faulty bearing, - Inspect qear ""it. gear. (A11 types)

- Reduce flows.

- Dilute sludge or reduce flow rate.

- Remove forelg" .aterial.

- correct a1ign.ent.

- Replace faulty parts.

OPERATING PROBLM 4: eicessive vibration.

4a. Improper lubrication. (A11 types)

- Check lubrication system.

4b. Improper adjumt- - vibration isolators. .ent of vlbrafion isolators.(All types)

4c. Discharge funnels - P o s i t i o n of funnels. may be contacting centrifuge.(All types1

4d. Portion of conveyor - inter ior of machine. flights may be plugged with solids causing imbaiaice. (Solid b o w l 1

4s. Gear box i o p r o p r l y - Gear box a1ignnr,,t. aligned.(l)il types)

4 f . Pi 1 low b l o c k - ,ns,Y"'t 1x.ariugs. h r a r i s i ) ~ damage. (1111 types1

- Correct lubrication.

- seposition slip joints at funnels.

- plush W t centrifuge.

- Align qear box.

- Replace twarinoe.

N a m

TIBLE 30 tcontinuedl

CEWTRIQUGATION lROUPLESHLWTING GUIDE

PROBAPLE CAUSE CHECK OR W I T O R RFASON CWRRk%XIVF ACTION

49. my1 cut Of balance. 1A11 types1

4h. Parts not tightly assembled. L A 1 1 types)

4i. "neve" wear of - Inspect conveyor. co0veyor. ISolld howl)

- Return rotating p r t a to manufacturer for rebalance.

- Tighten p r f s .

- Resurface, rebalance.

OPFRATIffi PROBLM 5 : Sudden increaae i n parer consumption.

5a. contact betreen . Solids plowsi lo& bar1 and accumulated for polished area

case.(All types) solids in centrifuge On Outer boul.

- Apply hard surfacing to to areas with wear.

5h. Effluent pipe - Check for free - Clear efflvent pipe. plugged.ll)Il type) discharge of solids.

OPERATING PROBLEW 6: Gradual increase in power CMsYmPtIOD.

6a. Conveyor blade wear. - Conveyor condition. - Resurface blades. [solid bowl)

OPERATIffi PROBLM 1 1 Spasmodic, svrging solids dischal-qe.

7a. Pool depth tm low. - Plate dam position. (solid bowl)

7h. Conveyor helix - Improper hard rouqh. (Solid howl) surfacing or corrosion.

lc. Feed pipe (if adjust- a l ~ l e ) f o ~ near drain- dqe deck.lSolid l u w l )

~ Refinish con~eyor blade areas.

- ewe feed pipe 10

e Ff 1 ~ien t end.

N

W m

TAPLE 30 (Continued )

CFNTRIFWITION .IROUBI.FSIIooTIffi GUIDE

PROBABLE CAUSE c n F a on M O N I ~ R RFASCW CORRECTIVE ACTION

7d. Yachine vibration - see Item 4. - see 1te. 4. excess ive (Solid barl, Disk-nazelel

OPERATING PROBLM 8: Centr i fuge s h u t s d a m (or w i l l not opera te ) .

ea. Blown fuses . ( A I 1 types 1

ab. Overload r e l a y t r ipped . (811 t y p e * )

Bc. H o t o r overheated. thermal p r o t e c t o r s tripped.lA11 types)

Bd. Torque c o n t r o l t r i p p e d . l r l l types1

8e. Vibra t ion switch tripped.11111 t y p e s )

- €use*.

- Overload relay.

- Thermal pro tec tors .

- See I t e m 3. Check torque ind ica tor .

- see 1te. 4.

- Overload or weak fuee. wy be wrong s i z e fuse.

- I n s u f f i c i e n t "enti la- t ion . Nigh a i r temperature.

- Torque i n d i c a t o r may be s tuck oc d e f e c t i v e .

- Switch m y be defec t ive .

- Replace fuses, f l u s h machine. I f fuse blows on start-up. mechanical inspec t ion is required.

- Plush machine, reset relay. I f r e l a y t r i p on r e s t a r t . mechanical Inspec t ion is required.

- Flush machine. reset thermal p r o t e c t o r s . Check f a n on motor.

- Reflace or repair.

- Replace.

REFERENCES

1.

2.

3 .

4.

5.

6.

7.

8.

9.

10.

11.

12.

USEPA. Environmental Regulations and Technology: The Electro- p l a t i n g Industry, EPA 625/10-80-001, 1980. 44 pp.

USEPA. Environmental Pol lu t ion Control Al te rna t ives : Cent ra l ized Waste Treatment Al te rna t ives for the E lec t rop la t ing Industry, EPA 625/5-81-017, 1981. 36 pp.

USEPA. Summary Report: Control and Treatment Technology for t h e Metal Finishing Industry, Ion Exchange, EPA 625/8-81-007, 1981. 46 PP.

USEPA. Summary Report: Control and Treatment Technology for the Metal Finishing Industry, Sul f ide P rec ip i t a t ion , EPA 625/8-80-003, 1980. 54 pp.

USEPA. Summary Report: Control Technology for the Metal F in ish ing Industry, Evaporators, EPA 625/8-79-002, 1979. 42 pp.

USEPA. Environmental Pol lu t ion Control Al te rna t ives : ECOnOmiCS of Wastewater Treatment Al te rna t ives for t he E lec t rop la t ing Industry, EPA 625/5-79-016, 1979. 72 pp.

USEPA. Summary Report: Control and Treatment Technology for t he Metal Finishing Industry, In-Plant Changes, EPA 625/8-82-008, 1982. 30 PP.

USEPA. Methods for Chemical Analysis of Water and Wastes, EPA 600/4-79-020, 1979.

American Publ ic Health Association. Standard Methods for the Exam- ina t ion of Water and Wastewater, 14 ed., American Publ ic Health Association, Washington, D.C., 1975. 1193 pp.

Federal Regis te r , 48(137), 32462-32488 ( J u l y 15, 1983).

T r e t o l i t e I n d u s t r i a l Bench Test ing Procedures Manual, T e t r o l i t e Corporation, S t . L o u i s , MO.

Cherry, K. F. P l a t ing Waste Treatment. Ann Arbor Science Publish- ers, InC., Ann Arbor, Michigan, 1982, 324 pp.

. 12.-~son, J. W. Technology and Economics of I n d u s t r i a l Pol lu t ion Abatement. I IEQ No. 76/22, I l l i n o i s I n s t i t u t e for Environmental Quality, Chicago, I l l i n o i s , 1976. 614 pp.

14. Water Pol lu t ion Control Federation. Operation of Wastewater Treat- ment P lan t s , Manual of P rac t i ce No. 11, Water Pol lu t ion Control Federation, Washington, D.C., 1976.

270

15 . Snoeyink, V. L. and D. Jenkins. Water Chemistry, John Wlley and Sons, Inc. , New York, NY, 1980.

16. Shinskey, F. G. pH and pIon Control i n Process and Waste Streams, John Wiley and Sons, Inc., New York, NY, 1973.

17. Scot t , M. c. sulfex@ - A New Process Technology. f o r Removal of Heavy Metals from Waste Streams. In: Proceedings of the 32nd I n d u s t r i a l Waste Conference, Purdue University, Ann Arbor Science Publ i shers , Inc., Ann Arbor, Michigan, 1977. pp. 622-629.

18. Thomas, M. J., and T. L. Theis. Ef fec ts of Selected Ions on t h e Removal of Chrome (111) Hydroxide. Journal Water Pol lu t ion Control Federation, 48 ( 9 ) : 2032-2045, 1976.

19. Pat te rson , J. W. E f f e c t of Carbonate Ion on P rec ip i t a t ion Treat- ment of Cadmium, Copper, Lead, and Zinc. In: Proceedings of t he 3 6 t h I n d u s t r i a l Waste Conference, Purdue Universi ty , Ann Arbor Science Publ ishers Inc., Ann Arbor, Michigan, 1975. pp. 132-150.

20. Pat te rson , J. W., J. J. Scala , and H. E. Allen. Heavy Metal T r e a t - ment by Carbonate Prec ip i ta t ion . In: Proceedings of t he 30th I n d u s t r i a l Waste Conference, Purdue Universi ty , Ann Arbor Science Publ ishers Inc., Ann Arbor, Michigan, 1975. pp. 132-150.

21. Weber, W. J., Jr. Physiochemical Processes f o r Water Qual i ty Con- t ro l . John Wiley and Sons, Inc., New York, New York, 1972. 640 PP .

22. Water Pol lu t ion Control Federation. Wastewater T r e a t m e n t P lan t Design, Manual of P rac t i ce 9 , water Pol lu t ion Control Federation, Washington, D.C., 1977. 560 pp.

23. Great Lakes - Upper Miss i ss ippi River Board of S t a t e Sani ta ry Engineers. Recommended Standards f o r Sewage Works, Health Educa- t i o n Service, Inc., Albany, New York, 1978.

24. Sexsmith, D. R., E. A. S a y i n e l l i , and J. S. Beecher. The U s e of Polymers f o r Water Treatment. Ind. Water Eng., Dec. 1969, pp. 18-24.

25. The American Ci ty and county, Dec. 1976, pp. 45-48.

26. BIF Technical Information, Polye lec t ro ly te Coagulant Aids and Flocculants , Dry and Liquid, Handling and Application, Apr i l 1971.

27. Adorjan, L. A. Some Aspects of Flocculation. Coal Preparat ion, sept./oct. 1968, pp. 171-176.

271

Documents

p2infohouse.org · FIELD MANUAL PERFORMANCE EVALUATION AND TROUBLESHOOTING AT METAL-FINISHING WASTEWATER TREATMENT FACILITIES T. N. Sargent, P.E. G. C. Patrick, P.E. E. H. Snider,