Coagulation Ly Thuyet Ve Keo Tu

8/7/2019 Coagulation Ly Thuyet Ve Keo Tu

1/41

Everything you want to know aboutCoagulation & Flocculation....

Zeta-Meter, Inc.


2/41

Fourth Edition

April 199 3

Copyright

Copyright by Zeta-Meter, Inc. 1 993, 1 991, 1 990,1988 . All rights reserved. No part of this publicationmay be reproduced, t ransm it ted , t ranscr ibed, s tored ina retrieval system, or translated into any language inany form by any m eans without the wri tten perm issionof Zeta-Meter, Inc.

Your Comments

We hop e this guide will be helpfu l. If you ha ve an ysu ggestions on h ow to make i t better, or if you ha veadd it ional informa tion you th ink would help other

readers , then please drop u s a note or g ive us a cal l .Future edit ions will incorporate your comments.

Address

Zeta-Meter, Inc.765 Middlebrook Avenu ePO Box 3008Stau nton , Virginia 2 4402

Telephone (540) 886-350 3Toll Free (USA) (800) 333-022 9Fax (540) 886-372 8E-Mail [email protected]

ht tp: / / www.zeta-meter .com

Credits

Conceived a nd writ ten by Louis RavinaDesigned and i l lustrated by Nicholas Moramarco


3/41

Chapter 4 _____________________ 19

Using Alum and Ferric CoagulantsTime Tested Coagulants

Alum inu m S u lfate (Alum )

pH Effects

Coagu lant Aids

Chapter 5 _____________________ 25

Tools for Dosage Control

J ar Tes tZeta Potential

St reaming Cu r rent

Tu rbidity an d Par t icle Coun t

Chapter 6 _____________________ 33

Tips on MixingBasics

Rapid Mixing

Flocculation

The Zeta Potential Experts ________ 37

About Zeta-Meter

Introduction _____________________ iii

A Word About This Guide

Chapter 1 ______________________ 1

The Electrokinetic ConnectionParticle Cha rge Prevent s Coagu lation

Microscopic Electrical Forces

Balancing Opposing Forces

Lowerin g the En ergy Barrier

Chapter 2 ______________________ 9

Four Ways to FlocculateCoagulate, Then Flocculate

Double Layer Compression

Cha rge Neu tra lization

Bridging

Colloid En trapm ent

Chapter 3 _____________________ 13

Selecting PolyelectrolytesAn Aid or Su bs titut e for Traditiona l

Coagulants

Picking the Best One

Cha ra cterizing Polymers

En h an cing Polymer Effectivenes s

Polymer Packaging and Feeding

Interferences

Contents


4/41

A Word About This Guide

The removal of su spen ded m at ter f rom

water is on e of th e ma jor goals of water

t reatmen t . Only dis infect ion is u sed more

often or cons idered more imp ortan t. In fact,

effective clar ification is rea lly n ecess ar y for

completely reliab le disin fection becau se

microorganisms are shielded by particles in

the water .

Clarification usually involves:

coa gu la t ion

floccu la t ion

s et t lin g

filt ra t ion

This gu ide focuses on coagulat ion an d

floccu lation : th e two key steps wh ich often

determine finished water quality.

Coagu lat ion control techn iques ha ve ad-

vanced slowly. Many plan t opera tors re-

mem ber when dosa ge control was ba sed

u pon a visu al evalu ation of th e floccu lation

ba sin an d th e clarifier. If th e operators

eyebal l evaluation found a deterioration inqual ity, then h i s common sense response

was to increase the coagulan t dose. This

remedy was based u pon the ass um pt ion

th at if a l it t le did s ome good, then more

ought to do bet ter , bu t i t often did worse.

The comp etency of a plant opera tor de-

pend ed on h is years of experience with tha t

sp ecific water su pp ly. By trial , error an d

oral t radi t ion, h e would eventu al ly encoun -

ter every type of problem and learn to deal

with it.

Rel iable inst ruments now help us under-

s tan d a nd control the clar i ficat ion process .

Our abi lity to meas u re tu rbidity, par t icle

count , zeta potent ial and s t reaming current

ma kes coagulat ion an d f loccu lat ion more of

a s cience, al though ar t a nd experience s t i llhave their place.

We make ze ta m eters an d h appen to be a

little biased in favor of zeta pote n tial. In

this gu ide, h owever , we ha ve at tem pted to

give you a fair picture of all of the tools at

your d isposa l, and how you can pu t them to

work.

Introduction

iii


5/41

1

Particle Charge Prevents Coagulation

The key to effective coagulation and floccu-

lat ion is a n u nd erstan ding of how individ-

u al colloids in teract with each oth er. Tu r-

bidity par t icles ra nge from a bou t .01 to 100

microns in s ize. Th e larger fraction is

relatively eas y to sett le or filter. The

smaller, colloidal fraction, (from .01 to 5

microns ), pres ent s the real cha llenge. Their

settl ing times are intolerably slow and they

eas ily esca pe filtra tion.

Th e beh avior of colloids in water is str ongly

influen ced by th eir electrokinetic cha rge.

Each colloidal part icle carries a l ike ch arge,

which in na tu re is u su al ly negat ive. This

like cha rge cau ses ad jacent pa r t icles to

repel each oth er an d pr event s effective

agglomera tion an d floccu lation . As a

resu lt , cha rged col loids tend to rema in

discrete , dispersed, and in suspension.

On the other hand, if the charge is signifi-

cant ly redu ced or elimina ted, then the

colloids will gath er t ogether. First formin g

sm all grou ps, th en larger aggregates an d

fina lly in to visible floc pa rticles wh ich s ettle

rapidly and filter easily.

Chapter 1

The Electrokinetic Connection

Charged Particles repel each other

Uncharged Particles are free to collide and aggre-gate.


6/41

2

Chapter 1 The Electrokinetic Connection

Microscopic Electrical Forces

The Double Layer

The dou ble layer m odel is u sed t o visu alizeth e ion ic environm ent in th e vicinity of a

cha rged colloid an d explains h ow electrical

repu lsive forces occu r. It is easier to u nd er-

s tan d this model as a s equen ce of s teps

tha t wou ld take place aroun d a s ingle

n egative colloid if the ions su rrou nd ing it

were sudd enly s t r ipped away.

We first look at th e effect of th e colloid on

th e positive ions , which a re often called

counter-ions . Initially, att ract ion from th e

n egative colloid cau ses som e of th e positive

ions to form a firmly at tach ed layer a roun dth e su rface of th e colloid. Th is layer of

counter- ions is known as the Stern layer.

Add itiona l positive ions are sti ll attra cted b y

th e negative colloid bu t n ow they are re-

pelled b y the positive Stern layer as well as

by other n earby posi t ive ions tha t are a lso

tryin g to approa ch th e colloid. A dyna mic

equ ilibriu m r esu lts, forming a diffu se layerof cou n ter-ion s. The diffu se positive ion

layer has a high concentrat ion near the

colloid which gra du ally decreas es with

distance u nt i l it reach es equ ilibr ium with

the n ormal coun ter- ion concentra t ion in

solut ion.

In a s imilar bu t opposi te fash ion, th ere is a

lack of negat ive ions in th e neighborh ood of

the s u rface, becaus e they are repelled by

th e n egative colloid. Negat ive ions ar e

called co-ions because they have the sam e

cha rge as th e colloid. Their concen tra tionwill gradu ally in creas e as th e repu lsive

forces of th e colloid are s creened out by th e

positive ions , u n til equ ilibriu m is again

reached with the co- ion concen trat ion in

solut ion.

Two Ways to Visualize the

Double Layer

The left view shows the

change in charge densityaround the colloid. Theright shows the distributionof positive and negativeions around the chargedcolloid.

Stern Layer

Diffuse Layer

Highly Negative

Colloid

Ions In Equilibrium

With Solution

Negative Co-Ion

Positive Counter-Ion


7/41

3

Double Layer Thickness

Th e diffu se layer can be visu alized as acharged a tmosph ere sur round ing the

colloid. At an y distan ce from th e su rface,

its cha rge dens ity is equa l to th e difference

in concen tra tion of positive an d n egative

ions a t th at p oint . Ch arge dens ity is great-

est n ear th e colloid a nd rapidly diminish es

towards zero as the concen trat ion of posi-

tive an d n egative ions m erge togeth er.

The at ta ched coun ter- ions in the Stern

layer an d th e charged a tm osphere in the

diffu se layer are wh at we refer to as th e

double layer.

The th icknes s of the d ouble layer depends

u pon th e concentrat ion of ions in solu t ion.

A h igher level of ion s mea n s more positive

ion s a re a vailable to neu tra lize the colloid.

The resu lt i s a th inn er doub le layer .

Decreasing the ionic concentration (bydi lu t ion, for exam ple) redu ces th e n u mb er

of positive ions an d a t h icker d oub le layer

resul t s .

The type of coun ter-ion will also influ ence

doub le layer thickness . Type refers to the

valence of th e positive coun ter-ion . For

ins tan ce, an equa l concentrat ion of alum i-

n u m (Al+3) ions will be much more effective

tha n sodium (Na+) ion s in n eu tra lizin g the

colloidal char ge and will resu lt in a th inn er

double layer.

Increas ing th e concentra t ion of ions or th eir

valence are both referred to as double layer

compression.

Variation of Ion Density in the Diffuse Layer

Increasing the level of ions in solution reduces thethickness of the diffuse layer. The shaded arearepresents the net charge density.

Distance From Colloid

IonConcentration

Diffuse Layer


IonConcentration

Diffuse Layer

Lower Level of Ions in Solution Higher Level of Ions in Solution

Level of ions in solution


8/41

4


Zeta Potent ial

The negative colloid and its positivelycha rged atm osph ere produce an electr ical

poten tial acros s th e diffu se layer. This is

highest at th e su rface an d drops off pro-

gressively with distance, approaching zero

at th e ou tside of th e diffu se layer. Th e

potential curve is useful because it indi-

cates the strength of the repulsive force

between colloids an d th e dis tan ce at wh ich

th ese forces come into play.

A pa rticular p oin t of interest on th e cur ve is

the p otent ial a t th e ju nct ion of the Stern

layer an d th e diffu se layer. Th is is kn ownas t he zeta potential. It is an imp ortant

featu re becaus e zeta potent ial can b e

meas u red in a fai r ly s imp le ma nn er , whi le

the su rface potent ial can not . Zeta potent ial

is an effective tool for coagulation controlbecau se cha nges in zeta p otent ial ind icate

changes in the repulsive force between

colloids.

The ra t io between zeta potent ial an d su r-

face potent ial depend s on d ouble layer

th icknes s. The low dissolved solids level

u su al ly foun d in water t reatmen t resu lts in

a r elatively large d oub le layer. In th is cas e,

zeta potential is a good approximation of

su rface potent ial . The s i tuat ion chan ges

with br ackish or sal ine waters; the h igh

level of ion s com pres ses th e dou ble layeran d the potent ial curve. Now the zeta

poten tial is on ly a fraction of th e su rface

potential .

Zeta Potential vs Surface Potential

The relationship between Zeta Potential and SurfacePotential depends on the level of ions in solution. Infresh water, the large double layer makes the zeta

potential a good approximation of the surfacepotential. This does not hold true for saline watersdue to double layer compression.


Zeta Potential

Surface Potential

Potential

Stern Layer

Diffuse Layer


Zeta Potential

Surface Potential

Potential

Stern Layer

Diffuse Layer

Fresh Water Saline Water


9/41

5

Electrostatic repulsion is always shown as apositive curve.

Balancing Opposing Forces

Th e DLVO Theory (na m ed a fter Derjagu in,

Landau, Verwery and Overbeek) is theclas sic explan ation of h ow particles inter-

act . It looks at the balan ce between two

opposin g forces - electrosta tic repu lsion

an d van der Waals at t ra ct ion - to explain

why som e colloids agglomera te an d floccu-

late while others will n ot.

Repulsion

Electrostatic repulsion becomes significant

when two par t icles app roach each other

and their electrical double layers begin to

overlap. En ergy is requ ired to overcome

this repu lsion an d force the p ar t iclestogeth er. The level of en ergy requ ired

increas es dra ma tical ly as th e par t icles are

driven closer an d closer togeth er. An

electrostat ic repu lsion curve is u sed to

ind icate the en ergy that m u st be overcome

if th e pa rticles a re to be forced togeth er.

The ma ximu m height of the cu rve is related

to the s u rface potent ial .

Attraction

Van der Waals at t ra ct ion b etween two

colloids is actu ally th e res u lt of forces

between individual molecules in eachcolloid. The effect is ad ditive; th at is, one

molecule of th e first colloid h as a van der

Waa ls a t t ract ion to each m olecule in the

second colloid. This is repea ted for each

molecule in t h e first colloid an d th e total

force is th e su m of all of th ese. An a ttra c-

tive energy curve is used to indicate the

variation in att ractive force with dista nce

between particles.

Van der Waals attraction is shown as a negativecurve.

Distance Between Colloids

RepulsiveEnergy

ElectricalRepulsion


AttractiveEnergy

Van der WaalsAttraction


10/41

6


The Energy Barrier

The DLVO theory combines the van derWaals a t t ract ion curve an d th e electrostat ic

repu lsion curve to explain th e tenden cy of

colloids to either r ema in discrete or to

floccu late. Th e combined cu rve is called

th e ne t interaction e nergy . At each dis-

tan ce, the sma ller energy is su btracted

from the larger to get the net interaction

energy. The n et value is then p lot ted -

above if repulsive, below if attractive - and

the curve is formed.

The net interaction curve can shift from

att ra ct ion to repulsion an d back to at t rac-t ion with increasing dis tan ce between

pa rticles. If th ere is a repu lsive section,

then this region is cal led th e energy barrier

and it s m aximu m height indica tes how

resista n t th e system is to effective coagula-

tion.

In ord er to agglomera te, two part icles on a

collision cou rse m u st h ave su fficient kinetic

energy (du e to their speed a nd ma ss) to

jump overthis barr ier . Once the energy

barr ier is cleared, th e net intera ct ion en ergy

is all attr active. No fu rth er repu lsive area sare encoun tered and a s a resu lt the par -

ticles agglomera te. This attra ctive region is

often r eferred to a s a n energy trap since the

colloids can be considered to be t ra pped

together by the van der Waals forces.

Interaction

The net interaction curve is formed by subtracting theattraction curve from the repulsion curve.

AttractiveEnergy

RepulsiveEnergy

ElectricalRepulsion


Net InteractionEnergy

EnergyBarrier

Energy Trap

van der WaalsAttraction


11/41

7

Lowering the Energy Barrier

For really effective coagulation, the energy

barrier should be lowered or completelyremoved so th at th e net interact ion is

always at t ract ive. This can be accom-

pl ish ed by either comp ressing the d ouble

layer or redu cing th e su rface cha rge.

Com pres s t he Double Layer

Double layer compression involves adding

sal ts to the system. As the ionic concentra -

t ion increases, the dou ble layer and the

repu lsion energy cu rves a re compressed

u nt i l there is n o longer an energy barr ier .

Particle agglomeration occurs rapidly under

these condi t ions becau se th e colloids canjust about fall into the van der Waals trap

without having to surm ount an energy

barr ier .

Floccu lation b y doub le layer comp ress ion is

also called salting outth e colloid. Add ing

ma ssive amou nts of sal t is an imp ract ical

technique for water t reatmen t , but th e

underlying concept should be understood,

an d ha s app licat ion toward wastewater

flocculation in brackish waters.

Compression

Double layer compression squeezes the repulsiveenergy curve reducing its influence. Further compres-sion would completely eliminate the energy barrier.

AttractiveEnergy


RepulsiveEnergy Electrical

Repulsion



EnergyBarrier

Energy Trap


12/41

8


Charge Reduction

Coagulant addition lowers the surface charge anddrops the repulsive energy curve. More coagulantcan be added to completely eliminate the energybarrier.

Lowe r the Surface Charge

In water t reatm ent , we lower the en ergybarr ier by adding coagulants to reduce the

su rface charge an d, consequen t ly, the zeta

potent ial. Two points a re importa nt h ere.

First , for all practical purposes, zeta poten-

tial is a direct measure of the surface charge

and we can u se zeta potent ia l measu re-

men ts to control charge neu tral izat ion.

Second, it is not necess ary to redu ce the

charge to ze ro. Ou r goal is to lower the

energy barr ier to the point wh ere the pa r-

ticle velocity from m ixin g allows th e colloidsto overwhelm it.

The energy barrier concept helps explain

why larger pa rticles will som etimes floccu-

late whi le sma l ler ones in th e sam e su spen -

sion esca pe. At iden tical velocities the

larger par t icles h ave a greater m ass an d

therefore more energy to get them over the

barr ier .

AttractiveEnergy


RepulsiveEnergy

ElectricalRepulsion



EnergyBarrier

Energy Trap


13/41

9

Coagulate, Then Flocculate

In wa ter clarification, th e term s coagulation

an d flocculation are sometimes used inter-

chan geably an d am bigu ous ly, bu t it is

bet ter to separate th e two in term s of

funct ion.

Coagul a t i on takes place when th e DLVO

energy barrier is effectively eliminated; this

lowerin g of th e en ergy barrier is a lso re-

ferred t o as destabilization .

Flocculat ion refers to the su ccessfu l

collisions that occur when the destabilized

part icles are d r iven toward each other by

the h ydrau l ic sh ear forces in th e rapid mix

an d floccu lation ba sins . Agglomer ates of a

few colloids th en qu ickly bridge together t o

form microflocs which in turn gather into

visible floc masses.

Real i ty is som ewhere in between. The lin e

between coagulation and flocculation is

often a somewh at blurry one. Most coagu -

lan ts can perform both fun ct ions a t once.

Th eir prim ary job is ch arge n eu tra lization

but they often ads orb onto more than one

colloid, formin g a bridge between th em a n d

helping th em to flocculate.

Coagu lat ion an d f loccu lat ion can be cau sed

by a n y of th e followin g:

dou ble layer compr ess ion

cha rge neu tral izat ion

bridging

colloid entra pm ent

Chapter 2

Four Ways to Flocculate

In t h e pages th at follow, each of these fou r

tools is discu ssed s epara tely, bu t the

solution to an y specific coagu lation -floccu -

lation problem will almost always involvethe s imul taneous u se of more than one of

these. Use these as a check lis t when

plann ing a test ing program to select an

efficient and economical coagulant system.


14/41

10

Chapter 2 Four Ways to Flocculate

Double Layer Compression

Doub le layer comp ress ion involves th e

ad dition of large qua n tit ies of an indifferen telectrolyte (e.g., sod ium ch loride). The

indifference refers to the fact that the ion

retains i ts ident i ty an d does not ad sorb to

th e colloid. This ch an ge in ionic concen tra -

t ion compress es the dou ble layer aroun d

th e colloid an d is often called salting out.

The DLVO th eory ind icates th at this r esu lts

in a lowerin g or eliminat ion of th e repu lsive

ener gy ba rrier. It is importa n t to realize

tha t sa l t ing out jus t compresses th e

colloid's sph ere of influen ce an d does n ot

necessa r i ly redu ce its cha rge.

In general, double layer compression is not

a practical coagulation technique for water

t reatm ent bu t i t can ha ve appl icat ion in

ind u str ial wastewater t rea tmen t i f waste

st ream s with d ivalent or t r ivalent coun ter-

ions ha ppen to be avai lable.

Compression

Flocculation by double layercompression is unusual, but

has some application inindustrial wastewaters.Compare this figure to theone on page 2.

Stern Layer

Diffuse Layer

Highly NegativeColloid

Ions In EquilibriumWith Solution


15/41

11

Charge Neutralization

Inorganic coagulan ts (su ch a s alum ) an d

cationic polymers often work throughcharge neutralization. It is a p ractical way

to lower th e DLVO en ergy barr ier a nd form

sta ble flocs. Cha rge neu tra lization in volves

adsorption of a positively charged coagulant

on th e su rface of th e colloid. Th is ch arged

surface coating neutralizes the negative

cha rge of th e colloid, resu lting in a n ear

zero n et char ge. Neutra lization is th e key to

optimizing treatment before sedimentation,

gran u lar m edia filtra tion or air flotat ion.

Charge neutralization alone will not neces-

sar i ly produce dra ma tic macroflocs (flocstha t can be seen with th e naked eye). This

is d emons trated by cha rge neu tral izing with

cationic polyelectrolytes in th e 50 ,000 -

200,00 0 m olecular weight ran ge. Microflocs

(which a re too sm all to be seen) may form

but will not aggregate quickly into visible

flocs.

Charge neutralization is easily monitored

and controlled using zeta potential . This isimp ortant becau se overdosing can reverse

the char ge on the colloid, an d redisperse i t

as a pos itive colloid. The resu lt is a p oorly

floccu lated system . The detriment al effect

of overdoing is especially noticeable with

very low molecular weight cationic polymer s

th at are ineffective a t b ridging.

Charge Reduction

Lowering the surface chargedrops the repulsive energy

curve and allows van derWaals forces to reduce theenergy barrier. Comparethis figure with that on theopposite page and the oneon page 2.

Stern Layer

Diffuse Layer

Ions In EquilibriumWith Solution

Slightly Negative

Colloid


16/41

12

Chapter 2 Four Ways to Flocculate

Bridging

Bridging occurs when a coagulan t forms

threa ds or f ibers which a t tach to severalcolloids , ca ptu r ing and binding them

together . Inorganic pr imary coagulants an d

organ ic polyelectrolytes b oth h ave the

cap ab ili ty of bridgin g. High er molecular

weights m ean longer molecules and more

effective bridging.

Bridging is often u sed in conjun ction with

charge neutralization to grow fast settl ing

and / or shear res i s tan t flocs . For ins ta nce ,

alu m or a low molecular weigh t cationic

polymer is firs t add ed u nd er rap id m ixing

conditions to lower the charge and allowmicroflocs to form. Th en a slight am oun t of

h igh m olecu lar weight p olymer, often a n

an ionic, can be add ed to br idge between th e

microflocs. The fact th at th e bridging

polymer is n egatively charged is n ot s ignifi-

can t becau se th e sm al l colloids h ave al-

ready been ca ptu red as microflocs.

Colloid Entrapment

Colloid entra pm ent involves a ddin g rela-

tively large doses of coagulan ts, u su allyalum inu m or i ron s al ts which pr ecipi tate as

hydrou s metal oxides. The amou nt of

coagu lan t u sed is far in excess of the

am oun t needed to neutra lize the cha rge on

th e colloid. Some cha rge neu tra lization

ma y occur b u t m ost of the col loids are

literally swept from th e bu lk of th e water b y

becoming enm eshed in th e set t ling hydrou s

oxide floc. This mech an ism is often ca lled

sw eep floc.

Sweep Floc

Colloids become enmeshed in the growing precipitate.

Bridging

Each polymer chain attaches to many colloids.


17/41

13

An Aid or Substitute forTraditional Coagulants

The class of coagu lan ts an d f loccu lan ts

known as polyelectrolytes (or polymers) is

becoming more an d more popu lar . A

proper dosa ge of th e right polyelectrolyte

can improve finished water qu ality while

significantly reducing sludge volume and

overall operating costs .

On a pr ice-per -poun d ba s is th ey a re mu ch

more expensive tha n inorganic coagulan ts ,su ch as alum , bu t overal l operat ing costs

can be lower becaus e of a redu ced need for

pH adjust ing chemicals and because of

lower s lud ge volu mes an d d isposa l costs .

In some cases they a re u sed to su pplement

t radi t ional coagulants whi le in others they

completely replace them.

Polyelectrolytes a re orga nic m acrom ole-

cu les. A polyelectrolyte is a polymer ; th at

is, i t is composed of many (poly) monomers

(mer) joined together. Polyelectrolytes ma y

be fabricated of one or more basic mono-

mer s (u su ally two). The degree of polymer i-

zat ion is th e nu mber of mon omers (bui lding

blocks) l ink ed togeth er to form one m ole-

cule, and can ra nge up to hun dreds of

t hous ands .

Picking the Best One

Becaus e of the n u mb er available and their

proprietary na tu re, it can be a real cha l-

lenge to select the best polyelectrolyte for a

sp ecific tas k. The followin g char acteristics

are u su al ly used to class ify them; man u fac-

tu rers will often p u bl ish some of these, bu t

n ot always with th e desired degree of deta il:

type (an ionic, non -ionic, or cat ionic)

m o le cu la r w eigh t

b a s ic m ole cu la r s t ru c t u r e

ch a r ge d en s it y su itab i lity for potable water t rea tment

Prelimin ary ben ch testing of polyelectrolytes

is a n imp ortant par t of the s elect ion proc-

ess , even when the polymer is us ed as a

flocculant aid for an inorganic coagulant.

Adding an untested polyelectrolyte without

kn owing th e opt imu m dosage, feed concen-

trat ion or mixing requ iremen ts can resu lt in

serious problems , includ ing filter clogging.

It is importan t to note that , within th e sam e

fam ily or type of polym er, th ere can be a

large differen ce in m olecu lar weight an d

cha rge den sity. For a sp ecific ap plication,

one m ember of a fam ily can h ave ju st th e

right combination of properties and greatly

outperform th e others .

Chapter 3

Selecting Polyelectrolytes


18/41

14

Chapter 3 Selecting Polyelectrolytes

Characterizing Polymers

Molecular Weight

The overall size of a polymer determ ines itsrelat ive u sefu lnes s for bridgin g. Size is

u su al ly meas u red as m olecular weight .

Manufacturers do not use a uniform

met hod to report molecu lar weight. For

this reas on, two similar polymers with th e

sam e pub lish ed molecular weight ma y

actu ally be qu ite differen t.

In ad dition, m olecu lar weight is on ly a

meas u re of average polymer length. Each

molecule in a d ru m of polymer is n ot the

sa me size. A wide ran ge can a nd will be

foun d in the sam e batch . This dis t r ibut ionof molecular weights is a n imp ortant prop-

er ty and can vary great ly.

Mo le c ular Ge ne ral

We ight Range De s c ript io n

10,000 ,000 or m ore Very High

1 ,0 00 ,0 00 t o 1 0,0 00 ,0 00 High

200 ,000 to 1 ,000,000 Med iu m

100 ,000 to 200 ,000 Low

50,000 to 100 ,000 Very Low

Les s th an 50 ,000 Very, Very Low

StructureTwo similar polyelectrolytes with th e sa me

composition of monomers, molecular

weight , an d ch arge chara cter is t ics can

perform different ly becau se of the way th e

mon omers are link ed together . For ex-

am ple, a produ ct with two monom ers A an d

B could have a regular alternation from A to

B or cou ld h ave grou ps of As followed by

grou ps of Bs.

Charge Density

Relative ch arge den sity is con trolled by th e

rat io of charged and uncharged monomers

u sed. The higher the residu al cha rge, the

h igher th e den sity. In genera l, th e relative

cha rge densi ty an d m olecular weight can -

not both be increas ed. As a resul t , the

ideal polyelectrolyte often involves a tradeoff

between ch arge densi ty and m olecular

weight.

Type o f Polym er

Polyelectrolytes ar e clas sified as n on-ionic,an ionic or cat ionic depending up on th e

residual cha rge on the p olymer in solu t ion.

Non-ionic polyelectrolytes are polymers

with a very low char ge dens ity. A typical

non -ionic is a polyacrylamide. Non -ionics

are used to flocculate solids through bridg-

ing.

Anionic polye lec t ro ly t es are negat ively

charged polymers and can be m anu fac tured

with a variety of cha rge den sities, from

pra ctically non -ionic to very stron gly an i-onic. Interm ediate charge densi t ies are

u su al ly the most u sefu l. Anionics are

norm ally u sed for br idging, to flocculate

solids . The acrylamide-ba sed an ionics with

very h igh molecular weights ar e very effec-

tive for th is.

Negative colloids can som etimes b e su c-

cessfully floccu lated with bridging-type lon g

cha in anionic polyelectrolytes. On e pos-

sible explan ation is th at a colloid with a n et

negat ive cha rge may actu al ly ha ve a mosa ic

of positive an d n egative regions . Areas ofpositive charge could serve as points of

at tach men t for th e n egat ive polymer.

Anionic polyelectrolytes m ay b e ca pa ble of

flocculating large particles, but a residual

haze of smaller colloids will almost always

remain. These mu st firs t ha ve their charge

neu tra lized in order to flocculate.

Cat ionic po lye lec t ro ly tes are positively

char ged polymers an d come in a wide range

of fam ilies, ch arge den sities a nd molecular

weigh ts . The var iety availab le offers great

flexibility in s olving sp ecific coagu lation a n d

floccu lat ion problems , bu t m akes select ing

the right polymer more complicated.

High molecular weight cationic polyelectro-

lytes can be th ought of as double acting

becau se th ey act in two ways: charge

neu tral izat ion an d br idging.


19/41

15

Direct Filtration with Cationic Polymers

It is often possible to eliminate or bypass conventional

flocculation and sedimentation when raw watersupplies are low in turbidity on a year-round basis.For new plants, this can mean a significant savings incapital cost. Coagulant treated water is then feddirectly to the filters in what is known as the directfiltration process. Cationic polymers are usually veryeffective in this type of service.

In this example, polymer dosage, filter effluentturbidity and filter head loss after 6 hours of operationwere plotted together. The minimum turbidity level isproduced by a dose of 7 mg/L at a corresponding zetapotential of +10 mV. A polymer dose of 3 mg/L wasselected as a more practical optimum because it

produces almost the same turbidity at a substantialsavings in polymer and with a much lower head lossthrough the filter. The result is a target zeta potentialof -1mV.

Cationic Polymer Screening

The true cost of a polymer is not its price per pound

but the cost per million gallons of water treated. Plotsof zeta potential versus polymer dosage can be usedto determine the relative dose levels of similarpolyelectrolytes.

In this example the target zeta potential was set at-5 mV. The corresponding doses are: 3 mg/L forPolymer A, 8 mg/L for Polymer B and 21 mg/L forPolymer C. The cost per million gallons ($/MG) isestimated by converting the dosage to pounds permillion gallons and then multiplying by the price perpound.

The result is $88/MG for Polymer A, $133/MG for

Polymer B and $88/MG for Polymer C. If all otherconsiderations are equal, then Polymers A & C areboth economical choices.

2 4 6 8 10 12

0

-5

-10

-15

-20

+5

+10

+15

+20

1

3

2

4

5

0.4

0.3

0.2

0.1

0.0

Polymer Dose, mg/L

ZetaPotential,mV

HeadLossAfter6Hours,f

t.

EffluentTurbidity,N

TU

ZetaPotential

Turbidity

FilterHeadLoss

+5

+10

0

-5

-10

-15

-20

5 10 15 20 25

ZetaPo

tential,mV

30

Polymer Dose, mg/L

Polymer A $2.00 / lb

Polymer C

$3.50 / lb

Polymer B

$0.50 / lb


20/41

16


Enhancing Polymer Effectiveness

Dual Polyme rSys tems

Two polymer s ca n h elp if no s ingle polymercan get the job done. Each h as a sp ecific

fu nct ion . For exam ple, a highly cha rged

cationic polymer can be added first to

n eu tra lize the cha rge on th e fine colloids,

an d form sm all microflocs. Th en a h igh

molecular weight anionic polymer can be

u sed t o mech an ically bridge th e microflocs

into large, rapidly settling flocs.

In water t reatm ent , du al polymer systems

ha ve the disadvanta ge tha t more carefu l

control is requ ired to ba lan ce the coun ter-

acting forces. Du al polymer s are morecommon in s ludge dewater ing, where

overdosing an d th e app earan ce of excess

polymer in the centrate or fi l trate is not as

impor tant .

Preconditioning

Inorgan ic coagu lan ts m ay be h elpfu l as a

coagulant aid when a polyelectrolyte alone

is not s u ccessful in d esta bilizing all th e

part icles . Pretreatment with inorganics can

also reduce th e cat ionic polymer dose an d

make it more stable, requiring less crit ical

control.

Preconditioning Polymers with AlumThe effect of preconditioning can be evaluated by

making plots of zeta potential versus polymer dosageat various levels of preconditioning chemical. In thisexample, the required dosage of cationic polymer wassubstantially reduced with 20 mg/L of alum while 10mg/l of alum was not effective.

+5

+10

0

-5

-10

-15

-20

5 15 20 25

ZetaPo

tential,mV

PolymerOnly

Polymer +20 mg/Lalum

Polymer +10 mg/Lalum

Polymer Dose, mg/L


21/41

17

Polymer Packaging and Feeding

Polyelectrolytes can be pu rchased in pow-

der , solu t ion an d emu lsion form. Each typehas a dvanta ges and d isadvantages . If

poss ible, feed facili t ies sh ould allow an y

type to be us ed.

Dry powderpo lymers are whit ish granular

powders , flakes or beads. Their tendency to

abs orb moisture from th e ai r and to s t ick to

feed screws, containers an d dru ms is a

ma jor nu isan ce. Dry polymers are also

difficult to wet and dissolve rather slowly.

Fifteen m inu tes to 1 hou r ma y be required.

Solut ion po lymers are often preferred todry powders becau se they are more conven-

ient. A little mixin g is us u ally su fficien t to

dilu te l iqu id polymers to feed str ength .

Active ingredients can vary from a few

percent to 50 percent .

Emuls ion polymers are a m ore recent

developm ent. They allow very high m olecu -

lar weight polymers to b e pu rcha sed in

convenient l iqu id form. Dilu tion with water

u nd er agi tat ion frees the gel par t icles in the

emu lsion a llowin g them to dissolve in t h e

water . Sometimes act ivators are requiredfor prepar at ion. Em u lsions are usu al ly

packa ged a s 2 0-30% a ct ive ingredients .

Pr epar ed ba t ches of polymer are n orma lly

u sed within 24 -48 h ours to prevent loss of

activity.

In addition, polymers are almost always

more effective when fed a s d ilu te s olu tion s

becau se they are easier to disperse an d

u niformly distribu te. Using a higher feed

s t rength may m ean th a t a h igher polymer

dose will be requ ired.

Typically, maximum feed strength is be-

tween 0 .01 to 0.05%, but ch eck with th e

polymer manufacturer for specific recom-

mendat ions .

Stock polymer solu t ions a re us u al ly made

u p to 0.1 to 0.5% as a good compromisebetween storage volume, batch life, and

viscosity. Dilu ting th e stock solution by

abou t 10:1 with water will usu al ly drop the

concentrat ion to th e recomm ended feed

level. The polymer an d dilu tion water

sh ould be blended in- l ine with a s tat ic

mixer or an edu ctor .

Using Cationic Polymers in Brackish Waters

In brackish waters, the correct cationic polymer doseis often concealed by the effect of double layercompression, which drops the zeta potential but not

the surface potential.

Diluting the treated sample with distilled water willgive an indication of whether enough polymer hasbeen added. If the surface potential is still high, thendilution will cause the zeta potential to increase andmore cationic is required.


Zeta PotentialAfter Dilution

Surface Potential

Potential

Stern Layer

Diffuse Layer

Zeta PotentialBefore Dilution


22/41

18


Interferences

Cationic polymers can react with negative

ions in solution, forming chemical bondswhich imp air their performan ce. Greater

doses are then required to ach ieve the sam e

degree of cha rge neu tra lization or bridging.

This is more noticeable in wastewater

t rea tment .

Examp les of interfer ing su bsta nces include

su lfides, h ydrosulfides a nd ph enolics , bu t

even chlorides ca n redu ce polymer effective-

nes s .

Even p H sh ould be considered, s ince

pa rticular p olymer families m ay performwell in s ome pH ran ges an d n ot others .

Phenol Interference

The effect of phenols on this cationic polymer wasevaluated by plotting zeta potential curves.

pH Effects

Zeta potential curves can be used to evaluate the

sensitivity of a cationic polymer to changes in pH. Forthis particular polymer the effect is quite large. It canbe much less for other products.

+5

+10

0

-5

-10

-15

ZetaPotential,mV

1 2 3 4 5 6

Polymer Dose, mg/L

pH 8

pH 10

+5

+10

0

-5

-10

-15

-20

ZetaPotential,mV

2 4 6 8 10 12

30 mg/LPhenol

NoPhenol

Polymer Dose, mg/L


23/41

19

Time Tested Coagulants

Alum inu m and fe r r ic compounds a re the

tradi t ional coagu lan ts for water a nd waste-

water t reatment . Both are from a fam ily

cal led m etal coagulan ts , an d both are s t ill

widely u sed today. In fact , man y plants u se

one of th ese exclu sively, an d h ave no

provision for polyelectrolyte addition.

Metal coagu lant s offer th e ad vant age of low

cost per pou nd . In a ddi t ion, select ion of

the opt imu m coagulan t is s imp le, s inceonly a few choices a re availab le. A distinct

disadvanta ge is the large s ludge volu me

produ ced, s ince s lud ge dewater ing an d

disposa l can be difficult an d expen sive.

Alu minu m a nd ferr ic coagu lants a re solu ble

sal ts . They are added in solut ion form an d

react with alkalinity in th e water t o form

ins olu ble hydrous oxides th at coagulate by

sweep floc and charge neutralization.

Metal coagu lan ts always r equire at ten t ion

to pH condi t ions an d cons iderat ion of the

alkalinity level in the raw and treated water.

Reason ab le dosa ge levels will frequ ent ly

resu lt in n ear opt imu m pH cond it ions. At

other t imes, ch emicals su ch as lime, soda

ash or sodiu m bicarbonate mu s t be added

to sup plement n atu ral alkal ini ty.

Chapter 4

Aluminum Sulfate (Alum)

Alum is one of th e mos t widely u sed coa gu-

lan ts an d will be us ed as a n exam ple of the

react ions th at occur with a metal coagu -

lan t . Ferr ic coagulan ts react in a general ly

s imila r m ann er , but th e ir opt imu m pH

ranges are different.

When alum inu m su lfate is add ed to water ,

hydrou s oxides of alum inu m a re formed.

The s imp lest of these is alum inu m h ydrox-

ide (Al(OH)3) which is a n insolub le precipi-tat e. Bu t severa l, more comp lex, positively

cha rged solu ble ions are a lso formed,

including:

Al6(OH)

15

+3

Al7(OH)

17

+4

Al8(OH)

20

+4

The proport ion of each will vary, depen ding

up on both the a lum d ose and the pH afte r

alum add it ion. To fu r ther complicate

mat te r s , u nder cer ta in condi t ions the

su lfat e ion (SO4-2

) ma y also become p ar t of the h ydrous a lum inu m complex by subs t i-

tu ting for som e of th e h ydroxide (OH-1 ) ions .

This will ten d to lower th e ch arge of the

h ydroxide com plex.

Using Alum and Ferric Coagulants


24/41

20

Chapter 4 Using Alum and Ferric Coagulants

How Alum Works

The m echan ism of coagu lat ion by alumincludes b oth cha rge neu tral izat ion an d

sweep floc. One or th e other m ay predomi-

na te, but each is always act ing to some

degree. It is probab le that ch arge neutra li-

zation takes place immediately after addi-

tion of alu m to water. The com plex, posi-

t ively charged h ydroxides of alum inu m tha t

rap idly form will adsorb to th e su rface of

the negative turbidity particles, neutralizing

th eir ch arge (an d zeta p otent ial) an d effec-

tively lowerin g or r em ovin g th e DLVO

energy barrier.

Simultaneously, aluminum hydroxide

precipitates will form. These add itiona l

particles enhance the rate of flocculation by

increasing the chances of a collision occur-

ring. Th e precipitate also grows indep end -

ently of the colloid population, enmeshing

colloids in th e sweep floc m ode.

The type of coagulation which predominates

is dependent on both th e a lum dose and

the pH after alu m ad di t ion. In general ,

sweep coagulat ion is th ought to predomi-

na te at alum d oses above 30 m g/ L; belowtha t , the dominan t form depends u pon both

dose and pH.

Alkalin ity is requ ired for th e alu m reaction

to su ccessfully proceed. Oth erwise, the pH

will be lowered to th e point wh ere solu ble

aluminum ion (Al+3) is formed instea d of

alum inu m hydroxide. Dissolved alum inu m

ion is an ineffective coagulan t an d can

cause dirty water problems in the distri-

bu t ion system. The react ion between alum

an d a lkalin ity is sh own b y the following:

600 300

Alum Alkalinity

Al2(SO

4)

3+ 3Ca(HCO

3)

2+ 6H

2O

Aluminum Carbonic

Hydroxide Acid

3CaSO4

+ 2Al(OH)3

+ 6H2CO

3

Estimating Alkalinity Requirem ent s

This equ at ion helps u s d evelop severals imple rules of thu mb abou t the relat ion

betweem a lum an d alkal ini ty.

Commercial alum is a crystalline material ,

with 14.2 water molecules (on average)

bound to each aluminum sulfate molecule.

The molecular weight of Al2(SO

4)

314 .2H

2O

is 600 .

Alkal ini ty is a meas u re of the a mou nt of

bicarbonate (HCO3

-1 ), car bon ate (CO3

-2 ) an d

hydroxide (OH -1 ) ion . Th e reaction sh ows

alkalinity in i ts bicarb ona te (HCO3 -) formwhich is typical at pH's b elow 8, bu t alka-

lin ity is always express ed in term s of th e

equivalent weight of calcium carbonate

(CaCO3), which has a molecular weight of

100 . Th e th ree Ca(HCO3)

2molecules th en

ha ve an equ ivalent m olecu lar weight of 3 x

100 or 300 as CaCO3.

The ru les of thu mb a re based on th e rat io

of 600 (alum) to 300 (alkalinity):

1 .0 mg/ L of comm ercia l a lum will

consu me ab out 0.5 m g/ L of alkal ini ty.

The r e s hou l d be a t le a s t 5 -10 m g / L o f

alkalinity rem aining after th e reaction

occurs to keep the pH near opt imu m.

Raw water a lka l in i ty shou ld be equal to

ha lf the expected alum dose plus 5 to 10m g/ L.

1.0 m g/ L of alkal ini ty expressed as CaCO3

is equivalent to:

0 .66 m g / L 85% qu ick lim e (CaO)

0 .78 mg/ L 95% hydra ted lime (Ca(OH)3)

0 .80 m g / L caus t ic s oda (NaOH)

1 .0 8 m g/ L s od a a s h (Na 2CO 3) 1 .52 m g/ L s od ium b ica r bona t e (NaHCO3)


25/41

21

Whe n t o Add Alkalinity

If na tu ral alkalinity is ins u fficient th en a ddart ificial alkalinity to ma intain th e des ired

level. Hydrat ed lime, cau stic soda , soda

ash or sodiu m bicarbonate m ay be used to

raise the alkalinity level.

Alkal ini ty shou ld a lways be add ed u p-

st ream , before alu m a ddi t ion, an d the

chem ical sh ould be com pletely diss olved by

the t ime the alum react ion tak es place.

Alu m rea cts instan tan eously and will

proceed to other end products if sufficient

alkalinity is not imm ediately availab le. Th is

requ iremen t is often ignored in a n effort tominimize tank s an d mixers , bu t poor

performan ce is th e pr ice tha t is paid.

When alum reacts with na tu ral alkalini ty,

the p H is decreased b y two different m ean s:

the bicarbonate alkalinity of the system is

lowered an d th e carbonic acid conten t is

increas ed. This is often an advan tage,

s ince opt imu m pH condi t ions for alum

coagulation are generally in the range of

abou t 5.0 to 7.0, whi le the pH ra nge of

mos t n a tura l waters is f rom abou t 6 .0 to

7.8.

At t imes, som e of the a lu m dose is actu al ly

being u sed solely to lower the p H to its

opt imu m valu e. In other words, a lower

alum dose would coagu late as effectively if

th e pH were lowered some other way. At

larger plants i t may be more economical to

add su lfu r ic acid ins tead.

It i s imp ortant to note tha t n ot al l sour ces

of artificial alkalinity have the same effect

on pH. Some produce carbonic acid when

they react an d lower the pH. Others do not .

Opt imum pH condi t ions should be taken

into a ccou nt when select ing a n alkal ini ty

source.

Zeta Potential Control of Alum Dose

There is no single zeta potential that will guarantee

good coagulation for every treatment plant. It willusually be between 0 and -10 mV but the target valueis best set by test, using pilot plant or actual operatingexperience.

Once the target ZP is established, then thesecorrelations are no longer necessary, except forinfrequent checks on a weekly, monthly, or seasonalbasis. Control merely involves taking a sample fromthe rapid mix basin and measuring the zeta potential.If the measured value is more negative than the targetZP, then increase the coagulant dose. If it is morepositive, then decrease it.

In this example a zeta potential of -3 mV correspondsto the lowest filtered water turbidity and would beused as the target ZP.

0

+5

-5

-10

-15

-20

-2510 20 30 40 50 60

0.0

0.1

0.2

0.3

0.4

0.5

Zeta-Potential

TurbidityZetaPo

tential,mV

Alum Dose, mg/L

TurbidityofFin

ishedWater,NTU


26/41

22


pH Effects

Charge

There is a s t rong relat ion between pH an dperforman ce, bu t th ere is n o s ingle opt i-

mu m pH for a specific water . Rather , there

is a n interrelat ion between pH a nd the type

of aluminum hydroxide formed. This in

turn de termines the charge on the hydrous

oxide complex. Oth er ions com e in to play

as well . The effect of pH on ch arge can be

evaluated u sing a zeta potent ial curve and

direct m easu remen t of zeta p otent ial is a

bet ter meth od of process control tha n p H.

Solubility of Aluminum

A secon d imp ortan t as pect of pH is i ts effecton s olu bili ty of th e alu m inu m (Al+3 ) ion .

Ins u fficient a lkalinity allows th e pH to dr op

to a point where th e alu minu m ion becomes

highly solub le. Dissolved alu min u m can

then pas s r ight th rough th e filters . After

fil t rat ion, p H is u su al ly adjus ted u pward for

corrosion con trol. The h igh er pH converts

dissolved alum inu m ion to ins olu ble alu mi-

nu m hydroxide which th en f loccu lates in

the d i s t r ibu t ion sys tem an d a lmos t guaran -

tees dirty water complaints .

Effect of pH on Alum FlocThe charge on alum floc is strongly effected by pH.This example shows the effect of pH on zeta potentialand residual color for a highly colored water. Thealum dose was held constant at 60 mg/L.

OverdosingChanges in zeta potential are good indicators ofoverdosing. This plant should operate at a zetapotential of about -5 mV. Overdosing produced azeta potential of +5 mV and is accompanied by amarked increase in residual aluminum in the finishedwater. Monitoring of turbidity is not effective forcontrol because the lag time between coagulantaddition and filtration is several hours.

-5

+5

-10

-15

0

20

40

ZetaPotential

ZetaPotential,mV

pH

ResidualColor

0

+10

60

80

54 6

+15

Color

0

+10

-5

-10

-15

0.0

0.2

0.4

ZetaPotential

Turbidityof Filtered

Water

ZetaPotential,mV

Alum Dose, mg/L

Turbidity,NTU

+5

+15

0

0.1

0.2

0.3

0.4

0.6

0.8

20 40 60 80 100 120

ResidualAluminum

ResidualAluminum


27/41

23

Cationic Coagulant Aid

Zeta potential curves can be used to evaluate the

charge neutralizing properties of cationic polymers.

Coagulant Aids

Tailoring Floc Charact eristic s

Polyelectrolytes which enh an ce th e floccu -lat ing act ion of metal coagulants (su ch a s

alum ), are ca lled coagulant aids or floccu-

lan t aids .

They provide a means of tailoring floc size,

set t l ing character is t ics and shear s t rength.

Coagulan t a ids ar e excellen t tools for

dealing with seasonal problem periods

when alu m a lon e is ineffective. They can

also be us ed to increas e plant capa city

without increasing physical plant size.

Ac t iva ted s i l i ca was u sed for th is pu rposebefore organic polymers became popular.

Activated silica produ ces large, den se, fast

set t ling alum flocs an d s imu ltan eously

tou ghen s it to a llow higher ra tes of fi ltra tion

or longer fi lter ru ns . A disad vanta ge is the

fairly precise control that is needed during

prep ara tion of th e activated s ilica. Silica is

prepared (activated) on-site by p art ially

neutralizing a sodium silicate solution, then

aging and diluting it .

Polyelectrolyte coagulan t aids ha ve the

advantages of activated sil ica and aresimpler to prepa re.

Cat i on ic p o l ym er s are ch arge neu tral izing

coagu lan t aids . They can redu ce the alum

or ferric dose while simultaneously increas-

ing floc size an d tou ghen ing it . Th ey also

redu ce the effect of su bsta nces th at inter-

fere with meta l coagulan ts. Long cha in,

high m olecu lar weight ca tion ics oper ate via

mecha nical br idging in add it ion to cha rge

neu tral izat ion. Cha rge neu tral izing can be

monitored using zeta potential , while the

bridging effect should be evaluated using

jar tes ting.

0

+5

-5

-10

-15

-20

-25

ZetaPo

tential,mV

Alum

Alum +CationicPolymer

10 20 30 40 50 60

Dose, mg/L


28/41

24


Wastewater Coagulation with Ferric Chloride

These curves are for coagulation of a municipaltrickling filter effluent. In this case, the optimum zetapotential is -11mv. Overdosing by only 5 mg/Lcauses a significant deterioration in performance, andis accompanied by a large change in zeta potential.Overdosing by 10 mg/L results in almost completedeterioration.

Non-Ionic Polymer Increases Settling Capacity

Jar tests were used to evaluate the effect of a very

small amount of non-ionic polymer on settling rates.Sedimentation velocities were converted to equivalentsettling basin overflow rates to illustrate the dramaticeffect of the polymer. With no polymer the turbiditywas 8 NTU at an overflow rate of 720 gallons per dayper square foot. After the polymer was added, theflow rate could be tripled with the settled waterturbidity actually improving at the same time.

10 20 30 40 50 60

0

-5

-10

-15

-20

+5

+10

+15

+20

5

15

10

20

100

60

40

20

Fe (III) Dose, mg/L

ZetaPotential,mV

ResidualTurbidity,NTU

O

rganicCarbonasCOD,mg/L

+25

-25

COD

ZetaPotential

Turbidity

0

80

25

0

Anionic and non-ionic polym ers a re

popular becau se a sm al l amou nt canincr eas e floc size severa l times . An ionic

an d n on-ionic polyelectrolytes work b est

because their very high molecular weight

promotes growth through mechan ica l

br idgin g. It is imp orta n t to allow m icroflocs

to form before add ing th ese polymer s. In

addi t ion, excess amounts can actual ly

inh ibit floccu lation . J ar testing is the best

way to establ ish the opt imu m d osage.

0

Turbidity

ofSettledWater,NTU

Overflow Rate, gal per day / sq. ft.

2

4

6

8

10

12

14

16

18

20

720 1440 2160 2880 3600

30 mg/LAlum

30 mg/LAlum +0.1 mg/LNon - IonicPolymer


29/41

25

Tools for Dosage Control

Chapter 5

Jar Test

An Unde rvalued Tool

We u su al ly ass ociate th e jar tes t wi th a

tedious dosage control tool tha t seems to

take an agonizingly long time to set up, run

an d then clean u p after . As a resul t the

tru e versa tili ty of th e hu mb le jar test is

often overlooked.

The following applications for jar testing

may be m ore impor tant th an it s u se in

rout ine dosage control. It is s tu dies su chas th ese where the jar tes t h as i ts major

value, s ince the resu lts ar e worth the t ime

an d effort to do th e test p roperly:

se lec tion of pr imary coagulant

com par is on o f coagu l an t a id s

opt imizing feed poin t for pH adjus t ing

chemicals

mixing energy and mixing t ime s tu dies

(rap id mix an d flocculation)

opt imizing feed point for coagulant aids

eva lua t ing d ilu t ion requi rements for

coagulants est imat ing set t ling velocit ies for sedimen -

tat ion bas in s izing

s tu dying e ffect of rap id changes in

mixin g energy

evalua ting effect of slu dge recycling

Jar Testing

A tedious but valuable tool which allows side by sidecomparison of many operating variables.


30/41

26

Chapter 5 Tools for Dosage Control

The Gator Jar

The original design was constructed of 1/4 inch thickplexiglas. It measures 11.5 cm square (inside) by anoverall depth of 21 cm. The sample tap is located 10cm below the water surface. Liquid volume is2000mL. The tap consists of a piece of soft tubingand a squeeze clamp, or a small plastic spigot.

Jar Test Apparatus

Many comm ercial laboratory jar tes t u ni tsare in u se today. The Phipps & Bird s ix

place gan g stirrer is very tra ditiona l. It ha s

1" x 3" flat blade pad dles a nd is u su al ly

u sed with 1.5 l iter glass beakers . A disad-

vantage is the large vortex that forms at

high s t i r r ing sp eeds.

Some provision is usually made for drawing

off a s ettled sa mp le from below th e water

su rface. Siphon type draw-offs are fre-

quen t ly used for glass beakers .

C a m p improved th e Phipps & Bird designby using 2-li ter glass beakers outfit ted with

vortex breaking s tators . These were espe-

cially suitable for mixing studies because

Cam p developed cu rves for mixin g in ten sity

(G) versu s s t i r rer speed u sing this d esign.

Gat or o r W agner j a r s are a relatively

recent imp rovement a nd are squ are plexi-

glas jars with a l iquid volu me of 2 l iters.

These a re n ow comm ercially availab le, an d

have several advantages over traditional

glas s beak ers. First , they are less fragile

and allow for easy insertion of a sample tap,thu s avoiding the need for a s iph on type

draw-off. Their squ are sha pe helps dam pen

rotat ion al velocity with out th e n eed for

stators while their plexiglas walls offer

mu ch grea ter therma l insu la t ion tha n

glass , thus minimizing temperature change

du ring test ing.


31/41

27

Settl ing Studies

The d ata from a well thought out jar tes tcan be u sed to realistically size settl ing

bas ins a nd to evalua te th e abi lity of coagu -

lan t aids to increas e the h ydrau lic capaci ty

of existing ba sins . Su rface overflow rate

(gallons per da y per squ are foot) is m ore

important than retent ion t ime in determin-

ing the h ydrau lic capa city of a s ettl ing

ba sin, a n d is calculated from th e velocity of

th e slowest s ettl ing floc tha t we wan t to

rem ove. In pra ctical term s, a settl in g ra te

of 1 cm / minu te is equ al to an overflow rate

of 360 gallons per day per square foot.

J ar test set t l ing data is obtained by drawing

off sa mp les a t a fixed dista n ce below the

water su rface, at var iou s t ime intervals

after the en d of rap id mixing an d floccula-

tion. Sett lin g velocity an d overflow ra te are

calcu lated for each t ime interval and the

sam ple reflects th e qu al ity tha t we can

expect at th at overfow rate.

The Gator jar ha s a convenient s am ple

dra w-off located 10 cm b elow th e water

su rface. Sam ples are u su al ly withd rawn at

intervals of 1, 2, 5, an d 10 minu tes af terthe mixers are s topped. These correspon d

to settl in g velocities of 10, 5 , 2 a n d 1 cm/

minu te which ar e equivalent to set t l ing

bas in overflow rates of 3600 , 1800 , 720 an d

360 gallons per da y per squ are foot .

Th e resu lts of th e overflow rate s tu dies can

be plotted on r egular (arith met ic) graph

paper, or on semi-logarithmic paper (with

tu rbidity on th e log scale) or on logar ithm ic

graph pap er . Actua l plant operat ing resu lts

can a lso be plot ted on the sa me graph s for

comparison.

See page 24, Non-Ionic Polymer Increases

Settling Capacity, for an exam ple of a

set t ling s tu dy.

G Curves for the Gator JarThese are for the Gator Jar, with a 1x3 inch Phippsand Bird stirrer paddle. See Chapter 6, Tips onMixingfor a discussion of the G value and its impor-tance as a rational measure of mixing intensly.

Predic ting Filtered Wate r Quality

For water t reatmen t , it i s imp ortant toremem ber tha t our u lt ima te goal is to

produ ce the best fi ltered water. We often

ass u me tha t the best set t led water will

always correspond to the best fi l tered

water , but tha t is not necessa r i ly so. In

fact , su bsta nt ial savings m ay be rea lized by

optimizing filtered water qu ality instea d.

Bench sca le test ing can be u sed in conjun c-

tion with jar tests to pred ict fil tered water

qu ality. Wh atm an # 40 filter paper (or

equ ivalent) provides a good simu lation . Use

fresh fil ter paper for each jar and discardthe first portion filtered.

5 10 20 50 100 200 5001

2

5

10

20

50

100

200

500

1000

Impeller Speed, RPM

G-value,

(sec-1

)

23 Co 3 Co


32/41

28


Jar Test Chec klist

Rou tine jar tes t procedu res sh ould betailored to closely match actual conditions

at the plant . This may take some experi-

men tat ion since full sca le mixing cond itions

are only app roxima ted in th e jar tes t a nd

treatm ent is on a b atch ins tead of a flow-

through bas i s .

Condi t ions to ma tch u p include:

sequence of chemica l addit ion

rapid mixing in tens i ty and t ime floccu lat ion intensi ty an d t ime

Visua lly evalu ated variables includ e: time for first floc forma tion

floc s ize

floc qua l ity s e t tling r a t e

Imm ediately after r ap id m ixing th e followin g

can be de te rmined:

ze ta pot en t ia l pH

Later , after slow m ixing an d s ettl ing,

sam ples can be carefu lly drawn off an d

an alyzed for:

t u r bid it y

c olor

filt e rability n um ber

pa r t ic le coun t

r e s idua l coagu l an t

filtered water turbidity

Limitations of Visual Evaluation

Routine jar testing is time consuming, and plantoperators often fall back on visual evaluation as abasis of comparison. Unfortunately, visual judgmentscan be surprisingly misleading. In the exampleabove, the visual evaluation (Poor, Fair, etc.) was ofabsolutely no help in selecting an economical alumdose.

0

+10

-5

-10

-15

-206 8 10 12 14 16

0.0

0.1

0.2

0.3

0.4

ZetaPotential


Water

ZetaPotential,mV

Alum Dose, mg/L

Turbidity,NTU

+5

+15

+20

Poor

PoorFair

Good

Best

Fair


33/41

29

Zeta Potential

Easily Understood

Zeta poten tial is a n excellent tool for coagu -lan t dosage control , and we are proud to

ha ve pioneered the u se of our inst ru men t in

water t reatm ent over 25 years ago. Opera-

tion of a Zeta-Meter is relatively simp le an d

only a s hort t ime is requ ired for each test .

The resu lts a re object ive an d repeata ble

and, most important ly, the techniques can

be eas ily learn ed.

Th e principle of opera tion is ea sy to u n der-

sta nd . A high qua lity stereoscop ic micro-

scope is u sed to comfortably observe tu r-

bidity par t icles inside a cha mb er cal led a nelectroph oresis cell . Electrodes placed in

each end of th e cell create an electric field

acros s i t . If th e tur bidity pa rticles ha ve a

cha rge, then th ey move in th e field with a

speed and direction which is easily related

to their zeta potential .

Zeta-Meter System 3.0

A standard parallel printercan be directly connectedfor a "hard copy" of your

test run.

Our Zeta-Meter 3.0 - Simple & Reliable

The Zeta-Meter 3.0 is a microprocessor-bas ed version of our popu lar inst ru men t .

The s am ple is poured into the cell, then the

electrodes a re ins er ted an d conn ected to

the Zeta-Meter 3.0 u ni t . Firs t , the ins t ru -

men t determines specific cond u ctan ce and

helps select the appropriate voltage to

app ly. Then , when th e electrodes are

energized the particles begin to move and

are t racked u sing a gr id in th e microscope

eyepiece.

Tracking simply involves pressing a button

an d h olding it down while a colloid trav-erses the gr id. When th e t rack but ton is

released th e ins t ru men t ins tan t ly calculates

an d displays th e zeta potential . A single

tracking takes a few second s, an d a com-

plete run takes only minutes .


34/41

30


A Practical De sign

The Zeta-Meter 3.0 is d esign ed to be m is-tak e-proof. It recogn izes imp ra ctical resu lts

an d t racking t imes tha t are too sh ort . A

clear butt on a llows th ese or oth er incon-

sis tent resu lts to b e deleted without losing

the rest of your d ata.

Stat is t ics are a lso ma inta ined b y the Zeta-

Meter 3.0, an d can be reviewed a t an y t ime.

Press ing a s ta tu s but ton causes the u ni t

to display th e total nu mb er of colloids

tracked, th eir average zeta poten t ial an d

stan dard deviat ion, a s tat is t ical measu re of

the s pread of the individu al data valu es.

A pr inter can be conn ected to the Zeta-

Meter 3.0 un it to record the ent i re ru n. The

outpu t sh ows the value obtained for each

colloid as well as a statistical summary of

the en t ire test .

Simplif ied Coagulant Dose Cont rol

The zeta potent ial tha t corresponds to

optimu m coa gulation will vary from p lant t o

plan t. The optim al valu e is often ca lled the

target zeta potent ial and is b est estab lish ed

first by correlation with jar tests or pilotun it s , an d th en with a c tua l p lan t per form-

ance .

Once a target value is set, routine control is

relatively simp le an d m erely in volves m eas -

uring the zeta potential of a sample from

th e flash m ix. If th e mea su red valu e is

more n egat ive tha n th e target value, just

increas e the primar y coagu lant dose. If i t is

more positive, then lower the dose.

Other Applications of Zeta Potential

You can also use zeta potential analysis forth e followin g u seful ap plications :

s c r een ing o f p r im ar y coagu lan t s

cost compa rison of cat ionic polyelectro-

lytes

op t im u m p H d et er m in a t ion s

checking qu al ity of delivered cat ionic

polyelectrolyte

opt imizing feed poin t for pH adjus t ing

chemicals

eva lu a t ing d ilu t ion/ mixing requirements

for ca tionic p olyelectrolytes


35/41

31

Streaming Current

On-Line Zet a-Pote nt ial . . . . Alm os t

A limitation on the zeta potential techniqueis th at i t is n ot a cont inu ous on-line m eas-

ur ing ins t ru ment . The s t reaming cur rent

detector was developed in resp onse to th is

need. Its ma in advanta ge is rapid detect ion

of plant u psets .

Stream ing cu rrent is real ly nothing more

than an other way to measure zeta poten-

tial , bu t i t is a ctu ally related to the zeta

potent ial of a s olid su rface, su ch a s th e

walls of a cylindrical tu be, an d n ot th e zeta

poten tial of th e tu rbidity or floc pa rticles.

Forcing a flow of water through a tube

ind u ces an electr ic cu rrent (cal led th e

stream ing current) an d voltage differen ce

(called the stream ing p otential) between the

end s of th e tub e. Some of th e particles in

th e liqu id will loosely adh ere to th e walls of

th e cylinder a n d will affect th e zeta poten -

tial of th e wall . As a resu lt th e stream ing

curren t or s t r eaming potent ial of the wa ll

will reflect to s ome d egree the zeta poten tial

of the p ar t icles in su spen sion.

Commercial InstrumentsMost commercial s t reaming cu rrent devices

u se a n oscillating flow to eliminat e ba ck-

groun d electrical signals. A piston in a

cylind er (called a boot) is very comm on.

The p is ton osci llates u p an d down a t a

relatively low frequency (about 4 cycles per

second) causing the sample to flow in an

al ternat ing fashion through the space

between th e piston an d cylin der. The flow

of water creates an AC stream ing cu rrent ,

du e to the zeta p otent ial of the cylind er an d

piston su rface. It is this curren t which is

meas u red an d am plified by the detector .

A Use ful Mon itor

Stream ing curren t is u sefu l as a n on-l inem on itor of zeta poten tial. However, it is

only an indicat ion becau se th e value is not

scaled. Tha t is , a cha nge in 10 s t ream ing

curren t u ni ts does not correspond direct ly

to a zeta poten tial cha nge of 10 m V. In

ad dition, t he zero position is often sh ifted

significan tly from t ru e zero and is not

s table.

St reaming cur rent an d s t reaming potent ia l

are both excellent ways to keep track of on-

line condi t ions, b u t sh ould be cal ibrated

with a zeta meter . A cha nge in th e s t ream-ing cu rrent tel ls you th at i t is t ime to tak e a

sam ple an d m easu re the ze ta p otent ia l.

Oscillating Piston Streaming Current DetectorThe AC signal is electrically amplified and conditionedto produce a signal that is proportional to the zetapotential.

SampleOut

SampleIn

Electrode

Piston

Boot

Electrode

AC

StreamingCurrent


36/41

32


Turbidity and Particle Count

Finish ed water tu rbidity is often u sed to

gauge the effectiveness of a water treatmentplant , bu t it is importan t to remem ber tha t

there is n o direct relat ion between the

amou nt of su spended ma t te r , o r num ber of

par t icles , an d th e tu rbidity of a s am ple.

Most turb id ity measu rements a re ba sed

u pon n ephelometry. A light beam is pass ed

through the water sample and the par t icles

in th e water scat ter th e light . The tu rbidi-

meter mea su res the intens ity of scat tered

light, usually at an angle of 90 to th e light

beam an d th e inten si ty is expressed in

s tan dard tu rb id ity uni t s .

The an gle of peak s cat ter and the am oun t

of l ight scattered are both influenced by thesize of th e pa rticles. Oth er factors , su ch as

the n a tu re and concent ra t ion of the par -

ticles will also effect th e tu rbidity mea su re-

m en t .

Particle size analyzers quantify the actual

par t icle coun t an d th e dis t r ibu t ion with

size. Par t icle coun ters are mu ch more

expensive than turbidimeters and are not

common ly foun d in water t reatm ent plants .

Particle Count vs. Turbidity

Simultaneous plots of zeta potential and turbidity areoften used to determine the target zeta potential andalum dose. The curve can shift if particle counts areused to judge performance instead. In this example,the target zeta potential was -5 millivolts usingturbidity as a criteria. This results in an alum dose of

45 mg/L. When particle count was used, the targetzeta potential is 0 millivolts, and the correspondingalum dose is 70 mg/L. Surprisingly, the particle countis almost triple at the lower dose determined by the

turbidity standard.

0

+10

-5

-10

0.0

0.2

0.4

ZetaPotential


Water

ZetaPotential,mV

Alum Dose, mg/L

Tu

rbidity,

NTU

+5

+15

0.6

0.8

20 40 60 80 100 120

1.0+20

0

+10

-5

-10

0

20

40

ZetaPotential

ZetaPotential,mV

Alum Dose, mg/L

RelativeParticleCount,percent

+5

+15

60

80

20 40 60 80 100 120

100

ParticleCount

+20


37/41

33

Basics

Mixing pa t terns are often complex an d a re

difficult to d escribe, s o we u su ally classify

mixers b ased u pon h ow closely they ap-

proximate one of two idealized flow pat-

tern s: comp lete mixing or plug flow.

Ideal plug f low mean s th a t each volum e of

water rema ins in th e reactor for exact ly the

sa me am oun t of t ime. If a slu g of dye were

injected into the flow as a tracer, then all

the dye would appear a t the out let at th esa m e time. Ideal plu g flow is very difficult

to ach ieve. In genera l, th e reaction bas in

mu st b e very long in comparison to i ts

width or diameter before the mixing pattern

begin s to a ppr oxima te plu g flow.

Com pl e t e m i x i ng bas ins are always in-

s tan ta neous ly b lended th roughout th e ir

ent i re volu me. As a resu lt , an incoming

volum e of water imm ediately loses i ts

ident i ty and is interm ixed with the water

th at entered previou sly. A complete-mix

reactor is also called a back-mix reactor

becau se its contents are always blended

ba ckwar ds with th e incom ing flow. If a slu g

of dye tracer was injected into the basin,

then some of i t would immediately appear

in the out goin g flow. The con centr ation of

dye would drop s teadi ly as th e dye in the

bas in backm ixed with the clear incoming

water an d was di lu ted by it .

Chapter 6

Tips on Mixing

Retention Time Curves

When several complete mix reactors are placed in aseries then the chance of the same particle quicklyexiting each basin is very small. The overall retentiontime pattern then changes and becomes moreregularly distributed around the average.

0 1.0 1.5 2.0 2.5 3.00.5

20

40

60

80

100

RelativeConcentration

Fraction of Average Retention Time

One Complete Mix Basin

Two

Three

Four

A PracticalApproximationof Plug Flow


38/41

34

Chapter 6 Tips on Mixing

Rapid Mixing

Plug Flow versus Complete Mix

Recomm enda t ions for th e best type of rapidmixing are confus ing at best . Th e ex-

trem ely fas t t imes for th e reaction of alum

with alkalinity (less th an 1 secon d) an d for

the ra pid format ion of the alu minu m h y-

droxide microfloc (less than 10 seconds)

ha ve produced a dvocates of in- l ine ins tan -

tan eous blend ers (plu g flow) as well as

tu bu lar plug flow reactors , all with reaction

times of a few secon ds . A plu g flow rea ctor

insu res tha t a n equa l amou nt of coagulant

is availab le to each pa rticle for an equ al

am oun t of t ime. This is imp ortant if the

reaction time is tru ly only a few second slong.

Others have found that the t radi t ional

complete-mix-type basin with turbine- or

propeller-type impellers is en tirely adequ ate

and have even recommended extending the

reaction time to several minutes (instead of

the s tan dard 3 0-60 seconds su gges ted in

some s ta te s t an dards) in order to enha nce

th e initial stages of floccu lation .

This a ppa rent conflict can be explained b y

cons iderin g the two poss ible types of coagu-lat ion: ch arge n eutra lizat ion a nd sweep

floc. For sweep coagu lation, extrem ely

sh ort mix t imes a re not required s ince most

of the colloids are captured by becoming

enm esh ed in th e growing precipi tate . For

alum , dosages above abou t 30 m g/ L wil l

produ ce sweep coagu lation. Lower dosa ges

resu lt in a combina t ion of sweep an d

cha rge neu tral izat ion or ju st cha rge neu -

t ral izat ion, depend ing on th e pH an d th e

alu m dos e. Clearly, th e choice of mixing

regimes is not easy.

Mixing Inten sity

Th e type an d inten sity of agitation is alsoimp ortant . Tu rbu lent mixing, char acter-

ized b y high velocity gradient s, is des irable

in order to provide sufficient energy for

inter pa rticle collisions . Propeller m ixers

are n ot as well su ited as tu rbine mixers .

Propellers put more energy into circulating

the ba sin contents , whi le tur bine mixers

sh ear the water , ind u cing the higher

velocity gradients and the small high

multidirectional velocity currents that

prom ote p art icle collisions .

High intensi ty throughou t th e rapid mixbas in ma y not be as impor tant a s th e sca le

an d intens ity of tu rbu lence at th e point of

coagu lan t addi t ion. In a turb ine- type

complete-mix basin, the mixing intensity

will not be un iform th roughou t th e basin.

Th e impeller disch arge zone occu pies

abou t 10 p er cent of the total volu me a nd

ha s a s hear inten si ty app roxima tely 2.5

t imes the average.

There is an upper l imit to mixing intensity

because high shear condi t ions can break

u p m icroflocs a n d d elay or pr event visiblefloc forma tion . Coagulan ts which act

through charge neutral izat ion can some-

times r ecover from th is du ring floccu lation

(examples inclu de a lu m an d low molecular

weigh t ca tionic polyelectrolytes). Bridging-

type floccu lan ts a re n ot as resi lient a nd

ma y not recover . This may be becau se the

long polymer cha ins are cu t , breaking th eir

pa rticle-to-particle bridges. The ch arge on

the polymer fragments may even interfere

with a t tempts to re-coagulate the system,

al though a polymer of opposite cha rge may

help.


39/41

35

Rules of Thum b for Rapid Mixing

The pu rpos e of rap id mixin g is t wo-fold.First , i t sh ould efficient ly dispers e th e

coagu lat ing chem icals . Second, and ju st as

imp ortant , i t sh ould provide condi t ions tha t

favor the formation of microfloc particles.

The ch ar acteristics of this m icrofloc will

determine the success of the flocculation

s tep .

It is difficult to offer hard and fast rules for

rapid mixers . In fact , man y rapid mix

alternatives provide satisfactory results.

Back -mix rea c tors genera lly work bes t if th ey follow th ese gu idelines:

Sta tors and squ are t anks fu nn el more

energy into mixing than into circulation.

Flat blade tu rbines provide more sh ear

inten si ty than propeller types.

Chemicals should be introduced at the

agitator blade level.

For sweep coagulat ion u sing alum or

ferric comp oun ds , G-valu es of between

300 and 1200 s ec-1 in combination with

retent ion t imes of 20 seconds to several

minu tes ha ve produ ced sat isfactory

resul ts .

When cationic polyelectrolytes a re u sed

as a p r imary coagulan t , then a G-valu e

ran ge of 400-700 is m ore appropriate .

Overly long reten tion times (lon ger tha n

1-2 minu tes), rapid cha nges in G valu e,

an d G values a bove 1000 ca n a ll be

detr imental .

Consider tapered velocity gradients to

provide a transition from rapid mixing to

flocculation. This m ay be importa nt if the p r ima ry coagulan t is a polymer.

Plug flo w reac tors ma y be more appropri-

ate i f cha rge neu tral izat ion p redominates

over sweep floc. G-valu es of 30 00-5 000

with retent ion t imes of abou t 1/ 2 second

are usual ly recommended.

G-Value

The G-valu e concept is a rou gh ap proxima -tion of mixin g inten sity. It is ba sed u pon

inp u t power, bas in volu me a nd viscosity.

For water , a t 1 0 OC, the G value can be

expressed in terms of horsepower per 1000

gallons of ba sin volu m e:

G = 387 x (HP per 1 000 gal)1/ 2

Practical G-Value RelationshipsThe relation between retention time and inputhorsepower per million gallons per day (MGD) of flowis linear for any selected G-value. This figure can be

used to estimate the G-value or the power require-ment to produce a specific G-value. It is calculated at10O C. The input horsepower will be a percentage ofthe mixer motor power, in the range of 70 - 85%.

0

InputHorsepowerperMGD

Retention Time - seconds30 60 90

1

2

3

4

5

G=1000

G=800

G=600

G=400

G=200


40/41

36

Chapter 6 Tips on Mixing

Flocculation

Rate of Flocculation

In genera l the ra te of floccu lation is con-trolled by two factors:

concen tra tion of pa rticles (both free

colloids a n d floc) velocity grad ient

A greater number of particles provides more

opportu nity for collisions to occur between

colloids or between floc p art icles an d col-

loids . This increas es the rate of captu re.

High er velocity gradient s also p rovide m ore

opportun it ies for collis ions , bu t th e sh ear

forces from too h igh a velocity gradient can

brea k u p larger flocs a n d will limit thema xim u m floc size.

Creat ing Collisions

The velocity gradients in a full scale floccu-

lat ion bas in can be created by a var iety of

mech an ism s, including baffled cham bers ,

rotat ing paddles , reciprocat ing blades an d

tu rbine- type mixers . Baffled cham bers are

lim ited becau se t he velocity gradient is

controlled only by the flow rate, while

mech an ical system s offer th e ab ili ty to

ind epende

Documents

Coagulation Ly Thuyet Ve Keo Tu