Treatment of Sugar Mill Waste Waters

8/18/2019 Treatment of Sugar Mill Waste Waters

1/9

Processing

TREATMENT

OF

SUGAR MILL WASTE WATERS

R. R. ~ i t h ~ a t e ,. S. Keniry and A. W. Strong

CSR Limited, Sydney, Australia

The system installed at Victoria Mill for the treatment of waste

waters is described and the design concgpts and operating procedures

are discussed. The system involves two stage treatment of wastes in

well-mixed ponds each with sludge re-cycle. The first or primary

stage is equipped with two small surface aerators and operates in a

primarily anaerobic condition with pH control and nutrient addition

The second stage is an aerobic activated sludge system. A third pond

is provided for pre-treatment of strong wastes prior to discharge to

the primary pond. Retention time in the primary pond is approximately

4 hours and slightly longer in the secondary pond. Suspended solids

concentrations in each pond are controlled at about 2500 mg/l with

excess sludge being disposed of to surrounding grazing land. In 1975

season the system handled an average daily volume of 3598 mh a s t e

containing on average 949 mg/l BOD5 and yielded final effluent

containing on average 6

mg/l

BOD5

and 58 mg/l suspended solids.

INTRODU TION

By the mid 1960 s it was apparent that increasing environmental

awareness would necessitate a reduction in the amounts of

BOD

and other

pollutants being discharged from many sugar mills in Queensland. In the

particular case of CSR mills, the conclusion was reached that some form of

treatment system would be required to treat, before discharge, effluents

other than condenser cooling waters.

Up to this time, there appeared to be little published work concerning

the treatment of waste waters containing mostly carbohydrate wastes.

Accordingly, in 1968 a development programme was commenced with the

aim of developing a process to reduce the

BOD

level of mill effluents to

about

30

mg/l at low suspended solids concentrations. The early stages

of the programme involved both laboratory scale and plant scale pilot

treatment trials, and culminated in the commissioning of a full-scale

treatment plant at Victoria Mill late in 1973 season. A similar, but smaller,

plant was commissioned at Goondi Mill in 1975 season, and a further plant

is under construction at Macknade Mill.

This paper briefly describes the full-scale liquid effluent treatment

system at Victoria Mill and its operating performance in 1975 season. Par-

ticular features of the current design and operating procedures, that have

evolved during the laboratory and pilot plant stage, and since commissioning

of the full scale system are also discussed.


2/9

PROCESSING

DESCRIPTION OF PL NT

A diagrammatic representation of the plant is given in Figure 1 and

the specification of major items of equipment is tabulated in TAble I Factory

waste waters, including spray pond overflow, are pumped to a preliminary

treatment plant which consists of two 1.83 m wide D.S.M. screens with

2 mm bar spacing to remove fibrous matter and then to a 27 m3 oil

separation tank to remove heavy greases.

The water then flows to the primary pond, except in the case of strong

-wastes, generated on weekends or at other times, which are diverted to the

weekend pond by means of motorised valves controlled from the factory.

In the primary pond wastes are partially degraded under primarily anaerobic

conditions with lime and nutrient addition, before being pumped to the

primary clarifier. The clear overflow from the primary clarifier flows to

the secondary pond, which operates as a conventional aerobic activated

sludge process. Mixed liquor from this pond overflows to the secondary

clarifier from which the final treated effluent overflows to lagoon Creek.

Sludge is recycled from each clarifier to the pond from which it originated.

Excess sludge from the primary treatment system is brought to an aerobic

state over a period of two to four days in the sludge stabilisation pond prior

to disposal by irrigation onto surrounding grazing land. Excess sludge from

the secondary pond is irrigated directly in the form of secondary clarifier

underflow. Under normal conditions approximately 10 hectares of land are

used for irrigation.

Treatment of wastes initially diverted to the weekend pond is discussed

later.

DESIGN CONCEPTS ND OPER TING PROCEDURES

Series Versus Single ond System

The results of early pilot plant trials in well mixed ponds indicated

that for weekday wastes of initial BOD5 of up; to 4,000 mg/l, the average

BOD, removal rate was greatest when the dissolved oxygen level in the

mixed liquor was zero. At zero oxygen concentration in mixed liquor the

rate of removal of BOD5 averaged 53 kg BOD5/kW/day, and was as high

as 85 kg BOD5/kW/day on occasions. At these high rates the process was

concluded to be substantially anaerobic, on the basis that the aerator oxygen

input rate at 30°C and zero dissolved oxygen concentration was only

33 6 kg/kW/day, as determined from both re-oxygenation and oxygen

depletion tests. The former method measures the reaeration of clean water

which has been chemically de-aerated and is usually termed the unsteady

state reaeration method . The latter method involves stimulating bacterial

growth so that the oxygen level falls to zero. As the oxygen level rises again

the aerator is stopped at selected points and the depletion by bacterial

activity measured. Residual BOD5 levels from the primarily anaerobic

process were generally much higher than those that could be obtained,

albeit at lower overall BOD5 removal rates, with an aerobic well mixed pond.


3/9

R . R . B AT H G AT E J . S . K E N IR Y A N D

A W

S TRO NG

'7-

Ba~ ass e nd Oil

Primary

Clarifier

Secondary

Pond

Secondary

Clarifier

Primary

F

Final Discharge

FIGURE 1

Waste water treatment system Victoria

Mill.

.


4/9


5/9

R.R. BATHGATE J. S. KENIRY AND A.W. STRONG

2513

The superior

BOD5

removal rates obtained in a well mixed anaerobic

pond offered a major potential advantage in terms of capital and operating

cost of full scale treatment systems. However, a full anaerobic system suffered

the disadvantages of objectionable odour and unsatisfactorily high

BOD5

in final effluent. The compromise solution was to adopt a two ponds n

series ystem where the primary pond was operated in an anaerobic

condition and the secondary pond was operated in an aerobic condition.

Selection of Aeration Mixing Devices

Floating surface aerators were selected in order to yield flexibility in

operation at the pilot plant stage by permitting variation of liquor levels.

The aerators were sized in order to supply complete mixing within the

ponds. Complete mixing of pond contents was selected in an effort to

maintain constant microbial activity at times of variable load. Aerators

instead of sub-surface mixers were used in 't he primary pond to assist in

odour and pH control.

Control o f Oxygen and Liquid Levels in Ponds

A problem frequently encountered in early trials was the formation

in the ponds of poor-settling or bulking sludge. The bulking of sludge was

associated primarily with the growth of filamentous organisms in aerobically

treated liquors containing significant concentrations of carbohydrates.

Observations indicated that the formation of bulking sludge was often

associated with the transition from aerobic to anaerobic conditions or vice

versa. Therefore a control system was installed to maintain aerobic condi-

tions in the secondary pond, whilst a level controller in the primary pond

maintains a minimum working volume by overriding the oxygen control

system signal and reducing flow to the secondary pond when the minimum

volume is approached. This minimum volume provides a buffer against

shock loads on the primary pond. The control system is shown as Figure 2.

It is assumed that facultative or anaerobic conditions are maintained

in the primary pond by using aerators which supply only about 25 of the

average oxygen demand in influent wastes. The large difference between

oxygen demand and supply ensures continuous anaerobic conditions.

The oxygen concentration in the secondary pond mixed liquor is

continuously monitored and controlled at the desired set point of

30

sat-

uration by regulating the flow from the primary to the secondary pond.

As a general rule oxygen concentration varies inversely with waste strength

and flow rate.

The pH of the primary pond mixed liquor is regulated by automatically

controlled dosing with milk of lime.

A

mixed liquor pH of 5.8 .0 gives

good removal efficiency and satisfies odour control requirements. The pH

of secondary pond mixed liquor requires no control and normally is in the

range 6.8

.3.


6/9

2514 PROCESSING

Th e phosphate and nitrogen co ntents of mill waste waters are inadequate

to satisfy microbial growth requirements.

Current practice is to add commercial fertilizers such as high analysis

superphosphate and aqua ammonia or di-ammonium phosphate to the

primary pond. Nutrients excess to primary pond requirements pass to the

secondary pond. The addition rate is regulated to maintain a concentration

of nitrogen and phosphorous of

1

mg/l each in secondary pond mixed

liquor.

T he settling characteristics of s econd ary pond sludge are very sensitive

I

to nutrient levels and if n o excess nutrien t is available from t he prima ry

pon d floc carryove r into final effluent is likely.

Consequently nutrient addition must be carefully balanced between

maintenance of adequate levels for bacterial growth and minimising the risk

of eutrophication below the final discharge point.

,

I

Level Transmitter 4 Oxygen Recorder Controller

2 Oxygen Meter 5 Automatic Control Valve

3 Multiply~ng Unit 6 Oxygen Measuring Electrode

FIGURE 2

reatment plant control system


7/9

R.R. BATHGATE

J . S .

KENIRY

AND A W

STRON 25

5

Weekend Wastes

Extremely strong wastes from washdowns and spillages are diverted

to the weekend pond for pre-treatment, because of the longer retention time

required to reduce the

BOD5 load to a level acceptable for feeding to the

secondary pond. After pre-treatment which includes pH control, nutrient

addition, and some primary sludge re-cycle, these wastes are slowly bled

back into the primary pond. The rate of bleed depends largely on the load

on the primary pond; the time to empty the weekend pond each week varies

between two and five days. Operating conditions in this pond are similar

to those in the primary pond,

aerators being used for mixing,

OD

reduction and for prevention of odours.

The combination of primary and weekend ponds provides a buffer

system which reduces the effects of variability in strengths of mill wastes

and allows smoother control of flow to the secondary pond.

Odour ontrol in Primary Pond

Anaerobic or septic conditions are usually accompanied by offensive

rotten egg type smells, but this is not the case for the primary pond. It is

considered that hydrogen sulphide and other gases are oxidised to odourless

compounds by introduction of oxygen, and that pH control prevents the

formation of a strongly reducing environment which favours the production

of sulphurous odours by sulphate reducing bacteria. Other smells caused

by partially reduced volatile carbohydrates which are normally produced

under anaerobic conditions, are minimised by maintaining pH

5 5

or greater

in the primary and weekend pond mixed liquor.

Pond Retention Times

Trial work indicated that for continuous treatment,

24

hours retention

in each pond of the series was sufficient to produce good quality effluent.

In actual operation the retention time in the primary pond varies between

19

and

32

hours and in the secondary pond between

26

and

40

hours

without noticeable effect on final effluent quality. We believe that shorter

retention times are possible and trial work is being undertaken to test for

the effect of shorter retention time.

ontrol o f Suspended Solids in Mixed Liquor

Suspended solids in both ponds are maintained between

2000 000

mg/l preferably close to

2500

mg/l. It has been found that a minimum

suspended solids of

1500 000

mg/l is required to prevent the growth

of single cell yeasts when loading is high. At suspended solids concentrations

in or above the upper end of the operating range, clarifier overloading can

occur if the flow rate is high. Higher concentrations also can cause poor

quality effluent

by

producing large quantities of fine floc when the waste

load decreases to a point where the population is starved for food; and can

aggravate sludge disposal problems when strong wastes lead to high bacterial

growth rates.

\ I

t


8/9

2516 PROCESSING

Pre-Season Sludge Preparation

Initially, sludge was formed by the addition of molasses to the sludge

stabilisation pond, with adequate nutrients, at such a rate that aerobic

conditions could be maintained by the 10 kW aerator. When bacterial growth

increased the suspended solids level to about 2000 mg/l, the contents of

this pond were pumped to a larger pond where growth continued with

further molasses and nutrient addition. This continued until sludge concen-

trations reached 2000 mg/l in both primary and secondary ponds, and

thus were available to commence full treatment of mill wastes. About five

weeks preparation time would normally be sufficient.

For the start of 1975 season, sludge held over from 1974 was fed with

molasses to generate further sludge. This reduced the preparation time from

five weeks to three weeks and reduced molasses requirements from 27 tonnes

to 16 tonnes.

T BLE II Operational data Victoria mill treatment plant 1975 season

Mill waste waters

Average daily volume 3598 m3

Average daily BOD5 content

3 97 tonnes

Treatment plant discharge

Average daily BOD5 concentration

daily results

<

20 mg /l BOD5 40

Average daily suspended solids concentration 48 mg /l

daily results

<

30 m g/l suspended solids 43

Average daily oxygen concentration 61 saturation

Average daily temperature 4 O

Average daily oil grease concentration < mg/l

Relevant operating data for the first 23 weeks of 1975 season are

presented in Table 11

The quality of waters discharged from the treatment plant during both

the 1974 and 1975 seasons was adversely affected by the presence of clay

eroded from the ponds walls. Measures taken during the 1975 slack season

reduced the amount of erosion to the extent that the 61 mg/l BOD5 and

213 mg/l suspended solids averages for 1974 were reduced considerably

to the levels achieved during 1975. With the further knowledge gained during

1975 season and further work during the 1976 slack season we expect the

quality of the final effluent will improve further.


9/9

R . R . B A T HG A T E J . S . K E N I R Y A N D A W STRONG 2517

A C K N O W L E D G E M E N T S

The authors express their appreciation to the Management of CSR

Limited for permission to publish this paper, and to the many CSR staff

particularly the late Mr.

J

R . Miller, who, over a number of years, have

contributed to the development and evaluation of the system described.

Th e company s consultants for the development of th e process were

Water and Trade Wastes Consultants Pty. Ltd. of 2 Barrack Street, Sydney,

N.S.W. 2000.

T R A T A M I E N T O D E L A S A G U A S D E D E S E C H O

D E L

C E N T R A L A Z U C A R E RO

R . R . Bathgate ,

J

S. Keniry

y

A. W. Strong

R SUM N

Se describe el sistema instalado en el Central Victoria para el

tratamiento de las aguas de desperdicio incluyendo en el trabajo

10s conceptos del disefio y la operacion. Para tratar 10s desechos

el sistema incluye dos etapas que se realizan en lagunas bajo plena

mezcla cada una con re-cic lo del lodo. La primera etapa va equipada

con dos pequefios aereadores de superficie

y

funciona principalmente

bajo condiciones anaerobicas con control del pH y adicion de 10s

nutrientes. La segunda etapa es un sistema aerobic0 activado de tra-

tamiento de 10s cienos. Hay una tercera laguna para el pre-trata-

miento de 10s desechos fuertes cuyo pre-tratamiento se realiza con

anterioridad a la descarga a la primera laguna. El tiempo de retencion

en la primera laguna es de

4

horas aproximadamente; y algo mas

prolongado en la segunda laguna. La

concentration

de solidos en

suspension en cada laguna se controla a unos 2500 mg/l; descar-

gando el exceso de 10s lodos a las tierras circundantes dedicadas a

pastos. Durante la campafia de 1975 el sistema trato un volumen

promedio diario de 3 598 md de desechos conteniendo en promedio

949 mg/l BOD y rindiendo un efluente final que contenia en pro-

medio 26 mg/l BODs y 58 mg/l de solidos en suspension.

Documents

Treatment of Sugar Mill Waste Waters