Treatment of Sugar Mill Waste Waters
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Transcript of Treatment of Sugar Mill Waste Waters
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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.
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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.
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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.
.
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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.
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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
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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
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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.
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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.