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Two-phase anaerobic treatment kinetics of palm oil wastes
Water Res Vol 19. No 5. pp. 66"-669. 1985 Pnnted in Great Britain. All rights reserved
0043-1354 85 $3.00 + 0 . 0 0 Copyright ,_- 1985 Pergamon Press Ltd
TECHNICAL NOTE TWO-PHASE ANAEROBIC T R E A T M E N T KINETICS OF PALM OIL WASTES W. J. NG I'*. K. K. WONG z and K. K. CHtY ~ ~National University of Singapore. Department of Civil Engineering. Singapore 0511 and :Universiti Pertanian Malaysia. Department of Environmental Science. Malaysia ( Receit'ed January 1984) Abstract--A laboratory-scale two-phase anaerobic digestion system was used to treat a palm oil mill effÊuent (POME) containing around 63,000rag 1-~ COD. Phase separation was accomplished through control of the hydraulic retention times of two reactors operated in series. Acid and methane phase biokinetic coefficients were evaluated. Steady state parameters indicate good process stability with high gas yields. Key words--two-phase, anaerobic, palm oil mill effluent, kinetics, hydraulic retention times
INTRODUCTION
merit was added. The characteristics of the palm oil mill effluent are given in Table 1. The reactors were seeded with digested sewage sludge from a domestic sewage treatment plant. Temperature of the reactors was maintained at 32=C. Mixing within each reactor was accomplished by continuously stirring with a propellor. There was no solids recycle. The digestion process was monitored by daily measurements of gas production and composition. The gas was collected by displacing a solution of 5~oH:SO+ and 20% Na,.SOa in a collection chamber. Gas composition was determined using a gas chromatograph. Other digestion parametres measured at regular intervals and according to Standard Methods (APHA, 1980) included alkalinity, total volatile acids, BODs, COD, total solid and suspended solids, pH, total Kjeldahl nitrogen and total phosphorus.
Palm oil mill effluent (POME) is a major agroindustrial waste in the Southeast Asian region. In Malaysia, for instance, the estimated BOD load which was discharged into receiving waters was 330 tonnes day -~ in 1980. This is equivalent to that generated by a population of 7.3 million people. Realising the seriousness of the situation, ways are actively being sought to reduce these organic loads. Recent studies have shown that anaerobic digestion of these wastes could result in significant reductions in BOD and C O D (Southworth, 1979; Chin and Tan, 1979a,b; Wong and Springer, 1981, Chin, 1981). These studies have concentrated on the conventional completely mixed digestion process. The anaerobic process is, however, essentially diphasic and the physiological and nutritional requirements of the acid and methane forming microorganisms are different. Results (Chin, 1981) have indicated that imbalances occurred between the two groups of bacteria at shorter retention times. Consequently, to ensure process stability, design retention times have to be high and this in turn leads to higher capital and operating costs. To reduce these, other process configurations are being investigated and this paper reports the results of a two-phase anaerobic fermentation system using POME.
RESULTS AND DISCUSSION
MATERIALS AND METHODS The two reactors, arranged in series, were made of plexiglass and each had a capacity of 161. Reactor I became the acid-phase digester. Part of its effluent was led into reactor 2+ the methane-phase digester. The digesters were fed on a continuous basis and no inorganic nutrient supple*To whom all correspondence should be addressed. 667 WR
19 ~--[
The P O M E investigated had a high organic content with a relatively large C O D : B O D ratio of 2.4. This indicated the presence of a large portion of difficult to digest organics. The success of the twophase process would depend on the ability of the micro-organisms to degrade these organics. Tables 2 and 3 show the steady state operating parameters for the acid- and methane-phase respectively. The increase in volatile acids in the acid-phase and the subsequent decrease in the methane-phase showed that phase separation was accomplished. Total volatile acid content in the acid-phase increased with increasing retention time. This probably was indicative of the difficult to degrade organics being broken down when the acid-forming microorganisms were given more time to do so. Significant C O D reductions of 54, 61, 67 and 70% were observed following the acid-phase with retention times of 1, 2, 4 and 6 days respectively.
668
Technical Note Table I. Characteristics of the palm oil mill effluent
Parameters pH Alkalinity (rag I - I ) Total volatile acid (rag 1-~ ) B O D 5 (rag 1-~ ) C O D (rag 1-t ) TS (rag I -~ ) TVS (rag 1-~ j TSS (rag t- ~) TVSS (mg l- E) T K N (rag l -t ) TP (rag I-~ )
Range
Average
4.05--4.15 980-1240 1440-1600 25,130-27,210 61,140-64,950 47.380--50.530 38,550-40.100 26,150--27.450 21.740-22,790 990-1050 280--315
4.1 1094 1503 26,222 62,934 48.431 39.339 26,456 22.149 1000 294
Overall COD reductions with the acid-phase operated at 1 day retention time while the methane phase was operated at 10, 20 and 30 days retention times were 78, 81 and 85~o respectively. Applying the following biokinetic model (Lawrence and McCarty, 1970) the biokinetic coefficients for two-phase anaerobic fermentation of POME were evaluated.
XO~ K, 1 (So_ S)=k-~ +~. 1 --
r(s0 - s)
= -
-
Oc 1
-
0F'"
(11 k~
XO, YkS
-
+
(2)
(3)
k,~
(K~ + S0 )
where k = maximum rate of substrate utilization per unit mass of biomass (day -*), Ks = half velocity constant, equal to the substrate concentration at one half the maximum growth rate (mg 1-* ),
Y = theoretical yield coefficient (rag VSS mg -~ BODs), ka= microbial decay coefficient (day -z) S = substrate concentration surrounding the biomass (mg l-t). So = influent substrate concentration (mg 1-~) X = concentration of active biomass (mg l-t), 0c = cell residence time (day), and 0~ i"= minimum cell regeneration time (day). The minimum cell regeneration time of the acid- and methane-phase was 1.31 and 2.48 days respectively for the two reactor system investigated. These are significantly shorter than the 15.3 days determined for mesophilic fermentation of POME in a single reactor mixculture system (Chin, 1981). A direct benefit that can be derived from the shorter retention times achievable in two-phase fermentation is the reduction in tankage requirements. This would cut down the capital investment costs of treatment. Gas yields of 0.98-2.21g-~ BOD5 utilized can be expected from the methane-phase. These values do not include the gas produced and the BOD5 utilized in the acid-phase but apply only to the methanephase. The energy yield from the methane-phase was found to range from 21816 to 44441Jg -~ BOD5 utilized depending on retention time. Although various methods have been proposed to provide for phase separation and these have included dialysis techniques (Hammer and Borchardt, 1966; Schaumberg and Kirsch, 1966) and the addition of inhibitors (Pohland and Mancy, 1969), population selection through manipulation of retention times (Ghosh et al., 1975; Pohland and Massey, 1975;
Table 2. Steady state p a r a m e t e r s - - a c i d - p h a s e Parameter
Acid -phase
Retention time (day) pH Alkalinity ( m g l -~ ) Total volatile acid (rag I -c ) B O D 5 (mg 1-1) C O D (mg 1 - ' ) TSS (ragl -] ) TVSS (mg 1-~ ) T K N (mg 1- i ) TP (rag 1-I) COD/BOD
1 6.0 2105 5458 11,340 29,030 16,980 13,337 740 200 2.56
2 6.0 2055 5906 9050 24,700 16.222 13.050 720 190 2.73
4 6.0 2155 6373 7500 21,006 14,961 12.842 740 180 2.80
6 6.0 2199 6868 7010 19,013 13,330 12,015 710 163 2.71
Table 3. Steady state p a r a m e t e r s - - m e t h a n e - p h a s e Parameter Retention time (day) pH Alkalinity (rag I -~ ) Total volatile acid ( m g l - t ) I~OD 5 (mg I -I) C O D (rag 1-I ) TSS (rag 1-j ) TVSS (mg I - t ) T K N (rag l -I ) TP (rag I - t ) G a s production rate (1 g - t B O D utilized) CH,(%) Energy yield (J g -t B O D utilized) COD/BOD
Acidphase
Methane-phase
1 6 2105 5458 11,340 29,030 16,980 13,337 740 200 --
10 7.2 4150 1215 3158 13,580 12,752 8557 427 90 0.98
20 7.2 4953 1095 1278 12,235 I 1,023 8233 330 75 1.49
----
60 21,8t6 4.30
6[ 33.722 9.57
30 7.2 4700 999 1180 9403 8129 7364 310 50 2.12 56.5 44,441 7.97
669
Technical Note
Waste composition Synthetic carbohydrate waste (1) Acid-phase (2) Methane-phase
Table 4. Two-phase anaerobic fermentation kinetic coefficients Y kj k g,* 0y" (g VSS g-~ ) (day -~ ) (g g-~ VSS-day) (mg I-~ ) (day) 0.15 0.28
---
7.20 0.48
400 4400
Palm oil mill effluent ( 1) Acid-phase 1.04 0.16 5.00 200,000 (2) Methane-phase 1,43 0.034 0.50 I0.666 Single reactor system 0.23 0.036 0.44 227 "Note: high values are due to the very high influent COD concentrations (62,934mg 1-* ). Massey and Pohland, 1978) has s h o w n greater p r o m ise on account of the technique's relative simplicity. Data from this study has indicated that phase separation by m a n i p u l a t i n g retention times is feasible for the anaerobic fermentation of P O M E . The concentration of fatty acids in the raw P O M E averaged at 1503 mg 1- ~ of which a significant portion would be long-chain acids. Nevertheless no inhibition of the acid-phase was observed. Following the acid-phase the average acid c o n c e n t r a t i o n was 5458 mg I- ~. The methane-phase, however, was not noticeably inhibited by this. The biokinetic coefficients evaluated are given in Table 4 a n d c o m p a r i s o n m a d e with those of other studies. Considerable differences were observed. The P O M E c o n t a i n e d a high c o n c e n t r a t i o n o f volatile suspended solids which were not completely degraded. The difficulty in separating the cell mass from influent VSS a n d the diversed nature o f the systems involved account for these differences. CONCLUSIONS The results indicate t h a t phase separation could be achieved by m a n i p u l a t i n g the retention times of the reactors and that P O M E can be effectively treated with a two-phase a n a e r o b i c fermentation system. T r e a t m e n t efficiency reached 85% with the (1 + 30) day system with no cell recycle. The significantly shorter m i n i m u m cell regeneration time of the twophase system should be of interest since reactor volumes can be reduced. REFERENCES
APHA (1980) Standard Methods for the Examination of Water and Wastewater, 15th Edition. American Public
Basis
9.58 × 10-: 1.41
COD COD
1.31 2.48 15.3
BOD BOD BOD
Reference Ghosh and Klass (1977)
This study
Chin (1981)
Health Association. American Water Work Association, Water Pollution Control Federation, Washington, DC. Chin K. K. (1981) Anaerobic treatment kinetics of palm oil sludge. Water Res. 15, 199-202. Chin K. K. and Tan G. T. (1977a) Anaerobic treatment of palm oil sludge. Proceedings of the Third Turkish-German Environmental Engineering Symposium, Istanbul, Turkey. Chin K. K. and Tan G. T. (1979b) An anaerobic treatment system for palm oil mill effluent. International Conference on Agricultural Engineering in National Development, Universiti Penanian Malaysia, Serdang, Malaysia. Ghosh S. and Klass D. L. (1977) Two-phase anaerobic digestion. Symposium on Clean Fuels from Biomass and Wastes, Orlando, FL. Ghosh S., Conrad J. R. and Klass D. L. (1975) Anaerobic acidogenesis of wastewater sludge. J. Wat. Pollut. Control Fed. 47, 30. Hammer M. S. and Borchardt J. A. (1966) Dialysis separation of sewage sludge digestion. J. san#. Engng Div. Am. Soc. cir. Engrs 95, 907. Lawrence A. W. and McCarty P. L. (1970) Unified basis for biological treatment design and operation. J. sanit. Engng Div. Am. Soc. cir. Engrs 96, 757-775. Massey M. L. and Pohland F. G. (1978) Phase separation of anaerobic stabilization by kinetic controls. J. Wat. Pollut. Control Fed. 50, 2204. Pohland F. G. and Mancy J. H. (1969) The use o f p H and pE measurements during methane biosynthesis. Biotechnol. Bioengng 11, 683. Pohland F. G. and Massey M. L. (1975) An application of process kinetics for phase separation of the anaerobic stabilization process. Prog. Wat. Technol. 7, 173. Schaumberg F. F. and Kirsch E. J. (1966) Anaerobic simulated mixed culture system. J. appl. Microbiol. 14, 761. Southworth A. (1979) Palm oil factory effluent treatment by anaerobic digestion in lagoons. Proc. 35th Ind. Waste Conf. Purdue Unit'. Wong K. K. and Springer A. M. (1981) A first-order kinetic model for designing anaerobic ponds in the treatment of palm oil mill effluent. Agric. Wastes 3, 35-42.
0043-1354 85 $3.00 + 0 . 0 0 Copyright ,_- 1985 Pergamon Press Ltd
TECHNICAL NOTE TWO-PHASE ANAEROBIC T R E A T M E N T KINETICS OF PALM OIL WASTES W. J. NG I'*. K. K. WONG z and K. K. CHtY ~ ~National University of Singapore. Department of Civil Engineering. Singapore 0511 and :Universiti Pertanian Malaysia. Department of Environmental Science. Malaysia ( Receit'ed January 1984) Abstract--A laboratory-scale two-phase anaerobic digestion system was used to treat a palm oil mill effÊuent (POME) containing around 63,000rag 1-~ COD. Phase separation was accomplished through control of the hydraulic retention times of two reactors operated in series. Acid and methane phase biokinetic coefficients were evaluated. Steady state parameters indicate good process stability with high gas yields. Key words--two-phase, anaerobic, palm oil mill effluent, kinetics, hydraulic retention times
INTRODUCTION
merit was added. The characteristics of the palm oil mill effluent are given in Table 1. The reactors were seeded with digested sewage sludge from a domestic sewage treatment plant. Temperature of the reactors was maintained at 32=C. Mixing within each reactor was accomplished by continuously stirring with a propellor. There was no solids recycle. The digestion process was monitored by daily measurements of gas production and composition. The gas was collected by displacing a solution of 5~oH:SO+ and 20% Na,.SOa in a collection chamber. Gas composition was determined using a gas chromatograph. Other digestion parametres measured at regular intervals and according to Standard Methods (APHA, 1980) included alkalinity, total volatile acids, BODs, COD, total solid and suspended solids, pH, total Kjeldahl nitrogen and total phosphorus.
Palm oil mill effluent (POME) is a major agroindustrial waste in the Southeast Asian region. In Malaysia, for instance, the estimated BOD load which was discharged into receiving waters was 330 tonnes day -~ in 1980. This is equivalent to that generated by a population of 7.3 million people. Realising the seriousness of the situation, ways are actively being sought to reduce these organic loads. Recent studies have shown that anaerobic digestion of these wastes could result in significant reductions in BOD and C O D (Southworth, 1979; Chin and Tan, 1979a,b; Wong and Springer, 1981, Chin, 1981). These studies have concentrated on the conventional completely mixed digestion process. The anaerobic process is, however, essentially diphasic and the physiological and nutritional requirements of the acid and methane forming microorganisms are different. Results (Chin, 1981) have indicated that imbalances occurred between the two groups of bacteria at shorter retention times. Consequently, to ensure process stability, design retention times have to be high and this in turn leads to higher capital and operating costs. To reduce these, other process configurations are being investigated and this paper reports the results of a two-phase anaerobic fermentation system using POME.
RESULTS AND DISCUSSION
MATERIALS AND METHODS The two reactors, arranged in series, were made of plexiglass and each had a capacity of 161. Reactor I became the acid-phase digester. Part of its effluent was led into reactor 2+ the methane-phase digester. The digesters were fed on a continuous basis and no inorganic nutrient supple*To whom all correspondence should be addressed. 667 WR
19 ~--[
The P O M E investigated had a high organic content with a relatively large C O D : B O D ratio of 2.4. This indicated the presence of a large portion of difficult to digest organics. The success of the twophase process would depend on the ability of the micro-organisms to degrade these organics. Tables 2 and 3 show the steady state operating parameters for the acid- and methane-phase respectively. The increase in volatile acids in the acid-phase and the subsequent decrease in the methane-phase showed that phase separation was accomplished. Total volatile acid content in the acid-phase increased with increasing retention time. This probably was indicative of the difficult to degrade organics being broken down when the acid-forming microorganisms were given more time to do so. Significant C O D reductions of 54, 61, 67 and 70% were observed following the acid-phase with retention times of 1, 2, 4 and 6 days respectively.
668
Technical Note Table I. Characteristics of the palm oil mill effluent
Parameters pH Alkalinity (rag I - I ) Total volatile acid (rag 1-~ ) B O D 5 (rag 1-~ ) C O D (rag 1-t ) TS (rag I -~ ) TVS (rag 1-~ j TSS (rag t- ~) TVSS (mg l- E) T K N (rag l -t ) TP (rag I-~ )
Range
Average
4.05--4.15 980-1240 1440-1600 25,130-27,210 61,140-64,950 47.380--50.530 38,550-40.100 26,150--27.450 21.740-22,790 990-1050 280--315
4.1 1094 1503 26,222 62,934 48.431 39.339 26,456 22.149 1000 294
Overall COD reductions with the acid-phase operated at 1 day retention time while the methane phase was operated at 10, 20 and 30 days retention times were 78, 81 and 85~o respectively. Applying the following biokinetic model (Lawrence and McCarty, 1970) the biokinetic coefficients for two-phase anaerobic fermentation of POME were evaluated.
XO~ K, 1 (So_ S)=k-~ +~. 1 --
r(s0 - s)
= -
-
Oc 1
-
0F'"
(11 k~
XO, YkS
-
+
(2)
(3)
k,~
(K~ + S0 )
where k = maximum rate of substrate utilization per unit mass of biomass (day -*), Ks = half velocity constant, equal to the substrate concentration at one half the maximum growth rate (mg 1-* ),
Y = theoretical yield coefficient (rag VSS mg -~ BODs), ka= microbial decay coefficient (day -z) S = substrate concentration surrounding the biomass (mg l-t). So = influent substrate concentration (mg 1-~) X = concentration of active biomass (mg l-t), 0c = cell residence time (day), and 0~ i"= minimum cell regeneration time (day). The minimum cell regeneration time of the acid- and methane-phase was 1.31 and 2.48 days respectively for the two reactor system investigated. These are significantly shorter than the 15.3 days determined for mesophilic fermentation of POME in a single reactor mixculture system (Chin, 1981). A direct benefit that can be derived from the shorter retention times achievable in two-phase fermentation is the reduction in tankage requirements. This would cut down the capital investment costs of treatment. Gas yields of 0.98-2.21g-~ BOD5 utilized can be expected from the methane-phase. These values do not include the gas produced and the BOD5 utilized in the acid-phase but apply only to the methanephase. The energy yield from the methane-phase was found to range from 21816 to 44441Jg -~ BOD5 utilized depending on retention time. Although various methods have been proposed to provide for phase separation and these have included dialysis techniques (Hammer and Borchardt, 1966; Schaumberg and Kirsch, 1966) and the addition of inhibitors (Pohland and Mancy, 1969), population selection through manipulation of retention times (Ghosh et al., 1975; Pohland and Massey, 1975;
Table 2. Steady state p a r a m e t e r s - - a c i d - p h a s e Parameter
Acid -phase
Retention time (day) pH Alkalinity ( m g l -~ ) Total volatile acid (rag I -c ) B O D 5 (mg 1-1) C O D (mg 1 - ' ) TSS (ragl -] ) TVSS (mg 1-~ ) T K N (mg 1- i ) TP (rag 1-I) COD/BOD
1 6.0 2105 5458 11,340 29,030 16,980 13,337 740 200 2.56
2 6.0 2055 5906 9050 24,700 16.222 13.050 720 190 2.73
4 6.0 2155 6373 7500 21,006 14,961 12.842 740 180 2.80
6 6.0 2199 6868 7010 19,013 13,330 12,015 710 163 2.71
Table 3. Steady state p a r a m e t e r s - - m e t h a n e - p h a s e Parameter Retention time (day) pH Alkalinity (rag I -~ ) Total volatile acid ( m g l - t ) I~OD 5 (mg I -I) C O D (rag 1-I ) TSS (rag 1-j ) TVSS (mg I - t ) T K N (rag l -I ) TP (rag I - t ) G a s production rate (1 g - t B O D utilized) CH,(%) Energy yield (J g -t B O D utilized) COD/BOD
Acidphase
Methane-phase
1 6 2105 5458 11,340 29,030 16,980 13,337 740 200 --
10 7.2 4150 1215 3158 13,580 12,752 8557 427 90 0.98
20 7.2 4953 1095 1278 12,235 I 1,023 8233 330 75 1.49
----
60 21,8t6 4.30
6[ 33.722 9.57
30 7.2 4700 999 1180 9403 8129 7364 310 50 2.12 56.5 44,441 7.97
669
Technical Note
Waste composition Synthetic carbohydrate waste (1) Acid-phase (2) Methane-phase
Table 4. Two-phase anaerobic fermentation kinetic coefficients Y kj k g,* 0y" (g VSS g-~ ) (day -~ ) (g g-~ VSS-day) (mg I-~ ) (day) 0.15 0.28
---
7.20 0.48
400 4400
Palm oil mill effluent ( 1) Acid-phase 1.04 0.16 5.00 200,000 (2) Methane-phase 1,43 0.034 0.50 I0.666 Single reactor system 0.23 0.036 0.44 227 "Note: high values are due to the very high influent COD concentrations (62,934mg 1-* ). Massey and Pohland, 1978) has s h o w n greater p r o m ise on account of the technique's relative simplicity. Data from this study has indicated that phase separation by m a n i p u l a t i n g retention times is feasible for the anaerobic fermentation of P O M E . The concentration of fatty acids in the raw P O M E averaged at 1503 mg 1- ~ of which a significant portion would be long-chain acids. Nevertheless no inhibition of the acid-phase was observed. Following the acid-phase the average acid c o n c e n t r a t i o n was 5458 mg I- ~. The methane-phase, however, was not noticeably inhibited by this. The biokinetic coefficients evaluated are given in Table 4 a n d c o m p a r i s o n m a d e with those of other studies. Considerable differences were observed. The P O M E c o n t a i n e d a high c o n c e n t r a t i o n o f volatile suspended solids which were not completely degraded. The difficulty in separating the cell mass from influent VSS a n d the diversed nature o f the systems involved account for these differences. CONCLUSIONS The results indicate t h a t phase separation could be achieved by m a n i p u l a t i n g the retention times of the reactors and that P O M E can be effectively treated with a two-phase a n a e r o b i c fermentation system. T r e a t m e n t efficiency reached 85% with the (1 + 30) day system with no cell recycle. The significantly shorter m i n i m u m cell regeneration time of the twophase system should be of interest since reactor volumes can be reduced. REFERENCES
APHA (1980) Standard Methods for the Examination of Water and Wastewater, 15th Edition. American Public
Basis
9.58 × 10-: 1.41
COD COD
1.31 2.48 15.3
BOD BOD BOD
Reference Ghosh and Klass (1977)
This study
Chin (1981)
Health Association. American Water Work Association, Water Pollution Control Federation, Washington, DC. Chin K. K. (1981) Anaerobic treatment kinetics of palm oil sludge. Water Res. 15, 199-202. Chin K. K. and Tan G. T. (1977a) Anaerobic treatment of palm oil sludge. Proceedings of the Third Turkish-German Environmental Engineering Symposium, Istanbul, Turkey. Chin K. K. and Tan G. T. (1979b) An anaerobic treatment system for palm oil mill effluent. International Conference on Agricultural Engineering in National Development, Universiti Penanian Malaysia, Serdang, Malaysia. Ghosh S. and Klass D. L. (1977) Two-phase anaerobic digestion. Symposium on Clean Fuels from Biomass and Wastes, Orlando, FL. Ghosh S., Conrad J. R. and Klass D. L. (1975) Anaerobic acidogenesis of wastewater sludge. J. Wat. Pollut. Control Fed. 47, 30. Hammer M. S. and Borchardt J. A. (1966) Dialysis separation of sewage sludge digestion. J. san#. Engng Div. Am. Soc. cir. Engrs 95, 907. Lawrence A. W. and McCarty P. L. (1970) Unified basis for biological treatment design and operation. J. sanit. Engng Div. Am. Soc. cir. Engrs 96, 757-775. Massey M. L. and Pohland F. G. (1978) Phase separation of anaerobic stabilization by kinetic controls. J. Wat. Pollut. Control Fed. 50, 2204. Pohland F. G. and Mancy J. H. (1969) The use o f p H and pE measurements during methane biosynthesis. Biotechnol. Bioengng 11, 683. Pohland F. G. and Massey M. L. (1975) An application of process kinetics for phase separation of the anaerobic stabilization process. Prog. Wat. Technol. 7, 173. Schaumberg F. F. and Kirsch E. J. (1966) Anaerobic simulated mixed culture system. J. appl. Microbiol. 14, 761. Southworth A. (1979) Palm oil factory effluent treatment by anaerobic digestion in lagoons. Proc. 35th Ind. Waste Conf. Purdue Unit'. Wong K. K. and Springer A. M. (1981) A first-order kinetic model for designing anaerobic ponds in the treatment of palm oil mill effluent. Agric. Wastes 3, 35-42.