World Journal of Microbiology and Biotechnology 6, 149-154
Anaerobic digestion of cassava starch factory effluent
V.B. Manilal, C.S. Narayanan and C. Balagopalan
Biomethanaflon of cassava starch factory effluent In a batch digester produced 130 I blogas/kg dry matter with an average methane content of 59%. About 63% CO0 was removed during 50 days. In semicontinuous digesters, gas production was 325 I/kg dry matter with a retention time of 33.3 days giving a COD reduction of 50%. Size of starter inoculum was important for good biogasification of the effluent. La blom6thanisatlon d'un effluent de fabrique d'amidon de manioc, en dlgesteur en milieu non-renouvel(~, a produit 1301 de bJogaz par kg de mati~re seche, avec une teneur moyenne en m(~thane de 50%. Environ 63% de la DCO a 6t(~ 61imin6e en 60 jours. En digesteur conduit en mode semi-continu, la production de gaz a atteint 325 }itres par kg de mati~re s(~che avec un temps moyen de r~tention de 33,3 jours. La r~ductlon de DCO 6tait de 50%. La faille de I'inoculum joue un r61e important dans la bonne biom6thanisation de I'effluent.
V.B. Manilal and C.S. Narayanan are with the Regional Research Laboratory, Trivandrum-695019, India; C. Balagopalan is with the Central Tuber Crops Research Institute (ICAR), Trivandrum-695017, India. V.B, Manilal is the corresponding author.
(~ 1990 Rapid Communications of Oxford Ltd.
A large number of starch factories based on cassava (Manihot esculenta Cranz) tubers are located in the cassava-growing tracts of tropical countries. The process of starch extraction requires voluminous amounts of water. This water is subsequently removed and discharged forming an effluent with high amounts of carbonaceous matter which pose a serious threat to the environment and quality of life. Anaerobic digestion is of particular importance in the effluent treatment system because of its potential in the stabilization of waste (Scammel 1975; Hakulinen & Mirja 1982; Braun & Huss 1982; Tesch et al. 1983). Coordinated metabolism by different bacterial groups efficiently decompose the hydrolysable carbohydrates into methane and CO z. Aerobic-anaerobic treatment of waste waters from maize and potato starch factories has been attempted by Frostell (1983). In the present study, the possibilities of utilizing cassava starch factory effluent for biomethanation was investigated in batch and semi-continuous digesters. During biogasification the composition of biogas, amount of gas output and the extent of reduction of pollution load were assessed.
Materials and Methods Feedstock Preparation A sample of effluent containing 6.2% total solids (TS) was collected from Lakshmi Starch Factory, Kundra, Kerala, India. The sample was amended with inorganic nitrogen source (urea) to give a C : N ratio approximately 30:1 (w/w). The final pH of the sample was adjusted to 7.0 using NaOH.
Digestion Apparatus Each single-stage digester was constructed in the laboratory. The mouth of a 7 1 bottle was tightly closed with a rubber stopper carrying a bent glass tube which served as the gas outlet. Single-stage, semi-continuous digesters were designed with 2 1 Erlenmeyer flasks. Each digester had three openings--two at the top and one at the bottom. One of the two openings at the top functioned as an inlet for the addition of fresh feedstock while the other acted as a gas outlet. The bottom opening was used as an outlet for the periodic removal of digested material.
149
V.B. Manilal, C.S. Narayanan and C. Balagopalan Total Nitrogen and Organic Carbon Analysis Total nitrogen in the sample was estimated by the micro-Kieldahl method. The organic carbon content of the feeding material was determined by oxidizing it to CO2 in the presence of 0 2 in a catalytic combustion tube at 900~ The CO 2 generated during the combustion was estimated by absorbing it in KOH.
Estimation of Chemical Oxygen Demand and Total Solids Chemical oxygen demand (COD) of the samples was determined by following the dichromate method described in AOAC (1975). Total solids in the samples were dried at 103~ according to the recommendations of American Public Health Association (APHA 1975).
Biogas Analysis The biogas was analysed by a gas chromatograph equipped with a thermal conductivity detector. Chromatographic separation of the components in the biogas was accomplished by gas chromatography using a Carbosieve column. Confirmation of peak identities was performed by combined gas chromatography/mass spectrometry.
Experimental Procedure Different quantities of cow dung were mixed with urea-amended and unamended feedstock to adjust the feedstock and cow dung ratio to 95:5, 90:10 and 80:20 (w/w). The materials were thoroughly mixed and 4 kg of the feedstock was introduced into each digester. Digesters were then sealed and incubated at 30~ The biogas generated was collected by the downward displacement of acidified water in measuring cylinders. One litre of digested sludge from the batch digester was used as a source of starter culture in the semi-continuous digester. Digestion of the sludge was further continued till the gas production ceased completely. The digested sludge taken in the semi-continuous digesters was thoroughly mixed with 20 g ( = 1300 mg COD), 40 g, 100 g, 120 g or 140 g feedstock at intervals of 96 or 144 h. Samples in the digesters were gently mixed four times daily for 5 rain using a magnetic stirrer. All experiments were done in triplicate at 30~ The biogas produced from the digesters was measured as in batch experiments.
Results Biomethanation in Batch Digesters Nature and composition of biogas. Mass spectral analysis of biogas obtained from anaerobic digesters showed the presence of methane and CO2 as major constituents with a small amount of butane being present. Influence of cow dung and urea in the production and composition of biogas. The period of gas production from the effluent was enhanced by increasing the quantity of cow dung (Figures 1 and 2). Since the effluent possessed negligible amounts of organic nitrogen, urea had to be added but this did not increase the period of gas production, except in samples containing 5% cow dung. Total gas production in both urea-amended and unamended samples increased with the quantity of cow dung being added (Table 1). The output of hiogas was sharply increased from 20 1/kg dry matter to 66 1/kg dry matter when cow dung was increased from 5% to 20% (w/w). Except for samples containing 5% cow dung, the total biogas production was higher in the unamended effluents compared with the ureaamended samples. The average methane content of the biogas generated from urea-amended samples was slightly higher than that from the unamended (Table 1). Methane
180
Biogas from cassava waste 1.0
1.0
~- 0.8
0.8
E
E b
~ 0.6
g
0.6
c o
c ..~
o
~ 0.4
0.2 ~
~ 0.2
0
0.4
10
20
30 40 (days)
50
60
70
0
1
10
Time of Incubation Figure 1. Effect of cow dung in the batch digestion of cassava starch factory effluent. 0 - - 5 % cow dung; D - - 1 0 % cow dung; ~ - - 2 0 % cow dung.
~
20
30 40 50 (days) Time of Incubation
60
70
Figure 2. Effect of cow dung in the batch digestion of cassava starch factory effluent (urea-amended). 0 - - 5 % cow dung; [ ] - - 1 0 % cow dung; / k - - 2 0 % cow dung.
Table 1. Influence of cow dung and urea on the blomethanation and reduction of pollution load of cassava starch factory effluent In batch digesters at 30~ Feedstocks
CSFE CSFE CSFE CSFE CSFE CSFE
+ + + + + +
5% CD 10% CD 20% CD 5% CD + urea 10% CD + urea 20% CD + urea
Total gas produced (I/kg dry matter)
Methane content (%)
TS reduction (%)
COO reducUon (%)
8 31 130 20 30 66
20 55 59 42 58 62
7 39 43 12 39 41
6 11 63 6 10 22
CSFE--Cassava starch factory effluent; CD---cow dung; TS--total solids; COD~chemical oxygen demand.
content of the biogas obtained from the urea-amended sample with 5% cow dung was at 42%, more than double the methane content of biogas collected from the unamended sample. About 3% increase in the methane content was observed in both the urea-amended samples containing 10% and 20% cow dung compared with the unamended. Control experiments were run with cow dung alone and all the values obtained on cow dung mixed effluents were corrected against the control. Reduction of total solids and chemical oxygen demand. The pollution load (total solids and COD) of amended and unamended effluent samples containing different proportions of cow dung was decreased during biomethanation (Table 1). The reduction of total solids and COD was increased with the increase in cow dung content. Urea amendment decreased the removal of COD in the samples contain ing 20% cow dung (22% as against 63% in unamended). However, in the samples
151
V.B. Manila], C.S. Narayanan and C. Balagopalan having 5% cow dung, the total solids reduction improved with the addition of urea.
Biomethanation in Semi-continuous Digesters In semi-continuous anaerobic digesters, cassava starch factory effluent was fermented to achieve maximum output of biogas and reduction of total solids with minimum retention time under ambient conditions. As the earlier batch digestion experiments showed, the rate of methane generation was directly proportional to the total biogas output, only the quantitative estimation of the biogas was followed in the semi-continuous digestion system. Fresh samples were added periodically which displaced the same amount of digested material to maintain the digester volume constant. Different feeding rates were tried and the results are given in Table 2. The maximum amount of biogas was obtained when the digester was fed with 7800 mg COD/1 of reactor volume.
Discussion Biogas from the agricultural wastes usually contains 60 to 70% (v/v) methane and 30 to 40% (v/v) CO2 (Cowley & Wase 1981) but yields of methane may be lower or higher with different feedstocks and technologies (Colleran et aL 1982; Silverio et aL 1986). The present study of biomethanation of cassava starch factory effluent resulted in the generation of biogas containing 59% (v/v) methane. There was an enhanced generation of biogas (Table 1) with a quantitative increase in cow dung which was mainly used as a source of inoculum bacteria.
Table 2. Blomethanatlon of cassava starch factory effluent in semi-continuous digesters at 30~ Feeding rate (rag CODltlme)
Incubation time (h)
Gas output (III of reactor volume)
pH
24 48 72
0.3 0.5 0.7
8.7 8.6 8.6
28 36 48
96 120 144
0.7 0.8 0.8
8.6 8.6 8.6
56 57 58
2600196 h
24 48 72 96
0.4 0.7 1.1 1.3
8.5 8.4 8.4 8.4
30 38 49 56
65001144 h
24 48 72 96 120 144
1.0 2.0 2.5 2.9 3.0 3.0
7.7 8.2 8.2 8.3 8.4 8.3
15 24 37 48 51 54
7800196 h
24 48 72 96
1.1 2.2 2.7 3.2
6.8 7.1 7.2 7.6
13 22 35 46
9100/96 h
24 48 72 96
0.9 1.6 2.2 2.6
5.9 5.8 6.0 6.2
12 16 33 43
1300/144 h
TS---total solids.
152
TS reductlon (%)
Biogas from cassava waste The process of biomethanation is optimal with a C : N ratio of around 30:1 (Cowley & Wase 1981; Ghose & D a s 1982; Wellinger 1985). Wastes containing high levels of carbohydrates usually promote the growth of acid-forming bacteria whose activity inhibits the methane-producing bacteria (Hansson 1982). Cassava starch factory effluent, being rich in carbohydrates with a negligible amount of proteins, may have promoted the growth of acid formers and not the methanogens, as evidenced by the low level of methane production in the digesters with only 5% cow dung (Table 1) or without cow dung at all (data not shown). Because of the nitrogen content of cow dung (C:N ratio 20:1), the mixing of higher proportions of cow dung may have increased the nitrogen content of the materials in digesters to support the growth of the methanogens. Although the production of methane was poor in those effluent samples mixed with 5% and 10% cow dung, significant amounts of solids were degraded during incubation. This shows that the carbohydrates were degraded by the acid-formers. However, a significant reduction of pollution load was achieved only in the digesters which were efficient in biomethanation. Although increased gas production was reported with higher loading rates of sewage sludge and other organics, biogas generation diminishes at higher loading rates than optimum (Loehr 1974). Results obtained on the semi-continuous digestion of cassava starch factory effluents indicated an optimum loading rate of 7800 mg COD/1 over 96 h (having a total retention time of 33.3 days) for maximum gas production and the maintenance of neutral pH under tested conditions. Data on total solids reduction suggests that the rate of solids reduction was extremely slow beyond 33.3 days of retention time (46%, 47.5% and 58% in 33.3, 40 and 300 days, respectively). About 50% of the total solids in the effluent was estimated as starch (Manilal et al. 1983) and the remaining 50% consisted of celluloses and hemicelluloses. During 33.3 days of retention time most of the easily degradable polysaccharides may have degraded and converted into biogas.
Acknowledgement We thank the Council of Scientific and Industrial Research, New Delhi, India for providing financial assistance.
References AOAC (AssOCIATIONOF OFFICIAL ANALYTICALCHEMISTS) I975 Official Methods of Analysis, 12th edn., ed. Hortwitz, W. pp. 608-611. Washington, DC: AOAC. APHA (AMERICANPUBLICHEALTHASSOCIATION)1975 Standard Methods for the Examination of Water and Waste Waler, 14th edn, pp. 9.,=98. Washington, DC: APHA. BRAUN, R. & HUSS, S. 1982 Anaerobic digestion of distillery effluents. Process Biochemistry 17, 25-27. COLLERAN, E., BARRY, M., WILKIE, A. &: NEWELL, P.J. 1982 Anaerobic digestion of agricultural waste using the upflow anaerobic filter design. ProcessBiochemislry 17~ 12-17. COWLEY, D. & WASE, D.A.J. 1981 Anaerobic digestion of farm wastes; a review- Part 1. Process Biochemistry16, 28-33. FROSTELL, B. 1983 Anaerobic~aerobic biological treatment of starch industry waste waters. Starch/Stiirke 35, 185-189. GHOSE, T.K. & DAS, D. 1982 Maximization of energy recovery in the biomethanation process--Part 2. Use of mixed residue in batch system. Process Biochemistry 17, 39-42. HAKULINEN,R. & 8AKINOJA-SALONEN,M. 1982 Treatment of pulp and paper industry waste water in an anaerobic fluidized bed reactor. Process Biochemistry 17, 18-22. HANSSON, G. 1982 End product inhibition in methane fermentations. Process Biochemistry 17, 45-49. LOEHR, R.C. 1974 Agricultural Waste Management. London: Academic Press. MANILAL, V.B., NARAYANAN,C.S. & BALAGOPALAN,C. 1983 Physico-chemical and microbiological characteristics of cassava starch factory effluents. Journal of Root Crops 9, 27-31. SCAMMELL, O.W. 1975 Anaerobic treatment of industrial wastes.Process Biochemistry 10; 34~36.
153
V.B. Manilal, C.S. Narayanan and C. Balagopalan SILVERIO, C.M., ANGLO,P.G., MONTERO,G.V., PACHECO,MA.V., ALAMIS,MA.L. & LuIs, V.S. JR 1986 Anaerobic treatment of distillery slops using an upfiow anaerobic filter reactor. ProcessBiochemistry21, 19~195. TESCH, W., SCHNEIDER, K. & BACHOFEN, 1983 Performance of an anaerobic filter for purification and methane production from waste water of a sugar refinery. Process Biochemistry18, 34-37. WELLINGER,A. 1985 Process parameters affecting methane production in mesophilic farm digesters. ProcessBiochemistry20, 131-137.
(Received as revised 9 January 1990; accepted 7 February 1990)
154
Anaerobic digestion of cassava starch factory effluent
V.B. Manilal, C.S. Narayanan and C. Balagopalan
Biomethanaflon of cassava starch factory effluent In a batch digester produced 130 I blogas/kg dry matter with an average methane content of 59%. About 63% CO0 was removed during 50 days. In semicontinuous digesters, gas production was 325 I/kg dry matter with a retention time of 33.3 days giving a COD reduction of 50%. Size of starter inoculum was important for good biogasification of the effluent. La blom6thanisatlon d'un effluent de fabrique d'amidon de manioc, en dlgesteur en milieu non-renouvel(~, a produit 1301 de bJogaz par kg de mati~re seche, avec une teneur moyenne en m(~thane de 50%. Environ 63% de la DCO a 6t(~ 61imin6e en 60 jours. En digesteur conduit en mode semi-continu, la production de gaz a atteint 325 }itres par kg de mati~re s(~che avec un temps moyen de r~tention de 33,3 jours. La r~ductlon de DCO 6tait de 50%. La faille de I'inoculum joue un r61e important dans la bonne biom6thanisation de I'effluent.
V.B. Manilal and C.S. Narayanan are with the Regional Research Laboratory, Trivandrum-695019, India; C. Balagopalan is with the Central Tuber Crops Research Institute (ICAR), Trivandrum-695017, India. V.B, Manilal is the corresponding author.
(~ 1990 Rapid Communications of Oxford Ltd.
A large number of starch factories based on cassava (Manihot esculenta Cranz) tubers are located in the cassava-growing tracts of tropical countries. The process of starch extraction requires voluminous amounts of water. This water is subsequently removed and discharged forming an effluent with high amounts of carbonaceous matter which pose a serious threat to the environment and quality of life. Anaerobic digestion is of particular importance in the effluent treatment system because of its potential in the stabilization of waste (Scammel 1975; Hakulinen & Mirja 1982; Braun & Huss 1982; Tesch et al. 1983). Coordinated metabolism by different bacterial groups efficiently decompose the hydrolysable carbohydrates into methane and CO z. Aerobic-anaerobic treatment of waste waters from maize and potato starch factories has been attempted by Frostell (1983). In the present study, the possibilities of utilizing cassava starch factory effluent for biomethanation was investigated in batch and semi-continuous digesters. During biogasification the composition of biogas, amount of gas output and the extent of reduction of pollution load were assessed.
Materials and Methods Feedstock Preparation A sample of effluent containing 6.2% total solids (TS) was collected from Lakshmi Starch Factory, Kundra, Kerala, India. The sample was amended with inorganic nitrogen source (urea) to give a C : N ratio approximately 30:1 (w/w). The final pH of the sample was adjusted to 7.0 using NaOH.
Digestion Apparatus Each single-stage digester was constructed in the laboratory. The mouth of a 7 1 bottle was tightly closed with a rubber stopper carrying a bent glass tube which served as the gas outlet. Single-stage, semi-continuous digesters were designed with 2 1 Erlenmeyer flasks. Each digester had three openings--two at the top and one at the bottom. One of the two openings at the top functioned as an inlet for the addition of fresh feedstock while the other acted as a gas outlet. The bottom opening was used as an outlet for the periodic removal of digested material.
149
V.B. Manilal, C.S. Narayanan and C. Balagopalan Total Nitrogen and Organic Carbon Analysis Total nitrogen in the sample was estimated by the micro-Kieldahl method. The organic carbon content of the feeding material was determined by oxidizing it to CO2 in the presence of 0 2 in a catalytic combustion tube at 900~ The CO 2 generated during the combustion was estimated by absorbing it in KOH.
Estimation of Chemical Oxygen Demand and Total Solids Chemical oxygen demand (COD) of the samples was determined by following the dichromate method described in AOAC (1975). Total solids in the samples were dried at 103~ according to the recommendations of American Public Health Association (APHA 1975).
Biogas Analysis The biogas was analysed by a gas chromatograph equipped with a thermal conductivity detector. Chromatographic separation of the components in the biogas was accomplished by gas chromatography using a Carbosieve column. Confirmation of peak identities was performed by combined gas chromatography/mass spectrometry.
Experimental Procedure Different quantities of cow dung were mixed with urea-amended and unamended feedstock to adjust the feedstock and cow dung ratio to 95:5, 90:10 and 80:20 (w/w). The materials were thoroughly mixed and 4 kg of the feedstock was introduced into each digester. Digesters were then sealed and incubated at 30~ The biogas generated was collected by the downward displacement of acidified water in measuring cylinders. One litre of digested sludge from the batch digester was used as a source of starter culture in the semi-continuous digester. Digestion of the sludge was further continued till the gas production ceased completely. The digested sludge taken in the semi-continuous digesters was thoroughly mixed with 20 g ( = 1300 mg COD), 40 g, 100 g, 120 g or 140 g feedstock at intervals of 96 or 144 h. Samples in the digesters were gently mixed four times daily for 5 rain using a magnetic stirrer. All experiments were done in triplicate at 30~ The biogas produced from the digesters was measured as in batch experiments.
Results Biomethanation in Batch Digesters Nature and composition of biogas. Mass spectral analysis of biogas obtained from anaerobic digesters showed the presence of methane and CO2 as major constituents with a small amount of butane being present. Influence of cow dung and urea in the production and composition of biogas. The period of gas production from the effluent was enhanced by increasing the quantity of cow dung (Figures 1 and 2). Since the effluent possessed negligible amounts of organic nitrogen, urea had to be added but this did not increase the period of gas production, except in samples containing 5% cow dung. Total gas production in both urea-amended and unamended samples increased with the quantity of cow dung being added (Table 1). The output of hiogas was sharply increased from 20 1/kg dry matter to 66 1/kg dry matter when cow dung was increased from 5% to 20% (w/w). Except for samples containing 5% cow dung, the total biogas production was higher in the unamended effluents compared with the ureaamended samples. The average methane content of the biogas generated from urea-amended samples was slightly higher than that from the unamended (Table 1). Methane
180
Biogas from cassava waste 1.0
1.0
~- 0.8
0.8
E
E b
~ 0.6
g
0.6
c o
c ..~
o
~ 0.4
0.2 ~
~ 0.2
0
0.4
10
20
30 40 (days)
50
60
70
0
1
10
Time of Incubation Figure 1. Effect of cow dung in the batch digestion of cassava starch factory effluent. 0 - - 5 % cow dung; D - - 1 0 % cow dung; ~ - - 2 0 % cow dung.
~
20
30 40 50 (days) Time of Incubation
60
70
Figure 2. Effect of cow dung in the batch digestion of cassava starch factory effluent (urea-amended). 0 - - 5 % cow dung; [ ] - - 1 0 % cow dung; / k - - 2 0 % cow dung.
Table 1. Influence of cow dung and urea on the blomethanation and reduction of pollution load of cassava starch factory effluent In batch digesters at 30~ Feedstocks
CSFE CSFE CSFE CSFE CSFE CSFE
+ + + + + +
5% CD 10% CD 20% CD 5% CD + urea 10% CD + urea 20% CD + urea
Total gas produced (I/kg dry matter)
Methane content (%)
TS reduction (%)
COO reducUon (%)
8 31 130 20 30 66
20 55 59 42 58 62
7 39 43 12 39 41
6 11 63 6 10 22
CSFE--Cassava starch factory effluent; CD---cow dung; TS--total solids; COD~chemical oxygen demand.
content of the biogas obtained from the urea-amended sample with 5% cow dung was at 42%, more than double the methane content of biogas collected from the unamended sample. About 3% increase in the methane content was observed in both the urea-amended samples containing 10% and 20% cow dung compared with the unamended. Control experiments were run with cow dung alone and all the values obtained on cow dung mixed effluents were corrected against the control. Reduction of total solids and chemical oxygen demand. The pollution load (total solids and COD) of amended and unamended effluent samples containing different proportions of cow dung was decreased during biomethanation (Table 1). The reduction of total solids and COD was increased with the increase in cow dung content. Urea amendment decreased the removal of COD in the samples contain ing 20% cow dung (22% as against 63% in unamended). However, in the samples
151
V.B. Manila], C.S. Narayanan and C. Balagopalan having 5% cow dung, the total solids reduction improved with the addition of urea.
Biomethanation in Semi-continuous Digesters In semi-continuous anaerobic digesters, cassava starch factory effluent was fermented to achieve maximum output of biogas and reduction of total solids with minimum retention time under ambient conditions. As the earlier batch digestion experiments showed, the rate of methane generation was directly proportional to the total biogas output, only the quantitative estimation of the biogas was followed in the semi-continuous digestion system. Fresh samples were added periodically which displaced the same amount of digested material to maintain the digester volume constant. Different feeding rates were tried and the results are given in Table 2. The maximum amount of biogas was obtained when the digester was fed with 7800 mg COD/1 of reactor volume.
Discussion Biogas from the agricultural wastes usually contains 60 to 70% (v/v) methane and 30 to 40% (v/v) CO2 (Cowley & Wase 1981) but yields of methane may be lower or higher with different feedstocks and technologies (Colleran et aL 1982; Silverio et aL 1986). The present study of biomethanation of cassava starch factory effluent resulted in the generation of biogas containing 59% (v/v) methane. There was an enhanced generation of biogas (Table 1) with a quantitative increase in cow dung which was mainly used as a source of inoculum bacteria.
Table 2. Blomethanatlon of cassava starch factory effluent in semi-continuous digesters at 30~ Feeding rate (rag CODltlme)
Incubation time (h)
Gas output (III of reactor volume)
pH
24 48 72
0.3 0.5 0.7
8.7 8.6 8.6
28 36 48
96 120 144
0.7 0.8 0.8
8.6 8.6 8.6
56 57 58
2600196 h
24 48 72 96
0.4 0.7 1.1 1.3
8.5 8.4 8.4 8.4
30 38 49 56
65001144 h
24 48 72 96 120 144
1.0 2.0 2.5 2.9 3.0 3.0
7.7 8.2 8.2 8.3 8.4 8.3
15 24 37 48 51 54
7800196 h
24 48 72 96
1.1 2.2 2.7 3.2
6.8 7.1 7.2 7.6
13 22 35 46
9100/96 h
24 48 72 96
0.9 1.6 2.2 2.6
5.9 5.8 6.0 6.2
12 16 33 43
1300/144 h
TS---total solids.
152
TS reductlon (%)
Biogas from cassava waste The process of biomethanation is optimal with a C : N ratio of around 30:1 (Cowley & Wase 1981; Ghose & D a s 1982; Wellinger 1985). Wastes containing high levels of carbohydrates usually promote the growth of acid-forming bacteria whose activity inhibits the methane-producing bacteria (Hansson 1982). Cassava starch factory effluent, being rich in carbohydrates with a negligible amount of proteins, may have promoted the growth of acid formers and not the methanogens, as evidenced by the low level of methane production in the digesters with only 5% cow dung (Table 1) or without cow dung at all (data not shown). Because of the nitrogen content of cow dung (C:N ratio 20:1), the mixing of higher proportions of cow dung may have increased the nitrogen content of the materials in digesters to support the growth of the methanogens. Although the production of methane was poor in those effluent samples mixed with 5% and 10% cow dung, significant amounts of solids were degraded during incubation. This shows that the carbohydrates were degraded by the acid-formers. However, a significant reduction of pollution load was achieved only in the digesters which were efficient in biomethanation. Although increased gas production was reported with higher loading rates of sewage sludge and other organics, biogas generation diminishes at higher loading rates than optimum (Loehr 1974). Results obtained on the semi-continuous digestion of cassava starch factory effluents indicated an optimum loading rate of 7800 mg COD/1 over 96 h (having a total retention time of 33.3 days) for maximum gas production and the maintenance of neutral pH under tested conditions. Data on total solids reduction suggests that the rate of solids reduction was extremely slow beyond 33.3 days of retention time (46%, 47.5% and 58% in 33.3, 40 and 300 days, respectively). About 50% of the total solids in the effluent was estimated as starch (Manilal et al. 1983) and the remaining 50% consisted of celluloses and hemicelluloses. During 33.3 days of retention time most of the easily degradable polysaccharides may have degraded and converted into biogas.
Acknowledgement We thank the Council of Scientific and Industrial Research, New Delhi, India for providing financial assistance.
References AOAC (AssOCIATIONOF OFFICIAL ANALYTICALCHEMISTS) I975 Official Methods of Analysis, 12th edn., ed. Hortwitz, W. pp. 608-611. Washington, DC: AOAC. APHA (AMERICANPUBLICHEALTHASSOCIATION)1975 Standard Methods for the Examination of Water and Waste Waler, 14th edn, pp. 9.,=98. Washington, DC: APHA. BRAUN, R. & HUSS, S. 1982 Anaerobic digestion of distillery effluents. Process Biochemistry 17, 25-27. COLLERAN, E., BARRY, M., WILKIE, A. &: NEWELL, P.J. 1982 Anaerobic digestion of agricultural waste using the upflow anaerobic filter design. ProcessBiochemislry 17~ 12-17. COWLEY, D. & WASE, D.A.J. 1981 Anaerobic digestion of farm wastes; a review- Part 1. Process Biochemistry16, 28-33. FROSTELL, B. 1983 Anaerobic~aerobic biological treatment of starch industry waste waters. Starch/Stiirke 35, 185-189. GHOSE, T.K. & DAS, D. 1982 Maximization of energy recovery in the biomethanation process--Part 2. Use of mixed residue in batch system. Process Biochemistry 17, 39-42. HAKULINEN,R. & 8AKINOJA-SALONEN,M. 1982 Treatment of pulp and paper industry waste water in an anaerobic fluidized bed reactor. Process Biochemistry 17, 18-22. HANSSON, G. 1982 End product inhibition in methane fermentations. Process Biochemistry 17, 45-49. LOEHR, R.C. 1974 Agricultural Waste Management. London: Academic Press. MANILAL, V.B., NARAYANAN,C.S. & BALAGOPALAN,C. 1983 Physico-chemical and microbiological characteristics of cassava starch factory effluents. Journal of Root Crops 9, 27-31. SCAMMELL, O.W. 1975 Anaerobic treatment of industrial wastes.Process Biochemistry 10; 34~36.
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(Received as revised 9 January 1990; accepted 7 February 1990)
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