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Ensilage of mangopeel for methane generation
Process
Wochemistry
28 (1993)
Ensilage
119-123
of Mangopeel
K. Madhukara,
Krishna
for Methane
Generation
Nand, N.R. Raju & H.R. Srilatha
Discipline of Microbiology and Sanitation, Central Food Technological Research Institute,
Mysore---
013, India
(Received 6 December 1991; revised manuscript received and accepted 19 May 1992)
Mangopeel is a waste product of mango processing which is available in large quantities in India during the harvest season, when disposal causes pollution problems. Ensilage of mangopeel for 6 months, the eflects of ensilage on its physico-chemical characteristics and methane generation by an anaerobic digestion process were studied. Ensilage helped in the pre-treatment of polymeric constituents and the conversion of major components of carbohydrates into volatile fatty acids. The ensilage mangopeel was a useful substrate for anaerobic digestion for methane generation, and the biogas yield was as high as 068 m3/kg volatile solids (VS) added, with a methane content of 52%. This compared favourably with 043 m3/kg VS added, and a methane content of 50%, for dried mangopeel. The ensilage of feedstock could therefore eliminate pre-processing such as milling and pulverisation. vegetable processing industries are seasonal, and as they decompose rapidly they are available for only a limited period of 2-3 months. The mechanical drying or sun-drying of these wastes (mangopeel, citruspeel, pineapple and tomato processing wastes), and their storage and further milling to produce feedstock add enormously to the cost of biogas production and make the entire process uneconomical. A simple and inexpensive method for preserving these wastes is needed, so that these resources may be used for energy generation throughout the year. The continuous use of an anaerobic digester and associated equipment makes this process highly economical for the production of biogas. Ensilage of forage crops is widely practised all over the world in modern agriculture but there are few reports in the literature on the preservation of fruit and vegetable processing wastes. Bryan and Parrish’ used ensilaged sorghum for ethanol pro-
INTRODUCTION Of all the alternative energy sources studied in recent years, biogas production appears to be the most favoured, especially when the feedstock is ago-industrial and food processing wastes. In addition to energy generation, it reduces pollution, and both liquid and solid residues may be utilised for aquaculture, and animal and poultry feeds. Anaerobic digestion of wastes from process plants is the only simple method which can be readily operated at farm, domestic and community levels for fuel production. The annual availability of these wastes amounts to 1005 million tonnes.’ A major part of this is mostly discarded and thus becomes a source of pollution. On occasions, disposal becomes expensive because of the high cost of transportation. The majority of wastes from the fruit and Corresponding Telex: X46-241
author: Dr K. Nand. Telephone FTRI IN.
ProcessBmchemistr_y
: 821-23539; 119
0032-9592/93/$6.00
0
1993 Else&r
Science
Publishers
Ltd,
England.
K. Madhukara
120
duction, and Barry and Colleran3 used stored sorghum effluent from tower silos and reported a methane yield of O-49 m3/kg chemical oxygen demand (COD) removed. Coble and Egg4 reported the performance of ensilaged sorghum in a twostage anaerobic digestion system, with the aim of developing an economically viable system. An ensilage process for the storage of mangopeel has been developed and the results of the preliminary studies are reported in this paper.
MATERIALS
AND METHODS
Mangopeel (1000 kg) collected from M/s. Fruitin Exports Ltd, Mysore, was stocked in a reinforced concrete silo, of 1-Om diameter and 1.25 m height, and compacted by applying pressure to remove air. The top portion was covered with a thick plastic sheet to protect the material from water. The ensilaged mangopeel was stored for 1 year and used periodically for chemical analysis and biogas generation. Mangopeel ensilaged for 6 months was used throughout the present study unless otherwise stated. Mangopeel was also sun-dried and shredded to a size of less than 5 mm using a hammer mill before feeding to the digester.
LABORATORY
DIGESTERS
Experiments were conducted in 6.5-litre glass fermentation vessels with an inlet for feeding and an outlet for biogas and effluent. The working volume of the digesters was 4 litres. They were wrapped in black plastic sheets to avoid algal growth. The digesters were initially charged with cowdung slurry diluted with water in the proportion of 4 : 5 together with a 10% inoculum obtained from biogas digesters that operated on mangopeel, cowdung, rabbit pellets and animal-house wastes, and were incubated undisturbed for 15 days for adaptation to anaerobic digestion. Digesters were grouped into two sets, for dry and ensilaged mangopeel separately, and a method involving a stepped decrease in cowdung and increase in mangopeel, both dry and ensilaged separately, in the proportion of 75 : 25, 50: 50,25 : 75 and 00: 100 was followed every week. The feed was prepared by blending the wastes to produce a homogeneous slurry, and was fed at the rate of 40 kg total solids (TS)/m3 day with a hydraulic retention time (HRT) of 30 days, supplemented with 2% urea by weight of total solids.
et al.
These digesters were operated in a semi-continuous mode of operation by loading once every 24 h.
CHEMICAL
ANALYSIS
The analysis of samples were carried out according to the methods of the American Public Health Association (APHA).” The volatile fatty acids (VFA), total solids (TS), volatile solids (VS) and pH of the effluent were analysed weekly. The volume of gas produced was recorded daily by water displacement, and its composition was analysed on a CIC (India) gas chromatograph equipped with a thermal conductivity detector, using a Poropak Q column, with an oven temperature of 60 “C.
RESULTS
AND DISCUSSIONS
The composition of fresh, dried and ensilaged mangopeel is listed in Table 1. Total carbohydrates, including cellulose, hemicellulose, and reducing and non-reducing sugars, of fresh and dry mangopeel approached 70 %. The ensilaged mangopeel contained only 25 % carbohydrate but the VFA content rose to the high level of 54 %. Higher percentages of lactic, butyric and propionic acid have normally been observed on ensilage of crop materials rich in water-soluble compounds.4.6 The ensilage of mangopeel for 6 months resulted in the conversion of 64 % of cellulose, hemicellulose and sugars into VFA. Although pectin, fat and proteins were partially hydrolysed, the lignin was not degraded. This increase in VFA was followed by a subsequent decrease in other constituents, but more detailed Table
1. Composition
Parameters
of Mangopeel EnsiIaged
Fresh
Dry
7447
Moisture (%) Total solids (%) Volatile solids (%) PH Ash (%)
25.53 85.86 4.00 14.14
70.07 29-93 9646 4.80 3-54
2644 73.56 96.24 4.80 3.45
% on dry weight basis Cellulose Hemicellulose Pectin Ether-soluble solids Protein Reducing sugar Non-reducing sugar Lignin
12-00 4.5 1 4.00 5.00 0.90 7.00 1.50 1 l-00
14-20 8.00 6-80 6.40 3.53 40.80 6.90 13.82
13.00 7.33 7-00 7.97 3.50 41-80 5.90 13.00
Ensilage qf mangopeel for methane generation
12345676 TIME
(weeks)
Fig. 1. Profiles of biogas production, methane content and pH during the start-up of anaerobic digestion of dry and ensildged mangopeel. 0, Ensilaged mangopeel; X : dry mangopeel.
cr
00 LOADING
I
60
00 RATE
(kg
I
100
1
120
TSlmS/day)
Fig. 2. Influence of loading rate on biogas generation effluent characteristics. 0, Ensilaged mangopeel; X, mangopeel.
and dry
investigations are needed on the biochemical changes and the types of microorganisms involved in the conversion of polymers to the various breakdown products. The pH of the mangopeel decreased rapidly from its initial value of pH 4.8 over the first 14 days. This was followed by a more gradual decrease, and after 3 months it became stabilised at pH 4-O. A similar trend during the ensilage of forage crops has been reported by a number of workers,4’7 although the pH declined to 3.9. The results of start-up experiments on the production of biogas, methane and pH changes are
121
given in Fig. 1. The pattern of biogas generation was the same for dry and ensilaged mangopeel. The initial biogas yield was low for both types of feedstock; it rose slowly and became almost stable after the fifth week. The methane content in the biogas followed a similar pattern, and gradually increased to 48-52 %. The pattern depicted in Fig. 1 was typical for start-up with fresh and dry mangopeel and tomato processing wastes.1+s The periodic examination of the effluent also showed a characteristic pattern, and the pH and VFA were always in the range of 6.45-7.20 and 70& 1200 mg/litre respectively. From the results of the start-up experiments, it can be concluded that ensilage of mangopeel for 6 months produced improved stability of the digesters, although the VFA content in the feed was very high. The total production of biogas depended on the availability of organic constituents of the feed to the digester. The effect of various loading rates in the range 40-120 kg TS/m” day at 30-day hydraulic retention time (HRT) was studied. For each loading rate the digesters were operated for 2 months and the effluent characteristics were determined for both types of feed. The rate and yield of biogas produced from dry and ensilaged mangopeel are depicted in Fig. 2. Biogas production increased with the loading rate. The yield was highest, however, at the lower loading rates of 6&80 kg TS/m3 day and then decreased gradually. This was associated with a drop in pH at higher loading rates and consequently increased levels of VFA (Fig. 2) for both dry and ensilaged mangopeel. A linear effect of loading rate on the rate of biogas generation and volatile solids utilisation was observed, but as the loading rate increased, the volatile solids content of the effluent also increased. The methane percentage decreased as the loading rate was increased for both feeds (Figure 2), but there was an increase in the rate of biogas production. Between 60 and 100 kg TS/m3 day loading rate of ensilaged and dry mangopeel, the yield of biogas was O&O.52 m3/kg VS added and 0.454.42 m3/kg VS added, respectively. A methane yield of O-24 m3/kg chemical oxygen demand (COD) added was reported for ensilaged sorghum by Coble and Egg.’ A high yield of biogas was always obtained with ensilaged mangopeel. A slurry containing 80 kg TS/m3 day could be handled easily for biogas production although the high concentrations of total solids led to handling problems and choking of the digesters. The influence of HRT’s on anaerobic digestion of both feeds was evaluated, and the results are
K. Madhukara
122
et al.
z$o~_y~_:;-~-i _ 1
0
10
20
30
40
50
60
TIME(doYs)
Fig. 4. Kinetics of biogas production Loading rate 80 kg TS/m3 day; HRT mangopeel ; X, dry mangopeel.
10
15-
20 HRT
Fig. 3. Influence
25
30
(days)
of hydraulic relenlion Lime (HRT) yield, methane content and volatile fatty acids. 0, mangopeel; x , dry mangopeel.
on biogas Ensilaged
summarised in Fig. 3. It is apparent that with 80 kg TS/m3 day loading rate and at ambient temperature the maximum yield of biogas was obtained at 20 days HRT with a methane content of 52%. At higher HRTs of 25-30 days, the gas production was similar and varied from 0.60 to 0.55 and from 0.42 to 0.43 m3/kg VS added for ensilaged and dry mangopeel respectively. Similar observations were recorded for the rate of biogas production (Fig. 3). However, at 15 and 10 days HRT the biogas yield and rate of biogas production declined markedly. At 10 days HRT, anaerobic digestion ceased because of excessive VFA production (Fig. 3). It was not possible to restore the anaerobic digestion by adjusting the pH of the digester slurry to 7.0. At 15 days HRT, a trend similar to that for 10 days HRT was observed, except that the biogas yield was higher, and the influence of VFA was less pronounced and it took longer for souring of the digester to occur. These results were in full agreement with the observations recorded for canteen wastes.’ The inhibition of biogas generation at higher HRT is associated with the increase in the level of VFA in the digesters and the consequent lowering of pH.9,“’ This is attributed to the washing out of much of the methanogenic population; these
and methane content. 20 days. 0, Ensilaged
microorganisms are very slow growers compared with acidogenic microorganisms, which grow even at low PH.‘,~~ The specific rates and yields of biogas production and reduction of volatile solids were always higher with the ensilaged mangopeel than the dry mangopeel. These results indicate that the anaerobic digestion of ensilaged mangopeel is a stable process and could be utilised for biogas production on a larger scale. To develop a stable process for biogas generation from ensilaged mangopeel, the kinetics of the process was studied using a loading rate of 80 kg TS/mX day and 20 days HRT at ambient temperature. Figure 4 depicts the variation of biogas and methane generation with time. Stable digestion was achieved throughout the study and the biogas production averaged 0.64 ma/kg VS for ensilaged mangopeel. Ensilage of mangopeel represents a promising method for storing wastes which originate from fruit and vegetable processing and other agroindustrial wastes which are highly seasonal. The entire quantity available during the season cannot be utilised for biofuel production at present. Ensilage is an old technique for storing forage crops for animal feed. Detailed investigations are needed to develop a simple and inexpensive method of ensilage of agro-industrial wastes and biomass for methane generation. This process also eliminates the pre-processing of feedstock such as milling and and the associated investment in pulverisation, equipment. ACKNOWLEDGEMENT The authors thank the Department of Nonconventional Energy Sources, Ministry of Energy, Government of India, for their financial support, the Area Co-ordinator, Discipline of Microbiology
Ensilage
of mangopeel
for
methane
and Sanitation, and Dr. Rajagopal Rao, Director, CFTRI, Mysore, for their keen interest in the work.
REFERENCES Anon., Final report of the work carried out on ‘Microbiological studies on the conversion of food processing and other related agro-industrial wastes for hiogas production’ (Proiect 5/2/41/85-BE) soonsored bv DNES, Government of India, 1989. Bryan, W.L. & Parrish, R.L., Solid state fermentation of sweet sorghum. ASAE Paper 82-3603. American Society of Agricultural Engineers, St Joseph, MI, 1982. Barry, M. & Colleran, E., Anaerobic digestion of silage effluent using an upflow fixed bed reactor. Agric. Wastes, 4 (1982) 231-9. Coble, C.G. & Egg, R., Ensilage storage of sorghum and high moisture biomass crops for anaerobic digestion of feed stocks. In Energy from Biomass and Wastes, X, ed. D.L. Klass. Elsevier Applied Science Publishers, London, and Institute of Gas Technology, Chicago, 1987, p. 1057.
8. 9.
10.
11.
12.
generation
123
Anon.: Standard Methods for the Examination of Water and Wastewater, 16th edn. American Public Health Association, New York, 1985. Woolford, M.K., The Silage Fermentation. Marcel Dekker New York, 1984. Linden, J.C., Henk, L.L., Murphy, V.G., Smith. D.H., Gabrielsen, B.C., Tengerdy, R.P. & Ezako, L., Preservation of potential f&-mentables in sweet sorghum by ensiling. Biotechnol. Bioenn., 30 (1987) 86G7. Sarada, R. & Nand, K., St&t-upof anaerobic digestion of tomato processing wastes for methane generation. Biol. Wastes, 30 (1989) 231-7. Beeman, R.E. & Suflita, J.M.. Environmental factors influencing methanogenesis in a shallow anoxic aquifer: a field and laboratory _ studv. ~ , _ J. Znd. Microbial., 5 (1990) 45-58. Summers, R., Bousfield, S. & Hobson, P.N., Uses and analysis of digested sludge. In Anaerobic Digestion, ed. D.A. Stafford, B.I. Wheatley & D.E. Hughes. Applied Science Publishers, London, 1980. Krishna Nand, Sumitra Devi, S., Prema Viswanath, Somayaji Deepak & Sarada, R., Anaerobic digestion of canteen wastes for biogas production: process optimization. Process Biochrm. 26 (1991) l-5. Ghosh, S. & Klass, D.L., Two phase anaerobic digestion. Process Biochem. 13 (1978) 15-20.
Wochemistry
28 (1993)
Ensilage
119-123
of Mangopeel
K. Madhukara,
Krishna
for Methane
Generation
Nand, N.R. Raju & H.R. Srilatha
Discipline of Microbiology and Sanitation, Central Food Technological Research Institute,
Mysore---
013, India
(Received 6 December 1991; revised manuscript received and accepted 19 May 1992)
Mangopeel is a waste product of mango processing which is available in large quantities in India during the harvest season, when disposal causes pollution problems. Ensilage of mangopeel for 6 months, the eflects of ensilage on its physico-chemical characteristics and methane generation by an anaerobic digestion process were studied. Ensilage helped in the pre-treatment of polymeric constituents and the conversion of major components of carbohydrates into volatile fatty acids. The ensilage mangopeel was a useful substrate for anaerobic digestion for methane generation, and the biogas yield was as high as 068 m3/kg volatile solids (VS) added, with a methane content of 52%. This compared favourably with 043 m3/kg VS added, and a methane content of 50%, for dried mangopeel. The ensilage of feedstock could therefore eliminate pre-processing such as milling and pulverisation. vegetable processing industries are seasonal, and as they decompose rapidly they are available for only a limited period of 2-3 months. The mechanical drying or sun-drying of these wastes (mangopeel, citruspeel, pineapple and tomato processing wastes), and their storage and further milling to produce feedstock add enormously to the cost of biogas production and make the entire process uneconomical. A simple and inexpensive method for preserving these wastes is needed, so that these resources may be used for energy generation throughout the year. The continuous use of an anaerobic digester and associated equipment makes this process highly economical for the production of biogas. Ensilage of forage crops is widely practised all over the world in modern agriculture but there are few reports in the literature on the preservation of fruit and vegetable processing wastes. Bryan and Parrish’ used ensilaged sorghum for ethanol pro-
INTRODUCTION Of all the alternative energy sources studied in recent years, biogas production appears to be the most favoured, especially when the feedstock is ago-industrial and food processing wastes. In addition to energy generation, it reduces pollution, and both liquid and solid residues may be utilised for aquaculture, and animal and poultry feeds. Anaerobic digestion of wastes from process plants is the only simple method which can be readily operated at farm, domestic and community levels for fuel production. The annual availability of these wastes amounts to 1005 million tonnes.’ A major part of this is mostly discarded and thus becomes a source of pollution. On occasions, disposal becomes expensive because of the high cost of transportation. The majority of wastes from the fruit and Corresponding Telex: X46-241
author: Dr K. Nand. Telephone FTRI IN.
ProcessBmchemistr_y
: 821-23539; 119
0032-9592/93/$6.00
0
1993 Else&r
Science
Publishers
Ltd,
England.
K. Madhukara
120
duction, and Barry and Colleran3 used stored sorghum effluent from tower silos and reported a methane yield of O-49 m3/kg chemical oxygen demand (COD) removed. Coble and Egg4 reported the performance of ensilaged sorghum in a twostage anaerobic digestion system, with the aim of developing an economically viable system. An ensilage process for the storage of mangopeel has been developed and the results of the preliminary studies are reported in this paper.
MATERIALS
AND METHODS
Mangopeel (1000 kg) collected from M/s. Fruitin Exports Ltd, Mysore, was stocked in a reinforced concrete silo, of 1-Om diameter and 1.25 m height, and compacted by applying pressure to remove air. The top portion was covered with a thick plastic sheet to protect the material from water. The ensilaged mangopeel was stored for 1 year and used periodically for chemical analysis and biogas generation. Mangopeel ensilaged for 6 months was used throughout the present study unless otherwise stated. Mangopeel was also sun-dried and shredded to a size of less than 5 mm using a hammer mill before feeding to the digester.
LABORATORY
DIGESTERS
Experiments were conducted in 6.5-litre glass fermentation vessels with an inlet for feeding and an outlet for biogas and effluent. The working volume of the digesters was 4 litres. They were wrapped in black plastic sheets to avoid algal growth. The digesters were initially charged with cowdung slurry diluted with water in the proportion of 4 : 5 together with a 10% inoculum obtained from biogas digesters that operated on mangopeel, cowdung, rabbit pellets and animal-house wastes, and were incubated undisturbed for 15 days for adaptation to anaerobic digestion. Digesters were grouped into two sets, for dry and ensilaged mangopeel separately, and a method involving a stepped decrease in cowdung and increase in mangopeel, both dry and ensilaged separately, in the proportion of 75 : 25, 50: 50,25 : 75 and 00: 100 was followed every week. The feed was prepared by blending the wastes to produce a homogeneous slurry, and was fed at the rate of 40 kg total solids (TS)/m3 day with a hydraulic retention time (HRT) of 30 days, supplemented with 2% urea by weight of total solids.
et al.
These digesters were operated in a semi-continuous mode of operation by loading once every 24 h.
CHEMICAL
ANALYSIS
The analysis of samples were carried out according to the methods of the American Public Health Association (APHA).” The volatile fatty acids (VFA), total solids (TS), volatile solids (VS) and pH of the effluent were analysed weekly. The volume of gas produced was recorded daily by water displacement, and its composition was analysed on a CIC (India) gas chromatograph equipped with a thermal conductivity detector, using a Poropak Q column, with an oven temperature of 60 “C.
RESULTS
AND DISCUSSIONS
The composition of fresh, dried and ensilaged mangopeel is listed in Table 1. Total carbohydrates, including cellulose, hemicellulose, and reducing and non-reducing sugars, of fresh and dry mangopeel approached 70 %. The ensilaged mangopeel contained only 25 % carbohydrate but the VFA content rose to the high level of 54 %. Higher percentages of lactic, butyric and propionic acid have normally been observed on ensilage of crop materials rich in water-soluble compounds.4.6 The ensilage of mangopeel for 6 months resulted in the conversion of 64 % of cellulose, hemicellulose and sugars into VFA. Although pectin, fat and proteins were partially hydrolysed, the lignin was not degraded. This increase in VFA was followed by a subsequent decrease in other constituents, but more detailed Table
1. Composition
Parameters
of Mangopeel EnsiIaged
Fresh
Dry
7447
Moisture (%) Total solids (%) Volatile solids (%) PH Ash (%)
25.53 85.86 4.00 14.14
70.07 29-93 9646 4.80 3-54
2644 73.56 96.24 4.80 3.45
% on dry weight basis Cellulose Hemicellulose Pectin Ether-soluble solids Protein Reducing sugar Non-reducing sugar Lignin
12-00 4.5 1 4.00 5.00 0.90 7.00 1.50 1 l-00
14-20 8.00 6-80 6.40 3.53 40.80 6.90 13.82
13.00 7.33 7-00 7.97 3.50 41-80 5.90 13.00
Ensilage qf mangopeel for methane generation
12345676 TIME
(weeks)
Fig. 1. Profiles of biogas production, methane content and pH during the start-up of anaerobic digestion of dry and ensildged mangopeel. 0, Ensilaged mangopeel; X : dry mangopeel.
cr
00 LOADING
I
60
00 RATE
(kg
I
100
1
120
TSlmS/day)
Fig. 2. Influence of loading rate on biogas generation effluent characteristics. 0, Ensilaged mangopeel; X, mangopeel.
and dry
investigations are needed on the biochemical changes and the types of microorganisms involved in the conversion of polymers to the various breakdown products. The pH of the mangopeel decreased rapidly from its initial value of pH 4.8 over the first 14 days. This was followed by a more gradual decrease, and after 3 months it became stabilised at pH 4-O. A similar trend during the ensilage of forage crops has been reported by a number of workers,4’7 although the pH declined to 3.9. The results of start-up experiments on the production of biogas, methane and pH changes are
121
given in Fig. 1. The pattern of biogas generation was the same for dry and ensilaged mangopeel. The initial biogas yield was low for both types of feedstock; it rose slowly and became almost stable after the fifth week. The methane content in the biogas followed a similar pattern, and gradually increased to 48-52 %. The pattern depicted in Fig. 1 was typical for start-up with fresh and dry mangopeel and tomato processing wastes.1+s The periodic examination of the effluent also showed a characteristic pattern, and the pH and VFA were always in the range of 6.45-7.20 and 70& 1200 mg/litre respectively. From the results of the start-up experiments, it can be concluded that ensilage of mangopeel for 6 months produced improved stability of the digesters, although the VFA content in the feed was very high. The total production of biogas depended on the availability of organic constituents of the feed to the digester. The effect of various loading rates in the range 40-120 kg TS/m” day at 30-day hydraulic retention time (HRT) was studied. For each loading rate the digesters were operated for 2 months and the effluent characteristics were determined for both types of feed. The rate and yield of biogas produced from dry and ensilaged mangopeel are depicted in Fig. 2. Biogas production increased with the loading rate. The yield was highest, however, at the lower loading rates of 6&80 kg TS/m3 day and then decreased gradually. This was associated with a drop in pH at higher loading rates and consequently increased levels of VFA (Fig. 2) for both dry and ensilaged mangopeel. A linear effect of loading rate on the rate of biogas generation and volatile solids utilisation was observed, but as the loading rate increased, the volatile solids content of the effluent also increased. The methane percentage decreased as the loading rate was increased for both feeds (Figure 2), but there was an increase in the rate of biogas production. Between 60 and 100 kg TS/m3 day loading rate of ensilaged and dry mangopeel, the yield of biogas was O&O.52 m3/kg VS added and 0.454.42 m3/kg VS added, respectively. A methane yield of O-24 m3/kg chemical oxygen demand (COD) added was reported for ensilaged sorghum by Coble and Egg.’ A high yield of biogas was always obtained with ensilaged mangopeel. A slurry containing 80 kg TS/m3 day could be handled easily for biogas production although the high concentrations of total solids led to handling problems and choking of the digesters. The influence of HRT’s on anaerobic digestion of both feeds was evaluated, and the results are
K. Madhukara
122
et al.
z$o~_y~_:;-~-i _ 1
0
10
20
30
40
50
60
TIME(doYs)
Fig. 4. Kinetics of biogas production Loading rate 80 kg TS/m3 day; HRT mangopeel ; X, dry mangopeel.
10
15-
20 HRT
Fig. 3. Influence
25
30
(days)
of hydraulic relenlion Lime (HRT) yield, methane content and volatile fatty acids. 0, mangopeel; x , dry mangopeel.
on biogas Ensilaged
summarised in Fig. 3. It is apparent that with 80 kg TS/m3 day loading rate and at ambient temperature the maximum yield of biogas was obtained at 20 days HRT with a methane content of 52%. At higher HRTs of 25-30 days, the gas production was similar and varied from 0.60 to 0.55 and from 0.42 to 0.43 m3/kg VS added for ensilaged and dry mangopeel respectively. Similar observations were recorded for the rate of biogas production (Fig. 3). However, at 15 and 10 days HRT the biogas yield and rate of biogas production declined markedly. At 10 days HRT, anaerobic digestion ceased because of excessive VFA production (Fig. 3). It was not possible to restore the anaerobic digestion by adjusting the pH of the digester slurry to 7.0. At 15 days HRT, a trend similar to that for 10 days HRT was observed, except that the biogas yield was higher, and the influence of VFA was less pronounced and it took longer for souring of the digester to occur. These results were in full agreement with the observations recorded for canteen wastes.’ The inhibition of biogas generation at higher HRT is associated with the increase in the level of VFA in the digesters and the consequent lowering of pH.9,“’ This is attributed to the washing out of much of the methanogenic population; these
and methane content. 20 days. 0, Ensilaged
microorganisms are very slow growers compared with acidogenic microorganisms, which grow even at low PH.‘,~~ The specific rates and yields of biogas production and reduction of volatile solids were always higher with the ensilaged mangopeel than the dry mangopeel. These results indicate that the anaerobic digestion of ensilaged mangopeel is a stable process and could be utilised for biogas production on a larger scale. To develop a stable process for biogas generation from ensilaged mangopeel, the kinetics of the process was studied using a loading rate of 80 kg TS/mX day and 20 days HRT at ambient temperature. Figure 4 depicts the variation of biogas and methane generation with time. Stable digestion was achieved throughout the study and the biogas production averaged 0.64 ma/kg VS for ensilaged mangopeel. Ensilage of mangopeel represents a promising method for storing wastes which originate from fruit and vegetable processing and other agroindustrial wastes which are highly seasonal. The entire quantity available during the season cannot be utilised for biofuel production at present. Ensilage is an old technique for storing forage crops for animal feed. Detailed investigations are needed to develop a simple and inexpensive method of ensilage of agro-industrial wastes and biomass for methane generation. This process also eliminates the pre-processing of feedstock such as milling and and the associated investment in pulverisation, equipment. ACKNOWLEDGEMENT The authors thank the Department of Nonconventional Energy Sources, Ministry of Energy, Government of India, for their financial support, the Area Co-ordinator, Discipline of Microbiology
Ensilage
of mangopeel
for
methane
and Sanitation, and Dr. Rajagopal Rao, Director, CFTRI, Mysore, for their keen interest in the work.
REFERENCES Anon., Final report of the work carried out on ‘Microbiological studies on the conversion of food processing and other related agro-industrial wastes for hiogas production’ (Proiect 5/2/41/85-BE) soonsored bv DNES, Government of India, 1989. Bryan, W.L. & Parrish, R.L., Solid state fermentation of sweet sorghum. ASAE Paper 82-3603. American Society of Agricultural Engineers, St Joseph, MI, 1982. Barry, M. & Colleran, E., Anaerobic digestion of silage effluent using an upflow fixed bed reactor. Agric. Wastes, 4 (1982) 231-9. Coble, C.G. & Egg, R., Ensilage storage of sorghum and high moisture biomass crops for anaerobic digestion of feed stocks. In Energy from Biomass and Wastes, X, ed. D.L. Klass. Elsevier Applied Science Publishers, London, and Institute of Gas Technology, Chicago, 1987, p. 1057.
8. 9.
10.
11.
12.
generation
123
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