International Research Journal of Environment Sciences________________________________ ISSN 2319–1414Vol. 4(6), 80-83, June (2015) Int. Res. J. Environment Sci. International Science Congress Association 80 Review Paper Anaerobic digestion of Vegetable waste for Biogas generation: A Review Patil V.S. and Deshmukh H.V.Lal Bahadur Shastri College of Arts, Science and Commerce, Satara-415002, M.S., INDIA Department of Microbiology, Yashavantrao Chavan Institute of Science, Satara-415002, M.S., INDIAAvailable online at: www.isca.in, www.isca.me Received 23rd Marhc 2015, revised 28th April 2015, accepted 6th June 2015 AbstractWorld’s vegetable production is strongly concentrated in Asia. India is next only to China in area and production of vegetables in the world. Vegetable waste is produced in huge quantities during the harvesting, poor and inadequate transportation, storage facilities, marketing practices and processing of vegetables. These serve as source of nuisance. The present methods used to dispose the vegetable wastes are inappropriate and cause environmental pollution. Biomethanation is an attractive option for vegetable waste treatment. Biomethanation is the anaerobic digestion of organic matter under controlled conditions of temperature, moisture and pH in an enclosed space to generate generates energy and nutrient rich effluent. The purpose of this review paper is to focus onto biomethanation studies on vegetable waste with respect to yield of biogas and pollution abatement studies. Keywords: Anaerobic digestion, biogas, vegetable waste, environmental pollution, etc. Introduction Vegetables in our daily diet are the source of proteins, vitamins, minerals, dietary fibers, micronutrients, antioxidants and phytochemicals. The diverse soil and climatic conditions in the country makes it possible to grow variety of vegetables. More than 40 kinds of vegetables belonging to different groups are grown in tropical, sub-tropical and temperate regions of the country. The vegetable markets produce plenty of vegetable waste per day. The disposal of these wastes is a very serious issue. The present vegetable waste disposal methods include dumping in municipal landfills, spreading on land and by feeding to animals. These unscientific methods results in environmental pollution. These methods results in land, water and air pollution. The treatment of vegetable wastes with the biological methods appears to be economical and also controls environmental pollution. Biomethanation is an attractive option among the biological methods since it generates biogas comprising mainly methane and carbon dioxide. Methane can be converted into electricity4,5and effluent from biomethanation plant have nutritive value6,7. Biogas yield and pollution abatement studies during anaerobic digestion of vegetable waste There are several reports on anaerobic digestion of vegetable waste for biomethanation potential. The anaerobic digestion of mixture of vegetable wastes was performed by Dhanalakshmi et al in a 2 L capacity single stage mesophilic anaerobic reactor. They operated the reactors at two different organic loading rates (OLR) 0.25 and 0.5gVS/l.d, with the HRT of 25days. The average biogas production was 0.150 and 0.300 L/day for two OLR rates respectively. Percent conversion of TS and VS was in the range of 53-62% and 62-67% respectively for two OLRs. The biogas yield was, 0.383 and 0.522 l/g TS added and 0.423 and 0.576 l/g VS added for the two OLR respectively. The methane content in the gas was around 63% and methane yield was 0.226 and 0.362 l CH4 / g VS added for the selected vegetables for the two OLR respectively. Dhanalakshmi and Ramanujam analyzed the performance of the vegetable waste based batch reactor and for determining cumulative biogas production for a given organic loading rate. The OLR used in experiment was in the range of 0.06 gm VS to 0.47 gm VS. Biogas production increased from OLR 0.06 gm VS to 0.26 gm VS and thereafter quantity decreased. Maximum Cumulative gas produced was found to be 3764 ml for 0.26 gm VS OLR. Babaeeand Shayegan10 conducted experiment to investigate the production of biogas from vegetable wastes by using anaerobic digestion process. They operated the digester at different organic feeding rates of 1.4, 2 and 2.75 kg VS/ m.d. The methane content of biogas was in the range of 49.7- 64% and biogas production rates was in the range of 0.12-0.4 m3 biogas/ (kg VS input). The methane yield was found to be 0.25 CH4/kg VS with 88% VS reduction at OLR of 1.4 kg VS/ .d. Biomethanation of vegetable waste was carried out in mesophilic conditions by Velmurugan and Ramanujam11 using a fed-batch laboratory scale reactor. The average methane content in the biogas was 65% and the Methane yield was 0.387 l CH4/g VS added for the selected types of wastes. Kumar et al12carried out a batch digestion of mixed vegetable market waste International Research Journal of Environment Sciences______________________________________________ ISSN 2319–1414 Vol. 4(6), 80-83, June (2015) Int. Res. J. Environment Sci. International Science Congress Association 81 using 5% Total Solid (TS) concentration in a laboratory-scale digester of 10 L capacity. The biogas yield of 0.15 m³ / kg TS was observed after 41 days. The maximum biogas production rate of 650 ml/hour was observed on day 25. The Chemical Oxygen Demand (COD) reduction observed during the present study was found to be about 65%. Biomethanation of vegetable waste at scale up studies has been carried by several workers. Kameswari et al13 studied biomethanation of vegetable market waste using bioreactor of capacity 30 tonnes per day. The biomethanation plant was designed for 30 tonnes per day, organic loading rate of 2.5 kg of VS/day/m3 with biogas generation of 2500 m3 of biogas per day. Biswas et al14 studied the anaerobic digestion of vegetable market waste in anaerobic digester of 10 dm3 capacities in a batch mode at an optimum temperature of 40C and 6.8 pH. They studied the effect of slurry concentration and that of the concentration of carbohydrate, protein and fat in the slurry on the biogas production rate and methane concentration in the biogas. A maximum concentration of methane in the order of 80% was obtained. Shashikant et al15 used 5 L capacity pyrex glass aspirator bottles for vegetable waste based Biomethanation. Biogas yield was 0.9 L/g VS added. Ranade et al16 studied biomethanation using market waste consisting of rotten vegetables using 25 L capacity lab scale biogas plant of floating dome design. The biogas yield was 175 L/day corresponding to 35 L/kg/day. Scale up studies on 200 L capacity biogas plant produced 302 L/kg/day. Several reports are available on biomethanation studies on vegetable component of kitchen waste. Lama et al17conducted a study at Kathmandu University, Nepal to determine the biomethanation potential of biodegradable portion of kitchen wastes from Kathmandu University Premises. Modified ARTI model compact biogas plant of 1 m digester and 0.75 m gasholder was used for biomethanation. The maximum methane gas and average maximum carbon dioxide recorded was found to be 65% and 58% respectively. The average gas production was found to be 173 L/day. Liu et al18 studied biomethanation of vegetable wastes collected from school canteen and vegetable leaves, stems and roots were mixed with the ratio of 1:1:1 using 15L capacity reactor made of plexiglass. The operational parameters for biomethanation were 100 to 128:4:1 C: N: P ratio, 35C temperature, OLR of 2.0 gVS/l.d. Biogas yield was in the range of 3.0 L /d to 3.5 L/d with 55% and 58% methane. Kitchen waste was used for biogas production by Voegeli et al19. The average daily gas production amount was found to be 290 L/d and 130 L/d when fed daily with 2 kg market waste. TS reduction of 84.9% for food waste and 72.8% for market waste with feeding period with 2 kg/d. VS reduction 92.2% and 85.3% for food and market waste respectively with 42.5 days HRT. COD reduction was approx. 83%. Chen et al20 established a 15 m3 capacity two-phase anaerobic digestion pilot plant for kitchen waste. They observed more than 80% VS reduction with the biogas yield of 0.6 m3/ kg VS. Biochemical methane potential (BMP) of vegetable waste is determined by several researchers. Labatut et al21 determined the BMP and biodegradability of an array of substrates with highly heterogeneous characteristics, with dairy manure. The results of about 175 individual BMP assays indicate that substrates rich in easily degradable carbohydrates yield the highest methane potential whereas the high lignocellulosic fraction exhibited the lowest yield. Co-digestion of dairy manure with easily degradable substrates increased the specific methane yields as compared to manure digestion only. Nguyen et al22 studied anaerobic digestion of organic fraction of municipal solid waste at a OLR 0.8kg VS/m3.d.The biogas yield obtained was 0.26m3/kg VS with 60 methane and 61% degradation of VS. Selinaand Joseph23 studied batch digestion of vegetable waste slurry in the laboratory using 2.5 L capacity bottle reactors for 60 days at ambient temperature conditions. The biogas yield from vegetable waste was found to be 0.391L/g of VS added. The methane content of the biogas was in the range of 67% during the first ten days and afterwards 68%-70%. The BMP was found to be 0.269 L CH4/g VS added. Ojolo et al24 examined the potential of vegetable component of municipal solid wastes. The batch fed 200dm capacity anaerobic digester was fed with 10-20 kg of vegetable and operated at 8-10 % TS of substrate, 40 days HRT and temperature of the fed substrate was within 29ºC and 33ºC. The average biogas yield varied from 5.15 dm/kg TS to 5.83 dm/kg TS. Davidsson et al25 determined the methane yield from 17 types of domestically organic fraction of municipal solid waste (OFMSW) in thermophilic digestion. The VS reduction was 80% and a methane yield of 300–400 Nm CH/ton VSin was obtained with 15 days HRT corresponding to about 70% of the methane potential. Many workers have used two phase anaerobic processes during the biomethanation studies on vegetable wastes. Zhu et al26studied biomethanation of Vegetable wastes using anaerobic sludge bed reactors (ASBR). The experiment was carried out at mesophilic temperature conditions (35C), HRT was 4 days and 20 days for hydrolytic reactor(HR) and methanogenic reactor(MR) respectively, OLR in HR and MR was 10/ g VS L and 5 g VS L-1 respectively. The biogas yield was about 660/g VS with 60±3% methane. Kamaraj27 studied vegetable market waste biomethanation process using biphasic anaerobic systems. The biogas and methane yields recorded were 511.8 and 352.7 l/ kg COD des respectively with a COD reduction of 75.6 % and 29.9 l/kg VMW fed. Rajeshwari et al28 studied biomethanation of solid vegetable market waste using two-stage digester: 100 L solid bed digester for hydrolysis and acidification and 24.6L Upflow anaerobic sludge blanket (UASB) reactor for biogas production. The COD reduction of 94% was achieved during UASB reactor operation at organic loading rate of 19.6 kg COD m day–3. Biswas et al29 developed a deterministic mathematical model topredict the characteristics of an anaerobic digester ofbiogas International Research Journal of Environment Sciences______________________________________________ ISSN 2319–1414 Vol. 4(6), 80-83, June (2015) Int. Res. J. Environment Sci. International Science Congress Association 82 generation satisfactorily. Mondal and Biswas30 performed comparative study on production of biogas from green vegetables wastes and dried vegetable wastes at different temperatures viz.20 C, 28 oC and 38 oC and solid concentrations (4%-8 %) in slurry by using two identical anaerobic bio digesters run in batch mode at pH 6.9. Maximum biogas (0.8 L/Kg DM/day) was produced at 6% solids conc. Dried vegetable waste showed 11.0 L biogas /Kg and green vegetable waste showed 6.5 L biogas /kg at 38 o C. Study showed high biogas yield and methane yield with the dried grinded vegetable was as compared to green vegetable wastes. The effect of catalysts on biogas yield by anaerobic digestion is studied by several scientists. Duran-Garcia et al31 assessed biogas production from peeling residues of potatoes, cabbage and carrots using several anaerobic digesters. They estimated the biogas yield, substrate pH and substrate concentration, using different types of catalysts. Kumar et al32 studied the effect of Ni(II), Zn(II) and Cd(II) on biogas production from potato waste and cattle manure mixture using fixed anaerobic batch digester. Maximum biogas was produced with 0.5:0.5 combination of potato waste and cattle manure. All the three heavy metals increased biogas yield over the control at 2.5 ppm concentration. The Cd++ at 2.5 ppm was found to produce maximum biogas in terms of volume. Conclusion Vegetable wastes have high carbohydrate and high moisture content and thus are a good substrate for the production of biogas through biomethanation process. Several researchers have studied biogas production from mixture of vegetable waste. Biogas yield reported by researchers is in the range of 0.360 L/g of VS to 0.9 L/g VS added. The temperature, pH and organic loading rates have a pronounced effect of vegetable waste biomethanation. The reactors of different designs also influence on the biogas yield. The use of appropriate catalysts is found to increase the biogas yield from anaerobic digestion of vegetable waste. Biomethanation process also reduces the load of organic pollutants in terms of reduction of TS, VS, BOD and COD. Thus, biomethanation appears to be an eco-friendly technology for the treatment of vegetable waste. 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