EFFECT OF PRE-TREATMENT ON THE ANAEROBIC CO-DIGESTION OF RICE STRAW AND VEGETABLE WASTE FOR BIOGAS PRODUCTION

Abstract

Co-digestion of rice straw and vegetable waste (represented by green mustard, Brassica juncea) was conducted in a laboratory scale experiment for biogas production run in batch mode. Rice straw and B. juncea were mixed at certain composition to obtain the C/N ratio of 20 and used as the raw material of the digestion process. Rice straw was subject to biological and physical pre-treatment prior to the digestion. Biological pre-treatment was conducted by mixing the rice straw with EM-4, a commercial enriched microbial source, and physical method of pre-treatment was done by grinding the material. Liquid effluent from an existing biodigester of cow manure was used as inoculums. Digestion process was conducted at mesophilic condition, 35oC. Biogas production was observed daily, whereas concentration of volatile fatty acids (VFA), total solid (TS), and volatile solid (VS) were observed weekly. Following the digestion for about 50 days, the average yields of biogas production were 0.016 L/g VS, 0.030 L/ g VS and 0.017 L/g VS, for non pre-treated, biologically pre-treated and physically pre-treated substrates, respectively. The highest production of biogas was achieved on the day 20 after which the production decreased gradually. The maximum methane concentration in all cases (with and without pre-treatment) was not significantly different. The decrease of pH in all system was observed in the first 15 days and then back gradually to neutral. Slight decrease of pH was again noticed at the end of digestion period, especially in the case of physical pretreated and non pretreated substrates. Residual volatile solid was found to be 33%, 88.5%, and 81.4% of the initial volatile solid for the biologically pretreated, physically pretreated and unpretreated substrates, respectively. Significant increase of biogas production in biological pretreated substrate is mainly attributed to the improvement of rice straw digestibility.