ANAEROBIC CO-DIGESTION OF VEGETABLE WASTE AND COW DUNG FOR BIOGAS PRODUCTION

Abstract

Energy crisis is the main hindrance for the economic development in Pakistan. Fortunately, Pakistan has enormous biomass resources available in the form of crop residues, sugarcane bagasse, food waste, wood, dung, feces and poultry litter etc. The present study was conducted to examine the potential for biogas production from cow dung and vegetable waste alone and when co-digested. For this reason batch and continuous digestions were performed in the present study. Both the digestion systems were performed in thermophilic range. Each experiment had three digesters, first one was filled with vegetable waste, second with 50% cow dung and 50% vegetable waste and third digester with cow dung only. For both the digestions substrates were pretreated with alkaline solution of Ca(OH)2 to buffer any sharp drop of pH. Due to this pH adjustment gas production was stable and increased tremendously. The results show that production of biogas from vegetable waste under thermophilic conditions in batch mode was 65.0 1/kg, from co-digestion it was 56.5 1/kg and from cow dung it was 47.7 l/kg. Average methane content in biogas after attaining stability was 62%, 55% and 50% for vegetable waste, co-digestion and cow dung respectively. In continuous digestion, biogas production from vegetable waste was 71.9 1/kg waste, from co-digestion it was 55.8 1/kg waste and from cow dung biogas production was 44.3 1/kg waste, with 15 day hydraulic retention time. Methane content was found to be 55%, 52% and 50% for vegetable waste, co-digestion and cow dung. It was noted that Alkaline pretreatment is necessary for waste like vegetables, which can degrade fast and can inhibit the digestion process by decreasing pH.