Turning neem leaves anti-microbial substrate to methane-producing co-substrate for the generation of clean and affordable electricity through biogas: A case study

Abstract

Environmental concerns necessitate forestation. Biomethanation of leaf litter of neem (LLN) was conducted to circumvent open burning. The effect of total tannin (TT) on biomethanation was investigated. We found the amount of TT (2.12%) and the total saponin content TSC (3.23%) as the anti-bacterial properties of LLN sufficient to inhibit bio-methanation. These anti-microbial characteristics were, thus, neutralized by co-digestion with fruit and vegetable wastes (FVW). We reinforced the bacteria by dilution with a liquid fraction of the digestate (LFD), resulting in a 97.6% increase in methane yield. The LFD was obtained from another digester using FVW as feedstock. The quality of biogas suffices production of clean and affordable energy. Maximum methane content, biogas, and methane yields of 64.2 %, 0.388 m3/kg VS, and 0.22 m3/kg VS were observed from the co-digestion of cattle dung (CD), LLN and FVW under mesophilic conditions at the volatile solids ratio of 1:1:1 and the overall total solids of the reactor of 9.36 %. The results closely matched that obtained from the co-digestion of FVW with food waste and with pig manure, proving LLN as a new co-substrate to FVW. The mean concentration of H2S (88.3 ppm) was below the prescribed value for clean and safe combustion. Experimental data were, hence employed to design and evaluate the performance of full-scale biogas plants to treat LLN and FVW being produced by the Hira Singh fruit and vegetable wholesale market in Delhi, India. It is the biggest of its kind in the country receiving 11,961 tons of fruits and vegetables while producing 150 tonnes of waste per day. With the government subsidies; a reduced (1.06 years) payback period can be achieved. The research presents the future scope for testing distinct toxic biomass.