Fermentation properties of agro-residues, leaf biomass and urban market garbage in a solid phase biogas fermenter

The decomposition and gas production pattern of eight unprocessed biomass feedstocks representing annual weeds, leaf litter, agro residues and market wastes were monitored in this laboratory study. Solid phase fermentation was effected with a weekly fed biomass bed sprinkled twice daily with recycled fermentor liquid to initiate and sustain biogas production from the decomposing biomass bed. Fermentors were fed from the top with gradually increasing feed rates to determine maximum feed rates sustainable. Feed rates of 1 g total solids (TS) l−1d−1 was possible which lead to pseudo steady state gas production rates between 0.26–0.98 l l−1 d−1 at specific gas yields of 0.18–0.44 l g−1 TS at 35–75% volatile solids (VS) destruction. Feedstocks such as paper mulberry (Broussenetia), Parthenium, Synedrella and urban garbage lost >50% VS in 30 d while paddy straw, bagasse and sugarcane trash exhibited lower VS loss (≥35%) in this period. During decomposition, bulky biomass feedstocks underwent compaction and obviated the need for a pretreatment step. Bulk densities rose manifold to reach between 150–350 g l−1 within 20 d. A higher decomposition rate, process optimization and use of pre-compacted feedstocks have the potential to increase the feed rates (0.96–1.93 g TS l−1d−1), quantity of feedstock held in the reactor as well as gas production rates. The current gas production rates and space economy in these fermentors compare well with Indian cattle dung fermentors (0.3–0.5 l l−1 d−1) and is attractive.