Two phase fermentation of whole leaf biomass to biogas

Abstract

Two phase (diphasic) fermentation of untreated whole leaf biomass involving a solid phase acidogenic system and an upflow anaerobic packed bed digester for methanogenesis has been studied. The solid phase acidogenic system consisted of a bed of biomass (22% solids) sprinkled daily with an aqueous suspension of biodegrading bacteria. Accumulation of volatile fatty acids (VFA) in the liquid lowered its pH to about 5 and suppressed methanogenesis in the biomass bed. A part of the leachate from the acidogenic digester was fed to the methanogenic digesters. Clear phase separation was obtained with fresh or dried leaf biomass when the total VFA levels exceeded 6 g L −1 (pH 5). At low VFA concentrations in the first stage significant levels of gas production occurred from the decomposing biomass bed indicating a possibility of solid phase fermentation without phase separation if VFA and pH levels are maintained within these critical limits. Specific gas yields amounting to about 290 and 190 L kg −1 dry solids were obtained for fresh and dry leaf biomass feedstocks, respectively, with a high methane content in the methanogenic reactors (>'70%). The results obtained indicate the potential for semi-continuous operation, a reduced solids retention time, a simpler feeding technique, the avoidance of floating problems, and a reduced requirement for pretreatment, which are all encountered in biogas digesters of conventional Indian design using untreated whole leaf biomass feedstocks.