Biogasification of solid wastes by two-phase anaerobic fermentation

Municipal, industrial and agricultural solid wastes, and biomass deposits cause large-scale pollution of land and water. Gaseous products of waste decomposition pollute the air and contribute to global warming. This paper describes the development of a two-phase fermentation system that alleviates methanogenic inhibition encountered with high-solids feeds, accelerates methane fermentation of solid bed, and captures methane (renewable energy) for captive use to reduce global warming. The innovative system consisted of a solid-bed reactor packed with simulated solid waste at a density of 160 kg/m3 and operated with recirculation of the percolated culture (bioleachate) through the bed. A rapid onset of solids hydrolysis, acidification, denitrification and hydrogen gas formation was observed under these operating conditions. However, these fermentative reactions stopped after about 2.5 months of solid-bed fermentation at which time total volatile fatty acids (VFA) concentration accumulated to 13,000 mg/l (as acetic) at pH 5, and the reactor head-gas consisted of 75% carbon dioxide, 20% nitrogen, 2% hydrogen and 3% methane. The VFA concentration and gas composition remained virtually constant for an additional 2.5 months of solid-bed fermentation indicating inhibition of the hydrolysis–acidification process. Inhibition of acidogenic fermentation was alleviated by moving the bioleachate to a separate methane-phase fermenter, and recycling methanogenic effluents at pH 7.5 to the solid bed. Coupled operation of the two reactors during the following 4.5 months of two-phase fermentation achieved methanogenic conversion of about 30% of the volatile solids (VS) content of the high-solids feed. Process operation was continuing.