Effect of Substrate to Inoculum Ratio on Bioenergy Recovery from Food Waste, Yard Waste, and Biosolids by High Solids Anaerobic Digestion

Residual biosolids from wastewater treatment facilities and the organic fraction of municipal solid waste (OFMSW), e.g., food and yard wastes (FW and YW), are difficult to manage due to increased waste generation and recent stringent regulations. The effect of substrate to inoculum ratio (S/I) on biochemical transformation mechanisms and biomethane production during high solids anaerobic digestion of FW, YW, and biosolids were investigated. Biochemical methane potential (BMP) assays were set up under mesophilic conditions at S/I of 1.0, 2.0, and 3.0 g OFMSW total solids (TS)/g inoculum TS. BMP with the lowest S/I (1.0) produced the highest cumulative CH₄ yield of 126 mL CH₄/(g VSadded), with 90% of its maximum yield by day 37, and a hydrolysis rate constant of 0.06/day. S/I 2.0 and 3.0 led to higher volatile fatty acid release than S/I 1.0. This resulted in pH-induced inhibition of methanogenesis, longer lag phases, and lower cumulative CH₄ yields [S/I = 2.0 at 45.6-mL CH₄/(g VSadded); S/I = 3.0 at 6.75-mL CH₄/(g VSadded). The results showed that accumulation of inhibitory intermediates could cause system failure due to mass transfer limitation under low moisture conditions when S/I > 1.0 based on TS.