Biogas production from high solids anaerobic co-digestion of food waste, yard waste and waste activated sludge

High-solids anaerobic co-digestion (HS-AcD) has the potential to recover energy and reduce environmental impacts of the organic fraction of municipal solid waste and waste activated sludge. We investigated the impact of substrate to inoculum (S/I) ratios, alkalinity sources (sodium bicarbonate and oyster shells), co-substrate mixing ratios and inoculum acclimation on HS-AcD of food waste, yard waste, and waste activated sludge using batch studies. Long-term HS-AcD performance was evaluated under the optimal conditions through a semi-continuous biodigester study with leachate recirculation. The digester with S/I = 1 using a mixture of crushed oyster shells and sodium bicarbonate as alkalinity sources had the highest methane yields (183 mL CH₄/g VS). Addition of waste activated sludge to food waste and yard waste alleviated acidification (pH 6.86 ± 0.12) during the start-up period, which and improved digester stability. Mixtures with FW/YW/WAS = 0.8:1.7:0.5 had higher methane yields (134 ± 15 mL CH₄/gVS) than mixtures with FW/YW/WAS = 1:1:1, but required a longer time (10 days) for self-recovery from volatile fatty acid inhibition. The use of an acclimated inoculum eliminated the lag time during start-up and produced 38% higher methane yield. In the semi-continuous biodigester, an average volatile solids reduction of 38% and methane yield of 186 mL/gVS was achieved. Improved performance in the semi-continuous biodigester compared with batch reactors was likely due to leachate recirculation, which can improve mass transfer of substrates to the microbial community. Digestate produced from HS-AcD of waste organics had a 1.7–2.3 fold higher nitrogen, similar phosphorous and 0.2–0.3 fold lower potassium content than commercially available bioorganic fertilizer.