Optimization of co-digestion ratios and operating parameters for enhanced methane yield from bio-waste in a plug flow reactor

Bio-waste represents an abundant renewable resource for sustainable methane production through anaerobic digestion; however, methane yield is strongly influenced by substrate composition and operating conditions. In this study, 105 unique co-digestion substrate combinations comprising kitchen waste (KW), cow manure (CM), flower waste (FW), sludge (SW), grass waste (GW), and paper waste (PW) were experimentally evaluated to identify the optimum feedstock combination for enhanced methane production. Preliminary screening was performed in 1 L batch reactors, followed by process optimization in a 10 L plug-flow reactor operated at a 14-day hydraulic retention period. The effects of substrate ratio, substrate-to-water ratio, slurry density, pH, and C: N ratio on biogas production were investigated. Among the evaluated combinations, the KW: CM mixture at a 4:1 ratio produced the best performance, achieving 700 mL biogas production with a methane concentration of 60%, an optimum C: N ratio of 25:1, and an optimum slurry density of 0.80 kg/L at a substrate-to-water ratio of 1:0.80. Compared with the other optimized co-digestion mixtures (KW: FW and FW: CM), the KW: CM mixture produced approximately 3–17% higher methane concentration and the highest cumulative biogas yield. The results demonstrate that optimizing slurry density and substrate composition significantly enhances anaerobic digestion efficiency and provides practical guidance for designing plug-flow digesters for decentralized bio-waste management and renewable energy production.