Optimizing biomethane production from co - digestion organic fraction of municipal solid waste and spent coffee grounds in anaerobic digestion

Abstract

Anaerobic digestion (AD) is a critical method to treat the rapidly growing amount of organic fraction of municipal solid waste (OFMSW) generated due to population growth and the expansion of the urbanization. This study focuses on optimizing biomethane production through anaerobic co-digestion of Organic Fraction of Municipal Solid Waste (OFMSW) and Spent Coffee Grounds (SCG). Laboratory-scale anaerobic digestion experiments were conducted using cow dung as inoculum. This study investigated to identify the optimum mixing ratio, hydraulic retention time (HRT), and inoculum/substrate ratio (I/S) for achieving the maximum biogas production while ensuring a high methane yield, using response surface methodology (RSM) and numerical optimization. The feedstock characterization showed that OFMSW had a total solids content of 20.2% and volatile solids of about 85%, while spent coffee grounds exhibited higher total solids (36%) and volatile solids (88.5%). The carbon-to-nitrogen ratios of OFMSW, SCG, and cow dung were 24.3, 26.4, and 27.7, respectively, indicating favorable nutrient balance for anaerobic digestion. The results showed that co-digestion significantly improved biogas yield and methane content compared to mono substrate digestion. The optimum biogas yields 0.63 L/gVS was achieved at an SCG fraction of 31.5% and 68.5% of OFMSW, HRT of 27 days, and ISR of 0.68. The optimum biomethane yield 0.42 L/gVS was obtained at an SCG fraction of 23.6% and76.4 % of OFMSW, HRT of 31 days, and I/S of 1.4 with methane content ranging from 55–60% and carbon dioxide ranging from 40–45 and less than 1% other trace gases. The findings confirm that co-digestion of OFMSW and SCG is an effective and sustainable approach for renewable energy generation, organic waste reduction, and improved waste management in urban areas.