Feedstock Balancing for Superior Biomethane Production and a Pathway to Sustainable Waste Valorization: Goat Manure and Rice Husk Co-Digestion Under Anaerobic Digestion

Anaerobic digestion (AD) is a common waste management method for producing renewable energy from biogas. However, animal manures typically have low C/N ratios, which can limit biogas recovery. This study aims to optimize biogas production through the co-digestion of goat manure (GM) with high-C/N lignocellulosic rice husk (RH) and sludge as the inoculum. Characterization of the substrate and inoculum revealed a low GM C/N ratio (GM = 21.3), whereas RH has a high-C/N (RH = 107.3). The volatile solids-to-total solids (VS/TS) ratios were around 82–85% for GM and RH. Batch experiments were conducted at different organic loading rates, with an inoculum-to-substrate ratio of 2:1 (mL:g), at 36 ± 1 °C for 65 days. This study investigates optimizing biomethane recovery by using serum-bottle biomethane potential (BMP) and compares kinetic performance and yields across different GM-to-RH ratios with the characteristics of the influent and effluent. The highest BMP values (mL CH4/gVS) occurred at 100% GM (245.1), followed by 90% GM (233.6) and 30% GM (232.5), indicating a strong synergy between GM and RH at specific mixing ratios. Kinetic modeling using both the modified Gompertz and first-order models effectively described digestion dynamics, allowing for estimation of potential lag phases and maximum production rates. These models aligned well with experimental data across substrates, aiding process design. Overall, the results show that strategic co-digestion of GM with RH can maximize methane recovery, with defined substrate ratios and a clear understanding of the kinetics essential for scale-up and sustainable biogas production.