Abstract

This study explores the optimization of biogas yield from the co-digestion of human excreta and kitchen waste using plant ash as a low-cost catalyst within a Taguchi L9 design. Anaerobic digestion of these abundant waste streams offers a sustainable energy pathway, but process instability and low methane yield remain challenges. Three parameters, temperature (30 ‒ 50 °C), pH (7.0 ‒ 7.6), and moisture content (70 ‒ 90%), were investigated. Analysis of variance showed temperature as the dominant factor, contributing about 68% of the variability, while pH and moisture had minor effects. Optimal conditions of 40 °C, pH 7.3, and 90% moisture content achieved a maximum yield of 130 mL/g VS. The addition of plant ash improved gas quality, increasing methane concentration from 60% to 68% and calorific value from 19.2 to 24.1 MJ m^-3. Kinetic modeling confirmed that the modified Gompertz equation best described the process (R² = 0.981), indicating rapid microbial acclimatization and stable methane generation. These results demonstrate that combining co-digestion with plant ash catalysis enhances buffering, accelerates degradation, and improves methane yield. The approach provides a practical and low-cost strategy for decentralized renewable energy production and sustainable waste management in resource-constrained regions.