Treatment of Sewage Sludge through Thermophilic-Thermophilic and Thermophilic-Mesophilic Anaerobic Digestion

This study evaluated the efficiency of a two-phase anaerobic digester using a substrate of raw and excess sludge. The system operated with a thermophilic acidogenic fermentation reactor (55 ± 1℃) followed by either a thermophilic methanogenic reactor (55 ± 1℃) or a mesophilic methanogenic reactor (35 ± 1℃). Over approximately 300 days, biogas production and volatile solids (VS) removal rates were measured in response to increased organic loading. Results indicated that while the systeｍ with thermophilic acidogenic and thermophilic methanogenic reactors was initially unstable, it stabilized and ultimately produced more gas than that of thermophilic acidogenic and mesophilic methanogenic reactors. However, both systems showed inconsistent biogas production with rising organic loading. The maximum methane production rate for the former was 0.57 m³/kg VS·d, about 9% higher than the latter, which achieved final VS removal efficiencies of 48.6% and 41.1%, respectively.Stability evaluations revealed that ammonia concentrations were higher in the system with thermophilic acidogenic and thermophilic methanogenic reactors, correlating with increased polymer decomposition efficiency. Additionally, the TVFAs (Total Volatile Fatty Acids)/Alkalinity ratio remained stable at approximately 0.4 for the system with thermophilic acidogenic and mesophilic methanogenic reactors, while that of thermophilic acidogenic and thermophilic methanogenic reactors showed decreasing stability at higher organic loads.