Effect of biogas recirculation strategy on biogas upgrading and process stability of anaerobic digestion of sewage sludge under slightly alkaline condition.

In this study, in-situ biogas upgrading was examined and compared in lab-scale semi-continuous anaerobic digestion (AD) systems coupled with biogas recirculation under neutral and slightly alkaline conditions. Results showed that neutral pH (~7.5) condition yielded 6-7% higher CH content and 52-53% less HS production, while slightly alkaline condition (pH ~8.0) achieved 14-15% higher CH content and 76-77% less HS production under biogas recirculation in comparison to the control (no biogas recirculation). Little effect was noticed on daily CH production rate at slightly alkaline condition under biogas recirculation. Endogenous free ammonia nitrogen-dissolved inorganic carbon-volatile fatty acids (FAN-DIC-VFAs) buffer system was found to regulate the pH and alkalinity in the bulk liquor. Therefore, biogas recirculation under slightly alkaline condition is proposed as an efficient and sustainable strategy for biogas upgrading during AD of sewage sludge.