Investigation of sludge reduction and biogas generation in high-rate anaerobic side-stream reactors for wastewater treatment

Abstract

The objective of this study is to investigate the mechanism of sludge reduction and biogas production in high-rate (<2.5 d) anaerobic side-stream reactors (ASSR) for minimizing sludge production during wastewater treatment. In the studied system, the daily generated activated sludge undergoes short anaerobic treatment in side-stream reactors and is returned to the main basin, instead of wastage. We chose 2–2.5 d for ASSRs based on the finding that activated sludge undergoes unique and effective hydrolysis during a short period of anaerobic digestion (AD), which is unavailable in aerobic digestion. The three activated sludge systems with 2.5 d ASSRs operated at 21–22 °C, 37 °C, and 55 °C showed the same efficiency of sludge reduction, indicating that for the purpose of minimizing sludge generation, ambient temperature will suffice for ASSR. Nevertheless, mesophilic ASSR showed significant and constant biogas production, which was absent in other ASSRs. These results indicate that in the activated sludge system with mesophilic ASSR, significant sludge reduction occurs inside ASSR. A short batch AD of activated sludge from this system also showed substantial methane production without anaerobic seed. These results suggests that activated sludge in this system already contains key anaerobic microbial community, making it possible to produce biogas in ASSR within such a short retention period. Based on these results, we propose a new schematic of wastewater treatment, which includes a high-rate ASSR and an anaerobic digester. This digester receives a small waste from ASSR, improving the speed and stability of AD compared to conventional AD.