Community Succession and Carbon Footprint Accounting of a Pilot-Scale Anaerobic Filter During Recovery After Long-Term Shutdown Under Low-Temperature Stress
The transition of municipal wastewater treatment from energy-intensive aerobic processes to energy-recovering anaerobic technologies is critical for achieving carbon neutrality. However, anaerobic treatment of low-strength wastewater faces operational challenges, especially after prolonged shutdowns and under low-temperature stress. This study investigated the recovery and performance of a pilot-scale anaerobic filter (AF) packed with volcanic rock media (effective volume 240 L) treating actual municipal wastewater. After a 234-day starvation period, the AF recovered within 63 days by extending hydraulic retention time from 8 to 12 hours, achieving total chemical oxygen demand (TCOD) and soluble chemical oxygen demand (SCOD) removal amounts of 58.6 mg/L and 21.5 mg/L, respectively. Following 100 days of low-temperature operation (influent temperature dropped to 11.7 °C), TCOD and SCOD removal amounts remained at 54.6 mg/L and 28.6 mg/L, respectively. Although methane production volume decreased, the specific methane yield was not significantly affected (0.276 L CH4/g TCODre during recovery vs. 0.244 L CH4/g TCODre under low temperature). Mechanistic analysis revealed that increased protein secretion in extracellular polymeric substances (EPS) was crucial for resistance to low-temperature stress, with protein content negatively correlated with temperature. Microbial community analysis showed that functional populations recovered in the order of hydrolytic acidogens, hydrogen-producing acetogens, and methanogens. Methanobacterium dominated after starvation, while Methanosaeta became dominant after low-temperature exposure due to its high affinity for acetate. Carbon footprint accounting indicated that recovering dissolved methane and coupling with nitrate-dependent methane oxidation is the optimal strategy for carbon reduction.