Cold-adapted membrane-aerated biofilm reactor for domestic wastewater treatment in northwest China: Pollutant removal performance and mechanistic insights

Membrane-aerated biofilm reactors (MABRs) have considerable potential for carbon and nitrogen removal from domestic wastewater. However, their long-term performance under high-altitude, low-temperature, and low-C/N conditions remains insufficiently understood. In this study, a pilot-scale MABR was continuously operated for 160 days to treat real domestic wastewater in a plateau region of Northwest China at an elevation of approximately 3500 m. The effects of hydraulic retention time (HRT), aeration mode, and nitrified liquor recirculation ratio were evaluated, and biofilm microbial communities were characterized by 16S rRNA gene sequencing. Under the optimized conditions of a 24 h HRT, S-3 aeration mode, and a 200% recirculation ratio, the average removal efficiencies of COD, NH 4 + -N, TN, NO 3 − -N, NO 2 − -N, and organic nitrogen reached 88.12 ± 1.54%, 94.20 ± 1.17%, 87.50 ± 1.92%, 61.03 ± 2.41%, 40.00 ± 2.76%, and 73.86 ± 2.05%, respectively. Effluent COD, NH 4 + -N, and TN met the Class 1 A limits of GB 18918–2002. Stable carbon and nitrogen removal was maintained at 8.5 °C without external carbon addition. Proteobacteria, Bacteroidota, and Firmicutes remained dominant throughout operation, while nitrifying and denitrifying taxa were selectively enriched after optimization. Notably, Nitrospira and Comamonas increased markedly and remained enriched during stable operation. Overall, optimized HRT, oxygen supply, and recirculation were associated with favorable biofilm redox conditions and functional microbial succession, supporting stable carbon and nitrogen removal under cold, low-C/N conditions. This study provides a practical basis for MABR operation in cold, high-altitude regions.