Environmental metrics and trade-off for aeration-driven nitrogen removal in a pilot-scale hybrid membrane-aerated biofilm reactor treating low-C/N municipal wastewater

Hybrid membrane-aerated biofilm reactor (MABR) has emerged as a promising upgrade for conventional nitrogen removal processes. This pilot-scale study provides a comprehensive technical and environmental performance evaluation of a hybrid MABR treating low-C/N municipal wastewater, benchmarked against a conventional modified Ludzack–Ettinger (MLE) process. The hybrid MABR demonstrated stable oxygen transfer rates of 13.6–14.2 g O₂/m²/day without external carbon addition and nitrifying liquid reflux, attaining a total nitrogen (TN) removal rate 3.6 times higher than the MLE system. Results from life cycle assessment (LCA) revealed that the hybrid MABR reduced overall environmental impacts and global warming potential (GWP) by approximately 70% and 75%, respectively, compared with the MLE process. While MABR-related components contributed less than 4% of total environmental impacts, operation under excessively low inlet airflow impaired nitrogen removal, thereby increasing GWP per unit of TN removed. These findings provide, for the first time at pilot scale, quantitatively substantiated evidence of environmental trade-offs between aeration efficiency and treatment capacity, as presented by life-cycle metrics. Scenario analysis further suggests potential environmental improvements through MABR expansion and renewable-energy integration. Microbial analysis also confirmed the stable biofilm-mediated nitrification–denitrification under carbon-limited conditions. Overall, the study provides quantitative insight into the energy–performance–environment nexus of hybrid MABR systems and establishes a framework for low-carbon operation and retrofitting of municipal wastewater treatment plants under low C/N conditions.