Comammox-based simultaneous nitrification and denitrification in mainstream membrane-aerated biofilm reactor - A...

This study applied a novel complete ammonium oxidation (comammox)-inclusive biological nitrogen removal (BNR) model to simulate total nitrogen (TN) removal and nitrous oxide (N₂O) production in mainstream membrane-aerated biofilm reactor (MABR) performing simultaneous nitrification and denitrification (SND). The results were compared with those from a conventional comammox-exclusive BNR model under the same operational conditions. The findings revealed that the MABR involving the functionality of comammox bacteria capable of two-step nitrification demonstrated superior TN removal performance compared to the MABR performing the conventional ammonium-oxidizing bacteria (AOB)-based SND. Although comammox bacteria cannot produce N₂O directly, the unexpected functional shift of heterotrophic denitrifying bacteria from an N₂O sink in the MABR with AOB-based SND to an N₂O source in the MABR with comammox-based SND resulted in even higher N₂O production factors in the latter driven by the abundant nitrite/nitrate supply provided by comammox bacteria performing two-step nitrification under specific conditions (e.g., O₂ surface loading ≥ 3.53 g-O₂/m²/d and hydraulic retention time (HRT) ≥ 0.7 d). By precisely regulating operational conditions (e.g., O₂ surface loading = 3.16 g-O₂/m²/d, influent C/N ratio = 6, HRT = 0.60 d, and steady-state biofilm thickness = 300 μm), the MABR performing comammox-based SND could achieve efficient TN removal (e.g., 81.3 %) with minimal N₂O production/emissions (e.g., 0.06 %). This study demonstrated the feasibility of establishing comammox-based SND in MABR and provided critical insights for advancing further research and design/control of sustainable BNR systems.