Contribution of nitrification and denitrification to nitrous oxide turnovers in membrane-aerated biofilm reactors (MABR): A model-based evaluation.

As a novel and sustainable technology, membrane-aerated biofilm reactors (MABR) performing simultaneous nitrification and denitrification face the challenge of undesirable nitrous oxide (NO) emission. Thereby, a comprehensive analysis of NO turnover pathways and the affecting parameters in MABR are demanded for NO mitigation strategies. In this work, a mathematical model describing three NO turnovers pathways was studied to uncover the underlying mechanisms and the impacts of operational conditions on NO turnovers in MABR system performing simultaneous nitrification and denitrification. The modeling results demonstrate that higher oxygen surface loading, longer hydraulic retention time (HRT) and lower influent chemical oxygen demand (COD) significantly induce higher NO production factor (0.18%-3.3%). NO turnovers are mainly regulated by the hydroxylamine (NHOH) pathway and heterotrophic bacteria (HB) denitrification, accounting for 76%-87% and 10%-21%, respectively. In contrast, the thicker biofilm (i.e., 400-600 μm) causes lower NO production factor (<0.13%), due to the shift of NO turnover pathways to the ammonium oxidizing bacteria (AOB) denitrification pathway (7.1%-9.3%) and HB denitrification (90.7%-92.9%). Meanwhile, the result of in-biofilm NO conversion rates shows that the NHOH pathway and HB denitrification become the predominant NO production pathway at the inner zone (0-160 μm) and the outer zone (290-350 μm) of the biofilm in MABR, respectively. The biofilm thickness at 160-280 μm can thus be regarded as an optimal zone to reduce NO production in MABR, due to more electrons preferentially used for NO reduction. The relatively low NO production factor (<0.5%) together with >80% total nitrogen (TN) removal in MABR can be achieved by controlling the oxygen surface loading (1.821-3.641 g/m/d) and influent COD concentrations (285-500 mg/L) within a certain range.