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 (SND) face the challenge of undesirable nitrous oxide (N2O) emission. Thereby, a comprehensive analysis of N2O turnover pathways and the affecting parameters in MABR are demanded for N2O mitigation strategies. In this work, a mathematical model describing three N2O turnovers pathways was studied to uncover the underlying mechanisms and the impacts of operational conditions on N2O turnovers in MABR system performing SND. The modelling results demonstrate that higher oxygen surface loading, longer hydraulic retention time (HRT) and lower influent chemical oxygen demand (COD) significantly induce higher N2O production factor (0.18%-3.3%). N2O turnovers are mainly regulated by the hydroxylamine (NH2OH) 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 N2O production factor (<0.13%), due to the shift of N2O 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 N2O conversion rates shows that the NH2OH pathway and HB denitrification become the predominant N2O 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 N2O production in MABR performing SND, due to more electrons preferentially used for N2O reduction. The relative low N2O production factor (80% total nitrogen (TN) removal in MABR can be achieved by controlling the oxygen surface loading (1.821-3.641 g/m2/d) and influent COD concentrations (285-500 mg/L) within a certain range.