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

Abstract

As a novel and sustainable technology, membrane-aerated biofilm reactors (MABR) performing simultaneous nitrification and denitrification 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 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 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, due to more electrons preferentially used for N₂O reduction. The relatively low N₂O 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/m²/d) and influent COD concentrations (285-500 mg/L) within a certain range.

Keywords: Mathematical modeling; Membrane-aerated biofilm reactors (MABR); Nitrous oxide (N(2)O) turnover pathways; Simultaneous nitrification and denitrification.