Modeling nitrous oxide emission from full-scale hybrid membrane aerated biofilm reactors (MABR).

Current published models for nitrous oxide (NO) emission in membrane aerated biofilm reactors (MABR) have several simplifications that are not representative of full-scale systems. This study developed an improved MABR NO model that captured commonly overlooked phenomena such as back diffusion of generated NO into MABR lumen gas and the recirculation of the NO laden lumen gas for tank mixing and biofilm thickness control. The improved model was validated with measured NO concentrations in the lumen gas phase and bulk mixed liquor in a full-scale hybrid MABR facility. The validated model was used to obtain insights into NO bioconversion pathways. Model predictions revealed that in the inner layers of the biofilm were hotspots of NO generation via the ammonium oxidizing organism activity. The NO transported to the outer biofilm layers was reduced via the heterotrophic denitrification pathway. The NO gas model predicted that up to 70 % of the NO carried by the recirculated lumen gas was scrubbed into the mixed liquor which was further denitrified. An NO emission ratio of 0.18 ± 0.01 % NON/N load was estimated for the full-scale MABR process which achieved up to 50 % removal of the influent N load, highlighting the potential of this technology to mitigate NO emissions when compared to conventional activated sludge.