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

Current published models for nitrous oxide (N2O) emission in membrane aerated biofilm reactors (MABR) have several simplifications that are not representative of full-scale systems. This study developed an improved MABR N2O model that captured commonly overlooked phenomena such as back diffusion of generated N2O into MABR lumen gas and the recirculation of the N2O laden lumen gas for tank mixing and biofilm thickness control. The improved model was validated with measured N2O 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 N2O bioconversion pathways. Model predictions revealed that in the inner layers of the biofilm were hotspots of N2O generation via the ammonium oxidizing organism activity. The N2O transported to the outer biofilm layers was reduced via the heterotrophic denitrification pathway. The N2O gas model predicted that up to 70 % of the N2O carried by the recirculated lumen gas was scrubbed into the mixed liquor which was further denitrified. An N2O emission ratio of 0.18 ± 0.01 % N2ON/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 N2O emissions when compared to conventional activated sludge.