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.