Modeling nitrous oxide emission from full-scale hybrid membrane aerated biofilm reactors (MABR)
Narasimman Lakshminarasimman, Mirzaman Zamanzadeh, Oliver Schraa, Wayne J. Parker
Abstract
• Current N 2 O models do not capture all mass transfer processes in full-scale MABRs • A model with N 2 O back diffusion and lumen gas sparging was developed • Predicted lumen gas and dissolved N 2 O matched observed values well • Longitudinal trends in N 2 O bioconversion pathways in plug flow tank were revealed • Modelling revealed 70% removal of lumen gas N 2 O through scrubbing into MABR bulk Current published models for nitrous oxide (N 2 O) emissions in membrane aerated biofilm reactors (MABR) have several simplifications that are not representative of full-scale systems. This study developed an improved MABR N 2 O model that captured commonly overlooked phenomena such as back diffusion of generated N 2 O into MABR lumen gas and the recirculation of the N 2 O laden lumen gas for tank mixing and biofilm thickness control. The improved model was validated with measured N 2 O 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 N 2 O bioconversion pathways. Model predictions revealed that in the inner layers of the biofilm were hotspots of N 2 O generation via the ammonium oxidizing organism activity. The N 2 O transported to the outer biofilm layers was reduced via the heterotrophic denitrification pathway. The N 2 O gas model predicted that up to 70% of the N 2 O carried by the recirculated lumen gas was scrubbed into the mixed liquor which was further denitrified. An N 2 O emission ratio of 0.18 ± 0.01% N 2 O-N/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 N 2 O emissions when compared to conventional activated sludge.