Nitrous Oxide Production in a Granule-based Partial Nitritation Reactor: A Model-based Evaluation.

Sustainable wastewater treatment has been attracting increasing attentions over the past decades. However, the production of nitrous oxide (NO), a potent GHG, from the energy-efficient granule-based autotrophic nitrogen removal is largely unknown. This study applied a previously established NO model, which incorporated two NO production pathways by ammonia-oxidizing bacteria (AOB) (AOB denitrification and the hydroxylamine (NHOH) oxidation). The two-pathway model was used to describe NO production from a granule-based partial nitritation (PN) reactor and provide insights into the NO distribution inside granules. The model was evaluated by comparing simulation results with NO monitoring profiles as well as isotopic measurement data from the PN reactor. The model demonstrated its good predictive ability against NO dynamics and provided useful information about the shift of NO production pathways inside granules for the first time. The simulation results indicated that the increase of oxygen concentration and granule size would significantly enhance NO production. The results further revealed a linear relationship between NO production and ammonia oxidation rate (AOR) (R = 0.99) under the conditions of varying oxygen levels and granule diameters, suggesting that bulk oxygen and granule size may exert an indirect effect on NO production by causing a change in AOR.