Long-term Assessment of N₂O Emission Factor in Full-scale Oxidation Ditch Reactor Considering Spatiotemporal Distribution

Abstract

In full-scale sewage treatment plants, long-term and high-frequency monitoring is required to mitigate nitrous oxide (N₂O) emissions. In this study, the profile of the dissolved N₂O concentration in a full-scale oxidation ditch reactor was investigated to determine the variation of the N₂O emission factor. It was found that the concentration of dissolved N₂O depended on microbial activity, which is affected by water temperature, dissolved oxygen concentration, and the dimensional relationship between the rotator and the inflow point. In the reactor, higher transcription levels of amoA mRNA and lower transcription levels of clade II type nosZ mRNA may be associated with N₂O production. The emission factor for removed dissolved inorganic nitrogen presented a mean value of 0.86% and a median of 0.19%. When N₂O production was promoted, gasification from the water surface was the most significant emission source, accounting for 52% of the total N₂O emitted, on average. The N₂O emission factor was often lower than 0.01% during stable operation; however, this factor was subject to sudden increases caused by nitrite accumulation.