Full-Scale Modeling Explaining Large Spatial Variations of Nitrous Oxide Fluxes in a Step-Feed Plug-Flow Wastewater Treatment Reactor

Nitrous oxide (N₂O) emission data collected from wastewater treatment plants (WWTPs) show huge variations between plants and within one plant (both spatially and temporarily). Such variations and the relative contributions of various N₂O production pathways are not fully understood. This study applied a previously established N₂O model incorporating two currently known N₂O production pathways by ammonia-oxidizing bacteria (AOB) (namely the AOB denitrification and the hydroxylamine pathways) and the N₂O production pathway by heterotrophic denitrifiers to describe and provide insights into the large spatial variations of N₂O fluxes in a step-feed full-scale activated sludge plant. The model was calibrated and validated by comparing simulation results with 40 days of N₂O emission monitoring data as well as other water quality parameters from the plant. The model demonstrated that the relatively high biomass specific nitrogen loading rate in the Second Step of the reactor was responsible for the much higher N₂O fluxes from this section. The results further revealed the AOB denitrification pathway decreased and the NH₂OH oxidation pathway increased along the path of both Steps due to the increasing dissolved oxygen concentration. The overall N₂O emission from this step-feed WWTP would be largely mitigated if 30% of the returned sludge were returned to the Second Step to reduce its biomass nitrogen loading rate.