10.1021/acs.est.5b02038.s001 Bing-Jie Ni Bing-Jie Ni Yuting Pan Yuting Pan Ben van den Akker Ben van den Akker Liu Ye Liu Ye Zhiguo Yuan Zhiguo Yuan Full-Scale Modeling Explaining Large Spatial Variations of Nitrous Oxide Fluxes in a Step-Feed Plug-Flow Wastewater Treatment Reactor American Chemical Society 2015 N 2O emission data Second Step N 2O production pathways biomass nitrogen loading rate water quality parameters nitrogen loading rate Nitrous Oxide Fluxes N 2O fluxes N 2O emission monitoring data WWTP NH 2OH oxidation pathway AOB denitrification pathway N 2O production pathway N 2O emission N 2O model wastewater treatment plants 2015-08-04 00:00:00 Journal contribution https://acs.figshare.com/articles/journal_contribution/Full_Scale_Modeling_Explaining_Large_Spatial_Variations_of_Nitrous_Oxide_Fluxes_in_a_Step_Feed_Plug_Flow_Wastewater_Treatment_Reactor/2144752 Nitrous oxide (N<sub>2</sub>O) 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 N<sub>2</sub>O production pathways are not fully understood. This study applied a previously established N<sub>2</sub>O model incorporating two currently known N<sub>2</sub>O production pathways by ammonia-oxidizing bacteria (AOB) (namely the AOB denitrification and the hydroxylamine pathways) and the N<sub>2</sub>O production pathway by heterotrophic denitrifiers to describe and provide insights into the large spatial variations of N<sub>2</sub>O fluxes in a step-feed full-scale activated sludge plant. The model was calibrated and validated by comparing simulation results with 40 days of N<sub>2</sub>O 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 N<sub>2</sub>O fluxes from this section. The results further revealed the AOB denitrification pathway decreased and the NH<sub>2</sub>OH oxidation pathway increased along the path of both Steps due to the increasing dissolved oxygen concentration. The overall N<sub>2</sub>O 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.