[N₂O Emission Factors from Wastewater Treatment Plants Based on Literature Statistics and Model Fitting]

The emission of nitrous oxide （N₂O） during wastewater treatment cannot be ignored. The analysis of statistical data from literature based on 126 empirical studies revealed that the geographical factors of wastewater treatment plants （WWTPs） had a significant impact on N₂O emission factors. However, the N₂O emission factors of WWTPs in all regions of the world were generally lower than the Intergovernmental Panel on Climate Change （IPCC） recommended values. In China, the N₂O emission factors （in N₂O-N/N_influent） of WWTPs were approximately 0.000 35-0.065 20 kg·kg^-1. Meanwhile, the N₂O emission factors of different wastewater treatment processes were also significantly different, especially since the sequencing batch reactor （SBR） process had higher emissions. The use of uniform default emission factors for accounting was prone to overestimate N₂O emissions, and it is recommended that countries conduct actual monitoring or modeling studies to develop categorical emission factors suitable for local conditions. In addition, the N₂O emission factor based on total nitrogen （TN） removal was weakly negatively correlated with TN removal in 126 empirical data, which was more in line with bioprocessing stoichiometry and could provide an accurate accounting method for N₂O. To this end, a digital twin model was developed to dynamically simulate a case anaerobic-anoxic-aerobic （AAO） WWTP to comprehensively quantify the dynamic emission behavior of N₂O, which demonstrated that N₂O emissions had significant seasonal and daily variability and were only equivalent to 11% of the calculated value of the emission factor based on the IPCC recommendation. Comparing the scatter linear fitting and categorical mean exponential fitting methods, it was found that the latter could more accurately reflect the negative correlation between the N₂O emission factors and the TN removal rate, and an exponential regression equation between the average N₂O emission factor based on the amount of TN removed and the TN removal rate was further developed to predict the N₂O emission. The dynamic simulation and categorical index fitting methods provided in this study are important references for the accurate accounting of N₂O emissions in similar WWTPs and provide help for understanding and responding to the N₂O emission problems.