Variability and Uncertainty Analysis of N₂O Emissions from WWTP to Improve the Accuracy of Emission Factors and the Design of Monitoring Strategies

Investigations on global nitrous oxide (N₂O) emissions from wastewater treatment plants (WWTPs) often overlook data variability and uncertainty by utilizing averaged emission factors (EF and N₂O emitted/total nitrogen load). The variability of the EF is system-specific: technology, operation, influent characteristics, and microbial community. The uncertainty of the EF is associated with the monitoring campaign: number of measuring locations, analytical methods, data frequency, and duration. This study quantified the spatial and temporal variability of N₂O emissions and EF from liquid measurements of N₂O in a surface-aerated WWTP over a 110-day campaign. The gas stripping model (k_La_N2O) yielded a minimum measurement error of 10.4% and 13.0% for the TN_load. Daily patterns of N₂O concentrations and emissions were not normally distributed but positively skewed, suggesting that log-distributions are more accurate descriptors (11.6% of the days emitted 50% of emissions). The EF was 0.30 ± 1.29% (n = 117602), and the higher standard deviation of the mean highlights the inaccuracy of normal distributions for N2O data sets, whereas the median and 2.5-97.5th quantiles and/or continuous log-distributions are more informative. An in silico analysis indicates that monitoring 5 out of 30 min instead of continuously provides reasonable accuracy and precision while allowing for six different monitoring locations.