A method to estimate the direct nitrous oxide emissions of municipal wastewater treatment plants based on the degree of nitrogen removal

The greenhouse gas nitrous oxide (N2O) is produced in activated sludge tanks as a byproduct of nitrification and heterotrophic denitrification. Insufficient knowledge on how microbiological N2O generation and degradation pathways impact N2O emissions in activated sludge tanks still hampers the development of effective mitigation strategies. Our research contributes to overcome this gap by quantifying N2O emissions through extensive measurement campaigns at ten full-scale wastewater treatment plants and correlating them to relevant operating parameters by multivariate regression analysis. Measurements revealed that N2O production depends mainly on the activity of nitrifying bacteria and is triggered by high ammonium concentrations. In contrast, well-performing heterotrophic denitrification plays a key role as a sink of N2O in activated sludge tanks. Following these patterns, low loaded plants achieving high nitrogen removal (83–92%) exhibited the lowest N2O emission intensity (0.0012 ± 0.001 kg N2O–N emitted per kg TKN in the influent wastewater). The regression analysis corroborated these results by revealing a negative linear correlation between the N2O emission factor and the total nitrogen removal degree of the plants. The regression model represents a novel estimation method that links N2O emissions with plant performance and provides a significant improvement over approaches applying fixed N2O emission factors.