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 (N ₂ O) is produced in activated sludge tanks as a byproduct of nitrification and heterotrophic denitrification. Insufficient knowledge on how microbiological N ₂ O generation and degradation pathways impact N ₂ O emissions in activated sludge tanks still hampers the development of effective mitigation strategies. Our research contributes to overcome this gap by quantifying N ₂ O 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 N ₂ O 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 N ₂ O in activated sludge tanks. Following these patterns, low loaded plants achieving high nitrogen removal (83-92%) exhibited the lowest N ₂ O emission intensity (0.0012 ± 0.001 kg N ₂ O-N emitted per kg TKN in the influent wastewater). The regression analysis corroborated these results by revealing a negative linear correlation between the N ₂ O emission factor and the total nitrogen removal degree of the plants. The regression model represents a novel estimation method that links N ₂ O emissions with plant performance and provides a significant improvement over approaches applying fixed N ₂ O emission factors.
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