An N₂O emissions model featuring newly integrated abiotic pathways in nitrification

Nitrification in biological wastewater treatment is a significant source of nitrous oxide (N₂O), a potent greenhouse gas (GHG). There are some models that describe the biological N₂O production process, but they don't include abiotic N₂O production pathways which, remarkably, contribute up to 50% of the total N₂O emissions under high nitrite (NO₂^-) conditions. This limitation frequently results in pronounced predictive biases under high influent ammonium (NH₄⁺) and intermediate NO₂^- conditions (such as in the partial nitrification + Anammox system), leading to the misidentification of N₂O emissions, undermining the development of effective mitigation strategies. To address this gap, a key abiotic N₂O production pathway was integrated into an existing model of nitrification which includes biological N₂O emissions. The upgraded model was systematically evaluated using literature-derived case studies, and can effectively predict the contributions of the abiotic pathway to N₂O emissions (49%), compared to experimental data (51%). A local sensitivity analysis confirms that the upgraded model has a resilience to perturbations within most parameters, although very high concentrations of NO₂^- (>1,000 mg N/L) necessitate a precise calibration of ammonium oxidation to nitrite (the AOB process), in which related parameters can more easily be measured in experiments. Moreover, a global sensitivity analysis demonstrates that dissolved oxygen (DO) and alkalinity are the most sensitive of the four key environmental factors (NH₄⁺, NO₂^-, DO and alkalinity) which control N₂O emissions.