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Heterotrophic denitrification consists of the four-step sequential reduction of nitrate to dinitrogen gas over nitrite, nitric oxide, and nitrous oxide. Oxidation processes, commonly of organic compounds, provide the electrons needed for the sequential reaction steps. The intracellular electron distribution is a competitive process among the four reduction steps. In this study, a model describing organic carbon oxidation and four-step denitrification through electron competition is proposed [Activated Sludge Model–Electron Competition (ASM–EC)]. The model describes denitrification rates as an analogy to how current intensity varies through a parallel set of resistors in electric circuits. The ASM–EC model was calibrated with data from batch experiments with heterotrophic denitrifying communities, where reduction of mixtures of nitrogen oxides was monitored, while different carbon sources were supplied in excess. The carbon sources included methanol, ethanol, acetate, and their ternary mixture. The electron distribution preference and electron uptake rates varied between the carbon sources and were captured by the model structure for most of the experiments. The ASM–EC model uses fewer parameters compared to existing state-of-the-art denitrification models and performed equally well in the tested scenarios. We advocate the use of this model for denitrification in the activated sludge model, which can easily be integrated in existing model structures, because it provides a parsimonious description of electron competition during denitrification.

Hydroxylamine (NH2OH) and nitrite (NO2–), intermediates during the nitritation process, can engage in chemical (abiotic) reactions that lead to nitrous oxide (N2O) generation. Here, we quantify the kinetics and stoichiometry of the relevant abiotic reactions in a series of batch tests under different and relevant conditions, including pH, absence/presence of oxygen, and reactant concentrations. The highest N2O production rates were measured from NH2OH reaction with HNO2, followed by HNO2reduction by Fe2+, NH2OH oxidation by Fe3+, and finally NH2OH disproportionation plus oxidation by O2. Compared to other examined factors, pH had the strongest effect on N2O formation rates. Acidic pH enhanced N2O production from the reaction of NH2OH with HNO2indicating that HNO2instead of NO2–was the reactant. In departure from previous studies, we estimate that abiotic N2O production contributes little (< 3% of total N2O production) to total N2O emissions in typical nitritation reactor systems between pH 6.5 and 8. Abiotic contributions would only become important at acidic pH (≤ 5). In consideration of pH effects on both abiotic and biotic N2O production pathways, circumneutral pH set-points are suggested to minimize overall N2O emissions from nitritation systems.