Nitrous oxide (N O) is emitted during microbiological nitrogen (N) conversion processes, when N O production exceeds N O consumption. The magnitude of N O production vs. consumption varies with pH and controlling net N O production might be feasible by choice of system pH. This article reviews how pH affects enzymes, pathways and microorganisms that are involved in N-conversions in water engineering applications. At a molecular level, pH affects activity of cofactors and structural elements of relevant enzymes by protonation or deprotonation of amino acid residues or solvent ligands, thus causing steric changes in catalytic sites or proton/electron transfer routes that alter the enzymes' overall activity. Augmenting molecular information with, e.g., nitritation or denitrification rates yields explanations of changes in net N O production with pH. Ammonia oxidizing bacteria are of highest relevance for N O production, while heterotrophic denitrifiers are relevant for N O consumption at pH > 7.5. Net N O production in N-cycling water engineering systems is predicted to display a 'bell-shaped' curve in the range of pH 6.0-9.0 with a maximum at pH 7.0-7.5. Net N O production at acidic pH is dominated by N O production, whereas N O consumption can outweigh production at alkaline pH. Thus, pH 8.0 may be a favourable pH set-point for water treatment applications regarding net N O production.

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 HNO2 reduction 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 HNO2 indicating that HNO2 instead 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.

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