Abiotic Nitrous Oxide (NO) Production Is Strongly pH Dependent, but Contributes Little to Overall NO Emissions in Biological Nitrogen Removal Systems.

Hydroxylamine (NHOH) and nitrite (NO), intermediates during the nitritation process, can engage in chemical (abiotic) reactions that lead to nitrous oxide (NO) 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 NO production rates were measured from NHOH reaction with HNO, followed by HNO reduction by Fe, NHOH oxidation by Fe, and finally NHOH disproportionation plus oxidation by O. Compared to other examined factors, pH had the strongest effect on NO formation rates. Acidic pH enhanced NO production from the reaction of NHOH with HNO indicating that HNO instead of NO was the reactant. In departure from previous studies, we estimate that abiotic NO production contributes little (< 3% of total NO production) to total NO 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 NO production pathways, circumneutral pH set-points are suggested to minimize overall NO emissions from nitritation systems.