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N2O measurements by liquid sensors in aerated tanks are an input to gas-liquid mass-transfer models for the prediction of N2O off-gas emissions. The prediction of N2O emissions from Water Resource Recovery Facilities (WRRFs) was evaluated by three different mass-transfer models using Benchmark Simulation Model 1 (BSM1) as a reference model. Inappropriate selection of mass-transfer model may result in miscalculation of carbon footprints based on soluble N2O online measurements. The film theory considers a constant mass-transfer expression, while more complex models suggest that emissions are affected by the aeration type, efficiency, and tank design characteristics. The differences among model predictions were 10–16% at dissolved oxygen (DO) concentration of 0.6 g/m3, when biological N2O production was the highest, while the flux of N2O was 20.0–24 kg N2O-N/d. At lower DO, the nitrification rate was low, while at DO higher than 2 g/m3, the N2O production was reduced leading to higher rates of complete nitrification and a flux of 5 kg N2O-N/d. The differences increased to 14–26% in deeper tanks, due to the pressure assumed in the tanks. The predicted emissions are also affected by the aeration efficiency when KLaN2O depends on the airflow instead of the KLaO2. Increasing the nitrogen loading rate under DO concentration of 0.50–0.65 g/m3 increased the differences in predictions by 10–20% in both alpha 0.6 and 1.2. A sensitivity analysis indicated that the selection of different mass-transfer models did not affect the selection of biochemical parameters for N2O model calibration. [ABSTRACT FROM AUTHOR]

Summary: Nitrous oxide (N2O) is emitted during microbiological nitrogen (N) conversion processes, when N2O production exceeds N2O consumption. The magnitude of N2O production vs. consumption varies with pH and controlling net N2O 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 N2O production with pH. Ammonia oxidizing bacteria are of highest relevance for N2O production, while heterotrophic denitrifiers are relevant for N2O consumption at pH > 7.5. Net N2O 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 N2O production at acidic pH is dominated by N2O production, whereas N2O consumption can outweigh production at alkaline pH. Thus, pH 8.0 may be a favourable pH set‐point for water treatment applications regarding net N2O production. [ABSTRACT FROM AUTHOR]

ABSTRACT Nitrous oxide (N2O), a by-product of biological nitrogen removal during wastewater treatment, is produced by ammonia-oxidizing bacteria (AOB) and heterotrophic denitrifying bacteria (HB). Mathematical models are used to predict N2O emissions, often including AOB as the main N2O producer. Several model structures have been proposed without consensus calibration procedures. Here, we present a new experimental design that was used to calibrate AOB-driven N2O dynamics of a mixed culture. Even though AOB activity was favoured with respect to HB, oxygen uptake rates indicated HB activity. Hence, rigorous experimental design for calibration of autotrophic N2O production from mixed cultures is essential. The proposed N2O production pathways were examined using five alternative process models confronted with experimental data inferred. Individually, the autotrophic and heterotrophic denitrification pathway could describe the observed data. In the best-fit model, which combined two denitrification pathways, the heterotrophic was stronger than the autotrophic contribution to N2O production. Importantly, the individual contribution of autotrophic and heterotrophic to the total N2O pool could not be unambiguously elucidated solely based on bulk N2O measurements. Data on NO would increase the practical identifiability of N2O production pathways. Biotechnol. Bioeng. 2017;114: 132-140. © 2016 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

N 2 O measurements by liquid sensors in aerated tanks are an input to gas-liquid mass-transfer models for the prediction of N 2 O off-gas emissions. The prediction of N 2 O emissions from Water Resource Recovery Facilities (WRRFs) was evaluated by three different mass-transfer models using Benchmark Simulation Model 1 (BSM1) as a reference model. Inappropriate selection of mass-transfer model may result in miscalculation of carbon footprints based on soluble N 2 O online measurements. The film theory considers a constant mass-transfer expression, while more complex models suggest that emissions are affected by the aeration type, efficiency, and tank design characteristics. The differences among model predictions were 10-16% at dissolved oxygen (DO) concentration of 0.6 g/m 3 , when biological N 2 O production was the highest, while the flux of N 2 O was 20.0-24 kg N 2 O-N/d. At lower DO, the nitrification rate was low, while at DO higher than 2 g/m 3 , the N 2 O production was reduced leading to higher rates of complete nitrification and a flux of 5 kg N 2 O-N/d. The differences increased to 14-26% in deeper tanks, due to the pressure assumed in the tanks. The predicted emissions are also affected by the aeration efficiency when K L a N 2 O depends on the airflow instead of the K L a O2 . Increasing the nitrogen loading rate under DO concentration of 0.50-0.65 g/m 3 increased the differences in predictions by 10-20% in both alpha 0.6 and 1.2. A sensitivity analysis indicated that the selection of different mass-transfer models did not affect the selection of biochemical parameters for N 2 O model calibration.

Nitrous oxide (N 2 O) is a potent greenhouse gas emitted during biological treatment of residual waters and can contribute significantly to the carbon footprint of the overall treatment, potentially offsetting energy-positive strategies. N 2 O production is mediated by three known biological pathways and through abiotic reactions, driven by biologically generated substances such as hydroxylamine and nitrite. The contributions of these different mechanism are determined by the environmental conditions and the resident microbial community. The newly discovered phenotypic diversity among aerobic ammonia oxidizers and the modularity of denitrifying pathway determines N 2 O emissions. Isotopic methods can be used to quantify N 2 O production pathways in water treatment systems, and mechanistic models can already predict N 2 O emissions, but limitations on their accuracy and precision still exist., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

Hydroxylamine (NH 2 OH) and nitrite (NO 2 - ), intermediates during the nitritation process, can engage in chemical (abiotic) reactions that lead to nitrous oxide (N 2 O) 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 N 2 O production rates were measured from NH 2 OH reaction with HNO 2 , followed by HNO 2 reduction by Fe 2+ , NH 2 OH oxidation by Fe 3+ , and finally NH 2 OH disproportionation plus oxidation by O 2 . Compared to other examined factors, pH had the strongest effect on N 2 O formation rates. Acidic pH enhanced N 2 O production from the reaction of NH 2 OH with HNO 2 indicating that HNO 2 instead of NO 2 - was the reactant. In departure from previous studies, we estimate that abiotic N 2 O production contributes little (< 3% of total N 2 O production) to total N 2 O 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 N 2 O production pathways, circumneutral pH set-points are suggested to minimize overall N 2 O emissions from nitritation systems.

Nitrous oxide (N 2 O) is emitted during microbiological nitrogen (N) conversion processes, when N 2 O production exceeds N 2 O consumption. The magnitude of N 2 O production vs. consumption varies with pH and controlling net N 2 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 2 O production with pH. Ammonia oxidizing bacteria are of highest relevance for N 2 O production, while heterotrophic denitrifiers are relevant for N 2 O consumption at pH > 7.5. Net N 2 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 2 O production at acidic pH is dominated by N 2 O production, whereas N 2 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 2 O production., (© 2018 Society for Applied Microbiology and John Wiley & Sons Ltd.)