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Nitrous oxide (N 2 O) can be emitted from wastewater treatment contributing to its greenhouse gas footprint significantly. Mathematical modeling of N 2 O emissions is of great importance toward the understanding and reduction of the environmental impact of wastewater treatment systems. This article reviews the current status of the modeling of N 2 O emissions from wastewater treatment. The existing mathematical models describing all the known microbial pathways for N 2 O production are reviewed and discussed. These included N 2 O production by ammonia-oxidizing bacteria (AOB) through the hydroxylamine oxidation pathway and the AOB denitrification pathway, N 2 O production by heterotrophic denitrifiers through the denitrification pathway, and the integration of these pathways in single N 2 O models. The calibration and validation of these models using lab-scale and full-scale experimental data is also reviewed. We conclude that the mathematical modeling of N 2 O production, while is still being enhanced supported by new knowledge development, has reached a maturity that facilitates the estimation of site-specific N 2 O emissions and the development of mitigation strategies for a wastewater treatment plant taking into the specific design and operational conditions of the plant. [ABSTRACT FROM AUTHOR]

Nitrous oxide (N2O) emissions from wastewater treatment plants vary substantially between plants, ranging from negligible to substantial (a few per cent of the total nitrogen load), probably because of different designs and operational conditions. In general, plants that achieve high levels of nitrogen removal emit less N2O, indicating that no compromise is required between high water quality and lower N2O emissions. N2O emissions primarily occur in aerated zones/compartments/periods owing to active stripping, and ammonia-oxidizing bacteria, rather than heterotrophic denitrifiers, are the main contributors. However, the detailed mechanisms remain to be fully elucidated, despite strong evidence suggesting that both nitrifier denitrification and the chemical breakdown of intermediates of hydroxylamine oxidation are probably involved. With increased understanding of the fundamental reactions responsible for N2O production in wastewater treatment systems and the conditions that stimulate their occurrence, reduction of N2O emissions from wastewater treatment systems through improved plant design and operation will be achieved in the near future. [ABSTRACT FROM AUTHOR]

Abstract: Ammonia-oxidising bacteria (AOB) are a major contributor to nitrous oxide (N2O) emissions during nitrogen transformation. N2O production was observed under both anoxic and aerobic conditions in a lab-scale partial nitritation system operated as a sequencing batch reactor (SBR). The system achieved 55±5% conversion of the 1g NH4 +-N/L contained in a synthetic anaerobic digester liquor to nitrite. The N2O emission factor was 1.0±0.1% of the ammonium converted. pH was shown to have a major impact on the N2O production rate of the AOB enriched culture. In the investigated pH range of 6.0–8.5, the specific N2O production was the lowest between pH 6.0 and 7.0 at a rate of 0.15±0.01mgN2O-N/h/g VSS, but increased with pH to a maximum of 0.53±0.04mgN2O-N/h/g VSS at pH 8.0. The same trend was also observed for the specific ammonium oxidation rate (AOR) with the maximum AOR reached at pH 8.0. A linear relationship between the N2O production rate and AOR was observed suggesting that increased ammonium oxidation activity may have promoted N2O production. The N2O production rate was constant across free ammonia (FA) and free nitrous acid (FNA) concentrations of 5–78mg NH3-N/L and 0.15–4.6mg HNO2-N/L, respectively, indicating that the observed pH effect was not due to changes in FA or FNA concentrations. [Copyright &y& Elsevier]

Nitrous oxide (N 2 O) is an important greenhouse gas and an ozone-depleting substance which can be emitted from wastewater treatment systems (WWTS) causing significant environmental impacts. Understanding the N 2 O production pathways and their contribution to total emissions is the key to effective mitigation. Isotope technology is a promising method that has been applied to WWTS for quantifying the N 2 O production pathways. Within the scope of WWTS, this article reviews the current status of different isotope approaches, including both natural abundance and labelled isotope approaches, to N 2 O production pathways quantification. It identifies the limitations and potential problems with these approaches, as well as improvement opportunities. We conclude that, while the capabilities of isotope technology have been largely recognized, the quantification of N 2 O production pathways with isotope technology in WWTS require further improvement, particularly in relation to its accuracy and reliability. [ABSTRACT FROM AUTHOR]

• Significant loss of total nitrogen is observed in an acidic aerobic sludge digester. • Nitrite is mainly reduced to NO and N 2 O under the acidic aerobic condition. • Rate of aerobic nitrite reduction is comparable to that of aerobic ammonia oxidation. • Controlling the nitrite reduction is critical to development of acidic sludge treatment. The acidic (i.e., pH ∼5) activated sludge process is attracting attention because it enables stable nitrite accumulation and enhances sludge reduction and stabilization, compared to the conventional process at neutral pH. Here, this study examined the production and potential pathways of nitric oxide (NO) and nitrous oxide (N 2 O) during acidic sludge digestion. With continuous operation of a laboratory-scale aerobic digester at high dissolved oxygen concentration (DO>4 mg O 2 L−1) and low pH (4.7±0.6), a significant amount of total nitrogen (TN) loss (i.e., 18.6±1.5% of TN in feed sludge) was detected. Notably, ∼40% of the removed TN was emitted as NO, with ∼8% as N 2 O. A series of batch assays were then designed to explain the observed TN loss under aerobic conditions. All assays were conducted with a low concentration of volatile solids (VS), i.e., VS<4.5 g L−1. This VS concentration is commensurate with the values commonly found in the aeration tanks of full-scale wastewater treatment systems, and thus no significant nitrogen loss should be expected when DO is controlled above 4 mg O 2 L−1. However, nitrite disappeared at a significant rate (with the chemical decomposition of nitrite excluded), leading to NO production in the batch assays at pH 5. The nitrite reduction could be associated with endogenous microbial activities, e.g., nitrite detoxification. The significant NO production illustrates the importance of aerobic nitrite reduction during acidic aerobic sludge digestion, suggesting this process cannot be neglected in developing acidic activated sludge technology. [Display omitted] [ABSTRACT FROM AUTHOR]

Several mathematical models have been proposed to describe nitrous oxide (N 2 O) production by ammonia oxidizing bacteria (AOB) under varying operational conditions. However, none of these N 2 O models are able to capture N 2 O dynamics caused by the variation of inorganic carbon (IC) concentration, which has recently been demonstrated to be a significant factor influencing N 2 O production by AOB. In this work, a mathematical model that describes the effect of IC on N 2 O production by AOB is developed and experimentally validated. The IC effect is considered by explicitly including the AOB anabolic process in the model, which is coupled to the catabolic process with the use of the Adenosine triphosphate (ATP) and Adenosine diphosphate (ADP) pools. The calibration and validation of the model were conducted using experimental data obtained with two independent cultures, including a full nitrification culture and a partial nitritation culture. The model satisfactorily describes the N 2 O data from both systems at varying IC concentrations. This new model enhances our ability to predict N 2 O production by AOB in wastewater treatment systems under varying IC conditions. [ABSTRACT FROM AUTHOR]

The effect of inorganic carbon (IC) on nitrous oxide (N 2 O) production by ammonia oxidizing bacteria (AOB) was investigated over a concentration range of 0–12 mmol C/L, encompassing typical IC levels in a wastewater treatment reactors. The AOB culture was enriched along with nitrite-oxidizing bacteria (NOB) in a sequencing batch reactor (SBR) to perform complete nitrification. Batch experiments were conducted with continuous carbon dioxide (CO 2 ) stripping or at controlled IC concentrations. The results revealed a linear relationship between N 2 O production rate (N 2 OR) and IC concentration ( R 2 = 0.97) within the IC range studied, suggesting a substantial effect of IC on N 2 O production by AOB. Similar results were also obtained with an AOB culture treating anaerobic sludge digestion liquor. The fundamental mechanism responsible for this dependency is unclear; however, in agreement with previous studies, it was observed that the ammonia oxidation rate (AOR) was also influenced by the IC concentration, which could be well described by the Monod kinetics. These resulted in an exponential relationship between N 2 OR and AOR, as previously observed in experiments where AOR was altered by varying dissolved oxygen and ammonia concentrations. It is therefore possible that IC indirectly affected N 2 OR by causing a change in AOR. The observation in this study indicates that alkalinity (mostly contributed by IC) could be a significant factor influencing N 2 O production and should be taken into consideration in estimating and mitigating N 2 O emissions in wastewater treatment systems. [ABSTRACT FROM AUTHOR]

Autotrophic ammonia oxidizing bacteria (AOB) have been recognized as a major contributor to N2O production in wastewater treatment systems. However, so far N2O models have been proposed based on a single N2O production pathway by AOB, and there is still a lack of effective approach for the integration of these models. In this work, an integrated mathematical model that considers multiple production pathways is developed to describe N2O production by AOB. The pathways considered include the nitrifier denitrification pathway (N2O as the final product of AOB denitrification with NO2- as the terminal electron acceptor) and the hydroxylamine (NH2OH) pathway (N2O as a byproduct of incomplete oxidation of NH2OH to NO2-). In this model, the oxidation and reduction processes are modeled separately, with intracellular electron carriers introduced to link the two types of processes. The model is calibrated and validated using experimental data obtained with two independent nitrifying cultures. The model satisfactorily describes the N2O data from both systems. The model also predicts shifts of the dominating pathway at various dissolved oxygen (DO) and nitrite levels, consistent with previous hypotheses. This unified model is expected to enhance our ability to predict N2O production by AOB in wastewater treatment systems under varying operational conditions. [ABSTRACT FROM AUTHOR]

Abstract: The quantification of nitrous oxide (N2O) emissions from open-surface wastewater treatment systems with surface aerators is difficult as emissions from the surface aerator zone cannot be easily captured by floating hoods. In this study, we propose and demonstrate a novel methodology to estimate N2O emissions from such systems through determination of the N2O transfer coefficient (k L a) induced by surface aerators based on oxygen balance for the entire system. The methodology is demonstrated through its application to a full-scale open oxidation ditch wastewater treatment plant with surface aerators. The estimated k L a profile based on a month-long measurement campaign for oxygen balance, intensive monitoring of dissolved N2O profiles along the oxidation ditch over a period of four days, together with mathematical modelling, enabled to determine the N2O emission factor from this treatment plant (0.52 ± 0.16%). Majority of the N2O emission was found to occur in the surface aerator zone, which would be missed if the gas hood method was applied alone. [Copyright &y& Elsevier]

Abstract: Limited availability of carbon sources has been regarded as an important factor leading to N2O accumulation during denitrification in wastewater treatment. By varying the carbon (methanol) loading rate to a methanol utilizing denitrifying culture in the presence of various electron acceptors (nitrate, nitrite, N2O and their combinations), this study quantitatively investigated the electron distribution among different nitrogen oxide reductases during denitrification. The results showed that electron competition occurs under not only carbon limiting but also carbon abundant conditions. The electron distribution among the nitrogen oxide reductases is affected by the carbon loading rate, with a lower fraction of electrons distributed to the N2O reductase with reduced carbon loading rate. N2O accumulation occurs when the electron flux going to nitrite reduction is higher than that going to N2O reduction. The study also showed that, for the culture investigated, the carbon to nitrogen ratio is not a key factor leading to N2O accumulation. [Copyright &y& Elsevier]

• First reported N 2 O mitigation strategy permanently implemented at full-scale. • 35% N 2 O emission reduction was achieved with 20% less operational cost. • N 2 O mitigation did not affect nutrient removal performance. • Validated multi-pathway N 2 O model at full-scale to evaluate mitigation strategies. • Proposed a standard approach for mitigating N 2 O emissions at full-scale plants. Mitigation of nitrous oxide (N 2 O) emissions is of primary importance to meet the targets of reducing carbon footprints of wastewater treatment plants (WWTPs). Despite of a large amount of N 2 O mitigation studies conducted in laboratories, full-scale implementation of N 2 O mitigation is scarce, mainly due to uncertainties of mitigation effectiveness, validation of N 2 O mathematical model, risks to nutrient removal performance and additional costs. This study aims to address the uncertainties by investigating the quantification, development and implementation of N 2 O mitigation strategies at a full-scale sequencing batch reactor (SBR). To achieve this, N 2 O emission dynamics, nutrient removal performance and operation of the SBR were monitored to quantify N 2 O emissions, and identify the N 2 O generation mechanisms. N 2 O mitigation strategies centered on reducing dissolved oxygen (DO) levels were consequently proposed and evaluated using a multi-pathway N 2 O production mathematical model before implementation. The implemented mitigation strategy resulted in a 35% reduction in N 2 O emissions (from the emission factor of 0.89 ± 0.05 to 0.58 ± 0.06%), which was equivalent to annual reduction of 2.35 tonne of N 2 O from the studied WWTP. This could be mainly attributed to reductions in N 2 O generated via the NH 2 OH oxidation pathway due to the lowering of DO level. As the first reported mitigation strategy permanently implemented at a full scale WWTP, it showcased that the mitigation of N 2 O emissions at full-scale is feasible and that widely accepted N 2 O mitigation strategies developed in laboratory studies are also likely effective in full-scale plants. Furthermore, the close agreement between the validated and predicted N 2 O emission factors (0.58% vs 0.55%, respectively), showed that the N 2 O mathematical model is a useful tool to evaluate N 2 O mitigation strategies at full-scale. Importantly this work demonstrated that N 2 O mitigation does not necessarily require additional operational cost to meet reduction targets. In contrast, the N 2 O mitigation applied here reduced energy requirements for aeration by 20%. Equally important, long-term monitoring identified that N 2 O mitigation did not affect the nutrient removal performance of the plant. Finally, with the knowledge acquired in this study, a standard approach for mitigating N 2 O emissions from full-scale treatment plants was proposed. Image, graphical abstract [ABSTRACT FROM AUTHOR]