Mathematical modeling of nitrous oxide production in an anaerobic/oxic/anoxic process

Understanding nitrogen removal and N2O emission mechanisms in an anaerobic-swing-anoxic-oxic (ASAO) continuous plug-flow system for low C/N municipal wastewater

This study aims to investigate effects of dissolved oxygen (DO) levels and aerated hydrodynamic retention time (HRT) on nitrogen removal and nitrous oxide (N2O) emissions in a novel anaerobic-swing-anoxic-oxic (ASAO) continuous plug-flow system for treating low carbon to nitrogen ratio municipal wastewater. The swing zones had varying DO levels and volumes, deciding the aerated HRT of the ASAO system. Results showed that low DO level (0.8-1.0 mg/L) and short aerated HRT led to high nitrogen removal performance (91.4 %-96.3 %) and low N2O emission factor (2.8 %). The simultaneous nitrification and denitrification (SND) in swing zones and endogenous denitrification in anoxic zones contributed to the nitrogen removal. Meanwhile, the SND and autotrophic denitrification processes were identified as the N2O sources. Low DO level enriched ammonia-oxidizing bacteria and enhanced the SND and autotrophic denitrification pathway. These findings suggest that the ASAO system is promising for reducing carbon emissions in municipal wastewater treatment.

Improving prediction of N2O emissions during composting using model-agnostic meta-learning

Nitrous oxide (N2O) represents a significant environmental challenge as a harmful, long-lived greenhouse gas that contributes to the depletion of stratospheric ozone and exacerbates global anthropogenic greenhouse warming. Composting is considered a promising and economically feasible strategy for the treatment of organic waste. However, recent research indicates that composting is a source of N2O, contributing to atmospheric pollution and greenhouse effect. Consequently, there is a need for the development of effective, cost-efficient methodologies to quantify N2O emissions accurately. In this study, we employed the model-agnostic meta-learning (MAML) method to improve the performance of N2O emissions prediction during manure composting. The highest R2 and lowest root mean squared error (RMSE) values achieved were 0.939 and 18.42 mg d-1, respectively. Five machine learning methods including the backpropagation neural network, extreme learning machine, integrated machine learning method based on ELM and random forest, gradient boosting decision tree, and extreme gradient boosting were adopted for comparison to further demonstrate the effectiveness of the MAML prediction model. Feature analysis showed that moisture content of structure material and ammonium concentration during composting process were the two most significant features affecting N2O emissions. This study serves as proof of the application of MAML during N2O emissions prediction, further giving new insights into the effects of manure material properties and composting process data on N2O emissions. This approach helps determining the strategies for mitigating N2O emissions.

Tracing and utilizing nitrogen loss in wastewater treatment: The trade-off between performance improvement, energy saving, and carbon footprint reduction

Biological nitrogen removal is widely applied to reduce the discharge of inorganic nitrogen and mitigate the eutrophication of receiving water. However, nitrogen loss is frequently observed in wastewater treatment systems, yet the underlying principle and potential enlightenment is still lacking a comprehensive discussion. With the development and application of novel biological technologies, there are increasing achievement in the deep understanding and mechanisms of nitrogen loss processes. This article reviews the potential and novel pathways of nitrogen loss, occurrence mechanisms, influential factors, and control strategies. A survey of recent literature showed that 3%∼73% of nitrogen loss beyond the nitrogen budget can be ascribed to the unintentional presence of simultaneous nitrification/denitrification, partial nitrification/anammox, and endogenous denitrification processes, under low dissolved oxygen (DO) and limited available organic carbon source at aerobic conditions. Key influential parameters, including DO, aeration strategies, solid retention time (SRT), hydraulic retention time (HRT), temperature and pH, significantly affect both the potential pathways of nitrogen loss and its quantitative contribution. Notably, the widespread and spontaneous growth of anammox bacteria is an important reason for ammonia escape at anaerobic/anoxic conditions, leading to 7%∼78% of nitrogen loss through anammox pathway. Moreover, the unwanted nitrous oxide (N2O) emission should also be considered as a key pathway in nitrogen loss. Future development of new nitrogen removal technologies is proposed to suppress the generation of harmful nitrogen losses and reduce the carbon footprint of wastewater treatment by controlling key influential parameters. Transforming "unintentional observation" to "intentional action" as high-efficiency and energy-efficient nitrogen removal process provides a new approach for the development of wastewater treatment.

Nitrous oxide emissions in novel wastewater treatment processes: A comprehensive review

The proliferation of novel wastewater treatment processes has marked recent years, becoming particularly pertinent in light of the strive for carbon neutrality. One area of growing attention within this context is nitrous oxide (N2O) production and emission. This review provides a comprehensive overview of recent research progress on N2O emissions associated with novel wastewater treatment processes, including Anammox, Partial Nitrification, Partial Denitrification, Comammox, Denitrifying Phosphorus Removal, Sulfur-driven Autotrophic Denitrification and n-DAMO. The advantages and challenges of these processes are thoroughly examined, and various mitigation strategies are proposed. An interesting angle that delve into is the potential of endogenous denitrification to act as an N2O sink. Furthermore, the review discusses the potential applications and rationale for novel Anammox-based processes to reduce N2O emissions. The aim is to inform future technology research in this area. Overall, this review aims to shed light on these emerging technologies while encouraging further research and development.

Establishment of nitrous oxide (N2O) dynamics model based on ASM3 model during biological nitrogen removal via nitrification

Nitrous oxide (N₂O), as one of the six greenhouse gases, is mainly produced in the biological nitrogen removal process of wastewater treatment plants (WWTPs). Establishing the N₂O kinetic model can provide insight into the N₂O generation mechanism and regulate its production. This work uses Activated Sludge Model NO.3 (ASM3) as the basic framework, combines organic storage with endogenous respiration theory, and couples ammonia-oxidizing bacteria (AOB) denitrification pathway and the NH₂OH/NOH model to establish a kinetic model. Meanwhile, the Sequencing Batch Reactor (SBR) process with artificial simulated urban domestic sewage was used as the carrier; MATLAB and EXCEL software were used as tools to establish a model calculation programme. The simulated values obtained by substituting the operating conditions of the SBR process into the model and the measured values of the SBR process were analysed. The correlation coefficient (R²) between the experimental values and simulated values obtained for the 5 components of COD, ammonia, nitrite, nitrate and total N₂O is 0.952, 0.996, 0.902, 0.991 and 0.956, respectively, which indicates that the N₂O kinetic model has great consistency, this further shows that the established model modelling mechanism is clear and accurate, and provides a new method for the N₂O dynamic model.

The role of hydroxylamine in promoting conversion from complete nitrification to partial nitrification: NO toxicity inhibition and its characteristics

This study investigated a strategy for hydroxylamine (NH₂OH) addition for promoting the conversion of complete nitrification to partial nitrification in a sequencing batch reactor (SBR). The results showed that continuous dosing of 5 mg-N/L NH₂OH into a complete nitrification reactor for 16 days led to an increase in the nitrite accumulation ratio (NAR) from 0.22% to 95.08% and a significant enhancement in the accumulation of NO and N₂O in the liquid. The maximum concentration of NO in each cycle rose with the increase of NAR during NH₂OH addition. With the stopping of NH₂OH addition, the partial nitrification disappeared progressively in 21 days. The analysis for microbial community showed that Nitrospira was the main NOB and its relative abundance decreased with NH₂OH addition and recovered after the cessation of NH₂OH addition. Accordingly, NH₂OH has a significant and reversible inhibition on Nitrospira and its essence might be related to NO toxicity.

Significant N2O emission from a high rate granular reactor for completely autotrophic nitrogen removal over nitrite

Expanded granular sludge bed (EGSB) reactors were rarely applied for complete ammonium removal over nitrite. In this study, a high ammonium loading rate of 3677 mg N/L/d was achieved in an EGSB reactor. Approximately 5.5-8.5% of influent ammonium was converted to nitrous oxide (N₂O) that is a potent greenhouse gas. Moreover, the percentage increased linearly with the increase in ammonium load. A model well matched the reactor dynamics. The model indicated that hydroxylamine (NH₂OH) oxidation contributed to over 40% of produced N₂O, and denitrification by ammonium oxidizing bacteria contributed to N₂O emission significantly. Furthermore, the model suggests that a low oxygen concentration can result in a low N₂O emission at the cost of a slightly low ammonium removal rate while influent organic matter play a minor role in reducing N₂O emission. This study shows that EGSB reactors are effective in ammonium removal. In addition, the emission of N₂O is significant.

A comprehensive model of N2O emissions in an anaerobic/oxygen-limited aerobic process under dynamic conditions

A comprehensive model for nitrous oxide (N₂O) emissions in an anaerobic/oxygen-limited aerobic (A/OLA) process is proposed here. This paper includes the following main innovations: (i) adding the phosphorus-accumulating organism (X_PAO) denitrification pathway to the contribution of N₂O emissions; (ii) considering the biological removal of organic matter and phosphorus and predicting the effect of influent phosphorus concentration on N₂O emissions via an increase in the influent phosphorus concentration; and (iii) determining the effect of X_PAO on N₂O production in a simultaneous nitrification, denitrification and phosphorus removal (SNDPR) system by sensitivity analysis. The results suggested that the simulated data matched the measured data well. The predominant pathways of N₂O emissions in the process of A/OLA were the ammonium-oxidizing bacterium (X_AOB) denitrification pathway and the heterotrophic bacterium (X_H) denitrification pathway, while the incomplete hydroxylamine (NH₂OH) oxidation pathway and the X_PAO denitrification pathway contributed less to N₂O emissions. The metabolic activity of X_PAO had a significant effect on N₂O emissions, and increasing the influent phosphorus concentration was beneficial for reducing the release of N₂O. This study is expected to provide a meaningful reference for reducing N₂O emissions in wastewater treatment engineering.

Syntrophic Growth of Geobacter sulfurreducens Accelerates Anaerobic Denitrification

Nitrate is considered as a contamination since it's over discharging to water incurs environmental problems. However, nitrate is an ideal electron sink for anaerobic pollutant degraders desiring electron acceptors due to the high redox potential. Unfortunately, not all degraders can directly reduce nitrate, and the anaerobic direct interspecies electron transfer (DIET) between degraders and denitrifiers has not been confirmed yet. Here we demonstrated that syntrophic growth of Geobacter sulfurreducens PCA with denitrifying microbial community at anaerobic condition eliminated the lag phase of 15 h and improved the denitrification rate by 13∼51% over a broad C/N ratio of 0.5 to 9. Quantitative PCR revealed that G. sulfurreducens selectively enhanced the expression of nirS coding for a cytochrome cd1-nitrite reductase, resulting in a fast and more complete denitrification. Geobacter also selectively enriched its potential denitrifying partners - Diaphorobacter, Delftia, and Shinella - to form spherical aggregates. More studies of the binary culture system need to be carried out to confirm the syntrophic mechanism of Geobacter and denitrifiers in the future. These findings extend our knowledge on understanding the anaerobic bacterial interspecies electron transfer in the denitrification process, which has broader implications in fast selection and stabilization of denitrifiers in wastewater treatment plant, and general understanding of ecology for nitrogen and metal cycling.

Characteristics of N2O production and hydroxylamine variation in short-cut nitrification SBR process

In order to study the characteristics of nitrous oxide (N₂O) production and hydroxylamine (NH₂OH) variation under oxic conditions, concentrations of NH₂OH and N₂O were simultaneously monitored in a short-cut nitrification sequencing batch reactor (SBR) operated with different influent ammonia concentrations. In the short-cut nitrification process, N₂O production was increased with the increasing of ammonia concentration in influent. The maximum concentrations of dissolved N₂O-N in the reactor were 0.11 mg/L and 0.52 mg/L when ammonia concentrations in the influent were 50 mg/L and 70 mg/L respectively. Under the low and medium ammonia load phases, the concentrations of NH₂OH-N in the reactor were remained at a low level which fluctuated around 0.06 mg/L in a small range, and did not change with the variation of influent NH₄⁺-N concentration. Based on the determination results, the half-saturation of NH₂OH in the biochemical conversion process of NH₂OH to NO₂^--N was very small, and the value of 0.05 mg NH₂OH-N/L proposed in the published literature was accurate. NH₂OH is an important intermediate in the nitrification process, and the direct determination of NH₂OH in the nitrification process was beneficial for revealing the kinetic process of NH₂OH production and consumption as well as the effects of NH₂OH on N₂O production in the nitrification process.

Calibration of the comprehensive NDHA-N2O dynamics model for nitrifier-enriched biomass using targeted respirometric assays

The NDHA model comprehensively describes nitrous oxide (N₂O) producing pathways by both autotrophic ammonium oxidizing and heterotrophic bacteria. The model was calibrated via a set of targeted extant respirometric assays using enriched nitrifying biomass from a lab-scale reactor. Biomass response to ammonium, hydroxylamine, nitrite and N₂O additions under aerobic and anaerobic conditions were tracked with continuous measurement of dissolved oxygen (DO) and N₂O. The sequential addition of substrate pulses allowed the isolation of oxygen-consuming processes. The parameters to be estimated were determined by the information content of the datasets using identifiability analysis. Dynamic DO profiles were used to calibrate five parameters corresponding to endogenous, nitrite oxidation and ammonium oxidation processes. The subsequent N₂O calibration was not significantly affected by the uncertainty propagated from the DO calibration because of the high accuracy of the estimates. Five parameters describing the individual contribution of three biological N₂O pathways were estimated accurately (variance/mean < 10% for all estimated parameters). The NDHA model response was evaluated with statistical metrics (F-test, autocorrelation function). The 95% confidence intervals of DO and N₂O predictions based on the uncertainty obtained during calibration are studied for the first time. The measured data fall within the 95% confidence interval of the predictions, indicating a good model description. Overall, accurate parameter estimation and identifiability analysis of ammonium removal significantly decreases the uncertainty propagated to N₂O production, which is expected to benefit N₂O model discrimination studies and reliable full scale applications.

Effects of influent COD/N ratios on nitrous oxide emission in a sequencing biofilm batch reactor for simultaneous nitrogen and phosphorus removal

The characteristics of N₂O emissions from an anaerobic/aerobic/anoxic (A/O/A) sequencing biofilm batch reactor (SBBR) were investigated under different influent COD/nitrogen (C/N) ratios (from 1-4). Results indicated that the C/N ratios affected the quantity of polyhydroxybutyrate (PHB) and residual organic substances after the anaerobic period, resulting in the largest N₂O emission during aerobic period occurred at a C/N of 2. Moreover, during the anoxic PHB-driven denitrification period, the rapid decline in the dissolved N₂O concentration indicated that the nitrite inhibition threshold for N₂O reduction increased with the increased C/N ratios, which means the higher influent C/N ratios could lower the inhibition of nitrite on N₂O reduction. Finally, more PHB and residual organic substances were provided to denitrification at a high C/N ratio, resulting in less total N₂O emission was achieved at a high C/N ratio in the A/O/A SBBR.

Strategies for enhanced deammonification performance and reduced nitrous oxide emissions

Deammonification's performance and associated nitrous oxide emissions (N₂O) depend on operational conditions. While studies have investigated factors for high performances and low emissions separately, this study investigated optimizing deammonification performance while simultaneously reducing N₂O emissions. Using a design of experiment (DoE) method, two models were developed for the prediction of the nitrogen removal rate and N₂O emissions during single-stage deammonification considering three operational factors (i.e., pH value, feeding and aeration strategy). The emission factor varied between 0.7±0.5% and 4.1±1.2% at different DoE-conditions. The nitrogen removal rate was predicted to be maximized at settings of pH 7.46, intermittent feeding and aeration. Conversely, emissions were predicted to be minimized at the design edges at pH 7.80, single feeding, and continuous aeration. Results suggested a weak positive correlation between the nitrogen removal rate and N₂O emissions, thus, a single optimizing operational set-point for maximized performance and minimized emissions did not exist.