A review on nitrous oxide (N2O) emissions during biological nutrient removal from municipal wastewater and sludge reject water

Quantifying, predicting, and mitigating nitrous oxide emissions in a full-scale partial nitritation/anammox reactor treating reject water

In this paper, a set of mathematical tools are developed and assembled to quantify, predict and virtually assess N2O emission mitigation strategies in partial nitritation (PN) / anammox (ANX) granular based reactors. The proposed approach is constructed upon a set of data pre-treatment methods, process simulation models, control tools (and algorithms) and key performance indicators to analyze, reproduce, and forecast the behavior of multiple operational variables within aerobic granular sludge systems. All these elements are tested on two full-scale data sets (#D1, #D2) collected over a period of four months (Sept-Dec 2023). Results show that data pretreatment is essential for noise reduction, filling data gaps, and ensuring smooth process simulations. The model accurately predicts (normalized RMSE< 1) multiple N oxidation states (NHx, NO2-, NO3-, N2O) and dissolved oxygen (DO), demonstrating its capability to describe bacterial behavior within the studied system. Special emphasis is placed on weak acid-base chemistry where pH is reliably reproduced, and it can be used for control purposes. Both biological and physico-chemical aspects are predicted at different time scales (months, days, minutes). While nitritation mainly occurred in the bulk, biofilm distribution showed inactive inner granule parts and increasing biomass (mostly ANX) towards the surface, with distinct organic concentrations. Gradients for multiple soluble compounds could also be reflected. Nitrifier denitrification (ND) is identified as the main N2O production pathway. The model revealed that the system was suffering from low ANX activity leading to NO2- accumulation. This in combination with low DO levels resulted in an unusually high emission factor (EF). The validation data set also yielded satisfactory results (normalized RMSE< 1). The scenario analysis revealed that modification of the operational parameters could improve the ANX activity and lead to N2O emission rates that are in line with what is normally expected from similar systems. The study includes a discussion on transitioning from process models to digital shadows/ twins for real-time process monitoring. Additionally, it emphasizes the necessity of evaluating reject water technologies from a plant-wide perspective.

Dynamically predicting nitrous oxide emissions in a full-scale industrial activated sludge reactor under multiple aeration patterns and COD/N ratios

The use of digital tools has become essential for quantifying and predicting greenhouse gas (GHG) emissions in urban wastewater treatment plants (WWTPs), enabling the development of operational regimes with a high probability of achieving net-zero targets. However, comprehensive studies documenting validation of model predictions-such as effluent quality, process economics, and emission factors-remain scarce within full-scale industrial settings. This paper aims to develop a decision support tool (DST) for (dynamically) predicting nitrous oxide (N2O) emissions in full-scale industrial activated sludge reactors (ASRs) and suggesting mitigation strategies. The DST, incorporating both biological and physico-chemical processes, was developed to address the unique characteristics of industrial wastewater. Specialized Gas-Liquid (G-L) mass transfer routines were also formulated to account for alternating anoxic and aerobic conditions in covered reactors. The proposed approach was validated using full-scale data collected at varying frequencies (from daily to minute intervals) during different campaigns at the largest industrial wastewater treatment system in Northern Europe. The DST was further tested across multiple aeration patterns and influent COD/N ratios. Results show that DST simulations can reproduce (daily) biological COD and nitrogen removal, sulfur transformations, and the physico-chemical precipitation of phosphorus with aluminum, achieving a deviation of 8.6 % over a six-week period. High-frequency (minute-level) dynamics for multiple nitrogen species (NHx, NO2-, NO3-, dissolved and gaseous N2O), dissolved oxygen (DO), and airflow were captured with a NRMSE of 0.16, 0.14 and 0.11 for three evaluated operational strategies (Baseline, Scenario #1 and #2), respectively. Both plant data and DST predictions indicate that the correlation (R2 up to 0.9) between emission factors (EFs) and influent COD/N ratios is significantly influenced by: i) oxygen supply dynamics (fast/slow) and ii) the duration of aeration periods. These EFs range from 0.2 % to 1.4 %. Analysis of derivatives identifies the denitrification (DEN) pathway as the primary contributor to N2O production, peaking at the anoxic phases, with the nitrifier-denitrification (ND) pathway contributing to a lesser extent at the end of aeration. Additionally, the DST generated response surfaces illustrating the key performance indicator (KPI) variations in EFs, nitrification capacity, effluent quality, and aeration energy consumption as functions of different aeration setpoints (DO and NO2-) across varying COD/N loads. The DST provided optimized strategies targeting those KPIs, which were successfully applied on site with improvements of most of the KPIs, achieving up to 71 % reductions of N2O emission (1.4 % to 0.4 %), potentially mitigating >15,000 tons CO2-e per year. These results demonstrate the DST's potential for broader applications in wastewater treatment processes.

Developing a simple activity test to estimate nitrous oxide production from sewage treatment

Developing an easy and precise method to estimate nitrous oxide (N2O) emissions from sewage treatment plants (STPs) is important for controlling greenhouse gas emissions. This study aimed to develop a simple N2O activity test to estimate the N2O profile in an STP. In addition to evaluating the stability of the developed test, the dissolved oxygen was measured in a gas-stripping-type N2O activity test to determine the experimental conditions under which N2O production could be estimated. When the developed method was applied in different STPs and compared with the actual generation quantity, the N2O production activity test demonstrated potential as a simple method for estimating the net N2O generation in the STP. Therefore, this study potentially provides an easy and effective method for quantifying N2O emissions from an STP.

Comprehensive assessment, intelligent prediction, and precise mitigation strategies for greenhouse gas emissions in full-scale wastewater treatment plants

Wastewater treatment plants (WWTPs) are major contributors to global anthropogenic greenhouse gas (GHG) emissions, with China ranks among the leading emitters. In the context of China's "dual-carbon" journey, precision quantification and predictive forecasting of GHG fluxes, particularly methane (CH4) and nitrous oxide (N2O)-are crucial for developing advanced mitigation strategies of WWTPs. To accurately assess GHG emissions, this study firstly introduced customized emission factors (EFs) to precisely evaluate the GHG emissions of a full - scale A2O - based WWTP in Beijing. This approach addressed the overestimation of emissions when using the IPCC's standard EFs. Additionally, the study proposed machine learning (ML) techniques to predict GHG fluxes based on routine wastewater quality parameters. Specifically, Long Short-Term Memory (LSTM) and Random Forest (RF) models showed the strong performance in predicting CH4 and N2O emissions, respectively. Moreover, our findings revealed distinct spatiotemporal patterns of GHG emission: CH4 emissions peak during the summer solstice, while N2O emissions rise during the winter months. For the first time, this study identified the nitrification biofilter in the advanced treatment unit as a significant direct source of N2O emissions. Eventhough, indirect CO2 emissions account for a dominant 57%-90% of the total GHG emissions. Scenario analyses revealed a strategic mitigation approach. Energy conservation emerged as the most effective measure, capable of reducing emissions by 23.41%, followed by heat recovery, which could cut emissions by 10.15%. In practical applications, improving energy efficiency is of utmost importance in real - world mitigation strategies. This highlights the significance of integrated approaches for achieving the sustainable development of WWTPs in the "dual - carbon" background.

Mechanism of nitrous oxide emission reduction in constructed wetlands based on plant harvesting management

The impact of plant harvesting on nitrous oxide (N2O) emission reduction in constructed wetlands (CWs) remains uncertain. This study focused on the Myriophyllum aquaticum wetland treating swine wastewater, with three different plant harvesting frequencies implemented: high harvesting (HF), low harvesting (LF), and no harvesting (CK). Results showed that compared to CK, cumulative N2O emissions decreased by 7.4 % (HF) and 18.5 % (LF), with only LF showing a significant reduction. The primary reductions in N2O emissions occurred during winter and summer, accounting for 26.2 %∼39.7 % and 14.0 %∼49.2 % of the total reductions, respectively. Crucial contributors to mitigating N2O emissions included water redox potential, sediment dissolved organic carbon, and ammonia nitrogen levels. Additionally, increased denitrification gene abundance in harvesting treatments reduced N2O emissions. These findings suggest that low-frequency plant harvesting, particularly in winter and summer, can significantly reduce N2O emissions from CWs.

Operational strategy for solid phase denitrification to achieve carbon balance between organic release and denitrification consumption

This study developed a pilot-scale vertical-baffled solid-phase denitrification reactor (VbSPDR) incorporating polycaprolactone (PCL) and ceramsite as fillers to balance organic carbon release and denitrification consumption. The Box-Behnken design was employed to assess the effects of hydraulic retention time (HRT), temperature, and influent nitrate concentration on nitrate removal efficiency and COD accumulation. Optimal conditions yielded a 95 % nitrate removal rate at 33 °C, an HRT of 1.22 h, and an influent nitrate concentration of 19 mg/L. Conversely, effluent COD concentration was minimized and dropped 2.5 mg/L at 13 °C, an HRT of 0.39 h, and an influent nitrate concentration of 19 mg/L. The PCL layer enriched hydrolysis-acidification and heterotrophic denitrifying bacteria by unclassified_f__Comamonadaceae and Acidovorax, while heterotrophic genera Phreatobacter thrived in ceramsite layer, enhancing the metabolism of COD over-released from PCL. These findings indicate that inorganic fillers can effectively enhance nitrate removal and controll effluent COD under varied operational parameters.

Advanced Technologies for Nitrogen Removal and Recovery from Municipal and Industrial Wastewater

Nitrogen pollution poses significant environmental challenges, contributing to eutrophication, soil acidification, and greenhouse gas emissions. This study explores advanced methods for nitrogen removal and recovery from municipal and industrial wastewater, with a focus on biological, chemical, and physical processes. Key processes, such as nitrification-denitrification and emerging technologies like shortcut nitrogen pathways, were analyzed for their efficiency, cost-effectiveness, and environmental benefits. This review highlights the integration of innovative techniques, including membrane systems and ammonia stripping, with traditional approaches to enhance nitrogen management. Emphasis is placed on optimizing operational conditions, such as pH, temperature, and carbon-to-nitrogen ratios, to achieve high removal rates while minimizing energy consumption and environmental impact. These findings underline the critical role of interdisciplinary strategies in addressing the challenges of nitrogen pollution and promoting sustainable wastewater management.

Effects of feeding and aeration strategies on N2O production and emission by an aerobic granular sludge system for municipal wastewater treatment

The effects of a single feeding cycle followed by a continuous aeration phase (ANDC) and a step-feeding cycle followed by intermittent aerobic/idle phases (ANDI) on the production and emission of nitrous oxide (N2O) from aerobic granular sludge (AGS) from real domestic sewage were studied. Higher N2O emissions were observed in the ANDI treatment, and 9.2 ± 4.1% of the influent TN was emitted as N2O, while in the ANDC treatment, 4.6 ± 2.5% of the influent TN was emitted as N2O. Both strategies were similar for carbon and total phosphorus removal; but ANDI was advantageous for ammonium nitrogen and total nitrogen removal. Regarding the microbial populations associated with N2O production, genera such as Thauera, a heterotrophic denitrifier, were found to have a relative abundance of 2.1% in ANDC and 3.8% in ANDI. Defluviccocus and Tetrasphaera, organisms capable of denitrification and phosphorus removal, especially the latter, were present in ANDC. Under ANDI conditions, these organisms may have been replaced by fast-growing organisms, such as Thauera. Principal component analysis (PCA) showed that incomplete denitrification was the dominant effect in the ANDC strategy. This may be related to the nitrate and phosphate concentrations and effluent characteristics (low C:N ratio). In the ANDI strategy, incomplete denitrification and low polyhydroxyalkanoate (PHA) consumption were the main effects. This is indicated by the high nitrite and phosphate concentrations. Therefore, according to the PCA results, the combination of the ANDC and ANDI strategies can play a crucial operational role in the dynamics of N2O production and emission, especially considering that real domestic wastewater was used in the present research.

The effect and mechanism of microplastics to the N2O emission in underground and aboveground wastewater treatment plants

This study investigated the effect and microbial mechanism of microplastics to the N2O emission in underground and aboveground wastewater treatment plants. The microplastics in the influent of Uwwtp and Awwtp were 4953 ± 558 and 2253 ± 563 particles/L. The microplastics rejection rate were 86.36 ± 0.02% (Awwtp) and 90.56 ± 0.02% (Uwwtp), respectively. The N2O concentration were 106.40 ± 134.17 nmol/L in Uwwtp and 53.58 ± 32.68 nmol/L in Awwtp. The N2O/NO3- ratio was 0.55% in the aerobic tank of Uwwtp. The N2O/NH4+ ratio was 0.39% in aerobic tank and 0.52% in secondary sedimentation tank of Awwtp. The microplastics were significantly correlated with NH4+ (p < 0.05) in Awwtp and significantly correlated with NH4+ (p < 0.01) and NO2-, NO3- (p < 0.01) in Uwwtps. Mental test results indicated that microplastics significantly correlated with the nitrifier and denitrifier in the systems. This suggest that microplastics could affect the nitrification and denitrification process in the two plants and thus affect the N2O emission. Microplastics in wwtps, i.e., the plastisphere, may be a novel microbial colonization site that could be vital to N2O emissions.

Utility of integrated papyrus-bivalve for bioremediation of aquaculture wastewater

Aquaculture generates substantial amount of residual feeds and faecal matter that accumulate in the culture environment and pollute effluent-receiving water, diminishing its ecological functioning. To devise means of treating nutrient-rich aquaculture wastewater, the efficiency of integrated papyrus-bivalve mesocosms in removing nutrients was evaluated. The mesocosms were fed on water (6600 L) from one brood-stock pond and allowed to settle for 2 weeks. Physico-chemical parameters, including nutrient level in influent and effluent of mesocosms, were analysed fortnightly using standard methods. The integrated papyrus-bivalve mesocosms had the highest removal of 69.5, 52.9, and 70.5% for total nitrogen (TN), total phosphorus (TP), and total dissolved solids (TDS), respectively, from the aquaculture wastewater. The removal efficiency of TN and TDS by the mesocosms followed the order: integrated papyrus-bivalve ˃ papyrus ˃ bivalves ˃ control, and removal efficiency for TP followed a similar trend, albeit integrated papyrus-bivalve≈papyrus. The nutrient retention (g/DMm2) in papyrus biomass was 24.09 and 2.93 in integrated papyrus-bivalve mesocosm and 20.77 and 3.81 in papyrus mesocosms for nitrogen and phosphorus, respectively. Meanwhile, the nutrient retention in bivalve biomass was 3.06 and 0.44 in integrated papyrus-bivalve and 1.95 and 0.34 in bivalve mesocosm, for nitrogen and phosphorus, respectively. The study confirms the synergistic functionality of papyrus and bivalves in treating aquaculture wastewater. Papyrus and bivalves in the mesocosms contributed to the bioremediation of wastewater through physical and biochemical processes, including filtration, nutrient uptake, and attachment surfaces for microorganisms. Therefore, we recommend integrated papyrus-bivalve bioremediation technology to fish farmers and policy makers.

Forecasting nitrous oxide emissions from a full-scale wastewater treatment plant using LSTM-based deep learning models

Nitrous oxide (N2O) emissions from wastewater treatment plants (WWTPs) exhibit significant seasonal variability, making accurate predictions with conventional biokinetic models difficult due to complex and poorly understood biochemical processes. This study addresses these challenges by exploring data-driven alternatives, using long short-term memory (LSTM) based encoder-decoder models as basis. The models were developed for future integration into a model predictive control framework, aiming to reduce N2O emissions by forecasting these over varying prediction horizons. The models were trained on 12 months and tested on 3 months of data from a full-scale WWTP in Amsterdam West, the Netherlands. The dataset encompasses seasonal peaks in N2O emissions typical for winter and spring months. The best performing model, featuring a 256-256 LSTM architecture, achieved the highest accuracy with test R2 values up to 0.98 across prediction horizons spanning 0.5 to 6.0 h ahead. Feature importance analysis identified past N2O emissions, influent flowrate, NH4+, NOx, and dissolved oxygen (DO) in the aerobic tank as most significant inputs. The observed decreasing influence of historical N2O emissions over extended prediction horizons highlights the importance and significance of process variables for the model's performance. The model's ability to accurately forecast short-term N2O emissions and capture immediate trends highlights its potential for operational use in controlling emissions in WWTPs. Further research incorporating diverse datasets and biochemical process inputs related to microbial activities in the N2O production pathways could improve the model's accuracy for longer forecasting horizons. These findings advocate for hybridising deep learning models with biokinetic and mechanistic insights to enhance prediction accuracy and interpretability.

Urbanization and greenhouse gas emissions from municipal wastewater in coastal provinces of China: Spatiotemporal patterns, driving factors, and mitigation strategies

Coastal cities, as hubs of social and economic activity, have witnessed rapid urbanization and population growth. This study explores the transformative changes in urban municipal wastewater treatment practices and their profound implications for greenhouse gas (GHG) emissions in Chinese coastal provinces. The approach employed in this study integrates comprehensive data analysis with statistical modeling to elucidate the complex interplay between urbanization, wastewater treatment practices, and GHG emissions. Results reveal a substantial surge in GHG emissions from coastal wastewater treatment, rising from 3367.1 Gg CO2e/yr in 1990-23644.8 Gg CO2e/yr in 2019. Spatially, the top 20 cities contribute 56.0% of emissions, with hotspots in the Bohai Sea Region, Yangtze River Delta, and Pearl River Delta. Initially dominated by emissions from untreated wastewater, post-2004, GHG emissions from treatment processes became the primary source, tied to electricity use. Growing population and urbanization rates escalated wastewater discharge, intensifying GHG emissions. From 1990 to 2019, average GHG intensity ranged between 320.5 and 676.6 g CO2e/m3 wastewater, with an annual increase of 12.3 g CO2e/m3. GHG intensity variations relate to the wastewater treatment rate, impacting CH4, N2O, and CO2 emissions, underscoring the need for targeted strategies to mitigate environmental impact.

Assessment and optimization of wastewater treatment plant in terms of effluent quality, energy footprint, and greenhouse gas emissions: An integrated modeling approach

Wastewater treatment plants (WWTPs) can benefit from utilizing digital technologies to reduce greenhouse gas (GHG) emissions and to comply with effluent quality standards. In this study, the GHG emissions and electricity consumption of a WWTP were evaluated via computer simulation by varying the dissolved oxygen (DO), mixed liquor recirculation (MLR), and return activated sludge (RAS) parameters. Three different measures, namely, effluent water quality, GHG emissions, and energy consumption, were combined as water-energy-carbon coupling index (WECCI) to compare the effects of the parameters on WWTPs, and the optimal operating condition was determined. The initial conditions of the A2O process were set to 4.0 mg/L of DO, 100 % MLR, and 90.7 % RAS. Eighty scenarios with various DO, MLR, and RAS were simulated under steady-state condition to optimize the biological treatment process. The optimal operating conditions were found to be 1.5 mg/L of DO, 190 % MLR, and 90.9 % RAS, which had the highest WECCI of 2.40 when compared to the WECCI of the initial condition (1.07). This optimal condition simultaneously reduced GHG emissions by 1348 kg CO2-eq/d and energy consumption by 11.64 MWh/d. This implies that controlling DO, MLR, and RAS through sensors, valves, and pumps offers a promising approach to operating WWTPs with reduced electricity consumption and GHG emissions while attaining effluent quality standards. Additionally, the nitrous oxide stripping rate exhibited linear relationships with the effluent total ammonia and nitrite concentrations in the aerobic reactor, suggesting that monitoring dissolved nitrogen compounds in the effluent and reactor could be a viable strategy to control MLR and DO in the biological reactor. The digital-based assessment and optimization tools developed in this study are expected to hold promise for application in broader environmental management efforts.

Increasing urine nitrification performance with sequential membrane aerated biofilm reactors

This study aimed to investigate whether separating organics depletion from nitrification increases the overall performance of urine nitrification. Separate organics depletion was facilitated with membrane aerated biofilm reactors (MABRs). The high pH and ammonia concentration in stored urine inhibited nitrification in the first stage and therewith allowed the separation of organics depletion from nitrification. An organics removal of 70 % was achieved at organic loading rates in the influent of 3.7 gCOD d-1 m-2. Organics depletion in a continuous flow stirred tank reactor (CSTR) for organics depletion led to ammonia stripping through diffused aeration of up to 13 %. Using an MABR, diffusion into the lumen amounted for 4 % ammonia loss only. In the MABR, headspace volume and therefore ammonia loss through the headspace was negligible. By aerating the downstream MABR for nitrification with the off-gas of the MABR for organics depletion, 96 % of the ammonia stripped in the first stage could be recovered in the second stage, so that the overall ammonia loss was negligibly low. Nitrification of the organics-depleted urine was studied in MABRs, CSTRs, and sequencing batch reactors in fed batch mode (FBRs), the latter two operated with suspended biomass. The experiments demonstrated that upstream organics depletion can double the nitrification rate. In a laboratory-scale MABR, nitrification rates were recorded of up to 830 mgNL-1 d-1 (3.1 gN m-2 d-1) with ambient air and over 1500 mgNL-1 d-1 (6.7 gN m-2 d-1) with oxygen-enriched air. Experiments with a laboratory-scale MABR showed that increasing operational parameters such as pH, recirculation flow, scouring frequency, and oxygen content increased the nitrification rate. The nitrification in the MABR was robust even at high pH setpoints of 6.9 and was robust against process failures arising from operational mistakes. The hydraulic retention time (HRT) required for nitrification was only 1 to 2 days. With the preceding organics depletion, the HRT for our system requires 2 to 3 days in total, whereas a combined activated sludge system requires 4 to 8 days. The N2O concentration in the off-gas increases with increasing nitrification rates; however, the N2O emission factor was 2.8 % on average and independent of nitrification rates. These results indicate that the MABR technology has a high potential for efficient and robust production of ammonium nitrate from source-separated urine.

Methane and nitrous oxide emissions from municipal wastewater treatment plants in China: A plant-level and technology-specific study

Wastewater treatment is an important source of greenhouse gases (GHGs). Yet large uncertainties remain in the quantification of GHG emissions from municipal wastewater treatment plants (MWWTPs) in China. A high-resolution and technology-specific emission inventory is still lacking to support mitigation strategies of MWWTPs. Here we develop a plant-level and technology-based MWWTP emission inventory for China covering 8703 plants and 19 treatment technology categories by compiling and harmonizing the most up-to-date facility-level databases. China's methane (CH4) and nitrous oxide (N2O) emissions from MWWTPs in 2020 are estimated to be 150.6 Gg and 22.0 Gg, respectively, with the uncertainty range of -30% to 37% and -30% to 26% at 95% confidence interval. We find an emission inequality across cities, with the richest cities emitting two times more CH4 and N2O per capita from municipal wastewater treatment than the poorest cities. The emitted CH4 and N2O are dominated by Anaerobic/Anoxic/Oxic-, Sequencing Batch Reactor-, Oxidation Ditch-, and Anoxic/Oxic-based MWWTPs of less than 20 years old. Considering the relatively young age structure of China's MWWTPs, the committed emissions highlight the importance of reducing on-site GHG emissions by optimization of operating conditions and innovation management. The emission differences among our estimates, previous studies, and the Intergovernmental Panel on Climate Change guidelines are largely attributed to the uncertainties in emission factors, implying the urgent need for more plant-integrated measurements to improve the accuracy of emission accounting.

Gas permeable membrane electrode assembly with in situ utilization of authigenic acid and base for transmembrane electro-chemisorption to enhance ammonia recovery from wastewater

Ammonia recovery from wastewater is of great significance for aquatic ecology safety, human health and carbon emissions reduction. Electrochemical methods have gained increasing attention since the authigenic base and acid of electrochemical systems can be used as stripper and absorbent for transmembrane chemisorption of ammonia, respectively. However, the separation of electrodes and gas permeable membrane (GPM) significantly restricts the ammonia transfer-transformation process and the authigenic acid-base utilization. To break the restrictions, this study developed a gas permeable membrane electrode assembly (GPMEA), which innovatively integrated anode and cathode on each side of GPM through easy phase inversion of polyvinylidene fluoride binder, respectively. With the GPMEA assembled in a stacked transmembrane electro-chemisorption (sTMECS) system, in situ utilization of authigenic acid and base for transmembrane electro-chemisorption of ammonia was achieved to enhance the ammonia recovery from wastewater. At current density of 60 A/m2, the transmembrane ammonia flux of the GPMEA was 693.0 ± 15.0 g N/(m2·d), which was 86 % and 28 % higher than those of separate GPM and membrane cathode, respectively. The specific energy consumption of the GPMEA was 9.7∼16.1 kWh/kg N, which were about 50 % and 25 % lower than that of separate GPM and membrane cathode, respectively. Moreover, the application of GPMEA in the ammonia recovery from wastewater is easy to scale up in the sTMECS system. Accordingly, with the features of excellent performance, energy saving and easy scale-up, the GPMEA showed good prospects in electrochemical ammonia recovery from wastewater.

Performance of wastewater treatment plants in emission of greenhouse gases

The present work has estimated greenhouse gas emissions in aerobic and anaerobic Wastewater Treatment Plants in Southern Italy. Greenhouse gases emissions from each treatment unit were calculated based on emission factors related to Chemical Oxygen Demand removal for biogenic CO2 and CH4 assessment and on Nitrogen removal for N2O. N2O, biogenic CO2, and CH4 emissions vary for aerobic and anaerobic-based WWTPs respectively from 73 kgCO2eq/PE*y for anaerobic plants to 91 kgCO2eq/PE*y for aerobic plants. In aerobic and anaerobic digestion systems WWTPs the contributions to CO2eq total emissions from N2O, CH4, biogenic CO2, and fossil CO2 are 30 %-33 %, 20 %-29 %, 22 %-25 %, and 26 %-16 %, respectively. N2O emissions from biological processes were found the most contributing sources of greenhouse gases while in the physical processes higher contribution is indirect carbon dioxide related to energy consumption. Compensatory measures are reported to reduce greenhouse gases emissions.

Critical evaluation of different mass transfer equations to model N2O emissions from water resource recovery facilities with diffuse aeration

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.

The roles of typical emerging pollutants on N2O emissions during biological nitrogen removal from wastewater

N2O as a potent greenhouse gas often generates in the biological nitrogen removal (BNR) processes during wastewater treatment, which makes BNR become an important greenhouse gas emission source. The emerging pollutants (EPs) are ubiquitous in wastewater and they have shown to influence the BNR processes. However, the deep discussion on potential impacts of EPs on N2O emissions during BNR is rare. Moreover, the experimental parameters for EPs investigation in most of literatures are generally not in line with real-world BNR processes, which calls for deep elucidating the roles of EPs on N2O production and emission. In this work, a critical review summarizes the existing literature about influences of typical EPs on N2O emissions and associated mechanisms during BNR, and it discusses the impacts of some easily overlooked factors, such as real EPs environmental concentrations, EPs bioaccumulation, and multiple EPs coexistence on N2O emissions. This review will provide an insight into exploring and mitigating threats posed by typical EPs on N2O emissions.

Regulatory of salinity on assembly of activated sludge microbial communities and nitrogen transformation potential in industrial plants of the lower Yangtze River basin

This study aims to thoroughly investigate the impact mode of salinity carried by industrial wastewater on the anaerobic-anoxic-oxic (A2O) sludge in wastewater treatment plants (WWTPs). Through comprehensive investigation of the A2O stage activated sludge (AS) from 19 industrial WWTPs in the downstream area of the Yangtze River, China, A total of 38 samples of anaerobic sludge and oxic sludge were collected and analyzed. We found that salinity stress significantly inhibits the growth of the AS community, particularly evident in the anaerobic sludge community. Furthermore, the high-saline environment induces changes in the structure and functional patterns of the AS community, leading to intensive interactions and resource exchanges among microorganisms. Halophilic microorganisms may play a crucial role in this process, significantly impacting the overall community structure, especially in the oxic sludge community. Additionally, salinity stress not only suppresses the nitrogen transformation potential of the AS but also leads to the accumulation of nitrite, thereby increasing the emission potential of both NO and N2O, exacerbating the greenhouse effect of the A2O process in industrial WWTPs. The findings of this study provide necessary theoretical support for maintaining the long-term stable operation of the A2O sludge system in industrial WWTPs, reducing carbon footprint, and improving nitrogen removal efficiency.

More applicable quantification of non-CO2 greenhouse gas emissions from wastewater treatment plants by on-site plant-integrated measurements

Wastewater treatment is an important source of non-CO2 greenhouse gases (GHGs). However, current quantification of these GHG emissions mainly employs unit-based measurements, where emissions from individual process units are identified, leading to large uncertainties of overall emissions. Here we introduce plant-integrated measurements, where emissions from the whole plant are measured through the off-gas pipelines of the enclosed facility, to quantify methane (CH4) and nitrous oxide (N2O) emissions from an underground municipal wastewater treatment plant (WWTP) in southern China. Our results show that the primary oxic tank contributes the largest in total CH4 and N2O emissions, with an average fraction of over 80 % and over 90 %, respectively. This can be attributed to the vigorous aeration process, which facilitates the transfer of dissolved CH4 and N2O from the liquid phase to the atmosphere through intensive air stripping. The plant-integrated measurements yield around 3-9 times higher emission factors of CH4 and N2O than the unit-based measurements. This difference in emission accounting is attributed to both varying survey durations of the two approaches and the omission of uncertain emission sources during unit-based measurements. The comparison between these two approaches indicates that plant-integrated measurements are more applicable for emission quantification of the whole plant whereas unit-based measurements provide insights into the emission characteristics of individual process units. More plant-integrated measurements are needed in the future for more accurate emission accounting of WWTPs.

Reduction of greenhouse gas emissions from closed activated sludge- to aerobic granular sludge-based biosystems via gas circulation

Greenhouse gas (GHG) emissions from biological treatment units are challenging wastewater treatment plants (WWTPs) due to their wide applications and global warming. This study aimed to reduce GHG emissions (especially N2O) using a gas circulation strategy in a closed sequencing-batch reactor when the biological unit varies from activated sludge (AS) to aerobic granular sludge (AGS). Results show that gas circulation lowers pH to 6.3 ± 0.2, facilitating regular granules but elevating total N2O production. From AS to AGS, N2O emission factor increased (0.07-0.86 %) due to decreasing ammonia-oxidizing rates while the emissions of CO2 (0.3 ± 0.1 kg-CO2/kg-chemical oxygen demand) and CH4 remained in the closed biosystem. The gas circulation decreased N2O emission factor by 63 ± 15 % after granulation higher than 44 ± 34 % before granulation, which is implemented by heterotrophic denitrification. This study provides a feasible strategy to enhance heterotrophic N2O elimination in the biological WWTPs.

Enhancing the relative abundance of comammox nitrospira in ammonia oxidizer community decreases N2O emission in nitrification exponentially

Comammox Nitrospira and canonical ammonia-oxidizing bacteria (cAOB) generally coexist in activated sludge. In present study, the effect of comammox Nitrospira on N2O production during nitrification of activated sludge was investigated. Comammox Nitrospira and cAOB were separately enriched in two nitrifying reactors, with respective relative abundance of approximately 98% in ammonia oxidizer community. The N2O emission factor (EF) of nitrification in comammox Nitrospira dominated reactor was 0.35%, consistently lower than that (2.2%) in cAOB dominated reactor. When increasing the relative abundance of comammox Nitrospira in ammonia oxidizer community, the N2O EF of nitrification decreased exponentially, which suggested that comammox Nitrospira not only decreased N2O production directly but also might have reduced N2O yield by cAOB. When cAOB dominated the ammonia oxidizer community of sludge, decreasing pH to 6.3, lowering DO to less than 0.5 mg/L, and increasing nitrite concentration enhanced N2O EF dramatically. When comammox Nitrospira became the dominant ammonia oxidizer, however, the N2O EF correlated to nitrite insignificantly and a low DO of 0.2 mg/L and weakly acidic pH (6.3) decreased N2O EF by approximately 70% and 60%, respectively. These results imply that enhancing the relative abundance of comammox Nitrospira in sludge is an effective way to reducing N2O emissions and can also offset the promoting effects of acidic pH, low DO, and high nitrite concentration on N2O production during nitrification.

Carbon emissions, wastewater treatment and aquatic ecosystems

As a nexus of environmental pollution, fossil fuel consumption and the global warming, carbon emissions are critical in China's long-term environmental strategies. In the water cycle, carbon is released during wastewater discharge, wastewater treatment, and subsequent changes in aquatic ecosystems. To gain a comprehensive understanding of this entire process, we investigate the intricate connections using balanced panel data from 261 prefecture-level cities in China spanning the period from 2000 to 2020. Each sample is quantified using 48 features derived from hydrosphere, biosphere, anthroposphere, atmosphere, pedosphere and lithosphere. This paper contributes to the relevant studies in the following ways: Firstly, to analyze the basic interaction within the water cycle, we utilize Structural Equation Modeling (SEM). Our results indicate a weak linear relationship between wastewater treatment and carbon emissions. We also substantiate the crucial role of the aquatic ecosystems in carbon fixation. Secondly, in order to comprehend the intricate interactions within the Earth system, we employ eight machine learning models to predict carbon emissions. We observe that extremely randomized trees (ET) exhibit the highest predictive accuracy among these models. Thirdly, in interpreting the ET model, we utilize Explainable artificial intelligence (XAI) techniques, including Shapley Additive Explanations (SHAP) and Accumulated Local Effects (ALE). Our 3D-SHAP analysis reveals heterogeneity in the emission effects of wastewater treatment across different sub-groups, indicating that emissions are especially sensitive to increased wastewater treatment in agricultural and tourism cities. Furthermore, 3D-SHAP analysis of the aquatic ecosystems exhibits a series of spikes, signifying that aquatic plants will abruptly lose their carbon storage ability once the degradation of the aquatic ecosystems exceeds a certain threshold. Finally, our ALE evaluation, depicting the dispersion tendency of feature importance, identifies the uncertainty of wastewater carbon release in agricultural and tourism cities, while also affirming the vulnerability of the aquatic ecosystems.

Ferric and sulfate coupled ammonium oxidation enhanced nitrogen removal in two-stage partial nitrification - Anammox/denitrification process for food waste liquid digestate treatment

Liquid digestate of food waste is an ammonium-, ferric- and sulfate-laden leachate produced during digestate dewatering, where the carbon source is insufficient for nitrogen removal. A two-stage partial nitrification-anammox/denitrification process was established for nitrogen removal of liquid digestate without pre-treatment (>300 d), through which nitrogen (95 %), biodegradable organics (100 %), sulfate (78 %) and iron (100 %) were efficiently removed. Additional ammonium conversion (20 %N) might be coupled with ferric and sulfate reduction, while produced nitrite could be further converted to di-nitrogen gas through anammox (75 %) and denitrification (25 %). Notably, since increasingly contribution of hydroxylamine producing nitrous oxide, and up-regulated expression of electron transfer and cytochrome c protein, the enhanced ammonium oxidation was probably conducted through extracellular polymeric substances-mediated electron transfer between sulfate/ferric-reducers and aerobic ammonium oxidizers. Thus, the established partial nitrification-anammox/denitrification process might be a cost-efficient nitrogen removal technology for liquid digestate, benefitting to domestic waste recycling and carbon neutralization.

Limitations of a biokinetic model to predict the seasonal variations of nitrous oxide emissions from a full-scale wastewater treatment plant

A biokinetic model based on BioWin's Activated Sludge Digestion Model (ASDM) coupled with a nitrous oxide (N2O) model was setup and calibrated for a full-scale wastewater treatment plant (WWTP) Amsterdam West, in the Netherlands. The model was calibrated using one year of continuous data to predict the seasonal variations of N2O emissions in the gaseous phase. This, according to our best knowledge, is the most complete full-scale data set used to date for this purpose. The results obtained suggest that the currently available biokinetic model predicted the winter, summer, and autumn N2O emissions well but failed to satisfactorily simulate the spring peak. During the calibration process, it was found that the nitrifier denitrification pathway could explain the observed emissions during all seasons while a combination of the nitrifier denitrification and incomplete heterotrophic denitrification pathways seemed to be dominant during the emissions peak observed during the spring season. Specifically, kinetic parameters related to free nitrous acid (FNA) displayed significant sensitivity leading to increased N2O production. The obtained values of two kinetic parameters, i.e., the FNA half-saturation during ammonia oxidising bacteria (AOB) denitrification and the FNA inhibition concentration related to heterotrophic denitrification, suggested a strong influence of the FNA bulk concentration on the N2O emissions and the observed seasonal variations. Based on the suboptimal performance and limitations of the biokinetic model, further research is needed to better understand the biochemical processes behind the seasonal peak and the influence of FNA.

Calculation of carbon emissions in wastewater treatment and its neutralization measures: A review

As the pursuit of "carbon neutrality" gains momentum, the emphasis on low-carbon solutions, emphasizing energy conservation and resource reuse, has introduced fresh challenges to conventional wastewater treatment approaches. Precisely evaluating carbon emissions in urban water supply and drainage systems, wastewater treatment plants, and establishing carbon-neutral operating models has become a pivotal concern in the future of wastewater treatment. Regrettably, limited research has been devoted to carbon accounting and the development of carbon-neutral strategies for wastewater treatment. In this review, to facilitate comprehensive carbon accounting, we initially recognizes direct and indirect carbon emission sources in the wastewater treatment process. We then provide an overview of several major carbon accounting methods and propose a carbon accounting framework. Furthermore, we advocate for a systemic perspective, highlighting that achieving carbon neutrality in wastewater treatment extends beyond the boundaries of wastewater treatment plants. We assess current technical measures both within and outside the plants that contribute to achieving carbon-neutral operations. Encouraging the application of intelligent algorithms for the multifaceted monitoring and control of wastewater treatment processes is paramount. Supporting resource and energy recycling is also essential, as is recognizing the benefits of synergistic wastewater treatment technologies. We advocate a systematic, multi-level planning approach that takes into account a wide range of factors. Our goal is to offer valuable insights and support for the practical implementation of water environment management within the framework of carbon neutrality, and to advance sustainable socio-economic development and contribute to a more environmentally responsible future.

Nitrous oxide emissions from two full-scale membrane-aerated biofilm reactors

The upcoming change of legislation in some European countries where wastewater treatment facilities will start to be taxed based on direct greenhouse gas (GHG) emissions will force water utilities to take a closer look at nitrous oxide (N2O) production. In this study, we report for the first time N2O emissions from two full-scale size membrane aerated biofilm reactors (MABR) (R1, R2) from two different manufacturers treating municipal wastewater. N2O was monitored continuously for 12 months in both the MABR exhaust gas and liquid phase. Multivariate analysis was used to assess process performance. Results show that emission factors (EFN2O) for both R1 and R2 (0.88 ± 1.28 and 0.82 ± 0.86 %) were very similar to each other and below the standard value from the Intergovernmental Panel on Climate Change (IPCC) 2019 (1.6 %). More specifically, N2O was predominantly emitted in the MABR exhaust gas (NTRexh) and was strongly correlated to the ammonia/um load (NHx,load). Nevertheless, the implemented Oxidation Reduction Potential (ORP) control strategy increased the bulk contribution (NTRbulk), impacting the overall EFN2O. A thorough analysis of dynamic data reveals that the changes in the external aeration (EA)/loading rate patterns suggested by ORP control substantially impacted N2O mass transfer and biological production processes. It also suggests that NTRexh is mainly caused by ammonia-oxidizing organisms (AOO) activity, while ordinary heterotrophic organisms (OHO) are responsible for NTRbulk. Different methods for calculating EFN2O were compared, and results showed EFN2O would range from 0.6 to 5.5 depending on the assumptions made. Based on existing literature, a strong correlation between EFN2O and nitrogen loading rate (R2 = 0.73) was found for different technologies. Overall, an average EFN2O of 0.86 % N2O-N per N load was found with a nitrogen loading rate >200 g N m-3 d-1, which supports the hypothesis that MABR technology can achieve intensified biological nutrient removal without increasing N2O emissions.

Enhanced nitrogen removal in sewage treatment is achieved by using kitchen waste hydrolysate without a significant increase in nitrous oxide emissions

Kitchen waste hydrolysate (KWH) is an effective replacement for commonly used carbon sources such as sodium acetate (NaAc) and glucose (Glu), in wastewater treatment plants (WWTPs) to enhance the total nitrogen (TN) removal efficiency in sewage and reduce the operating cost of WWTPs. However, KWH utilization introduces complex organic matter that may lead to increased nitrous oxide (N2O) emissions, compared with that of NaAc and Glu, causing significant damage to the atmosphere. Therefore, this study aims to compare the effects of KWH, Glu, and NaAc on N2O emissions in sewage treatment. The results indicated that KWH introduction did not lead to a significant increase in N2O emissions, with a conversion rate of only 5.61 %. Compared with raw sludge, the addition of only Glu and NaAc significantly increased the abundance of the nar G gene, indicating that the readily degradable carbon sources initiated denitrification at a faster rate than KWH. When KWH was added, there was a notable increase in the abundance of genes associated with partial nitrification and denitrification (nir K, hzo, and nos Z). In contrast, Glu and NaAc did not have a significant effect on the nos Z gene. The results suggested that KWH supplementation was more effective to reduce N2O to N2. Moreover, the KWH addition significantly increased the microbial diversity in the sludge and promoted the presence of shortcut nitrification and denitrification bacteria (Comamonadaceae) and denitrification bacteria (Rhodobacteraceae), further indicating the potential of KWH for enhanced denitrification and reduced N2O emissions. Overall, to the best of our knowledge, this is the first study that demonstrated KWH, as a novel and complex organic carbon source, can be safely used in sewage treatment processes to improve the pollutant removal efficiency without causing a significant increase in N2O emissions.

Dynamics of CO2, CH4, and N2O in Ria Formosa coastal lagoon (southwestern Iberia) and export to the Gulf of Cadiz

A first characterization of greenhouse gases had been carried out to study their role and impact in a productive transitional coastal system of the southern Portugal - Ria Formosa lagoon. To this purpose, the partial pressure of CO2 (pCO2) and the concentration of dissolved CH4 and N2O have been measured. Two surveys were carried out during 2020, at low tide under typical conditions of Spring (March) and end of Summer (October). The samplings sites were distributed along the costal lagoon covering: i) inner areas with strong human impact (influence of different flows of treated wastewater discharges); and ii) main channels in connection with the main inlets to study the exchanges with the ocean. In general, the highest values of the three greenhouse gases were found at the inner studied areas, especially affected by the disposal of treated effluents from wastewater treatment plans, in October. The mean water - atmosphere fluxes of the CO2, CH4 and N2O are positive, showing that the study area acts as a source of these gases to the atmosphere. On the other hand, it was calculated a rough estimation of the three gases globally exported from Ria Formosa to the ocean, through the main six inlets to evaluate the magnitude of the supply of these gases from Ria Formosa to the adjacent ocean. The mean CO2, CH4 and N2O horizontal water fluxes exported from all the inlets of Ria Formosa to the Gulf of Cadiz for both seasons, during low water, are 8.7 ± 3.9 mmol m-2 s-1, 8.0 ± 3.5 μmol m-2 s-1 and 3.2 ± 1.5 μmol m-2 s-1, which corresponds to a mass transport through the inlets section of 0.7 ± 0.7 kg s-1, 0.2 ± 0.2 g s-1 and 0.2 ± 0.3 g s-1 respectively. From these estimates, as expected, the higher mass transport was found at the larger and deeper inlets (Faro-Olhão and Armona).

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.

Reduction of sewage sludge and N2O emissions by an Oxic Settling Anaerobic (OSA) process: The case study of Corleone (Italy) wastewater treatment plant

Biosolid management is becoming one of the most crucial issues for wastewater treatment plant (WWTP) operators. The application of the Oxic Settling Anaerobic (OSA) process allows the minimisation of excess sludge production. This study compares conventional activated sludge (CAS) and OSA layouts in a full-scale WWTP (namely, Corleone - Italy). Extensive monitoring campaigns were conducted to assess treatment performances regarding carbon and nutrient removal, greenhouse gas (GHG) emissions, excess sludge production, and biomass activity (by means of respirometric analysis). Results showed that the effluent quality consistently met the Italian discharge limits. However, with the implementation of the OSA process, there was a decrease in ammonium removal efficiency, which could be attributed to reduced nitrifier activity related to reduced biomass production and extended anaerobic conditions affecting the nitrification process. On the other hand, the OSA configuration significantly increased phosphorus removal, indicating a high phosphorus content in the resulting waste sludge. A worsening of the sludge settling properties was observed with the OSA configuration likely due to decreased EPS concentrations. The sludge production in the OSA configuration decreased by 17.3 % compared to CAS. Nitrous-oxide measurements did not show a variation between CAS and OSA configurations, confirming that the OSA process can be a suitable solution for reducing WWTP's carbon footprint.

Modelling greenhouse gas emissions from biological wastewater treatment by GPS-X: The full-scale case study of Corleone (Italy)

Greenhouse gas (GHG) emissions from wastewater treatment plants (WWTPs) can affect climate change and must be measured and reduced. Mathematical modelling is an attractive solution to get a tool for GHG mitigation. However, although many efforts have been made to create reliable tools that can simulate "sustainable" full-scale WWTP operation, these studies are not considered complete enough to include GHG emissions and energy consumption of biological processes under long-term dynamic conditions. In this study, activated sludge model no. 1 (ASM1) was modified to model nitrous oxide (N2O) emissions with a plant-wide modelling approach. The model is novel compared to the state of the art since it includes three steps denitrification, all N2O production pathways and its stripping in an ASM1. The model has been calibrated and validated through long-term water quality and short-term N2O emissions data collected from Corleone (Italy) WWTP. Different dissolved oxygen (DO) concentrations and return sludge (RAS) ratios were tested with dynamic simulations to optimise the full-scale WWTP. The scenarios have been compared synergistically with effluent quality, direct GHG emissions, and energy footprint by the water-energy‑carbon coupling index (WECCI). This modelling study is novel as it fully covers long-term calibration/validation of the model with N2O measurements and tests the dynamic optimisation. Decision-makers and operators can use this new model to optimise GHG emissions and treatment costs.

Decoupling the simultaneous effects of NO2-, pH and free nitrous acid on N2O and NO production from enriched nitrifying activated sludge

In the pursuit of energy and carbon neutrality, nitrogen removal technologies have been developed featuring nitrite (NO2-) accumulation. However, high NO2- accumulations are often associated with stimulated greenhouse gas (i.e., nitrous oxide, N2O) emissions. Furthermore, the coexistence of free nitrous acid (FNA) formed by NO2- and proton (pH) makes the consequence of NO2- accumulation on N2O emissions complicated. The concurrent three factors, NO2-, pH and FNA may play different roles on N2O and nitric oxide (NO) emissions simultaneously, which has not been systematically studied. This study aims to decouple the effects of NO2- (0-200 mg N/L), pH (6.5-8) and FNA (0-0.15 mg N/L) on the N2O and NO production rates and the production pathways by ammonia oxidizing bacteria (AOB), with the use of a series of precisely executed batch tests and isotope site-preference analysis. Results suggested the dominant factors affecting the N2O production rate were NO2- and FNA concentrations, while pH alone played a relatively insignificant role. The most influential factor shifted from NO2- to FNA as FNA concentrations increased from 0 to 0.15 mg N/L. At concentrations below 0.0045 mg HNO2-N/L, nitrite rather than FNA played a significant role stimulating N2O production at elevated nitrite concentrations. The inhibition effect of FNA emerged with further increase of FNA between 0.0045-0.015 mg HNO2-N/L, weakening the promoting effect of increased nitrite. While at concentrations above 0.015 mg HNO2-N/L, FNA inhibited N2O production especially from nitrifier denitrification pathway with the level of inhibition linearly correlated with the FNA concentration. pH and the nitrite concentration regulated the production pathways, with elevated pH promoting the nitrifier nitrification pathway, while elevated NO2- concentrations promoting the nitrifier denitrification pathway. In contrast to N2O, NO emission was less susceptible to FNA at concentrations up to 0.015 mg N/L but was stimulated by increasing NO2- concentrations. This study, for the first time, distinguished the effects of pH, NO2- and FNA on N2O and NO production, thereby providing support to the design and operation of novel nitrogen removal systems with NO2- accumulation.

Impact of aged and virgin microplastics on sedimentary nitrogen cycling and microbial ecosystems in estuaries

Microplastics (MPs) entering the environment undergo complex weathering (aging) processes, however, the impacts of aged MPs on estuarine nitrogen cycling and microbial ecosystems remain largely unknown. In this study, a 50 days microcosm experiment was conducted to investigate the response of sedimentary nitrogen (N) transformation processes, N2O emission and microbial communities to virgin and aged MPs (PE and PS) exposure. We found that aged MPs influenced sediment nitrogen turnover more rapidly and profoundly than virgin MPs and showed type and dose-response effect. During the first 10 days, higher concentration (3 % by weight of sediment) aged MPs (both PS and PE) treatments significantly promoted denitrification (ANOVA, P < 0.05), while virgin MPs treatments had weak effect on denitrification, compared with the control (P > 0.05). Moreover, higher concentration aged PS-MPs remarkably enhanced N2O emission on the 10th day, while N2O was consumed in the control. After 50 days incubation, there was an overall increase in nirK gene abundance exposed to MPs, and nosZ gene copies in aged PS treatments were around twice that in the control based on qPCR (P < 0.05). The function prediction also showed significant elevation of relative abundance of denitrification and DNRA relevant genes in bacterial community. In addition, aged PS treatment (3 %) recruited specific bacterial and archaeal assemblies, with Sedimenticolaceae, Lentimicrobiaceae, SCGC_AAA011-D5, SG8-5, Lokiarchaeia, and Odinarchaeia selectively enriched in the treatment. Our study highlighted that virgin and aged MPs had different impact on sediment nitrogen cycling, and the ecological risks of aged MPs should be concerned since all MPs eventually get weathered when they enter the environment.

An Advanced Synergy of Partial Denitrification-Anammox for Optimizing Nitrogen Removal from Wastewater: A Review

Anammox is a widely adopted process for energy-efficient removal of nitrogen from wastewater, but challenges with NOB suppression and NO₃^- accumulation have led to a deeper investigation of this process. To address these issues, the synergy of partial denitrification and anammox (PD-anammox) has emerged as a promising solution for sustainable nitrogen removal in wastewater. This paper presents a comprehensive review of recent developments in the PD-anammox system, including stable performance outcomes, operational parameters, and mathematical models. The review categorizes start-up and recovery strategies for PD-anammox and examines its contributions to sustainable development goals, such as reducing N₂O emissions and saving energy. Furthermore, it suggests future trends and perspectives for improving the efficiency and integration of PD-anammox into full-scale wastewater treatment system. Overall, this review provides valuable insights into optimizing PD-anammox in wastewater treatment, highlighting the potential of simultaneous processes and the importance of improving efficiency and integration into full-scale systems.

A new strategy to simultaneous removal and recovery of nitrogen from wastewater without N2O emission by heterotrophic nitrogen-assimilating bacterium

Biological assimilation that recovery the nitrogen from wastewater in the form of biomass offers a more environmentally friendly solution for the limitations of the conventional wastewater treatments. This study reported the simultaneous removal and recovery of nitrogen from wastewater without N₂O emission by a heterotrophic nitrogen-assimilating Acinetobacter sp. DN1 strain. Nitrogen balance, biomass qualitative analysis, genome and enzyme studies have been performed to illustrate the mechanism of nitrogen conversion by strain DN1. Results showed that the ammonium removal followed one direct pathway (GOGAT/GDH) and three indirect pathways (NH₄⁺ → NH₂OH → NO → NO₂^- → NH₄⁺ → GOGAT/GDH; NH₄⁺ → NH₂OH → NO → NO₂^- → NO₃^- → NO₂^- → NH₄⁺ → GOGAT/GDH; NH₄⁺ → NH₂OH → NO → NO₃^- → NO₂^- → NH₄⁺ → GOGAT/GDH). Nitrogen balance and biomass qualitative analysis showed that over 70 % of the ammonium in the wastewater was converted into intracellular nitrogen-containing compounds and stored in the cells of strain DN1. Traditional denitrification pathway was not detected and the ammonium was removed through assimilation, which makes it more energy-saving for nitrogen recovery when compared with Haber-Bosch process. This study provides a new direction for simultaneous nitrogen removal and recovery without N₂O emission by the heterotrophic nitrogen-assimilating bacterium.

Metabolic Performance and Fate of Electrons during Nitrate-Reducing Fe(II) Oxidation by the Autotrophic Enrichment Culture KS Grown at Different Initial Fe/N Ratios

Autotrophic nitrate-reducing Fe(II)-oxidizing (NRFeOx) microorganisms fix CO2 and oxidize Fe(II) coupled to denitrification, influencing carbon, iron, and nitrogen cycles in pH-neutral, anoxic environments. However, the distribution of electrons from Fe(II) oxidation to either biomass production (CO2 fixation) or energy generation (nitrate reduction) in autotrophic NRFeOx microorganisms has not been quantified. We therefore cultivated the autotrophic NRFeOx culture KS at different initial Fe/N ratios, followed geochemical parameters, identified minerals, analyzed N isotopes, and applied numerical modeling. We found that at all initial Fe/N ratios, the ratios of Fe(II)oxidized to nitratereduced were slightly higher (5.11 to 5.94 at Fe/N ratios of 10:1 and 10:0.5) or lower (4.27 to 4.59 at Fe/N ratios of 10:4, 10:2, 5:2, and 5:1) than the theoretical ratio for 100% Fe(II) oxidation being coupled to nitrate reduction (5:1). The main N denitrification product was N2O (71.88 to 96.29% at Fe/15N ratios of 10:4 and 5:1; 43.13 to 66.26% at an Fe/15N ratio of 10:1), implying that denitrification during NRFeOx was incomplete in culture KS. Based on the reaction model, on average 12% of electrons from Fe(II) oxidation were used for CO2 fixation while 88% of electrons were used for reduction of NO3- to N2O at Fe/N ratios of 10:4, 10:2, 5:2, and 5:1. With 10 mM Fe(II) (and 4, 2, 1, or 0.5 mM nitrate), most cells were closely associated with and partially encrusted by the Fe(III) (oxyhydr)oxide minerals, whereas at 5 mM Fe(II), most cells were free of cell surface mineral precipitates. The genus Gallionella (>80%) dominated culture KS regardless of the initial Fe/N ratios. Our results showed that Fe/N ratios play a key role in regulating N2O emissions, for distributing electrons between nitrate reduction and CO2 fixation, and for the degree of cell-mineral interactions in the autotrophic NRFeOx culture KS. IMPORTANCE Autotrophic NRFeOx microorganisms that oxidize Fe(II), reduce nitrate, and produce biomass play a key role in carbon, iron, and nitrogen cycles in pH-neutral, anoxic environments. Electrons from Fe(II) oxidation are used for the reduction of both carbon dioxide and nitrate. However, the question is how many electrons go into biomass production versus energy generation during autotrophic growth. Here, we demonstrated that in the autotrophic NRFeOx culture KS cultivated at Fe/N ratios of 10:4, 10:2, 5:2, and 5:1, ca. 12% of electrons went into biomass formation, while 88% of electrons were used for reduction of NO3- to N2O. Isotope analysis also showed that denitrification during NRFeOx was incomplete in culture KS and the main N denitrification product was N2O. Therefore, most electrons stemming from Fe(II) oxidation seemed to be used for N2O formation in culture KS. This is environmentally important for the greenhouse gas budget.

Analysis of bacterial community distribution characteristics in the downstream section of a cross confluence in a polluted urban channel

Channel confluences are common in urban rivers and caused complex hydrodynamic conditions in the downstream section, significantly influencing the distribution of pollutants and the microbial community. So far, the principles of bacterial community assembly and their linkages with environmental factors are poorly understood. In the present study, the hydrodynamic and pollution conditions were investigated in a typical channel confluence of an urban river in the Yangtze River delta area, China, and their impacts on the bacterial community structure in the water and sediment were characterized using 16S rRNA gene high-throughput sequencing technology. Based on the results, the flow velocity was the crucial factor influencing the dispersal of nutrients, organic compounds, and bacterial communities in the river water. Moreover, the sediments exhibited higher α-diversity and bacterial richness for nitrogen and sulfur cycling than the water. In addition to flow velocity, the contents of total organic carbon, total phosphorus, and heavy metals determined the sediment bacterial communities at varying depths. The predictive analysis of functional gene category indicated differences between the water and sediment communities in metabolic potentials and pathogen risk and provided guidance for water pollution control and the eco-remediation of urban rivers.

Strategy of nitrate removal in anaerobic ammonia oxidation-dependent processes

The anaerobic ammonium oxidation (anammox), a microbial process that is considered as a low-cost and high efficient wastewater treatment, has received extensive attention with an attractive application prospect. The anammox process reduces nitrite (NO₂^-) to nitrogen gas (N₂) with ammonium (NH₄⁺) as the electron donor. However, some nitrate (NO₃^-) equivalent to 11% of total nitrogen (TN) is generated in this process, which limits the development of anammox. To overcome this problem, many efforts have been made in this regard, mainly combining with other biological treatment methods (denitrification, denitrifying anaerobic methane oxidation, etc.), introducing the substance into anammox process, etc. Herein, we summarized a detailed review of previous researches on the removal of NO₃^- in the anammox-dependent processes. It is hoped that this review could serve as valuable guidance in future research and practical applications of anammox.

Investigating global surface temperature from the perspectives of environmental, demographic, and economic indicators: current status and future temperature trend

Anthropogenic activities have increased atmospheric concentrations of greenhouse gas emissions, which have observably increased global temperature. Recognizing it as one of the most critical issues caused by human activities, this study investigates the effects of environmental, demographic, and economic indicators on global and regional temperature. For this purpose, advanced and powerful machine learning techniques, such as ANN, CNN, SVM, and LSTM, are employed using the data from 1980 to 2018 of the aforementioned regions to predict and forecast global and regional temperatures in Africa, Asia, Europe, North America, and South America. First, the predicted results were found very close to the actual surface temperature, confirming that environmental, economic, and demographic indicators are critical drivers of climate change. Second, this study forecasted global temperature from 2023 to 2050 and regional temperature from 2022 to 2050. The results also predicted a considerable increase in global temperature and regional temperature in the forthcoming years. Particularly, Asia and Africa may experience extreme weather in the future with an increase of more than 1.6 °C. Based on the findings of this study, the major implications have been that maintaining greenhouse gas emissions, balancing economic development, urbanization, and environmental quality while reducing fossil fuel energy consumption will ensure climate mitigation. The findings demand an alteration in human behavior regarding fossil fuel energy consumption to control greenhouse gas emissions, which is the most significant contributor to climate change.

Plant-wide modeling of a metropolitan wastewater treatment plant to reduce energy consumption and carbon footprint

A real metropolitan wastewater treatment plant (RWWTP) serving a population equivalent of 1.55 million was modeled to reduce energy consumption and carbon footprint (CFP). An approach was proposed to handle the dilution factor and partial aeration due to discontinuous air diffuser locations in the Bardenpho-5 configuration. Various operational, structural, and configurational modifications were evaluated. Results indicated that management scenarios might provide conflicting outcomes for different targets. Reduced energy consumption may not result in lower CFP at the same time. Moreover, operational changes that would impact total nitrogen (TN) concentrations and N₂O release may significantly impact CFP. A policy of using a modified Bardenpho-5 process with reduced internal recycle (IR) ratio, waste activated sludge (WAS), and return activated sludge (RAS) flow rates provided the lowest CPF. Modified Bardenpho-5 process and replacing belt thickeners with gravity thickeners supplied the highest savings in energy consumption. Overall, up to 14% and 20% reductions were possible in the energy consumption and CFP of the plant, respectively. The RWWTP may save up to 10% in energy expenses annually by operational modifications.

Automatic control and optimal operation for greenhouse gas mitigation in sustainable wastewater treatment plants: A review

In order to promote low-carbon sustainable operational management of the wastewater treatment plants (WWTPs), automatic control and optimal operation technologies, which devote to improving effluent quality, operational costs and greenhouse gas (GHG) emissions, have flourished in recent years. There is no consensus on the design procedure for optimal control/operation of sustainable WWTPs. In this review, we summarize recent researches on developing control and optimization strategies for GHG mitigation in WWTPs. Faced with the fact that direct carbon dioxide (CO₂) emissions (considered biological origin) are generally not included in the carbon footprint of WWTPs, direct emissions (nitrous oxide (N₂O), methane (CH₄)) and indirect emissions are paid much attention. Firstly, the plant-wide models with GHG dynamic simulation, which are employed to design and evaluate the automatic control schemes as well as representative studies on identifying key factors affecting GHG emissions or comprehensive performance are outlined. Then, both traditional and advanced control methods commonly used in GHG mitigation are reviewed in detail, followed by the multi-objective optimization practices of control/operational parameters. Based on the mentioned control and (or) optimization strategies, a novel design framework for the optimal control/operation of sustainable WWTPs is proposed. The findings and design framework proposed in the paper will provide guidance for GHG mitigation and sustainable operation in WWTPs. It is foreseeable that more accurate and appropriate plant-wide models together with flexible control methods and intelligent optimization strategies will be developed to satisfy the upgrading requirements of WWTPs in the future.

Bioaugmentation performances with a powerful strain for nitrogen removal without N2O accumulation

N₂O is regarded as an inevitable intermediate during nitrogen removal, especially for wastewater treatment plants where good operating conditions would be required to mitigate N₂O releasing, which generally causes a high treatment cost. In this study, a novel bacterium capable of removing nitrogen without N₂O accumulation was isolated and identified as Citrobacter freundii XY-1. The nitrogen removal characteristics, nitrogen removal pathway, bioaugmentation in different reactors as well as microbial diversity were investigated. Results showed that 99.42% of NH+ 4-N and 95% of total organic carbon could be removed within 48 h with the corresponding removal rates being 4.03 mg/(L·h) and 39.42 mg/(L·h), respectively. It was inferred that traditional denitrification and N₂O generation do not exist in the pathway of removing nitrogen by XY-1 based on isotope analysis and functional genes detection. Bioaugmentations of XY-1 in both sequencing batch reactor and biological aerated filter significantly promoted the performances of nitrogen removal. The microbial diversity indicated that the relative abundance of strain XY-1 ranged from 45% to 66%, predominating throughout the running period. Overall, XY-1 could become an incredibly important candidate for the upgrading of wastewater treatment plants.

Techno-economic analysis of sidestream ammonia removal technologies: Biological options versus thermal stripping

Over the past twenty years, various commercial technologies have been deployed to remove ammonia (NH₄-N) from anaerobic digestion (AD) liquors. In recent years many anaerobic digesters have been upgraded to include a pre-treatment, such as the thermal hydrolysis process (THP), to produce more biogas, increasing NH₄-N concentrations in the liquors are costly to treat. This study provides a comparative techno-economic assessment of sidestream technologies to remove NH₄-N from conventional AD and THP/AD dewatering liquors: a deammonification continuous stirred tank reactor (PNA), a nitrification/denitrification sequencing batch reactor (SBR) and thermal ammonia stripping process with an ammonia scrubber (STRIP). The SBR and PNA were based on full-scale data, whereas the STRIP was designed using a computational approach to achieve NH₄-N removals of 90-95%. The PNA presented the lowest whole-life cost (WLC) over 40 years, with £7.7 M, while the STRIP had a WLC of £43.9 M. This study identified that THP dewatering liquors, and thus a higher ammonia load, can lead to a 1.5-3.0 times increase in operational expenditure with the PNA and the SBR. Furthermore, this study highlighted that deammonification is a capable and cost-effective nitrogen removal technology. Processes like the STRIP respond to current pressures faced by the water industry on ammonia recovery together with targets to reduce nitrous oxide emissions. Nevertheless, ammonia striping-based processes must further be demonstrated in WWTPs and WLC reduced to grant their wide implementation and replace existing technologies.

N2O production and emission pathways in anammox biofilter for treating wastewater with low nitrogen concentrations

Although anaerobic ammonia oxidation (anammox) is a cost-effective nitrogen removal process, nitrous oxide (N₂O) production will greatly reduce the advantages of this process. It is important to identify the N₂O emission pathways and then reduce the N₂O production in anammox system. To date, very limited research has been done to investigate the N₂O production and N₂O emission pathways in anammox biofilter. In this study, N₂O production were investigated under different filtration rates in anammox biofilter for treating wastewater with low nitrogen concentrations, and N₂O emission pathways were analyzed with batch tests using N₂O microsensor and stable isotope mass spectrometry. The results showed N₂O production increased with the increase of filtration rates in anammox biofilter, where the N₂O emission factor increased from 0.012 % at 1.0 m/h to 0.496 % at 3.0 m/h. And the optimal operation condition was at filtration rate of 1.5 m/h, where NH₄⁺-N and NO₂^--N removal efficiencies reached 99 % and N₂O concentration was the lowest. qPCR showed that anammox bacteria, nitrifying bacteria and denitrifying bacteria were all present in anammox biofilter, with anammox bacteria in the highest abundance. And nitrifying bacteria and denitrifying bacteria provided the possibility of N₂O production. The batch tests and stable isotope mass spectrometry analysis indicated that nitrifier denitrification, hydroxylamine oxidation and endogenous heterotrophic denitrification were N₂O production pathways in aerobic zone and anoxic zone of anammox biofilter, respectively. In addition, batch tests under different conditions showed no oxygen environment could reduce N₂O production. Therefore, the production of N₂O in anammox system is a problem that cannot be ignored and should be paid more attention to.

Exploring GHG emissions in the mainstream SCEPPHAR configuration during wastewater resource recovery

The wastewater sector paradigm is shifting from wastewater treatment to resource recovery. In addition, concerns regarding sustainability during the operation have increased. In this sense, many water utilities have become aware of the potential GHG emissions during the operation of wastewater treatment. This study assesses the nitrous oxide and methane emissions during the long-term operation of a novel wastewater resource recovery facility (WRRF) configuration: the mainstream SCEPPHAR. The long-term N₂O and CH₄ emission factors calculated were in the low range of the literature, 1 % and 0.1 %, respectively, even with high nitrite accumulation in the case of N₂O. The dynamics and possible sources of production of these emissions are discussed. Finally, different aeration strategies were implemented to study the impact on the N₂O emissions in the nitrifying reactor. Results showed that operating the pilot-plant under different dissolved oxygen concentrations (between 1 and 3 g O₂ m^-3) did not have an effect on the N₂O emission factor. Intermittent aeration was the aeration strategy that most mitigated the N₂O emissions in the nitrifying reactor, obtaining a reduction of 40 % compared to the normal operation of the pilot plant.

Methane and nitrous oxide concentrations and fluxes from heavily polluted urban streams: Comprehensive influence of pollution and restoration

Streams draining urban areas are usually regarded as hotspots of methane (CH₄) and nitrous oxide (N₂O) emissions. However, little is known about the coupling effects of watershed pollution and restoration on CH₄ and N₂O emission dynamics in heavily polluted urban streams. This study investigated the CH₄ and N₂O concentrations and fluxes in six streams that used to be heavily polluted but have undergone different watershed restorations in Southwest China, to explore the comprehensive influences of pollution and restoration. CH₄ and N₂O concentrations in the six urban streams ranged from 0.12 to 21.32 μmol L^-1 and from 0.03 to 2.27 μmol L^-1, respectively. The calculated diffusive fluxes of CH₄ and N₂O were averaged of 7.65 ± 9.20 mmol m^-2 d^-1 and 0.73 ± 0.83 mmol m^-2 d^-1, much higher than those in most previous reports. The heavily polluted streams with non-restoration had 7.2 and 7.8 times CH₄ and N₂O concentrations higher than those in the fully restored streams, respectively. Particularly, CH₄ and N₂O fluxes in the fully restored streams were 90% less likely than those found in the unrestored ones. This result highlighted that heavily polluted urban streams with high pollution loadings were indeed hotspots of CH₄ and N₂O emissions throughout the year, while comprehensive restoration can effectively weaken their emission intensity. Sewage interception and nutrient removal, especially N loadings reduction, were effective measures for regulating the dynamics of CH₄ and N₂O emissions from the heavily polluted streams. Based on global and regional integration, it further elucidated that increasing environment investments could significantly improve water quality and mitigate CH₄ and N₂O emissions in polluted urban streams. Overall, our study emphasized that although urbanization could inevitably strengthen riverine CH₄ and N₂O emissions, effective eco-restoration can mitigate the crisis of riverine greenhouse gas emissions.

A review of enhanced municipal wastewater treatment through energy savings and carbon recovery to reduce discharge and CO2 footprint

Municipal wastewater treatment that mainly performed by conventional activated sludge (CAS) process faces the challenge of intensive aeration-associated energy consumption for oxidation of organics and ammonium, contributing to significant directly/indirectly greenhouse gas (GHG) emissions from energy use, which hinders the achievement of carbon neutral, the top priority mission in the coming decades to cope with the global climate change. Therefore, this article aimed to offer a comprehensive analysis of recently developed biological treatment processes with the focus on reducing discharge and CO2 footprint. The biotechnologies including "Zero Carbon", "Low Carbon", "Carbon Capture and Utilization" are discussed, it suggested that, by integrating these processes with energy-saving and carbon recovery, the challenges faced in current wastewater treatment plants can be overcome, and a carbon-neutral even be possible. Future research should investigate the integration of these methods and improve anammox contribution as well as minimize organics lost under different scales.