Carbon neutrality of wastewater treatment - A systematic concept beyond the plant boundary

Enterobacter sp. HIT-SHJ4 isolated from wetland with carbon, nitrogen and sulfur co-metabolism and its implication for bioremediation

Both autotrophic and heterotrophic denitrification are known as important bioprocesses of microbe-mediated nitrogen cycle in natural ecosystems. Actually, mixotrophic denitrification co-driven by organic matter and reduced sulfur substances are also common, especially in hypoxic environments such as estuarine sediments. However, carbon, nitrogen and sulfur co-metabolism during mixotrophic denitrification in natural water ecosystems has rarely been reported in detail. Therefore, this study investigated the co-metabolism of carbon, nitrogen and sulfur using samples collected from four distinct natural water ecosystems. Results demonstrated that samples from various sources all exhibited the ability for co-metabolism of carbon, nitrogen and sulfur. Microbial community analysis showed that Pseudomonas and Paracoccus were dominant bacteria ranging from 65.6% to 75.5% in mixotrophic environment. Enterobacter sp. HIT-SHJ4, a mixotrophic denitrifying strain which owned the capacity for co-metabolism of carbon, nitrogen and sulfur, was isolated and reported here for the first time. The strain preferred methanol as its carbon source and demonstrated remarkable efficiency for removing sulfide and nitrate with below 100 mg/L sulfide. Under weak acid conditions (pH 6.5-7.0), it exhibited enhanced capability in converting sulfide to elemental sulfur. Its bioactivity was evident within a temperature from 25 °C to 40 °C and C/N ratios from 0.75 to 3. This study confirmed the widespread presence of microbial-mediated synergistic carbon, nitrogen and sulfur metabolism in natural aquatic ecosystems. HIT-SHJ4 emerges as a novel strain, shedding light on carbon, nitrogen and sulfur co-metabolism in natural water bodies. Furthermore, it also serves as a promising candidate microorganism for in-situ ecological remediation, particularly in dealing with contamination posed by nitrate, sulfide, and organic matter.

Multi-biomimetic Double Interlaced Wetting Janus Surface for Efficient Fog Collection

Various methods to solve water scarcity have attracted increasing attention. However, most existing water harvesting schemes have a high demand for preparation methods and costs. Here, a multi-biomimetic double interlaced wetting Janus surface (DIWJS) was prepared by laser for effective fog collection. The as-prepared surfaces are composed of superhydrophilic points/hydrophobic substrates on the A-side and superhydrophilic stripes/hydrophobic substrates on the B-side. The interlaced wettability and superhydrophilic points on the A side are conducive to capture and permeation of droplets. The superhydrophilic stripes and interlaced wettability on the B-side are conducive to transportation and shedding of droplets. Therefore, the overall fog collection process is accelerated. The proposal of smart farm model validates broad application prospects of DIWJS. This work provides an advanced and multi-biomimetic surface and provides important insights for green, low-cost, and versatile strategies to solve water scarcity issues.

Re-vitalizing wastewater: Nutrient recovery and carbon capture through microbe-algae synergy using omics-biology

Increasing amounts of wastewater is the most pervasive and challenging environmental problem globally. Conventional treatment methods are costly and entail huge energy, carbon consumption and greenhouse gas emissions. Owing to their unique ability of carbon capturing and resource recovery, microalgae-microbiome based treatment is a potential approach and is widely used for carbon-neutral wastewater treatment. Microalgae-bacteria synergy (i.e., the functionally beneficial microbial synthetic communities) performs better and enhances carbon-sequestration and nutrient recovery from wastewater treatment plants. This review presents a comprehensive information regarding the potential of microalgae-microbiome as a sustainable agent for wastewater and discusses synergistic approaches for effective nutrient removal. Moreover, this review discusses, the role of omics-biology and Insilco approaches in unravelling and understanding the algae-microbe synergism and their response toward wastewater treatment. Finally, it discusses various microbiome engineering approaches for developing the effective microalgae-bacteria partners for carbon sequestration and nutrient recovery from wastewater, and summarizes future research perspectives on microalgae-microbiome based bioremediation.

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.

Deciphering carbon emissions in urban sewer networks: Bridging urban sewer networks with city-wide environmental dynamics

As urbanization accelerates, understanding and managing carbon emissions from urban sewer networks have become crucial for sustainable urban water cycles. This review examines the factors influencing greenhouse gas (GHG) emissions within urban sewage systems, analyzing the complex effects between water quality, hydrodynamics, and sewer infrastructure on GHG production and emission processes. It reveals significant spatiotemporal heterogeneity in GHG emissions, particularly under long-term scenarios where flow rates and temperatures exhibit strong impacts and correlations. Given the presence of fugitive and dissolved potential GHGs, standardized monitoring and accounting methods are deemed essential. Advanced modeling techniques emerge as crucial tools for large-scale carbon emission prediction and management. The review identifies that traditional definitions and computational frameworks for carbon emission boundaries fail to fully consider the inherent heterogeneity of sewers and the dynamic changes and impacts of multi-source pollution within the sewer system during the urban water cycle. This includes irregular fugitive emissions, the influence of stormwater systems, climate change, geographical features, sewer design, and the impacts of food waste and antibiotics. Key strategies for emission management are discussed, focusing on the need for careful consideration of approaches that might inadvertently increase global emissions, such as ventilation, chemical treatments, and water management practices. The review advocates for an overarching strategy that encompasses a holistic view of carbon emissions, stressing the importance of refined emission boundary definitions, novel accounting practices, and comprehensive management schemes in line with the water treatment sector's move towards carbon neutrality. It champions the adoption of interdisciplinary, technologically advanced solutions to mitigate pollution and reduce carbon emissions, emphasizing the importance of integrating cross-scale issues and other environmentally friendly measures in future research directions.

Insights into feasibility and microbial characterizations on simultaneous elimination of dissolved methane from anaerobic effluents and nitrate/nitrite reduction in a conventional anoxic reactor with magnetite

Discovery of nitrate/nitrite-dependent anaerobic methane oxidation (DAMO) challenges the conventional biological treatment processes, since it provides a possibility of simultaneously mitigating dissolved methane emissions from anaerobic effluents and reducing additional carbon sources for denitrification. Due to the slow growth of specialized DAMO microbes, this possibility has been just practiced with biofilms in membrane biofilm reactors or granular sludge in membrane bioreactors. In this study, simultaneous elimination of dissolved methane from anaerobic effluents and nitrate/nitrite reduction was achieved in a conventional anoxic reactor with magnetite. Calculations of electron flow balance showed that, with magnetite the eliminated dissolved methane was almost entirely used for nitrate/nitrite reduction, while without magnetite approximately 52 % of eliminated dissolved methane was converted to unknown organics. Metagenomic sequencing showed that, when dissolved methane served as an electron donor, the abundance of genes for reverse methanogenesis and denitrification dramatically increased, indicating that anaerobic oxidation of methane (AOM) coupled to nitrate/nitrite reduction occurred. Magnetite increased the abundance of genes encoding the key enzymes involved in whole reverse methanogenesis and Nir and Nor involved in denitrification, compared to that without magnetite. Analysis of microbial communities showed that, AOM coupled to nitrate/nitrite reduction was proceeded by syntrophic consortia comprised of methane oxidizers, Methanolinea and Methanobacterium, and nitrate/nitrite reducers, Armatimonadetes_gp5 and Thauera. With magnetite syntrophic consortia exchanged electrons more effectively than that without magnetite, further supporting the microbial growth.

Should wastewater treatment plants' operational mode radically change to minimize GHG emissions?

The operation of municipal wastewater treatment plants (WWTPs) invariably results in significant emission of greenhouse gases (i.e., CH4, N2O, and CO2) into the atmosphere. We propose to consider a radical change in the way municipal WWTPs are operated, with the aim of minimizing GHG emissions while recycling most of the nutrient mass. The means to this end are to reduce the WWTP energy demand while maximizing the recovery of resources (phosphorus, ammonia, methane). The suggested concept involves operating the activated sludge process at a low sludge retention time (SRT < 2 d), i.e., under conditions that maximize the heterotrophic mass yield and eliminate nitrification. The ammonia concentration that remains in the water (considering N in the excess sludge and struvite production in the sludge-dewatering supernatant line) would be separated from the WWTP effluents using a unique ion-exchange material (ZnHCF), which would be regenerated using a low-volume 4 M NaCl solution. The ammonia would be then stripped at high pH and re-adsorbed by an acidic solution for reuse as fertilizer. The high bacterial yield and lack of nitrification in the aerobic step are expected to boost methane yield 3-4-fold, induce lower oxygen consumption, and most importantly, yield much lower N2O release. An approximate energy mass balance shows the concept to merit further consideration, owing to the potential significant reduction in N2O(g) emissions and recovery of resources. Empirical work followed by LCA is required to corroborate the hypothesis presented herein.

Sprayed Oil-Water Microdroplets as a Hydrogen Source

Liquid water provides the largest hydrogen reservoir on the earth's surface. Direct utilization of water as a source of hydrogen atoms and molecules is fundamental to the evolution of the ecosystem and industry. However, liquid water is an unfavorable electron donor for forming these hydrogen species owing to its redox inertness. We report oil-mediated electron extraction from water microdroplets, which is easily achieved by ultrasonically spraying an oil-water emulsion. Based on charge measurement and electron paramagnetic resonance spectroscopy, contact electrification between oil and a water microdroplet is demonstrated to be the origin of electron extraction from water molecules. This contact electrification results in enhanced charge separation and subsequent mutual neutralization, which enables a ∼13-fold increase of charge carriers in comparison with an ultrapure water spray, leading to a ∼16-fold increase of spray-sourced hydrogen that can hydrogenate CO2 to selectively produce CO. These findings emphasize the potential of charge separation enabled by spraying an emulsion of liquid water and a hydrophobic liquid in driving hydrogenation reactions.

Spatially-assembled binary carbon anode synergizing directional electron transfer and enriched microbe accommodation for wastewater treatment and energy conversion: From simulation to experiments

Bioelectrochemical systems (BESs) hold prospects in wastewater energy and resource recovery. Anode optimization is important for simultaneous enhancement of wastewater energy conversion and effluent quality in BESs. In this study, a multi-physics model coupling fluid flow, organic degradation and electrochemical process was constructed to guide the design and optimization of BES anodes. Based on the multi-physics simulation, spatially-assembled binary carbon anodes composed of three-dimensional carbon mesh skeleton and granular activated carbon were proposed and established. The granular activated carbon conducive to microbe accommodation played a vital role in improving effluent water quality, while the carbon mesh skeleton favoring electron collection and transfer could enhance the bioelectricity output. With an average chemical oxygen demand (COD) removal rate of 0.442 kg m-3 d-1, a maximum power density of 20.6 W m-3 was achieved in the optimized composite anode BES, which was 25% and 154% higher than carbon mesh skeleton BES and granular activated carbon BES. Electroactive bacteria were enriched in composite anodes and performed important functions related to microbial metabolism and energy production. The spatially-assembled binary carbon anode with low carbon mesh packing density was more cost-effective with a daily energy output per anode cost of 221 J d-1 RMB-1. This study not only provides a cost-efficient alternative anode to simultaneously improve organic degradation and power generation performance, but also demonstrates the potential of multi-physics simulation in offering theoretical support and prediction for BES configuration design as well as optimization.

A multi-criteria decision framework for circular wastewater systems in emerging megacities of the Global South

Lima faces increasing water stress due to demographic growth, climate change and outdated water management infrastructure. Moreover, its highly centralized wastewater management system is currently unable to recover water or other resources. Hence, the primary aim of this study is to identify suitable wastewater treatment alternatives for both eutrophication mitigation and indirect potable reuse (IPR). For eutrophication mitigation, we examined MLE, Bardenpho, Step-feed, HF-MBR, and FS-MBR. For IPR, we considered secondary treatment+UF + RO + AOP or MBR + RO + AOP. These alternatives form part of a WWTP network at a district level, aiding Lima's pursuit of a circular economy approach. This perspective allows reducing environmental impacts through resource recovery, making the system more resilient to disasters and future water shortages. The methods used to assess these scenarios were Life Cycle Assessment for the environmental dimension; Life Cycle Costing for the economic perspective; and Multi-Criteria Decision Analysis to integrate both the quantitative tools aforementioned and qualitative criteria for social and techno-operational dimensions, which combined, strengthen the decision-making process. The decision-making steered towards Bardenpho for eutrophication abatement when environmental and economic criteria were prioritized or when the four criteria were equally weighted, while HF-MBR was the preferred option when techno-operational and social aspects were emphasized. In this scenario, global warming (GW) impacts ranged from 0.23 to 0.27 kg CO2eq, eutrophication mitigation varied from 6.44 to 7.29 g PO4- equivalent, and costs ranged between 0.12 and 0.17 €/m3. Conversely, HF-MBR + RO + AOP showed the best performance when IPR was sought from the outset. In the IPR scenario, GW impacts were significantly higher, at 0.46-0.51 kg CO2eq, eutrophication abatement was above 98 % and costs increased to ca. 0.44 €/m3.

Predictive modeling based on artificial neural networks for membrane fouling in a large pilot-scale anaerobic membrane bioreactor for treating real municipal wastewater

Membrane fouling is the primary obstacle to applying anaerobic membrane bioreactors (AnMBRs) in municipal wastewater treatment. This issue holds critical significance as efficient wastewater treatment serves as a cornerstone for achieving environmental sustainability. This study uses machine learning to predict membrane fouling, taking advantage of rapid computational and algorithmic advances. Based on the 525-day operation data of a large pilot-scale AnMBR for treating real municipal wastewater, regression prediction was realized using multilayer perceptron (MLP) and long short-term memory (LSTM) artificial neural networks under substantial variations in operating conditions. The models involved employing the organic loading rate, suspended solids concentration, protein concentration in extracellular polymeric substance (EPSp), polysaccharide concentration in EPS (EPSc), reactor temperature, and flux as input features, and transmembrane pressure as the prediction target output. Hyperparameter optimization enhanced the regression prediction accuracies, and the rationality and utility of the MLP model for predicting large-scale AnMBR membrane fouling were confirmed at global and local levels of interpretability analysis. This work not only provides a methodological advance but also underscores the importance of merging environmental engineering with computational advancements to address pressing environmental challenges.

Increase metabolic heat to compensate for low temperature in activated sludge systems

The efficient operation of activated sludge systems is frequently hindered by low temperatures, and extensive research has been conducted to overcome this difficulty. However, the effect of varying temperatures on heat generation during substrate degradation remains unclear. In this study, results from laboratory-scale reactors show that sludge generated 5.36 ± 0.58 J/mg COD, 4.45 ± 0.24 J/mg COD, and 4.22 ± 0.26 J/mg COD at 10 °C, 20 °C, and 30 °C under aerobic conditions, respectively. Similarly, the sludge generated 4.05 ± 0.31 J/mg COD, 2.37 ± 0.15 J/mg COD, and 2.89 ± 0.18 J/mg COD under anoxic conditions. Despite the decreased respiration rates and hence reduced pollutant removal efficiency, sludge exhibited effective heat generation at low temperatures. Results from the full-scale plant also show a negative correlation between the heat generation capacity of microorganisms and the temperatures. 14.2 °C is considered the critical wastewater temperature for microorganisms' heat generation to offset the investigated plant's heat dissipation. This observation verified that thermal compensation for low temperatures was also significant in the full-scale plant. The mechanism of low-temperature compensation is attributed to non-growth processes being less dependent on temperature than growth processes, resulting in slow microbial growth but high heat generation at low temperatures. These findings provide valuable insights into the design and sustainable operation of wastewater treatment plants.

Insights into current bio-processes and future perspectives of carbon-neutral treatment of industrial organic wastewater: A critical review

With the rise of the concept of carbon neutrality, the current wastewater treatment process of industrial organic wastewater is moving towards the goal of energy conservation and carbon emission reduction. The advantages of anaerobic digestion (AD) processes in industrial organic wastewater treatment for bio-energy recovery, which is in line with the concept of carbon neutrality. This study summarized the significance and advantages of the state-of-the-art AD processes were reviewed in detail. The application of expanded granular sludge bed (EGSB) reactors and anaerobic membrane bioreactor (AnMBR) were particularly introduced for the effective treatment of industrial organic wastewater treatment due to its remarkable prospect of engineering application for the high-strength wastewater. This study also looks forward to the optimization of the AD processes through the enhancement strategies of micro-aeration pretreatment, acidic-alkaline pretreatment, co-digestion, and biochar addition to improve the stability of the AD system and energy recovery from of industrial organic wastewater. The integration of anaerobic ammonia oxidation (Anammox) with the AD processes for the post-treatment of nitrogenous pollutants for the industrial organic wastewater is also introduced as a feasible carbon-neutral process. The combination of AnMBR and Anammox is highly recommended as a promising carbon-neutral process for the removal of both organic and inorganic pollutants from the industrial organic wastewater for future perspective. It is also suggested that the AD processes combined with biological hydrogen production, microalgae culture, bioelectrochemical technology and other bio-processes are suitable for the low-carbon treatment of industrial organic wastewater with the concept of carbon neutrality in future.

Mechanistic insights into synergistic facilitation of copper/zinc ions and dewatered swine manure-derived biochar on anaerobic digestion of swine wastewater

Biochar-assisted anaerobic digestion (AD) has been proposed an advanced system for swine wastewater (SW) management. However, the effects of metallic nutrients in SW, such as copper/zinc ions (Cu2+/Zn2+), on the biochar-assisted AD of SW are not well understood. This study investigated the influences of individual Cu2+/Zn2+ or dewatered swine manure-derived biochar, as well as their combined additions, on the AD of SW. The results showed that exposure to 50 mg/L Cu2+/Zn2+ temporary inhibited methane production, but the addition of 20 g/L biochar alleviated this inhibition by shortening the methanogenic lag time and increasing methane yield. Following a period of acclimation, both Cu2+/Zn2+ and biochar promoted methane production, although metagenomic analysis revealed distinct mechanisms underlying their promotion. Cu2+/Zn2+ enhanced ATP processing, including electron exchange between NADH/NAD+ and succinate/fumarate transformation, by 26.0-35.8%. Additionally, the gene encoding Coenzyme M methylation was upregulated by 36.2% along with enrichments of Methanocullus and Methanosarcina, contributing to accelerated hydrolysis and methanogenesis rates by 54.7% and 44.8%, respectively. On the other hand, biochar mainly stimulated bacterial F-type ATPase activities by 28.4%, likely facilitating direct interspecies electron transfer between Geobacter and Methanosarcina for syntrophic methanogenesis. The combined addition of Cu2+/Zn2+ and biochar resulted in "win-win" benefits, significantly increasing the maximum methane production rate from 40.3 mL CH4/d to 53.7 mL CH4/d. Moreover, the introduction of biochar into AD of SW facilitated the transformation of more Cu2+/Zn2+ from a reducible Fe-Mn oxides form to a residual form, which potentially reduced the metallic toxicity of the digestate for soil amendment. The findings of this study provide novel insights into understanding the synergistic impacts of heavy metals and biochar in regulating SW during AD, as well as the management of associated digestate.

A comprehensive framework for eco-environmental impact evaluation of wastewater treatment plants: Integrating carbon footprint, energy footprint, toxicity, and economic assessments

The need for clear and straightforward guidelines for carbon footprint (CFP) and energy footprint (EFP) evaluations is critical due to the non-transparent and misleading results that have been reported. This study aims to address this gap by integrating CFP, EFP, toxicity, and economic assessments to evaluate the eco-environmental impacts of wastewater treatment plants (WWTPs). The results indicate that the total CFP was below 0.6 kg CO2/kg COD removed, which is attributed to CO2 offset and biogas recovery. However, site-specific EFP varied considerably from 482.7 to 2294 kgCO2/kWh due to design differences of WWTPs and their aeration and mixing energy demand (46.96-66.1%). The use of crude oil and natural gas for electricity generation significantly increased EFP, CFP, and carcinogenic human toxicity. In contrast, a combined heat and power (CHP) installation enabled energy recovery ranging from 12.09% to 65.65%. Construction costs dominated the highest share of total costs (85.43%), with indirect construction costs (42.9%) and operation labor costs (61.4%) being the primary elements in the total net costs. It is worth noting that site-specific CO2 emission factors were used in the calculations to decrease model uncertainty. However, to improve modeling reliability, we recommend modifying the regional CO2 emission factor and focusing on emerging technologies to recover energy and biogas.

Insight into the Pseudocapacitive Behavior of Electroactive Biofilms in Response to Dynamic-Controlled Electron Transfer and Metabolism Kinetics for Current Generation in Water Treatment

Electroactive biofilms (EBs) engage in complex electron transfer and storage processes involving intracellular and extracellular mediators with temporary electron storage capabilities. Consequently, electroactive biofilms exhibit pseudocapacitive behaviors during substrate degradation processes. However, comprehensive systematic research in this area has been lacking. This study demonstrated that the pseudocapacitive property was an intrinsic characteristic of EBs. This property represents dynamic-controlled electron transfer and is critical in current generation, unlike noncapacitive responses. Nontransient charge and discharge experiments revealed a correlation between capacitive charge accumulation and current generation in EBs. Additionally, analysis of substrate degradation suggested that the maximum power density (Pmax) changed with the kinetic constants of COD degradation, with pseudocapacitances of EBs directly proportional to Pmax. The interaction networks of key latent variables were evaluated through partial least-squares path modeling analysis. The results indicated that cytochrome c was closely associated with the formation of pseudocapacitance in EBs. In conclusion, pseudocapacitance can be considered a valuable indicator for assessing the complex electron transfer behavior of EBs. Pseudocapacitive biofilms have the potential to efficiently regulate biological reactions and serve as a promising carbon-neutral and renewable strategy for energy generation and storage. An in-depth understanding of the intrinsic property of pseudocapacitive behavior in EBs can undoubtedly advance the development of this concept in the future.

Prediction of biological nutrients removal in full-scale wastewater treatment plants using H2O automated machine learning and back propagation artificial neural network model: Optimization and comparison

The effective control of total nitrogen (ETN) and total phosphorus (ETP) in effluent is challenging for wastewater treatment plants (WWTPs). In this work, automated machine learning (AutoML) (mean square error = 0.4200 ∼ 3.8245, R2 = 0.5699 ∼ 0.6219) and back propagation artificial neural network (BPANN) model (mean square error = 0.0012 ∼ 6.9067, R2 = 0.4326 ∼ 0.8908) were used to predict and analyze biological nutrients removal in full-scale WWTPs. Interestingly, BPANN model presented high prediction performance and general applicability for WWTPs with different biological treatment units. However, the AutoML candidate models were more interpretable, and the results showed that electricity carbon emission dominated the prediction. Meanwhile, increasing data volume and types of WWTP hardly affected the interpretable results, demonstrating its wide applicability. This study demonstrated the validity and the specific advantages of predicting ETN and ETP using H2O AutoML and BPANN model, which provided guidance on the prediction and improvement of biological nutrients removal in WWTPs.

The magic of algae-based biochar: advantages, preparation, and applications

Compared with other biomass sources, the use of algae as a raw material to prepare biochar (BC) has important advantages including safety, high yield and economy. The protein content of algae cells is as high as 3.2 mg DCW/L, and the graphitic-N and N-O functional groups generated by the pyrolysis of proteins could effectively activate free radicals. Combined with the generated pore structure, the electron transfer/exchange capability was enhanced, which is conducive to improving its catalytic performance. Algae as a natural N source, the manuscript analyzed the surface properties and physicochemical properties of algae-based BC, and investigated its degradation effect on organic/inorganic pollutants in wastewater. Subsequently, the effect of nitrogen-doped BC on the adsorption/catalysis capacity was discussed. Finally, the directed preparation of algae-based BC applied in different scenarios was summarized. Algae-based BC has the property of N doping, which broadens its application efficiency in the environmental field. Overall, this manuscript reviews how to achieve efficient utilization of algae-based BC in wastewater.

Wastewater Treatment for Carbon Dioxide Removal

Wastewater treatment is notorious for its hefty carbon footprint, accounting for 1-2% of global greenhouse gas (GHG) emissions. Nonetheless, the treatment process itself could also present an innovative carbon dioxide removal (CDR) approach. Here, the calcium (Ca)-rich effluent of a phosphorus (P) recovery system from municipal wastewater (P recovered as calcium phosphate) was used for CDR. The effluent was bubbled with concentrated CO2, leading to its mineralization, i.e., CO2 stored as stable carbonate minerals. The chemical and microstructural properties of the newly formed minerals were ascertained by using state-of-the-art analytical techniques. FTIR identified CO3 bonds and carbonate stretching, XRF and SEM-EDX measured a high Ca concentration, and SEM imaging showed that Ca is well distributed, suggesting homogeneous formation. Furthermore, FIB-SEM revealed rhombohedral and needle-like structures and TEM revealed rod-like structures, indicating that calcium carbonate (CaCO3) was formed, while XRD suggested that this material mainly comprises aragonite and calcite. Results imply that high-quality CaCO3 was synthesized, which could be stored or valorized, while if atmospheric air is used for bubbling, a partial direct air capture (DAC) system could be achieved. The quality of the bubbled effluent was also improved, thus creating water reclamation and circular economy opportunities. Results are indicative of other alkaline Ca-rich wastewaters such as effluents or leachates from legacy iron and steel wastes (steel slags) that can possibly be used for CDR. Overall, it was identified that wastewater can be used for carbon mineralization and can greatly reduce the carbon footprint of the treatment process, thus establishing sustainable paradigms for the introduction of CDR in this sector.

Environmental assessment of on-site source-separated wastewater treatment and reuse systems for resource recovery in a sustainable sanitation view

Sustainable sanitation solutions are necessary for promoting public health and environmental security. In this study, on-site domestic wastewater treatment (WWT) systems used for households in rural and peri-urban areas of Brazil were compared in different scenarios from a life cycle assessment (LCA) perspective. The evaluated scenarios represented different practices in wastewater management, such as direct discharge into the soil, rudimentary treatment, septic tank, public sewerage system, and source separation of wastewater streams for water, nutrient, and organic matter recovery. The WWT technologies considered in the proposed scenarios of source-separated wastewater streams were as follows: an evapotranspiration tank (TEvap) and composting toilet for blackwater, a modified constructed wetland (EvaTAC) for greywater, and a storage tank for urine. LCA was performed in this study according to the ISO standards to assess the environmental impacts at both midpoint and endpoint levels. The results show that on-site source-separated wastewater treatment systems with resource recovery result in significant reductions in environmental impacts compared to scenarios with precarious conditions or 'end-of-pipe' solutions. For example, regarding the human health damage category, the scenarios involving resource recovery, including systems such as EvaTAC, TEvap, composting toilet, and urine storage tank, demonstrate significantly lower values (-0.0117 to -0.0115 DALY) compared to scenarios with rudimentary cesspits and septic tanks (0.0003 to 0.001 DALY). We conclude that the focus should be beyond mere pollution aspects and instead concentrate on the benefits of the co-products, which are: avoiding the extraction and consumption of valuable and increasingly scarce raw materials, such as potable water, and production of synthetic fertilizer. Furthermore, it is highly recommended that an LCA of sanitation systems synergistically integrates the WWT process, the constructive aspects, and the resource recovery potential.

A Critical Review of Data Science Applications in Resource Recovery and Carbon Capture from Organic Waste

Municipal and agricultural organic waste can be treated to recover energy, nutrients, and carbon through resource recovery and carbon capture (RRCC) technologies such as anaerobic digestion, struvite precipitation, and pyrolysis. Data science could benefit such technologies by improving their efficiency through data-driven process modeling along with reducing environmental and economic burdens via life cycle assessment (LCA) and techno-economic analysis (TEA), respectively. We critically reviewed 616 peer-reviewed articles on the use of data science in RRCC published during 2002-2022. Although applications of machine learning (ML) methods have drastically increased over time for modeling RRCC technologies, the reviewed studies exhibited significant knowledge gaps at various model development stages. In terms of sustainability, an increasing number of studies included LCA with TEA to quantify both environmental and economic impacts of RRCC. Integration of ML methods with LCA and TEA has the potential to cost-effectively investigate the trade-off between efficiency and sustainability of RRCC, although the literature lacked such integration of techniques. Therefore, we propose an integrated data science framework to inform efficient and sustainable RRCC from organic waste based on the review. Overall, the findings from this review can inform practitioners about the effective utilization of various data science methods for real-world implementation of RRCC technologies.

A municipal wastewater treatment plant "drinking beer" for reduction of cost and carbon emission

In wastewater treatment plants (WWTPs), external carbon sources are often required due to low C/N influent. However, the use of external carbon sources can increase treatment costs and cause large carbon emissions. Beer wastewater, which contains a substantial amount of carbon, is often treated separately in China, consuming significant energy and cost. However, most studies using beer wastewater as an external carbon source are still on a laboratory scale. To address this issue, this study proposes using beer wastewater as an external carbon source in an actual WWTP to reduce operating costs and carbon emissions while achieving a win-win situation. The denitrification rate of beer wastewater was found to be higher than that of sodium acetate , resulting in improved treatment efficiency of the WWTP. Specifically, COD, BOD₅, TN, NH₄⁺-N and TP increased by 3.4%, 1.6%, 10.8%, 1.1%, and 1.7%, respectively. Additionally, the treatment cost and carbon emission per 10 000 tons of wastewater treated were reduced by 537.31 yuan and 2.27 t CO₂, respectively. These results indicate that beer wastewater has significant utilization potential and provide a reference for using different types of production wastewater in WWTPs. This study's findings demonstrate the feasibility of implementing this approach in an actual WWTP setting.

In a quest for high-efficiency mainstream partial nitritation-anammox (PN/A) implementation: One-stage or two-stage?

Partial nitritation-anammox (PN/A) process is known as an energy-efficient technology for wastewater nitrogen removal, which possesses a great potential to bring wastewater treatment plants close to energy neutrality with reduced carbon footprint. To achieve this goal, various PN/A processes implemented in a single reactor configuration (one-stage system) or two separately dedicated reactors configurations (two-stage system) were explored over the past decades. Nevertheless, large-scale implementation of these PN/A processes for low-strength municipal wastewater treatment has a long way to go owing to the low efficiency and effectiveness in nitrogen removal. In this work, we provided a comprehensive analysis of one-stage and two-stage PN/A processes with a focus on evaluating their engineering application potential towards mainstream implementation. The difficulty for nitrite-oxidizing bacteria (NOB) out-selection was revealed as the critical operational challenge to achieve the desired effluent quality. Additionally, the operational strategies of low oxygen commonly adopted in one-stage systems for NOB suppression and facilitating anammox bacteria growth results in a low nitrogen removal rate (NRR). Introducing denitrification into anammox system was found to be necessary to improve the nitrogen removal efficiency (NRE) by reducing the produced nitrate with in-situ utilizing the organics from wastewater itself. However, this may lead to part of organics oxidized with additional oxygen consumed in one-stage system, further compromising the NRR. By applying a relatively high dissolved oxygen in PN reactor with residual ammonium control, and followed by a granules-based anammox reactor feeding with a small portion of raw municipal wastewater, it appeared that two-stage system could achieve a good effluent quality as well as a high NRR. In contrast to the widely studied one-stage system, this work provided a unique perspective that more effort should be devoted to developing a two-stage PN/A process to evaluate its application potential of high efficiency and economic benefits towards mainstream implementation.

Simultaneous partial nitrification, Anammox and nitrate-dependent Fe(II) oxidation (NDFO) for total nitrogen removal under limited dissolved oxygen and completely autotrophic conditions

Sustainable nitrogen removal from wastewater at reduced energy and/or chemical consumptions is challenging. This paper investigated, for the first time, the feasibility of coupled partial nitrification, Anammox and nitrate-dependent Fe(II) oxidation (NDFO) for sustainable autotrophic nitrogen removal. With NH₄⁺-N as the only nitrogen-containing compound in the influent, near-complete nitrogen removal (a total of 97.5 % with a maximal total nitrogen removal rate of 6.64 ± 2.68 mgN/L/d) was achieved in a sequencing batch reactor for a 203-d operation without organic carbon source addition and forced aeration. Anammox (predominated by Candidatus Brocadia) and NDFO bacteria (such as Denitratisoma) were successfully enriched, with total relative abundances up to 11.54 % and 10.19 %, respectively. Dissolved oxygen (DO) concentration was a key factor affecting the coupling of multi (ammonia oxidization, Anammox, NDFO, iron-reduction, etc.) bacterial communities, resulting in different total nitrogen removal efficiencies and rates. In batch tests, the optimal DO concentration was 0.50-0.68 mg/L with a maximal total nitrogen removal efficiency of 98.7 %. Fe(II) in the sludge not only competed with nitrite oxidizing bacteria for DO to prevent complete nitrification, but promoted the transcription of NarG and NirK genes (10.5 and 3.5 times higher than the group without Fe(II) addition) as indicated by the reverse transcription quantitative polymerase chain reaction (RT-qPCR), resulting in increased NDFO rate (by 2.7 times) and promoted NO₂^--N generated from NO₃^--N, which back fed the Anammox process, achieving near-complete nitrogen removal. The reduction of Fe(III) by iron-reducing bacteria (IRB) and hydrolytic and fermentative anaerobes enabled a sustainable Fe(II)/Fe(III) recycling, avoiding the need in continuous Fe(II) or Fe (III) dosage. The coupled system is expected to benefit the development of novel autotrophic nitrogen removal processes with neglectable energy and material consumptions for the treatment of wastewater with low organic carbon and NH₄⁺-N contents in underdeveloped regions, such as decentralized rural wastewaters.

Sustainability for wastewater treatment: bioelectricity generation and emission reduction

This review covers the technological measures of a self-sustainable anaerobic up-flow sludge blanket (UASB) system compared with an aerobic activated sludge process (ASP) for wastewater treatment plants (WWTPs). The ASP requires a huge amount of electricity and chemicals and also results in the emission of carbon. The UASB system, instead, is based on greenhouse gas (GHG) emission reduction and is associated with biogas production for cleaner electricity. WWTPs including the ASP system are not sustainable due to the massive financial power required for clean wastewater. When the ASP system was used, the amount of production was estimated to be 10658.98 tonnes CO₂eq-d^- of carbon dioxide. Whereas it was 239.19 tonnes CO₂eq-d^-1 with the UASB. The UASB system is advantageous over the ASP system as it has a high production of biogas, needs low maintenance, yields a low amount of sludge, and is also a source of electricity that can be used as a power source for the WWTPs. Also, the UASB system produces less biomass, and this helps in reducing costs and maintaining work. Moreover, the aeration tank of the ASP needs 60% of energy distribution; on the other hand, the UASB consumes less energy, approximately 3-11%.

Water-climate change extended nexus contribution to social welfare and environment-related sustainable development goals in China

Climate change exacerbates uncertainties in water resource management, water supply, and treatment that are energy intensive and then exert great pressure on climate change mitigation; hence, interrelated and contradictory characteristics within the water-climate change (WC) nexus system are needed to be studied. The nexus thinking and coordination of WC would impact many realistic practices and assist in sustainable socioeconomic development since traditional single-target policies have sometimes been out of function. Hence, the ability to direct water production and use as well as climate change mitigation has become a hotspot recently. Furthermore, we find that there has been no complete research on reviewing the impacts of the WC nexus in different areas on the Sustainable Development Goals (SDGs). Hence, this paper builds a core nexus of WC and then analyzes those effects on social and environmental aspects in many areas, including sewage treatment, energy transition, waste treatment, land management, and ocean management. This paper discusses how WC interlinkages are utilized to realize SDGs in those areas. Moreover, uncertainties derived from exogenous hydrology, climate change, and anthropogenic endogenous systems for realistic problems appeal to gradually increasing concern. Finally, implications offer valuable guidelines for integrated management of water and carbon emissions, as well as sustainable socioeconomic development in the future.

Metatranscriptional characterization of metabolic dynamics in anaerobic membrane bioreactor producing methane from low-strength wastewater

An anaerobic membrane bioreactor (AnMBR) with media is an emerging carbon-neutral biotechnology for low-strength wastewater (LSWW) treatment and methane recovery. Understanding metabolic dynamics among methanogens and syntrophic bacteria is important in optimizing the design and operation of AnMBR. However, little is known about it, especially in media-attached microbial communities. This study explored metabolic dynamics to compare media-attached and suspended conditions. Accordingly, metagenomes and metatranscriptomes from AnMBRs with polymeric media and fed with different influent concentrations (350 and 700 mg-COD/L) were analyzed. Metabolic dynamics were profoundly influenced by the different growth habitats and influent conditions, although the applied influent concentrations are within the range of typical LSWW. Metabolic dynamics prediction results suggest that media-attached-growth habitats may have provided a more favorable microenvironment for methanogens to grow and produce methane, especially under low influent conditions. These findings provide significant implications for optimizing floating media design and operation of AnMBR-producing methane from LSWW.

Co-composting of faecal sludge and carbon-rich wastes in the earthworm's synergistic cooperation system: Performance, global warming potential and key microbiome

Composting is an effective alternative for recycling faecal sludge into organic fertilisers. A microflora-earthworm (Eisenia fetida) synergistic cooperation system was constructed to enhance the composting efficiency of faecal sludge. The impact of earthworms and carbon-rich wastes (rice straw (RS) and sawdust (S)) on compost properties, greenhouse gas emissions, and key microbial species of composting were evaluated. The addition of RS or S promoted earthworm growth and reproduction. The earthworm-based system reduced the volatile solid of the final substrate by 13.19-16.24 % and faecal Escherichia coli concentrations by 1.89-3.66 log10 cfu/g dry mass compared with the earthworm-free system. The earthworm-based system increased electrical conductivity by 0.322-1.402 mS/cm and reduced C/N by 56.16-64.73 %. The NH₄⁺:NO₃^- ratio of the final faecal sludge and carbon-rich waste was <0.16. The seed germination index was higher than 80 %. These results indicate that earthworms contribute to faecal sludge maturation. Earthworm addition reduced CO₂ production. The simultaneous addition of earthworms and RS system (FRS2) resulted in the lowest global warming potential (GWP). The microbial diversity increased significantly over time in the RS-only system, whereas it initially increased and later decreased in the FRS2 system. Cluster analysis revealed that earthworms had a more significant impact on the microbial community than the addition of carbon-rich waste. Co-occurrence networks for earthworm-based systems were simple than those for earthworm-free systems, but the major bacterial genera were more complicated. Highly abundant key species (norank_f_Chitinophagaceae and norank_f_Gemmatimonadaceae) are closely related. Microbes may be more cooperative than competitive, facilitating the conversion of carbon and nitrogen in earthworm-based systems. This work has demonstrated that using earthworms is an effective approach for promoting the efficiency of faecal sludge composting and reducing GWP.

The long overlooked microalgal nitrous oxide emission: Characteristics, mechanisms, and influencing factors in microalgae-based wastewater treatment scenarios

Microalgae-based wastewater treatment is particularly advantageous in simultaneous CO₂ sequestration and nutrients recovery, and has received increasing recognition and attention in the global context of synergistic pollutants and carbon reduction. However, the fact that microalgae themselves can generate the potent greenhouse gas nitrous oxide (N₂O) has been long overlooked, most previous research mainly regarded microalgae as labile organic carbon source or oxygenic approach that interfere bacterial nitrification-denitrification and the concomitant N₂O production. This study, therefore, summarized the amount and rate of N₂O emission in microalgae-based systems, interpreted in-depth the multiple pathways that lead to NO formation as the key precursor of N₂O, and the pathways that transform NO into N₂O. Reduction of nitrite could take place in either the cytoplasm or the mitochondria to form NO by a series of enzymes, while the NO could be enzymatically reduced to N₂O at the chloroplasts or the mitochondria respectively under light and dark conditions. The influences of abiotic factors on microalgal N₂O emission were analyzed, including nitrogen types and concentrations that directly affect the nitrogen transformation routes, illumination and oxygen conditions that regulate the enzymatic activities related to N₂O generation, and other factors that indirectly interfere N₂O emission via NO regulation. The uncertainty of microalgae-based N₂O emission in wastewater treatment scenarios were emphasized, which would be particularly impacted by the complex competition between microalgae and ammonia oxidizing bacteria or nitrite oxidizing bacteria over ammonium or inorganic carbon source. Future studies should put more efforts in improving the compatibility of N₂O emission results expressions, and adopting consistent NO detection methods for N₂O emission prediction. This review will provide much valuable information on the characteristics and mechanisms of microalgal N₂O emission, and arouse more attention to the non-negligible N₂O emission that may impair overall greenhouse gas reduction efficiency in microalgae-based wastewater treatment systems.

Anaerobic membrane bioreactor for carbon-neutral treatment of industrial wastewater containing N, N-dimethylformamide: Evaluation of electricity, bio-energy production and carbon emission

The feasibility of anaerobic membrane bioreactor (AnMBR) for the treatment of N, N-dimethylformamide (DMF)-containing wastewater was theoretically compared with the conventional activated sludge (CAS) process in this study. The electricity consumption and expenditure, bio-energy production and CO₂ emission were investigated using the operational results of a lab-scale AnMBR operated in a long-term operation. The AnMBR was capable of producing bio-methane from wastewater and generated 3.45 kWh/m³ of electricity as recovered bio-energy while the CAS just generated 1.17 kWh/m³ of electricity from the post-treatment of excessive sludge disposal. The large quantity of bio-methane recovered by the AnMBR can also be sold as sustainable bioresource for the use of household natural gas with a theoretical profit gain of 29,821 US$/year, while that of the CAS was unprofitable. The AnMBR was also demonstrated to significantly reduce the carbon emission by obtaining a theoretical negative CO₂ production of -2.34 kg CO₂/m³ with the recycle of bio-energy while that for the CAS was 4.50 kg CO₂/m³. The results of this study demonstrate that the AnMBR process has promising potential for the carbon-neutral treatment of high-strength DMF-containing wastewater in the future.

Carbon Emission Accounting and the Carbon Neutralization Model for a Typical Wastewater Treatment Plant in China

To reduce carbon emissions and achieve carbon neutrality in China, it is pivotal to explore low-carbon wastewater treatment processes and carbon-neutral wastewater treatment plants (WWTPs). This study investigated the Beijing Gaobeidian WWTP to explore the current energy consumption and carbon emission status of representative WWTPs in China. Furthermore, it explored a possible low-carbon operating model. Results show that the current total energy consumption of Gaobeidian WWTP is 280,717 MWh/y, while its energy recovery is 268,788 MWh/y. As a result, the energy neutralization ratio is 95.8%, and the plant is close to reaching energy neutrality. The carbon emission of this plant is 446,468 t/y. However, it reduced its carbon emissions by 252,994 t/y and reached only 56.7% of carbon neutrality. Although the plant almost reached energy neutrality, it has a long way to go before reaching carbon neutrality. It was found that a subsequent increase in the recovery of residual heat from secondary effluent can increase the energy and carbon neutralization ratio to 523.1% and 219.0%, respectively, meaning that the WWTP can become a power production unit and a carbon sink. This study can provide a reference for exploring efficient energy use and reaching carbon neutrality for domestic WWTPs.

Data-driven analysis on immobilized microalgae system: New upgrading trends for microalgal wastewater treatment

Microalgal immobilization is receiving increasing attention as one of the most viable alternatives for upgrading conventional wastewater treatment. However, an in-depth discussion of the state-of-the-art and limitations of available technologies is currently lacking. More importantly, the reason for the hesitant development of immobilized microalgae for wastewater treatment remains unclear, which hinders its practical application. Thus, comprehensively understanding and evaluating details on immobilized microalgae is urgently needed, especially for the current advances of immobilization of microalgae in wastewater treatment over the last few decades. In this review, scientometric approach is used to explore research hotspots and visualize emerging trends. Data-driven analysis is used to scientifically and methodically determine hotspots in the current research on immobilized microalgal wastewater treatment, along with that the implicit inner connection underlying the frequent co-occurring terms was explored in depth. Four hotspots focusing on immobilized microalgae for wastewater treatment were identified, mainly demonstrating: (1) main factors including light, temperature and immobilization methods would majorly affect the treatment performance of immobilized microalgae; (2) immobilized microalgae membrane bioreactor, immobilized microalgae-based microbial fuel cell and immobilized microalgae-based bed reactor are three dominant treatment systems; (3) immobilized microalgae have a higher robustness and tolerance for treating various types of wastewater; and (4) a complete sustainable circle from wastewater treatment to resource conversion via the immobilized microalgae can be achieved. Finally, several new directions and new perspectives that expose the necessity for fulfilling further research and fundamental gaps are pointed out. Taken together, this review provides helpful information to facilitate the development of innovative and feasible immobilized microalgal technologies thus increasing their viability and sustainability.

Biowastes for biodegradable bioplastics production and end-of-life scenarios in circular bioeconomy and biorefinery concept

Due to global urbanization, industrialization, and economic development, biowastes generation represents negative consequences on the environment and human health. The use of generated biowastes as a feedstock for biodegradable bioplastic production has opened a new avenue for environmental sustainability from the circular (bio)economy standpoint. Biodegradable bioplastic production can contribute to the sustainability pillars (environmental, economic, and social). Furthermore, bioenergy, biomass, and biopolymers production after recycling of biodegradable bioplastic can help to maintain the energy-environment balance. Several types of biodegradable bioplastic, such as starch-based, polyhydroxyalkanoates, polylactic acid, and polybutylene adipate terephthalate, can achieve this aim. In this review, an overview of the main biowastes valorization routes and the main biodegradable bioplastic types of production, application, and biodegradability are discussed to achieve the transition to the circular economy. Additionally, end-of-life scenarios (up-cycle and down-cycle) are reviewed to attain the maximum environmental, social, and economic benefit from biodegradable bioplastic products under biorefinery concept.

Challenges in carbon footprint evaluations of state-of-the-art municipal wastewater resource recovery facilities

Wastewater treatment is an important source of direct and indirect greenhouse gas (GHG) emissions, which some wastewater operators report and account for CO₂-eq impacts through carbon footprint evaluations. We investigated the challenges with GHG emissions' accounting of three state-of-the-art energy-efficient wastewater resource recovery facilities (WRRFs) and reviewed their CO₂ accounting reports. Our study aimed to highlight the major contributors and factors to estimate emissions, including direct N₂O and CH₄ emissions and propose recommendations for public reporting of CO₂ accounting of WRRFs. We categorised emissions as direct (scope 1), background (scope 2), downstream and avoided emissions (scope 3A and 3B) and evaluated how a change in emission factor may affect how close the WRRFs are to reaching CO₂ neutrality. The results show that electricity consumption and direct emissions constitute between 20 and 70% of actual CO₂-eq emissions and therefore need careful consideration. All three plants have increasingly offset scope 2 emissions over 2014-2019, resulting in a total reduction of approximately 3211 tons CO₂-eq, corresponding to 72% of their needed cuts by 2030 set by the Danish government. No standard factors are used across the plants to estimate emissions. We propose some general recommendations that wastewater operators can apply to correctly report and account for CO₂-eq emissions. We also recommend that operators move their long-term focus from CO₂ neutrality to CO₂-eq reduction and make an effort to measure and quantify scope 1 direct emissions properly. A tax on N₂O emissions should be introduced in future policies.

The trade-off between N2O emission and energy saving through aeration control based on dynamic simulation of full-scale WWTP

This study investigated the trade-off between energy saving and N₂O emission reduction of WWTP under the precise control of dissolved oxygen (DO) concentration through model simulation. A long-term dynamic model for full-scale WWTP GHG emissions was established and calibrated with monitored year-round hourly water quality data to quantify the annual GHG emissions from WWTP. Results showed that N₂O dominated the direct emission, up to 76.1%, and the variability of N₂O generation could better be revealed by dynamic simulation. Furthermore, GHG emissions of the WWTP were mainly contributed by electric energy, among which the blower consumes the most electricity. To reduce the electricity consumption of blowers, improve mechanical efficiency and reduce DO concentration should be considered. DO setting played a significant role in the N₂O and CH₄ emission, electricity consumption and effluent quality, which was challenging to balance. The ultralow-oxygen (0-1/0.2-1 mg/L) and low oxygen (1-2 mg/L) control strategies were proposed, and their effects on total GHG emissions and effluent water quality were discussed. If the anaerobic environment (DO<0.2 mg/L)could be avoided, the control frequency (high and low) of the DO set-point did not have a significant effect on the emissions of N₂O and CH₄ and the effluent quality. The ultralow-oxygen strategy (0.2-1 mg/L) with a high-frequency control strategy achieved the lowest GHG emissions under the current energy mix. However, by 2050, as the energy supply gets cleaner, the total GHG emissions of WWTPs with ultralow-oxygen aeration (0.2-1 mg/L) will exceed low-oxygen aeration by 3.6%-4.2%, as N₂O dominates 61.6%. Therefore, considering the trade-off between N₂O emission and energy saving in WWTP, ultralow-oxygen aeration is a transition scheme to cleaner energy.