Emerging investigators series: revisiting greenhouse gas mitigation from conventional activated sludge and anaerobic-based wastewater treatment systems

Siming Chen , Moustapha Harb , Pooja Sinha and Adam L. Smith *
Civil & Environmental Engineering, Astani Department of Civil and Environmental Engineering, University of Southern California, 3620 South Vermont Avenue, Los Angeles, CA 90089, USA. E-mail: smithada@usc.edu; Tel: +1 213 740 0473

First published on 25th September 2018

---

---

Siming Chen

Siming Chen is a Ph.D. student in Environmental Engineering at the University of Southern California. He holds a B.S. degree in Environmental Engineering from China University of Mining and Technology and an M.S. degree in Environmental Engineering from the University of Southern California. His research focuses on post-treatment of mainstream anaerobic effluents, with emphasis on bioelectrochemical systems that recover energy from dissolved methane. His research interests are on energy and other resource recovery during wastewater treatment.

Moustapha Harb

Dr. Moustapha Harb is currently a Postdoctoral Research Associate in the Department of Civil and Environmental Engineering at the University of Southern California. He holds B.S. and M.S. degrees in Civil and Environmental Engineering from the University of Houston and a Ph.D. in Environmental Science and Engineering from King Abdullah University of Science and Technology (KAUST). Dr. Harb's research interests involve the development of anaerobic biotechnologies, mitigating the impact of microbial contaminants, and understanding the fate of micropollutants in wastewater environments.

Pooja Sinha

Pooja Sinha is an Environmental Engineer in Training with project experience in wastewater treatment and force main design. She has additional research experience in aerobic granular sludge technology (AGS), stormwater quality, and microbial community analysis of anaerobic systems. Her primary areas of focus include water reuse, process engineering, stormwater treatment, and pump stations. Ms. Sinha holds her B.S. in Biotechnology from SRM University and her M.S. in Environmental Engineering from USC.

Adam L. Smith

Dr. Adam L. Smith is an Assistant Professor in the Astani Department of Civil and Environmental Engineering at the University of Southern California. He received his M.S.E. and Ph.D. from the University of Michigan in Environmental Engineering in 2011 and 2014, respectively. He received his B.S. in Civil Engineering from Marquette University in 2009. The Smith Research Group focuses on biotechnologies for resource recovery from waste streams.

---

Water impact Conventional and mainstream anaerobic wastewater processes remain difficult to compare in terms of their global warming potential (GWP) due to unresolved variability in high-impact GHG emissions. Here, we identify knowledge gaps in those GHG contributors, highlight potential mitigation strategies, and provide a basis for the direct comparison of CAS and anaerobic wastewater treatment.

1. Introduction

As the risks of climate change become increasingly acute, the necessity for accurate greenhouse gas (GHG) accounting has led to a renewed focus on wastewater management as an emissions source. The most widely employed wastewater treatment methods, namely aerobic (i.e., activated sludge-based processes) and mainstream anaerobic processes, both significantly contribute to GHG generation in their current forms of implementation. Mainstream anaerobic processes, which lessen WWTP energy costs and biosolids generation, are receiving renewed interest as an alternative to aerobic processes.¹ The EPA Inventory of GHG Emissions and Sinks estimates that US wastewater treatment plants (WWTPs) accounted for approximately 0.3% of overall emissions in 2016, with CH₄ and N₂O accounting for 3 and 5 MMT CO₂ equivalents, respectively.² CH₄ and N₂O are of particular concern due to their relatively high 100-year global warming potentials (34 and 298 CO_2eq, respectively),³ with WWTPs currently estimated to be the sixth largest contributor of N₂O emissions worldwide (approximately 3%).⁴ Despite the magnitude of N₂O emissions from WWTPs, our understanding of formation mechanisms and ability to model or predict emissions remains lacking.^5,6

Today, conventional activated sludge (CAS) processes coupled with anaerobic digestion are widely used in domestic wastewater treatment, despite their high energy requirements (up to 3% of overall US electricity consumption) and lack of large-scale energy and nutrient recovery.⁷ Although anaerobic digestion significantly offsets WWTP energy demands and reduces sludge handling requirements, it is unclear whether this conventional approach will remain attractive in light of recent advances in mainstream anaerobic treatment.⁸ Mainstream anaerobic systems directly recover energy via biogas production, produce drastically less sludge, and have been proven viable at a range of operational temperatures.⁹ Taking advantage of their favorably warm climate, several Latin American countries have long incorporated mainstream anaerobic processes, specifically upflow anaerobic sludge blankets (UASBs), for full-scale domestic wastewater management.^10,11 The integration of membrane separation and anaerobic treatment (i.e., anaerobic membrane bioreactors (AnMBRs)) has greatly expanded interest in mainstream anaerobic processes worldwide.^12,13 This has led to recent advances in the technology's application at ambient temperatures, as well as promising testing at the pilot-scale.^14,15 However, loss of dissolved CH₄ in effluents is an outstanding concern. Such losses not only reduce energy recovery, but also pose severe environmental impacts due to GHG emissions.¹ A direct comparison of GHG emissions between aerobic and anaerobic processes is vital to help stakeholders navigate a potential transition to anaerobic treatment.

Several review papers have been published to address GHG emissions from conventional WWTPs, many of which have focused specifically on N₂O emissions.^5,16–18 For mainstream anaerobic treatment, a recent review by Crone et al. evaluated dissolved effluent CH₄ while discussing technologies for recovery.¹⁹ Despite the significant contributions of the aforementioned reviews, N₂O and CH₄ emission quantifications for the purpose of directly comparing CAS and mainstream anaerobic treatment systems' GWP remain unavailable. The primary purpose of the current study is to systematically focus on outstanding knowledge gaps in GHG emissions limiting direct comparability of CAS and mainstream anaerobic treatment. The issues specifically evaluated in this work include CAS WWTP total GHG estimation, pathway-associated N₂O generation mechanisms in aerobic-based WWTPs, and dissolved CH₄ recovery efficiency for anaerobic system effluents.

2. Anthropogenic GHG emissions from CAS WWTPs

2.1. Overview of GHG generation in wastewater treatment processes

GHG emissions are attributable to essentially every unit process in conventional aerobic wastewater treatment coupled with anaerobic digestion (Fig. 1). Here, we categorize these emissions as either direct or indirect, where direct emissions include GHGs physically produced by either in-plant or downstream environmental processes and where indirect emissions include electrical energy demands and chemical inputs of the system. From the perspective of indirect emissions, aeration tanks comprise more than 40% of total plant energy demand^7,20,21 and are often reported as contributing most significantly to overall GHG emissions (approximately 0.298 kg CO_2e per m³ based on reported U.S. energy carbon footprint of 0.472 kg CO₂ kW⁻¹ h⁻¹).^22–25 Sidestream processes for primary and waste activated sludges also contribute indirect GHG emissions via energy demand and chemical addition during dewatering, transportation, land application, and landfilling,^26,27 which can account for between 0.134 to 0.167 kg CO_2e per m³ of domestic wastewater.²⁸ It should be noted, however, that biogas production from anaerobic digestion can significantly offset indirect GHG emissions by lessening reliance on a potentially GHG emission heavy primary energy mix.^24,26

Regarding direct GHG emissions, N₂O generated during denitrification, either in anoxic tanks (in the case of CAS with biological nitrogen removal) or in the receiving aquatic environment when nitrate-rich effluent is released (in the absence of on-site anoxic treatment), is considered the primary source of direct GHG emissions.² The EPA's Inventory of US Greenhouse Gases and Sinks reports this N₂O emission source as part of effluent emissions due to the majority of plants not employing biological nitrogen removal. However, in scenarios where nitrogen removal is achieved, these emissions are largely confined to within the plant footprint.²⁹ Aeration tanks are also responsible for direct N₂O generation as a result of incomplete nitrification, with their contribution to total N₂O footprint being recently identified as potentially much higher than previously considered.² In addition to N₂O, aeration tanks are also responsible for significant generation of CO₂ due to microbial degradation of organic carbon, however, these direct CO₂ emissions are not traditionally considered in GHG accounting because of their biogenic origin.³⁰ Despite this, recent research has shown that approximately 14% of total organic carbon in municipal wastewater is actually of non-biogenic origins due to domestic use of soaps and detergents, leading to an underestimation of direct GHG emissions.^31,32 Quantifying direct emissions of CAS WWTPs has proven to be the most challenging aspect of GHG estimation, as quantification methods and assumptions are wide ranging in existing literature and governmental reports. In the following, GHG emissions are normalized to volume of domestic wastewater (DWW) treated to better compare parallel studies, regardless of differences in treatment process.

2.2. Common findings of different quantification methods for plant-wide GHG emissions

Two common approaches have been reported for quantifying direct GHG emissions: (1) emission factor-based methods derived from dynamic modeling and (2) actual emission values determined from on-site measurement. Generally, model-based studies have reported high variability in overall plant emissions (from 0.24 to 2.4 kg CO_2e per m³ DWW), with contradictory findings regarding the primary source of emissions.^20,33–35 A clear consensus has thus not yet been reached identifying the major contributors to total GHG emissions.⁶ Models are typically constrained to specific plant configurations and feature inconsistent emission factors. Nonetheless, one common observation is that N₂O emissions contribute the greatest uncertainty in emissions estimation,^33,36 particularly due to its excessive GWP and the lack of a comprehensive mechanism-based model of formation.⁶

On-site quantification methods have included sampling and subsequent lab analysis,^22,23,37 on-line off-gas collection and analysis,^38–41 and tracer dispersion monitoring.⁴² Normalization of on-site sampling methodology has enabled the comparison of different treatment processes, such as activated sludge, oxidation ditches, anaerobic/anoxic/aerobic processes (A²O), and reverse A²O.^22,37 Research taking advantage of covered treatment units and direct off-gas on-line analyzers showed correlation between operational conditions and GHG emissions. Such trends include emission increases with seasonal water temperature variation,³⁸ changes in aeration rates,³⁹ discharge of reject water to influent streams,³⁹ length of anoxic/oxic phases (in sequencing batch reactors (SBRs)),⁴⁰ and influent nitrite variations.⁴¹ The use of less conventional approaches, such as tracer addition and dispersion monitoring, have generally been less accurate compared to on-site measurement. A study by Yoshida et al., for example, that utilized tracers and mobile cavity ring-down spectroscopy sampling, found large variations in emissions over multiple campaigns, with CH₄ generation ranging from 4.99 to 92.3 kg h⁻¹ and N₂O from 0.37 to 10.5 kg h⁻¹.⁴²

Although aeration energy consumption is the primary contributor to indirect GHG emissions,^33,38,40 direct emission rates remain less clear and are a significant obstacle to achieving a plant-wide understanding.^22,23,37 Further, aeration control strategies impact both energy consumption and N₂O generation (affected by DO levels), implying that tradeoffs exist between direct and indirect GHG emissions.²⁰ Existing literature has reported between 0 and 14.6% of nitrogen entering WWTPs being converted to N₂O,^{5,26,28,40,42} contributing 1% to 78.4% of overall plant carbon footprints.^{23,24,28,38,40} Based on this extreme variability, a more thorough evaluation of literature addressing N₂O emissions in aerobic-based WWTPs is necessary.

2.3. Specific direct GHG emission sources in CAS WWTPs

3. GHG emission management for mainstream anaerobic treatment

Anaerobic processes are considered a sustainable and energetically favorable alternative to conventional aerobic processes. Anaerobic processes directly convert organics to methane-rich biogas and eliminate energy requirements associated with aeration.⁹ However, the release of dissolved CH₄ along with discharged effluents remains a significant implementation concern, severely increasing GHG emissions while concomitantly reducing potential energy recovery.^1,19 Such losses, which in extreme cases account for up to 90% of total produced CH₄, pose a severe environmental threat if mainstream anaerobic treatment becomes the norm.¹¹ Still, successful mitigation of these emissions would enable anaerobic treatment with less GHGs than CAS processes, providing impetus for advancing dissolved CH₄ recovery technologies.⁹²

3.1. GHG emission sources in anaerobic bioreactors

Given that the majority of GHG emission-related research on anaerobic treatment has been conducted at the bench- and pilot-scale,^93–106 full-scale indirect CO₂-based emissions estimates for electricity consumption remain largely unconfirmed. Nonetheless, energy balances of mainstream anaerobic treatment are generally expected to significantly improve upon current CAS.¹⁰⁷ In addition, an objective comparison of GHG emissions between CAS and mainstream anaerobic processes requires inclusion of downstream nutrient removal processes for anaerobic systems (e.g., partial nitritation–anammox). Such nitrogen removal processes have been shown to emit even higher levels of N₂O than CAS, as reviewed by Massara et al.,¹⁶ and necessitate further process optimization to be successfully mitigated.

Even accounting for these uncertainties, the most significant GHG-associated threat from mainstream anaerobic treatment remains effluent CH₄ losses. CH₄ saturation relative to Henry's law in anaerobic effluents has been observed to range between a factor of 1.0 to 5.2, resulting in the loss of 10–90% of total CH₄ produced.¹⁹ Recent work has demonstrated robust operation (i.e., COD removal) at temperatures as low as 6 °C. However, such low temperatures exacerbate GHG emission concerns by inherently increasing CH₄ solubility.⁹⁶ Overall, CH₄ solubility at low temperatures is largely responsible for increasing trends in dissolved CH₄ concentrations across all anaerobic bioreactor system types (Fig. 3a), despite reactor configuration and biogas composition also playing a role. Although the integration of membrane filtration in AnMBRs has improved effluent quality at such low temperatures, similar CH₄ oversaturation is still observed.^101,106 Studies by Smith et al.^106,108 on low-temperature AnMBR operation, specifically, have documented the likelihood that high methanogenic activity in membrane biofilms are responsible for dissolved CH₄ oversaturation. Experimental findings suggested that as systems increasingly relied on membrane biofilm-based treatment at decreased temperatures, biofilm methanogens directly emitted CH₄ into the effluent.¹⁰⁶

Although limitations of gas–liquid transfer rates have been identified as an obstacle for recovery, in situ biogas sparging readily achieves gas–liquid equilibrium and maximizes CH₄ evolution to headspace. Several recent studies have demonstrated effluent CH₄ saturation factors of close to 1 and/or reduced dissolved CH₄ content by up to 50% by employing in situ biogas sparging.^98,103 AnMBRs operating at temperatures above 20 °C have also shown relatively low saturation factors (in the range of 1.0 to 1.1).^100,109,110 Yet, other work has shown CH₄ saturation exceeding a factor of 1.5 or greater, even when biogas sparging is sufficiently utilized.

The strongest deviation above CH₄ saturation level was observed for scenarios at 15 °C and lower (Fig. 3b). In some scenarios, no biogas CH₄ was produced, with all produced CH₄ being evolved in the effluents (Fig. 3c).^96,106 Given that this phenomenon is likely caused by disproportionate biofilm-based CH₄ production, it cannot be easily mitigated by reactor biogas sparging/stripping.^101,106 Using 34 as a standard GWP factor for CH₄, GHG emissions from dissolved CH₄ were calculated to be in the range of 0.281 to 2.82 kg CO_2e per m³ DWW. This is generally comparable to the wide-ranging rates of CAS (0.24 to 2.4 kg CO_2e per m³) and will ultimately necessitate downstream CH₄ recovery technologies.

3.2. Utilizing membrane contactors for dissolved CH₄ recovery

Of the physiochemical-driven methods examined for dissolved CH₄ removal and/or recovery from anaerobic effluents, the most widely tested involves membrane contactors for effluent CH₄ desorption. Relevant operational parameters of these systems include membrane properties, contact area, gas/liquid flow rates, flow direction, vacuum pressure, and gas/liquid supply sources (shell or lumen). In the following, we provide a comparative analysis of the energy use/recovery potential of the two primary modes of membrane contactor operation, namely sweep gas- and vacuum-based desorption.

3.3. Analysis of energy demands and recovery by membrane contactors

Superior removal rates are achievable by vacuum degasification as compared to sweep gas operation.¹¹⁵ Further, vacuum desorption has specific advantages associated with direct on-site CH₄ use, which are not achievable by sweep gas contactors. Such advantages, however, must be evaluated in comparison with the greater energy requirements of vacuum-driven transmembrane pressure (TMP). Given the knowledge gaps in literature from the perspective of energy use and recovery, a comparative analysis of the practical limitations of each CH₄ recovery method is necessary to assess each technology's economic feasibility and overall GWP.

As summarized in Table 1, CH₄ concentrations for sweep gas driven membrane contactors are generally less than 2.4% of total off-gas volume, with only one case demonstrating relatively high concentrations of 23.2% with a polypropylene HFMC operated at low gas to liquid (G/L) ratios.^{97,110–112,114–116} In most cases, sweep gas driven membrane contactors exhibited increasing effluent removal efficiencies at higher gas to liquid (G/L) ratios, which also lead to decreased CH₄ off-gas purity. The majority of studies to date investigating sweep gas membrane contactor use have had a primary objective of reducing effluent concentrations to eliminate combustion risks in downstream discharge piping. Therefore, the systems were not operated to achieve optimal off gas concentrations. Without post-removal purification of sweep gas, limited approaches are available for utilization, as CH₄ concentrations are generally too low for even basic off-gas flaring (i.e., >5% CH₄). However, recent developments have shown that direct combustion via thermal/catalytic flow reversal reactors, regenerative/catalytic oxidation, or lean burn-gas turbine combustion can be achieved at CH₄ concentrations as low as 1%.¹¹⁷ Implementing such air-based off-gas in on-site cogeneration plant engines has been proposed previously for anaerobic digester dewatering process gasses.⁵¹ These applications, in combination with optimization of G/L ratios, could lead to sweep gas membrane contactors being a viable option for effluent CH₄ GHG mitigation.

---

A recent review by Crone et al.¹⁹ calculated an energy input to recovery ratio of 1.0 for effluent CH₄ recovery using vacuum driven membrane contactors. However, given the high variability in existing literature associated with operational parameters of vacuum degasification, a more comparable evaluation of these variables is necessary. Table 2 provides a normalized summary of studies on vacuum-driven membrane contactors, their energy requirements, and the potential energy content of recovered CH₄ using a unified methodology. Results of multiple studies suggest that systems operated at TMPs between 14 and 50 kPa are generally energy positive while maintaining CH₄ recovery rates between 60 and 90%.^94,113–115 Further, CH₄ recovery in different scenarios did not improve substantially with increasing vacuum pressure, proving that low vacuum scenarios are generally effective. Based on these observations, operating vacuum driven membrane contactors at relatively low vacuum pressures (<50 kPa) can enable the entire HFMC system to be energy neutral/positive while achieving sufficient effluent GHG reduction.

---

3.4. Biological approaches

Biological oxidation is a promising alternative strategy for CH₄ removal from anaerobic effluents. The most common technique is the downflow hanging sponge (DHS). This method has proven highly effective by numerous studies, as previously reviewed.¹⁹ In comparison, another potentially promising treatment system that has yet to be fully investigated is known as denitrifying anaerobic CH₄ oxidation (DAMO), and is reviewed herein.

3.5. Implications of physical vs. biological systems for CH₄ mitigation

It should be noted that the aforementioned biological approaches, while potentially requiring less energy input than membrane contactors and still mitigating GHG emissions, do not capture CH₄ for energy recovery. Although this is a significant limitation for DHS systems, DAMO's use of CH₄ as an electron donor for nitrate reduction provides an alternative route to its utilization when nitrogen removal is required (e.g., effluent discharge to nitrogen-sensitive waterways). MFCs, although only recently demonstrated for methane, could be advantageous over both physical CH₄ recovery (using membrane contactors) and other biological approaches due to their ability to directly recover energy.

4. Future GHG management perspectives

4.1. N₂O as an energy recovery oxidant

N₂O, as a powerful oxidant, has the potential to be selectively produced in wastewater treatment processes (e.g., via coupled aerobic–anoxic side-stream nitrogen removal) and serve as a combustion oxidant in combination with CH₄.^144,145 In conventional WWTPs, N₂O off-gas collection could be accomplished via the installation of covers on treatment unit processes, however this may be impractical for nitrification due to the large volume of gas produced by aeration. Although selective reduction of N₂O has been practiced industrially, similar processes may not be cost-effective in large application to low N₂O containing gases as they require catalysts and high reaction temperatures.¹⁴⁶ Nevertheless, with consistent N₂O production and improved collection efficiency in newly designed processes,⁸⁴ this could be a worthwhile future research topic.

4.2. Technologies for targeting CO₂ capture

Given the potential significance of non-biogenic CO₂ sources in municipal wastewater influents, it is important to also consider possible means for direct CO₂ sequestration or capture. However, considering the relatively high solubility of CO₂ in water and its potential cost of recovery, the treatment of dissolved CO₂in situ using emerging technologies has recently become a topic of interest. For example, phototrophic technologies relying on algae and/or phototrophic bacteria could promote carbon fixation while simultaneously achieving nutrient removal.¹⁴⁷ Alternatively, carbonic anhydrase, a ubiquitous enzyme capable of catalyzing the hydration of CO₂ into bicarbonate and hydrogen at high rates, could potentially be incorporated into engineered systems to sequester carbon directly.¹⁴⁷ Microbially assisted electrolytic systems also have the potential to sequester and convert CO₂ to bicarbonate using either wastewater or seawater as the electrolyte while producing beneficial products such as H₂.^147,148

4.3. Methanotroph-based recovery of high-value end products

Methanotrophs can be metabolically engineered to synthesize a range of high-value products including single cell proteins, biopolymers (e.g., polyhydroxyalkanoates, PHB), soluble metabolites (e.g., methanol, formaldehyde, formate), lipids, lycopene, C30 carotenoid, lactic acid and exopolysaccharides.^149–152 Methane oxidation is a multi-step process in which CH₄ is oxidized to methanol, formaldehyde, formate, and CO₂ sequentially. Given that methanol dehydrogenase is located in the periplasmic membrane of methanotrophs, methanol must be transported out of the cell membrane in order for subsequent processes to ensue. Based on this, methanotrophs can be genetically engineered or supplemented with inhibitors to suppress this dehydrogenase and stop CH₄ oxidation at methanol,¹⁵³ which can then be collected and enriched for use in MFCs (currently commercially available and used as portable electricity sources). The application of these processes, among others, could be especially useful when remote electricity generation is needed, the energy for which could be supplied exclusively from treated wastewater.

4.4. Methane adsorbent-related technology

Recent studies have demonstrated that structures with high adsorption capacity and packing density can be used to adsorb and store CH₄. For example, an investigation by Bagheri et al.¹⁵⁴ demonstrated that microporous activated carbon generated from corn cobs was capable of high levels of CH₄ adsorption (150 v/v). Other materials such as constructed multilayer graphene nanostructures (MGNs) with optimized layer distances were able to satisfy the U.S. department of engineering target for adsorbents (180 v/v).¹⁵⁵ Although the aforementioned materials require pressures of >100 psi to effectively sorb CH₄ into their structures (reducing their viability from an energy and GHG footprint perspective), the recent synthesis of a monolithic metal–organic framework has proven capable of reaching a CH₄ packing density of 259 v/v at pressures previously comparable to those of half of its capacity.¹⁵⁶ The continuous improvement of adsorptive materials and the potential for their exploration at lower sorption pressures could lead to viable use for CH₄ capture, purification, and transport from wastewater effluents.

5. Perspectives on the direct comparison of CAS- and anaerobic-based GHG emissions

Based on a normalized analysis of existing literature, herein we provide a parallel assessment of CAS-based and anaerobic-based mainstream wastewater treatment using their dominant GHG sources and assuming equivalent levels of treatment for effluents (i.e., nitrogen and COD removal). Recent studies have shown that approximately 0.298 kg CO_2e per m³ of GHG emissions come from electricity usage in conventional WWTPs (based on 0.472 kg CO₂ kW⁻¹ h⁻² energy carbon footprint),^23,24,40 with 50% of that energy demand being consumed by aeration.⁷ It is estimated that roughly 25% of plant electricity use can be offset by energy produced from sludge digestion.⁷ Taking these values into account, total fossil fuel-generated emissions from conventional treatment would be on the order of 0.224 kg CO_2e per m³ DWW. Total fugitive CH₄ emissions were also included at an average of 0.095 kg CO_2e m⁻³.⁵¹

Assuming full-scale mainstream anaerobic treatment energy demands are comparable to those of conventional WWTPs before considering aeration requirements, it can be anticipated that their electricity consumption accounts for approximately 0.149 kg CO_2e per m³ DWW. The amount of energy achievable from direct biogas recovery (headspace) was further calculated based on methane loss values extracted from Fig. 3 at 25 °C and 10 °C (15 and 50 mg CH₄ L⁻¹, respectively). Assuming a 95% conversion of incoming COD (430 mg L⁻¹) to CH₄, energy density of 55.5 MJ kg⁻¹, conversion efficiency to electricity of 35%, and a CO₂ emission factor from electricity usage of 0.472 kg CO₂ kW⁻¹ h⁻¹,¹⁵⁷ it is estimated that anaerobic mainstream treatment electricity-associated GHG footprints could be reduced to below 0.02 kg CO_2e per m³ at 10 °C, while actually achieving energy positive operation at 25 °C (−0.073 kg CO_2e per m³). Based on these calculations, it can be concluded that mainstream anaerobic treatment has the potential to more significantly offset electricity-associated GHG emissions when compared to conventional WWTPs with anaerobic digestion (0.224 kg CO_2e per m³). However, without downstream CH₄ recovery, anaerobic effluents would contribute GHG emissions of approximately 0.51 and 1.70 kg CO_2e per m³ at 25 °C and 10 °C, respectively (using CH₄ GWP of 34 and dissolved methane-temperature relationship obtained from Fig. 3).

N₂O emissions from the nitrogen removal process in WWTPs have been identified as the most widely varying and least predictable of GHG sources (ranging from 0 to 14.6% of incoming nitrogen). Nonetheless, an analysis of several representative full-scale studies of conventional anoxic–oxic processes revealed an average emission factor of 1.5%,^{37,38,42,158–160} resulting in N₂O emissions of 0.281 kg CO_2e per m³ DWW for conventional WWTPs (assuming influent of 20 mg N L⁻¹ and N₂O GWP of 298). If nitritation coupled with anammox is employed as the nitrogen removal process for mainstream anaerobic treatment and an average nitrogen to N₂O ratio of 2.8% is used (estimated from full-scale nitration–anammox studies),^{76,77,161,162} N₂O emissions for anaerobic treatment can be calculated as 0.529 kg CO_2e per m³ WW. The relatively higher emissions observed for nitration–anammox have been attributed to a lack of process optimization for N₂O mitigation,¹⁶ which can likely be improved upon significantly in future research.

Therefore, total GHG emissions from CAS WWTPs are significantly lower than mainstream anaerobic systems, even at 25 °C (0.599 vs. 0.966 kg CO_2e per m³ WW). Anaerobic treatment GHG footprints would likely be exacerbated at lower ambient temperatures, reaching up to 2.25 CO_2e per m³ at 10 °C, if no effluent recovery was employed. As outlined in this review, however, emerging techniques for both nitrogen and dissolved CH₄ removal/recovery could effectively negate these outstanding issues. If, for example, energy-efficient dissolved CH₄ recovery is employed and comparable N₂O emissions are achieved through nitrogen removal processes optimization, mainstream anaerobic system GHG footprints would easily drop below those calculated for current CAS WWTP processes. Further, recent research has implicated mainstream anaerobic effluents as likely to become acceptable for direct irrigation reuse from a microbial safety perspective.^163,164 This application of nutrient-rich treated effluents could negate the necessity of nitrogen removal, essentially allowing for the elimination of direct N₂O emissions from mainstream anaerobic treatment and such systems to approach carbon neutrality.

6. Conclusions

Existing literature on WWTP GHGs has reported broadly varying total emissions ranging from 0.243 to 2.4 kg CO_2e per m³ WW. A unified and comprehensive plant-wide approach inclusive of all direct and indirect emissions is necessary for accurate WWTP carbon footprint interpretation. Overall, the most significant obstacle facing GHG mitigation in CAS WWTPs is associated with understanding N₂O generation, whereas for the sustainability of mainstream anaerobic wastewater treatment, dissolved CH₄ emissions are of greatest concern. Other specific observations of this review are summarized as follows:

• N₂O emissions are both dominant and highly variable in conventional aerobic-based WWTPs (0 to 95% of N for lab-scale and 0 to 14.6% of N for full-scale), with several critical factors influencing this variability including: DO, pH, nitrite, carbon source availability and ammonium loading.

• More research is specifically needed in elucidating the pathways involved in N₂O formation (i.e., nitrifier denitrification, heterotrophic denitrification, and hydroxylamine oxidation) at different operational conditions, which can be then used to correlate practical mitigation strategies with specific processes and configurations.

• Dissolved CH₄ contributions in mainstream anaerobic treatment account for the majority of GHG emissions. Anaerobic system GHG emissions are inversely correlated with operational temperature due to increasing CH₄ solubility and supersaturation (section 3.1).

• Analysis of membrane contactors for physical dissolved CH₄ removal showed that for sweep gas systems, gas to liquid (G/L) ratio is a critical parameter influencing CH₄ removal efficiency and off-gas CH₄ concentration. Vacuum driven membrane contactors, although capable of high-quality gas recovery, require operation at TMPs below 50 kPa to achieve energy neutrality.

• Several emerging methods for dissolved CH₄ recovery are likely to play significant roles in future management of dissolved CH₄. For example, DAMO combined with anammox could allow for the simultaneous removal of both nitrogen and CH₄ from anaerobic effluents.

To significantly reduce WWTP GHG emissions, future research on CAS must focus on N₂O management strategies to minimize emissions. For anaerobic systems, both efficient CH₄ and nitrogen resource recovery must be achieved without introducing incidental increases in N₂O generation. The accomplishment of this goal appears to be within reach, given the prospects of emerging CH₄ recovery processes and likelihood of effluent reuse.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Siming Chen is partially funded by the China Scholarship Council. Moustapha Harb is partially funded by Water for Agriculture grant no. 2016-68007-25044 from the USDA National Institute of Food and Agriculture. The research was supported by the University of Southern California.

References

1. A. L. Smith, L. B. Stadler, L. Cao, N. G. Love, L. Raskin and S. J. Skerlos, Navigating wastewater energy recovery strategies: a life cycle comparison of anaerobic membrane bioreactor and conventional treatment systems with anaerobic digestion, Environ. Sci. Technol., 2014, 48, 5972–5981 CrossRef CAS PubMed .
2. EPA, Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990–2016, U.S. Environmental Protection Agency, 2018 Search PubMed.
3. T. F. Stocker, D. Qin, G. Plattner, M. Tignor, S. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P. Midgley, Climate change 2013: the physical science basis. Intergovernmental panel on climate change, working group I contribution to the IPCC fifth assessment report (AR5), New York, 2013 Search PubMed.
4. D. Gupta and S. K. Singh, Greenhouse gas emissions from wastewater treatment plants: a case study of Noida, J. Water Sustainability, 2012, 2, 131–139 CAS .
5. M. J. Kampschreur, H. Temmink, R. Kleerebezem, M. S. Jetten and M. C. van Loosdrecht, Nitrous oxide emission during wastewater treatment, Water Res., 2009, 43, 4093–4103 CrossRef CAS PubMed .
6. G. Mannina, G. Ekama, D. Caniani, A. Cosenza, G. Esposito, R. Gori, M. Garrido-Baserba, D. Rosso and G. Olsson, Greenhouse gases from wastewater treatment—A review of modelling tools, Sci. Total Environ., 2016, 551, 254–270 CrossRef PubMed .
7. P. L. McCarty, J. Bae and J. Kim, Domestic wastewater treatment as a net energy producer–can this be achieved?, Environ. Sci. Technol., 2011, 45(17), 7100–7106 CrossRef CAS PubMed .
8. B. D. Shoener, C. Zhong, A. D. Greiner, W. O. Khunjar, P. Y. Hong and J. S. Guest, Design of anaerobic membrane bioreactors for the valorization of dilute organic carbon waste streams, Energy Environ. Sci., 2016, 9, 1102–1112 RSC .
9. A. L. Smith, L. B. Stadler, N. G. Love, S. J. Skerlos and L. Raskin, Perspectives on anaerobic membrane bioreactor treatment of domestic wastewater: a critical review, Bioresour. Technol., 2012, 122, 149–159 CrossRef CAS PubMed .
10. E. Giraldo, M. Pena, C. Chernicharo, J. Sandino and A. Noyola, Anaerobic sewage treatment technology in Latin-America: A selection of 20 years of experiences, Proceedings of the Water Environment Federation, 2007, vol. 2007, pp. 5208–5228 Search PubMed.
11. C. A. Chernicharo, J. B. Van Lier, A. Noyola and T. B. Ribeiro, in Anaerobic Biotechnology: Environmental Protection and Resource Recovery, World Scientific, 2015, pp. 263–296 Search PubMed.
12. H. Ozgun, R. K. Dereli, M. E. Ersahin, C. Kinaci, H. Spanjers and J. B. van Lier, A review of anaerobic membrane bioreactors for municipal wastewater treatment: integration options, limitations and expectations, Sep. Purif. Technol., 2013, 118, 89–104 CrossRef CAS .
13. H. Lin, W. Peng, M. Zhang, J. Chen, H. Hong and Y. Zhang, A review on anaerobic membrane bioreactors: applications, membrane fouling and future perspectives, Desalination, 2013, 314, 169–188 CrossRef CAS .
14. Z. Lei, S. Yang, Y. Li, W. Wen, X. C. Wang and R. Chen, Application of anaerobic membrane bioreactors to municipal wastewater treatment at ambient temperature: A review of achievements, challenges, and perspectives, Bioresour. Technol., 2018, 267, 756–768 CrossRef CAS PubMed .
15. C. Shin and J. Bae, Current status of the pilot-scale anaerobic membrane bioreactor treatments of domestic wastewaters: A critical review, Bioresour. Technol., 2018, 247, 1038–1046 CrossRef CAS PubMed .
16. T. M. Massara, S. Malamis, A. Guisasola, J. A. Baeza, C. Noutsopoulos and E. Katsou, A review on nitrous oxide (N₂O) emissions during biological nutrient removal from municipal wastewater and sludge reject water, Sci. Total Environ., 2017, 596, 106–123 CrossRef PubMed .
17. Y. Law, L. Ye, Y. Pan and Z. Yuan, Nitrous oxide emissions from wastewater treatment processes, Philos. Trans. R. Soc., B, 2012, 367, 1265–1277 CrossRef CAS PubMed .
18. B.-J. Ni and Z. Yuan, Recent advances in mathematical modeling of nitrous oxides emissions from wastewater treatment processes, Water Res., 2015, 87, 336–346 CrossRef CAS PubMed .
19. B. C. Crone, J. L. Garland, G. A. Sorial and L. M. Vane, Significance of dissolved methane in effluents of anaerobically treated low strength wastewater and potential for recovery as an energy product: A review, Water Res., 2016, 104, 520–531 CrossRef CAS PubMed .
20. D. Mamais, C. Noutsopoulos, A. Dimopoulou, A. Stasinakis and T. Lekkas, Wastewater treatment process impact on energy savings and greenhouse gas emissions, Water Sci. Technol., 2015, 71, 303–308 CrossRef CAS PubMed .
21. D. Rosso, L. E. Larson and M. K. Stenstrom, Aeration of large-scale municipal wastewater treatment plants: state of the art, Water Sci. Technol., 2008, 57, 973–978 CrossRef PubMed .
22. X. Yan, L. Li and J. Liu, Characteristics of greenhouse gas emission in three full-scale wastewater treatment processes, J. Environ. Sci., 2014, 26, 256–263 CrossRef CAS .
23. S. Masuda, S. Suzuki, I. Sano, Y.-Y. Li and O. Nishimura, The seasonal variation of emission of greenhouse gases from a full-scale sewage treatment plant, Chemosphere, 2015, 140, 167–173 CrossRef CAS PubMed .
24. D. Kyung, M. Kim, J. Chang and W. Lee, Estimation of greenhouse gas emissions from a hybrid wastewater treatment plant, J. Cleaner Prod., 2015, 95, 117–123 CrossRef CAS .
25. EIA, State Electricity Profiles-2016, U.S. Energy Information Administration, 2018 Search PubMed.
26. K. Oshita, T. Okumura, M. Takaoka, T. Fujimori, L. Appels and R. Dewil, Methane and nitrous oxide emissions following anaerobic digestion of sludge in Japanese sewage treatment facilities, Bioresour. Technol., 2014, 171, 175–181 CrossRef CAS PubMed .
27. Y. C. Chen and J. Kuo, Potential of greenhouse gas emissions from sewage sludge management: a case study of Taiwan, J. Cleaner Prod., 2016, 129, 196–201 CrossRef CAS .
28. L. Snip, Quantifying the greenhouse gas emissions of wastewater treatment plants, Environ. Sci., 2010, 8–13 Search PubMed .
29. A. Cadwallader and J. M. VanBriesen, Incorporating Uncertainty into Future Estimates of Nitrous Oxide Emissions from Wastewater Treatment, J. Environ. Eng., 2017, 143, 04017029 CrossRef .
30. S. Eggleston, L. Buendia, K. Miwa, T. Ngara and K. Tanabe, 2006 IPCC guidelines for national greenhouse gas inventories, Institute for Global Environmental Strategies Hayama, Japan, 2006 Search PubMed.
31. L. Y. Tseng, A. K. Robinson, X. Zhang, X. Xu, J. Southon, A. J. Hamilton, R. Sobhani, M. K. Stenstrom and D. Rosso, Identification of preferential paths of fossil carbon within water resource recovery facilities via radiocarbon analysis, Environ. Sci. Technol., 2016, 50, 12166–12178 CrossRef CAS PubMed .
32. D. R. Griffith, R. T. Barnes and P. A. Raymond, Inputs of fossil carbon from wastewater treatment plants to US rivers and oceans, Environ. Sci. Technol., 2009, 43, 5647–5651 CrossRef CAS PubMed .
33. C. Sweetapple, G. Fu and D. Butler, Identifying key sources of uncertainty in the modelling of greenhouse gas emissions from wastewater treatment, Water Res., 2013, 47, 4652–4665 CrossRef CAS PubMed .
34. M. B. Shahabadi, L. Yerushalmi and F. Haghighat, Estimation of greenhouse gas generation in wastewater treatment plants–Model development and application, Chemosphere, 2010, 78, 1085–1092 CrossRef PubMed .
35. J. Keller and K. Hartley, Greenhouse gas production in wastewater treatment: process selection is the major factor, Water Sci. Technol., 2003, 47, 43–48 CrossRef CAS PubMed .
36. B. J. Ni and Z. G. Yuan, Recent advances in mathematical modeling of nitrous oxides emissions from wastewater treatment processes, Water Res., 2015, 87, 336–346 CrossRef CAS PubMed .
37. K.-L. Hwang, C.-H. Bang and K.-D. Zoh, Characteristics of methane and nitrous oxide emissions from the wastewater treatment plant, Bioresour. Technol., 2016, 214, 881–884 CrossRef CAS PubMed .
38. M. Daelman, E. M. van Voorthuizen, L. Van Dongen, E. Volcke and M. Van Loosdrecht, Methane and nitrous oxide emissions from municipal wastewater treatment–results from a long-term study, Water Sci. Technol., 2013, 67, 2350–2355 CrossRef CAS PubMed .
39. A. Rodríguez-Caballero, I. Aymerich, M. Poch and M. Pijuan, Evaluation of process conditions triggering emissions of green-house gases from a biological wastewater treatment system, Sci. Total Environ., 2014, 493, 384–391 CrossRef PubMed .
40. A. Rodríguez-Caballero, I. Aymerich, R. Marques, M. Poch and M. Pijuan, Minimizing N₂O emissions and carbon footprint on a full-scale activated sludge sequencing batch reactor, Water Res., 2015, 71, 1–10 CrossRef PubMed .
41. M. R. Daelman, E. M. van Voorthuizen, U. G. van Dongen, E. I. Volcke and M. C. van Loosdrecht, Seasonal and diurnal variability of N₂O emissions from a full-scale municipal wastewater treatment plant, Sci. Total Environ., 2015, 536, 1–11 CrossRef CAS PubMed .
42. H. Yoshida, J. Mønster and C. Scheutz, Plant-integrated measurement of greenhouse gas emissions from a municipal wastewater treatment plant, Water Res., 2014, 61, 108–118 CrossRef CAS PubMed .
43. H. D. Monteith, H. R. Sahely, H. L. MacLean and D. M. Bagley, A rational procedure for estimation of greenhouse-gas emissions from municipal wastewater treatment plants, Water Environ. Res., 2005, 77, 390–403 CrossRef CAS PubMed .
44. M. D. Short, A. Daikeler, K. Wallis, W. L. Peirson and G. M. Peters, Dissolved methane in the influent of three Australian wastewater treatment plants fed by gravity sewers, Sci. Total Environ., 2017, 599, 85–93 CrossRef PubMed .
45. Y. Liu, X. Cheng, X. X. Lun and D. Z. Sun, CH4 emission and conversion from A(2)O and SBR processes in full-scale wastewater treatment plants, J. Environ. Sci., 2014, 26, 224–230 CrossRef CAS .
46. C. E. Y. Kwok, D. Muller, C. Caldow, B. Lebegue, J. G. Monster, C. W. Rella, C. Scheutz, M. Schmidt, M. Ramonet, T. Warneke, G. Broquet and P. Ciais, Methane emission estimates using chamber and tracer release experiments for a municipal waste water treatment plant, Atmos. Meas. Tech., 2015, 8, 2853–2867 CrossRef CAS .
47. J. H. Wang, J. Zhang, H. J. Xie, P. Y. Qi, Y. G. Ren and Z. Hu, Methane emissions from a full-scale A/A/O wastewater treatment plant, Bioresour. Technol., 2011, 102, 5479–5485 CrossRef CAS PubMed .
48. P. M. Czepiel, P. M. Crill and R. C. Harriss, Methane emissions from municipal waste-water treatment processes, Environ. Sci. Technol., 1993, 27, 2472–2477 CrossRef CAS .
49. A. Aboobakar, M. Jones, P. Vale, E. Cartmell and G. Dotro, Methane Emissions from Aerated Zones in a Full-Scale Nitrifying Activated Sludge Treatment Plant, Water, Air, Soil Pollut., 2014, 225, 10 CrossRef .
50. Z. Alshboul, J. Encinas-Fernandez, H. Hofmann and A. Lorke, Export of Dissolved Methane and Carbon Dioxide with Effluents from Municipal Wastewater Treatment Plants, Environ. Sci. Technol., 2016, 50, 5555–5563 CrossRef CAS PubMed .
51. M. R. J. Daelman, E. M. van Voorthuizen, U. van Dongen, E. I. P. Volcke and M. C. M. van Loosdrecht, Methane emission during municipal wastewater treatment, Water Res., 2012, 46, 3657–3670 CrossRef CAS PubMed .
52. C. Schaum, T. Fundneider and P. Cornel, Analysis of methane emissions from digested sludge, Water Sci. Technol., 2016, 73, 1599–1607 CrossRef CAS PubMed .
53. E. A. Scheehle and M. R. Doorn, Improvements to the US wastewater methane and nitrous oxide emissions estimates, US EPA, Washington, DC, 2001 Search PubMed.
54. P. Czepiel, P. Crill and R. Harriss, Nitrous oxide emissions from municipal wastewater treatment, Environ. Sci. Technol., 1995, 29, 2352–2356 CrossRef CAS PubMed .
55. E. Bock, I. Schmidt, R. Stüven and D. Zart, Nitrogen loss caused by denitrifying Nitrosomonas cells using ammonium or hydrogen as electron donors and nitrite as electron acceptor, Arch. Microbiol., 1995, 163, 16–20 CrossRef CAS .
56. L. Poughon, C. G. Dussap and J. B. Gros, Energy model and metabolic flux analysis for autotrophic nitrifiers, Biotechnol. Bioeng., 2001, 72, 416–433 CrossRef CAS PubMed .
57. G. Tallec, J. Garnier, G. Billen and M. Gousailles, Nitrous oxide emissions from secondary activated sludge in nitrifying conditions of urban wastewater treatment plants: effect of oxygenation level, Water Res., 2006, 40, 2972–2980 CrossRef CAS PubMed .
58. G. Tallec, J. Garnier, G. Billen and M. Gousailles, Nitrous oxide emissions from denitrifying activated sludge of urban wastewater treatment plants, under anoxia and low oxygenation, Bioresour. Technol., 2008, 99, 2200–2209 CrossRef CAS PubMed .
59. Y. Wang, X. Lin, D. Zhou, L. Ye, H. Han and C. Song, Nitric oxide and nitrous oxide emissions from a full-scale activated sludge anaerobic/anoxic/oxic process, Chem. Eng. J., 2016, 289, 330–340 CrossRef CAS .
60. R. M. Rathnayake, M. Oshiki, S. Ishii, T. Segawa, H. Satoh and S. Okabe, Effects of dissolved oxygen and pH on nitrous oxide production rates in autotrophic partial nitrification granules, Bioresour. Technol., 2015, 197, 15–22 CrossRef CAS PubMed .
61. R. K. Hynes and R. Knowles, Production of nitrous oxide by Nitrosomonas europaea: effects of acetylene, pH, and oxygen, Can. J. Microbiol., 1984, 30, 1397–1404 CrossRef CAS .
62. A. Terada, S. Sugawara, K. Hojo, Y. Takeuchi, S. Riya, W. F. Harper Jr, T. Yamamoto, M. Kuroiwa, K. Isobe and C. Katsuyama, Hybrid nitrous oxide production from a partial nitrifying bioreactor: hydroxylamine interactions with nitrite, Environ. Sci. Technol., 2017, 51, 2748–2756 CrossRef CAS PubMed .
63. R. A. Kester, W. De Boer and H. J. Laanbroek, Production of NO and N₂O by pure cultures of nitrifying and denitrifying bacteria during changes in aeration, Appl. Environ. Microbiol., 1997, 63, 3872–3877 CAS .
64. L. Peng, B.-J. Ni, L. Ye and Z. Yuan, The combined effect of dissolved oxygen and nitrite on N₂O production by ammonia oxidizing bacteria in an enriched nitrifying sludge, Water Res., 2015, 73, 29–36 CrossRef CAS PubMed .
65. X. Quan, M. Zhang, P. G. Lawlor, Z. Yang and X. Zhan, Nitrous oxide emission and nutrient removal in aerobic granular sludge sequencing batch reactors, Water Res., 2012, 46, 4981–4990 CrossRef CAS PubMed .
66. M. J. Kampschreur, N. C. Tan, R. Kleerebezem, C. Picioreanu, M. S. Jetten and M. C. V. Loosdrecht, Effect of dynamic process conditions on nitrogen oxides emission from a nitrifying culture, Environ. Sci. Technol., 2007, 42, 429–435 CrossRef .
67. H. Zheng, K. Hanaki and T. Matsuo, Production of nitrous oxide gas during nitrification of wastewater, Water Sci. Technol., 1994, 30, 133–141 CrossRef CAS .
68. J. Burgess, R. Stuetz, S. Morton and T. Stephenson, Dinitrogen oxide detection for process failure early warning systems, Water Sci. Technol., 2002, 45, 247–254 CrossRef CAS PubMed .
69. J. H. Ahn, T. Kwan and K. Chandran, Comparison of partial and full nitrification processes applied for treating high-strength nitrogen wastewaters: microbial ecology through nitrous oxide production, Environ. Sci. Technol., 2011, 45, 2734–2740 CrossRef CAS PubMed .
70. A. Aboobakar, E. Cartmell, T. Stephenson, M. Jones, P. Vale and G. Dotro, Nitrous oxide emissions and dissolved oxygen profiling in a full-scale nitrifying activated sludge treatment plant, Water Res., 2013, 47, 524–534 CrossRef CAS PubMed .
71. T. J. Goreau, W. A. Kaplan, S. C. Wofsy, M. B. McElroy, F. W. Valois and S. W. Watson, Production of NO₂⁻and N₂O by nitrifying bacteria at reduced concentrations of oxygen, Appl. Environ. Microbiol., 1980, 40, 526–532 CAS .
72. R. Yu, M. J. Kampschreur, M. C. v. Loosdrecht and K. Chandran, Mechanisms and specific directionality of autotrophic nitrous oxide and nitric oxide generation during transient anoxia, Environ. Sci. Technol., 2010, 44, 1313–1319 CrossRef CAS PubMed .
73. W.-M. Xie, B.-J. Ni, W.-W. Li, G.-P. Sheng, H.-Q. Yu and J. Song, Formation and quantification of soluble microbial products and N₂O production by ammonia-oxidizing bacteria (AOB)-enriched activated sludge, Chem. Eng. Sci., 2012, 71, 67–74 CrossRef CAS .
74. L. Peng, B.-J. Ni, D. Erler, L. Ye and Z. Yuan, The effect of dissolved oxygen on N2O production by ammonia-oxidizing bacteria in an enriched nitrifying sludge, Water Res., 2014, 66, 12–21 CrossRef CAS PubMed .
75. K. Y. Park, Y. Inamori, M. Mizuochi and K. H. Ahn, Emission and control of nitrous oxide from a biological wastewater treatment system with intermittent aeration, J. Biosci. Bioeng., 2000, 90, 247–252 CrossRef CAS PubMed .
76. C. M. Castro-Barros, M. Daelman, K. Mampaey, M. Van Loosdrecht and E. Volcke, Effect of aeration regime on N₂O emission from partial nitritation-anammox in a full-scale granular sludge reactor, Water Res., 2015, 68, 793–803 CrossRef CAS PubMed .
77. M. J. Kampschreur, W. R. van der Star, H. A. Wielders, J. W. Mulder, M. S. Jetten and M. C. van Loosdrecht, Dynamics of nitric oxide and nitrous oxide emission during full-scale reject water treatment, Water Res., 2008, 42, 812–826 CrossRef CAS PubMed .
78. Y. Law, P. Lant and Z. Yuan, The effect of pH on N₂O production under aerobic conditions in a partial nitritation system, Water Res., 2011, 45, 5934–5944 CrossRef CAS PubMed .
79. K. Hanaki, Z. Hong and T. Matsuo, Production of nitrous oxide gas during denitrification of wastewater, Water Sci. Technol., 1992, 26, 1027–1036 CrossRef CAS .
80. M. Thörn and F. Sörensson, Variation of nitrous oxide formation in the denitrification basin in a wastewater treatment plant with nitrogen removal, Water Res., 1996, 30, 1543–1547 CrossRef .
81. Y. Zhou, M. Pijuan, R. J. Zeng and Z. Yuan, Free nitrous acid inhibition on nitrous oxide reduction by a denitrifying-enhanced biological phosphorus removal sludge, Environ. Sci. Technol., 2008, 42, 8260–8265 CrossRef CAS PubMed .
82. A. Soler-Jofra, B. Stevens, M. Hoekstra, C. Picioreanu, D. Sorokin, M. C. van Loosdrecht and J. Pérez, Importance of abiotic hydroxylamine conversion on nitrous oxide emissions during nitritation of reject water, Chem. Eng. J., 2016, 287, 720–726 CrossRef CAS .
83. R. Schulthess, M. Kühni and W. Gujer, Release of nitric and nitrous oxides from denitrifying activated sludge, Water Res., 1995, 29, 215–226 CrossRef CAS .
84. H. Gao, M. Liu, J. S. Griffin, L. Xu, D. Xiang, Y. D. Scherson, W.-T. Liu and G. F. Wells, Complete nutrient removal coupled to nitrous oxide production as a bioenergy source by denitrifying polyphosphate-accumulating organisms, Environ. Sci. Technol., 2017, 51, 4531–4540 CrossRef CAS PubMed .
85. X. Zhu and Y. Chen, Reduction of N2O and NO generation in anaerobic− aerobic (low dissolved oxygen) biological wastewater treatment process by using sludge alkaline fermentation liquid, Environ. Sci. Technol., 2011, 45, 2137–2143 CrossRef CAS PubMed .
86. Z. Hu, J. Zhang, S. Li and H. Xie, Impact of carbon source on nitrous oxide emission from anoxic/oxic biological nitrogen removal process and identification of its emission sources, Environ. Sci. Pollut. Res., 2013, 20, 1059–1069 CrossRef CAS PubMed .
87. K. Song, W. F. Harper, T. Hori, S. Riya, M. Hosomi and A. Terada, Impact of carbon sources on nitrous oxide emission and microbial community structure in an anoxic/oxic activated sludge system, Clean Technol. Environ. Policy, 2015, 17, 2375–2385 CrossRef CAS .
88. X. Zhang, X. Wang, J. Zhang, X. Huang, D. Wei, W. Lan and Z. Hu, Reduction of nitrous oxide emissions from partial nitrification process by using innovative carbon source (mannitol), Bioresour. Technol., 2016, 218, 789–795 CrossRef CAS PubMed .
89. M. Zheng, Y. Tian, T. Liu, T. Ma, L. Li, C. Li, M. Ahmad, Q. Chen and J. Ni, Minimization of nitrous oxide emission in a pilot-scale oxidation ditch: generation, spatial variation and microbial interpretation, Bioresour. Technol., 2015, 179, 510–517 CrossRef CAS PubMed .
90. J. E. Burgess, B. B. Colliver, R. M. Stuetz and T. Stephenson, Dinitrogen oxide production by a mixed culture of nitrifying bacteria during ammonia shock loading and aeration failure, J. Ind. Microbiol. Biotechnol., 2002, 29, 309–313 CrossRef CAS PubMed .
91. J. Foley, D. De Haas, Z. Yuan and P. Lant, Nitrous oxide generation in full-scale biological nutrient removal wastewater treatment plants, Water Res., 2010, 44, 831–844 CrossRef CAS PubMed .
92. F. Cakir and M. Stenstrom, Greenhouse gas production: a comparison between aerobic and anaerobic wastewater treatment technology, Water Res., 2005, 39, 4197–4203 CrossRef CAS PubMed .
93. G. Lettinga, A. De Man, A. Van der Last, W. Wiegant, K. Van Knippenberg, J. Frijns and J. Van Buuren, Anaerobic treatment of domestic sewage and wastewater, Water Sci. Technol., 1993, 27, 67–73 CrossRef CAS .
94. W. M. Bandara, H. Satoh, M. Sasakawa, Y. Nakahara, M. Takahashi and S. Okabe, Removal of residual dissolved methane gas in an upflow anaerobic sludge blanket reactor treating low-strength wastewater at low temperature with degassing membrane, Water Res., 2011, 45, 3533–3540 CrossRef CAS PubMed .
95. C. Souza, C. Chernicharo and S. Aquino, Quantification of dissolved methane in UASB reactors treating domestic wastewater under different operating conditions, Water Sci. Technol., 2011, 64, 2259–2264 CrossRef CAS PubMed .
96. W. M. Bandara, T. Kindaichi, H. Satoh, M. Sasakawa, Y. Nakahara, M. Takahashi and S. Okabe, Anaerobic treatment of municipal wastewater at ambient temperature: Analysis of archaeal community structure and recovery of dissolved methane, Water Res., 2012, 46, 5756–5764 CrossRef CAS PubMed .
97. J. Cookney, E. Cartmell, B. Jefferson and E. McAdam, Recovery of methane from anaerobic process effluent using poly-di-methyl-siloxane membrane contactors, Water Sci. Technol., 2012, 65, 604–610 CrossRef CAS PubMed .
98. J. Giménez, N. Martí, J. Ferrer and A. Seco, Methane recovery efficiency in a submerged anaerobic membrane bioreactor (SAnMBR) treating sulphate-rich urban wastewater: evaluation of methane losses with the effluent, Bioresour. Technol., 2012, 118, 67–72 CrossRef PubMed .
99. R. Yoo, J. Kim, P. L. McCarty and J. Bae, Anaerobic treatment of municipal wastewater with a staged anaerobic fluidized membrane bioreactor (SAF-MBR) system, Bioresour. Technol., 2012, 120, 133–139 CrossRef CAS PubMed .
100. R. Pretel, A. Robles, M. Ruano, A. Seco and J. Ferrer, Environmental impact of submerged anaerobic MBR (SAnMBR) technology used to treat urban wastewater at different temperatures, Bioresour. Technol., 2013, 149, 532–540 CrossRef CAS PubMed .
101. A. L. Smith, S. J. Skerlos and L. Raskin, Psychrophilic anaerobic membrane bioreactor treatment of domestic wastewater, Water Res., 2013, 47, 1655–1665 CrossRef CAS PubMed .
102. L. Ren, Y. Ahn and B. E. Logan, A two-stage microbial fuel cell and anaerobic fluidized bed membrane bioreactor (MFC-AFMBR) system for effective domestic wastewater treatment, Environ. Sci. Technol., 2014, 48, 4199–4206 CrossRef CAS PubMed .
103. J. Gouveia, F. Plaza, G. Garralon, F. Fdz-Polanco and M. Peña, A novel configuration for an anaerobic submerged membrane bioreactor (AnSMBR. Long-term treatment of municipal wastewater under psychrophilic conditions), Bioresour. Technol., 2015, 198, 510–519 CrossRef CAS PubMed .
104. M. Harb, Y. Xiong, J. Guest, G. Amy and P.-Y. Hong, Differences in microbial communities and performance between suspended and attached growth anaerobic membrane bioreactors treating synthetic municipal wastewater, Environ. Sci.: Water Res. Technol., 2015, 1, 800–813 RSC .
105. N. Matsuura, M. Hatamoto, H. Sumino, K. Syutsubo, T. Yamaguchi and A. Ohashi, Recovery and biological oxidation of dissolved methane in effluent from UASB treatment of municipal sewage using a two-stage closed downflow hanging sponge system, J. Environ. Manage., 2015, 151, 200–209 CrossRef CAS PubMed .
106. A. Smith, S. Skerlos and L. Raskin, Anaerobic membrane bioreactor treatment of domestic wastewater at psychrophilic temperatures ranging from 15 C to 3 C, Environ. Sci.: Water Res. Technol., 2015, 1, 56–64 RSC .
107. C. Shin, P. L. McCarty, J. Kim and J. Bae, Pilot-scale temperate-climate treatment of domestic wastewater with a staged anaerobic fluidized membrane bioreactor (SAF-MBR), Bioresour. Technol., 2014, 159, 95–103 CrossRef CAS PubMed .
108. A. L. Smith, S. J. Skerlos and L. Raskin, Membrane biofilm development improves COD removal in anaerobic membrane bioreactor wastewater treatment, Microb. Biotechnol., 2015, 8, 883–894 CrossRef CAS PubMed .
109. J. B. Gimenez, N. Marti, J. Ferrer and A. Seco, Methane recovery efficiency in a submerged anaerobic membrane bioreactor (SAnMBR) treating sulphate-rich urban wastewater: Evaluation of methane losses with the effluent, Bioresour. Technol., 2012, 118, 67–72 CrossRef CAS PubMed .
110. J. Cookney, A. Mcleod, V. Mathioudakis, P. Ncube, A. Soares, B. Jefferson and E. J. McAdam, Dissolved methane recovery from anaerobic effluents using hollow fibre membrane contactors, J. Membr. Sci., 2016, 502, 141–150 CrossRef CAS .
111. A. McLeod, B. Jefferson and E. J. McAdam, Toward gas-phase controlled mass transfer in micro-porous membrane contactors for recovery and concentration of dissolved methane in the gas phase, J. Membr. Sci., 2016, 510, 466–471 CrossRef CAS .
112. S. Wongchitphimon, W. Rongwong, C. Y. Chuah, R. Wang and T.-H. Bae, Polymer-fluorinated silica composite hollow fiber membranes for the recovery of biogas dissolved in anaerobic effluent, J. Membr. Sci., 2017, 540, 146–154 CrossRef CAS .
113. W. Bandara, M. Ikeda, H. Satoh, M. Sasakawa, Y. Nakahara, M. Takahashi and S. Okabe, Introduction of a degassing membrane technology into anaerobic wastewater treatment, Water Environ. Res., 2013, 85, 387–390 CrossRef CAS PubMed .
114. M. Henares, M. Izquierdo, J. Penya-Roja and V. Martínez-Soria, Comparative study of degassing membrane modules for the removal of methane from Expanded Granular Sludge Bed anaerobic reactor effluent, Sep. Purif. Technol., 2016, 170, 22–29 CrossRef CAS .
115. M. Henares, M. Izquierdo, C. Gabaldón and V. Martínez-Soria, Dissolved methane recovery from an anaerobic effluent using a PDMS hollow fiber membrane contactor, Soria, Spain, 2016, p. 30 Search PubMed.
116. W. Rongwong, S. Wongchitphimon, K. Goh, R. Wang and T.-H. Bae, Transport properties of CO₂ and CH₄ in hollow fiber membrane contactor for the recovery of biogas from anaerobic membrane bioreactor effluent, J. Membr. Sci., 2017, 541, 62–72 CrossRef CAS .
117. J. K. Stolaroff, S. Bhattacharyya, C. A. Smith, W. L. Bourcier, P. J. Cameron-Smith and R. D. Aines, Review of Methane Mitigation Technologies with Application to Rapid Release of Methane from the Arctic, Environ. Sci. Technol., 2012, 46, 6455–6469 CrossRef CAS PubMed .
118. M. Hatamoto, H. Yamamoto, T. Kindaichi, N. Ozaki and A. Ohashi, Biological oxidation of dissolved methane in effluents from anaerobic reactors using a down-flow hanging sponge reactor, Water Res., 2010, 44, 1409–1418 CrossRef CAS PubMed .
119. N. Matsuura, M. Hatamoto, H. Sumino, K. Syutsubo, T. Yamaguchi and A. Ohashi, Closed DHS system to prevent dissolved methane emissions as greenhouse gas in anaerobic wastewater treatment by its recovery and biological oxidation, Water Sci. Technol., 2010, 61, 2407–2415 CrossRef CAS PubMed .
120. M. Hatamoto, T. Miyauchi, T. Kindaichi, N. Ozaki and A. Ohashi, Dissolved methane oxidation and competition for oxygen in down-flow hanging sponge reactor for post-treatment of anaerobic wastewater treatment, Bioresour. Technol., 2011, 102, 10299–10304 CrossRef CAS PubMed .
121. M. A. van Kessel, K. Stultiens, M. F. Slegers, S. G. Cruz, M. S. Jetten, B. Kartal and H. J. O. den Camp, Current perspectives on the application of N-damo and anammox in wastewater treatment, Curr. Opin. Biotechnol., 2018, 50, 222–227 CrossRef CAS PubMed .
122. X. Chen, J. Guo, Y. Shi, S. Hu, Z. Yuan and B.-J. Ni, Modeling of simultaneous anaerobic methane and ammonium oxidation in a membrane biofilm reactor, Environ. Sci. Technol., 2014, 48, 9540–9547 CrossRef CAS PubMed .
123. X. Chen, J. Guo, G.-J. Xie, Y. Liu, Z. Yuan and B.-J. Ni, A new approach to simultaneous ammonium and dissolved methane removal from anaerobic digestion liquor: A model-based investigation of feasibility, Water Res., 2015, 85, 295–303 CrossRef CAS PubMed .
124. C. Cai, S. Hu, J. Guo, Y. Shi, G.-J. Xie and Z. Yuan, Nitrate reduction by denitrifying anaerobic methane oxidizing microorganisms can reach a practically useful rate, Water Res., 2015, 87, 211–217 CrossRef CAS PubMed .
125. Y. Shi, S. H. Hu, J. Q. Lou, P. L. Lu, J. Keller and Z. G. Yuan, Nitrogen Removal from Wastewater by Coupling Anammox and Methane-Dependent Denitrification in a Membrane Biofilm Reactor, Environ. Sci. Technol., 2013, 47, 11577–11583 CrossRef CAS PubMed .
126. X. Chen, J. Guo, G. J. Xie, Z. Yuan and B. J. Ni, Achieving complete nitrogen removal by coupling nitritation-anammox and methane-dependent denitrification: A model-based study, Biotechnol. Bioeng., 2016, 113, 1035–1045 CrossRef CAS PubMed .
127. X. Chen, Y. Liu, L. Peng, Z. Yuan and B.-J. Ni, Model-based feasibility assessment of membrane biofilm reactor to achieve simultaneous ammonium, dissolved methane, and sulfide removal from anaerobic digestion liquor, Sci. Rep., 2016, 6, 25114 CrossRef CAS PubMed .
128. M. H. Winkler, K. Ettwig, T. Vannecke, K. Stultiens, A. Bogdan, B. Kartal and E. Volcke, Modelling simultaneous anaerobic methane and ammonium removal in a granular sludge reactor, Water Res., 2015, 73, 323–331 CrossRef CAS PubMed .
129. B. Hu, Z. He, S. Geng, C. Cai, L. Lou, P. Zheng and X. Xu, Cultivation of nitrite-dependent anaerobic methane-oxidizing bacteria: impact of reactor configuration, Appl. Microbiol. Biotechnol., 2014, 98, 7983–7991 CrossRef CAS PubMed .
130. C. Kampman, T. L. G. Hendrickx, F. A. Luesken, T. A. van Alen, H. J. M. Op den Camp, M. S. M. Jetten, G. Zeeman, C. J. N. Buisman and H. Temmink, Enrichment of denitrifying methanotrophic bacteria for application after direct low-temperature anaerobic sewage treatment, J. Hazard. Mater., 2012, 227, 164–171 CrossRef PubMed .
131. B. L. Zhu, J. Sanchez, T. A. van Alen, J. Sanabria, M. S. M. Jetten, K. F. Ettwig and B. Kartal, Combined anaerobic ammonium and methane oxidation for nitrogen and methane removal, Biochem. Soc. Trans., 2011, 39, 1822–1825 CrossRef CAS PubMed .
132. J. Ding, Y.-Z. Lu, L. Fu, Z.-W. Ding, Y. Mu, S. H. Cheng and R. J. Zeng, Decoupling of DAMO archaea from DAMO bacteria in a methane-driven microbial fuel cell, Water Res., 2017, 110, 112–119 CrossRef CAS PubMed .
133. M. J. McAnulty, V. G. Poosarla, K.-Y. Kim, R. Jasso-Chávez, B. E. Logan and T. K. Wood, Electricity from methane by reversing methanogenesis, Nat. Commun., 2017, 8, 15419 CrossRef CAS PubMed .
134. S. Chen and A. L. Smith, Methane-driven microbial fuel cells recover energy and mitigate dissolved methane emissions from anaerobic effluents, Environ. Sci.: Water Res. Technol., 2018, 4, 67–79 RSC .
135. J. Myung, P. E. Saikaly and B. E. Logan, A two-staged system to generate electricity in microbial fuel cells using methane, Chem. Eng. J., 2018, 352, 262–267 CrossRef CAS .
136. B. E. Logan, B. Hamelers, R. Rozendal, U. Schröder, J. Keller, S. Freguia, P. Aelterman, W. Verstraete and K. Rabaey, Microbial fuel cells: methodology and technology, Environ. Sci. Technol., 2006, 40, 5181–5192 CrossRef CAS PubMed .
137. K. Rabaey and W. Verstraete, Microbial fuel cells: novel biotechnology for energy generation, Trends Biotechnol., 2005, 23, 291–298 CrossRef CAS PubMed .
138. K. Rabaey, K. Van de Sompel, L. Maignien, N. Boon, P. Aelterman, P. Clauwaert, L. De Schamphelaire, H. T. Pham, J. Vermeulen and M. Verhaege, Microbial fuel cells for sulfide removal, Environ. Sci. Technol., 2006, 40, 5218–5224 CrossRef CAS PubMed .
139. W. van Hees, A bacterial methane fuel cell, J. Electrochem. Soc., 1965, 112, 258–262 CrossRef .
140. P. Girguis and C. E. Reimers, Methane-powered microbial fuel cellsUS Pat., Application No. 12/994,598, PCT/US2009/003209, 2011 Search PubMed .
141. Z. J. Ren, Microbial fuel cells: Running on gas, Nat. Energy, 2017, 2, 1 Search PubMed .
142. B. E. Logan, M. J. Wallack, K.-Y. Kim, W. He, Y. Feng and P. Saikaly, Assessment of microbial fuel cell configurations and power densities, Environ. Sci. Technol. Lett., 2015, 2(8), 206–214 CrossRef CAS .
143. Z. Ge and Z. He, Long-term performance of a 200 liter modularized microbial fuel cell system treating municipal wastewater: treatment, energy, and cost, Environ. Sci.: Water Res. Technol., 2016, 2, 274–281 RSC .
144. Y. D. Scherson, S.-G. Woo and C. S. Criddle, Production of nitrous oxide from anaerobic digester centrate and its use as a co-oxidant of biogas to enhance energy recovery, Environ. Sci. Technol., 2014, 48, 5612–5619 CrossRef CAS PubMed .
145. Y. D. Scherson, G. F. Wells, S.-G. Woo, J. Lee, J. Park, B. J. Cantwell and C. S. Criddle, Nitrogen removal with energy recovery through N₂O decomposition, Energy Environ. Sci., 2013, 6, 241–248 RSC .
146. Y. Jin, M. C. Veiga and C. Kennes, Bioprocesses for the removal of nitrogen oxides from polluted air, J. Chem. Technol. Biotechnol., 2005, 80, 483–494 CrossRef CAS .
147. B. Shoener, I. Bradley, R. Cusick and J. Guest, Energy positive domestic wastewater treatment: the roles of anaerobic and phototrophic technologies, Environ. Sci.: Processes Impacts, 2014, 16, 1204–1222 RSC .
148. G. H. Rau, H. D. Willauer and Z. J. Ren, The global potential for converting renewable electricity to negative-CO 2-emissions hydrogen, Nat. Clim. Change, 2018, 8, 621 CrossRef .
149. P. J. Strong, S. Xie and W. P. Clarke, Methane as a resource: can the methanotrophs add value?, Environ. Sci. Technol., 2015, 49, 4001–4018 CrossRef CAS PubMed .
150. P. J. Strong, B. Laycock, S. N. S. Mahamud, P. D. Jensen, P. A. Lant, G. Tyson and S. Pratt, The opportunity for high-performance biomaterials from methane, Microorganisms, 2016, 4, 11 CrossRef PubMed .
151. W. Guo, D. Li, R. He, M. Wu, W. Chen, F. Gao, Z. Zhang, Y. Yao, L. Yu and S. Chen, Synthesizing value-added products from methane by a new Methylomonas, J. Appl. Microbiol., 2017, 123, 1214–1227 CrossRef CAS PubMed .
152. I. Y. Hwang, A. D. Nguyen, T. T. Nguyen, L. T. Nguyen, O. K. Lee and E. Y. Lee, Biological conversion of methane to chemicals and fuels: technical challenges and issues, Appl. Microbiol. Biotechnol., 2018, 102, 3071–3080 CrossRef CAS PubMed .
153. A. Taylor, P. Molzahn, T. Bushnell, C. Cheney, M. LaJeunesse, M. Azizian and L. Semprini, Immobilization of Methylosinus trichosporium OB3b for methanol production, J. Ind. Microbiol. Biotechnol., 2018, 1–11 CAS .
154. N. Bagheri and J. Abedi, Adsorption of methane on corn cobs based activated carbon, Chem. Eng. Res. Des., 2011, 89, 2038–2043 CrossRef CAS .
155. J.-J. Chen, W.-W. Li, X.-L. Li and H.-Q. Yu, Improving biogas separation and methane storage with multilayer graphene nanostructure via layer spacing optimization and lithium doping: a molecular simulation investigation, Environ. Sci. Technol., 2012, 46, 10341–10348 CrossRef CAS PubMed .
156. T. Tian, Z. X. Zeng, D. Vulpe, M. E. Casco, G. Divitini, P. A. Midgley, J. Silvestre-Albero, J. C. Tan, P. Z. Moghadam and D. Fairen-Jimenez, A sol-gel monolithic metal-organic framework with enhanced methane uptake, Nat. Mater., 2018, 17, 174–179 CrossRef CAS PubMed .
157. E. I. Administration and G. P. Office, International Energy Outlook 2016: With Projections to 2040, Government Printing Office, 2016 Search PubMed.
158. J. Sommer, G. Ciplak, A. Linn, E. Sumer, G. Benckiser and J. Ottow, Quantification of emitted and retained N₂O in a municipal waste water treatment plant with activated sludge and nitrifying-denitrifying units, Agribiological Research, 1998, 51(1), 59–73 CAS .
159. J. H. Ahn, S. Kim, H. Park, B. Rahm, K. Pagilla and K. Chandran, N₂O emissions from activated sludge processes, 2008− 2009: results of a national monitoring survey in the United States, Environ. Sci. Technol., 2010, 44, 4505–4511 CrossRef CAS PubMed .
160. S. Sun, Z. Bao and D. Sun, Study on emission characteristics and reduction strategy of nitrous oxide during wastewater treatment by different processes, Environ. Sci. Pollut. Res., 2015, 22, 4222–4229 CrossRef CAS PubMed .
161. J. Desloover, H. De Clippeleir, P. Boeckx, G. Du Laing, J. Colsen, W. Verstraete and S. E. Vlaeminck, Floc-based sequential partial nitritation and anammox at full scale with contrasting N2O emissions, Water Res., 2011, 45, 2811–2821 CrossRef CAS PubMed .
162. M. Kampschreur, R. Poldermans, R. Kleerebezem, W. Van Der Star, R. Haarhuis, W. Abma, M. Jetten and M. Van Loosdrecht, Emission of nitrous oxide and nitric oxide from a full-scale single-stage nitritation-anammox reactor, Water Sci. Technol., 2009, 60, 3211–3217 CrossRef CAS PubMed .
163. M. Harb and P.-Y. Hong, Anaerobic Membrane Bioreactor Effluent Reuse: A Review of Microbial Safety Concerns, Fermentation, 2017, 3, 39 CrossRef .
164. M. Harb and P. Y. Hong, Molecular-based detection of potentially pathogenic bacteria in membrane bioreactor (MBR) systems treating municipal wastewater: a case study, Environ. Sci. Pollut. Res., 2017, 24, 5370–5380 CrossRef CAS PubMed .
165. M. Henares, P. Ferrero, P. San-Valero, V. Martínez-Soria and M. Izquierdo, Performance of a polypropylene membrane contactor for the recovery of dissolved methane from anaerobic effluents: Mass transfer evaluation, long-term operation and cleaning strategies, J. Membr. Sci., 2018, 563, 926–937 CrossRef CAS .
166. R. Jain, Method for economic evaluation of membrane-based air separation, Gas Sep. Purif., 1989, 3, 123–127 CrossRef CAS .
167. G. Luo, W. Wang and I. Angelidaki, A new degassing membrane coupled upflow anaerobic sludge blanket (UASB) reactor to achieve in-situ biogas upgrading and recovery of dissolved CH4 from the anaerobic effluent, Appl. Energy, 2014, 132, 536–542 CrossRef CAS .

This journal is © The Royal Society of Chemistry 2018