Application Of Bioelectrochemical Systems Research Articles

Analyzing microbial communities and metabolic functions in estuaries affected by human activity to uncover bioelectrochemical system potential

In the context of bioelectrochemical systems (BES), a profound understanding of the taxonomic composition within indigenous microbial communities is crucial. Their metabolic versatility assumes a foundational role in dictating the functionality of BES, yielding diverse applications for bioenergy production and bioremediation in anthropogenically impacted estuaries. Employing high-throughput sequencing of the V3-V4 region of 16S rRNA and predictive metagenomic analyses of three different estuaries samples (eutrophicated urban reservoir, mud volcano impacted river, and its river mouth), this study delves into the intricate interplay between these communities and their metabolic functions in response to human-induced perturbations, with a specific focus on their applicability to BES. Our findings illuminate remarkable distinctions among the three samples, manifesting in variations in microbial composition and functional metabolic pathways within the respective ecosystems. The mud volcano-affected river, characterized by the highest microbial diversity, exhibited the most diverse array of functional metabolic processes, prominently dominated by chemoheterotrophy (67.43%). In contrast, the river mouth displayed a distinct penchant for autotrophy (33.33%). The eutrophicated urban reservoir showcased a balanced coexistence of chemoheterotrophy (41.18%) and autotrophy (35.29%). Regarding metabolic potential for BES application, most of metabolism potential were associated with anodic activity, while cathodic activity was restricted to autotrophy and its relatives. The results suggested that the microbial communities anthropogenically impacted estuaries were potentially used for anodic and cathodic reactions in BES as electrogens to generate bioelectricity and reduce of pollutants.

Engineering hybrid conductive electrochemically active biofilms enable efficient interfacial electron transfer and syntrophic carbon metabolism

Supply of carbon-based nanomaterials (e.g., carbon nanotubes, CNTs) to develop highly conductive electrochemically active biofilms (EABs) is a potential strategy for facilitating extracellular electron transfer (EET) in bioelectrochemical systems (BESs). Understanding of the underlying CNTs-mediated EET behaviors is helpful to further advance the practical application of BESs. Here, the cognitive influence of CNTs on bioelectrocatalytic activity and electron transfer efficiency of EABs were elucidated. CNTs can be embedded into EABs to form hybrid conductive biofilms (CNTs/EABs), achieving a high current density (7.4 ± 1.40 A m−2) and excellent coulombic recovery (46.0 ± 2.70 %) over 100 days of steady operation. The supply of CNTs can mitigate the dependence of exoelectrogens (such as Geobacter) on outer membrane cytochromes (OMCs) and conductive pili due to their down-regulated genes expression in CNTs/EABs, but it can significantly improve microbial carbon metabolism because physically high-conductive CNTs can establish rapid EET pathways, which may reduce the necessity for cells to invest metabolic energy in producing conductive pili and cytochromes that are required in the absence of CNTs. Such enhancement in electron transfer rate may be caused by the interfacial interaction between OMCs and CNTs, resulting in an order of magnitude higher than in the control (5.5 ± 1.60 s−1vs. 0.28 ± 0.04 s−1) and without compromising of mass diffusion. This study provides comprehensive insight into the role of carbon-based nanomaterials in provoking interfacial electron transfer and renewable energy recovery.

Elucidating the impacts of cobalt (II) ions on extracellular electron transfer and pollutant degradation by anodic biofilms in bioelectrochemical systems during industrial wastewater treatment

Electrogenic biofilms in bioelectrochemical systems (BES) are critical in wastewater treatment. Industrial effluents often contain cobalt (Co2+); however, its impact on biofilms is unknown. This study investigated how increasing Co2+ concentrations (0–30 mg/L) affect BES biofilm community dynamics, extracellular polymeric substances, microbial metabolism, electron transfer gene expression, and electrochemical performance. The research revealed that as Co2+ concentrations increased, power generation progressively declined, from 345.43 ± 4.07 mW/m2 at 0 mg/L to 160.51 ± 0.86 mW/m2 at 30 mg/L Co2+. However, 5 mg/L Co2+ had less effect. The Co2+ removal efficiency in the reactors fed with 5 and 10 mg/L concentrations exceeded 99% and 94%, respectively. However, at 20 and 30 mg/L, the removal efficiency decreased substantially, likely because of reduced biofilm viability. FTIR indicated the participation of biofilm functional groups in Co2+ uptake. XPS revealed Co2+ presence in biofilms as CoO and Co(OH)2, indicating precipitation also aided removal. Cyclic voltammetry and electrochemical impedance spectroscopy tests revealed that 5 mg/L Co2+ had little impact on the electrocatalytic activity, while higher concentrations impaired it. Furthermore, at a concentration of 5 mg/L Co2+, there was an increase in the proportion of the genus Anaeromusa-Anaeroarcus, while the genus Geobacter declined at all tested Co2+ concentrations. Additionally, higher concentrations of Co2+ suppressed the expression of extracellular electron transfer genes but increased the expression of Co2+-resistance genes. Overall, this study establishes how Co2+ impacts electrogenic biofilm composition, function, and treatment efficacy, laying the groundwork for the optimized application of BES in remediating Co2+-contaminated wastewater.

Interspecies ecological competition rejuvenates decayed Geobacter electroactive biofilm.

Bioelectrochemical systems (BESs) exploit electroactive biofilms (EABs) for promising applications in biosensing, wastewater treatment, energy production, and chemical biosynthesis. However, during the operation of BESs, EABs inevitably decay. Seeking approaches to rejuvenate decayed EABs is critical for the sustainability and practical application of BESs. Prophage induction has been recognized as the primary reason for EAB decay. Herein, we report that introducing a competitive species of Geobacter uraniireducens suspended prophage induction in Geobacter sulfurreducens and thereby rejuvenated the decayed G. sulfurreducens EAB. The transcriptomic profile of G. sulfurreducens demonstrated that the addition of G. uraniireducens significantly affected the expression of metabolism- and stress response system-related genes and in particular suppressed the induction of phage-related genes. Mechanistic analyses revealed that interspecies ecological competition exerted by G. uraniireducens suppressed prophage induction. Our findings not only reveal a novel strategy to rejuvenate decayed EABs, which is significant for the sustainability of BESs, but also provide new knowledge for understanding phage-host interactions from an ecological perspective, with implications for developing therapies to defend against phage attack.

A review of autotrophic denitrification for groundwater remediation: A special focus on bioelectrochemical reactors

Groundwater is an important resource that can help in climate change adaptation. However, the pollution of these aquifers with nitrate is a widespread problem of growing concern. Biological denitrification using inorganic electron donors shows significant advantages in treating nitrate-polluted groundwater where organic matter presence is negligible. However, mass transfer limitations and secondary contamination seem to be the major hinderance to spread the use of these technologies. This could be solved by the use of bioelectrochemical systems (BES), which emerge as an attractive technology to solve these problems due to the reported low energy demand and high denitrification rates. However, technical and operational issues must be considered to replicate these results at full-scale. This review summarizes the biological basis of autotrophic denitrification and the key aspects of its application in bioelectrochemical systems. In addition, an estimation of the capital costs required for the implementation of a BES considering different population sizes and initial nitrate concentration in the groundwater is made.

Ultrasonic Spray Coating to Optimize Performance of Bio-Electrochemical Systems.

This work investigates the optimization of carbon-based electrodes employed in bio-electrochemical systems (BES) through the deposition of nanostructured layers of poly(3,4-ethylene-dioxy-thiophene) poly(styrene-sulfonate) (PEDOT:PSS) on commercial carbon paper electrodes via ultrasonic spray coating (USC). This innovative application of USC demonstrated that uniform and controlled depositions of PEDOT:PSS can be successfully performed on carbon-based electrodes. To this end, the morphology and spatial uniformity of depositions were verified via scanning electron microscopy and Raman spectroscopy. Electrochemical characterizations of fabricated electrodes demonstrated a more than two-fold increase in the electrochemical active surface area with respect to bare carbon paper. A lab-scale experiment on BES was performed, selecting microbial fuel cells (MFCs) as the reference devices. Devices featuring USC-deposited PEDOT:PSS electrodes showed a three-fold-higher energy recovery with respect to control cells, reaching a maximum value of (13 ± 2) J·m-3. Furthermore, the amount of PEDOT:PSS required to optimize MFCs' performance is in line with values reported in the literature for other deposition methods. In conclusion, this work demonstrates that USC is a promising technique for application in BES.

Implementation of cytochrome c proteins and carbon nanotubes hybrids in bioelectrodes towards bioelectrochemical systems applications.

Multiheme cytochrome c (Cyt c) can function as a redox protein on electrode to accomplish bioelectrocatalysis. However, the direct electron transfer (DET) between the redox site of Cyt c and electrode is low due to the large coupling distance. A close proximity or a connection pathway from the deeply buried active site to the protein surface can be established by modifying the electrode with carbon nanotubes (CNTs) to improve the DET. Therefore, the isolated Cyt c has been assembled or casted with CNTs by various processes to form Cyt c-CNTs bioelectrodes that can be further applied to biosensing and bioanalysis. These strategies can be transplanted to the fabrication of biofilm-CNTs based electrodes by complexing the out membrane (OM) Cyt c of natural electricigen with CNTs to realize the application of the electrochemical properties of "in vivo" Cyt c to bioelectrochemical systems (BESs). This review intends to highlight the preparation strategies of bioelectrodes that have been well studied in electrochemical biosensors and improving approaches of the DET from the CNTs surface to Cyt c in their hybrids. The efficient fabrication processes of the biofilm-CNTs based electrodes that can be considered as "in vivo" Cyt c-CNTs based electrodes for BES designs are also summarized, aiming to provide an inspiration source and a reference to the related studies of BES downstream.

Engineering cell-electrode interfacial electron transfer to boost power generation of electroactive biofilm

Electroactive biofilm plays a crucial rule in the electron transfer efficiency of bio-electrochemical systems (BES). However, the slow rate of interfacial electron transfer (IET) restricts practical applications of various BES. Here, a modular engineering strategy was developed to enhance IET rate. Firstly, to accelerate the low transmembrane electron transfer rate caused by insulative cell membrane of Shewanella oneidensis, pili-based artificial conductive nanowires and outer-membrane c-cytochrome OmcF from Geobacter sulfurreducens were heterologously expressed to construct transmembrane electron conduits. Secondly, to improve the low electron transfer rate from S. oneidensis to anode owing to the poor electron collection ability of the anode, N-doped carbon nanotubes and polyriboflavin were used to increase the surface area and active sites of the anode. Thirdly, to further reduce the resistance due to the low conductivity of biofilm, the polydopamine was coated in situ to construct high-speed conductive networks, obtaining an unprecedented power density of 5233.7 ± 364.7 mW/m2, ∼83.1-fold higher than that of the wild-type strain (62.2 ± 5.5 mW/m2), and the maximum coulomb efficiency of ESR1 @PDA was 85.4%, which, to the best of our knowledge, is one of the highest output power densities and coulomb efficiencies that have ever reported in the genetically engineered Shewanella strains. This study demonstrated an integrated biotic-electrode modular engineering strategy to boost power generation of electroactive biofilm via synthetic biology and material engineering.

Enhancement mechanism of biocathodes on nitrification in the bioelectrochemical system from a microbial perspective

Biocathodes in bioelectrochemical systems (BESs) have been reported to enhance nitrogen (N) removal. However, the complex multi-parameter variables in BESs have increased the difficulty of studying the enhanced mechanism of nitrification using biocathodes. This study aimed to construct a range of potentiostatic biocathodes (−400, −200, ＋200, and ＋400 mV vs Ag/AgCl) with a dissolved O2 concentration of 2–3 mg L−1 using a potentiostat to further clarify the mechanism. The mechanism of how biocathodes influence microorganisms was elucidated from multiple perspectives, including variations in N concentration, electrochemistry, high-throughput sequencing, and PICRUSt2 prediction. The biocathodes were found to increase the nitrification rate by improving the abundance of nitrification-related bacteria (unidentified_Nitrospiraceae, Thauera, Nitrosomonas, Prosthecobacter, and Stenotrophomonas), the expression of genes related to ammonia oxidation and nitrite oxidation (pmoABC, hao, nirk, ncd2, and npd), and the activity of ammonia oxidase (AMO) and nitrite oxidase (NOR) in the biocathodes. The results of this study might provide crucial academic support and demonstration in facilitating the application of BESs and realizing the deep treatment of wastewater.

Enhancement of Ammonium Oxidation at Microoxic Bioanodes.

Bioelectrochemical systems (BESs) are considered to be energy-efficient to convert ammonium, which is present in wastewater. The application of BESs as a technology to treat wastewater on an industrial scale is hindered by the slow removal rate and lack of understanding of the underlying ammonium conversion pathways. This study shows ammonium oxidation rates up to 228 ± 0.4 g-N m-3 d-1 under microoxic conditions (dissolved oxygen at 0.02-0.2 mg-O2/L), which is a significant improvement compared to anoxic conditions (120 ± 21 g-N m-3 d-1). We found that this enhancement was related to the formation of hydroxylamine (NH2OH), which is rate limiting in ammonium oxidation by ammonia-oxidizing microorganisms. NH2OH was intermediate in both the absence and presence of oxygen. The dominant end-product of ammonium oxidation was dinitrogen gas, with about 75% conversion efficiency in the presence of a microoxic level of dissolved oxygen and 100% conversion efficiency in the absence of oxygen. This work elucidates the dominant pathways under microoxic and anoxic conditions which is a step toward the application of BESs for ammonium removal in wastewater treatment.

Bio-electrochemical system analysis and improvement: A technical review

Bio-electrochemical systems (BESs) use electrode–microorganism interactions for biotechnology applications such as electricity generation, wastewater treatment, bioremediation, and producing valuable products. The crossover between biotic and abiotic factors is the driving force of BES operations. The interaction of these factors determines the overall efficiency of BES in various applications. Scientists are investigating various aspects for modulating these factors to increase the efficiency and use of the BES across a broader spectrum. In this review, the integration of BESs with electrochemical capacitors, use of synthetic and biosurfactants, application of BESs in biogas generation, utilization of 3D electrodes in BESs, and combining BESs with thermoelectric regulators have been thoroughly discussed. The potential applications of BESs and research gaps in various domains were also covered. This review will be beneficial to researchers better understand and therefore apply the modelling and optimization techniques for BESs constructively, ultimately contributing to the circular cascading bioeconomy.

Electrogenetic control of gene expression in Shewanella oneidensis MR-1 using Arc-dependent transcriptional promoters

Electrochemically active bacteria (EAB) are capable of electrically interacting with electrodes, enabling their application in bioelectrochemical systems (BESs). As the performance of BES is related to the metabolic activities of EAB, the development of methods to control their metabolic activities is important to facilitate BES applications. A recent study found that the EAB Shewanella oneidensis MR-1 uses the Arc system to regulate the expression of catabolic genes in response to electrode potentials, suggesting that a methodology for electrical control of gene expression in EAB, referred to as electrogenetics, can be developed by using electrode potential-responsive, Arc-dependent transcriptional promoters. Here, we explored Arc-dependent promoters in the genomes of S.oneidensis MR-1 and Escherichia coli to identify electrode potential-responsive promoters that are differentially activated in MR-1cells exposed to high- and low-potential electrodes. LacZ reporter assays using electrode-associated cells of MR-1 derivatives revealed that the activities of promoters located upstream of the E.coli feo gene (Pfeo) and the MR-1 nqrA2 (SO_0902) gene (Pnqr2) were significantly increased when S.oneidensis cells were exposed to electrodes poised at+0.7V and-0.4V (versus the standard hydrogen electrode), respectively. Additionally, we developed a microscopic system for in situ monitoring of promoter activity in electrode-associated cells and found that Pnqr2 activity was persistently induced in MR-1cells associated with an electrode poised at-0.4V. Our results indicate that these electrode potential-responsive promoters enable efficient regulation of gene expression in EAB, providing a molecular basis for the development of electrogenetics.

Three-dimensional macroporous structure and surface carbon-coating modification of stainless-steel anode boost bioelectrocatalytic performance

Bioelectrochemical systems (BESs) is a promising and sustainable technology for wastewater treatment, and the practical application of BESs generally requires scalable three-dimensional (3D) electrodes. In this study, we present a stainless-steel crochet mesh electrode (P-SS) with 3D macroporous structure, and modified by carbon coating through pyrolyzing residual lubricating oil in the mesh production process. The effect of 3D macroporous structure and surface carbon-coating modification on the improvement of electrode bioelectrocatalytic performance was investigated during the study. When the electrode was used as the anode in BESs reactors, a superior bioeletrocatalytic current was generated, reaching a maximum projected current density of 9.95 mA·cm−2 and achieving a highest geometric current density of 1.89 mA·cm−2 ever recorded. We found the 3D macroporous structure and surface carbon-coating modification enabled the electrode well mass transfer, excellent biocompatibility and fluent electron transport, finally boosted the superior bioeletrocatalytic performance. We also verified the electrode has a remarkable stackability as suggested by the bioelectrocatalytic current increasing linearly with the increase of electrode thickness. In addition, the acceptable capital cost (＜6500 $·m−3), adequate mechanical strength and feasible industrial process of this novel electrode all indicate its great prospects for the scaling-up of BESs.

A review on bioelectrochemical systems for emerging pollutants remediation: A computational approaches

A wide range of pollutant compounds are discharged into the environment that they need suitable remediations. For solving the problem, the bioelectrochemical systems (BESs) are green technology suitable for the pollutant treatments. The induced electric current in BESs can stimulate microbes metabolism and increasing electron transfer. It has been reported that the performance of BESs in reducing wastewater emerging pollutants (EPs) reaches more than 90 %. In the BESs, the emerging pollutants as carbon sources plays a significant role between competition and bacterial community and their metabolism. Although some BESs have been presented for the treatment systems, little attention has been paid to the computational approaches and kinetics studies of EPs remediation. Therefore, the current review summarizes the application of BESs for emerging pollutants removal. Moreover, we have provided the predominant microorganisms as well as the microbial metabolism and computational approaches in the biodegradation of EPs. In this review, density functional theory (DFT), molecular dynamics (MD) simulations, computational fluid dynamics (CFD), artificial intelligence or artificial neural networks (ANNs), and quantum mechanics/molecular mechanics (QM/MM) simulations have presented for the BESs. For interpretation of the BESs Michaelis-Menten, First-order kinetic and second order models are suitable kinetic models.

Integrating bioelectrochemical system with aerobic bioreactor for organics removal and caustic recovery from alkaline saline wastewater

Bioelectrochemical systems (BES) are increasingly being explored as an auxiliary unit process to enhance conventional waste treatment processes. This study proposed and validated the application of a dual-chamber bioelectrochemical cell as an add-on unit for an aerobic bioreactor to facilitate reagent-free pH-correction, organics removal and caustic recovery from an alkaline and saline wastewater. The process was continuously fed (hydraulic retention time (HRT) of 6 h) with a saline (25 g NaCl/L) and alkaline (pH 13) influent containing oxalate (25 mM) and acetate (25 mM) as the target organic impurities present in alumina refinery wastewater. Results suggested that the BES concurrently removed the majority of the influent organics and reduced the pH to a suitable range (9–9.5) for the aerobic bioreactor to further remove the residual organics. Compared to the aerobic bioreactor, the BES enabled a faster removal of oxalate (242 ± 27 vs. 100 ± 9.5 mg/L.h), whereas similar removal rates (93 ± 16 vs. 114 ± 23 mg/L.h, respectively) were recorded for acetate. Increasing catholyte HRT from 6 to 24 h increased the caustic strength from 0.22% to 0.86%. The BES enabled caustic production at an electrical energy demand of 0.47 kWh/kg-caustic, which is a fraction (22%) of the electrical energy requirement for caustic production using conventional chlor-alkali processes. The proposed application of BES holds promise to improve environmental sustainability of industries in managing organic impurities in alkaline and saline waste streams.

Recent Advances in Bioelectrochemical Systems for Nitrogen and Phosphorus Recovery Using Membranes.

Bioelectrochemical systems (BESs) have emerged as a technology that is able to recover resources from different kinds of substrates, especially wastewater. Nutrient recovery, mostly based on membrane reactor configuration, is a clear niche for BES application. The recovery of nitrogen or phosphorus allows for treatment of wastewater while simultaneously collecting a concentrated stream with nutrients that can be reintroduced into the system, becoming a circular economy solution. The aim of this study is to review recent advances in membrane-based BESs for nitrogen and phosphorus recovery and compare the recovery efficiencies and energy requirements of each system. Finally, there is a discussion of the main issues that arise from using membrane-based BESs. The results presented in this review show that it would be beneficial to intensify research on BESs to improve recovery efficiencies at the lowest construction cost in order to take the final step towards scaling up and commercialising this technology.

Endogenous electric field accelerates phenol degradation in bioelectrochemical systems with reduced electrode spacing.

Reducing the electrode spacing in bioelectrochemical systems (BESs) are widely reported to improve power output, which was mainly attributed to the decrease of ohmic resistance (Rohm) for a long time. Here we found the change of endogenous electric field (EF) intensity was the key to improve electroactivity in response to a reduced electrode spacing, which also accelerated phenol biodegradation. Correlation and principal components analysis revealed that the microbial community of electroactive biofilm (EAB) was independent of Rohm, while the EF intensity was found closely related to most of predominant genera. A strong EF selectively enriched phenol-degrading bacteria Comamonas in suspension and Geobacter in EAB, contributed to the improvement of degradation efficiency. EF also induced the secretion of extracellular polymeric substances, protected EAB from being inactivated by phenol. Our findings highlighted the importance of EF intensity on BESs performance, providing new insights into the design and application of BESs in wastewater treatment.

Bioelectrochemical System Application for Pesticides Removal: A mini-review

The widespread use of pesticides for agricultural purposes results in their presence in the environment. Pesticide occurrence is causing potential negative impacts on human health and environmental deterioration in general. One of the interdisciplinary field to implement a technology for converting and conserving resources is bioelectrochemistry. An integrated system based on bioelectrochemical processes, namely Bioelectrochemical System (BES), has been well developed to date. The application of BES to remove various materials in the bioremediation process has been well established in the combination with diverse technologies. The set up of integrated BES with Granular Activated Carbon (GAC) is one of the promising solutions to enhance the removal of the pesticide in water. The BES in situ has been implemented to remove contaminants in groundwater. This mini-review discusses the recent development of pesticide removal and environmental remediation using bioelectrochemical approaches, offering new insight into the big scope of green technology application.

Microbial electrochemical bioremediation of petroleum hydrocarbons (PHCs) pollution: Recent advances and outlook

Soil contamination by petroleum hydrocarbons (PHCs) causes various acute problems such as crop growth, yield, and grain quality. Many Physico-chemical and biological approaches are employed to remove the PHCs from polluted soil. However, some of these approaches are too slow, or ineffective, and/or environmentally unfriendly. Indeed, bioelectrochemical systems (BES) combine biological methods and electrochemistry in the context of a redox reaction. BES is gaining attention from scientists and policymakers as a promising technology for the remediation of soil contaminated with PHCs. This review discusses BES working principles, mechanisms, design configuration, operational parameters, and advances in BES applications to remediate PHCs from contaminated soil efficiently. The role of biosurfactants and biochar in enhancing PHC degradation in soil using BES mediating extracellular electron transfer (EET) and biofilm formation is highlighted. Furthermore, recent innovations in this field, technical and economic challenges and limitations in scaling-up BES, and future perspectives are discussed. This review suggests that biochar-based single-chamber air-cathode reactors are preferred because of their lower cost compared to the other classic configurations. Additional efforts are needed in the design of BES reactors, soil characteristics, and seasonal variations in BES performance over a long period of operation to improve the efficiency of soil remediation and power production as well as to apply it on a large scale.

Response of electroactive biofilms from real wastewater to metal ion shock in bioelectrochemical systems

The electrochemical activity of bioelectrochemical systems (BESs) was proven to be dependent on the stability of electroactive biofilms (EABs), but the response of EABs based on real wastewater to external disturbances is not fully known. Herein, we used real wastewater (beer brewery wastewater) as a substrate for culturing EABs and found that current generation, biomass, redox activity and extracellular polymeric substances (EPS) content in those EABs were lower as compared to EABs cultured with synthetic wastewaters (acetate and glucose). However, the EABs from the beer brewery wastewater showed moderate anti-shock resistance capability. The proteins and humic acid in loosely bound EPS (LB-EPS) exhibited a positive linear relationship with current recovery after Ag+ shock, indicating the importance of LB-EPS for protecting the EABs. Fluorescence and Fourier transform infrared spectroscopy integrated with two-dimensional correlation spectroscopy verified that the spectra of the protein-like region of LB-EPS changed considerably under the interference of Ag+ concentration and the CO group of humic acid or proteins was mainly responsible for binding with Ag+ to attenuate its toxicity to the EABs. This is the first study revealing the underlying molecular mechanism of EABs cultured with real wastewater against external heavy metal shock and provides useful insights into enhancing the application of BESs in future water treatment.