Over one-fourth of the world's population lacks safe, reliable drinking water access. To ensure adequate water supply, arid communities are increasingly tapping into unconventional water sources, which are often saline and require energy-intensive treatment. For the one billion people who suffer from both lack of water and electricity, incorporating renewable energy can help solve this water-energy nexus challenge. This article provides a comprehensive review of solar-thermal membrane distillation, a desalination and water purification method that can convert source waters of a wide range of salinities to potable water using thermal energy harvested from sunlight. Membrane distillation (MD) is a thermally driven separation process with vapor passage through a porous, hydrophobic membrane. This thermal desalination technology can incorporate abundant, low-grade heat to reduce its energy needs and overall costs. Furthermore, solar-thermal MD's niche application is in treating hypersaline water sources. Herein, we review current lab-scale and pilot-scale studies on solar-thermal MD systems while comparing direct solar-thermal MD, indirect solar-thermal MD, and hybrid solar MD systems as well as solar concentration opportunities. Feedwater in standard solar-thermal MD processes is heated indirectly using high-heat transfer configurations. Comparatively, solar energy directly heats the membrane surface in direct solar-thermal MD. Hybrid solar MD systems convert solar energy into electricity and thermal energy to be stand-alone systems. Current challenges and commercialization limitations of solar MD technology, such as membrane wetting, system scale-up technology, and high energy consumption are also addressed with proposals for possible solutions. We conclude by highlighting successful pilot studies to show where solar-thermal MD is most equipped to meet water and electricity access challenges.

Partial desalination of brackish waters by nanofiltration (NF) is a lower-energy alternative than near-complete (high rejection) desalination by reverse osmosis (RO) in cases where required water quality is less than that of drinking water. Such is the case for a non-potable application like the irrigation of salt-tolerant food crops where there is often an inverse relationship between crop yield and irrigation water salinity. In our previous work on partial desalination for cultivation of salt tolerant crops in hydroponic (soilless) controlled environment agriculture (CEA), we presented modeling simulations of the NF process for conventional one- and two-stage process configurations with several commercially-available NF membranes for producing partially-desalted water. Performance assessment was based on specific energy consumption (SEC), water recovery (%), and product-water salinity as total dissolved solids (TDS), matching that of crop salinity tolerance. In this article, motivated by further decreasing SEC and increasing recovery, we build on that prior work and provide additional modeling simulations for several emerging NF process configurations: (i) a one-stage (1S) NF with continuous partial concentrate recycle (1S-CR) as a means of increasing recovery; (ii) a 1S NF with internally staged design (ISD) with combinations of different NF elements (e.g., high rejection and high permeability) situated within a single-stage pressure vessel as a means of creating stages within a stage; (iii) a closed circuit NF (CC-NF) with a one-stage system operated in a semi-batch mode as a means of increasing recovery with periodic cycles of complete continuous concentrate recycle followed by concentrate flush; (iv) a two-stage (2S) system with various distributions of numbers of elements in the second stage, and (v) a three-stage (3S) NF system to increase recovery over that of a 2S-NF. We have found that most of these emerging NF process configurations can produce a target product-water TDS at a generally higher recovery and/or lower SEC than the conventional process configurations assessed earlier. We have provided a comparative assessment of these processes for two brackish groundwater sources by highlighting the ratio of recovery/SEC, given that a lower SEC would lower operating costs and a higher recovery would reduce waste brine disposal costs. While process configurations providing higher recovery/SEC ratios include 3S-NF and CC-NF, there are potential constraints to consider such as additional process components for 3S-NF as well as operational challenges and higher product-water TDS for CC-NF. We have also provided guidance in specific membrane selection for providing a targeted product-water TDS for a given crop.

Water scarcity is a critical challenge in West Texas, where agriculture and the oil and gas industry compete for limited water resources. The oil and gas industry generates approximately 32

In an era of increasing scarcity of freshwater resources linked to climate change-induced regional droughts, there is growing interest in utilizing brackish water resources for agriculture. Given that agricultural use accounts for about 70 % of freshwater withdrawals globally, there is a critical need to further expand the potential use of brackish waters in food crop production. There is an abundance of brackish water resources (e.g., inland brackish groundwaters; seawater-impacted surface waters such as bays, estuaries, and river deltas; coastal groundwater aquifers impacted by seawater intrusion). Major constraints in using brackish waters for irrigation are the salt tolerance of a given crop, reduced crop yield with increasing salinity, and potential soil salinization in traditional land-based agriculture; however, the latter can be alleviated by the practice of hydroponics (soilless cultivation). Desalinated brackish waters can be used in hydroponic cultivation of salt-tolerant high value crops under controlled environment agriculture (CEA) to increase productivity, if environmental impacts of desalinated brine are mitigated. One potential approach to overcoming regional freshwater scarcity in agricultural irrigation is the application of membrane-based desalination technologies to brackish waters. Compared with conventional (high-rejection/non-discriminate) desalination technologies (e.g., seawater reverse osmosis), partial desalination technologies can reduce water treatment costs, energy utilization, and carbon footprint while providing fit-for-purpose water with tailored quality for hydroponics CEA to match water salinity with plant salt tolerance. Besides overall salinity, there are specific ions that can either exert toxicity on certain plants (e.g., Cl−) or serve as plant nutrients (e.g., K+, Ca2+). Given that there are existing membrane-based desalination technologies that could potentially be operated in a modified mode with new membrane products, we herein investigate nanofiltration (NF) and electrodialysis/electrodialysis reversal (ED/EDR) as viable partial desalination technologies. NF is generally considered as an ion fractionation process in terms of retention of divalent and permeation of monovalent ions, thus providing partial desalting. While multiple ED/EDR stages may be used to achieve a high degree of desalination, ED/EDR operation under a reduced electrode voltage and/or a single stage, may provide a targeted salinity reduction. This review explores the potential of NF and ED/EDR for partial desalting of brackish water sources as well as assessing their ion-selectivity and limitations thereof. The aim is to highlight benefits of partial desalination in providing water from brackish water sources for food crops in a hydroponics CEA system.

The widespread application of electrodialysis is constrained by the high cost of ion exchange membranes, necessitating the development of affordable alternatives. This study focuses on the fabrication and performance evaluation of cation exchange membranes made from polyethersulfone (PES) and sulfonated polyethersulfone (sPES). Membranes were synthesized through phase inversion with varying solvent evaporation times, using N-Methyl-2-Pyrrolidone (NMP) as the solvent. The structural and functional modifications were confirmed using FTIR, XPS, and AFM techniques. Performance tests identified optimal electrodialysis results for PES membranes with a 3 h solvent evaporation time and for sPES membranes with a 1 h evaporation time. Under varying operational conditions, including applied voltage, flow rates, and feed solutions, sPES membranes demonstrated superior performance, underscoring their potential for cost-effective brackish water desalination applications.

Electrodialysis (ED) is a viable technology for treating unconventional waters due to its higher chemical and mechanical stability and less propensity to fouling and scaling compared to other membrane technologies. With the advances in electro-driven separation processes, ED has become a multipurpose technology capable of brine treatment, mineral recovery, chemical production, and desalination. Modeling has assisted in developing new ion-exchange membranes (IEMs) and optimizing operating parameters to enhance ED performance. Still, there is an opportunity to extend ED modeling for resource recovery by including computational-aided membrane development that will help identify new selective IEMs and reduce the experimental testing by acting as a membrane screening tool. Additionally, a multifunctional optimization approach using machine learning or artificial intelligence will enable the simulation and optimization of experimental parameters, leading to reduced experimental testing and minimized levelized cost of production.

As the demand for high-recovery desalination grows, zero liquid discharge (ZLD) technologies have gained attention for their ability to minimize waste and maximize water recovery, an especially critical goal for inland desalination. This study presents the first field-scale application of the emerging technology salt-free electrodialysis metathesis (SF-EDM) at the Kay Bailey Hutchison Desalination Plant in El Paso, TX, to treat real brackish reverse osmosis (RO) brine at high hydraulic recoveries. The pilot evaluated batch, semi-continuous, and pretreated configurations to assess hydraulic recovery, specific energy consumption, and ion separation. The findings illuminate that SF-EDM achieved hydraulic recoveries up to 90 %, with total system recovery exceeding 98 % when integrated with RO. The process maintained directional separation of calcium and sulfate ions, limiting calcium sulfate precipitation. These results validate SF-EDM as a viable high recovery brine management strategy. From a zero liquid discharge perspective, further valorization of concentrate streams could push the total system recovery beyond 98 %, highlighting its potential for sustainable inland desalination.

This study evaluated the potential of photothermal carbon black (CB) coatings to use direct solar light as an energy source for pervaporation desalination. We used simulated sunlight to irradiate the CB-coated membrane and measured water flux and salt rejection. The feed was recirculated at a rate of 1 mL/s to replicate the industrial recirculation condition. We observed no statistically significant difference in water fluxes of the CB-coated membranes tested with and without a simulated sunlight irradiation of 0.8 sun intensity (0.82 +/- 0.34 and 0.79 +/- 0.16 kg/(h m2), respectively). Furthermore, the average permeances of the uncoated and coated membranes were similar. The results indicate that with rapid recirculating large volume feed photothermal heating from CB coatings is insufficient to increase the pervaporation flux of water because the heat generated by the CB heating on the surface of the membrane is largely dissipated into the bulk of the recirculating feed.

Bipolar membrane electrodialysis enables the in-situ production of high value products (e.g., acid and base) from clean brine, which is essential for a sustainable future. A technoeconomic assessment (TEA) was conducted on extraction of value from brine using the WaterTAP framework to identify optimal cost across a wide design space. In the power constrained regime, increasing supplied salt concentration does not necessarily result in reduced cost or increased NaOH concentration. A detailed analysis elucidates the critical roles of water dissociation, limiting currents, and sodium diffusion play in shaping the landscape of levelized cost. Among these, water splitting predominantly influences the TEA outcomes across most of the optimal design space. Sensitivity analysis further demonstrates that membrane properties controlling water dissociation significantly impact the unit cost. The results indicate that innovations targeting improvements in water disassociation should be prioritised to effectively reduce the levelized cost of product production.

Since reverse osmosis (RO) is often limited to a water recovery of 70-85 %, implementing a secondary process like electrodialysis metathesis (EDM) to treat concentrate can increase system water recovery. This study explores salt-free electrodialysis metathesis (SF-EDM) as an alternative to conventional EDM to investigate its performance in the zero discharge desalination (ZDD) process. SF-EDM utilizes monovalent-selective ion exchange membranes, eliminating the need for sodium chloride (NaCl) as a substitution solution. This approach generates one diluate stream and two highly soluble concentrate streams enriched in calcium and sulfate, respectively. In this article, a series of lab-scale tests based on synthetic and real RO concentrate were performed to investigate how SF-EDM performs operating at high water recovery with a feedwater with high scaling potential. We critically analyze the performance in terms of salinity reduction, selectivity, and energy consumption. Results of the study show that without the addition of NaCl, the process was able to selectively separate sulfate and calcium into two concentrate streams achieve a salinity reduction of 93 % and a total system water recovery (RO and SF-EDM) of 90 %. A technoeconomic analysis found SF-EDM to be 80 % of the cost of conventional EDM making it a competitive technology for brackish water RO concentrate management.

As a vapor pressure-driven process, pervaporation (PV) shares several of the advantages of membrane distillation (MD), such as the ability to tackle high salinity waters and the possibility of integrating low grade heat sources to reduce energy consumption. Membrane scaling and pore wetting remain strong limitations to the implementation of MD desalination. In comparison, dense, non-porous PV membranes are considered. In this study, PV membranes made from NEXARTM, a sulfonated pentablock copolymer, were evaluated and compared to polytetrafluoroethylene (PTFE) MD membranes in a vacuum configuration. The membranes were tested using three solutions: 32 g L-1 sodium chloride (NaCl), a brackish water (8.4 g L-1) of high scaling potential, and 5.5 g L-1 NaCl with 1 mM sodium dodecyl sulfate. The NEXARTM membrane achieved a permeance of 93.1±44.6 kg m-2 h-1 bar-1 for the 32 g L-1 brine, which was almost 20% higher than the PTFE MD membrane. This permeance decreased in the presence of foulants; however, in contrast with the MD membrane, where scaling and surfactants induced pore wetting, the salt rejection for the NEXARTM PV membrane was constant at >99% for all water types. These results emphasize the robustness of PV as a process to deal with challenging saline waters.

Study region: The Middle Rio Grande (MRG), defined by the portion of the basin from Elephant Butte Reservoir in New Mexico to the confluence with the Rio Conchos in Far West Texas, U.S.A. and Northern Chihuahua, Mexico.Study focus: The future of water for the MRG and many other arid and semi-arid regions of the world is challenged by a changing climate, agricultural intensification, growing urban pop-ulations, and a segmented governance system in a transboundary setting. The core question for such settings is: how can water be managed so that competing agricultural, urban, and envi-ronmental sectors can realize a sustainable future? We synthesize results from interdisciplinary research aimed at "water futures", considering possible, probable, and preferable outcomes from the known drivers of change in the MRG in a stakeholder participatory mode. We accomplished this by developing and evaluating scenarios using a suite of scientifically rigorous computer models, melded with the input from diverse stakeholders.New hydrological insights for the region: Under likely scenarios without significant interventions, relatively cheap and easy to access water will be depleted in about 40 years. Interventions to mitigate this outcome will be very costly. A new approach is called for based on "adaptive cooperation" among sectors and across jurisdictions along four important themes: information sharing, water conservation, greater development and use of alternative water sources, and new limits to water allocation/withdrawals coupled with more flexibility in uses.

Centralized water infrastructure has, over the last century, brought safe and reliable drinking water to much of the world. But climate change, combined with aging and underfunding, is increasingly testing the limits of-and reversing gains made by-these large-scale water systems. To address these growing strains and gaps, we must assess and advance alternatives to centralized water provision and sanitation. The water literature is rife with examples of systems that are neither centralized nor networked, but still meet water needs of local communities in important ways, including: informal and hybrid water systems, decentralized water provision, community-based water management, small drinking water systems, point-of-use treatment, small-scale water vendors, and packaged water. Our work builds on these literatures by proposing a convergence approach that can integrate and explore the benefits and challenges of modular, adaptive, and decentralized ("MAD") water provision and sanitation, often foregrounding important advances in engineering technology. We further provide frameworks to evaluate justice, economic feasibility, governance, human health, and environmental sustainability as key parameters of MAD water system performance.

Electro-driven separation processes offer several potential advantages over pressure-driven separation processes such as reverse osmosis for water reuse and desalination, including energy savings for low-salinity waters, cation or anion selectivity, and versatility for fit-for-purpose treatment. In this perspective, we review technologies for electro-driven separation processes and evaluate their prospect for marginalized water sources and fit-for-purpose water treatment, which include improving freshwater sustainability, protecting environmental flows, and improving recycling of industrial process streams and municipal wastewater reuse. We discuss critical aspects related to application, implementation, and techno-economic evaluation of electro-driven separation technologies. Electro-driven processes provide viable options to enhance a circular water economy by reducing salinity and selectively separating contaminants while recovering valuable products with increased environmental sustainability.

Pervaporation is a vapor pressure-driven membrane desalination process that can desalinate water with greater total dissolved solids than conventional reverse osmosis. This review analyzes the performance (flux and per-meance) of membrane materials used for pervaporation desalination. Poly(vinyl alcohol) (PVA), silica/silicate, graphene oxide (GO), and zeolite were the most frequently used materials to synthesize pervaporation desali-nation membranes. PVA is the most common material and it yields a relatively high permeance. Surface free energies of different materials were evaluated as well, to analyze the scaling/fouling propensity of the current common pervaporation desalination membranes. PVA is found to be more likely to experience scaling by gypsum while adding organic silica/silicate or GO has the potential to mitigate this issue. When comparing PVA and polyvinylidene fluoride, the hydrophobic polymer is more likely to experience scaling/fouling than hydrophilic polymers. These results indicate that future development in membranes for high-efficiency pervaporation desalination may benefit from emphasizing materials with higher hydrophilicity.

Fluorotelomer alcohols (FTOHs) are one of the major classes of per- and polyfluoroalkyl substances (PFAS). Due to their potential toxicity, persistence, and ubiquitous presence in the environment, some common PFAS are voluntarily phased out; while FTOHs are used as alternatives to conventional PFAS. FTOHs are precursors of perfluorocarboxylic acids (PFCAs) and therefore they are commonly detected in water matrices, which eventually indicate PFAS contamination in drinking water supplies and thus a potential source of human exposure. Even though studies have been conducted nationwide to evaluate the degree of FTOHs in the water environment, robust monitoring is lacking because of the unavailability of simple and sustainable analytical extraction and detection methods. To fill the gap, we developed and validated a simple, rapid, minimal solvent use, no clean-up, and sensitive method for the determination of FTOHs in water by stir bar sorptive extraction (SBSE) coupled with thermal desorption-gas chromatography-mass spectrometry (TD-GC-MS). Three commonly detected FTOHs (6:2 FTOH, 8:2 FTOH, and 10:2 FTOH) were selected as the model compounds. Factors such as extraction time, stirring speed, solvent composition, salt addition, and pH were investigated to achieve optimal extraction efficiency. This “green chemistry” based extraction provided good sensitivity and precision with low method limits of detection ranging from 2.16 ng/L to 16.7 ng/L and with an extraction recovery ranging 55%–111%. The developed method were tested on tap water, brackish water, and wastewater influent and effluent. 6:2 FTOH and 8:2 FTOH were detected in two wastewater samples at 78.0 and 34.8 ng/L, respectively. This optimized SBSE-TD-GC-MS method will be a valuable alternative to investigate FTOHs in water matrices.

Water matrix composition impacts water treatment performance. However, matrix composition impacts have rarely been studied for electrochemical water treatment processes, and the correlation between the composition and the treatment efficiency is lacking. This work evaluated the electrochemical reduction of nitrate (ERN) using different complex water matrices: groundwater, brackish water, and reverse osmosis (RO) concentrate/brine. The ERN was conducted using a tin (Sn) cathode because of the high selectivity towards nitrogen evolution reported for Sn electrocatalysts. The co-existence of calcium (Ca2+), magnesium (Mg2+), and carbonate (CO32) ions in water caused a 4-fold decrease in the nitrate conversion into innocuous nitrogen gas due to inorganic scaling formation on the cathode surface. XRF and XRD analysis of fouled catalyst surfaces detected brucite (Mg (OH)(2)), calcite (CaCO3), and dolomite (CaMg(CO3)(2)) mineral scales formed on the cathode surface. Surface scaling created a physical barrier on the electrode that decreased the ERN efficiency. Identifying these main sources of ERN inhibition was key to devising potential fouling mitigation strategies. For this reason, the chemical softening pre-treatment of a real brackish water was conducted and this significantly increased nitrate conversion and faradaic efficiency during subsequent ERN treatment, leading to a lower electric energy consumption per order. Understanding the ionic foulant composition responsible for influencing electrochemicallydriven technologies are the first steps that must be taken to move towards niche applications such as decentralized ERN. Thus, we propose either direct ERN implementation in regions facing high nitrate levels in soft waters, or a hybrid softening/nitrate removal system for those regions where high nitrate and high-water hardness appear simultaneously.

Membrane separation has enjoyed tremendous advances in relevant material and engineering sciences, making it the fastest growing technology in water treatment. Although membranes as a broadspectrum physical barrier have great advantages over conventional treatment processes in a myriad of applications, the need for higher selectivity and specificity in membrane separation is rising as we move to target contaminants at trace concentrations and to recover valuable chemicals from wastewater with low energy consumption. In this review, we discuss the drivers, fundamental science, and potential enabling materials for high selectivity membranes, as well as their applications in different water treatment processes. Membrane materials and processes that show promise to achieve high selectivity for water, ions, and small molecules-as well as the mechanisms involved-are highlighted. We further identify practical needs, knowledge gaps, and technological barriers in both material development and process design for high selectivity membrane processes. Finally, we discuss research priorities in the context of existing and future water supply paradigms.

Electrodialysis (ED) desalination performance of different conventional and laboratory-scale ion exchange membranes (IEMs) has been evaluated by many researchers, but most of these studies used their own sets of experimental parameters such as feed solution compositions and concentrations, superficial velocities of the process streams (diluate, concentrate, and electrode rinse), applied electrical voltages, and types of IEMs. Thus, direct comparison of ED desalination performance of different IEMs is virtually impossible. While the use of different conventional IEMs in ED has been reported, the use of bioinspired ion exchange membrane has not been reported yet. The goal of this study was to evaluate the ED desalination performance differences between novel laboratory‑scale bioinspired IEM and conventional IEMs by determining (i) limiting current density, (ii) current density, (iii) current efficiency, (iv) salinity reduction in diluate stream, (v) normalized specific energy consumption, and (vi) water flux by osmosis as a function of (a) initial concentration of NaCl feed solution (diluate and concentrate streams), (b) superficial velocity of feed solution, and (c) applied stack voltage per cell-pair of membranes. A laboratory‑scale single stage batch-recycle electrodialysis experimental apparatus was assembled with five cell‑pairs of IEMs with an active cross-sectional area of 7.84 cm2. In this study, seven combinations of IEMs (commercial and laboratory-made) were compared: (i) Neosepta AMX/CMX, (ii) PCA PCSA/PCSK, (iii) Fujifilm Type 1 AEM/CEM, (iv) SUEZ AR204SZRA/CR67HMR, (v) Ralex AMH-PES/CMH-PES, (vi) Neosepta AMX/Bare Polycarbonate membrane (Polycarb), and (vii) Neosepta AMX/Sandia novel bioinspired cation exchange membrane (SandiaCEM). ED desalination performance with the Sandia novel bioinspired cation exchange membrane (SandiaCEM) was found to be competitive with commercial Neosepta CMX cation exchange membrane.