Combining air recharging and membrane superhydrophobicity for fouling prevention in membrane distillation

Bio-Inspired Functional Materials for Environmental Applications

With the global population expected to reach 9.7 billion by 2050, there is an urgent need for advanced materials that can address existing and developing environmental issues. Many current synthesis processes are environmentally unfriendly and often lack control over size, shape, and phase of resulting materials. Based on knowledge from biological synthesis and assembly processes, as well as their resulting functions (e.g., photosynthesis, self-healing, anti-fouling, etc.), researchers are now beginning to leverage these biological blueprints to advance bio-inspired pathways for functional materials for water treatment, air purification and sensing. The result has been the development of novel materials that demonstrate enhanced performance and address sustainability. Here, an overview of the progress and potential of bio-inspired methods toward functional materials for environmental applications is provided. The challenges and opportunities for this rapidly expanding field and aim to provide a valuable resource for researchers and engineers interested in developing sustainable and efficient processes and technologies is discussed.

Nanofilament-Coated Superhydrophobic Membranes Show Enhanced Flux and Fouling Resistance in Membrane Distillation

Membrane distillation (MD) is an important technique for brine desalination and wastewater treatment that may utilize waste or solar heat. To increase the distillation rate and minimize membrane wetting and fouling, we deposit a layer of polysiloxane nanofilaments on microporous membranes. In this way, composite membranes with multiscale pore sizes are created. The performance of these membranes in the air gap and direct contact membrane distillation was investigated in the presence of salt solutions, solutions containing bovine serum albumin, and solutions containing the surfactant sodium dodecyl sulfate. In comparison to conventional hydrophobic membranes, our multiscale porous membranes exhibit superior fouling resistance while attaining a higher distillation flux without using fluorinated compounds. This study demonstrates a viable method for optimizing MD processes for wastewater and saltwater treatment.

Enhanced Anti-Wetting Methods of Hydrophobic Membrane for Membrane Distillation

Increasing issues of hydrophobic membrane wetting occur in the membrane distillation (MD) process, stimulating the research on enhanced anti-wetting methods for membrane materials. In recent years, surface structural construction (i.e., constructing reentrant-like structures), surface chemical modification (i.e., coating organofluorides), and their combination have significantly improved the anti-wetting properties of the hydrophobic membranes. Besides, these methods change the MD performance (i.e., increased/decreased vapor flux and increased salt rejection). This review first introduces the characterization parameters of wettability and the fundamental principles of membrane surface wetting. Then it summarizes the enhanced anti-wetting methods, the related principles, and most importantly, the anti-wetting properties of the resultant membranes. Next, the MD performance of hydrophobic membranes prepared by different enhanced anti-wetting methods is discussed in desalinating different feeds. Finally, facile and reproducible strategies are aspired for the robust MD membrane in the future.

Surface Functional Groups Affect Iron (Hydr)oxide Heterogeneous Nucleation: Implications for Membrane Scaling

Because of its favorable thermodynamics and fast kinetics, heterogeneous solid nucleation on membranes triggers early-stage mineral scaling. Iron (hydr)oxide, a typical membrane scale, initially forms as nanoparticles that interact with surface functional groups on membranes, but these nanoscale phenomena are difficult to observe in real time. In this study, we utilized in situ grazing incidence small angle X-ray scattering and ex situ atomic force microscopy to examine the heterogeneous nucleation of iron (hydr)oxide on surface functional groups commonly used in membranes, including hydroxyl (OH), carboxyl (COOH), and fluoro (F) groups. We found that, compared to nucleation on hydrophilic OH- and COOH-surfaces, the high hydrophobicity of an F-modified surface significantly reduced the extents of both heterogeneously and homogeneously formed iron (hydr)oxide nucleation. Moreover, on the OH-surface, the high functional group density of 0.76 nmol/cm² caused faster heterogeneous nucleation than that on a COOH-surface, with a density of 0.28 ± 0.04 nmol/cm². The F-surface also had the highest heterogeneous nucleation energy barrier (26 ± 0.6 kJ/mol), followed by COOH- (23 ± 0.8 kJ/mol) and OH- (20 ± 0.9 kJ/mol) surfaces. The kinetic and thermodynamic information provided here will help us better predict the rates and extents of early-stage scaling of iron (hydr)oxide nanoparticles in membrane processes.

Hierarchically Structured Nanoparticle-Free Omniphobic Membrane for High-Performance Membrane Distillation

The functional loss of membranes caused by pore wetting, mineral scaling, or structural instability is a critical challenge in membrane distillation (MD), which primarily hinders its practical applications. Herein, we propose a novel and facile strategy to fabricate omniphobic membranes with exceptionally robust MD performance. Specifically, a substrate with a hierarchical re-entrant architecture was constructed via spray-water-assisted non-solvent-induced phase separation (SWNIPS), followed by a direct fluorinated surface decoration via "thiol-ene" click chemistry. Deionized (DI) water contact angle measurements revealed an ultrahigh surface water contact angle (166.8 ± 1.8°) and an ultralow sliding angle (3.6 ± 1.1°) of the resultant membrane. Destructive abrasion cycle and ultrasonication tests confirmed its structural robustness. Moreover, the membrane possessed excellent wetting resistance, as evidenced by the prevention of membrane pore penetration by all low-surface-tension testing liquids, allowing stable long-term MD operation to treat brine wastewater with a surfactant content of 0.6 mM. In a desalination experiment using shale gas wastewater, the omniphobic membrane exhibited robust MD performance, achieving a high water recovery ratio of ∼60% without apparent changes in water flux and permeate conductivity over the entire membrane process. Overall, our study paves the way for a nanoparticle-free methodology for the scalable fabrication of high-performance MD membranes with surface omniphobicity and structural robustness in hypersaline wastewater treatment.

Biofouling control of membrane distillation for seawater desalination: Effect of air-backwash and chemical cleaning on biofouling formation

This study explored the applicability of chemical cleaning and air-backwash to alleviate biofouling on seawater membrane distillation (SWMD). Membrane performance and wettability properties maintained at optimum duration and frequency of the treatments, as indicated by low permeate conductivity throughout the tests. The cleaning of the membrane using 2% NaOH by immersing the membrane for 30 min after 240 min operation removed the biofouling layer, indicated by low permeate conductivity of 370 µScm^-1 after cleaning. However, more frequent membrane cleaning led to membrane damage, more severe wetting, and membrane hydrophobicity reduction. Ten-second air-backwash after 240 min of operation was also effective in controlling the biofouling, particularly when conducted at air pressure of 1 bar. More frequent air-backwash resulted in more aggravated inorganic fouling and accelerated biofouling formation due to the recurring introduction of air, leading to rapid membrane wetting.

Bio-inspired super liquid-repellent membranes for membrane distillation: Mechanisms, fabrications and applications

With the aggravation of the global water crisis, membrane distillation (MD) for seawater desalination and hypersaline wastewater treatment is highlighted due to its low operating temperature, low hydrostatic pressure, and theoretically 100% rejection. However, some issues still impede the large-scale applications of MD technology, such as membrane fouling, scaling and unsatisfactory wetting resistance. Bio-inspired super liquid-repellent membranes have progressed rapidly in the past decades and been considered as one of the most promising approaches to overcome the above problems. This review for the first time systematically summarizes and analyzes the mechanisms of different super liquid-repellent surfaces, their preparation and modification methods, and anti-wetting/fouling/scaling performances in the MD process. Firstly, the topology theories of in-air superhydrophobic, in-air omniphobic and underwater superoleophobic surfaces are illustrated using different models. Secondly, the fabrication methods of various super liquid-repellent membranes are classified. The merits and demerits of each method are illustrated. Thirdly, the anti-wetting/fouling/scaling mechanisms of super liquid-repellent membranes are summarized. Finally, the conclusions and perspectives of the bio-inspired super liquid-repellent membranes are elaborated. It is anticipated that the systematic review herein can provide readers with foundational knowledge and current progress of super liquid-repellent membranes, and inspire researchers to overcome the challenges up ahead.

How Multilayered Feathers Enhance Underwater Superhydrophobicity

Inspired by ducks, we demonstrate that air pockets within stacked layers of porous superhydrophobic feathers can withstand up to five times more water pressure compared to a single feather. In addition to natural duck feathers, this "layer effect" was replicated with synthetic feathers created by laser cutting micrometric slots into aluminum foil and imparting a superhydrophobic nanostructure. It was revealed that adding layers promotes an increasingly redundant pathway for water impalement, which serves to pressurize the enclosed air pockets. This was validated by creating a probabilistic pore impalement model and also by filling the feathers with an incompressible oil, rather than air, to suppress the layer effect. In addition to revealing a utility of natural duck feathers, our findings suggest that multilayered engineered surfaces can maintain air pockets at high pressures, useful for reducing the drag and fouling of marine structures or enhancing desalination membranes.

A Critical Review of Membrane Wettability in Membrane Distillation from the Perspective of Interfacial Interactions

Hydrophobic membranes used in membrane distillation (MD) systems are often subject to wetting during long-term operation. Thus, it is of great importance to fully understand factors that influence the wettability of hydrophobic membranes and their impact on the overall separation efficiency that can be achieved in MD systems. This Critical Review summarizes both fundamental and applied aspects of membrane wetting with particular emphasis on interfacial interaction between the membrane and solutes in the feed solution. First, the theoretical background of surface wetting, including the relationship between wettability and interfacial interaction, definition and measurement of contact angle, surface tension, surface free energy, adhesion force, and liquid entry pressure, is described. Second, the nature of wettability, membrane wetting mechanisms, influence of membrane properties, feed characteristics and operating conditions on membrane wetting, and evolution of membrane wetting are reviewed in the context of an MD process. Third, specific membrane features that increase resistance to wetting (e.g., superhydrophobic, omniphobic, and Janus membranes) are discussed briefly followed by the comparison of various cleaning approaches to restore membrane hydrophobicity. Finally, challenges with the prevention of membrane wetting are summarized, and future work is proposed to improve the use of MD technology in a variety of applications.

A comparison between superhydrophobic surfaces (SHS) and slippery liquid-infused porous surfaces (SLIPS) in application

Slippery liquid-infused porous surfaces inspired by the Nepenthes pitcher plant exhibit excellent performances and are known for their extremely low contact angle hysteresis (<5°) and smooth surface. In contrast, superhydrophobic surfaces (SHS) exhibit poor pressure stability, difficulty in self-healing, and difficulty in removing low surface tension liquids or organic solvents, which can affect the stable air layer. Thus, these issues can be avoided through the replacement of SHS with slippery liquid infused porous surfaces (SLIPS). In this review, the theoretical models of SHS and SLIPS are classified initially, and several design standards for the preparation of SLIPS are briefly described. Then, we focus on comparing the differences in the application of SHS and SLIPS, such as pressure stability, transparency, and droplet manipulation. However, there are still some problems that need to be improved during the preparation of SLIPS, such as the evaporation of the lubricant layer, the use of a lubricant layer of toxic perfluoropolyether and other substances, and easily lost nanostructured lubricant layer. Accordingly, several new improved methods are proposed in this review, and finally, the potential applications and development prospects of SLIPS are presented.

Long-Running Comparison of Feed-Water Scaling in Membrane Distillation

Membrane distillation (MD) has shown promise for concentrating a wide variety of brines, but the knowledge is limited on how different brines impact salt scaling, flux decline, and subsequent wetting. Furthermore, past studies have lacked critical details and analysis to enable a physical understanding, including the length of experiments, the inclusion of salt kinetics, impact of antiscalants, and variability between feed-water types. To address this gap, we examined the system performance, water recovery, scale formation, and saturation index of a lab-scale vacuum membrane distillation (VMD) in long-running test runs approaching 200 h. The tests provided a comparison of a variety of relevant feed solutions, including a synthetic seawater reverse osmosis brine with a salinity of 8.0 g/L, tap water, and NaCl, and included an antiscalant. Saturation modeling indicated that calcite and aragonite were the main foulants contributing to permeate flux reduction. The longer operation times than typical studies revealed several insights. First, scaling could reduce permeate flux dramatically, seen here as 49% for the synthetic brine, when reaching a high recovery ratio of 91%. Second, salt crystallization on the membrane surface could have a long-delayed but subsequently significant impact, as the permeate flux experienced a precipitous decline only after 72 h of continuous operation. Several scaling-resistant impacts were observed as well. Although use of an antiscalant did not reduce the decrease in flux, it extended membrane operational time before surface foulants caused membrane wetting. Additionally, numerous calcium, magnesium, and carbonate salts, as well as silica, reached very high saturation indices (>1). Despite this, scaling without wetting was often observed, and scaling was consistently reversible and easily washed. Under heavy scaling conditions, many areas lacked deposits, which enabled continued operation; existing MD performance models lack this effect by assuming uniform layers. This work implies that longer times are needed for MD fouling experiments, and provides further scaling-resistant evidence for MD.

The Application of Submerged Modules for Membrane Distillation

This paper deals with the efficiency of capillary modules without an external housing, which were used as submerged modules in the membrane distillation process. The commercial hydrophobic capillary membranes fabricated for the microfiltration process were applied. Several constructional variants of submerged modules were discussed. The influence of membrane arrangement, packing density, capillary diameter and length on the module performance was determined. The effect of process conditions, i.e., velocity and temperature of the streams, on the permeate flux was also evaluated. The submerged modules were located in the feed tank or in the distillate tank. It was found that much higher values of the permeate flux were obtained when the membranes were immersed in the feed with the distillate flowing inside the capillary membranes. The efficiency of submerged modules was additionally compared with the conventional membrane distillation (MD) capillary modules and a similar performance of both constructions was achieved.

Review on Blueprint of Designing Anti-Wetting Polymeric Membrane Surfaces for Enhanced Membrane Distillation Performance

Recently, membrane distillation (MD) has emerged as a versatile technology for treating saline water and industrial wastewater. However, the long-term use of MD wets the polymeric membrane and prevents the membrane from working as a semi-permeable barrier. Currently, the concept of antiwetting interfaces has been utilized for reducing the wetting issue of MD. This review paper discusses the fundamentals and roles of surface energy and hierarchical structures on both the hydrophobic characteristics and wetting tolerance of MD membranes. Designing stable antiwetting interfaces with their basic working principle is illustrated with high scientific discussions. The capability of antiwetting surfaces in terms of their self-cleaning properties has also been demonstrated. This comprehensive review paper can be utilized as the fundamental basis for developing antiwetting surfaces to minimize fouling, as well as the wetting issue in the MD process.

Implementation of two multiphase flow methods in modeling wetting of microporous hydrophobic membranes

Pore wetting phenomenon plays a critical role in a porous media and is critical in various processes. For instance, liquid entry pressure (LEP) is one of the critical characteristics of hydrophobic membranes used in membrane distillation (MD) processes. In this study, pore-scale models were developed to assess the accuracy of two multiphase flow computational fluid dynamics (CFD) methods, as modeling tools for predicting two-phase flow in microporous MD membranes. Finite element method (FEM)-based phase field (PF) method (which was applied in the COMSOL package) and finite volume method (FVM)-based volume of fluid (VOF) method (which was applied in Star-CCM+) were the selected CFD tools for the implementation. The boundary conditions of the models were first set based on the experimental procedure for measuring the LEP, as given in the literature. Then, the models were used to capture the LEP under the gradually increased water pressure. Critical tuning of CFD parameters of each tool (such as mesh size, mesh type, and interface thickness) was conducted to investigate their influence on the LEP prediction accuracy and the water/air interface representation at the pore entrance. CFD model results were presented and compared with both experimental LEP data and the calculated value using the Young-Laplace equation (YLE). Both CFD tools were capable of capturing the water/air interface. LEP result from the VOF model showed good agreement with the experimental data, but the PF model overestimated the LEP value closer to the theoretical YLE value. For both approaches, the adjustment of the interface thickness was critical. In the VOF method, a realistic interface thickness could be achieved by adjusting both mesh size and time step simultaneously. In contrast, PF simulations were less mesh sensitive. The accuracy of the VOF model was better due to its mass conservation condition at the interface.

A review of membrane development in membrane distillation for emulsified industrial or shale gas wastewater treatments with feed containing hybrid impurities

Investigations on membrane materials for membrane distillation (MD) and its applications have been ongoing since the 1990s. However, a lack of materials that produce robustly stable and up-to-the-mark membranes for MD for different industrial applications remains an ongoing problem. This paper provides an overview of materials developed for MD applications. Although key aspects of published articles reviewed in this paper pertain to MD membranes synthesized for desalination, future MD can also be applied to organic wastewater containing surfactants with inorganic compounds, either with the help of hybrid treatment processes or with customized membrane materials. Many industrial discharges produce effluents at a very high temperature, which is an available driving force for MD. However, there remains a lack of cost-effective membrane materials. Amphiphobic and omniphobic membranes have recently been developed for treating emulsified and shale gas produced water, but the problem of organic fouling and pore wetting remains a major challenge, especially when NaCl and other inorganic impurities are present, which further deteriorate separation performance. Therefore, further advancements in materials are required for the treatment of emulsified industrial wastewater containing surfactants, salts, and for oil or shale gas wastewater for its commercialized reuse. Integrated MD systems, however, may represent a major change in shale gas wastewater and emulsified wastewater that are difficult to treat.

Fouling and wetting in the membrane distillation driven wastewater reclamation process - A review

Fouling and wetting of membranes are significant concerns that can impede the widespread application of the membrane distillation (MD) process during high-salinity wastewater reclamation. Fouling, caused by the accumulation of undesirable materials on the membrane surface and pores, causes a decrease in permeate flux. Membrane wetting, the direct permeation of the feed solution through the membrane pores, results in reduced contaminant rejection and overall process failure. Lately, the application of MD for water recovery from various types of wastewaters has gained increased attention among researchers. In this review, we discuss fouling and wetting phenomena observed during the MD process, along with the effects of various mitigation strategies. In addition, we examine the interactions between contaminants and different types of MD membranes and the influence of different operating conditions on the occurrence of fouling and wetting. We also report on previously investigated feed pre-treatment options before MD, application of integrated MD processes, the performance of fabricated/modified MD membranes, and strategies for MD membrane maintenance during water reclamation. Energy consumption and economic aspects of MD for wastewater recovery is also discussed. Throughout the review, we engage in dialogues highlighting research needs for furthering the development of MD: the incorporation of MD in the overall wastewater treatment and recovery scheme (including selection of appropriate membrane material, suitable pre-treatment or integrated processes, and membrane maintenance strategies) and the application of MD in long-term pilot-scale studies using real wastewater.

Slippery for scaling resistance in membrane distillation: A novel porous micropillared superhydrophobic surface

Scaling in membrane distillation (MD) is a key issue in desalination of concentrated saline water, where the interface property between the membrane and the feed become critical. In this paper, a slippery mechanism was explored as an innovative concept to understand the scaling behavior in membrane distillation for a soluble salt, NaCl. The investigation was based on a novel design of a superhydrophobic polyvinylidene fluoride (PVDF) membrane with micro-pillar arrays (MP-PVDF) using a micromolding phase separation (μPS) method. The membrane showed a contact angle of 166.0 ± 2.3° and the sliding angle of 15.8 ± 3.3°. After CF₄ plasma treatment, the resultant membrane (CF₄-MP-PVDF) showed a reduced sliding angle of 3.0°. In direct contact membrane distillation (DCMD), the CF₄-MP-PVDF membrane illustrated excellent anti-scaling in concentrating saturated NaCl feed. Characterization of the used membranes showed that aggregation of NaCl crystals occurred on the control PVDF and MP-PVDF membranes, but not on the CF₄-MP-PVDF membrane. To understand this phenomenon, a "slippery" theory was introduced and correlated the sliding angle to the slippery surface of CF₄-MP-PVDF and its anti-scaling property. This work proposed a well-defined physical and theoretical platform for investigating scaling problems in membrane distillation and beyond.

Wetting phenomena in membrane distillation: Mechanisms, reversal, and prevention

Membrane distillation (MD) is a rapidly emerging water treatment technology; however, membrane pore wetting is a primary barrier to widespread industrial use of MD. The primary causes of membrane wetting are exceedance of liquid entry pressure and membrane fouling. Developments in membrane design and the use of pretreatment have provided significant advancement toward wetting prevention in membrane distillation, but further progress is needed. In this study, a broad review is carried out on wetting incidence in membrane distillation processes. Based on this perspective, the study describes the wetting mechanisms, wetting causes, and wetting detection methods, as well as hydrophobicity measurements of MD membranes. This review discusses current understanding and areas for future investigation on the influence of operating conditions, MD configuration, and membrane non-wettability characteristics on wetting phenomena. Additionally, the review highlights mathematical wetting models and several approaches to wetting control, such as membrane fabrication and modification, as well as techniques for membrane restoration in MD. The literature shows that inorganic scaling and organic fouling are the main causes of membrane wetting. The regeneration of wetting MD membranes is found to be challenging and the obtained results are usually not favorable. Several pretreatment processes are found to inhibit membrane wetting by removing the wetting agents from the feed solution. Various advanced membrane designs are considered to bring membrane surface non-wettability to the states of superhydrophobicity and superomniphobicity; however, these methods commonly demand complex fabrication processes or high-specialized equipment. Recharging air in the feed to maintain protective air layers on the membrane surface has proven to be very effective to prevent wetting, but such techniques are immature and in need of significant research on design, optimization, and pilot-scale studies.

Mixed Matrix Polytetrafluoroethylene/Polysulfone Electrospun Nanofibrous Membranes for Water Desalination by Membrane Distillation

The electrospinning technique was used successfully to fabricate nanofibers of polysulfone (PSF) in which polytetrafuoroethylene nanoparticles (PTFE NPs) were embedded. The size of the PTFE NPs is only 1.7 to 3.6 times smaller than the nanofiber diameter. The transition from hydrophobic to superhydrophobic character of the bead-free PSF electrospun nanofiber mats occurred with a PTFE NPs loading in the range 12-18% of the PSF weight. Transmission electron microscopy images revealed protruding nanosized asperities on the fiber surface due to the embedded PTFE NPs in the PSF matrix. For low PTFE NPs content in PSF matrix (<6% of the polymer weight), the PTFE NPs were arranged one by one in a single file along the PSF nanofiber axis. The structural characteristics of the nanofibers and electrospun nanofibrous membranes (ENMs) were studied by means of different techniques and their relationship with the PTFE NPs loading in PSF were discussed. The PSF/PTFE ENMs were tested in desalination by direct contact membrane distillation (DCMD) and the obtained performance was discussed in terms of the ENMs structural characteristics. Competitive permeate fluxes, as high as 39.5 kg/m²h, with stable low permeate electrical conductivities (<7.145 μS/cm) for 30 g/L NaCl aqueous solution and transmembrane temperature of 60 °C were achieved without detecting any interfiber space wetting.

Inorganic fouling mitigation by salinity cycling in batch reverse osmosis

Enhanced fouling resistance has been observed in recent variants of reverse osmosis (RO) desalination which use time-varying batch or semi-batch processes, such as closed-circuit RO (CCRO) and pulse flow RO (PFRO). However, the mechanisms of batch processes' fouling resistance are not well-understood, and models have not been developed for prediction of their fouling performance. Here, a framework for predicting reverse osmosis fouling is developed by comparing the fluid residence time in batch and continuous (conventional) reverse osmosis systems to the nucleation induction times for crystallization of sparingly soluble salts. This study considers the inorganic foulants calcium sulfate (gypsum), calcium carbonate (calcite), and silica, and the work predicts maximum recovery ratios for the treatment of typical water sources using batch reverse osmosis (BRO) and continuous reverse osmosis. The prediction method is validated through comparisons to the measured time delay for CaSO₄ membrane scaling in a bench-scale, recirculating reverse osmosis unit. The maximum recovery ratio for each salt solution (CaCO₃, CaSO₄) is individually predicted as a function of inlet salinity, as shown in contour plots. Next, the maximum recovery ratios of batch and conventional RO are compared across several water sources, including seawater, brackish groundwater, and RO brine. Batch RO's shorter residence times, associated with cycling from low to high salinity during each batch, enable significantly higher recovery ratios and higher salinity than in continuous RO for all cases examined. Finally, representative brackish RO brine samples were analyzed to determine the maximum possible recovery with batch RO. Overall, the induction time modeling methodology provided here can be used to allow batch RO to operate at high salinity and high recovery, while controlling scaling. The results show that, in addition to its known energy efficiency improvement, batch RO has superior inorganic fouling resistance relative to conventional RO.

Sodium Hydroxide Production from Seawater Desalination Brine: Process Design and Energy Efficiency

The ability to increase pH is a crucial need for desalination pretreatment (especially in reverse osmosis) and for other industries, but processes used to raise pH often incur significant emissions and nonrenewable resource use. Alternatively, waste brine from desalination can be used to create sodium hydroxide, via appropriate concentration and purification pretreatment steps, for input into the chlor-alkali process. In this work, an efficient process train (with variations) is developed and modeled for sodium hydroxide production from seawater desalination brine using membrane chlor-alkali electrolysis. The integrated system includes nanofiltration, concentration via evaporation or mechanical vapor compression, chemical softening, further ion-exchange softening, dechlorination, and membrane electrolysis. System productivity, component performance, and energy consumption of the NaOH production process are highlighted, and their dependencies on electrolyzer outlet conditions and brine recirculation are investigated. The analysis of the process also includes assessment of the energy efficiency of major components, estimation of system operating expense and comparison with similar processes. The brine-to-caustic process is shown to be technically feasible while offering several advantages, that is, the reduced environmental impact of desalination through lessened brine discharge, and the increase in the overall water recovery ratio of the reverse osmosis facility. Additionally, best-use conditions are given for producing caustic not only for use within the plant, but also in excess amounts for potential revenue.

Superhydrophobic membrane: progress in preparation and its separation properties

Superhydrophobic membrane that is highly resistant to wetting by aqueous solution has gained great attention because of its potential to be applied in many emerging membrane processes such as membrane gas absorption (MGA) and membrane distillation (MD). Numerous approaches have been proposed to obtain membranes with superhydrophobic surface from materials with various degrees of hydrophobicity. This paper then reviews the progress in superhydrophobic membrane preparation and its separation properties. A brief description of superhydrophobicity is firstly presented. Preparation methods of the superhydrophobic membrane are subsequently reviewed, including direct processing method and surface modification of the existing membrane. Finally, the separation properties and challenges of superhydrophobic membranes are discussed. This article could provide an insight for further development of superhydrophobic membrane.

A review of polymeric membranes and processes for potable water reuse

Conventional water resources in many regions are insufficient to meet the water needs of growing populations, thus reuse is gaining acceptance as a method of water supply augmentation. Recent advancements in membrane technology have allowed for the reclamation of municipal wastewater for the production of drinking water, i.e., potable reuse. Although public perception can be a challenge, potable reuse is often the least energy-intensive method of providing additional drinking water to water stressed regions. A variety of membranes have been developed that can remove water contaminants ranging from particles and pathogens to dissolved organic compounds and salts. Typically, potable reuse treatment plants use polymeric membranes for microfiltration or ultrafiltration in conjunction with reverse osmosis and, in some cases, nanofiltration. Membrane properties, including pore size, wettability, surface charge, roughness, thermal resistance, chemical stability, permeability, thickness and mechanical strength, vary between membranes and applications. Advancements in membrane technology including new membrane materials, coatings, and manufacturing methods, as well as emerging membrane processes such as membrane bioreactors, electrodialysis, and forward osmosis have been developed to improve selectivity, energy consumption, fouling resistance, and/or capital cost. The purpose of this review is to provide a comprehensive summary of the role of polymeric membranes in the treatment of wastewater to potable water quality and highlight recent advancements in separation processes. Beyond membranes themselves, this review covers the background and history of potable reuse, and commonly used potable reuse process chains, pretreatment steps, and advanced oxidation processes. Key trends in membrane technology include novel configurations, materials and fouling prevention techniques. Challenges still facing membrane-based potable reuse applications, including chemical and biological contaminant removal, membrane fouling, and public perception, are highlighted as areas in need of further research and development.