A framework for better understanding membrane distillation separation process

The role of membrane technology in palm oil mill effluent (POME) decontamination: Current trends and future prospects

This article reviews the role of membrane systems in treating palm oil mill effluent (POME), a waste generated by the palm industry. The review focuses on various membrane systems such as microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO), highlighting their effectiveness in removing pollutants and recovering water. Special attention is given to hybrid systems integrating membrane bioreactors (MBRs) and other advanced processes to enhance fouling control, improve water quality, and promote sustainability. Several case studies and quantitative data have demonstrated the reduction of chemical oxygen demand (COD), total suspended solids (TSS), and biological oxygen demand (BOD), illustrating the impact of these technologies. This comprehensive review also explores recent advancements, such as the integration of Zero Liquid Discharge (ZLD) processes, providing insights into the benefits and challenges of membrane technology for POME treatment. This article aims to inform future research and guide industrial applications toward more sustainable and efficient wastewater management in the palm oil industry.

Graphene Membrane for Water-Related Environmental Application: A Comprehensive Review and Perspectives

Graphene-based materials can be potentially utilized for separation membranes due to their unique structural properties such as precise molecular sieving by interlayer spacing or pore structure and excellent stability in harsh environmental conditions. Therefore, graphene-based membranes have been extensively demonstrated for various water treatment applications, including desalination, water extraction, and rare metal ion recovery. While most of the utilization has still been limited to the laboratory scale, emerging studies have dealt with scalable approaches to show commercial feasibility. This review summarizes the recent studies on diverse graphene membrane fabrications and their environmental applications related to water-containing conditions in addition to the molecular separation mechanism and critical factors related to graphene membrane performance. Additionally, we discuss future perspectives and challenges to provide insights into the practical applications of graphene-based membranes on the industrial scale.

An Opportunity for Synergizing Desalination by Membrane Distillation Assisted Reverse-Electrodialysis for Water/Energy Recovery

Industry, agriculture, and a growing population all have a major impact on the scarcity of clean-water. Desalinating or purifying contaminated water for human use is crucial. The combination of thermal membrane systems can outperform conventional desalination with the help of synergistic management of the water-energy nexus. High energy requirement for desalination is a key challenge for desalination cost and its commercial feasibility. The solution to these problems requires the intermarriage of multidisciplinary approaches such as electrochemistry, chemical, environmental, polymer, and materials science and engineering. The most feasible method for producing high-quality freshwater with a reduced carbon footprint is demanding incorporation of industrial low-grade heat with membrane distillation (MD). More precisely, by using a reverse electrodialysis (RED) setup that is integrated with MD, salinity gradient energy (SGE) may be extracted from highly salinized MD retentate. Integrating MD-RED can significantly increase energy productivity without raising costs. This review provides a comprehensive summary of the prospects, unresolved issues, and developments in this cutting-edge field. In addition, we summarize the distinct physicochemical characteristics of the membranes employed in MD and RED, together with the approaches for integrating them to facilitate effective water recovery and energy conversion from salt gradients and freshwater.

Molecular Dynamics Simulation of Membrane Distillation for Different Salt Solutions in Nanopores

Nanoporous membranes offer significant advantages in direct contact membrane distillation applications due to their high flux and strong resistance to wetting. This study employs molecular dynamics simulations to explore the performance of membrane distillation in a single nanopore, mainly focusing on wetting behavior, liquid entry pressure, and membrane flux variations across different concentrations and types of salt solutions. The findings indicate that increasing the NaCl concentration enhances the wetting of membrane pores, thereby decreasing the entry pressure of the solution. However, at the same salt concentration, the differences in wetting and liquid entry pressure among various salts, including CaCl2, KCl, NaCl, and LiCl, are minimal. The presence of hydrated ions significantly reduces membrane flux. As the concentration of NaCl solutions increases, the number of hydrated ions rises, thereby lowering the membrane flux of the salt solution. Furthermore, the type of salt has a pronounced effect on the structure of hydrated ions. Solutions with Ca2+ and Li+ exhibit the smallest first-layer radius of hydrated ions. Under the same salt concentration, KCl solutions demonstrate the highest membrane distillation flux, while CaCl2 solutions show the lowest flux.

An in-depth analysis of membrane distillation research (1990-2023): Exploring trends and future directions through bibliometric approach

This bibliometric analysis offers a comprehensive investigation into membrane distillation (MD) research from 1990 to 2023. Covering 4389 publications, the analysis sheds light on the evolution, trends, and future directions of the field. It delves into authorship patterns, publication trends, prominent journals, and global contributions to reveal collaborative networks, research hotspots, and emerging themes within MD research. The findings demonstrate extensive global participation, with esteemed journals such as Desalination and the Journal of Membrane Science serving as key platforms for disseminating cutting-edge research. The analysis further identifies crucial themes and concepts driving MD research, ranging from membrane properties to strategies for mitigating membrane fouling. Co-occurrence analysis further highlights the interconnectedness of research themes, showcasing advancements in materials, sustainable heating strategies, contaminant treatment, and resource management. Overlay co-occurrence analysis provides temporal perspective on emerging research trends, delineating six key topics that will likely shape the future of MD. These include innovations in materials and surface engineering, sustainable heating strategies, emerging contaminants treatment, sustainable water management, data-driven approaches, and sustainability assessments. Finally, the study serves as a roadmap for researchers and engineers navigating the dynamic landscape of MD research, offering insights into current trends and future trajectories, ultimately aiming to propel MD technology towards enhanced performance, sustainability, and global relevance.

Molecularly imprinted polymer composite membranes: From synthesis to diverse applications

This review underscores the fundamentals of MIP-CMs and systematically summarizes their synthetic strategies and applications, and potential developments. MIP-CMs are widely acclaimed for their versatility, finding applications in separation, filtration, detection, and trace analysis, as well as serving as scaffolds in a range of analytical, biomedical and industrial contexts. Also characterized by extraordinary selectivity, remarkable sensitivity, and outstanding capability to bind molecules, those membranes are also cost-effective, highly stable, and configurable in terms of recognition and, therefore, inalienable in various application fields. Issues relating to the potential future for the paper are discussed in the last section with the focus on the improvement of resource practical application across different areas. Hence, this review can be seen as a kind of cookbook for the design and fabrication of MIP-CMs with an intention to expand the scope of their application.

Research on the Performance and Computational Fluid Dynamics Numerical Simulation of Plate Air Gap Membrane Distillation Module

Membrane distillation (MD) is widely used in the field of seawater desalination. Among its various sub-categories, air gap membrane distillation (AGMD) stands out due to its high thermal efficiency and compatibility with low-grade heat sources. This study delves into the impact of varying operating conditions on AGMD performance, employing numerical simulations which are grounded in experimental validation. The objective was to enhance the performance of AGMD, mitigate polarization phenomena, and provide a reference for optimizing membrane component design. The results show that the agreements between the simulated and the experimental values were high. When increasing the feed temperature and decreasing the coolant temperature, the impact of polarization phenomena on the performance of AGMD was reduced. The mass flux, Total Permeate Concentration (TPC), and heat flux increased by 81.69%, 36.89%, and 118.01%, respectively, when the feed temperature was increased from 50 °C to 75 °C. When the coolant temperature decreased from 22 °C to 7 °C, the mass flux increased by 37.06%. The response surface analysis revealed that the feed temperature has significant influence on AGMD performance, and there is a noticeable interaction between the feed temperature and coolant temperature. These findings will play key roles in practical applications.

Carbon-Based Nanocomposite Membranes for Membrane Distillation: Progress, Problems and Future Prospects

The development of an ideal membrane for membrane distillation (MD) is of the utmost importance. Enhancing the efficiency of MD by adding nanoparticles to or onto a membrane's surface has drawn considerable attention from the scientific community. It is crucial to thoroughly examine state-of-the-art nanomaterials-enabled MD membranes with desirable properties, as they greatly enhance the efficiency and reliability of the MD process. This, in turn, opens up opportunities for achieving a sustainable water-energy-environment nexus. By introducing carbon-based nanomaterials into the membrane's structure, the membrane gains excellent separation abilities, resistance to various feed waters, and a longer lifespan. Additionally, the use of carbon-based nanomaterials in MD has led to improved membrane performance characteristics such as increased permeability and a reduced fouling propensity. These nanomaterials have also enabled novel membrane capabilities like in situ foulant degradation and localized heat generation. Therefore, this review offers an overview of how the utilization of different carbon-based nanomaterials in membrane synthesis impacts the membrane characteristics, particularly the liquid entry pressure (LEP), hydrophobicity, porosity, and membrane permeability, as well as reduced fouling, thereby advancing the MD technology for water treatment processes. Furthermore, this review also discusses the development, challenges, and research opportunities that arise from these findings.

Recent developments in solar-powered membrane distillation for sustainable desalination

The freshwater shortage continues to be one of the greatest challenges affecting our planet. Although traditional membrane distillation (MD) can produce clean water regardless of climatic conditions, the process wastes a lot of energy. The technique of solar-powered membrane distillation (SPMD) has received a lot of interest in the past decade, thanks to the development of photothermal materials. SPMD is a promising replacement for the traditional MD based on fossil fuels, as it can prevent the harmful effects of emissions on the environment. Integrating green solar energy with MD can reduce the cost of the water purification process and secure freshwater production in remote areas. At this point, it is important to consider the most current progress of the SPMD system and highlight the challenges and prospects of this technology. Based on this, the background, recent advances, and principles of MD and SPMD, their configurations and mechanisms, fabrication methods, advantages, and current limitations are discussed. Detailed comparisons between SPMD and traditional MD, assessments of various standards for incorporating photothermal materials with desirable properties, discussions of desalination and other applications of SPMD and MD, and energy consumption rates are also covered. The final section addresses the potential of SPMD to outperform traditional desalination technology while improving water production without requiring a significant amount of electrical or high-grade thermal energy.

Membrane distillation assisting food production processes of thermally sensitive food liquid items: a review

Physical separation technologies have become important tool for processing in the current food manufacturing industries, especially for the products containing bioactive compounds thanks to their health benefits in costumers. As for the processing of bioactive food ingredients implies the implementation of integrated systems oriented to their separation, fractionation, and recovery. In this field, membrane distillation (MD), which is a thermally driven membrane process, has been proposed as an alternative for the separation and concentration of liquid food items. In principle, MD can separate water and volatile compounds from aqueous feed solutions through a permeate that passes across microporous hydrophobic membranes. The separation via MD is thanks to the vapor pressure difference on both membrane sides. In this review, we analyzed the ongoing experimental efforts aimed to recover and purify food bioactive compounds from the concentration of fruit juices and extracts using MD. Also, the processing of dairy products, concentration of food by-products, and ethanol production and its removal from beverages using MD have been reviewed. Additionally, a feedback on the distinct membrane module configurations and membrane requirements for successful operation is addressed.

Enhancing the Permeate Flux Improvement of Direct Contact Membrane Distillation Modules with Inserted S-Ribs Carbon-Fiber Filaments

Three widths of manufacturing S-ribs carbon-fiber filaments acting as turbulence promoters were implemented into the flow channel of direct contact membrane distillation (DCMD) modules to augment the permeate flux improvement in the present study. Attempts to reduce the disadvantageous temperature polarization effect were made by inserting S-ribs turbulence promoters in improving pure water productivity, in which both heat- and mass-transfer boundary layers were diminished due to creating vortices in the flow pattern and increasing turbulence intensity. The temperature polarization coefficient ttemp was studied and found to enhance device performance (less thermal resistance) under inserting various S-ribs carbon-fiber thicknesses and operating both cocurrent- and countercurrent-flow patterns. The permeate fluxes in the DCMD modules with inserted S-ribs carbon-fiber turbulence promoters were investigated theoretically by developing the mathematical modeling equations and were conducted experimentally with various design and operating parameters. The theoretical predictions and experimental results exhibited a great potential to considerably achieve permeate flux enhancement in the new design of the DCMD system. The DCMD module with inserted S-ribs carbon-fiber turbulence promoters in the flow channel could provide a relative permeate flux enhancement up to 37.77% under countercurrent-flow operations in comparisons with the module of using the empty channel. An economic consideration on both permeate flux enhancement and power consumption increment for the module with inserted S-ribs carbon-fiber filaments was also delineated.

Review of Hybrid Membrane Distillation Systems

Membrane distillation (MD) is an attractive separation process that can work with heat sources with low temperature differences and is less sensitive to concentration polarization and membrane fouling than other pressure-driven membrane separation processes, thus allowing it to use low-grade thermal energy, which is helpful to decrease the consumption of energy, treat concentrated solutions, and improve water recovery rate. This paper provides a review of the integration of MD with waste heat and renewable energy, such as solar radiation, salt-gradient solar ponds, and geothermal energy, for desalination. In addition, MD hybrids with pressure-retarded osmosis (PRO), multi-effect distillation (MED), reverse osmosis (RO), crystallization, forward osmosis (FO), and bioreactors to dispose of concentrated solutions are also comprehensively summarized. A critical analysis of the hybrid MD systems will be helpful for the research and development of MD technology and will promote its application. Eventually, a possible research direction for MD is suggested.

Patterned dense Janus membranes with simultaneously robust fouling, wetting and scaling resistance for membrane distillation

Membrane fouling, wetting and scaling are three prominent challenges that severely hinder the practical applications of membrane distillation (MD). Herein, polyamide/polyvinylidene fluoride (PA/PVDF) Janus membrane comprising a hydrophobic PVDF substrate and a patterned dense PA layer by reverse interfacial polymerization (R-IP) was developed. Direct contact MD experiments demonstrated that PA/PVDF Janus membrane could exhibit simultaneously superior resistance towards surfactant-induced wetting, oil-induced fouling and gypsum-induced scaling without compromising flux. Importantly, the size-sieving effect, rather than the breakthrough pressure of the membrane, was revealed as the critical factor that probably endowed its resistance to wetting. Furthermore, a unique possible anti-scaling mechanism was unveiled. The superhydrophilic patterned dense PA layer with strong salt rejection capability not only prevented scale-precursor ions from intruding the substrate but also resulted in the high surface interfacial energy that inhibited the adhesion and growth of gypsum on the membrane surface, while its relatively low surface -COOH density benefited from R-IP process further ensured the membrane with a low scaling propensity. This study shall provide new insights and novel strategies in designing high-performance MD membranes and enable robust applications of MD facing the challenges of membrane fouling, wetting and scaling.

Advanced Development of Molecularly Imprinted Membranes for Selective Separation

Molecularly imprinted membranes (MIMs), the incorporation of a given target molecule into a membrane, are generally used for separating and purifying the effective constituents of various natural products. They have been in use since 1990. The application of MIMs has been studied in many fields, including separation, medicine analysis, solid-phase extraction, and so on, and selective separation is still an active area of research. In MIM separation, two important membrane performances, flux and permselectivities, show a trade-off relationship. The enhancement not only of permselectivity, but also of flux poses a challenging task for membranologists. The present review first describes the recent development of MIMs, as well as various preparation methods, showing the features and applications of MIMs prepared with these different methods. Next, the review focuses on the relationship between flux and permselectivities, providing a detailed analysis of the selective transport mechanisms. According to the majority of the studies in the field, the paramount factors for resolving the trade-off relationship between the permselectivity and the flux in MIMs are the presence of effective high-density recognition sites and a high degree of matching between these sites and the imprinted cavity. Beyond the recognition sites, the membrane structure and pore-size distribution in the final imprinted membrane collectively determine the selective transport mechanism of MIM. Furthermore, it also pointed out that the important parameters of regeneration and antifouling performance have an essential role in MIMs for practical applications. This review subsequently highlights the emerging forms of MIM, including molecularly imprinted nanofiber membranes, new phase-inversion MIMs, and metal-organic-framework-material-based MIMs, as well as the construction of high-density recognition sites for further enhancing the permselectivity/flux. Finally, a discussion of the future of MIMs regarding breakthroughs in solving the flux-permselectivity trade-off is offered. It is believed that there will be greater advancements regarding selective separation using MIMs in the future.

Simultaneous ammonium and water recovery from landfill leachate using an integrated two-stage membrane distillation

Resources recovery from landfill leachate (LFL) has been attracting growing attention instead of merely purifying the wastewater. An integrated two-stage membrane distillation (ITMD) was proposed to simultaneously purify LFL and recover ammonia in this study. The results showed that organics could be always effectively rejected by the ITMD regardless of varying feed pH, with COD removal higher than 99%. With feed pH increased from 8.64 to 12, the ammonia migration (50-100%) and capture (36-75%) in LFL were considerably enhanced, boosting the separated ammonia enrichment to 1.3-1.7 times due to the improved ammonium diffusion. However, the corresponding membrane flux of the first MD stage decreased from 13.7 to 10.5 L/m²·h. Elevating feed pH caused the deprotonation of NOM and its binding with inorganic ions, constituting a complex fouling layer on the membrane surface in the first MD stage. In contrast, the membrane permeability and fouling of the second MD were not affected by feed pH adjustment because only volatiles passed through the first MD. More importantly, it was estimated that ITMD could obtain high-quality water and recover high-purity ammonium from LFL with relatively low ammonium concentration at an input cost of $ 2-3/m³, which was very competitive with existing techniques. These results demonstrated that the ITMD can be a valuable candidate strategy for simultaneous water purification and nutrient recovery from landfill leachate.

Development of Hydrophobic Coal-Fly-Ash-Based Ceramic Membrane for Vacuum Membrane Distillation

Membrane distillation is an emerging separation technology with a high separation factor in water desalination. Ceramic membranes are increasingly used in membrane distillation because of high thermal and chemical stabilities. Coal fly ash is a promising ceramic membrane material with low thermal conductivity. In this study, three hydrophobic coal-fly-ash-based ceramic membranes were prepared for saline water desalination. The performances of different membranes in membrane distillation were compared. The effects of membrane pore size on permeate flux and salt rejection were researched. The coal-fly-ash-based membrane showed both a higher permeate flux and a higher salt rejection than the alumina membrane. As a result, using coal fly ash as the material for membrane fabrication can effectively increase the performance when applied to MD. Increasing the membrane pore size improved the permeate flux, but reduced the salt rejection. When the mean pore size increased from 0.15 μm to 1.57 μm, the water flux rose from 5.15 L·m^-2·h^-1 to 19.72 L·m^-2·h^-1, but the initial salt rejection was reduced from 99.95% to 99.87%. The hydrophobic coal-fly-ash-based membrane with a mean pore size of 0.18 μm exhibited a water flux of 9.54 L·m^-2·h^-1 and a salt rejection of higher than 98.36% in membrane distillation.

Treatment to surfactant containing wastewater with membrane distillation membrane with novel sandwich structure

Surfactant containing wastewater widely exists in textile industry, which hardly to be treated by membrane technology due to its high in salinity and wetting potential. In this study, PVDF membrane was modified by constructing a PDMS-SiO₂-PDMS "sandwich" structure on top of its surface via coating to achieve resistance to surfactant induced wetting. The "sandwich" layer was optimized based on the membrane performance during membrane distillation. Compared to the pristine PVDF membrane with contact angle of 92°, the water contact angle of the membrane with a "sandwich" layer of 0.44 μm increased to 153°. For the feed contained 0.5 wt% NaCl and 0.25 wt% surfactant, there was no membrane wetting occurred during the experiment period using the membrane with a "sandwich" structure, in comparison to the pristine PVDF membrane being wetted from beginning. For a challenge experiment to the developed membrane lasting for 100 h using a surfactant containing feed, there is no wetting sign observed and the stable flux is 20 kg·m^-2·h^-1.

Nanoparticle-Enhanced PVDF Flat-Sheet Membranes for Seawater Desalination in Direct Contact Membrane Distillation

In this study, hydrophobic functionalized carbon nanotubes (fCNTs) and silica nanoparticles (fSiO₂NPs) were incorporated into polyvinylidene fluoride (PVDF) flat-sheet membranes to improve their performance in membrane distillation (MD). The performance of the as-synthesized membranes was evaluated against commercial reference polytetrafluoroethylene (PTFE) flat-sheet membranes. The water contact angle (WCA) and liquid entry pressure (LEP) of the PVDF membrane were compromised after incorporation of hydrophilic pore forming polyvinylpyrrolidone (PVP). These parameters were key in ensuring high salt rejections in MD processes. Upon incorporation of fCNTS and fSiO₂NPs, WCA and LEP improved to 103.61° and 590 kPa, respectively. Moreover, the NP additives enhanced membrane surface roughness. Thus, an increase in membrane roughness improved WCA and resistance to membrane wetting. High salt rejection (>99%) and stable fluxes (39.77 kg m^-2 h^-1) were recorded throughout a 3 h process evaluation where 3.5 wt% NaCl solution was used as feed. These findings were recorded at feed temperature of 60 ℃. Evidently, this study substantiated the necessity of high feed temperatures towards high rates of water recovery.

Nickel Chalcogenide Nanoparticles-Assisted Photothermal Solar Driven Membrane Distillation (PSDMD)

Developing photothermal solar driven membrane distillation (PSDMD) is of great importance in providing fresh water for remote off-grid regions. The production of freshwater through the PSDMD is driven by the temperature difference between feed and distillate sides created via the addition of efficient photothermal nanostructures. Here we proposed nickel sulfides and nickel tellurium nanoparticles (NPs) to be loaded into the polymeric membrane to enhance its performance. Ag and CuSe NPs are also considered for comparison as they are previously used for membrane distillation (MD). Our theoretical approach showed that all of the considered NPs increased the temperature of the PVDF membrane by around a few degrees. NiS and NiTe2 NPs are the most efficient solar light-to-heat converters compared to NiTe and NiS2 NPs due to their efficient absorption over the visible range. PVDF membrane loaded with 25% of NiCs NPs and a porosity of 32% produced a transmembrane vapor flux between 22 and 27 L/m²h under a 10-times-amplified sun intensity. Under the same conditions, the PVDF membrane loaded with CuSe and Ag NPs produced 15 and 18 L/m²h of vapor flux, respectively. The implantation of NPs through the membrane not only increased its surface temperature but also possessed a high porosity which provided a higher distillation and energy efficiency that reached 58% with NiS NPs. Finally, great agreement between our theoretical model and experimental measurement is obtained.

Desalination technologies, membrane distillation, and electrospinning, an overview

Water is a critical component for humans to survive, especially in arid lands or areas where fresh water is scarce. Hence, desalination is an excellent way to effectuate the increasing water demand. Membrane distillation (MD) technology entails a membrane-based non-isothermal prominent process used in various applications, for instance, water treatment and desalination. It is operable at low temperature and pressure, from which the heat demand for the process can be sustainably sourced from renewable solar energy and waste heat. In MD, the water vapors are gone through the membrane's pores and condense at permeate side, rejecting dissolved salts and non-volatile substances. However, the efficacy of water and biofouling are the main challenges for MD due to the lack of appropriate and versatile membrane. Numerous researchers have explored different membrane composites to overcome the above-said issue, and attempt to develop efficient, elegant, and biofouling-resistant novel membranes for MD. This review article addresses the 21st-century water crises, desalination technologies, principles of MD, the different properties of membrane composites alongside compositions and modules of membranes. The desired membrane characteristics, MD configurations, role of electrospinning in MD, characteristics and modifications of membranes used for MD are also highlighted in this review.

Research Progress of Water Treatment Technology Based on Nanofiber Membranes

In the field of water purification, membrane separation technology plays a significant role. Electrospinning has emerged as a primary method to produce nanofiber membranes due to its straightforward, low cost, functional diversity, and process controllability. It is possible to flexibly control the structural characteristics of electrospun nanofiber membranes as well as carry out various membrane material combinations to make full use of their various properties, including high porosity, high selectivity, and microporous permeability to obtain high-performance water treatment membranes. These water separation membranes can satisfy the fast and efficient purification requirements in different water purification applications due to their high filtration efficiency. The current research on water treatment membranes is still focused on creating high-permeability membranes with outstanding selectivity, remarkable antifouling performance, superior physical and chemical performance, and long-term stability. This paper reviewed the preparation methods and properties of electrospun nanofiber membranes for water treatment in various fields, including microfiltration, ultrafiltration, nanofiltration, reverse osmosis, forward osmosis, and other special applications. Lastly, various antifouling technologies and research progress of water treatment membranes were discussed, and the future development direction of electrospun nanofiber membranes for water treatment was also presented.

Recent advances and prospects in electrochemical coupling technologies for metal recovery from water

With the development of our society, the desire to recover valuable metal resources from metal-containing wastewaters or natural water bodies is becoming increasingly stronger nowadays. To overcome the limitations of single techniques, coupling technologies with synergistic effects are attracting increasing attention regarding metal resource recovery from water with particular interest in electrochemical coupling technologies in view of the advantages of electrochemical methods. This state-of-the-art review comprehensively presented the mechanisms and performance of electrochemical coupling systems for metal recovery from water. To give a clear overview of current research trends, technologies coupled with electrochemical processes can be categorized into six main types: electrochemical techniques, membrane modules, adsorption/extraction techniques, sonication technologies, energy supply techniques and others. The electrochemical coupling system has shown synergistic advantages (e.g., improving metal recovery efficiency, reducing energy consumption) over single technologies. We then discuss the remaining challenges, present corresponding solutions, and put forward future directions for current electrochemical coupled systems towards metal recovery. This review is conducive to broadening the potential applications of electrochemical coupling processes for metal recovery and sustainable water treatment.

New Materials and Phenomena in Membrane Distillation

In recent decades, membrane-based processes have been extensively applied to a wide range of industrial processes, including gas separation, food industry, drug purification, and wastewater treatment. Membrane distillation is a thermally driven separation process, in which only vapour molecules transfer through a microporous hydrophobic membrane. At the operational level, the performance of membrane distillation is negatively affected by wetting and temperature polarization phenomena. In order to overcome these issues, advanced membranes have been developed in recent years. This review, which focuses specifically on membrane distillation presents the basic concepts associated with the mass and heat transfer through hydrophobic membranes, membrane properties, and advances in membrane materials. Photothermal materials for solar-driven membrane distillation applications are also presented and discussed.

Structural design of the electrospun nanofibrous membrane for membrane distillation application: a review

Although membrane distillation (MD) is a promising technology for water desalination and industrial wastewater treatment, the MD process is not widely applied in the global water industry due to the lack of a suitable membrane for the MD process. The design and appropriate manufacture are the most important factors for MD membrane optimization. The well-designed porous structure, superhydrophobic surface, and pore-wetting prevention of the membrane are vital properties of the MD membrane. Nowadays, electrospinning that is capable of manufacturing membranes with superhydrophobic or omni phobic properties is considered a promising technology. Electrospun nanofibrous membranes (ENMs) possess the characteristics of cylindrical morphology, re-entrant structure, and easy-shaping for a specific purpose, benefiting the membrane design and modification. Based on that, this review investigates the current state and future progress of the superhydrophobic, multi-layer, and omniphobic ENMs manufactured with various structural designs for seawater desalination and wastewater purification. We expect that this paper will provide some recommendations and guidance for further fabrication research and the configuration design of ENMs in the MD process for seawater desalination and wastewater purification.

A Zero-Brine Discharge Seawater Desalination Using a Pilot-Scale Membrane Distillation System Integrated with Crystallizer

The management of brines generated from reverse osmosis operation remains a critical challenge requiring new approaches and processes to limit the impact of brine discharge onto ecosystems and to enhance both water and valuable resource recovery. The treatment of real seawater reverse osmosis (SWRO) brines (45,000 ppm TDS) obtained from a local Singaporean desalination plant with a crystallizer integrated pilot-scale membrane distillation unit (MDC) was studied. Commercial STOMATE® hollow fiber membranes were used in vacuum membrane distillation (VMD) configuration, leading to an average flux of around 3.7 L/m2-h at a permeate vacuum of 80 kPa and an average feed temperature of 65 °C. Consistent separation operations were achieved for the treatment of real SWRO brine over a period of 280 h; this led to a water recovery of >95% and to the collection of salt slurries, containing up to ~10−20 wt% of moisture, from the crystallizer. This approach demonstrates the potential of MDC systems to achieve zero brine discharge efficiently from seawater desalination systems, providing an environmentally friendly alternative to manage brines by increasing water recovery and generating salt slurries of economic value.

Dielectrophoresis-Based Universal Membrane Antifouling Strategy toward Colloidal Foulants

Membrane fouling compromises the benefits of membrane technology, leading to its performance deterioration and incremental cost. Coupling with an electric field has been attractive but is limited by the electrical dependence of the electrophoresis (EP) mechanism and undesired faradic reactions. This study reports a universal dielectrophoresis-based (DEP) membrane antifouling strategy for electronegative, electropositive, and neutral colloidal foulants, which depends on the particle polarizability rather than its charge. The porous Ni@PVDF model electroconductive membrane was fabricated to construct a nonuniform electric field inducing DEP, while applying a low voltage avoided side electrochemical reactions. For electronegative SiO₂(-) and electropositive Al₂O₃(+) particles with a lower relative permittivity than the medium water (78), the membrane permeability all remarkably increased by 90.1% under AC/DC (±1.0 V) fields. By contrast, serious membrane fouling occurred for the BaTiO₃ colloids with a higher relative permittivity (∼2000). Notably, the permittivity of nearly all colloids in wastewater treatment is much less than that of water, which makes the dielectrophoresis-based antifouling strategy universal. The theoretical simulation systematically analyzed the forces on particles including DEP, EP, and others, indicating that the formed protected area on the membrane pore wall by DEP forces prevented the irreversible membrane blockage of colloids and facilitated loose cake layer formation for alleviating membrane fouling. In brief, this work reported a hopeful concept for dielectrophoresis-based membrane antifouling and verified its antifouling mechanism.

A review of the potential of conventional and advanced membrane technology in the removal of pathogens from wastewater

Consumption of pathogenic contaminated water has claimed the lives of many people. Hence, this scenario has emphasized the urgent need for research methods to avoid, treat and eliminate harmful pathogens in wastewater. Therefore, effective water treatment has become a matter of utmost importance. Membrane technology offers purer, cleaner, and pathogen-free water through the water separation method via a permeable membrane. Advanced membrane technology such as nanocomposite membrane, membrane distillation, membrane bioreactor, and photocatalytic membrane reactor can offer synergistic effects in removing pathogen through the integration of additional functionality and filtration in a single chamber. This paper also comprehensively discussed the application, challenges, and future perspective of the advanced membrane technology as a promising alternative in battling pathogenic microbial contaminants, which will also be beneficial and valuable in managing pandemics in the future as well as protecting human health and the environment. In addition, the potential of membrane technology in battling the ongoing global pandemic of coronavirus disease 2019 (COVID-19) was also discussed briefly.

Heat-Integration of Solar-Heated Membrane Distillation and Fuel Cell for Desalination System Based on the Dynamic Optimization Approach

The heat integration feasibility of the proton exchange membrane fuel cell (PEMFC) coupled with the solar-heated direct contact membrane distillation (DCMD) module is evaluated in this study. The additional waste heat from the PEMFC increases the DCMD system’s ability to produce fresh water and electricity. Two systems units to be assessed mainly include a flat plate solar collector, a heat storage tank with an internal heat exchanger, and the DCMD module with and without the PEMFC module. The importance of daily operation continuity is emphasized through a preliminary dynamic simulation and proper sizing of the solar-heated DCMD distillation. Sensitivity analysis is implemented to analyze the relationship between the essential variables and the daily freshwater production. The design variables of both configurations are rigorously optimized in terms of minimum unit production cost (UPC). The proposed heat integration feasibility is evaluated to obtain critical insights on the design strategy of the hybrid systems.

Super Water-Extracting Gels for Solar-Powered Volatile Organic Compounds Management in the Hydrological Cycle

Water-soluble volatile organic compounds (VOCs) are widely spread in the natural hydrological cycle, contaminating potential water sources, and leading to unexpected ecological hazards. However, water-purification technologies toward VOCs are energy-intensive and present unsatisfactory purity of the obtained water. The fundamental challenge is to differentiate the motion of water and VOC molecules by separators. Here, the concept of a super water-extracting gel (SWEG) for VOC-management and water purification via direct solar distillation is proposed. The strong hydrogen bonding effect in the hypercrosslinked hydrophilic polymeric networks enables the SWEG to extract water from VOC-containing water, which rejects the VOC solutes while allowing water through for interfacial evaporation. The obtained SWEG achieves a VOCs removal ratio up to 99.99% by solar distillation under 1 sun. A solar water-purification system is also demonstrated to produce clean water, which surpasses other competitive technologies based on electricity.

Water Flux Prediction in Direct Contact Membrane Distillation Subject to Inorganic Fouling

Freshwater is a limited resource, which has driven the development of new purification and water-reuse technologies. One promising technology for water treatment is membrane distillation (MD). One of the main problems of MD, and of many desalination technologies, is membrane fouling, which reduces the performance of the membrane. This work presents a mathematical model that aims to predict distillate fluxes in direct-contact MD when fouling occurs as salts are deposited onto the membrane surface, forming an inorganic fouling layer. The mathematical model uses a heat- and mass-transfer formulation for prediction of the distillate flux under steady state conditions, and it is combined with the cake-filtration theory to represent the distillate fluxes after the onset of membrane fouling. Model results agree well with experimental observation of distillate fluxes, both before (~12–14 kg m⁻² h⁻¹) and after the onset of membrane fouling, with root-mean-square errors smaller than 1.4 kg m⁻² h⁻¹ in all the experiments. These results suggest that the cake-filtration theory can be used to represent water flux decline in MD membranes prone to inorganic fouling. From our experiments and from the modelling exercise, we found that the onset of membrane failure was relatively constant; the precipitation reaction constant is conditioned by the physicochemical interaction between the feed solution and the membrane; and the rate of flux decline after membrane fouling depends on flow conditions as well as on the precipitation compound. However, the proposed model has limitations that must be addressed in future investigations to validate it under a wider range of operating conditions, for membranes composed by other materials and with different feed solutions to address organic, biological, and/or colloidal fouling, which typically occur under real conditions.

Membrane cleaning in membrane distillation of reverse osmosis concentrate generated in landfill leachate treatment

As a thermally induced membrane separation process, membrane distillation (MD) has drawn more and more attention to the advantages of treating hypersaline wastewaters, especially the concentrate from the reverse osmosis (RO) process. One of the major obstacles in widespread MD application is the membrane fouling. We investigated the feasibility of direct contact membrane distillation (DCMD) for landfill leachate reverse osmosis concentrate (LFLRO) brine treatment and systematically assessed the efficiency of chemical cleaning for DCMD after processing LFLRO brine. The results showed that 80% water recovery rate was achieved when processing the LFLRO brine by DCMD, but membrane fouling occurred during the DCMD process, and manifested as the decreasing of permeate flux and the increasing of permeate conductivity. Analysis revealed that the serious flux reduction was primarily caused by the fouling layer, which consisted of organic matter and inorganic salts. Five cleaning methods were investigated for membrane cleaning, including hydrogen chloride (HCl)-sodium hydroxide (NaOH), ethylene diamine tetraacetic acid (EDTA)-NaOH, citric acid, sodium hypochlorite (NaClO) and sodium dodecyl sulphate (SDS) cleaning. Among the chemical cleaning methods investigated, the 3 wt.% SDS cleaning showed the best efficiency at recovering the performance of fouled membranes.

Membrane Distillation for Wastewater Treatment: A Mini Review

Water serves as an indispensable part of human life and production. On account of the overexploitation of traditional water sources, the demand for wastewater recycling is expanding rapidly. As a promising water treatment process, membrane distillation (MD) has been utilized in various wastewater treatments, such as desalination brine, textile wastewater, radioactive wastewater, and oily wastewater. This review summarized the investigation work applying MD in wastewater treatment, and the performance was comprehensively introduced. Moreover, the obstructions of industrialization, such as membrane fouling, membrane wetting, and high energy consumption, were discussed with the practical investigation. To cope with these problems, various strategies have been adopted to enhance MD performance, including coupling membrane processes and developing membranes with specific surface characteristics. In addition, the significance of nutrient recovery and waste heat utilization was indicated.