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Polyvinylidene fluoride (PVDF) electrospun nanofibrous membranes (ENMs), consolidated with a heatpress process and of various thicknesses were fabricated and tested in a DCMD cell at five different operating conditions. Membranes as thin as 27 μm were sufficiently robust for evaluation and gave a transmembrane flux as high as 60 L/h m 2 . An analytical model was created to estimate the optimal membrane thickness for DCMD operations. It is found that the value of optimal thickness increases with reduced heat transfer coefficients; decreased feed inlet temperature; increased membrane permeability; and increased salinity. Even for 10% NaCl the predicted optimum was estimated to be 13 μm which was too thin for experimental confirmation. Based upon this analysis but with due allowance for the variation of heat transfer coefficients with temperature dependent physical properties a single Matlab model was created to fit the five sets of data. With the introduction of a structural derivation factor, which reflected the experimentally determined variation of porosity and pore size with thickness, the model was found to fit the data very well. [ABSTRACT FROM AUTHOR]

Membrane distillation (MD) is a promising thermally-driven desalination process for various applications in food, pharmaceutical, textile, and chemical industries. However, membrane fouling and wetting are still major challenges, which limit the stability of MD membranes over long-term use. From 2005, researchers have developed many strategies to enhance membrane fouling resistance by simultaneously engaging reactions, but these works have not been systematically reviewed and categorized. Therefore, this review aims to identify research gaps and provide an overview of the state-of-the-art reactive MD processes, which are categorized broadly based on two configurations: 1 – " hybrid" involving coupling MD with an oxidation or biological process as a pre-treatment step and 2 – " integrated" involving immobilizing reactive materials on the membrane. The reactive MD processes were sub-classified into five categories based on reaction technology then analysed in terms of working mechanisms, advantages, limitations, and their application potentials. It shows that minimizing fouling in both hybrid and integrated processes can be attributed to the interaction of the organics/degraded by-products with the membrane surface/immobilized reactive layer, and the capability of the pollutants to be oxidized. In addition, depending on the types of the coupled process, oxidation can be accompanied by other mechanisms, such as electrostatic repulsion and electro flotation which is the most promising mechanism. Overall, despite the current developments in hybrid and integrated MD processes, many challenges exist including flux decline in the case of the complex feed solutions, a trade-off between oxidation and other accompanied mechanisms (e., electrostatic repulsion), and the possibility of membrane alteration. Future opportunities can be explored in terms of dedicated fouling studies, energy efficiency, removing/converting waste streams to valuable by-products, durability of the process equipment against oxidation, and targeting unique applications such as the treatment of concentrated organic waste streams. [Display omitted] • Reactive MD categorized into "hybrid" and "integrated" configurations. • Oxidation processes are a major focus in reactive MD. • Challenges include membrane fouling and material degradation. • Reactive MD demonstrates highly efficient treatment simultaneous with desalination. • Future studies should focus on fouling, reaction efficiency and targeted applications. [ABSTRACT FROM AUTHOR]

Membrane distillation (MD) is emerging as a promising technology for treating the reverse osmosis brines. However, limited cases were reported on the design of a large-scale direct contact MD (DCMD) system. The practical scale-up options for modular matrix design in a multi-element MD system and a multi-subsystem MD train were explored by commercial flowsheet simulator Aspen Plus. Compared to the benchmark single module system, which showed drastically deteriorating membrane performance with increasing membrane area, the multi-element DCMD system with modules in parallel matrix was found to perform better with water production improved slightly but specific power consumption ( SPC ) greatly reduced down to 0.5% of the single module system. The optimal matrix was obtained at module number of eight for a 20 m 2 module due to trade-off relationship between module specifications and effective process driving force in MD. Supported by theoretical analysis, it was found that the matrix array pattern had no influence on the performance of the multi-subsystem DCMD train. Further investigation showed that an 18-subsystem MD train with a membrane area of 200 m 2 achieved a 16 times water production rate with only 10% of the SPC , as compared to that of single system with the same membrane area. [ABSTRACT FROM AUTHOR]

Inspired by mussels’ byssus with remarkable adhesive power that is neither degraded nor deformed in the marine environment, dopamine coating has emerged as an option for membrane surface modification. With its many advantages, direct-contact membrane distillation (DCMD) appears to hold potential for the recovery of high quality water from produced water. However, membrane fouling and pore wetting are challenging issues of long-term DCMD operations due to the presence of oils and surfactants in these waters. Hence, it is paramount to develop robust membranes with anti-fouling and anti-wetting properties for effective produced water treatment. In this study, we fabricated a composite hollow fiber membrane by single-step co-deposition of polydopamine (PDA)/polyethylenimine (PEI) onto the outer surface of a commercial hydrophobic polyvinylidene fluoride (PVDF) substrate. The successful co-deposition was verified using different characterization techniques. The composite membrane exhibited Janus wettability (Janus membranes have opposing properties at an interface) with its modified outer surface being in-air hydrophilic/underwater superoleophobic for preventing organics adhesion while insuring that unmodified pores beneath the surface remained hydrophobic for vapor transport. The anti-fouling and anti-wetting properties of the modified membrane were investigated via DCMD experiments by feeding a series of low surface tension solutions. In comparison to the pristine PVDF membrane, the modified membrane exhibited promising wetting resistant property against different surfactant types and excellent fouling resistant property against nonionic and cationic surfactants. Moreover, the modified membrane presented a promising long-term performance when feeding a cationic surfactant-stabilized petroleum-in-water emulsion mimicking produced water, during which a stable flux and excellent permeate quality were maintained throughout 7 days of operation. The efficacious combined effects of a hydration layer and protonated amine-functional groups on the PDA/PEI grafted layer could prevent membrane fouling and pore wetting. The results suggest that the mussel-inspired composite PVDF membrane could potentially be used for long-term water recovery from produced water via DCMD. [ABSTRACT FROM AUTHOR]

Aiming to optimize the system-level heat utilization, a pilot-scale direct contact membrane distillation desalination system integrated with heat recovery (DCMD–HX) was studied using Aspen Plus. An implicit expression of gain output ratio (GOR) was derived to reveal the interplay of heat utilization and process parameters including operating conditions, module specifications as well as membrane properties in the DCMD–HX desalination system. Compared to operating temperatures, the feed/permeate recirculating flowrates were identified as the most influential operational factors affecting the GOR. In the current settings, the maximal GOR of 6.0 was observed at low and equivalent feed- and permeate-side flowrates regardless of module specifications. Low flowrates, however, resulted in undesirable low water productivity, which was consistent with the trade-off relationship observed between the heat utilization efficiency and water recovery rate in MD. Employing membranes with high heat-transfer resistance (low conductivity and thicker membrane wall) helped to improve the GOR up to 32%. Simulated results also showed that the GOR value increased by 1.3-fold with the preheater parameter Δ T HX varying from 5 to 0 °C. The non-linear scale-up relationship existed between the membrane area and heat utilization (i.e., GOR) was also observed, indicating the possible uncertainty in accurately predicting the GOR value for industrial-scale desalination systems based on lab-scale module testing. [ABSTRACT FROM AUTHOR]

Membrane devices, membrane processes and membrane-based conventional chemical engineering processes have achieved extraordinary levels of process intensification (PI). Generally membrane-based devices require smaller equipment to achieve a given device production rate. Further they can eliminate dispersion-based operation and achieve extraordinary selectivity. Exploiting the compartmentalization of two regions on two sides of the membrane, membrane devices can combine two processes carried out on two sides of the membrane in one membrane device. Selected membrane processes and their applications are briefly reviewed here in terms of the level of PI achieved. The membrane processes selected are generally recently commercialized or being commercialized or have great potential for commercialization: membrane bioreactor; membrane gas–liquid contacting; membrane solvent extraction; forward osmosis; pressure-retarded osmosis; membrane distillation; membrane distillation bioreactor. Examples of potential process intensification by membrane processes are briefly illustrated for processing of lignocellulose to biofuels in a biorefinery and for produced water treatment. [ABSTRACT FROM AUTHOR]

This study focused on the effect of sodium alginate on the performance of direct contact membrane distillation (DCMD). The feed solution contained various combinations of sodium chloride, sodium sulfate and calcium chloride along with bovine serum albumin, xanthan gum and sodium alginate. Unlike findings from the majority of prior studies that suggested the presence of alginate in feed solution caused the deterioration of membrane process performance, our results indicated that sodium alginate exhibited anti-scaling properties and water flux enhancing effect. However, this interesting phenomenon was exhibited by sodium alginate under particular conditions only. Experiments performed with other organic foulants such as xanthan gum did not display the same trend. It is believed that the presence of a hydrophilic layer (calcium alginate gel), which is much less thermal conductive as compared to the PTFE membrane, on the top of the membrane could reduce the amount of heat dissipated due to evaporative cooling or reduce conductive heat loss in the membrane, thus enhancing the thermal efficiency of the system. • Sodium alginate exhibited anti-scaling properties and water flux enhancing effect • Reduction of conductive heat loss with the presence of hydrophilic layer • The degree of improvement was governed by the consistency of the gel layer formed [ABSTRACT FROM AUTHOR]

• An overview of polymer-based membranes for membrane contacting process is presented. • Current challenges include limited performances, pore wetting and membrane fouling. • Membrane-centred strategies to high-performing antiwetting/fouling membranes are proposed. • Biocatalytic, mixed-matrix, composite, Janus and omniphobic membranes are promising. • Future research is on fouling, materials development and process design optimizations. The gas–liquid membrane contactor (GLMC) is a promising technology for gas absorption and stripping with many competitive advantages over conventional contacting processes. At the centre of this technology, membranes are instrumental in offering a high contact area at the gas-liquid interface. Today, polymer-based membranes are the mainstream for GLMC applications owing to their well-established fabrication techniques and economic advantages. Herein, we aim to review the current progress in polymer-based membranes for GLMC processes. Specifically, we first provide a basic overview of the principles and background of GLMC. Next, we identify three of the biggest Achilles heels of polymer-based membranes for GLMC processes, namely, (1) membrane performances, (2) pore wetting, and (3) membrane fouling. To a certain extent, we contend that a large part of these challenges can be resolved by optimizing the membrane designs. As such, in the next section, we focus our attention on high-performance, antiwetting and antifouling membranes as potential solutions to address these challenges. From a membrane-centred perspective, the scope of our discussion covers extensively from membrane materials, morphologies, structures and configurations, surface modifications to novel membrane designs including composite and mixed-matrix membranes. Notably, as membrane fouling studies are still in the preliminary stage, attractive membrane designs such as Janus and omniphobic membranes are borrowed from membrane distillation process to gain greater insights into the potential solutions that can be adapted for GLMC processes. [ABSTRACT FROM AUTHOR]

Membrane distillation (MD) has been touted as a promising technology for niche applications such as desalination of surfactant- and oil-containing feed streams. Hitherto, the deployment of conventional hydrophobic MD membranes for such applications is limited and unsatisfactory. This is because the presence of surfactants and oils in aqueous feed streams reduces the surface-tension of these media significantly and the attachment of these contaminants onto hydrophobic membrane surfaces often leads to membrane fouling and pore wetting, which compromises on the quantity and quality of water recovered. Endowing MD membranes with surfaces of special wettabilities has been proposed as a strategy to combat membrane fouling and pore wetting. This involves the design of local kinetic energy barriers such as multilevel re-entrant surface structures, surfaces with ultralow surface-energies, and interfacial hydration layers to impede transition to the fully-wetted Wenzel state. This review critiques the state-of-the-art fabrication and surface-modification methods as well as practices used in the development of omniphobic and Janus MD membranes with specific emphasis on the advances, challenges, and future improvements for application in challenging surfactant- and oil-containing feed streams. Unlabelled Image • This review focuses on MD membrane development for low-surface-tension feed streams. • The development of omniphobic and Janus membranes are critically reviewed. • The impacts of surface chemistry and surface structures are analyzed. • This review lays the groundwork for future MD membrane development for niche applications. [ABSTRACT FROM AUTHOR]

Highlights • Novel thermoelectric coupled sweeping gas membrane distillation (T-SGMD) hybrid. • Hybrid membrane distillation for decentralized water treatment, dehumidification and space cooling. • Latent heat cooling provides for water treatment simultaneously. • Impact of membrane area, cool air recycle and module orientation on T-SGMD efficiency. • Interplay among power consumption, space cooling capacity and condensate rate. Abstract The current concept for cooling the indoors is far from ideal with respect to the total energy consumed and waste discharged. A novel concept for improving the energy efficiency is proposed via hybridizing the heat pump with a membrane distillation (MD) unit for simultaneous space cooling and water treatment. MD is well-acknowledged for utilizing low-quality waste heat for water treatment, which makes it feasible for coupling with a heat pump to make use of both the hot and cold reservoirs of the pump. Accordingly, the objective of the current effort was to investigate via experiments the efficacy of a thermoelectric heat pump coupled with a sweep-gas MD system (T-SGMD) by measuring the cooling capacity, condensate production and power consumption. The results from this study can be extended to other heat pumps. Three key highlights emanated from this study. Firstly, condensate production per unit energy consumed can be doubled with the T-SGMD system relative to thermoelectric dehumidification alone. Secondly, cool air recycle affected the condensate flux the most without a drastic loss of cooling compared to other tested parameters during the operation of the T-SGMD. Lastly, the T-SGMD system was able to provide an increase in condensate produced per unit energy without a loss in cooling capacity per unit energy input. These advantages of coupling heat pumps with MD, leveraging on the current advancements in MD, is promising for a hybridized system for decentralized water treatment, dehumidification and space cooling. [ABSTRACT FROM AUTHOR]

Porous hydrophobic polyvinylidene fluoride (PVDF) membranes have been extensively used in direct-contact membrane distillation (DCMD) processes. However, these PVDF membranes are vulnerable to membrane fouling and pore wetting in low surface tension feeds, restricting its application for water recovery from challenging industrial wastewaters. Therefore, it is of paramount importance to engineer fouling- and wetting-resistant MD membranes for robust long-term applications. In this study, a superoleophobic composite hollow fiber membrane with sandwich structure has been developed via accelerated oxidant-induced polydopamine (PDA) deposition on both the outer and inner surfaces of a commercial hydrophobic PVDF membrane under slightly acidic conditions (pH = 5). The modified surface prevents organics adhesion ascribing to its underwater superoleophobicity while the unmodified pores remain hydrophobic for vapor transport. The long-term robustness of the PDA-decorated membrane in highly saline feeds containing low surface tension contaminants has been evaluated via bench-scale DCMD experiments. In contrast to the pristine PVDF membrane, the PDA-decorated membrane exhibits excellent fouling- and wetting-resistant properties in different surfactant solutions as well as oil-in-water emulsion. The PDA-decorated membrane has also been used for seawater desalination, during which it maintains a stable flux and high salt rejection rate. Furthermore, the PDA-decorated membrane presents a flux enhancement of up to 70% over the pristine PVDF membrane in 3.5 wt% NaCl solution at 333 K. This study demonstrates the potential of the PDA-decorated membrane for extended DCMD applications such as water recovery from industrial wastewater containing low surface tension substances. [ABSTRACT FROM AUTHOR]

Polyvinylidene fluoride (PVDF)/polytetrafluoroethylene (PTFE) hollow fiber membranes were developed via thermally induced phase separation (TIPS) method for direct contact membrane distillation (DCMD). The effects of PTFE addition on the thermal behavior of the dope mixtures and membrane formation were investigated. It was found that the crystallization of PVDF was significantly enhanced with increased nucleation sites provided by PTFE particles, leading to promoted formation of smaller spherulites in a greater density. Furthermore, the improved uniformity and increased amount of cavity between the spherical crystallites coherently facilitated the formation of smaller pores ranging from 0.08 to 0.12 μm. With certain PTFE loading, the membranes exhibited improved porosity, water permeability and hydrophobicity as well as enhanced tensile strength of 9.4 ± 0.3 MPa. To examine the DCMD performance, the membranes were tested under various conditions using 3.5 wt% NaCl solution. A stable permeation flux of 28.3 kg m − 2 h − 1 at the feed temperature of 60 °C with 99.99% NaCl rejection for over 50 h of operation was achieved, which is comparable with similar type of PVDF membranes while the newly developed membrane exhibited better mechanical strength. This study suggests that the as-spun PVDF/PTFE hollow fiber membranes have potential for DCMD applications. [ABSTRACT FROM AUTHOR]

Membrane distillation (MD) is an attractive technology for seawater reverse osmosis (SWRO) brine treatment. Submerged MD (S-MD) offers an additional advantage of a compact system compared to cross-flow MD. This study evaluated the performances of three different S-MD configurations; submerged direct contact membrane distillation (S-DCMD), submerged vacuum direct contact membrane distillation (S-VDCMD) and submerged vacuum membrane distillation (S-VMD) for SWRO brine treatment. A 13–77% higher water flux was obtained by S-MDs with vacuum incorporation (S-VMD and S-VDCMD) compared to S-DCMD, attributed to higher driving force. Evaluation on the influence of feed concentration and permeate temperature revealed that S-MD with high vacuum was significantly affected by feed concentration. Meanwhile S-DCMD was more severely affected by feed temperature losses, based on the tendency of membrane pore crystallization formation. The crystallization tendency on the membrane surface was influenced by the presence of vacuum pressure. A repeated cycle of S-DCMD with membrane air-backwashing was effective for flux recovery and to reduce membrane crystallization, enabling to concentrate SWRO brine by 2.8 times of volume concentration factor. [ABSTRACT FROM AUTHOR]

Membrane distillation (MD) is a promising desalination technology as it is less sensitive to feed's salinity and can handle high-salinity water. However, the industrial application of MD is restricted by the limited availability of cost-effective hydrophobic membranes. This study developed a simple method to fabricate hydrophobic MD membranes through the coating of a fluoroethylene vinyl ether polymer called LUMIFLON™ LF-200. The fluoropolymer was effectively coated on porous nitrocellulose (NC) and nylon membrane through dip-coating followed by a thermal crosslinking. The coating layer was thin and uniform, covering the surface of the whole membrane matrix, while the high porosity and the microstructure of the membranes were still mostly preserved. After modification, the originally hydrophilic NC and nylon membranes were transformed into hydrophobic, with water contact angles of ∼130°. The 12LF-NC membrane showed a high flux of 10.1 kg m−2 h−1 with salt rejection >99.99% over 120 h in the direct contact MD fed with 10 wt% NaCl solution, which outperformed the commercial PVDF membrane. This coating method has the potential to modify various substrates to produce MD membranes with good performance, which broadens the availability of appropriate membranes for MD application. [Display omitted] • Fluorinated copolymer, LUMIFLON LF-200 is used to prepare hydrophobic MD membranes. • LF-200 was simply dip-coated on hydrophilic substrates and crosslinked under mild heating. • The WCAs of modified nitrocellulose and nylon membranes increased to ∼130° from 0°. • The 12LF-NC membrane had excellent flux >10 kg m−2 h−1 treating 10 wt% salt water. • This method has high efficiency, wide versatility and the potential to scale up. [ABSTRACT FROM AUTHOR]

Increasing demand for oil and gas leads to the generation of substantial amount of produced water, bringing about deleterious impacts on the environment. Direct-contact membrane distillation (DCMD) could be a possible option for dewatering oil-in-water (O/W) emulsions because of many benefits brought by the DCMD process. However, these low surface tension solutions pose some difficult issues such as membrane fouling and pore wetting. The mechanisms involved are not fully understood due to the lack of study of the interaction between the emulsions and the membrane surface in the DCMD domain. To address the challenges, this study aims at developing a fundamental understanding of the relationship between surfactant-stabilized O/W emulsions and polyvinylidene fluoride (PVDF) membrane surface in DCMD operations. Effects of surfactant types (Span 20, Tween 20, and sodium dodecyl sulfate), oil concentration, and oil types (petroleum and vacuum pump oil) were systematically studied to better understand the fouling and wetting mechanisms involved. The results reveal that surfactant concentration and hydrophobicity had an influence on the membrane fouling and wetting behaviors. Surfactants with a lower hydrophilic-lipophilic balance (HLB) value could make the PVDF membrane surface less hydrophobic and cause less severe fouling by restraining the adsorption of oil droplets on the membrane surface. These findings suggest that membrane surface modification is required to achieve anti-fouling and anti-wetting properties to make DCMD an energy-efficient and effective technology for treating produced water. [ABSTRACT FROM AUTHOR]

Solvent resistant membrane distillation (SR-MD) is a novel technology to effectively separate water from waste streams containing solvents with high boiling points, avoiding the high chemical costs and release of harmful gas in conventional treatments. Ceramic membranes are promising for this process as they are highly stable chemically but they require modification to tune the hydrophilicity to hydrophobicity. However, the widely used silanization for hydrophobic modification often involves expensive chemicals and may produce toxic substances. Here, we present a new hydrophobic modification method with the use of fatty acid chloride (FAC), as a greener and cheaper alternative to silanes. In the grafting reaction, the acyl chloride groups react with the –OH groups on the ceramic membranes to form strong ester bonds. We successfully grafted stearoyl chloride (SC) and palmitoyl chloride (PC) on the ceramic tubular membranes. With long carbon chains grafted, the modified membranes exhibited high hydrophobicity with a water contact angle >141° and a liquid entry pressure >3 bar. When tested in SR-MD with a feed solution containing 50 wt% dimethyl sulfoxide, the PC-modified membrane demonstrated a flux of 3.2 kg m−2 h−1, with rejection >98% and separation factor >110 at 60 °C, and a high flux of 4.5 kg m−2 h−1 with rejection >96% at 70 °C. The FAC-modified membranes allow a cost-effective treatment of challenging wastewater containing organic solvents through SR-MD. [Display omitted] • Hydrophobic ceramic membranes for solvent resistant MD were synthesized through fatty acid chloride grafting. • The acyl chloride groups formed strong ester bonds on ceramic surface and the modified membranes had high hydrophobicity. • Flux of 3.2 kg m−2 h−1, rejection >98% and separation factor >110 can be achieved when treating 50 wt% DMSO in SR-MD. • The hydrophobicity and separation performance are comparable to ceramic membrane grafted with silanes. • This study presents a greener and cheaper surface modification chemistry to synthesize hydrophobic ceramic membranes. [ABSTRACT FROM AUTHOR]

Membrane distillation (MD) technology is being extensively studied to address operational challenges such as undesired thermal efficiency and scaling phenomenon in recovering valuable solutes and minimizing brine disposal. This study has explored the working mechanisms of utilizing gas–liquid two-phase flow to enhance heat transfer and mitigate scaling formation in MD concentration process, based on the quantification of heat-transfer coefficients and local scaling-resistance associated with bubble size properties. With the aid of direct observation and statistical analysis on the bubble characteristics in a specially-designed direct contact MD (DCMD) module, it was found that the bubbles with small mean bubble size and narrow size distribution were preferred for creating even flow distribution, intensifying mixing and enhancing surface shear rate. Compared to non-bubbling DCMD, the heat-transfer coefficient and temperature polarization coefficient ( TPC ) reached up to 2.30- and 2.13-fold, respectively, at an optimal gas flowrate of 0.2 L min −1 . With the theoretical expressions for local scaling resistance derived based on the resistance-in-series model, the relative permeation flux ( J w/o|t=t1 /J w/o|t=0 ) in non-bubbling MD was quantified and found to rapidly decline by 65% as the concentration process progressed, consistent with the increasing trend of the ratio of local scaling resistance to the overall resistance ( r fl /r ov ). Fortunately, the introduction of gas bubbles has shown benefits for supersaturation brine concentrating MD process – remarkably decreased the local-scaling resistance due to bubble-intensified shear stress and enhanced hydrodynamics. Also, the total water removal for the brine concentration process was significantly improved by 131% and the discharged brine volume was reduced accordingly at appropriately selected gas flow rates. Nevertheless, at inappropriately high gas flowrates, high energy consumption and potential fiber breakage should be avoided. [ABSTRACT FROM AUTHOR]

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. [Display omitted] • Concepts and mechanisms of bio-inspired super liquid-repellent surfaces were elaborated. • Methods to develop super liquid-repellent MD surfaces were summarized and discussed. • Their MD performance and mechanisms were summarized and analysed. • The perspectives of these super liquid-repellent membranes were illustrated. [ABSTRACT FROM AUTHOR]

• The progress of photothermal membrane distillation (PMD) is reviewed. • PMD mechanisms and membrane requirements are summarized. • Photothermal materials for developing PMD membranes are reviewed. • Diverse approaches to prepare PMD membranes are classified and discussed. • PMD membrane performance and their applications are reviewed and discussed. The conventional membrane distillation (MD) process is accompanied by large energy consumption, low thermal efficiency and inevitable requirements of centralized infrastructures, which impede its practical applications, especially in the offshore and remote off-grid areas. Thanks to the rapid development of efficient photothermal materials over the last decade, a new photothermal membrane distillation (PMD) process has emerged to harness abundant solar energy and localize heating on the membrane-feed water interface via photothermal effects. Driven by the temperature difference across the PMD membrane, water vapor can be generated on the membrane-feed surface, transported through membrane pores and condensed at permeate side to obtain freshwater, thus tackling the challenge of obtaining clean water using green energy. The PMD process avoids heating the entire bulk feed water and feed transportation from heat units to membrane modules, which save substantial amounts of energy. The interfacial localized heating intrinsically mitigates the temperature polarization across the membrane. The latent heat from vapor condensation can be effectively recovered via multi-level PMD configurations. As great efforts have been made to exploit PMD process, it is imperative to review the state-of-the-art progress of PMD and shed light on its future trend. Here, we briefly illustrate PMD mechanisms and membrane requirements, photothermal materials feasible for developing PMD membranes along with their light-to-heat mechanisms. This is followed by reviewing diverse approaches to prepare PMD membranes, which are classified into one-step fabrication and multi-step modification methods. Comprehensive discussion about PMD membrane performance in different configurations and their small pilot-scaled applications are provided. The effects of operational parameters and module designs are discussed in Section 6. Finally, the current challenges and future perspectives of PMD process are emphasized with the aim of providing guidance for future works. [Display omitted] [ABSTRACT FROM AUTHOR]

• Solvent resistant membrane distillation was tested for organic wastewater separation. • Hydrophobic ceramic membranes were obtained through chemical grafting. • The grafted membranes performed excellently in water-dimethyl sulfoxide separation. • Rejection >98% can be achieved for a feed containing <65 wt% dimethyl sulfoxide. • The permeate product can be sent to bioreactor if feed concentration is <20 wt%. The treatment of organic waste or wastewater with high organic solvent content has been challenging in industries as it cannot be done effectively using conventional wastewater treatment technologies such as biodegradation and advanced oxidation process. Solvent resistant membrane distillation (SR-MD) was proposed as an energy-efficient alternative to treat these waste streams but its application is hampered by the lack of solvent-resistant membranes, and there is a research gap in studying the feeds with water-solvent mixtures. In this work, ceramic tubular membranes with different pore sizes and structures were molecularly grafted with 1H,1H,2H,2H-perfluorodecyltriethoxysilane to obtain hydrophobic ceramic membranes for SR-MD. The modified membranes exhibited excellent hydrophobicity and solvent resistant properties, and they were tested for SR-MD performance with a wide range of dimethyl sulfoxide (DMSO) feed concentrations, from 3.5 to 85 wt%. The membranes exhibited a high DMSO rejection of >98% and the separation factor of >170, with permeation flux >4.4 kg m−2 h−1 when the DMSO concentration in feed was below 65 wt%. The separation performance was found strongly dependent on the evaporation step and the vapour-liquid equilibrium near the interface. The DMSO rejection was also comparable to pervaporation while the permeation flux was much higher at the feed concentration of 50 wt%. This study establishes the strategy of using SR-MD as a promising membrane process in treating complex industrial wastes with high organic solvent content. [Display omitted] [ABSTRACT FROM AUTHOR]

Hydrophobic membranes suffer from wetting and scaling when treating hypersaline brine using membrane distillation (MD). Thus omniphobic membranes with re-entrant surfaces have been developed by diverse modifications to enhance anti-wetting/scaling properties. However, those post-modifications usually sacrifice membrane permeability in spite of improved stability. In this work, omniphobic membranes were fabricated by decorating polyvinylidene fluoride (PVDF) nanofibrous membranes with different silica nanoparticles (SiNPs) followed by fluorination. Compared to unmodified nanofibrous and commercial PVDF membranes, both smaller SiNPs coated membrane #P-Min and larger SiNPs coated membrane #P-Max can perfectly sustain liquids with low surface tensions on their surfaces without wetting. In contrast to #P-Max with an oil contact angle of 136 ± 1°, #P-Min exhibited better oleophobicity with an oil contact angle of 152 ± 1°. In addition, the rigidity of their omniphobic properties was confirmed by testing in harsh conditions. Moreover, #P-Min successfully overcame the trade-off relation between membrane permeability and rejection with an enhanced MD flux due to more effective water evaporation area on membrane surface and a higher membrane porosity. While #P-Max was wetted by a hypersaline feed solution composed of 25 wt% NaCl, #P-Min could maintain a stable flux of 15 L m−2h−1 in a continuous 7-h MD operation, which proved its excellent performance for brine treatment. Furthermore, #P-Min exhibited less scaling tendency when feed was a saturated gypsum solution. Compared to #P-Max , the better anti-scaling properties of #P-Min should be due to its higher surface energy barrier, better slippery property and less solid-liquid contact area, which effectively impede crystal nucleation and attachment. Image 1 • Effects of nano-scaled roughness on membrane omniphobicity were studied. • As-prepared omniphobic membranes were stable under harsh conditions. • Omniphobic membrane coated with smaller NPs exhibited better performance. • It should be due to high surface energy barrier, slippery property and less solid-liquid contact area. [ABSTRACT FROM AUTHOR]

Membrane distillation (MD) has attracted increasing attention for seawater and brackish water desalination, wastewater treatment, and concentration of juices and pharmaceutical solutions due to its feasibility of using low-grade energy and achieving high solute rejection. However, the successful implementation of MD technology is still impeded by membrane wetting resulted from membrane scaling by inorganic crystals and membrane fouling by organic matters in the feed solutions, especially when dealing with challenging wastewaters. Herein, this study describes a novel strategy to develop a robust and superhydrophobic membrane via electrospinning followed by electrospray to enhance membrane anti-wetting properties caused by scaling and fouling. The robust superhydrophobic membrane PDMS-3 comprised a superhydrophobic surface layer fabricated by electrospraying a PVDF/PDMS/silica fume blended solution and an electrospun nanofibrous PVDF support. The newly developed membrane PDMS-3 not only exhibited an outstanding superhydrophobicity with a water contact angle of 170⁰ ± 1⁰, but also showed a unique anti-abrasive feature, which still possessed a slippery superhydrophobic property after 40 abrasion cycles. Moreover, the DCMD tests revealed that this rigid superhydrophobic surface rendered PDMS-3 with the best anti-scaling and anti-fouling properties compared to the nanofibrous and commercial PVDF membranes. It could achieve a superior MD flux of 28 kg m−2 h−1and a stable salt rejection above 99.99% during a continuous 160-h DCMD operation when the feed and permeation solutions were 3.5 wt% NaCl solution at 333K and distillate water at 293 K, respectively. These results underline the importance of anti-abrasive superhydrophobic membrane development to deal with wastewaters containing diverse inorganic and organic matters via MD. • The robust and superhydrophobic composite membranes were fabricated by electrospinning and electrospray. • The abrasive resistance of as-developed superhydrophobic membranes were examined. • PDMS-3 with the highest silica content exhibits the best anti-abrasive property. • Compared to nanofibrous PVDF membrane, PDMS-3 exhibits better anti-scaling and anti-fouling properties. • The surface property of PDMS-3 is significant in determining its long-term stability in MD. [ABSTRACT FROM AUTHOR]