A cost-effective and high efficient Janus membrane for the treatment of oily brine using membrane distillation

Membrane distillation technology could utilize low-grade heat to desalinate brine, but the membrane material often suffers from disadvantages of low permeation flux and weak robustness to contaminants. To address these issues, the commercial polytetrafluoroethylene (PTFE) membrane was modified by cost-effective chemicals of tannic acid and (3-Aminopropyl)-triethoxysilane (APTES) to construct hydrophilic/underwater superoleophobic nano-rough structures on the surface to enhance its flux and oil-fouling resistance in direct contact membrane distillation (DCMD). The results show that a high underwater oil contact angle of 180° is observed to the membrane surface due to the rough nanostructures functionalized by abundant hydroxyl groups. Despite the additional mass transfer resistance provided by the rough nanostructures, the flux was increased noticeably. This is mainly attributed to the strong interactions between the abundant hydroxyl groups of hydrophilic layer surface and water molecules, leading to a part of free water (FW) staying at intermediate transition state (IW). The mass transfer resistance of the hydrophilic layer itself is reduced as a consequence of decreased evaporation enthalpy of water, thereby increasing the flux. Moreover, while the flux of the pristine membrane is reduced by 84.18%, the flux of Janus membrane remains the same when treating mineral oil brine emulsions with oil concentration up to 1500 ppm in comparison with the result for 35 g/L brine solution, indicating that the Janus membrane is safe from the oil contamination. Our work provides a fine guidance for membrane distillation to treat high oily brine. .

Hypercrosslinked Hydrogel Composite Membranes Targeted for Removal of Volatile Organic Compounds via Selective Solution-Diffusion in Membrane Distillation

Membrane distillation (MD) has attracted considerable interest in hypersaline wastewater treatment. However, its practicability is severely impeded by the ineffective interception of volatile organic compounds (VOCs), which seriously affects the product water quality. Herein, a hypercrosslinked alginate (Alg)/aluminum (Al) hydrogel composite membrane is facilely fabricated via Alg pregel formation and ionic crosslinking for efficient VOC interception. The obtained MD membrane shows a sufficient phenol rejection of 99.52% at the phenol concentration of 100 ppm, which is the highest rejection among the reported MD membranes. Moreover, the hydrogel composite membrane maintains a high phenol interception (>99%), regardless of the feed temperature, initial phenol concentration, and operating time. Diffusion experiments and molecular dynamics simulation verify that the selective diffusion is the dominant mechanism for VOCs-water separation. Phenol experiences a higher energy barrier to pass through the dense hydrogel layer compared to water molecules as the stronger interaction between phenol-Alg compared with water-Alg. Benefited from the dense and hydratable Alg/Al hydrogel layer, the composite membrane also exhibits robust resistance to wetting and fouling during long-term operation. The superior VOCs removal efficiency and excellent durability endow the hydrogel composite membrane with a promising application for treating complex wastewater containing both volatile and nonvolatile contaminants.

Advancements in Nanoenabled Membrane Distillation for a Sustainable Water-Energy-Environment Nexus

The emergence of nano innovations in membrane distillation (MD) has garnered increasing scientific interest. This enables the exploration of state-of-the-art nano-enabled MD membranes with desirable properties, which significantly improve the efficiency and reliability of the MD process and open up opportunities for achieving a sustainable water-energy-environment (WEE) nexus. This comprehensive review provides broad coverage and in-depth analysis of recent innovations in nano-enabled MD membranes, focusing on their role in achieving desirable properties, such as strong liquid-repellence, high resistance to scaling, fouling, and wetting, as well as efficient self-heating and self-cleaning functionalities. The recent developments in nano-enhanced photothermal-catalytic applications for water-energy co-generation within a single MD system are also discussed. Furthermore, the bottlenecks are identified that impede the scale-up of nanoenhanced MD membranes and a future roadmap is proposed for their sustainable commercialiation. This holistic overview is expected to inspire future research and development efforts to fully harness the potential of nano-enabled MD membranes to achieve sustainable integration of water, energy, and the environment.

Recent Advances in Dopamine-Based Membrane Surface Modification and Its Membrane Distillation Applications

Surface modification of membranes is essential for improving flux and resistance to contamination for membranes. This is of great significance for membrane distillation, which relies on the vapor pressure difference across the membrane as the driving force. In recent years, biomimetic mussel-inspired substances have become the research hotspots. Among them, dopamine serves as surface modifiers that would achieve highly desirable and effective membrane applications owing to their unique physicochemical properties, such as universal adhesion, enhanced hydrophilicity, tunable reducibility, and excellent thermal conductivity. The incorporation of a hydrophilic layer, along with the utilization of photothermal properties and post-functionalization capabilities in modified membranes, effectively addresses challenges such as low flux, contamination susceptibility, and temperature polarization during membrane distillation. However, to the best of our knowledge, there is still a lack of comprehensive and in-depth discussions. Therefore, this paper systematically compiles the modification method of dopamine on the membrane surface and summarizes its application and mechanism in membrane distillation for the first time. It is believed that this paper would provide a reference for dopamine-assisted membrane separation during production, and further promote its practical application.

Mechanism of Silica Nanoparticle-Induced Particulate Fouling in Vacuum Membrane Distillation

Membrane distillation (MD) is a process driven by the vapor pressure difference dependent on temperature variation, utilizing a hydrophobic porous membrane. MD operates at low pressure and temperature, exhibiting resilience to osmotic pressure. However, a challenge arises as the membrane performance diminishes due to temperature polarization (TP) occurring on the membrane surface. The vacuum MD process leverages the application of a vacuum to generate a higher vapor pressure difference, enhancing the flux and mitigating TP issues. Nevertheless, membrane fouling leads to decreased performance, causing membrane wetting and reducing the ion removal efficiency. This study investigates membrane fouling phenomena induced by various silica nanoparticle sizes (400, 900, and 1300 nm). The patterns of membrane fouling, as indicated by the flux reduction, vary depending on the particle size. Distinct MD performances are observed with changes in the feed water temperature and flow rate. When examining the membrane fouling mechanism for particles with a porosity resembling actual particulate materials, a fouling form similar to the solid type is noted. Therefore, this study elucidates the impact of particulate matter on membrane fouling under diverse conditions.

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.

Engineering Multinanochannel Polymer-Intercalated Graphene Oxide Membrane for Strict Volatile Sieving in Membrane Distillation

Graphene oxide (GO) membranes enabled by subnanosized diffusion channels are promising to separate small species in membrane distillation (MD). However, the challenge of effectively excluding small volatiles in MD persists due to the severe swelling and subsequent increase in GO interlamination spacing upon direct contact with the hot feed. To address this issue, we implemented a design in which a polymer is confined between the GO interlaminations, creating predominantly 2D nanochannels centered around 0.57 nm with an average membrane pore size of 0.30 nm. Compared to the virginal GO membrane, the polymer-intercalated GO membrane exhibits superior antiswelling performance, particularly at a high feed temperature of 60 °C. Remarkably, the modified membrane exhibited a high flux of approximately 52 L m-2 h-1 and rejection rates of about 100% for small ions and 98% for volatile phenol, with a temperature difference of 40 °C. Molecular dynamics simulations suggest that the sieving mechanisms for ions and volatiles are facilitated by the narrowed nanochannels within the polymer network situated between the 2D nanochannels of GO interlaminations. Concurrently, the unrestricted permeation of water molecules through the multinanochannel GO membrane encourages high-flux desalination of complex hypersaline wastewater.

Performance investigation of poly(vinylidene fluoride-cohexafluoropropylene) membranes containing SiO2 nanoparticles in a newly designed single vacuum membrane distillation system

The current study focuses on the development of a superhydrophobic poly(vinylidene fluoride-cohexafluoropropylene) nanocomposite membrane suitable for vacuum membrane distillation by incorporating SiO2 nanoparticles. At loading hydrophobic nano-SiO2 particle concentration (0.50-1.50 wt.%), the developed nanocomposite membranes are optimized in terms of vacuum membrane distillation performance. The influence of temperature, vacuum pressure, and feed water flow is studied for desalinating high-salinity brine. The results show that the developed vacuum distillation membrane is capable of 95% salt rejection during the treatment of a highly saline feed (65,000 ppm) at fixed flow rates of 120 L/h saline feed and different operating conditions consisting of feed inlet temperatures ranging from 40°C to 70°C and distillate inlet temperatures of 7-15°C. The vacuum membrane distillation process achieves 0.38-1.66% water recovery with increasing concentration factor, meaning that recovery is increased, and shows a specific electrical energy consumption of 5.16-23.90 kWh/m3 for product water. Overall, the newly designed membrane demonstrates suitability for a vacuum membrane distillation system. PRACTITIONER POINTS: Desalinate high-salinity brine (TDS > 35,000 ppm) using a vacuum membrane distillation system. A hydrophobic PVDF-HFP/SiO2 nanocomposite membrane development for vacuum membrane distillation. A newly designed single vacuum membrane distillation system for RO brine treatment.

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.

Concept and analysis of hybrid reversal multi-stage flash and membrane distillation desalination system

The concept and analysis of integrating membrane distillations (MD) with reversal once-through Multistage Flash (RV-MSF) desalination is presented. The analysis is based on numerical simulation. The MD vessels are integrated into the terminal ends of the RV-MSF system to leverage the thermal energy associated with these terminal streams. Hybridisation at the last MSF stage, i.e. by replacing the brine cooler, contributes marginally to the overall production rate which amounts to 2%. However, it is found that hybridisation at stage one, i.e. utilising the energy of the MSF reject brine can increase the overall production rate by 65%. For seawater feed temperature of 80 oC and 24 MSF stages, 5 MD vessels in series can be integrated with the RV-MSF process. This ultimate hybridisation helped improve the recovery ratio from 7 to 23%, decreasing the specific cooling water requirement from 23 to 12 kg/kg and reducing the specific energy consumption from 129 to 41 kWh/m3 with respect to the stand-alone RV-MSF system. However, this achievement incurs an additional specific area for heat transfer which increased from 29 to 65 m2/(kg/s). This is because a large number of MD modules are incorporated into the hybridisation.

A Comparative Analysis of Pervaporation and Membrane Distillation Techniques for Desalination Utilising the Sweeping Air Methodology with Novel and Economical Pervaporation Membranes

This study used the sweeping air approach to conduct a comparative analysis of pervaporation (PV) and membrane distillation (MD) in the context of desalinating saline/hypersaline water. An experimental setup of the sweeping air arrangement was designed and built at a laboratory size to conduct the research. The desalination process using PV used innovatively designed cellulose acetate (CA) membranes specifically adapted for this purpose. Conversely, in the studies involving MD, hydrophobic polytetrafluoroethylene (PTFE) membranes were utilised. CA membranes were fabricated in our laboratory using the phase inversion approach. The physicochemical characteristics of the membranes were assessed using many methodologies, including FTIR spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), contact angle measurement, and water uptake analysis. This facilitated a more comprehensive comprehension of the impact of the alkaline treatment on these features. The variables that were examined included the kind of membrane, the pore size of the PTFE membrane, the composition of the casting solution of CA, the concentration of the feed solution, the temperature of the feed, and the temperature of the condenser cooling water. The morphologies of the membranes were examined using SEM. The study's findings indicated that the use of MD resulted in a greater flow and a remarkable percentage of salt rejection (% SR). Furthermore, it was observed that the flux was positively correlated with the feed temperature, while it exhibited an inverse relationship with the cooling water temperature. Moreover, it was observed that the impact of the pore size of the PTFE membrane on the desalination process was found to be minimal. The most optimal outcomes obtained were 13.35 kg/m2 h with a percentage salt rejection (% SR) of 99.86, and 17.96 kg/m2 h with a % SR of 99.83 at a temperature of 70 °C, while using MD and PV technologies, respectively. Furthermore, both methods demonstrated the capability to desalinate very salty solutions with a salinity level of up to 160 g/L, thereby yielding potable water in a single step.

Application of Machine Learning to Characterize the Permeate Quality in Pilot-Scale Vacuum-Assisted Air Gap Membrane Distillation Operation

Membrane distillation (MD) is a thermal desalination technique proposed for the valorization of residual brines that other operations such as reverse osmosis cannot treat. Previous studies have shown that vacuum-assisted air gap (V-AGMD) operation in commercial multi-envelope modules improves the performance of MD noticeably. However, the permeate quality at pilot scale has not been thoroughly characterized so far. The aim of this study is, therefore, to assess and model the effect of the main operating conditions (feed flow rate, inlet temperatures, and feed salinity) on the permeate quality. Results from different steady-state experiments allowed to estimate descriptive metrics such as the salt rejection factor (SRF) and the membrane leak ratio (MLR). Given their non-linear behavior, these metrics were subsequently modeled using artificial neural networks (ANN) to estimate the permeate quality in the whole scope of operating conditions. Acceptable SRF results with MLR values lower than 0.2% confirmed the validity of MD as an operation for the treatment of concentrated brines, although the salinity of the resulting permeate does not comply in all cases with that permitted for human consumption.

Review and Analysis of Heat Transfer in Spacer-Filled Channels of Membrane Distillation Systems

Membrane distillation (MD) is an attractive process for the concentration of seawater brines. Modelling and simulation of membrane distillation processes requires a better knowledge of the heat transfer coefficients in spacer-filled channels which are usually determined by applying empirical correlations for the Nusselt number. In this study, first, a comprehensive literature review on heat transfer correlations was conducted. It was found that the empirical correlations often used for MD simulation result in strongly varying Nusselt numbers that differ by up to an order of magnitude at low Reynolds numbers. Then, heat transfer in spacer-filled channels was investigated experimentally in a membrane distillation system using an aluminum plate instead of a flat-sheet membrane. Numerous tests were carried out with sodium chloride solutions in a wide range of salinities, between 1 g/kg and 95 g/kg, and temperatures, between 30 °C and 80 °C, yielding high heat transfer coefficients in a range of 1500 to 8300 W/(m2K) at relatively low Reynolds numbers, between 100 and 1500, clearly showing the influence of the spacers on heat transfer. A new empirical Nusselt correlation (Nu=0.158Re0.652Pr0.277) was derived which represents the experimental data with a deviation of 10% and is valid for 100 Collapse

Key Words

• brine concentration
• heat transfer
• membrane distillation
• modelling
• spacer-filled channel

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MESH Headings Collapse

Grants

• 1321 KSB Stiftung (KSB Foundation)

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Affiliation(s)

Sebastian Schilling
Heike Glade Engineering Thermodynamics, University of Bremen, 28359 Bremen, Germany

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Productivity and Thermal Performance Enhancements of Hollow Fiber Water Gap Membrane Distillation Modules Using Helical Fiber Configuration: 3D Computational Fluid Dynamics Modeling

Although hollow fiber water gap membrane distillation (HF-WGMD) units offer certain advantages over other MD desalination systems, they still require enhancements in terms of distillate flux and productivity. Therefore, this work proposes a novel configuration by incorporating the helical turns of HF membranes within the water gap channel of the HF-WGMD modules. A fully coupled 3D CFD model is developed and validated to simulate the multifaceted energy conservations and diffusion mechanisms that are inherent to the transport phenomena in the proposed HF-WGMD module. Single and double helical HF membrane designs with different numbers of turns are compared to the reference modules of single and double straight HF membrane designs under various operational conditions. At a feed temperature of 70 °C, a noteworthy 11.4% enhancement in the distillate flux is observed when employing 20 helical turns, compared to the single straight HF membrane module. Furthermore, the specific productivity revealed a maximum enhancement of 46.2% when using 50 helical turns. The thermal performance of the proposed HF-WGMD module shows higher energy savings of up to 35% in specific thermal energy consumption for a one-stage module. Using three stages of single helical modules can increase the gain output ratio from 0.17 for the single stage to 0.37, which represents an increase of 117.6%. These findings indicate the high potential of the proposed approach in advancing the performance of HF-WGMD systems.

Nanocomposite Hydrogel Engineered Janus Membrane for Membrane Distillation with Robust Fouling, Wetting, and Scaling Resistance

Membrane distillation (MD) is considered to be rather promising for high-salinity wastewater reclamation. However, its practical viability is seriously challenged by membrane wetting, fouling, and scaling issues arising from the complex components of hypersaline wastewater. It remains extremely difficult to overcome all three challenges at the same time. Herein, a nanocomposite hydrogel engineered Janus membrane has been facilely constructed for desired wetting/fouling/scaling-free properties, where a cellulose nanocrystal (CNC) composite hydrogel layer is formed in situ atop a microporous hydrophobic polytetrafluoroethylene (PTFE) substrate intermediated by an adhesive layer. By the synergies of the elevated membrane liquid entry pressure, inhibited surfactant diffusion, and highly hydratable surface imparted by the hydrogel/CNC (HC) layer, the resultant HC-PTFE membrane exhibits robust resistance to surfactant-induced wetting and oil fouling during 120 h of MD operation. Meanwhile, owing to the dense and hydroxyl-abundant surface, it is capable of mitigating gypsum scaling and scaling-induced wetting, resulting in a high normalized flux and low distillate conductivity at a concentration factor of 5.2. Importantly, the HC-PTFE membrane enables direct desalination of real hypersaline wastewater containing broad-spectrum foulants with stable vapor flux and robust salt rejection (99.90%) during long-term operation, demonstrating its great potential for wastewater management in industrial scenarios.

Experimental Investigation of a Plate-Frame Water Gap Membrane Distillation System for Seawater Desalination

This study presented a detailed investigation into the performance of a plate-frame water gap membrane distillation (WGMD) system for the desalination of untreated real seawater. One approach to improving the performance of WGMD is through the proper selection of cooling plate material, which plays a vital role in enhancing the gap vapor condensation process. Hence, the influence of different cooling plate materials was examined and discussed. Furthermore, two different hydrophobic micro-porous polymeric membranes of similar mean pore sizes were utilized in the study. The influence of key operating parameters, including the feed water temperature and flow rate, was examined against the system vapor flux and gained output ratio (GOR). In addition, the used membranes were characterized by means of different techniques in terms of surface morphology, liquid entry pressure, water contact angle, pore size distribution, and porosity. Findings revealed that, at all conditions, the PTFE membrane exhibits superior vapor flux and energy efficiency (GOR), with 9.36% to 14.36% higher flux at a 0.6 to 1.2 L/min feed flow rate when compared to the PVDF membrane. The copper plate, which has the highest thermal conductivity, attained the highest vapor flux, while the acrylic plate, which has an extra-low thermal conductivity, recorded the lowest vapor flux. The increasing order of GOR values for different cooling plates is acrylic < HDPE < copper < aluminum < brass < stainless steel. Results also indicated that increasing the feed temperature increases the vapor flux almost exponentially to a maximum flux value of 30.36 kg/m2hr. The system GOR also improves in a decreasing pattern to a maximum value of 0.4049. Moreover, a long-term test showed that the PTFE membrane, which exhibits superior hydrophobicity, registered better salt rejection stability. The use of copper as a cooling plate material for better system performance is recommended, while cooling plate materials with very low thermal conductivities, such as a low thermally conducting polymer, are discouraged.

Progress on Improved Fouling Resistance-Nanofibrous Membrane for Membrane Distillation: A Mini-Review

Nanofibrous membranes for membrane distillation (MD) have demonstrated promising results in treating various water and wastewater streams. Significant progress has been made in recent decades because of the development of sophisticated membrane materials, such as superhydrophobic, omniphobic and Janus membranes. However, fouling and wetting remain crucial issues for long-term operation. This mini-review summarizes ideas as well as their limitations in understanding the fouling in membrane distillation, comprising organic, inorganic and biofouling. This review also provides progress in developing antifouling nanofibrous membranes for membrane distillation and ongoing modifications on nanofiber membranes for improved membrane distillation performance. Lastly, challenges and future ways to develop antifouling nanofiber membranes for MD application have been systematically elaborated. The present mini-review will interest scientists and engineers searching for the progress in MD development and its solutions to the MD fouling issues.

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.

Electrospun Poly (Vinylidene Fluoride-Co-Hexafluoropropylene) Nanofiber Membranes for Brine Treatment via Membrane Distillation

The major challenge for membrane distillation (MD) is the membrane wetting resistance induced by pollutants in the feed solution. The proposed solution for this issue was to fabricate membranes with hydrophobic properties. Hydrophobic electrospun poly (vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) nanofiber membranes were produced for brine treatment using the direct-contact membrane distillation (DCMD) technique. These nanofiber membranes were prepared from three different polymeric solution compositions to study the effect of solvent composition on the electrospinning process. Furthermore, the effect of the polymer concentration was investigated by preparing polymeric solutions with three different polymer percentages: 6, 8, and 10%. All of the nanofiber membranes obtained from electrospinning were post-treated at varying temperatures. The effects of thickness, porosity, pore size, and liquid entry pressure (LEP) were studied. The hydrophobicity was determined using contact angle measurements, which were investigated using optical contact angle goniometry. The crystallinity and thermal properties were studied using DSC and XRD, while the functional groups were studied using FTIR. The morphological study was performed with AMF and described the roughness of nanofiber membranes. Finally, all of the nanofiber membranes had enough of a hydrophobic nature to be used in DCMD. A PVDF membrane filter disc and all nanofiber membranes were applied in DCMD to treat brine water. The resulting water flux and permeate water quality were compared, and it was discovered that all of the produced nanofiber membranes showed good behavior with varying water flux, but the salt rejection was greater than 90%. A membrane prepared from DMF/acetone 5-5 with 10% PVDF-HFP provided the perfect performance, with an average water flux of 44 kg.m^-2.h^-1 and salt rejection of 99.8%.

Integrated Electrocoagulation, Ultrafiltration, Membrane Distillation, and Crystallization for Treating Produced Water

Produced water (PW) generated from hydraulic fracturing operations was treated using an integrated electrocoagulation, ultrafiltration, membrane distillation, and crystallization process (EC UF MDC). The aim was to determine the viability of this integrated process for maximizing water recovery. The results obtained here indicate that optimizing the various unit operations could lead to increased recovery of PW. Membrane fouling limits all membrane separation processes. A pretreatment step to suppress fouling is essential. Here, removal of total suspended solids (TSS) and total organic carbon (TOC) was achieved by electrocoagulation (EC) followed by ultrafiltration (UF). The hydrophobic membrane used in membrane distillation may be fouled by dissolved organic compounds. Reducing membrane fouling is essential to increase the long-term durability of the membrane distillation (MD) system. In addition, combining membrane distillation with crystallization (MDC) can help reduce scale formation. By inducing crystallization in the feed tank, scale formation on the MD membrane was suppressed. The integrated EC UF MDC process can impact Water Resources/Oil & Gas Companies. Conservation of surface and groundwater is possible by treating and reusing PW. Additionally, treating PW reduces the amount of PW disposed in Class II disposal wells and promotes more environmentally sustainable operations.

Treatment of Aniline Wastewater by Membrane Distillation and Crystallization

Aniline is a highly toxic organic pollutant with "carcinogenic, teratogenic and mutagenesis" characteristics. In the present paper, a membrane distillation and crystallization (MDCr) process was proposed to achieve zero liquid discharge (ZLD) of aniline wastewater. Hydrophobic polyvinylidene fluoride (PVDF) membranes were used in the membrane distillation (MD) process. The effects of the feed solution temperature and flow rate on the MD performance were investigated. The results showed that the flux of the MD process was up to 20 L·m^-2·h^-1 and the salt rejection was above 99% under the feeding condition of 60 °C and 500 mL/min. The effect of Fenton oxidation pretreatment on the removal rate of aniline in aniline wastewater was also investigated, and the possibility of realizing the ZLD of aniline wastewater in the MDCr process was verified.

Interlayered Interface of a Thin Film Composite Janus Membrane for Sieving Volatile Substances in Membrane Distillation

Hypersaline wastewater treatment using membrane distillation (MD) has gained significant attention due to its ability to completely reject nonvolatile substances. However, a critical limitation of current MD membranes is their inability to intercept volatile substances owing to their large membrane pores. Additionally, the strong interaction between volatile substances and MD membranes underwater tends to cause membrane wetting. To overcome these challenges, we developed a dual-layer thin film composite (TFC) Janus membrane through electrospinning and sequential interfacial polymerization of a polyamide (PA) layer and cross-linking a polyvinyl alcohol/polyacrylic acid (PP) layer. The resulting Janus membrane exhibited high flux (>27 L m^-2 h^-1), salt rejection of ∼100%, phenol rejection of ∼90%, and excellent resistance to wetting and fouling. The interlayered interface between the PA and PP layer allowed the sieve of volatile substances by limiting their dissolution-diffusion, with the increasing hydrogen bond network formation preventing their transport. In contrast, small water molecules with powerful dynamics were permeable through the TFC membrane. Both experimental and molecular dynamics simulation results elucidated the sieving mechanism. Our findings demonstrate that this type of TFC Janus membrane can serve as a novel strategy to design next-generation MD membranes against volatile and non-volatile contaminants, which can have significant implications in the treatment of complex hypersaline wastewater.

Novel Approach to Landfill Wastewater Treatment Fouling Mitigation: Air Gap Membrane Distillation with Tin Sulfide-Coated PTFE Membrane

This study addressed the fouling issue in membrane distillation (M.D.) technology, a promising method for water purification and wastewater reclamation. To enhance the anti-fouling properties of the M.D. membrane, a tin sulfide (TS) coating onto polytetrafluoroethylene (PTFE) was proposed and evaluated with air gap membrane distillation (AGMD) using landfill leachate wastewater at high recovery rates (80% and 90%). The presence of TS on the membrane surface was confirmed using various techniques, such as Field Emission Scanning Electron Microscopy (FE-SEM), Fourier Transform Infrared Spectroscopy (FT-IR), Energy Dispersive Spectroscopy (EDS), contact angle measurement, and porosity analysis. The results indicated the TS-PTFE membrane exhibited better anti-fouling properties than the pristine PTFE membrane, and its fouling factors (FFs) were 10.4-13.1% compared to 14.4-16.5% for the PTFE membrane. The fouling was attributed to pore blockage and cake formation of carbonous and nitrogenous compounds. The study also found that physical cleaning with deionized (DI) water effectively restored the water flux, with more than 97% recovered for the TS-PTFE membrane. Additionally, the TS-PTFE membrane showed better water flux and product quality at 55 °C and excellent stability in maintaining the contact angle over time compared to the PTFE membrane.

Janus Membrane with Hydrogel-like Coating for Robust Fouling and Wetting Resistance in Membrane Distillation

Membrane distillation (MD) is a promising technique for water reclamation from hypersaline wastewater. However, fouling and wetting of the hydrophobic membranes are two prominent challenges for the widespread application of MD. Herein, we developed an antiwetting and antifouling Janus membrane comprising a hydrogel-like polyvinyl alcohol/tannic acid (PVA/TA) top layer and a hydrophobic polytetrafluoroethylene (PTFE) membrane substrate via a facile and benign strategy combining mussel-amine co-deposition with the shrinkage-rehydration process. Interestingly, the vapor flux of the Janus membrane was not compromised, though a microscale PVA/TA layer was introduced, possibly due to the high water uptake and reduced water evaporation enthalpy of the hydrogel-like structure. Moreover, the PVA/TA-PTFE Janus membrane sustained stable MD performance while treating a challenging saline feed containing surfactants and mineral oils. The robust wetting resistance arises from the synergistic effects of the elevated liquid entry pressure (1.01 ± 0.02 MPa) of the membrane and the retardation of surfactant transport to the substrate PTFE layer. Meanwhile, the hydrogel-like PVA/TA layer hinders oil fouling due to its strongly hydrated state. Furthermore, the PVA/TA-PTFE membrane exhibited improved performance in purifying shale gas wastewater and landfill leachate. This study provides new insights into the facile design and fabrication of promising MD membranes for hypersaline wastewater treatment.

Aliquots of MIL-140 and Graphene in Smart PNIPAM Mixed Hydrogels: A Nanoenvironment for a More Eco-Friendly Treatment of NaCl and Humic Acid Mixtures by Membrane Distillation

The problem of water scarcity is already serious and risks becoming dramatic in terms of human health as well as environmental safety. Recovery of freshwater by means of eco-friendly technologies is an urgent matter. Membrane distillation (MD) is an accredited green operation for water purification, but a viable and sustainable solution to the problem needs to be concerned with every step of the process, including managed amounts of materials, membrane fabrication procedures, and cleaning practices. Once it is established that MD technology is sustainable, a good strategy would also be concerned with the choice of managing low amounts of functional materials for membrane manufacturing. These materials are to be rearranged in interfaces so as to generate nanoenvironments wherein local events, conceived to be crucial for the success and sustainability of the separation, can take place without endangering the ecosystem. In this work, discrete and random supramolecular complexes based on smart poly(N-isopropyl acrylamide) (PNIPAM) mixed hydrogels with aliquots of ZrO(O₂C-C₁₀H₆-CO₂) (MIL-140) and graphene have been produced on a polyvinylidene fluoride (PVDF) sublayer and have been proven to enhance the performance of PVDF membranes for MD operations. Two-dimensional materials have been adhered to the membrane surface through combined wet solvent (WS) and layer-by-layer (LbL) spray deposition without requiring further subnanometer-scale size adjustment. The creation of a dual responsive nanoenvironment has enabled the cooperative events needed for water purification. According to the MD's rules, a permanent hydrophobic state of the hydrogels together with a great ability of 2D materials to assist water vapor diffusion through the membranes has been targeted. The chance to switch the density of charge at the membrane-aqueous solution interface has further allowed for the choice of greener and more efficient self-cleaning procedures with a full recovery of the permeation properties of the engineered membranes. The experimental evidence of this work confirms the suitability of the proposed approach to obtain distinct effects on a future production of reusable water from hypersaline streams under somewhat soft working conditions and in full respect to environmental sustainability.

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.

Plasma-Induced Superhydrophobicity as a Green Technology for Enhanced Air Gap Membrane Distillation

Superhydrophobicity has only recently become a requirement in membrane fabrication and modification. Superhydrophobic membranes have shown improved flux performance and scaling resistance in long-term membrane distillation (MD) operations compared to simply hydrophobic membranes. Here, we introduce plasma micro- and nanotexturing followed by plasma deposition as a novel, dry, and green method for superhydrophobic membrane fabrication. Using plasma micro- and nanotexturing, commercial membranes, both hydrophobic and hydrophilic, are transformed to superhydrophobic featuring water static contact angles (WSCA) greater than 150° and contact angle hysteresis lower than 10°. To this direction, hydrophobic polytetrafluoroethylene (PTFE) and hydrophilic cellulose acetate (CA) membranes are transformed to superhydrophobic. The superhydrophobic PTFE membranes showed enhanced water flux in standard air gap membrane distillation and more stable performance compared to the commercial ones for at least 48 h continuous operation, with salt rejection >99.99%. Additionally, their performance and high salt rejection remained stable, when low surface tension solutions containing sodium dodecyl sulfate (SDS) and NaCl (down to 35 mN/m) were used, showcasing their antiwetting properties. The improved performance is attributed to superhydrophobicity and increased pore size after plasma micro- and nanotexturing. More importantly, CA membranes, which are initially unsuitable for MD due to their hydrophilic nature (WSCA ≈ 40°), showed excellent performance with stable flux and salt rejection >99.2% again for at least 48 h, demonstrating the effectiveness of the proposed method for wetting control in membranes regardless of their initial wetting properties.

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.

Tuning PVDF Membrane Porosity and Wettability Resistance via Varying Substrate Morphology for the Desalination of Highly Saline Water

Here, we report the fabrication of a series of highly efficient polyvinylidene fluoride (PVDF) membranes via substrate morphology variations. A wide range of sandpaper grit sizes (150-1200) were utilized as casting substrates. The effect of the penetration of abrasive particles present on the sandpapers on the casted polymer solution was tuned, and the impact of these particles on porosity, surface wettability, liquid entry pressure and morphology were investigated. The membrane distillation performance of the developed membrane on sandpapers was evaluated for the desalination of highly saline water (70,000 ppm). Interestingly, the utilization of cheap and widely available sandpapers as a substrate for casting can not only help in tuning the MD performance, but also in producing highly efficient membranes with stable salt rejection (up to 100%) and a 210% increase in the permeate flux over 24 h. The findings in this study will help in delineating the role of substrate nature in controlling the produced membrane characteristics and performance.

Applicability of Composite Magnetic Membranes in Separation Processes of Gaseous and Liquid Mixtures-A Review

Recent years have shown a growing interest in the application of membranes exhibiting magnetic properties in various separation processes. The aim of this review is to provide an in-depth overview of magnetic membranes that can be successfully applied for gas separation, pervaporation, ultrafiltration, nanofiltration, adsorption, electrodialysis, and reverse osmosis. Based on the comparison of the efficiency of these separation processes using magnetic and non-magnetic membranes, it has been shown that magnetic particles used as fillers in polymer composite membranes can significantly improve the efficiency of separation of both gaseous and liquid mixtures. This observed separation enhancement is due to the variation of magnetic susceptibility of different molecules and distinct interactions with dispersed magnetic fillers. For gas separation, the most effective magnetic membrane consists of polyimide filled with MQFP-B particles, for which the separation factor (α_rat O₂/N₂) increased by 211% when compared to the non-magnetic membrane. The same MQFP powder used as a filler in alginate membranes significantly improves water/ethanol separation via pervaporation, reaching a separation factor of 12,271.0. For other separation methods, poly(ethersulfone) nanofiltration membranes filled with ZnFe₂O₄@SiO₂ demonstrated a more than four times increase in water flux when compared to the non-magnetic membranes for water desalination. The information gathered in this article can be used to further improve the separation efficiency of individual processes and to expand the application of magnetic membranes to other branches of industry. Furthermore, this review also highlights the need for further development and theoretical explanation of the role of magnetic forces in separation processes, as well as the potential for extending the concept of magnetic channels to other separation methods, such as pervaporation and ultrafiltration. This article provides valuable insights into the application of magnetic membranes and lays the groundwork for future research and development in this area.

Innovative Membrane Technologies for the Treatment of Wastewater Polluted with Heavy Metals: Perspective of the Potential of Electrodialysis, Membrane Distillation, and Forward Osmosis from a Bibliometric Analysis

A bibliometric analysis, using the Scopus database as a source, was carried out in order to study the scientific documents published up to 2021 regarding the use of electrodialysis, membrane distillation, and forward osmosis for the removal of heavy metals from wastewater. A total of 362 documents that fulfilled the search criteria were found, and the results from the corresponding analysis revealed that the number of documents greatly increased after the year 2010, although the first document was published in 1956. The exponential evolution of the scientific production related to these innovative membrane technologies confirmed an increasing interest from the scientific community. The most prolific country was Denmark, which contributed 19.3% of the published documents, followed by the two main current scientific superpowers: China and the USA (with 17.4% and 7.5% contributions, respectively). Environmental Science was the most common subject (55.0% of contributions), followed by Chemical Engineering (37.3% of contributions) and Chemistry (36.5% of contribution). The prevalence of electrodialysis over the other two technologies was clear in terms of relative frequency of the keywords. An analysis of the main hot topics identified the main advantages and drawbacks of each technology, and revealed that examples of their successful implementation beyond the lab scale are still scarce. Therefore, complete techno-economic evaluation of the treatment of wastewater polluted with heavy metals via these innovative membrane technologies must be encouraged.

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.

Surface Modification of UiO-66 on Hollow Fibre Membrane for Membrane Distillation

The hydrophobicity of metal-organic frameworks (MOFs) is critical in enhancing the separation process in membrane distillation. Herein, a new superhydrophobic University of Oslo 66 (UiO-66) MOFs was successfully constructed on the top of alumina hollow fibre (AHF) membrane for desalination purposes. The fabrication methodology of the membrane involved in situ growth of pure crystalline UiO-66 on top of AHF and post-synthetic modification by fluorosilane grafting. The resultant membrane was characterised to study the physicochemical properties of the pristine and modified membrane. A superhydrophobic UiO-66 with a contact angle of 163.6° and high liquid entry pressure was obtained by introducing a highly branched fluorocarbon chain while maintaining its crystallinity. As a result, the modified membrane achieved 14.95 L/m²∙h water flux and 99.9% NaCl rejection with low energy consumption in the direct contact membrane distillation process. Furthermore, the high surface energy contributed by UiO-66 is maximised to produce the maximum number of accessible sites for the grafting process. The synergistic effect of surface hydrophobicity and porous UiO-66 membrane in trapping water vapour shows great potential for desalination application.

Anti-Wetting Performance of an Electrospun PVDF/PVP Membrane Modified by Solvothermal Treatment in Membrane Distillation

Membrane distillation (MD) is attractive for water reclamation due to the fact of its unique characteristics. However, membrane wetting becomes an obstacle to its further application. In this paper, a novel hydrophobic polyvinylidene fluoride/poly(vinyl pyrrolidone) (PVDF/PVP) membrane was fabricated by electrospinning and solvothermal treatment. The electrospun membranes prepared by electrospinning showed a multilevel interconnected nanofibrous structure. Then, a solvothermal treatment introduced the micro/nanostructure to the membrane with high roughness (Ra = 598 nm), thereby the water contact angle of the membrane increased to 158.3 ± 2.2°. Owing to the superior hydrophobicity, the membrane presented high resistance to wetting in both NaCl and SDS solutions. Compared to the pristine PVDF membrane, which showed wetting with a flux decline (120 min for 0.05 mM surfactant solution treatment), the prepared membrane showed outstanding stability over 600 min, even in 0.2 mM surfactant solutions. These results confirm a simple method for anti-wetting hydrophobic membrane preparation, which presented universal significance to direct contact membrane distillation (DCMD) for industrial application.

Fouling Control Strategies for High Concentrated Liquid Desiccants Concentrating Using Membrane Distillation

Air conditioning using a liquid desiccant (LD) is an energy-efficient air purification and cooling system. However, high energy is required to concentrate or regenerate the LD. This study aimed to investigate the characteristics of membrane fouling in more detail and determine control strategies for LD concentrating using membrane distillation (MD). Two different LDs-lithium chloride (LiCl) and potassium formate (HCOOK)-were used. Because LDs require high concentrations by nature (i.e., 40 wt% for LiCl and 70 wt% for HCOOK), the concentration was started from half of those concentrations. This resulted in a flux decline with severe membrane fouling during the concentration using MD. Different membrane fouling mechanisms were also observed, depending on the LD type. Three different physical membrane fouling control methods, including water flushing (WF), air backwashing (AB), and membrane spacer (SP), were introduced. Results showed that WF was the most effective. Both AB and SP showed a marginal change to no cleaning; however, an initial flux with SP was about 1.5 times higher than no cleaning. Therefore, WF combined with the SP could maintain a high flux and a low fouling propensity in the treatment of a high-concentration solution using MD.

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.

Janus Distillation Membrane via Mussel-Inspired Inkjet Printing Modification for Anti-Oil Fouling Membrane Distillation

In this work, inkjet printing technology was used to print a thin layer of a hydrophilic solution containing polydopamine as a binder and polyethyleneimine as a strong hydrophilic agent on a commercial hydrophobic membrane to produce a Janus membrane for membrane distillation. The pristine and modified membranes were tested in a direct-contact membrane distillation system with mineral oil-containing feedwater. The results revealed that an integrated and homogenous hydrophilic layer was printed on the membrane with small intrusions in the pores. The membrane, which contained three layers of inkjet-printed hydrophilic layers, showed a high underwater oil contact angle and a low in-air water contact angle. One-layer inkjet printing was not robust enough, but the triple-layer coated modified membrane maintained its anti-oil fouling performance even for a feed solution containing 70 g/L NaCl and 0.01 v/v% mineral oil concentration with a flux of around 20 L/m²h. This study implies the high potential of the inkjet printing technique as a facile surface modification strategy to improve membrane performance.

Multistage Surface-Heated Vacuum Membrane Distillation Process Enables High Water Recovery and Excellent Heat Utilization: A Modeling Study

Surface-heated membrane distillation (MD) enhances the energy efficiency of desalination by mitigating temperature polarization (TP). However, systematic investigations of larger scale, multistage, surface-heated MD system with high water recovery and heat recycling are limited. Here, we explore the design and performance of a multistage surface-heated vacuum MD (SHVMD) with heat recovery through a comprehensive finite difference model. In this process, the latent heat of condensation is recovered through an internal heat exchanger (HX) using the retentate from one stage as the condensing fluid for the next stage and an external HX using the feed as the condensing fluid. Model results show that surface heating enhances the performance compared to conventional vacuum MD (VMD). Specifically, in a six-stage SHVMD process, 54.44% water recovery and a gained output ratio (GOR) of 3.28 are achieved with a surface heat density of 2000 W m^-2, whereas a similar six-stage VMD process only reaches 18.19% water recovery and a GOR of 2.15. Mass and energy balances suggest that by mitigating TP, surface heating increases the latent heat trapped in vapor. The internal and external HXs capture and reuse the additional heat, which enhances the GOR values. We show for SHVMD that the hybrid internal/external heat recovery design can have GOR value 1.44 times higher than that of systems with only internal or external heat recovery. Furthermore, by only increasing six stages to eight stages, a GOR value as high as 4.35 is achieved. The results further show that surface heating can reduce the energy consumption of MD for brine concentration. The multistage SHVMD technology exhibits a promising potential for the management of brine from industrial plants.

Wettability Studies of Capillary PTFE Membranes Applied for Membrane Distillation

In the present study, the membrane distillation (MD) process was studied with the use of commercial polytetrafluoroethylene (PTFE) capillary membranes. For this purpose, both solutions with NaCl concentrations up to 300 g/L and brines contaminated with oil (70-120 mg/L) were used as feeds. The membrane's wetting resistance was tested by conducting long-term experiments for over 3500 h. Using detailed studies, it has been shown that increasing the salt concentration from 35 to 300 g/L resulted in a 50% reduction in the permeate flux. Nevertheless, the membranes retained 100% of the salt rejection after 2000 h of the module's operation. Moreover, it has been found that performing the MD process with brines contaminated with oil (120 mg/L) led to the wetting of some membranes pores, which it turn resulted in an increase in the distillate's conductivity to 450 µS/cm after 700 h running the process. The mechanism of pore wetting by oil droplets adsorbed on the membrane's surface was presented. Finally, the proposed method of membrane cleaning with the use of a solvent allowed restoring the initial MD module's performance. Consequently, both the permeate flux and distillate conductivity were stable during the MD of the feed containing 35 g/L of NaCl over the next 280 h.

A Comparison between Various Polymeric Membranes for Oily Wastewater Treatment via Membrane Distillation Process

Oily wastewater (OW) is detrimental towards the environment and human health. The complex composition of OW needs an advanced treatment, such as membrane technology. Membrane distillation (MD) gives the highest rejection percentage of pollutants in wastewater, as the membrane only allows the vapor to pass its microporous membrane. However, the commercial membranes on the market are less efficient in treating OW, as they are prone to fouling. Thus, the best membrane must be identified to treat OW effectively. This study tested and compared the separation performance of different membranes, comparing the pressure-driven performance between the membrane filtration and MD. In this study, several ultrafiltration (UF) and nanofiltration (NF) membranes (NFS, NFX, XT, MT, GC and FILMTEC) were tested for their performance in treating OW (100 ppm). The XT and MT membranes (UF membrane) with contact angles of 70.4 ± 0.2° and 69.6 ± 0.26°, respectively, showed the best performance with high flux and oil removal rate. The two membranes were then tested for long-term performance for two hours with 5000 ppm oil concentration using membrane pressure-filtration and MD. The XT membrane displayed a better oil removal percentage of >99%. MD demonstrated a better removal percentage; the flux reduction was high, with average flux reduction of 82% compared to the membrane pressure-filtration method, which experienced a lower flux reduction of 25%. The hydrophilic MT and XT membranes have the tendency to overcome fouling in both methods. However, for the MD method, wetting occurred due to the feed penetrating the membrane pores, causing flux reduction. Therefore, it is important to identify the performance and characteristics of the prepared membrane, including the best membrane treatment method. To ensure that the MD membrane has good anti-fouling and anti-wetting properties, a simple and reliable membrane surface modification technique is required to be explored. The modified dual layer membrane with hydrophobic/hydrophilic properties is expected to produce effective separation in MD for future study.

Evaluation of a Hybrid Moving Bed Biofilm Membrane Bioreactor and a Direct Contact Membrane Distillation System for Purification of Industrial Wastewater

Integrated wastewater treatment processes are accepted as the best option for sustainable and unrestricted onsite water reuse. In this study, moving bed biofilm reactor (MBBR), membrane bioreactor (MBR), and direct contact membrane distillation (DCMD) treatment steps were integrated successively to obtain the combined advantages of these processes for industrial wastewater treatment. The MBBR step acts as the first step in the biological treatment and also mitigates foulant load on the MBR. Similarly, MBR acts as the second step in the biological treatment and serves as a pretreatment prior to the DCMD step. The latter acts as a final treatment to produce high-quality water. A laboratory scale integrated MBBR/MBR/DCMD experimental system was used for assessing the treatment efficiency of primary treated (PTIWW) and secondary treated (STIWW) industrial wastewater in terms of permeate water flux, effluent quality, and membrane fouling. The removal efficiency of total dissolved solids (TDS) and effluent permeate flux of the three-step process (MBBR/MBR/DCMD) were better than the two-step (MBR/DCMD) process. In the three-step process, the average removal efficiency of TDS was 99.85% and 98.16% when treating STIWW and PTIWW, respectively. While in the case of the two-step process, the average removal efficiency of TDS was 93.83% when treating STIWW. Similar trends were observed for effluent permeate flux values which were found, in the case of the three-step process, 62.6% higher than the two-step process, when treating STIWW in both cases. Moreover, the comparison of the quality of the effluents obtained with the analysed configurations with that obtained by Jeddah Industrial Wastewater Treatment Plant proved the higher performance of the proposed membrane processes.

Design Parameters of a Direct Contact Membrane Distillation and a Case Study of Its Applicability to Low-Grade Waste Energy

In the design of membrane distillation systems, the effect of different heat transfer coefficient models on the transmembrane flux seems to have been overlooked thus far. Interestingly, the range of discrepancy in the results of the transmembrane flux is wide, especially in the laminar flow region, where MD is often operated. This can be inferred by studying the design and parameters of the direct contact membrane distillation system. In this study, the physical and physiochemical properties that affect the design of MD are comprehensively reviewed, and based on the reviewed parameters, an MD design algorithm is developed. In addition, a cost analysis of the designed MD process for low-grade-energy fluids is conducted. As a result, a total unit product cost of USD 1.59/m³, 2.69/m³, and 15.36/m³ are obtained for the feed velocities of 0.25, 1 and 2.5 m/s, respectively. Among the design parameters, the membrane thickness and velocity are found to be the most influential.

Fluoropolymer Membranes for Membrane Distillation and Membrane Crystallization

Fluoropolymer membranes are applied in membrane operations such as membrane distillation and membrane crystallization where hydrophobic porous membranes act as a physical barrier separating two phases. Due to their hydrophobic nature, only gaseous molecules are allowed to pass through the membrane and are collected on the permeate side, while the aqueous solution cannot penetrate. However, these two processes suffer problems such as membrane wetting, fouling or scaling. Membrane wetting is a common and undesired phenomenon, which is caused by the loss of hydrophobicity of the porous membrane employed. This greatly affects the mass transfer efficiency and separation efficiency. Simultaneously, membrane fouling occurs, along with membrane wetting and scaling, which greatly reduces the lifespan of the membranes. Therefore, strategies to improve the hydrophobicity of membranes have been widely investigated by researchers. In this direction, hydrophobic fluoropolymer membrane materials are employed more and more for membrane distillation and membrane crystallization thanks to their high chemical and thermal resistance. This paper summarizes different preparation methods of these fluoropolymer membrane, such as non-solvent-induced phase separation (NIPS), thermally-induced phase separation (TIPS), vapor-induced phase separation (VIPS), etc. Hydrophobic modification methods, including surface coating, surface grafting and blending, etc., are also introduced. Moreover, the research advances on the application of less toxic solvents for preparing these membranes are herein reviewed. This review aims to provide guidance to researchers for their future membrane development in membrane distillation and membrane crystallization, using fluoropolymer materials.

Breaking the Saturated Vapor Layer with a Thin Porous Membrane

The main idea of membrane distillation is to use a porous hydrophobic membrane as a barrier that isolates vapor from aqueous solutions. It is similar to the evaporation process from a free water surface but introduces solid-liquid interfaces and solid-vapor interfaces to a liquid-vapor interface. The transmembrane mass flux of a membrane-distillation process is affected by the membrane's intrinsic properties and the temperature gradient across the membrane. It is interesting and important to know whether the evaporation process of membrane distillation is faster or slower than that of a free-surface evaporation under the same conditions and know the capacity of the transmembrane mass flux of a membrane-distillation process. In this work, a set of proof-of-principle experiments with various water surface/membrane interfacial conditions is performed. The effect and mechanism of membrane-induced evaporation are investigated. Moreover, a practical engineering model is proposed based on mathematical fitting and audacious simplification, which reflects the capacity of transmembrane flux.

Experimental Performance of a Membrane Desorber with a H2O/LiCl Mixture for Absorption Chiller Applications

For absorption cooling cycles using water as a refrigerant, H₂O/LiCl mixtures are suitable for replacing conventional H₂O/LiBr mixtures. In addition, membrane devices can be used to develop compact and lighter absorption systems, and they can operate with H₂O/LiCl mixtures. The present paper describes an experimental evaluation of a membrane desorber/condenser operating at atmospheric pressure. Two operation modes were analyzed: continuous cycle operation and intermittent operation. For the first operation mode, the maximum desorption rate was 3.49 kg/h·m², with a solution temperature of 90.3 °C and a condensation temperature of 25.1 °C. The lowest desorption rate value was 0.26 kg/h·m², with a solution temperature of 75.4 °C and a condensation temperature of 40.1 °C. In the second mode, after three operating hours, the refrigerant fluid produced, per 1 m² of membrane area, 7.7, 5.6, 4.3, and 2.2 kg, at solution temperatures of 90.3, 85.3, 80.4, and 75.4 °C, respectively. A one-dimension heat and mass transfer model is presented. The calculated values of desorption rate and outlet temperatures were compared with the experimental data; a square correlation coefficient of 0.9929 was reached for the desorption rate; meanwhile, for the outlet solution temperatures and the outlet cooling-water temperatures, a square correlation coefficient up to 0.9991 was achieved. The membrane desorber has the advantages of operating at atmospheric-pressure conditions, high condensation temperature, the ability to use different saline solution working mixtures, and different operation methods. These advantages can lead to new absorption systems.

Pore-Level Multiphase Simulations of Realistic Distillation Membranes for Water Desalination

Membrane distillation (MD) is a thermally driven separation process that is operated below boiling point. Since the performance of MD modules is still comparatively low, current research aims to improve the understanding of the membrane structure and its underlying mechanisms at the pore level. Based on existing realistic 3D membrane geometries (up to 0.5 billion voxels with 39nm resolution) obtained from ptychographic X-ray computed tomography, the D3Q27 lattice Boltzmann (LB) method was used to investigate the interaction of the liquid and gaseous phase with the porous membrane material. In particular, the Shan and Chen multi-phase model was used to simulate multi-phase flow at the pore level. We investigated the liquid entry pressure of different membrane samples and analysed the influence of different micropillar structures on the Wenzel and Cassie-Baxter state of water droplets on rough hydrophobic surfaces. Moreover, we calculated the liquid entry pressure required for entering the membrane pores and extracted realistic water contact surfaces for different membrane samples. The influence of the micropillars and flow on the water-membrane contact surface was investigated. Finally, we determined the air-water interface within a partially saturated membrane, finding that the droplet size and distribution correlated with the porosity of the membrane.

Friction and Heat Transfer in Membrane Distillation Channels: An Experimental Study on Conventional and Novel Spacers

The results of an experimental investigation on pressure drop and heat transfer in spacer-filled plane channels, which are representative of Membrane Distillation units, are presented and discussed. Local and mean heat transfer coefficients were obtained by using Thermochromic Liquid Crystals and Digital Image Processing. The performances of a novel spacer geometry, consisting of spheres that are connected by cylindrical rods, and are hereafter named spheres spacers, were compared with those of more conventional woven and overlapped spacers at equal values of the Reynolds number Re (in the range ~150 to ~2500), the pitch-to-channel height ratio, the flow attack angle and the thermal boundary conditions (two-side heat transfer). For any flow rate, the novel spacer geometry provided the least friction coefficient and a mean Nusselt number intermediate between those of the overlapped and the woven spacers. For any pressure drop and for any pumping power, the novel spacer provided the highest mean Nusselt number over the whole Reynolds number range that was investigated. The influence of buoyancy was also assessed for the case of the horizontal channels. Under the experimental conditions (channel height H ≈ 1 cm, ΔT ≈ 10 °C), it was found to be large in empty (spacer-less) channels that were up to Re ≈ 1200 (corresponding to a Richardson number Ri of ~0.1), but it was much smaller and limited to the range Re < ~500 (Ri < ~0.5) in the spacer-filled channels.

Unique Behaviors and Mechanism of Highly Soluble Salt-Induced Wetting in Membrane Distillation

Scaling-induced wettinggreatly limits the application of membrane distillation (MD) for the desalination of high-salinity feed. Although highly soluble salts (e.g., NaCl) have high concentrations in this water, their scaling-induced wetting remains overlooked. To unravel the elusive wetting behaviors of highly soluble salts, in this study, we systematically investigated the scaling formation and wetting progress by in situ observation with optical coherence tomography (OCT). Through examining the influence of salt type and vapor flux on the wetting behavior, we revealed that highly soluble salt-induced wetting, especially under high vapor flux, shared several unique features: (1) occurring before the bulk feed reached saturation, (2) no scale layer formation observed, and (3) synchronized wetting progress on the millimeter scale. We demonstrated that a moving scale layer caused these interesting phenomena. The initial high vapor flux induced high concentration and temperature polarizations, which led to crystallization at the gas-liquid interface and the formation of an initial scale layer. On the one hand, this scale layer bridged the water into the hydrophobic pores; on the other hand, it blocked the membrane pores and reduced the vapor flux. In this way, the decreased vapor flux mitigated the concentration/temperature polarizations, and consequently led to the dissolution of the feed-facing side of the scale layer. This dissolution prevented the membrane pores from being completely blocked, facilitating the transportation and crystallization of salts at the distillate-facing side of the scale layer (i.e., the gas-liquid interface), thus the proceeding of the wetting layer.

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.

Fouling and Performance Investigation of Membrane Distillation at Elevated Recoveries for Seawater Desalination and Wastewater Reclamation

This study reports on the impact of elevated recovery (i.e., 80%, 85%, and 90%) on the fouling and performance of air gap membrane distillation (AGMD) with real seawater and landfill leachate wastewater samples using polytetrafluoroethylene (PTFE) polymer membranes. Increasing the feed temperature from 55 °C to 65 °C improved the water flux of seawater and wastewater and shortened the operating time by 42.8% for all recoveries. The average water flux in the 80%, 85%, and 90% recovery experiments at the 65 °C feed temperature was 32%, 37.32%, and 36.7% higher than the case of 55 °C for the same recoveries. The water flux decline was more severe at a higher temperature and recovery. The highest flux decline was observed with a 90% recovery at 65 °C feed temperature, followed by an 85% recovery at 65 °C. Close examination of the foulants layer revealed that seawater formed a cake fouling layer made predominantly of metal oxides. In contrast, the landfill leachate fouling was a combination of pore blocking and cake formation, consisting mainly of carbonous and nitrogenous compounds. Physical cleaning with deionized (DI) water at 55 °C and 65 °C and chemical cleaning with hydrogen peroxide (H2O2) were investigated for their efficiency in removing membrane foulants. Analytical results revealed that seawater fouling caused membrane pore blockage while wastewater fouling formed a porous layer on the membrane surface. The results showed that membrane cleaning with hydrogen peroxide restored >97% of the water flux. Interestingly, the fouling factor in seawater tests was 10%, while it was 16% for the wastewater tests.