Recent advances in membrane distillation processes: Membrane development, configuration design and application exploring

Fabrication and Surface Interactions of Super-Hydrophobic Silicon Carbide for Membrane Distillation

Hydrophilic silicon carbide was modified by surface deposition of a super-hydrophobic coating that is based on perfluorosilanes. The modification was proven to yield membrane surfaces with contact angles that were higher than 145° and to be stable under hydrothermal conditions. The measurement of the isosteric heat of adsorption of water and toluene by microgravimetry showed that, after modification, the membrane material was fully covered by a low-energy surface, which is consistent with the fluorocarbon moieties that were introduced by the modification. The same modification method was applied to a commercial multichannel SiC membrane tube (nominal pore size = 0.04 µm), which was tested in a direct contact membrane distillation apparatus. The membrane was permeable to water vapour and volatiles, but it showed full rejection for salt ions and organic pollutants with low vapour pressure (such as ibuprofen and caffeine). Moreover, the membrane was reusable, and its performances were stable with no sign of pore wetting over 8 h of filtration.

High-Flux Direct-Contact Pervaporation Membranes for Desalination

To solve the pore-wetting problems of membrane distillation, we developed a series of three-layer composite pervaporation (PV) membranes that could be applied in direct contact mode. Specifically, a porous poly(vinylidene fluoride) (PVDF) layer was placed on top of a polytetrafluoroethylene (PTFE) microfiltration membrane using the nonsolvent-induced phase inversion method, and then a poly(vinyl alcohol) (PVA) dense layer was coated on the PVDF/PTFE substrate. The hydrophobic PTFE layer prevented the permeate side of the PVA/PVDF/PTFE membrane from being wetted with cooling water. The PVDF intermediate layer acted as a glue to the PVA and PTFE layers. Also, the PVA dense layer provided a high water flux, salt rejection, and antifouling property. As a result, a high water flux of 44.5 ± 3.0 kg/(m² h) with the NaCl rejection of >99.99% were achieved for the direct-contact pervaporation (DCPV) membranes when separating a 3.5 wt % NaCl solution at 75.0 ± 0.9 °C using a cooling water stream of 20.6 ± 0.3 °C. Moreover, when the NaCl solution contained 10 mg/L Tween20 (acting as a contaminant), a stable water flux of 45.8 ± 0.6 kg/(m² h) was maintained for at least 24 h, indicating excellent antifouling property. Furthermore, when the permeate side was under vacuum, the water flux increased to 83.4 ± 6.5 kg/(m² h), similar to the highest reported data of all the existing PV desalination membranes. More importantly, the easy-to-scale-up fabrication method indicated great potential of DCPV membranes for commercialization.

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

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

Hydrophobic nanostructured wood membrane for thermally efficient distillation

Current membrane distillation (MD) is challenged by the inefficiency of water thermal separation from dissolved solutes, controlled by membrane porosity and thermal conductivity. Existing petroleum-derived polymeric membranes face major development barriers. Here, we demonstrate a first robust MD membrane directly fabricated from sustainable wood material. The hydrophobic nanowood membrane had high porosity (89 ± 3%) and hierarchical pore structure with a wide pore size distribution of crystalline cellulose nanofibrils and xylem vessels and lumina (channels) that facilitate water vapor transportation. The thermal conductivity was extremely low in the transverse direction, which reduces conductive heat transport. However, high thermal conductivity along the fiber enables efficient thermal dissipation along the axial direction. As a result, the membrane demonstrated excellent intrinsic vapor permeability (1.44 ± 0.09 kg m^-1 K^-1 s^-1 Pa^-1) and thermal efficiency (~70% at 60°C). The properties of thermal efficiency, water flux, scalability, and sustainability make nanowood highly desirable for MD applications.

Online Conductimetric Flow-Through Analyzer Based on Membrane Diffusion for Ammonia Control in Wastewater Treatment Process

Ammonia is a necessary monitoring parameter that should be controlled within an optimum range in the whole process of wastewater treatment and recycling, but few reliable real-time monitoring technologies are available currently under such harsh water conditions. This study proposes a continuous conductometric flow-through analyzer for ammonia monitoring (CFAA) in the wastewater treatment process. It is developed based on the gas diffusion mechanisms, and the proposed analytical principle has been validated in which the real-time conductivity increment rate is linearly proportional to the real-time ammonia concentration in the sample. This method could be generally applicable in monitoring a wide ammonia concentration range (10.2 μg L^-1 to 500 mg L^-1), and it is capable of achieving long-term ammonia monitoring by periodic renewal of the receiving solution. The potential impact factors and corresponding calibration principles are also developed to avoid tedious ongoing calibration. The field application results demonstrate that CFAA can effectively and directly achieve real-time and average ammoniacal nitrogen monitoring at different treatment stages regardless of the complexity of wastewater, not requiring any sample pretreatment. Compared with other ammonia online monitoring technologies, the proposed CFAA shows remarkable advantages in high reliability, durability, and accuracy, especially under severe monitoring condition. It can be a useful monitoring tool for continuous ammonia control in the wastewater treatment process.

CFD Investigation of Spacer-Filled Channels for Membrane Distillation

The membrane distillation (MD) process for water desalination is affected by temperature polarization, which reduces the driving force and the efficiency of the process. To counteract this phenomenon, spacer-filled channels are used, which enhance mixing and heat transfer but also cause higher pressure drops. Therefore, in the design of MD modules, the choice of the spacer is crucial for process efficiency. In the present work, different overlapped spacers are investigated by computational fluid dynamics (CFD) and results are compared with experiments carried out with thermochromic liquid crystals (TLC). Results are reported for different flow attack angles and for Reynolds numbers (Re) ranging from ~200 to ~800. A good qualitative agreement between simulations and experiments can be observed for the areal distribution of the normalized heat transfer coefficient. Trends of the average heat transfer coefficient are reported as functions of Re for the geometries investigated, thus providing the basis for CFD-based correlations to be used in higher-scale process models.

Liquid–Liquid Continuous Extraction and Fractional Distillation for the Removal of Organic Compounds from the Wastewater of the Oil Industry

This is the first study to carry out a laboratory-scale assay to assess the potentiality of continuous liquid–liquid extraction with dichloromethane (CLLEDCM) and high-power fractional distillation (HPFD) as a treatment to decontaminate the wastewater generated by the petroleum industry (WW). The analytical parameters of treated wastewater (TWW) evidenced a remarkable quality improvement compared to the original WW. CLLEDCM–HPFD yielded 92.4%–98.5% of the WW mass as more environmentally friendly water. Compared to the original values determined in the WW, total petroleum hydrocarbon (TPH) decreased by 95.0%–100.0%, and the chemical oxygen demand (COD) decreased by 90.5%–99.9%. Taking into account the yield of the treated water, the amount of pollutant removed, and the risks of each process, the order of the potentiality of these treatments, from highest to lowest, was HPFD > CLLEDCM–HPFD > CLLEDCM. CLLEDCM treatment alone produced TWW with poorer quality, and the CLLEDCM–HPFD sequence involved the greatest consumption of time and energy (0.390–0.905 kWh/kg). CLLEDCM-only was the least effective treatment because the TWW obtained failed to comply with the regulations of oil-producing countries.

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

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

Pathways and challenges for efficient solar-thermal desalination

Solar-thermal desalination (STD) is a potentially low-cost, sustainable approach for providing high-quality fresh water in the absence of water and energy infrastructures. Despite recent efforts to advance STD by improving heat-absorbing materials and system designs, the best strategies for maximizing STD performance remain uncertain. To address this problem, we identify three major steps in distillation-based STD: (i) light-to-heat energy conversion, (ii) thermal vapor generation, and (iii) conversion of vapor to water via condensation. Using specific water productivity as a quantitative metric for energy efficiency, we show that efficient recovery of the latent heat of condensation is critical for STD performance enhancement, because solar vapor generation has already been pushed toward its performance limit. We also demonstrate that STD cannot compete with photovoltaic reverse osmosis desalination in energy efficiency. We conclude by emphasizing the importance of factors other than energy efficiency, including cost, ease of maintenance, and applicability to hypersaline waters.

Adsorption-assisted transport of water vapour in super-hydrophobic membranes filled with multilayer graphene platelets

The effects of confinement of multilayer graphene platelets in hydrophobic microporous polymeric membranes are here examined. Intermolecular interactions between water vapour molecules and nanocomposite membranes are envisaged to originate assisted transport of water vapour in membrane distillation processes when a suitable filler-polymer ratio is reached. Mass transport coefficients are estimated under different working conditions, suggesting a strong dependence of the transport on molecular interactions. Remarkably, no thermal polarization is observed, although the filler exhibits ultrahigh thermal conductivity. In contrast, enhanced resistance to wetting as well as outstanding mechanical and chemical stability meets the basic requirements of water purification via membrane distillation. As a result, a significant improvement of the productivity-efficiency trade-off is achieved with respect to the pristine polymeric membrane when low amounts of platelets are confined in spherulitic-like PVDF networks.

One-step tailoring surface roughness and surface chemistry to prepare superhydrophobic polyvinylidene fluoride (PVDF) membranes for enhanced membrane distillation performances

Superhydrophobic polyvinylidene fluoride (PVDF) membrane is a promising material for membrane distillation. Existing approaches for preparing superhydrophobic PVDF membrane often involve separate manipulation of surface roughness and surface chemistry. Here we report a one-step approach to simultaneously manipulate both the surface roughness and surface chemistry of PVDF nanofibrous membranes for enhanced direct-contact membrane distillation (DCMD) performances. The manipulation was realized in a unique solvent-thermal treatment process, during which a treatment solution containing alcohols was involved. We demonstrate that by using different chain-length alcohols in the treatment solvent, surface roughness can be promoted by creating nanofin structures on the PVDF nanofibers using an alcohol which has moderate affinity with PVDF. Meanwhile, surface chemistry can be tuned by adjusting the fraction distribution of crystal phases (nonpolar α phase and polar β phase) in the membrane using different alcohols. PVDF membranes with different surface wettabilities were used to evaluate the effects of surface roughness and surface energy on the DCMD performances. Combining both low surface energy and multi-scale surface roughness, pentanol-treated PVDF membrane achieved best anti-water property (water contact angle of 164.1° and sliding angle of 8.1°), and exhibited superior water flux and enhanced anti-wetting ability to low-surface-tension feed in the DCMD application.

Antiwetting and Antifouling Janus Membrane for Desalination of Saline Oily Wastewater by Membrane Distillation

In this study, we develop Janus membranes comprising a hydrophilic zwitterionic polymer layer and an omniphobic (all-liquid-repelling) porous substrate that simultaneously possess fouling and wetting resistances. An omniphobic membrane was first fabricated by attaching silica nanoparticles (SiNPs) to the fibers of a quartz fiber mat, creating multilevel re-entrant structures, followed by surface fluorination to reduce the surface energy. The Janus membrane was then fabricated by grafting a zwitterionic polymer brush layer via surface-initiated atom-transfer radical-polymerization (ATRP) on the omniphobic substrate. Membrane characterizations, including Fourier-transform infrared spectroscopy, fluorescence microscopy, and contact angle measurements, confirm that the surface hydrophilicity can be finely tuned by adjusting the duration of the ATRP reaction. Also, the zwitterionic polymer brush layer was confined on the top surface of the Janus membrane, rendering the surface hydrophilic, while the remaining part of the Janus membrane remained omniphobic, resisting the wicking of low-surface-tension liquids including ethanol and hexane. A static oil-fouling test showed that crude oil droplets irreversibly fouled an omniphobic membrane (without a hydrophilic top layer) in water. In contrast, oil droplets placed on the Janus membrane in air were immediately desorbed upon its immersion in water. Finally, we performed direct-contact membrane distillation (MD) experiments using a crude-oil-in-saline (NaCl) water emulsion as a feed solution, simulating highly saline oily wastewater. The Janus membrane exhibited superior wetting and fouling resistances, with a stable water flux and nearly perfect salt rejection, while an omniphobic membrane failed in the desalination process. Our work highlights the great potential of antiwetting and antifouling Janus membranes for water reclamation from challenging industrial wastewaters through MD.

Pressure-driven and thermally-driven membrane operations for the treatment of arsenic-contaminated waters: A comparison

The presence of arsenic in water beyond the admitted limits is becoming an important concern for many Countries. Methods usually employed for the arsenic removal from water are based on coagulation followed by filtration, ion-exchange, adsorption. Drawbacks like the use of chemicals and the production of sludges (in case of coagulation) have, however, to be mentioned. Membrane operations, based on the features of membrane materials involved in the separation, do not need chemicals and are easy to scale-up, due to their modularity. In this contribution, the potential of membrane operations for the treatment of arsenic-polluted water is presented and discussed. In particular, two classes of membrane operations are illustrated and compared, the pressure-driven, like Nanofiltration (NF) and Reverse Osmosis (RO) and the thermally-driven ones, like Membrane Distillation (MD).

The Influence of Talc Addition on the Performance of Polypropylene Membranes Formed by TIPS Method

The effect of talc addition on the morphology of capillary membranes formed by a thermally induced phase separation (TIPS) method was investigated in the presented work. The usability of such formed membranes for membrane distillation was evaluated. Two types of commercial capillary polypropylene membranes, fabricated for microfiltration process, were applied in the studies. A linear arrangement of polymer chains was obtained in the walls of membranes formed without a talc addition. In the case of membranes blended with talc, the linear structure was disordered, and a more porous structure was obtained. The changes in morphology enhanced the mechanical properties of blended membranes, and their lower thermal degradation was observed during 350 h of membrane distillation studies. Long-term studies confirmed the stability of talc dispersion in the membrane matrix. A leaching of talc from polypropylene (PP) membranes was not found during the membrane distillation (MD) process.

Development of A Novel Corrugated Polyvinylidene difluoride Membrane via Improved Imprinting Technique for Membrane Distillation

Membrane distillation (MD) is an attractive technology for desalination, mainly because its performance that is almost independent of feed solute concentration as opposed to the reverse osmosis process. However, its widespread application is still limited by the low water flux, low wetting resistance and high scaling vulnerability. This study focuses on addressing those limitations by developing a novel corrugated polyvinylidene difluoride (PVDF) membrane via an improved imprinting technique for MD. Corrugations on the membrane surface are designed to offer an effective surface area and at the same time act as a turbulence promoter to induce hydrodynamic by reducing temperature polarization. Results show that imprinting of spacer could help to induce surface corrugation. Pore defect could be minimized by employing a dual layer membrane. In short term run experiment, the corrugated membrane shows a flux of 23.1 Lm⁻²h⁻¹ and a salt rejection of >99%, higher than the referenced flat membrane (flux of 18.0 Lm⁻²h⁻¹ and similar rejection). The flux advantage can be ascribed by the larger effective surface area of the membrane coupled with larger pore size. The flux advantage could be maintained in the long-term operation of 50 h at a value of 8.6 Lm⁻²h⁻¹. However, the flux performance slightly deteriorates over time mainly due to wetting and scaling. An attempt to overcome this limitation should be a focus of the future study, especially by exploring the role of cross-flow velocity in combination with the corrugated surface in inducing local mixing and enhancing system performance.

Electrospun Nanofiber Membranes Incorporating PDMS-Aerogel Superhydrophobic Coating with Enhanced Flux and Improved Antiwettability in Membrane Distillation

Electrospun nanofiber membranes (ENMs) have garnered increasing interest due to their controllable nanofiber structure and high void volume fraction properties in membrane distillation (MD). However, MD technology still faces limitations mainly due to low permeate flux and membrane wetting for feeds containing low surface tension compounds. Perfluorinated superhydrophobic membranes could be an alternative, but it has negative environmental impacts. Therefore, other low surface energy materials such as silica aerogel and polydimethylsiloxane (PDMS) have great relevancy in ENMs fabrication. Herein, we have reported the high flux and nonwettability of ENMs fabricated by electrospraying aerogel/polydimethylsiloxane (PDMS)/polyvinylidene fluoride (PVDF) over electrospinning polyvinylidene fluoride- co-hexafluoropropylene (PVDF-HFP) membrane (E-PH). Among various concentrations of aerogel, the 30% aerogel (E-M3-A30) dual layer membrane achieved highest superhydrophobicity (∼170° water contact angle), liquid entry pressure (LEP) of 129.5 ± 3.4 kPa, short water droplet bouncing performance (11.6 ms), low surface energy (4.18 ± 0.27 mN m^-1) and high surface roughness ( R_a: 5.04 μm) with re-entrant structure. It also demonstrated nonwetting MD performance over a continuous 7 days operation of saline water (3.5% of NaCl), high antiwetting with harsh saline water containing 0.5 mM sodium dodecyl sulfate (SDS, 28.9 mN m^-1), synthetic algal organic matter (AOM).

Performance Comparison between Polyvinylidene Fluoride and Polytetrafluoroethylene Hollow Fiber Membranes for Direct Contact Membrane Distillation

Increasing water demand coupled with projected climate change puts the Southwestern United States at the highest risk of water sustainability by 2050. Membrane distillation offers a unique opportunity to utilize the substantial, but largely untapped geothermal brackish groundwater for desalination to lessen the stress. Two types of hydrophobic, microporous hollow fiber membranes (HFMs), including polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), were evaluated for their effectiveness in direct contact membrane distillation (DCMD). Water flux and salt rejection were measured as a function of module packing density and length in lab-scale systems. The PVDF HFMs generally exhibited higher water flux than the PTFE HFMs possibly due to thinner membrane wall and higher porosity. As the packing density or module length increased, water flux declined. The water production rate per module, however, increased due to the larger membrane surface area. A pilot-scale DCMD system was deployed to the 2nd largest geothermally-heated greenhouse in the United States for field testing over a duration of about 22 days. The results demonstrated the robustness of the DCMD system in the face of environmental fluctuation at the facility.

Effective treatment of reverse osmosis concentrate from incineration leachate using direct contact membrane distillation coupled with a NaOH/PAM pre-treatment process

Reverse osmosis is frequently used to process biologically treated leachate from municipal solid waste incineration plants. Reverse osmosis concentrate from incineration leachate (ROCIL) contains extremely high concentrations of monovalent and divalent ions (e.g. Na⁺, K⁺, Mg²⁺, and Ca²⁺) and some refractory organic pollutants (e.g. humic substances). In this study, lab-scale direct contact membrane distillation (DCMD) coupled with pre-treatment was applied to treat ROCIL. NaOH and polyacrylamide (PAM) chemical precipitation and coagulation pretreatment effectively removed Ca²⁺ and Mg²⁺ (>99%) from the ROCIL, which also significantly improved the treatment efficiency of DCMD and slowed down membrane fouling caused by Mg₅(CO₃)₄(OH)₂·4H₂O and CaCO₃ scaling on the membrane surface. During the long-term operation of DCMD, ROCIL was concentrated 21 times and nearly all of the inorganic ions (>99.9%) and organic matter (>99%) were removed from the pre-treated ROCIL. A strong interaction occurred due to the accumulation of humic substances and metal ions in the feed solution, which lead to inorganic and organic scaling deposited on the membrane surface and pores, but the wetting phenomenon was not serious. These results demonstrated that DCMD coupled with NaOH/PAM pre-treatment can be a potential alternative for further treatment and concentration of ROCIL to obtain high quality water.

On-line solvent exchange system: Automation from extraction to analysis

Removal of organic solvent from sample extracts is required before analysis by reversed phase HPLC to preserve chromatographic performance and allow for bigger injection volumes, boosting sensitivity. Herein, an automated on-line extraction evaporation procedure is integrated with HPLC analysis. The evaporation occurs inside a 200 μm microfluidic channel confined by a vapor permeable membrane. A feedback control algorithm regulates evaporation rate keeping the output flow rate constant. The evaporation process across this membrane was firstly characterized with water/solvent mixtures showing organic solvent removal capabilities. This system allowed continuous methanol, ethanol and acetonitrile removal from samples containing up to 80% organic solvent. An evaporative injection procedure was developed demonstrating the use of the device for fully integrated extract reconstitution coupled to HPLC analysis, applied to analysis of the antibiotic chloramphenicol in milk samples. Sample reconstitution and collection was performed in less than 10 min and can be executed simultaneously to HPLC analysis of the previous sample in a routine workflow, thus having minimal impact on the total sample analysis time when run in a sequence.

Mechanical Vibration for the Control of Membrane Fouling in Direct Contact Membrane Distillation

One of the biggest challenges for direct contact membrane distillation (DCMD) in treating wastewater from flue gas desulfurization (FGD) is the rapid deterioration of membrane performance resulting from precipitate fouling. Chemical pretreatment, such as lime-soda ash softening, has been used to mitigate the issue, however, with significant operating costs. In this study, mechanical vibration of 42.5 Hz was applied to lab-scale DCMD systems to determine its effectiveness of fouling control for simulated FGD water. Liquid entry pressure and mass transfer limit of the fabricated hollow fiber membranes were determined and used as the operational constraints in the fouling experiments so that the observed membrane performance was influenced solely by precipitate fouling. Minimal improvement of water flux was observed when applying vibration after significant (~16%) water-flux decline. Initiating vibration at the onset of the experiments prior to the exposure of foulants, however, was promising for the reduction of membrane fouling. The water-flux decline rate was reduced by about 50% when compared to the rate observed without vibration. Increasing the module packing density from 16% to 50% resulted in a similar rate of water-flux decline, indicating that the fouling propensity was not increased with packing density in the presence of vibration.

Water and Wastewater Treatment Systems by Novel Integrated Membrane Distillation (MD)

The scarcity of freshwater has been recognized as one of the main challenges people must overcome in the 21st century. The adoption of an environmentally friendly, cost-effective, and energy-efficient membrane distillation (MD) process can mitigate the pollution caused by industrial and domestic wastes. MD is a thermally driven process based on vapor–liquid equilibrium, in which the separation process takes place throughout a microporous hydrophobic membrane. The present paper offers a comprehensive review of the state-of-the-art MD technology covering the MD applications in wastewater treatment. In addition, the important and sophisticated recent advances in MD technology from the perspectives of membrane characteristics and preparation, membrane configurations, membrane wetting, fouling, and renewable heat sources have been presented and discussed.

Capillary Polypropylene Membranes for Membrane Distillation

Only nonwetted porous membranes can be used in membrane distillation. The possibility of application in this process the capillary polypropylene membranes manufactured by thermally-induced phase separation was studied. The performance of a few types of membranes available commercially was presented. The resistance of the membranes to wetting was tested in the continuous process of water desalination. These studies were carried out for 1000 h without module cleaning. The presence of scaling layer on the membranes surface was confirmed by Scanning Electron Microscope observations. Both the permeate flux and distillate conductivity were almost not varied after the studied period of time, what indicates that the used membranes maintained their nonwettability, and the negative influence of scaling was limited. The role of surface porosity on the pore wetting and influence of membrane wettability on the quality of the distillate obtained were discussed.

Enhancement of energy utilization using nanofluid in solar powered membrane distillation

Nanofluids have excellent solar energy utilization efficiency due to the localized surface plasmon resonance phenomenon. In this study, photothermal nanofluids were employed as the feed solution for energy harvesting in solar powered membrane distillation. Ten different nanofluids were compared and TiN (titanium nitride) was chosen following UV-Vis-NIR-waveband (ultraviolet-visible-near-infrared) optical absorption analysis, zeta potential measurement, and membrane distillation flux testing. Desalination experiments were conducted using a range of TiN concentrations and solar radiation powers. The results showed that water flux and solar energy utilization efficiency increased with increasing TiN content. Compared to the base fluid (35 g/L NaCl aqueous solution), flux increased from 0.47 to 0.74 kg/(m²∙h), while energy utilization efficiency improved from 32.1% to 50.5% for 100 mg/L TiN nanofluid. Flux also increased with the increasing of solar radiation power markedly. With 5 kW/m² solar radiation power, the flux reached 2.77 kg/(m²∙h). Furthermore, the permeate water produced was of excellent quality contained less than 10 mg/L salinity when using 35 g/L NaCl feed solution. And no nanoparticles were detected transport through the membrane during the process. The nanofluid enhanced solar powered membrane distillation represents a promising perspective for better solar energy utilization.

Flexible Fire-Resistant Photothermal Paper Comprising Ultralong Hydroxyapatite Nanowires and Carbon Nanotubes for Solar Energy-Driven Water Purification

Efficient utilization of abundant solar energy for clean water generation is considered a sustainable and environment friendly approach to mitigate the global water crisis. For this purpose, this study reports a flexible fire-resistant photothermal paper by combining carbon nanotubes (CNTs) and fire-resistant inorganic paper based on ultralong hydroxyapatite nanowires (HNs) for efficient solar energy-driven water steam generation and water purification. Benefiting from the structural characteristics of the HN/CNT photothermal paper, the black CNT surface layer exhibits a high light absorbability and photothermal conversion capability, the HN-based inorganic paper acts as a thermal insulator with a high temperature stability, low thermal conductivity, and interconnected porous structure. By combining these advantages, high water evaporation efficiencies of 83.2% at 1 kW m^-2 and 92.8% at 10 kW m^-2 are achieved. In addition, the HN/CNT photothermal paper has a stable water evaporation capability during recycling and long-time usage. The promising potential of the HN/CNT photothermal paper for efficient production of drinkable water from both actual seawater and simulative wastewater samples containing heavy metal ions, dyes, and bacteria is also demonstrated. The highly flexible HN/CNT photothermal paper is promising for application in highly efficient solar energy-driven seawater desalination and wastewater purification.

Fluorographite-co-polydimethylsiloxane coated polyvinylidene-fluoride membrane for desalination of highly saline water with humic acid in direct contact membrane distillation

Membranes with amphiphobic properties are crucial for desalination of concentrated saline water with organic foulant by membrane distillation. In this research, we coated Fluorographite (FGi) particles on polyvinylidene-fluoride (PVDF) flat sheet membranes with 0.8% v/v polydimethylsiloxane (PDMS) as binder by filtration coating method. We evaluated its amphiphobicity via contact angles which were observed by DI water, canola oil and organic solutions as mimicking for oleophobicity. Surface modifications by FGi particles on the surface of membranes was introduced with PDMS binder which brought bifunctionality of amphiphobicity to enhance hydrophobicity and oleophobicity. We also investigated performance of coated membranes in direct contact membrane distillation (DCMD) with 1 molar concentrated NaCl solution along with humic acid being organic foulant and compared with virgin membrane (M1 membrane) and Liquid entry pressure (LEP) was also evaluated. LEP was measured by dynamic method which shown considerable improvement in coated membranes. M3 membrane showed overall better performance in terms of salt rejection higher than 99.87% and flux as compared to M1 membrane. Furthermore, the evaluation of surface characterizations was done by FESEM with EDX, AFM and FTIR. Surface morphology confirmed the coating of FGi on membrane surface while EDX evidenced increase in atomic percent of fluorine in turn F:C ratio.

Emerging Membrane Technologies for Water and Energy Sustainability: Future Prospects, Constrains and Challenges

The increasing demand for global energy consumption expedites major opportunities for the innovation of green energy technologies. Addressing the issue of sustainable energy is highly crucial for societies in order to maintain secure and balanced future progress in the economy and ecologically. Recently, there has been a growing interest in the development of improved and efficient sustainable energy technologies that are capable of reducing the global environmental footprint. The growing knowledge of hybrid techniques contributes to a decrease in the use of environmental resources while generating energy. However, various factors including the availability of natural resources, and different economic policies restrict the development of sustainable energies. Water and energy are the two major aspects for progressing towards a sustainable future. Recently, membrane-based technologies have begun to play an essential role in the advancement of sustainable energy and water demands. In this review article, the opportunities for membrane technologies dealing with water and energy sustainability have been analyzed.