Membrane Distillation Research Articles

Improved in-situ characterization for the scaling-induced wetting in membrane distillation: Unraveling the role of crystalline morphology

Despite being recognized as a promising technique for treating high salinity water, membrane distillation (MD) has been plagued by the scaling of sparingly soluble salts. The growth of crystals can not only create additional resistance to evaporating water at the feed-membrane interface, but also alter the hydrophobic network to bridge the feed and distillate (i.e., result in the phenomenon of wetting). When recognizing the uncertain behaviors of calcium sulfate (CaSO4) scaling in MD, this study was motivated to ascertain whether the crystal-membrane interactions could be dependent on the variation in crystalline morphology. In particular, optical coherence tomography (OCT) was employed to characterize the scaling-induced wetting via a direct-observation-through-the-membrane (DOTM) mode, which mitigated the effects of developing an external scaling layer on resolving the crystal-membrane interactions. The improved in-situ characterization suggests that the crystalline morphology of CaSO4 could be effectively regulated by varying the stoichiometry of crystallizing ions; the richness of calcium in the aqueous environment for crystallization would be in favor of weakening the crystal-membrane interactions. The stoichiometry-dependent growth of CaSO4 crystals can be exploited to develop an effective strategy for preventing the hydrophobic network from being wetted or irreversibly damaged.

Multi-hierarchical structured PTFE membrane for liquid desiccant dewatering via membrane distillation

Poly (tetrafluoroethylene) (PTFE) can be used for robust separation membranes owing to its exceptional chemical and thermal stabilities. Simultaneously achieving construction of hierarchical pore structures and re-entrant surface microstructures during the fabrication of PTFE membrane holds significant importance in addressing the issues of low flux and membrane wetting in membrane distillation (MD). Herein, we proposed a flexible method for manufacturing hierarchically structured PTFE membrane by combining electrospinning/spraying with sintering/welding. The present approach facilitated the controlled fabrication of a superhydrophobic PTFE membrane with slippery surface (exhibiting a water contact angle of 153.5°, and an ultralow sliding angle of 7.5°) without using fluoride-based solvents or external nanoparticles. Moreover, the anti-wetting mechanism and performance stability of the PTFE membranes in MD were thoroughly investigated. In liquid desiccant regeneration experiments, the 20 wt% LiCl solution was successfully concentrated to 28.64 wt%. Furthermore, the PTFE membrane demonstrated a stable flux (>30 kg m−2 h−1) and relatively low permeate conductivity (<15 μs cm−1) during treating simulated seawater for 30 h. The present work offers a new approach to fabricate multi-hierarchical structured superhydrophobic membranes for desalination and regeneration of liquid desiccants.

Enhancing flux rates and energy efficiency in membrane distillation through stearic acid-electrosprayed membranes

This study investigates the effects of stearic acid-electrosprayed coatings on commercial PTFE, PVDF, and PA membrane substrates for membrane distillation (MD). Drawing inspiration from prior research involving carbon nanotube modifications, stearic acid was selected for its ability to increase hydrophobicity and reduce heat transfer resistance. The stearic acid was electrosprayed from an ethanol solution onto the substrates, which were subsequently characterized by contact angle, liquid entry pressure, and thickness measurements. The results showed a notable flux increase of 131% for PTFE and 17% for PVDF, but a decrease of 56% for PA. A heat transfer model suggests that stearic acid improves flux by lowering heat transfer resistance in PTFE and PVDF substrates. This study highlights the potential of stearic acid coatings to enhance MD performance, providing valuable insights into improving flux and energy efficiency in MD applications.

MOF-based photothermal Janus membrane with robust anti-fouling performance for enhanced membrane distillation

As a promising desalination technology, photothermal membrane distillation (PMD) is confronted with challenges such as low freshwater yield and membrane fouling. Herein, we have introduced a copper-based metal-organic framework (Cu-CAT) into a hydrophilic/superhydrophobic polyvinylidene fluoride (PVDF) Janus tree-like nanofiber membrane (TLNMs) to construct photothermal Janus TLNMs (P-Janus TLNMs) for enhanced PMD. Given the exceptional photothermal properties of the hydrophilic Cu-CAT layer, coupled with the fine pore structure and elevated porosity characteristics of the superhydrophobic PVDF TLNMs, P-Janus TLNMs have demonstrated outstanding performance when integrated into a PMD system. Upon exposure to solar illumination of 1 kW·m−2, the P-Janus TLNMs achieved a remarkable surface temperature of 88 °C, yielding a permeation flux of 1.46 L m−2 h−1, a salt rejection rate exceeding 99.99 %, and an energy utilization rate of 81 %. Furthermore, after 20 days of continuous operation with simulated seawater containing oil and surfactant, the P-Janus TLNMs presented a salt rejection of 99.9 % and a stable permeation flux of 1.71 L m−2 h−1 at the feed/permeate temperatures of 24/20 °C. This work introduces a novel membrane with immense potential for boosting permeation flux and enhancing anti-fouling properties in PMD, thereby advancing the frontier of sustainable seawater desalination.

Multi-objective optimization of a solar-biomass multi-generation system with dual-stage desalination for brine minimization

A hybrid solar-biomass multi-generation system feeding an off-grid community is proposed in this study. An organic Rankine Cycle (ORC) generates electricity by harnessing the thermal energy supplied by the hybrid heat generation system. Part of the produced electricity is utilized to meet the community's electricity needs, while another fraction powers a brackish water reverse osmosis unit (RO). The waste heat from the ORC is recovered to produce domestic hot water and to drive a membrane distillation (MD) unit for RO brine minimization. A multi-objective optimization of the proposed system is conducted using the NSGA-II algorithm and TOPSIS decision-making tool considering plant's overall energy efficiency, exergy efficiency, and the total exergoeconomic cost of the useful products as objective functions. The study's results indicate that the overall optimal solution is achieved using R1336mzz(Z), with corresponding energy efficiency, exergy efficiency and hourly exergoeconomic cost of 45.31%, 5.39%, and 87.68 €/h, respectively. Moreover, the unitary costs associated with the useful products are 0.207 €/kWh, 0.029 €/kWh, 0.683 €/m3, and 3.36 €/m3 for the produced electricity, domestic hot water, RO permeate and MD distillate, respectively. In terms of sustainability, the system is not only renewable-driven but also achieves a 21% reduction in brine discharge through heat recovery at the ORC condenser compared to single-stage RO desalination. Additionally, using R1336mzz(Z) with its low global warming potential of 2 significantly lowers CO2 equivalent emissions compared to R245-fa. Lastly, sensitivity analysis indicates that hybridization ratios of up to 80% can result in a 10.6% reduction in CO2 emissions originating from biomass combustion compared to biomass-only solutions.

Membrane fouling in integrated forward osmosis and membrane distillation systems – A review

With the increase in water consumption, especially by the industrial and agricultural sectors, more strategies capable of increasing the recovery and reuse of water resources have been sought. An example of technology that assists in this challenge is membrane separation processes. However, systems that use this type of separation may be limited by the phenomenon of fouling. Integrated systems such as forward osmosis (FO) with membrane distillation (MD) have several advantages, such as obtaining a high-quality permeate from raw wastewater, the constant regeneration of the draw solution, which assists in continuous operation, and the possibility of association with biological tanks that enable nutrients recovery. However, these systems face limitations related to low flux values caused by membrane fouling. Therefore, the present review seeks to study fouling in integrated FO-MD systems to understand the main challenges of applying this technology and achieving higher permeate flux values. From the most recent studies, many strategies are being evaluated, such as different membrane materials, deposition of antifouling materials on membrane surface, feed temperature, draw solution substances, concentration and temperature, use of spacers and new modules configurations. That way, the low flux from those systems may be bypassed, enabling FO-MD systems scale up.

Engineering morphological architecture of superhydrophobic/hydrophilic composite membrane for efficient photothermal membrane distillation

The viability and efficacy of photothermal membrane distillation (PMD) is still uncertain due to its inherent energy-efficiency and throughput mass flux limitation. Herein, we develop a delamination-free multilayer photothermal membrane that simultaneously imparts slashed mass transfer resistance, enhanced photothermal effect and strong water-repellency by engineering morphological architecture. Using a one-step programmed dual-channel electrospinning followed by an electrostatic spraying technique, the proposed membrane is composited by a thick water-intrudable hydrophilic supporting layer, a thin hydrophobic layer, and an ultrathin Ti3C2Tx MXene-engineered superhydrophobic layer. It is proposed that the morphological architecture engineering could render mitigation of mass transfer resistance, firm water-repellency, and robust heat localization. Hence, in addition to superior flux of 1.27 L m−2 h−1 (inlet feed/permeate at 20/20 °C) and 15.89 L m−2 h−1 (inlet feed/permeate at 50/20 °C), prominent solar efficiency (76.34 % and 96.45 %) of the proposed composite membrane (DS15-M) also was achieved during PMD operation (1.0 kW m−2). Moreover, the DS15-M showcased not only robust wetting resistance and long-term consistency, but also obviously mitigated temperature polarization effect during the PMD operation. This research work emphasizes the important role of morphological architecture in rendering performance enhancement, which is a significant implication in engineering membrane architecture for PMD application.

Direct Contact Membrane Distillation of Artificial Urine for Sugar Beet Production in a Hydroponic System

Direct Contact Membrane Distillation of Artificial Urine for Sugar Beet Production in a Hydroponic System

Direct Contact Membrane Distillation of Artificial Urine and Its Application in Plasticizing Lunar Regolith

Direct Contact Membrane Distillation (DCMD) uses low heat sources to separate water from urea, which was then used as a plasticizer in regolith-based cement to make it more workable. The work investigated separating potable water and urea from artificial urine using DCMD and then characterizing the products. Water was successfully separated from the artificial urine solution as characterized by density, conductivity, pH, and substance concentrations. The concentrated urine solution was used in regolith-based cement cured under vacuum at temperatures that simulated temperatures that would be expected in construction on the Moon. Workability and other properties were improved by replacing water with concentrated urine solution in the mix.

Process intensification in the direct contact membrane distillation (DCMD) desalination by patterning membrane surface: A CFD study

In this research, the impact of pattern geometry on the intensification of the performance of surface patterned membranes for saline water desalination by direct contact membrane distillation (DCMD) is investigated by computational fluid dynamics (CFD) simulation. The Comsol Multiphysics software was applied to solve the governing transport equations for heat, momentum, and mass transfer. The result of the model was validated by the published experimental data, and the maximum deviation was less than 10%. The target was to maximize the permeate flux by altering surface pattern geometry and dimensions. Based on the previous studies, a prism pattern was chosen in this work, and the influences of pattern type (3 types), pattern dimension (25-150 µm valley depth), and the distance between the valleys (0-400 µm) were studied on the temperature polarization coefficient (TPC) and DCMD permeate flux. The results showed that the pattern with a valley depth of 25 µm and a distance between the valleys of 300 µm had the best performance in DCMD operation. In this situation and feed temperature of 80°C, a TPC of 0.78 and a water flux of 49.3 kg/m2.h were attained. The characteristics of flow close to the patterned membrane surface were also investigated, and it was observed that there are weak shear stresses in the lower zone of the valleys, while stronger shear stresses are created in the upper regions that are responsible for improving the TPC and water flux in the patterned membranes.

A numerical method for simulating variable density flows in membrane desalination systems

We present a novel method for simulating unsteady, variable density, fluid flows in membrane desalination systems. By assuming the density varies only with concentration and temperature, the scheme decouples the solution of the governing equations into two sequential blocks. The first solves the governing equations for the temperature and concentration fields, which are used to compute all thermophysical properties. The second block solves the conservation of mass and momentum equations for the velocity and pressure. We show that this is computationally more efficient than schemes that iterate over the full coupled equations in one block. We verify that the method achieves second-order spatial–temporal accuracy, and we use the method to investigate buoyancy-driven convection in a desalination process called vacuum membrane distillation. Specifically, we show that with gravity properly oriented, variations in temperature and concentration can trigger a double-diffusive instability that enhances mixing and improves water recovery. We also show that the instability can be strengthened by providing external heating.

Numerical simulations of conductive heating vacuum membrane distillation: Quantitative analysis of shunted heat flows and preventive strategy of salt crystallization

Numerical simulations of conductive heating vacuum membrane distillation: Quantitative analysis of shunted heat flows and preventive strategy of salt crystallization

Review on advanced techniques in zero liquid discharge water desalination via humidification-dehumidification within thermally-driven transport reactors

The article examines an advanced zero liquid discharge (ZLD) desalination method focusing on humidification-dehumidification (HDH) in thermally driven transport reactors. The limited availability of water, particularly in dry locations, has prompted the advancement of several desalination techniques. Reverse Osmosis (RO) is a prevalent membrane technology in the market; however, it has restrictions regarding brine disposal. ZLD technologies strive to mitigate environmental consequences by transforming saltwater into drinkable water and precious salts while minimizing waste. Membrane Distillation and Crystallization (MD-C) is notable as a highly efficient Zero Liquid Discharge (ZLD) technique. Despite its high energy consumption, it can combine MD-C with renewable or waste energy sources to improve sustainability. Freeze Desalination (FD) was reviewed for its economic efficiency, especially when utilizing cold energy from liquefied natural gas (LNG). Hybrid systems that combine Forward Osmosis (FO) and membrane distillation with condensation (MD-C) can potentially improve water recovery and decrease energy usage. The HDH desalination process imitates the natural precipitation process, examines its ability to operate at low temperatures, and highlights its potential for integration with renewable energy sources. The review discusses the HDH design, the effect of salinity on its performance, and the different dehumidification technologies. It emphasizes the significance of creative methods in achieving sustainable water management.

Modification of poly (vinylidene fluoride-co-hexafluoropropylene) membranes by sulfonated poly (ether ether keton) coating for membrane distillation of textile wastewater

The textile and related industries can produce a large amount of wastewater with organic dyes pollutants which can cause uncertain environmental impacts. In this study, the improved poly (vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) hollow fiber membranes were used in a continuous air gap membrane distillation (AGMD) operation for wastewater treatment of the Hamadan textile factory, Iran. The porous membranes were fabricated by a phase-inversion process and the inner surface was coated by a thin layer of sulfonated poly (ether ether keton) (SPEEK). After SPEEK coating, the water contact angle of the membrane decreased from about 93° to 57°. The SPEEK coated membrane presented mean pore size and overall porosity of 10 nm and 82.2%, respectively. For the wastewater treatment, the improved membrane presented the water flux and color rejection of 24.7 kg/m2 h and 99.8% during 72 h of the AGMD operation, respectively. Additionally, the treated wastewater reduced to the turbidity of ∼2 NTU, total dissolved solid (TDS) of 16 mg/L and chemical oxygen demand (COD) of 10 mg/L. The coated membrane presented a reasonable anti-fouling property with a flux recovery ratio (FRR) of 91.7% after 72 h of the operation. Hence, the improved PVDF-HFP membrane can be a potential alternative for the removal of organic dyes from wastewater via a membrane distillation process.

Green kaolin-tailored hydrophobic composite membrane for efficient desalination of high-salinity water

This study reports a novel approach to tune the surface and activate ceramic-polymeric based membranes from widely available and naturally occurring kaolin materials for the desalination of highly saline water via membrane distillation. Electrospraying was used to endow the membrane surface with a fine distribution of silica particles to build on roughness and enhance the grafting of long-chain (F21) fluoroalkyl silane (FAS). Interestingly, the adopted approach significantly transformed the surface properties from hydrophilic with water contact angle WCA ∼ 40° to superhydrophobic nature with WCA ∼ 160° along with significant increase in the liquid entry pressure. In air-gap membrane distillation with 70,000 ppm NaCl feed solution, the developed membranes exhibited a stable salt rejection (∼ 99.99 %) and a high water permeate flux of 9.3 kg/m2h. Notably, the membranes developed via the adopted approach outperformed direct FAS-modified kaolin membranes and other reported ceramic-based membranes. Hence, electrosprayed silica and subsequent FAS treatment approach on the membrane surface can effectively activate and make way for actual applications of the ceramic-based membranes for the desalination of highly saline water.

Development of Polyamide-Modified Membranes for Solar-Driven Seawater Desalination System

In response to the escalating global water crisis, this study introduces the development of polyamide-modified membranes (PA-PES, PA-PP, and PA-PTFE) through interfacial polymerization to enhance the efficiency of a passive solar desalination system. FTIR analysis and morphological characterization showed that a thin polyamide film formed above the modified membranes using m-phenylene diamine (MPD) and trimesoyl chloride (TMC). Notable improvements were observed in its productivity and distillate salinity by integrating these modified membranes into the membrane distiller of the system. Mainly, the PA-PES membrane achieved productivity of 764.56 ml/m2-h and reduced salinity to as low as 2 g NaCl/L. Despite challenges in salinity reduction, possibly due to residual chlorides, this study demonstrates the potential of polyamide-modified membranes in advancing solar-driven desalination, offering a promising solution to mitigate global water scarcity. This research paves the way for further advancements in sustainable desalination technology, emphasizing the need for continued optimization and exploration of membrane-based systems.

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

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

Pressurizing Distillate Membrane Distillation Assisted by Impedance-Based Liquid Entry Pressure Measurement for Effective Wetting Mitigation

Pressurizing Distillate Membrane Distillation Assisted by Impedance-Based Liquid Entry Pressure Measurement for Effective Wetting Mitigation

Insights into gypsum and silica scaling behaviors of dense Janus membranes in membrane distillation: The effect of varied surface properties

Dense Janus membranes (JMs) have emerged as potential candidates to address the tough scaling issue in hypersaline wastewater treatments for membrane distillation (MD). However, behaviors and mechanisms of various scalants on dense JMs have not been systematically investigated. Herein, we designed three dense JMs to simultaneously compare their anti-scaling properties against gypsum and silica in MD and focus on the critical role of varied surface property in scaling control. Results showed that dense JMs with highly positive gypsum-membrane interfacial energy and with strong electrical repulsion against silica species were more effective in resisting gypsum and silica, respectively. Scaled membrane characterization, quartz crystal microbalance monitoring, interfacial energy analysis and AFM force measurement revealed the distinct anti-scaling mechanisms. Highly hydrophilic surfaces without specific interactions with scaling ions provided robust thermodynamic barriers and less kinetically affected local supersaturations, preventing gypsum nucleation and growth. In contrast, positively charged surfaces favorably attracted negatively charged silica species with high reactivity, promoting subsequent polymerization with silicic acid that kinetically dominated silica scaling. Moreover, coexisting oversaturated gypsum and silica influenced the scaling behavior of each other, and surface properties of the preferentially formed scaled layer might also matter in scaling process. This study shall provide useful insights into the anti-scaling mechanisms of dense JMs in MD and highlight the potential of dense surface design as an effective scaling control strategy to pave the way for potential MD applications.

Flexible operation of nuclear hybrid energy systems for load following and water desalination

Nuclear hybrid energy systems (NHES) have the potential to provide dependable and emission-free electricity to the grid while also increasing the flexibility and reliability of the electrical grid. Molten salt reactor (MSR) technology can provide consistent, carbon-free electricity while also increasing efficiency, security, and sustainability and reducing nuclear waste. This study investigates the integration of Molten Salt Reactors (MSR) and conventional Pressurized Water Reactors (PWR) with desalination technologies: Direct Contact Membrane Distillation (DCMD), Multi-Stage Flash Distillation (MSFD), and Reverse Osmosis (RO). Dynamic first-principles models were developed and tested using real grid data from the New York Independent System Operator. The results demonstrate that nuclear power is capable of flexibly responding to changing grid demand while simultaneously producing clean water, particularly during periods of low electricity demand. The MSR-RO system was found to be the most efficient in electricity generation and water production, and all hybrid systems reduced CO2 emissions by 356,000 to 682,000 tons annually. Economic analysis reveals that nuclear desalination technologies are cost-competitive with conventional systems, especially when paired with RO. These findings confirm the technical feasibility and environmental benefits of nuclear hybrid systems for sustainable electricity and water production.