Understanding the impact of membrane properties and transport phenomena on the energetic performance of membrane distillation desalination

Enhanced distillate production of stepped solar still via integration with multi-stage membrane distillation

Arid and coastal regions, such as Egypt, face significant challenges in meeting potable water demands driven by population growth and sustainable development. Energy-efficient desalination technologies are urgently needed to reduce reliance on conventional energy sources. This study introduces a novel integration of a thermal membrane with a stepped solar still, leveraging basin heat to drive membrane distillation (MD). The system employs multiple membrane layers and water-vapor absorbent wicks to capture vapor and transfer latent heat, enabling cascading preheating of brine layers. A validated mathematical model assessed the system's performance under varying conditions, accommodating up to ten stages. Results reveal that increasing MD stages significantly enhances productivity, achieving a peak daily output of 47 L/m2 day-five times that of a passive multi-basin solar still. However, productivity gains plateau beyond three stages, indicating an optimal balance between complexity and efficiency. The system's performance is affected by ambient and seasonal conditions, including wind speed and temperature fluctuations observed during trials in June, May, and December, requiring additional modifications for stabilization. This study demonstrates the potential of integrating MD with solar stills as a scalable, energy-efficient desalination solution for arid regions. Future research should focus on optimizing stage configurations and evaluating economic feasibility for large-scale applications.

Comparison of Different Polymeric Membranes in Direct Contact Membrane Distillation and Air Gap Membrane Distillation Configurations

Membrane distillation (MD) is an evolving thermal separation technique most frequently aimed at water desalination, compatible with low-grade heat sources such as waste heat from thermal engines, solar collectors, and high-concentration photovoltaic panels. This study presents a comprehensive theoretical-experimental evaluation of three commercial membranes of different materials (PE, PVDF, and PTFE), tested for two distinct MD modules-a Direct Contact Membrane Distillation (DCMD) module and an Air Gap Membrane Distillation (AGMD) module-analyzing the impact of key operational parameters on the performance of the individual membranes in each configuration. The results showed that increasing the feed saline concentration from 7 g/L to 70 g/L led to distillate flux reductions of 12.2% in the DCMD module and 42.9% in the AGMD one, averaged over the whole set of experiments. The increase in feed temperature from 65 °C to 85 °C resulted in distillate fluxes up to 2.36 times higher in the DCMD module and 2.70 times higher in the AGMD one. The PE-made membrane demonstrated the highest distillate fluxes, while the PVDF and PTFE membranes exhibited superior performance under high-salinity conditions in the AGMD module. Membranes with high contact angles, such as PTFE with 143.4°, performed better under high salinity conditions. Variations in operational parameters, such as flow rate and temperature, markedly affect the temperature and concentration polarization effects. The analyses underscored the necessity of a careful selection of membrane type for each distillation configuration by the specific characteristics of the process and its operational conditions. In addition to experimental findings, the proposed heat and mass transfer-reduced model showed good agreement with experimental data, with deviations within ±15%, effectively capturing the influence of operational parameters. Theoretical predictions showed good agreement with experimental data, confirming the model's validity, which can be applied to optimization methodologies to improve the membrane distillation process.

Solar-Driven Thin Air Gap Membrane Distillation with a Slippery Condensing Surface

Membrane-based desalination is essential for mitigating global water scarcity; yet, the process is energy-intensive and heavily reliant on fossil fuels, resulting in substantial carbon emissions. To address the challenges of treating seawater, produced water, brackish groundwater, and wastewater, we have developed a thin air gap membrane distillation (AGMD) system featuring a novel slippery condensing surface. The quasi-liquid slippery surface facilitates efficient condensate water droplet removal, allowing for the implementation of a 1 mm thin air gap. This advancement has led to a 2-fold increase in permeate flux without lowering the thermal efficiency while preventing permeate flooding. Furthermore, the thin AGMD system, employing a cost-effective zirconium nitride/poly(vinylidene fluoride) (ZrN-PVDF) composite membrane, has been demonstrated for solar-driven desalination. Experimental results indicate that reducing the air gap from 2 to 1 mm enhances the permeate flux by 150%.

Investigating Permselectivity in PVDF Mixed Matrix Membranes Using Experimental Optimization, Machine Learning Segmentation, and Statistical Forecasting

This research examines the correlation between interfacial characteristics and membrane distillation (MD) performance of copper oxide (Cu) nanoparticle-decorated electrospun carbon nanofibers (CNFs) polyvinylidene fluoride (PVDF) mixed matrix membranes. The membranes were fabricated by a bottom-up phase inversion method to incorporate a range of concentrations of CNF and Cu + CNF particles in the polymer matrix to tune the porosity, crystallinity, and wettability of the membranes. The resultant membranes were tested for their application in desalination by comparing the water vapor transport and salt rejection rates in the presence of Cu and CNF. Our results demonstrated a 64% increase in water vapor flux and a salt rejection rate of over 99.8% with just 1 wt % loading of Cu + CNF in the PVDF matrix. This was attributed to enhanced chemical heterogeneity, porosity, hydrophobicity, and crystallinity that was confirmed by electron microscopy, tensiometry, and scattering techniques. A machine learning segmentation model was trained on electron microscopy images to obtain the spatial distribution of pores in the membrane. An Autoregressive Integrated Moving Average with Explanatory Variable (ARIMAX) statistical time series model was trained on MD experimental data obtained for various membranes to forecast the membrane performance over an extended duration.

Membrane Design Criteria and Practical Viability of Pressure-Driven Distillation

Pressure-driven distillation (PD) is a novel desalination technology based on hydraulic pressure driving force and vapor transport across a hydrophobic porous membrane. In theory, PD offers near-perfect rejection for nonvolatile solutes, chlorine resistance, and the ability to decouple water and solute transport. Despite its advantages, pore wetting and the development of a reverse transmembrane temperature difference are potential critical concerns in PD, with the former compromising the salt rejection and the latter reducing or even eliminating the driving force for vapor transport. We herein present an analysis to evaluate the practical viability and membrane design principles of PD with a focus on the dependence of flux and salt rejection (SR) on membrane properties. By modeling the mass transfer in a PD process under different conditions, we arrive at two important conclusions. First, a practically detrimental reverse transmembrane temperature difference does not develop in PD under all relevant circumstances and is thus not a practical concern. Second, for a PD process to achieve an acceptable SR, the membrane pores should be at the nanometer scale with a highly uniform pore size distribution. This analysis demonstrates the practical viability of PD and provides the principles for designing robust and high-performance PD membranes.

Toward a universal framework for evaluating transport resistances and driving forces in membrane-based desalination processes

Desalination technologies using salt-rejecting membranes are a highly efficient tool to provide fresh water and augment existing water supplies. In recent years, numerous studies have worked to advance a variety of membrane processes with different membrane types and driving forces, but direct quantitative comparisons of these different technologies have led to confusing and contradictory conclusions in the literature. In this Review, we critically assess different membrane-based desalination technologies and provide a universal framework for comparing various driving forces and membrane types. To accomplish this, we first quantify the thermodynamic driving forces resulting from pressure, concentration, and temperature gradients. We then examine the resistances experienced by water molecules as they traverse liquid- and air-filled membranes. Last, we quantify water fluxes in each process for differing desalination scenarios. We conclude by synthesizing results from the literature and our quantitative analyses to compare desalination processes, identifying specific scenarios where each process has fundamental advantages.

Silver sulfide decorated carbonaceous sawdust/ES-PANI composites as salt-resistant solar steam generator

Solar steam generation (SSG) is a potential approach for resolving the global water and energy crisis while causing the least amount of environmental damage. However, using adaptable photothermal absorbers with salt resistance through a simple, scalable, and cost-effective production approach is difficult. Herein, taking advantage of the ultra-fast water transportation in capillaries, and the large seawater storage capacity of wood, we develop a highly efficient natural evaporator. The wood wastes (sawdust) were carbonized at low temperatures to fabricate a green and low-cost carbonaceous porous material (CW). To enhance the salt resistance in high saline water, this evaporator was coated with polyaniline emeraldine salt (ES-PANI) which was synthesized through facile and cost-effective one-step oxidation of aniline. Furthermore, the composite was decorated with silver sulfide to increase the evaporation rate which reached up to 1.1 kg m⁻² h⁻¹ under 1 sun irradiation with 91.5% efficiency. Besides, the evaporator performs exceptionally well over 10 cycles due to the salt resistance capability of ES-PANI which generates a “Donnan exclusion” effect against cations in saline water. The Ag₂S@PANI/CW evaporator may be a viable large-scale generator of drinking water due to its high efficiency for energy conversion, simple and low-cost fabrication approach, salt-resistance, and durability.

Solar steam generation (SSG) is a potential approach for resolving the global water and energy crisis while causing the least amount of environmental damage.

Desalinating a real hyper-saline pre-treated produced water via direct-heat vacuum membrane distillation

Membrane distillation (MD) is an emerging thermal desalination technology capable of desalinating waters of any salinity. During typical MD processes, the saline feedwater is heated and acts as the thermal energy carrier; however, temperature polarization (as well as thermal energy loss) contributes to low distillate fluxes, low single-pass water recovery and poor thermal efficiency. An alternative approach is to integrate an extra thermal energy carrier as part of the membrane and/or module assembly, which can channel externally provided heat directly to the membrane-feedwater interface and/or along the feed channel length. This direct-heat delivery has been demonstrated to increase single-pass water recovery and enhance the overall thermal efficiency. We developed a bench-scale direct-heated vacuum MD (DHVMD) process to desalinate pre-treated oil and gas "produced water" with an initial total dissolved solids of 115,500 ppm at a feed temperature ranging between 24 and 32 °C. We evaluated both water flux and specific energy consumption (SEC) as a function of water recovery. The system achieved a 50% water recovery without significant scaling, with an average flux >6 kg m^-2 hr^-1 and a SEC as low as 2,530 kJ kg^-1. The major species of mineral scales (i.e., NaCl, CaSO₄, and SrSO₄) that limited the water recovery to 68% were modeled in terms of thermodynamics and identified by scanning electron microscopy and energy-dispersive X-ray spectroscopy. In addition, we further developed and employed a physics-based process model to estimate temperature, salinity, water transport and energy flows for full-scale vacuum MD and DHVMD modules. Model results show that a direct-heat input rate of 3,600 W can increase single-pass water recovery from 2.1% to 3.1% while lowering the thermal SEC from 7,800 kJ kg^-1 to 6,517 kJ kg^-1 in an unoptimized module. Finally, the scaling up potential of DHVMD process is briefly discussed.

Towards sustainable circular brine reclamation using seawater reverse osmosis, membrane distillation and forward osmosis hybrids: An experimental investigation

Desalination and wastewater treatment technologies require an effective solution for brine management to ensure environmental sustainability, which is closely linked with efficient process operations, reduction of chemical dosages, and valorization of brines. Within the scope of desalination brine reclamation, a circular system consisting of seawater reverse osmosis (SWRO), membrane distillation (MD), and forward osmosis (FO) three-process hybrid is investigated. The proposed design increases water recovery from SWRO brine (by MD) and dilutes concentrated brine to seawater level (by FO) for SWRO feed. It ultimately reduces SWRO process brine disposal and improves crystallization efficiency for a zero-liquid discharge application. The operating range of the hybrid system is indicated by a seawater volumetric concentration factor (VCF) ranging from 1.0 to 2.2, which covers practical and sustainable operation in full-scale applications. Within the proposed VCF range, different operating conditions of the MD and FO processes were evaluated in series with concentrated seawater as well as real SWRO brine from a full-scale desalination plant. Water quality and membrane surface were analyzed before and after experiments to assess the impact of the SWRO brine. Despite their low concentration (0.13 mg/L as phosphorous), antiscalants present in SWRO brine alleviated the flux decline in MD operations by 68.3% compared to operations using seawater concentrate, while no significant influence was observed on the FO process. A full spectrum of water quality analysis of real SWRO brine and Red Sea water is made available for future SWRO brine reclamation studies. The operating conditions and experimental results have shown the potential of the SWRO-MD-FO hybrid system for a circular brine reclamation.

Charge-Gradient Hydrogels Enable Direct Zero Liquid Discharge for Hypersaline Wastewater Management

Zero liquid discharge (ZLD), which maximizes water recovery and eliminates environmental impact, is an urgent wastewater management strategy for alleviating freshwater shortage. However, because of the high concentration of salts and broad-spectrum foulants in wastewater, a huge challenge for ZLD is lack of a robust membrane-based desalination technology that enables direct wastewater recovery without costly pretreatment processes. Here, a paradigm-shift membrane distillation (MD) strategy is presented, wherein the traditional hydrophobic porous membrane is replaced with a hydrophilic nonporous charge-gradient hydrogel (CGH) membrane that possesses hypersaline tolerance, fouling/scaling-free properties, and negligible vapor transfer resistance inside the membrane, simultaneously. Therefore, the CGH-based MD with high water flux enables direct desalination of hypersaline wastewater (130 g L^-1 ) containing broad-spectrum foulants (500 mg L^-1 ) during continuous long-term operation (200 h), and this technology paves a promising way to direct ZLD for wastewater management.

Stack Thermo-Osmotic System for Low-Grade Thermal Energy Conversion

Thermo-osmotic energy conversion (TOEC) technology, developed from membrane distillation, is an emerging method that has the potential of obtaining electricity efficiently from a low-grade heat source but faces the difficult problem of pump power loss. In this study, we build a novel TOEC system with a multistage architecture that can work without pump assistance. The experiment system, made of cheap commercial materials, can obtain a power density of 1.39 ± 0.25 W/m², with a heating temperature of 80 °C, and its efficiency increased linearly with the total stage number. A theory calculation shows that a 30-stage system with a specific membrane and a working pressure of 5.0 MPa can obtain an efficiency of 2.72% with a power density of 14.0 W/m². By a molecular dynamics simulation, it is shown that a high-performance membrane has the potential to work at 40 MPa. This study proves that TOEC technology is a practical and competitive approach to covert low-grade thermal energy into power efficiently.

Feasibility of membrane distillation process for potable water reuse: A barrier for dissolved organic matters and pharmaceuticals

In this study, the feasibility of the membrane distillation (MD) process as a wastewater reclamation system for portable reuse was investigated. The flux was stably maintained at about 20 L/m²h (LMH) at ΔT 30 °C, compared to higher flux at ΔT 50 °C, which showed a rapid decrease in the flux due to severe fouling. MD produced excellent quality of potable water satisfied the drinking water standards of Korea from effluent of sewage treatment plant (ESTP). The fractions of the hydrophobic OC (HOC) and chromatographic DOC (CDOC) from LC-OCD analysis was firstly suggested to understand different organic transport during the MD process. The transport of organic matters across the MD membrane mitigated at low operation temperature and the transported organics in all the tested waters were mostly volatile low molecular weight organics, aromatic amino acids. All of thirteen selected pharmaceuticals were completely removed by MD, regardless of their properties. In order to retard the membrane fouling of the MD process, coagulation and filtration pre-treatments were applied. The pre-treatment process coupled MD process could successfully remove impurities including NH₄-N without severe membrane fouling. Moreover, coagulation pretreatment reduced transport of ammonia due to decrease in pH.

Multifunctional nanocoated membranes for high-rate electrothermal desalination of hypersaline waters

Surface heating membrane distillation overcomes several limitations inherent in conventional membrane distillation technology. Here we report a successful effort to grow in situ a hexagonal boron nitride (hBN) nanocoating on a stainless-steel wire cloth (hBN-SSWC), and its application as a scalable electrothermal heating material in surface heating membrane distillation. The novel hBN-SSWC provides superior vapour permeability, thermal conductivity, electrical insulation and anticorrosion properties, all of which are critical for the long-term surface heating membrane distillation performance, particularly with hypersaline solutions. By simply attaching hBN-SSWC to a commercial membrane and providing power with an a.c. supply at household frequency, we demonstrate that hBN-SSWC is able to support an ultrahigh power intensity (50 kW m^-2) to desalinate hypersaline solutions with exceptionally high water flux (and throughput), single-pass water recovery and heat utilization efficiency while maintaining excellent material stability. We also demonstrate the exceptional performance of hBN-SSWC in a scalable and compact spiral-wound electrothermal membrane distillation module.

Simulation Study on Direct Contact Membrane Distillation Modules for High-Concentration NaCl Solution

Membrane distillation technology, as a new membrane-based water treatment technology that combines the membrane technology and evaporation process, has the advantages of using low-grade heat, working at atmospheric pressure with simple configuration, etc. In this study, heat and mass transfer were coupled at the membrane surfaces through the user-defined function program. The effects of feed temperature, feed velocity and permeate velocity on temperature polarization were mainly investigated for a high-concentration NaCl solution. The temperature polarization was increased with the increase of feed temperature and the decrease of feed and permeate velocity. The effects of temperature, inlet velocity and solution concentration on the evaporation efficiency of the membrane module for co- and counter-current operations were investigated in detail. The counter-current operation performed better than co-current operation in most cases, except for the condition where the NaCl concentration was relatively low or the module length was long enough. In addition, the optimal membrane thickness for both PVDF and PTFE was studied. The optimal membrane thickness was found in the range of 10 to 20 μm, which corresponded to the highest permeate flux for the selected materials, pore size distribution, and operation conditions. Membrane material with lower thermal conductivity and larger porosity was prone to get higher permeate flux and had larger optimal membrane thickness. Increasing feed velocity or feed temperature could decrease the optimal membrane thickness.

Minimum Net Driving Temperature Concept for Membrane Distillation

In this study, we analyzed the heat requirement of membrane distillation (MD) to investigate the trade-off between the evaporation efficiency and driving force efficiency in a single effect MD system. We found that there exists a non-zero net driving temperature difference that maximizes efficiency. This is the minimum net driving temperature difference necessary for a rational operational strategy because below the minimum net driving temperature, both the productivity and efficiency can be increased by increasing the temperature difference. The minimum net driving temperature has a similar magnitude to the boiling point elevation (~0.5 °C for seawater), and depends on the properties of the membrane and the heat exchanger. The minimum net driving temperature difference concept can be used to understand the occurrence of optimal values of other parameters, such as flux, membrane thickness, and membrane length, if these parameters are varied in a way that consequently varies the net driving temperature difference.

Membrane Technologies in Wastewater Treatment: A Review

In the face of water shortages, the world seeks to explore all available options in reducing the over exploitation of limited freshwater resources. One of the surest available water resources is wastewater. As the population grows, industrial, agricultural, and domestic activities increase accordingly in order to cater for the voluminous needs of man. These activities produce large volumes of wastewater from which water can be reclaimed to serve many purposes. Over the years, conventional wastewater treatment processes have succeeded to some extent in treating effluents for discharge purposes. However, improvements in wastewater treatment processes are necessary in order to make treated wastewater re-usable for industrial, agricultural, and domestic purposes. Membrane technology has emerged as a favorite choice for reclaiming water from different wastewater streams for re-use. This review looks at the trending membrane technologies in wastewater treatment, their advantages and disadvantages. It also discusses membrane fouling, membrane cleaning, and membrane modules. Finally, recommendations for future research pertaining to the application of membrane technology in wastewater treatment are made.

Multistage and passive cooling process driven by salinity difference

Space cooling in buildings is anticipated to rise because of an increasing thermal comfort demand worldwide, and this calls for cost-effective and sustainable cooling technologies. We present a proof-of-concept multistage device, where a net cooling capacity and a temperature difference are demonstrated as long as two water solutions at disparate salinity are maintained. Each stage is made of two hydrophilic layers separated by a hydrophobic membrane. An imbalance in water activity in the two layers naturally causes a non-isothermal vapor flux across the membrane without requiring any mechanical ancillaries. One prototype of the device developed a specific cooling capacity of up to 170 W m^-2 at a vanishing temperature difference, considering a 3.1 mol/kg calcium chloride solution. To provide perspective, if successfully up-scaled, this concept may help satisfy at least partially the cooling needs in hot, humid regions with naturally available salinity gradients.

Electrothermally Driven Membrane Distillation for Low-Energy Consumption and Wetting Mitigation

Membrane distillation (MD) is a promising alternative approach for desalination, especially for high-salinity brines. Its application has been limited by its high operational cost because of the energy consumption required for hydraulic circulation and heating the entire circulating feed. Localized heating of the feed by Joule heating diminishes energy consumption, but the potential charging on the electrothermal material surface causes water splitting and membrane degradation in high-salinity environments. Herein, a novel reverse Joule-heating air gap MD method was designed in which an electrothermal material was placed at the air gap, isolating itself from saline water. Even though the Joule-heating layer was at the air gap side, 90.56% of heat flowed into the saline water for heating the feed. The opposite temperature gradient in the membrane matrix as opposed to conventional MD-mitigated membrane wetting was caused by capillary condensation. This novel electrothermal-driven MD configuration is worthy to be introduced into applications.

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.

Relating Organic Fouling in Membrane Distillation to Intermolecular Adhesion Forces and Interfacial Surface Energies

This study investigates the fouling mechanisms in membrane distillation, focusing on the impact of foulant type and membrane surface chemistry. Interaction forces between a surface-functionalized particle probe simulating a range of organic foulants and model surfaces, modified with different surface energy materials, were measured by atomic force microscopy. The measured interaction forces were compared to those calculated based on the experimentally determined surface energy components of the particle probe, model surface, and medium (i.e., water). Surfaces with low interfacial energy exhibited high attractive interaction forces with organic foulants, implying a higher fouling potential. In contrast, hydrophilic surfaces (i.e., surfaces with high interfacial energy) showed the lowest attractive forces with all types of foulants. We further performed fouling experiments with alginate, humic acid, and mineral oil in direct contact membrane distillation using polyvinylidene fluoride membranes modified with various materials to control membrane surface energy. The observed fouling behavior was compared to the interaction force data to better understand the underlying fouling mechanisms. A remarkable correlation was obtained between the evaluated interaction force data and the fouling behavior of the membranes with different surface energy. Membranes with low surface energy were fouled by hydrophobic, low surface tension foulants via "attractive" and subsequent "adsorptive" interaction mechanisms. Furthermore, such membranes have a higher fouling potential than membranes with high or ultralow surface energy.

Membrane distillation for industrial wastewater treatment: Studying the effects of membrane parameters on the wetting performance

Substantial amounts of trace hazardous elements have been detected in industrial wastewater (e.g fluoride > 900 mg/L). Feed water characteristics, operational parameters, and membrane properties are major factors affecting flux and rejection of the MD process. Membrane parameters such as membrane material type and pore size have been investigated. Fluoride ion rejection was selected to setup a methodology to remove trace elements from wastewater by adjusting the membrane parameters in DCMD. Study of the fouling thickness of the MD membrane using pH and feed water composition revealed that a PVDF membrane with a smooth surface holds a thicker fouling layer, which enhances fluoride rejection while reducing the permeate flux. On the other hand, PTFE and PP membranes showed higher mass transfer and higher wetting performance, respectively. Therefore,a PVDF membrane with low organic feed water at higher alkaline pH can be utilized to obtain high-quality permeate, while PTFE can provide the highest flux with acceptable permeate water quality. Therefore, this methodology can be applied toidentify the optimum membrane to fit the required permeate flux, rejection requirements,and operating pH to treat any kind of non-volatileinorganic pollutants from industrial wastewater.

Energy Efficiency and Performance Limiting Effects in Thermo-Osmotic Energy Conversion from Low-Grade Heat

Low-grade heat energy from sources below 100 °C is available in massive quantities around the world, but cannot be converted to electricity effectively using existing technologies due to variability in the heat output and the small temperature difference between the source and environment. The recently developed thermo-osmotic energy conversion (TOEC) process has the potential to harvest energy from low-grade heat sources by using a temperature difference to create a pressurized liquid flux across a membrane, which can be converted to mechanical work via a turbine. In this study, we perform the first analysis of energy efficiency and the expected performance of the TOEC technology, focusing on systems utilizing hydrophobic porous vapor-gap membranes and water as a working fluid. We begin by developing a framework to analyze realistic mass and heat transport in the process, probing the impact of various membrane parameters and system operating conditions. Our analysis reveals that an optimized system can achieve heat-to-electricity energy conversion efficiencies up to 4.1% (34% of the Carnot efficiency) with hot and cold working temperatures of 60 and 20 °C, respectively, and an operating pressure of 5 MPa (50 bar). Lower energy efficiencies, however, will occur in systems operating with high power densities (>5 W/m²) and with finite-sized heat exchangers. We identify that the most important membrane properties for achieving high performance are an asymmetric pore structure, high pressure resistance, a high porosity, and a thickness of 30 to 100 μm. We also quantify the benefits in performance from utilizing deaerated water streams, strong hydrodynamic mixing in the membrane module, and high heat exchanger efficiencies. Overall, our study demonstrates the promise of full-scale TOEC systems to extract energy from low-grade heat and identifies key factors for performance optimization moving forward.