Search Results (107)

This study experimentally investigates a novel hybrid system integrating thermoelectric generators (TEGs) with direct contact membrane distillation (DCMD) for simultaneous low-grade heat recovery, electricity generation, and water desalination. Commercial TEG modules were sandwiched between heat spreaders to transfer thermal energy from a source (approx. 140 °C) to a cooling sink, driving saline water evaporation through a hydrophobic membrane. A validated mathematical model showed strong agreement with the experimental results. The system achieved freshwater mass fluxes of 8–9.5 kg/m²/h and electrical power outputs density of 25–35 W/m². Increasing heat input (450–700 W) significantly enhanced freshwater production and electrical output, improving the Gain Output Ratio (GOR) and reducing Specific Energy Consumption (SEC). While higher feed salinity (up to 35,000 ppm) measurably declined mass flux and thermal efficiency, thermoelectric generation and thermal resistance remained largely unaffected. Energy and exergy efficiencies showed moderate sensitivity to operating conditions, while the Water–Electrical Energy Cogeneration Index (WEeCI) increased at high salinity, highlighting the robust contribution of electricity generation. These results demonstrate the potential of the TEG–DCMD system for the sustainable co-generation of water and power from industrial waste heat or renewable thermal sources. Full article

The present review summarizes a vast body of literature on the subject of Membrane Distillation (MD), with a special emphasis on the existing results and correlations for the transmembrane transport of heat and mass. The issue of saltwater physical properties is also discussed in depth, whereas the advective transport of heat and salt concentration in the feed and permeate compartments is only briefly mentioned but is beyond the scope of this review. The paper does not aim to provide a complete treatment of the subject of MD, which can be found in other publications. Rather, it suggests the data and correlations most suitable for the range of operating conditions typically expected in actual units implementing Direct Contact Membrane Distillation (DCMD), including hollow fiber designs, with a view to assist model development. The focus is on MD for water desalination, although some results may apply well to other fields. Full article

A new DCMD module design that introduces an insulation barrier of negligible thickness to divide the open duct of the hot-saline feed into two subchannels for dual-flow operation was investigated. This configuration enables one subchannel to operate in a cocurrent-flow mode and the other in a countercurrent-flow recycling mode, thereby significantly enhancing the permeate flux. Theoretical and experimental investigations were conducted to develop modeling equations capable of predicting the permeate flux in DCMD modules. These studies demonstrated the technical feasibility of minimizing temperature polarization effects while improving flow characteristics to boost permeate flux. Results indicated that increasing both convective heat-transfer coefficients and residence time generally improved device performance. The dual-flow operation increased fluid velocity and extended residence time, leading to reduced heat-transfer resistance and enhanced heat-transfer efficiency. Theoretical predictions and experimental results consistently showed that the absorption flux improved by up to 40.77% under the double-flow operation with internal recycling configuration compared to a single-pass device of identical dimensions. The effects of inserting the insulation barrier on permeate flux enhancement, power consumption, and overall economic feasibility were also discussed. Full article

Membrane distillation (MD) is a promising technology for reducing the volume of high-salinity brines generated from desalination plants, yet limited knowledge exists regarding its fouling behavior under long-term operation. In this study, fouling was investigated through the autopsy of a hollow fiber MD module operated for 120 days in a direct contact membrane distillation (DCMD) configuration using real desalination brine. Despite stable salt rejection exceeding 99%, a gradual decline in flux and permeability was observed, indicating progressive fouling and partial wetting. Post-operation analyses, including SEM, EDS, ICP-OES, and FT-IR, revealed that the dominant foulants were inorganic scales, particularly calcium carbonate (CaCO₃), with minor contributions from suspended particles (SiO₂, Fe) and organic matter. Fouling was more severe in the inlet and inner regions of the module due to intensified temperature and concentration polarization, which promoted supersaturation and scale deposition. These combined effects led to a reduction in membrane hydrophobicity and liquid entry pressure, ultimately accelerating partial wetting and performance deterioration. The findings provide valuable insights into the spatial fouling behavior and mechanisms in MD systems, highlighting the importance of hydrodynamic optimization and fouling mitigation strategies for long-term brine concentration applications. Full article

Ammonia recovery from synthetic thermophilic anaerobic digestate was achieved through Direct Contact Membrane Distillation (DCMD) using hydrophobic flat-sheet membranes under different operating conditions. The influence of temperature gradients (0 °C, 20 °C, 35 °C, and 45 °C) and pH levels of the thermophilic anaerobic sludge (7.8, 8.2, 9, and 12) was investigated. The process utilized a DCMD setup with hydrophobic PTFE membranes of 0.22 µm nominal pore radius, and receiving solutions consisting of deionized water and 1 M H₂SO₄. The best results were obtained with isothermal distillation and high pH levels in the feed. Isothermal distillation at 65 °C (a temperature gradient of 0 °C), with 1 M H₂SO₄ as the receiving solution, and at pH levels 8.2 and 12, yielded NH₃ recoveries of 36.4 ± 1.6% and 100.0 ± 0.1%, respectively. Under the same conditions, the molar fluxes were 0.63 ± 0.01 mol TAN m⁻² h⁻¹ and 1.84 ± 0.01 mol TAN m⁻² h⁻¹, respectively. It is worth noting that some very low depositions on the membrane were detected, leading to changes in the surface morphology, as confirmed by atomic force microscopy. Full article

Plastic waste and industrial residues like steel slag pose significant environmental challenges, with limited recycling solutions. This study investigates a sustainable approach by repurposing waste polystyrene and steel slag into composite membranes via electrospinning for membrane distillation applications. Steel slag incorporation enhanced membrane porosity, hydrophobicity, and thermal stability, with process optimization performed through response surface methodology by varying slag content (0–10 wt%), voltage (15–30 kV), and feed rate (0.18–10 mL·h⁻¹). Optimized membranes achieved a reduced fiber diameter (1.172 µm), high porosity (82.3%), and superior hydrophobicity (contact angle 102.2°). Mechanical performance improved with a 12% increase in tensile strength and a threefold rise in liquid entry pressure over pure polystyrene membranes, indicating greater durability and wetting resistance. In direct contact membrane distillation, water flux improved by 15% while maintaining salt rejection above 98%. Under photothermal membrane distillation, evaporation rates rose by 69% and solar-to-thermal conversion efficiency by 60% compared to standard PVDF membranes. These results demonstrate the feasibility of transforming waste materials into high-performance, durable membranes, offering a scalable and eco-friendly solution for sustainable desalination. Full article

Direct Contact Membrane Distillation–Crystallization (DCMD-Cr) is a synergistic technology for zero liquid discharge (ZLD) and resource recovery from high-salinity brines. In this study, DCMD-Cr was integrated to desalinate real oilfield-produced water (PW) with an initial salinity of 156,700 mg/L. The PW was concentrated to its saturation point of 28 wt.% via DCMD, and the integrated crystallization increased the overall water recovery from 42.0% to 98.9%, with a decline in water flux and salt rejection, mainly due to vapor pressure lowering and scaling. The precipitated salts in the crystallization unit were recovered and identified using different techniques. The results indicated that 91% of the crystals are sodium chloride, and less than 5% are calcium sulfate. A techno-economic analysis (TEA) was performed to evaluate the economic feasibility of the integrated DCMD-Cr process with a 500,000 gallons per day (GDP) capacity. The results showed that the crystallization operating cost was dominant at USD 0.50 per barrel, while the capital cost was only USD 0.04 per barrel. The economic viability can be enhanced by recovering value-added byproducts and using renewable or waste heat, which can reduce the total cost to USD 0.50 per barrel. Full article

Commercial hydrophobic membranes encounter severe problems such as membrane wetting and membrane fouling under extreme conditions, which affect membrane separation performance. To enhance the anti-fouling abilities of hydrophobic membranes, a composite membrane with omniphobic characteristics was fabricated successfully in this paper. Titanium dioxide (TiO₂) nanoparticles were in-situ grown via the hydrothermal synthesis method, and then fluorosilane with low surface energy was grafted on polyvinylidene fluoride (PVDF) membranes. Subsequently, the morphologies, chemical compositions, wetting properties and structural parameters of composite membranes were characterized systematically. Various contaminants were added to the feed to investigate the anti-fouling and anti-wetting performances of the composite membrane in membrane distillation tests. The results showed that butyl titanate was first hydrolyzed to form titanium hydroxide (Ti(OH)₄) and then it was dehydrated to form TiO₂ in the hydrothermal environment. TiO₂ crystals continued to grow and formed rough morphology with micro-nano synergistic distribution, which is similar to a “sunflower” disk composed of cubic clusters and nanopillars. Meanwhile, fluorosilane successfully was grafted onto TiO₂. The contact angles of deionized water, 0.4 mM sodium dodecyl sulfate (SDS) solution and 0.2% v/v mineral oil emulsion on the composite membrane surface were 167.3°, 162.0° and 158.5°, respectively, endowing the composite membrane with excellent omniphobic features. In direct contact membrane distillation (DCMD) tests, the composite membrane exhibited a relatively stable membrane permeate flux, and the salt rejection rate almost reached 100%. The mixture, consisting of inorganic salts, organic substances, surfactants and oil emulsions, was used as feed. In contrast, the commercial PVDF membrane flux decreased drastically and even dropped to 0 due to the membrane fouling and wetting. As for the pristine PVDF membrane, the membrane surface was covered with pollutants and membrane pores were blocked. Therefore, it was proved that the omniphobic composite membrane possesses outstanding anti-fouling and anti-wetting performance. Full article

Though the pursuit of sustainable desalination processes with high water recovery has intensified the research interest in membrane distillation (MD), the influence of module connection configuration on performance stability remains poorly explored. The current study provided a comprehensive multiparameter assessment of hollow fibre membrane modules connected in parallel and series in direct contact membrane distillation (DCMD) for the first time. The configurations were evaluated under varying process parameters such as temperature (50–70 °C), flow rates (22.1–32.3 mL·s⁻¹), magnesium concentration as scalant (1.0–4.0 g·L⁻¹), and flow direction (co-current and counter-current), assessing their influence on temperature gradients (∆T), flux and pH stability, salt rejection, and crystallisation. Interestingly, the parallel module configuration maintained high operational stability with uniform flux and temperature differences (∆T) even at high recovery factors (>75%). On one hand, the serial configuration experienced fluctuating ∆T caused by thermal and concentration polarisation, causing an early crystallisation (abrupt drop in feed conductivity). Intensified polarisation effects with accelerated crystallisation increased the membrane risk of wetting, particularly at high recovery factors. Despite these changes, the salt rejection remained relatively high (99.9%) for both configurations across all tested conditions. The findings revealed that acidification trends caused by MgSO₄ were configuration-dependent, where the parallel setup-controlled rate of pH collapse. This study presented a novel framework connecting membrane module architecture to mass and heat transfer phenomena, providing a transformative DCMD module configuration design in water desalination. These findings not only provide the critical knowledge gaps in DCMD module configurations but also inform optimisation of MD water desalination to achieve high recovery and stable operation conditions under realistic brine composition. Full article

Theoretical and experimental investigations were conducted to predict permeate flux in direct contact membrane distillation (DCMD) modules equipped with turbulence promoters. These DCMD modules operate at moderate temperatures (45 °C to 60 °C) using a hot saline feed stream while maintaining a constant temperature for the cold inlet stream. The temperature difference between the two streams creates a gradient across the membrane surfaces, leading to thermal energy dissipation due to temperature polarization effects. To address this challenge, 3D-printed turbulence promoters were incorporated into the DCMD modules. Acting as eddy promoters, these structures aim to reduce the temperature polarization effect, thereby enhancing permeate flux and improving pure water productivity. Various designs of promoter-filled channels—with differing array configurations and geometric shapes—were implemented to optimize flow characteristics and further mitigate polarization effects. Theoretical predictions were validated against experimental results across a range of process parameters, including inlet temperatures, volumetric flow rates, hydraulic diameters, and flow configurations, with deviations within 10%. The DCMD module with the inserted 3D-printed turbulence promoters in the flow channel could provide a relative permeate flux enhancement up to 91.73% under the descending diamond-type module in comparison with the module of using the no-promoter-filled channel. The modeling equations demonstrated technical feasibility, particularly with the use of both descending and ascending hydraulic diameters of 3D-printed turbulence promoters inserted into the saline feed stream, as compared to a module using an empty channel. Full article

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. Full article

The concept of integrating mechanical vapor compression (MVC) with direct contact membrane distillation (DCMD) is presented and analyzed. The hybrid system utilizes the DCMD to harvest the thermal energy of the MVC reject brine to preheat a portion of the seawater intake and simultaneously produce additional fresh water. Based on the operating temperature, the hybrid system requires specific energy consumption between 9.6 to 24.3 kWh/m³, which is equivalent to 25 to 37% less than the standalone MVC. Similarly, the freshwater production of the hybrid system can range between 1.03 and 1.1 kg/h, which is equivalent to a 3% and 10% increase relative to the standalone MVC when operating at brine temperatures of 50 and 90 °C, respectively. However, this enhancement is achieved at the expense of an average of 60% larger total surface area. This is partially due to the incorporation of the surface area of the MD modules and mostly to reduced temperature differences. Altering the permeate-to-feed ratio of the DCMD module led to a marginal change in the overall production without any enhancement in the compression power consumption. Increasing the MD module length by 50% resulted in a 3% enlargement in the overall production rate and a 10% reduction in power consumption. A modified hybrid structure that additionally utilizes the distillate heat is sought. A 5% increase in water production at the expense of a 45% rise in the specific compression energy of the modified structure over the original hybrid system is obtained. Full article

Membrane pore wetting remains a significant challenge to achieving the stable operation and commercialization of membrane distillation processes. This study quantitatively assessed membrane surface hydrophobicity to investigate its impact on the performance and water production cost of an MD system. Membranes with a similar pore wetting resistance but differing in surface hydrophobicity and pore diameter were examined. A direct contact membrane distillation unit was modeled, and the water flux results were compared with laboratory experiments to validate the model. The validated model was subsequently employed to simulate a seawater desalination plant with a designed capacity of 20 m³/day. The results demonstrated that membranes with a higher surface hydrophobicity and bigger pore sizes achieved higher water flux, increasing from 0.6 kg/m²·h to 2.5 kg/m²·h, and significantly reduced water production costs from NZD$13.5/m³ to $3.9/m³. This research highlights the importance of optimizing membrane surface properties and microstructures to advance MD applications. Full article

Treatment of extremely saline water such as the brine rejected from reverse osmosis water desalination plants, and produced water from shale oil and non-conventional gas extraction, is considered a global problem. Consequently, in this work, hollow fiber membrane distillation (HFMD) is experimentally evaluated for desalinating extremely saline water of a salinity ranging from 40,000 to 130,000 ppm. For the purpose of comparison, the HFMD is also tested for desalinating brackish (3000–12,000 ppm) and sea (25,000–40,000 ppm) water. Firstly, the HFMD is tested at two values of feed water temperature (65 and 76 °C) and flow rate (600 and 850 L/h). The experimental results showed that the HFMD productivity significantly increases when the temperature of feed water increases. Increasing the feed water flow rate also has a positive effect on the productivity of HFMD. It is also concluded that the productivity of the HFMD is not significantly affected by increasing the salt concentration when brackish and sea water are used. The productivity also slightly decreases with increasing the salt concentration when extremely saline water is used. The decrement in the productivity reaches 27%, when the salt concentration increases from 40,000 to 130,000 ppm. Based on the conducted economic analysis, the HFMD shows a good potential for desalinating extremely saline water especially when the solar collector is used as a heat source. In this case, the cost per liter of freshwater is reduced by 21.7–23.1% when the evacuated tube solar collectors are used compared to the system using electrical heaters. More reduction in the cost per liter of freshwater is expected when a high capacity solar-powered HFMD plant is installed. Full article

Adequate water supplies are crucial for missions to the Moon, since water is essential for astronauts’ health. Ionic liquids (ILs) have been investigated for processing metal oxides, the main components of lunar regolith, to separate oxygen and metals. The IL must be diluted in the process. Recycling this diluted IL post-processing is important to reduce the materials required in resupply missions. In addition, water will be needed in lunar greenhouses for growing food and aiding in sustaining a habitable environment. Direct contact membrane distillation (DCMD) is a new technology for water purification that was examined in this study for its feasibility to concentrate IL. Hydrophobic membranes composed of polytetrafluoroethylene (PTFE) and polyvinylidene (PVDF) were found to hold promise in separating solutes from water to concentrate a diluted IL solution and to recover water. A bench-scale DCMD system was employed to test this method at temperatures of 50 °C, 65 °C, and 80 °C. Hence, the benefits and limitations of DCMD with PTFE and PVDF membranes were explored for the aqueous IL 1-ethyl-3 methylimidazolium hydrogen sulfate for DCMD performed at different temperatures. Full article