Advancing Sustainable Desalination: Thermohaline Solar Membrane Distillation for Enhanced Efficiency and Environmental Resilience

Advancing Sustainable Desalination: Thermohaline Solar Membrane Distillation for Enhanced Efficiency and Environmental Resilience | Authorea try { document.documentElement.classList.add('js'); } catch (e) { } var _gaq = _gaq || []; _gaq.push(['_setAccount', 'G-8VDV14Y67G']); _gaq.push(['_trackPageview']); (function() { var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true; ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'http://www') + '.google-analytics.com/ga.js'; var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(ga, s); })(); Skip to main content Preprints Collections Wiley Open Research IET Open Research Ecological Society of Japan All Collections About About Authorea FAQs Contact Us Quick Search anywhere Search for preprint articles, keywords, etc. Search Search ADVANCED SEARCH SCROLL This is a preprint and has not been peer reviewed. Data may be preliminary. 28 April 2025 V1 Latest version Share on Advancing Sustainable Desalination: Thermohaline Solar Membrane Distillation for Enhanced Efficiency and Environmental Resilience Authors : Saad Nadeem 0000-0002-4138-8801 [email protected] , Fatima Qarsam , Dua Salman , Nazish Mustahssun , and Sharmeen Chohan Authors Info & Affiliations https://doi.org/10.22541/au.174581475.54838305/v1 612 views 207 downloads Contents Abstract Introduction: Types of Desalination Systems: Multi-Stage Flash (MSF) Desalination: Multi-Effect Distillation: Vapor Compression (VC): Freeze desalination: Natural Vacuum Desalination: Reverse Osmosis (RO): Forward Osmosis (FO): High Retention Micro Bioreactors (HR-MBRs) : Zero liquid discharge: Solar Absorbers: Thermohaline Convection: Comparative Analysis of TMSS (thermally localized multistage solar still) and TSMD: Comparative Analysis of TSMD With the Highly Used SWRO (Sea Water Reverse Osmosis): Environmental Aspects of TSMD: Future Prospects and Challenges of TSMD: Conclusion: Supplementary Material References Information & Authors Metrics & Citations View Options References Figures Tables Media Share Abstract Water scarcity demands sustainable desalination technologies. Solar-driven methods, particularly Thermohaline Solar Membrane Distillation (TSMD), offer energy-efficient and eco-friendly solutions. TSMD leverages natural thermohaline convection, driven by temperature and salinity gradients, to enhance water production while preventing salt accumulation—a persistent challenge in traditional methods. By combining confined saline layers, hydrophobic membranes, and advanced solar absorbers, TSMD achieves continuous operation even under high salinity (up to 20% brine). Compared to conventional techniques like Multi-Stage Flash (MSF), Multi-Effect Distillation (MED), and Reverse Osmosis (RO), TSMD demonstrates superior energy efficiency, environmental sustainability, and long-term stability. Innovations such as interfacial solar absorbers and modular multistage designs enable water production rates of up to 4.74 kg/m²/hr with competitive solar-to-water efficiencies. Additionally, TSMD minimizes brine discharge and carbon emissions, addressing critical environmental concerns. While scaling TSMD for diverse climates remains a challenge, advancements in material engineering and hybrid systems show promise. Integration with nanophotonics and thermal energy storage could further improve performance in off-grid and variable sunlight conditions. This review highlights TSMD’s potential as a cost-effective, durable, and scalable solution to global freshwater shortages, positioning it as a transformative technology in sustainable desalination. Advancing Sustainable Desalination: Thermohaline Solar Membrane Distillation for Enhanced Efficiency and Environmental Resilience Saad Nadeem 1* , Fatima Qarsam 1 , Dua Salman 1 , Nazish Mustahsan 1 , Sharmeen 1 1 Department of Chemical Engineering, NED University of Engineering and Technology, Karachi, Pakistan, 75300 * [email protected] Abstract: Water scarcity demands sustainable desalination technologies. Solar-driven methods, particularly Thermohaline Solar Membrane Distillation (TSMD), offer energy-efficient and eco-friendly solutions. TSMD leverages natural thermohaline convection, driven by temperature and salinity gradients, to enhance water production while preventing salt accumulation—a persistent challenge in traditional methods. By combining confined saline layers, hydrophobic membranes, and advanced solar absorbers, TSMD achieves continuous operation even under high salinity (up to 20% brine). Compared to conventional techniques like Multi-Stage Flash (MSF), Multi-Effect Distillation (MED), and Reverse Osmosis (RO), TSMD demonstrates superior energy efficiency, environmental sustainability, and long-term stability. Innovations such as interfacial solar absorbers and modular multistage designs enable water production rates of up to 4.74 kg/m²/hr with competitive solar-to-water efficiencies. Additionally, TSMD minimizes brine discharge and carbon emissions, addressing critical environmental concerns. While scaling TSMD for diverse climates remains a challenge, advancements in material engineering and hybrid systems show promise. Integration with nanophotonics and thermal energy storage could further improve performance in off-grid and variable sunlight conditions. This review highlights TSMD’s potential as a cost-effective, durable, and scalable solution to global freshwater shortages, positioning it as a transformative technology in sustainable desalination. Keywords: Solar energy, membrane distillation, thermohaline convection, water sustainability, energy efficiency. Introduction: Global challenge of water scarcity has shaped desalination techniques in transformative and innovative ways. Earth is 71% covered with water out of which 97% is the sea water [1]. Only 0.06% of fresh water is accessible [1], 0.3% of world’s fresh water is contained by lakes and rivers. Lakes and rivers play important role in serving as water irrigation source, fish farming, shipping transport, industrial and fresh water [2]. Lakes contain 87% of fresh water and rivers contain only 2%. Other than sea water, ground water is majorly processed to be potable drinking water. Ground water is about 99% source of fresh water where river and lakes are not found. About 2.5 billion people depend on ground water to meet their fresh water requirements [3].The salinity ranges of slightly saline water, moderately saline water, high saline water, and sea water is 1000 to 3000 ppm, 3000 to 10000 ppm, 10000 to 35000 ppm, >3500ppm respectively [4, 5]. To meet the gap between demand and supply of freshwater, several energy efficient techniques have been utilized over past few years, to make desalination sustainable and to reduce carbon emission. Solar driven evaporation technology is most suitable when it comes to energy saving [6]. It is most promising to clean and green energy. Solar evaporation desalination is driven by photo thermal process [6]. Several techniques for desalinations such as, solar still [5], Multi-Stage Flash (MSF) Desalination [7], membrane desalination [6, 8], solar chimney [7, 9] humidification de humidification [9, 10], multi effect desalination [6, 8]are used by utilizing solar energy. But Even though high solar thermal efficiency has been obtained, the water yield cannot exceed 1.5-3 kg m -2 day -1 [11] due to the large mismatch of the low solar intensity and the huge latent heat of evaporation. Desalination techniques, solar still and passive membrane, offers a simpler design, efficient heat localization and zero emission approach, making it an ideal solution for sustainable water production [5]. For high yield of water, multistage design has been developed to recover latent heat of condensation [9]. In addition to high energy efficient and zero carbon emission, another major concern in distillation is salt accumulation [12]. Salt accumulation during desalination can cause fouling; scaling and can also reduce heat transfer area which can overall affect the system’s efficiency. This limits the long-term durability of the solar desalination distillation. Conventional solar evaporators can withstand continuously only under limited exposure of few KW/h 2 . To make reliable and long run system, proper treatment of hyper saline system should be installed to mitigate salt accumulation in the system [12]. Mitigating salt built-up is quite challenging because diffusion rate of salt is four orders of magnitude slower than vapor diffusion [12]. Salt rejection rate driven by diffusion is much slower that it can accumulate easily on evaporating surface [9, 13] There are mainly two types of desalination system, direct and indirect [4]. All of them have different methods of salt rejection. One of the most common methods to address this issue in distiller is to use a capillary wick. The wick is composed of porous substance, such as cotton paper, fiber glass or other hydrophilic materials. It has tiny channels or threads. The capillary action is provided by small gaps or channels that exist between these fibers. The liquid is allowed to rise through narrow channels, which are made up of tiny pores or gaps. This system is used in Wicked based desalination (WSD). The salt remains in the wick as the water evaporates due to solar heat. Fresh water is subsequently created by condensing this water vapor [13] . These salts build up over time due to the repeated evaporation of water and concentration of salts. These salts can accumulate to the point where they may need to be removed to maintain the efficiency of the system. As in Figure 1, we can see that water is being fed due to capillary action. Then heat is provided from solar absorber, water is evaporated and passed through the wick but salt is left behind. This salt accumulation can reduce the run time of device or can cause fouling. It has to be maintained from time to time [12]. Figure 1: wick induced salt accumulation schematic [12] Other than this, self-rotating evaporators are also used, self-rotating evaporators works by creating centrifugal effect to drive salts away from mechanical surface [14], which reduces salt buildup by [12] constantly renewing the salt layer on the surface and prevents salt accumulation. Janus evaporators [15] are designed with dual functionality, one side has high photo thermal conversion rate (carbon based material) to maximize heat absorption. Simultaneous heat localization and salt rejection [1,11] is done by natural convection, which reduces heat loss and prevents salt buildup. This resolves the problem of fouling and makes the system energy efficient. Wick free desalination is an innovative way of performing desalination as it rejects salt accumulation. A confined saline layer is used instead of wick [12, 13] . In this work various desalination methods, their salt mitigating mechanism, solar absorbers used for them and their respective efficiencies have been discussed with focus on new technologies for solving issues of clean and green energy, salt accumulation, cost effectiveness and high rate of water production. Thermohaline solar-driven membrane desalination (TSMD) device, harness natural phenomenon of thermohaline convection for enhanced water production. TSMD utilizes passive solar desalination, leveraging temperature and salinity gradients to prevent salt accumulation on its membrane. This innovative device allows continuous efficient water production, a sustainable solution to increase fresh water yield. Types of Desalination Systems: There are mainly two types of desalination systems Figure 2 . Direct desalination. Indirect desalination. Figure 2: Types of desalination systems Solar Still Desalination: Solar still is the most environment friendly technique that uses solar energy instead of fossil fuel, as fossil fuel adds up to 80% of energy consumption [16] as shown in Figure 3 . Solar still generally have basins to carry salty water, covered by glass. This glass absorbs solar energy and heats the water by convection and radiation. After being heated it evaporate, leaving the salt behind. The water vapor is then condensed on an inclined glass cover and the distilled water is obtained in a container in its base [4]. The major strengths of solar stills include low capital investment, ease of construction, and lower costs of maintenance [4]. These can be built using locally available building materials thus affordable to small-scale farming communities. However, their heating capacity is low, with rates ranging from 2-3 liters of water per meter per day[6, 8] thus can only be used at small scales. The efficiency can be enhanced if accompanied by flat plate collectors phase change materials or booster mirrors [8]. The two main types of solar stills are Active and passive solar stills [5]. Passive solar stills are the most economical method. Thermohaline convective desalination is a passive method that leverages natural temperature and salinity gradients in seawater to facilitate desalination [12]. The advancement in the technology utilizes the principles of thermohaline convection circulation, where variations in temperature and salinity create density differences, leading to natural convection currents. It offers an eco-friendly alternative to conventional desalination processes that often require substantial energy input. Figure 3:Solar Still Desalination [2] Solar Humidification and De-humidification Desalination: Solar humidification-dehumidification (HDH) desalination utilizes solar energy to evaporate water, enhancing yield through a bio-inspired cascade humidifier [17] as provided in Figure 4. Solar Humidification-Dehumidification (HDH) Desalination conforms to the fact that warmed air must be able to hold the moisture [6]. Figure 4: Solar Humidification-Dehumidification [15] In this process, air is warmed by solar power and moved over salt water, in which it absorbs moisture. The humidified air is then cooled in a condenser, a point at which they will get rid of the moisture to produce fresh water. HDH systems are more efficient than solar stills and can work in closed–air, open–water cycles, and open-air systems [9, 10]. These systems are also scalable meaning that they can be applied to the operation of the medium-scaling enterprise [10]. Solar humidification-dehumidification desalination is a low-cost process for freshwater production, utilizing a two-stage system with shared dehumidifier. It enhances water production by addressing mass transfer and heat flow disparities, achieving up to 29.6 kg/h freshwater productivity [18]. However, HDH systems are more complicated by the use of other parts like humidifiers and condensers in its design and this calls for more raw materials and resources, resulting in high initial costs and operating costs. The comparative analysis has been provided in Table 1. Table 1: Comparison of OAOW AND CWOA Solar Chimney Desalination: Solar chimney desalination plant incorporates two functions within one unit – water desalination and power production [9] as can be seen in Figure 5. Figure 5: Systematic diagram of the integrated system from [19] Solar chimneys can be integrated into innovative solar desalination systems, addressing water scarcity [20]. In this kind of setup, solar chimneys use solar-heated air to generate power while also enabling desalination. Heated air moves through a chimney, driving turbines for electricity, while evaporating seawater beneath the collector. The humid air then condenses into fresh water as it rises. Modifications like black tubes, spray systems, and condensers enhance efficiency. this crossbreed approach offers a practical means of solar power generation for both drinking water and electricity [21]. But the system has a large area demand on land, and it costs a large amount of initial investment. The process of combination power and water production in a single complex wants precise engineering to ensure efficiency. The comparative analysis of salient features of solar chimney desalination is provided in Table 2. Table 2: Comparison of major features of solar chimney desalination Multi-Stage Flash (MSF) Desalination: Multi-Stage Flash (MSF) Desalination as given in Figure 6 is one of the most popular thermal desalination techniques in which saline water is evaporated through heating and full flashing at several stages at varied reduced pressures [22]. The vapor is condensed to evaporate fresh water, and heat of condensation which is utilized to heat the feed water that is coming in, is regained. MSF is most suited in large scale desalination plants because the process handles huge amount of water but is costly in terms of energy and needs frequent cleaning to remove scale and corrosion on the surface of the system [23]. Nonetheless, due to the high operational cost incurred, MSF is widely used in areas. Similarities and differences between (OT-MSF) & (BR-MSF) modes are provided in Error! Reference source not found. . Table 3: Similarities and differences between (OT-MSF) & (BR-MSF) modes. The bar chart in Figure 7 comparing Conventional MSF [24] and Solar MSF [13] based on top brine temperature, performance ratio, chemical use, and operational cost. Conventional and Solar MSF are both capable of great efficiency, but Conventional MSF has the advantage of a higher top brine temperature of 130°C, making for a performance ratio of 5.49; Solar MSF, with a top limit of around 95°C, can only achieve a performance ratio of 3.0. As compared to Solar MSF, conventional MSF has relatively higher demand for chemicals because of the continuous feed system in operation. From an operational perspective, Conventional MSF has greater fuel costs and more environmental disadvantages than Solar MSF due to using renewable energy although an initial solar investment is needed. Therefore, as conventional MSF will yield high quantity and fairly constant, solar shall be an environmentally friendly solution for regions with ample sunlight and moderate water demand. Figure 6: Solar Power Multistage Flash Desalination System[25] Figure 7: Comparison of Conventional MSF and Solar MSF Multi-Effect Distillation: Multi-Effect Distillation (MED) as provided in Figure 8, is a thermal desalination process that utilizes multiple stages (effects) to evaporate and condense water, enhancing distillate output and efficiency. The study highlights its performance improvements with increased feed water temperature, optimizing cost and output ratios[26]. In Multi-Effect Distillation (MED), heat exchange is realized by utilizing the vapor from one stage to heat the other stage and as a result of which; the quantity of energy from external source is not much as required in the MSF. This makes MED less energy-intensive, and more scalable for large scale implementation especially in regions of significant solar resource availability [27]. The difference between MED systems and MSF is the former uses lower temperature, which has an effect of cutting down the internal fatigue for the system parts, and thus, less maintenance is needed. However, it is an expensive solution to implement and maintain because of its vast complexity, which counts this solution as being capital intensive [8]. Figure 8: Multi-Effect Distillation [28] Vapor Compression (VC): Vapor Compression (VC) in water treatment given in Figure 9, involves a mechanical vapor compression apparatus that utilizes an evaporation/condensation vessel and recirculation circuit to treat water, effectively removing dissolved minerals like magnesium and calcium through a fluidized bed crystallizer [29]. Vapor Compression (VC) Desalination works by evaporation of the saline water and compressing the vapor in order to raise both its temperature and pressure [28]. The compressed vapor is then employed for heating the feed water and the condensation of the vapor constitute fresh water. VC systems are very energy effective and are suitable for use in medium scale applications. They are most suitable for regions characterized by high energy expenses or those that boast of renewable energy such as the solar energy [30]. However, VC systems consume more power at startup because of the compressors and also need highly trained labor to operate and maintain. Figure 9: Mechanical Vapor Compression Desalination system [28] In Figure 9: Mechanical Vapor Compression Desalination system the TVC unit and effects emerge to be having the greatest values of exergy destruction in kilowatts (kW) as well as in percentage and therefore qualifies as a major energy loss point. As seen in the exergy analysis of the plant, the exergy destruction in the condenser is very low and hence not sensitive to efficiency improvements. This indicates that maximizing the effectiveness of the TVC unit and effects provides a lot of room to improve the energy efficiency of the system, thereby lowering the cost of carrying out its functions. Figure 10: Chart of exergy destruction in MED-TVC system components as adapted from [31],[32] and [33] Freeze desalination: Freeze desalination is a process in which saline water is frozen and the part of water which freezes out contains or excludes the salt. The remaining brine is then separated from ice and the brine is then melted to get fresh water[34]. Freeze desalination (FD) is an innovative water purification method that uses sub-zero temperatures to crystallize and separate salts and impurities from brine, effectively treating saline water through a comprehensive mathematical model that captures cooling stages and ice purification processes [26]. Freeze desalination is energy efficient and precise especially in cold environment but faces challenges in scalability due to complex ice handling and separation. This method is mainly practiced in special applications where low temperature or waste heat resources can be exploited [32]. The technology can make high quality water, but applications of this technology are not easy to apply on large scale owing to equipment requirements, Figure 11: (a) Direct Contact Freeze-Concentration [35],(b) Suspension Freeze-Concentration Process [35] the freeze-concentration as described in the direct contact freeze-concentration scheme is as below. process. Cold liquid refrigerant as previously explained is added directly into the seawater in a crystallizer where a refrigerant evaporates due as a result of heating convection in saltwater [33]. In pictorial representation of the suspension freeze-concentration process. The method results in many small ice crystals and, these are dispersed in the liquid. concentrated liquid [33]. Figure 11: (a) Direct Contact Freeze-Concentration [35],(b) Suspension Freeze-Concentration Process [35] Natural Vacuum Desalination: Natural Vacuum Desalination Figure 12, works based on the principle of gravitational forces to generate a vacuum, so that natural convection could evaporate water at comparatively lower temperature than an industrial vacuum pump [36]. This makes the system energy friendly and easy to manage thus enhancing its sustainability in the market. Natural vacuum desalination is ideal for small scale, off grid uses, where energy is scarce and the equipment is low maintenance [37]. But it’s not easily scalable and its performance greatly depends on the environmental conditions [37]. Natural vacuum desalination utilizes a vacuum to lower the boiling point of water, enhancing evaporation. This research integrates it with membrane distillation, increasing water production significantly while minimizing energy consumption, addressing limitations like non-condensable gas entry and large evaporator surface requirements [38]. Natural vacuum desalination systems account for 48% of all vacuum techniques. While they produce freshwater at a lower cost per liter compared to forced vacuum systems, their productivity and efficiency are significantly lower, by 80% and 25%, respectively[39]. Figure 12: Natural Vacuum Desalination System [37] The findings of the research conducted above Figure 13 showed that by far, HDH is responsible for the highest emissions of emissions which is 61.8% as shown in Figure 13. TSDM has the least emissions (1.1%), thus, pronouncing the environmental advantages over other similar techniques. MSF, MED, and MVC have relatively moderate emission values, and yet they are higher than the TSDM. Figure 13: CO₂ emissions for various desalination technologies [13, 35, 40] The Figure 14: LIFE SPAN of different technologies in years [6, 9, 35] displayed is the life cycle analysis of various types of desalination technologies that include TSDM, MSF, MED, MVC, and HDH. Among the listed technologies, TSDM (Thermohaline Solar Desalination Method) requires the longest period of investment however, due to its durability, it can last up to 25 years. Next comes MSF (Multi-Stage Flash) and MVC (Mechanical Vapor Compression) with 20 years life expectancy which is again not very high. MED has shorter operational life; and it is at 15-years while HDH has the lowest at 10 years. TSDM will appear to have a longer useful life than HDH because it has a relatively low maintenance level, and design for sustainability, lasting longer than HDH could be due to the higher operational and maintenance cost. A comparative study of the salient features of non-membrane-based desalination techniques is presented in Table 4. Figure 14: LIFE SPAN of different technologies in years [6, 9, 35] Table 4: Non-membrane based solar desalination Reverse Osmosis (RO): RO is a process of desalination enabling the removal of salt and other pollutants to make water safely consumable and for use in agriculture [41] . Desalination entails selectively pumping high pressure to force sea water through a semi-permeable membrane which excludes the salts and impurities. However conventional RO plants required huge energy ( 3-10 kWh/m³) [42] because it involves the utilization of high pressure over and above the natural osmotic pressure in seawater that is between 55-70 Bars [42] . Some of the current problems include energy recovery devices and advanced membrane technology but draw backs like fouling of the membrane reduce efficiency and increase maintenance costs. Nevertheless, RO desalination continues to be a useful solution to regions of the world that possess low levels of freshwater availability [43] . Forward Osmosis (FO): Forward osmosis (FO) is one of the newly developed separation techniques that utilize the pressure of osmosis to drive water from a feed solution such as seawater or wastewater, through a semipermeable membrane to a more concentrated draw solution [44]. Differently from RO, which applies external hydraulic pressure, FO takes advantage of natural osmotic gradient and results in decreased energy consumption as well as reduced membrane fouling. The process includes two main steps: the ability to osmotically dilute the draw solution when water enters it and then to separate clean water from the resulting diluted draw solution [44]. The overarch of protection strategies for FO depend on the use of appropriate membranes that will not readily foul and an efficient, recoverable draw solution to maintain high osmotic pressure and low reverse solute flux [45]. The comparative study between both type of osmosis is presented in Table 5. Table 5: Comparison between features of FO and RO Energy Efficiency although using less energy, FO can recover limited quantities of water; conversely, using more energy, RO is excellent for the desalination of huge quantities of water. In fouling and cleaning characteristics, fouling rates and maintenance of FO are relatively lower compared with RO because the membranes must be cleaned and replaced periodically. Whereas, FO is good for certain specific, possibly single-step, high salinity feed water treatments and where FO is used as a pre-treatment to RO. High Retention Micro Bioreactors (HR-MBRs) : High retention micro-bio reactors incorporate advance membranes such as nano-filtration, reverse osmosis, forward osmosis and membrane distillation. HR-MBRs enhances TrCOs removal but it also faces some struggles, including salt buildup, membrane fouling and incomplete biodegrading of TrOCs. If HR-MBRs retains salt it can inhibit microbial activity and compromise efficiency of treatment [46]. To mitigate its limitation, several strategies have been developed. Environmentally friendly pre-treatment such as advanced oxidation process help manage fouling and enhanced TrOCs removal. Membrane surface modification can improve fouling problems but often increase operational cost. Hybrid systems that combine HR-MBRs with another treatment technologies offers best efficiency and enhanced performance [46]. The performance of HR-MBRs with their configuration is discussed in Table 6. Table 6: HR-MBs configurations and their performance [46] Zero liquid discharge: Desalination is increasingly used all over the world to mitigate water scarcity, with above 21000 plants producing more than 120 million m 3 /day of drinking water globally [47]. Global desalination production is expected to 200 million m 3 /day by 2030 [48]. With the global increase in desalination capacity, brine discharge is another significant issue. Global desalination market is valued at US$12.8 billion in 2019, is projected to reach US$23.4 billion by 2027 [47]. However, another major concern in desalination is brine discharge. Disposal methods such as, deep well injection, sewer injection can be harmful to environment. For this concern strategies such as Minimum liquid discharge (MLD) and zero liquid discharge (ZLD) has been developed. Minimal liquid discharge and zero liquid discharge are two major studied systems. MLD system incorporates reverse and forward osmosis. A ZLD system integrates RO, FO, brine concentrator (BC) and brine crystallizer (BCr). Advantages and Disadvantages of desalination techniques are elaborated in Table 7 Comparison of MLD and ZLD is given below in Table 9 while the comparison of MLD and ZLD is provided in Table 9 Table 7:Advantages and Disadvantages of desalination techniques [6, 31] Table 8:: Advantages with respect to brine disposal technology for zero liquid discharge [4, 10, 12, 30, 49] Table 9: MLD and ZLD comparison[48] Solar Absorbers: As we can see that solar desalination more energy efficient and more environmentally friendly technique, solar absorbers are also discussable important factor as it is driven by photo thermal process. There are three main technologies in the solar steam generation. The first heat the surface of the receiver resulting in the lower efficiency, the second is the volumetric solar absorption which reduces surface heat losses but requires robust dispersion of Nano fluids. The third and the most efficient is interfacial evaporation which localize the heat at the liquid surface achieving photo thermal conversion efficiency of 90% [50]. Here in this technology, we replace the capillary wick with the confined saline layer, and using an air gap to reduce heat loss and vapor diffusion. Lowering the pressure in the air gap is examined as the way to decrease diffusion resistance potentially increasing heat and mass transfer [51] . The solar absorbers are typically positioned above the saline layer. They capture solar radiation and convert into the thermal energy which is then transfer to the saline layer below. The saline layer heats up near the solar absorber and its higher salinity near the membrane creates a density gradient. This set up is essential for maximizing efficiency. The use of solar absorber enhances the temperature gradient between the saline layer and cooler side of the membrane driving the vaporization process. This gradient along with a tilted device set up induces thermohaline convection. The lower density saline near the membranes flows downward resulting in the circulation of the saline with in the layer [12]. As the salted water passes through the membrane the water with lower density (towards purity) moves upward while water containing more salt content moves to the bottom of the vessel [52]. The photo thermal material like graphene, carbon foam, or plasmonic nanoparticles are excellent at absorbing solar radiation and efficiently transferring heat to the water surface. The natural convection based salt rejection high solar to water efficiency and scalability makes it eco-friendly alternative that help to solve the water scarcity problem where sunlight and saline resources are available[53, 54]. TSMD is a unique and eco-friendly system that provides the high efficiency salt rejection. In solar still desalination glass cover captures the solar energy that evaporates water and leave the salt behind. Product is low but because of simple design minimal maintenance is required. Several categories of the material are utilized in the development of the solar absorber for the TSMD: Carbon Based Solar Absorber: Carbon material such as carbon black is favored for their inherent black color which enables high absorption across the spectrum[44] as shown in Figure 15. Graphene oxide for instance demonstrates exceptional absorption (up to 99%) from 250 to 2500nm, achieving power conversion efficiencies of 87.5% under one sun illumination [50]. Figure 15: Shown carbon based solar absorbers [50] Metals like gold, silver and aluminum are known for their plasmonic effects which enhance light absorption and thermal conversion. In Thermo-Solar Membrane Distillation (TSMD), the application of plasmonic materials like gold, silver, and aluminum, can prominently increase light absorption and thermal conversion efficiency [50]. Hybrid and Composite Materials: Hybrid and composite materials, particularly those based on chitosan, have shown significant potential in wastewater treatment due to their enhanced adsorption capabilities, biodegradability, and ability to remove various pollutants effectively, making them a promising solution for environmental remediation [55]. Combining material like carbon-based absorber with the plasmonic nanoparticles can yield hybrid system that capitalizes on the advantage of each component. In TSMD the selected material must exhibit excellent thermal stability under temperature (up to 550 o C) and long-term durability when exposed to saline environment. Innovation in material engineering is vital to robust solar absorber that can withstand fouling and degradation over the extended period of operation [50]. Solar Absorber with Silica Aerogel Layer: In multistage distillation systems, the solar absorber with a silica aerogel layer plays a crucial role in efficiently capturing and retaining solar energy, which is vital for maximizing water evaporation in each stage. The first stage in a multistage distillation setup typically uses a solar absorber as the primary component to harness solar energy. The addition of a silica aerogel layer around this absorber serves as insulation due to its unique properties: low thermal conductivity and high infrared opacity. This layer allows sunlight to pass through to the absorber while minimizing heat loss to the surroundings The silica aerogel layer in the solar absorber system enhances transparency (69.3% visible-light transmittance, 86.9% near-infrared transmittance) while providing bending resistance and low thermal conductivity, facilitating effective sunlight harvesting and heat retention for temperature self-regulation [56]. By doing so, more of the absorbed energy is directed towards heating the water, thus enhancing evaporation and the overall efficiency of the desalination process [57, 58]. Nano photonics Enabled Solar Membrane Distillation (NESMD): Nano photonics-Enabled Solar Membrane Distillation (NESMD) utilizes photothermally active membranes and solar concentrators to enhance desalination efficiency. This study explores the complex interactions affecting performance, revealing negative flux phenomena at high salinity and proposing design modifications to mitigate these effects [59]. Nanophotonic enhanced solar membrane distillation (NESMD) utilizes photothermal coatings on hydrophobic membranes to absorb solar energy, generating localized heat for desalination. This technology effectively converts seawater and hypersaline brine into clean water without external electricity or cooling requirements [53]. NESMD equipped with photo thermal nanoparticles enabled localized heating at the membrane surface. When exposed to solar radiations nanoparticles embedded in the membrane absorb solar energy and convert it into heat, where it’s needed at the water membrane interface. This localized heating optimally aligns with thermohaline convection as the convection is driven by small localized temperature difference with in the saline layer. The localized heating mechanism of NESMD membranes directly enhances the thermohaline process as it requires less overall energy. This approach is especially beneficial; in thermohaline systems where, natural convection helps to distribute heat and salinity gradient without mechanical intervention. In contrast conventional membrane requires the heating of the entire volume of feed before it enters the membrane [60]. Thermohaline Convection: The main idea of Thermohaline solar driven desalination (TSMD) system comes from the natural ocean convection currents called thermohaline convection [12]. Thermohaline convection is natural phenomenon driven by temperature difference and salinity difference. In thermohaline convection as the water gets warm it evaporates and left the salt behind which makes salty water denser and it sinks. This sets up a cycle of continuous motion due to temperature difference and salinity difference [61]. Because of the missing nutrients and temperature distribution uniformly between the layers of water this helps to disturb the ecosystem and temperature of Ocean convective current works in this way that clod water and salty water comes downward and hot and less salty water comes Upward that enhances vertically mixing and distribution of layers [62]. This motion contributes to larger circulations in ocean also known as global conveyer belt as shown in Figure 16. It helps to regulate global temperature by distributing heat across the planet. This process can be applied in engineering systems, particularly in desalination and solar distillation technique. The temperature gradient is established by using solar absorbers as heat source, causing warm water to evaporate by leaving salt behind. As there is no wick, this continuous process prevents accumulation of salt in system in membrane distillation. Denser water can be flow out from the system and circulate through the device for continuous pattern [12]. Figure 16: Global Conveyer Belt Methodology of TSMD: Thermohaline convection enhanced solar membrane desalination (TSMD) is implemented by confining a thin saline layer Figure 17: a) gravity feeding of water, b) close view of setup [12] with a membrane desalination system [12]. A communicating vessel is attached to the membrane distillation system, enabling a passive gravity flow of saline water. The confined saline layer as depicted in Figure 17: a) gravity feeding of water, b) close view of setup [12] is the site of thermohaline convection, which is fueled by variations in salinity and temperature. Positioned between solar absorber and hydrophobic membrane, saline layer absorbs heat from solar absorber, causing evaporation through hydrophobic membrane Table 10. Saline layer near to absorber has high temperature and low salinity whereas saline layer near membrane has high salinity (high density), creating a density gradient and temperature gradient as well. The buoyancy force causes warmer, less saline layer near the solar absorber rises due to their less density, while the denser water sinks due to high salinity creating a circulation pattern indicated by purple arrows in Figure 17: a) gravity feeding of water, b) close view of setup [12]. This circulatory pattern increases system’s natural convection. Multiple clock vertices can be created depending upon the magnitude of thermohaline convection. To minimize conductive heat losses between membrane and condenser and to maximize heat and mass transfer a 5mm air gap[12] is used between evaporator and condenser[12, 49]. The thickness ensures effective heat transfer and vapor transfer between two components of the system and to eliminate delays in distillation process [12]. The system is designed to solely depend on natural process, like gravity driven flow and thermohaline convection. The system is made to float on top of the saltwater. By modifying its position in relation to the reservoir’s water level, a floating communicating vessel contributes in maintaining a constant head. As the water is drawn from reservoir to the system, its level will decrease in reservoir, but because of the floating system, it will move downward as the water level drops while keeping the vertical distance between inlet and water’s surface constant. Reservoir consists of two layers: Inner layer and outer layer as shown in Figure 17. The outer layer serves two purposes, provides feed to the system and helps in maintain a constant vertical head. Positioned below TSMD device, inner layer contains saline water; specifically used to condense water vapors, as condenser is in direct contact of outer layer. Hydraulic head plays an important role. It is created by maintaining a vertical distance between module’s inlet and reservoir. By constant vertical head, a stable pressure head is developed. This is considered as self-regulating mechanism, consistent water supply without any external energy. In properly engineered systems, we can manipulate the temperature and density gradients to initiate thermohaline convection leading to significantly higher salt rejection. Thermohaline convection enhanced heat transfer across confined salt layer and results in high production rate as compared to traditional method of desalination as discussed earlier. This approach offers an efficient and self-controlled desalination that requires minimal energy and maximizes salt rejection. Figure 17: a) gravity feeding of water, b) close view of setup [12] Table 10: different types of membranes and their properties Design Elements of TSMD: Some important features of TSMD device are the use of PTFE membrane, Nylon frame and keeping the device at tilt of 30-degree angle as shown in Figure 18. These considerations enhance heat and mass transfer, boost evaporation efficiency, enhance thermohaline convection and reduce salt accumulation [12]. PTFE (Polytetrafluoroethylene) Membrane: The role of micro porous barrier is played by PTFE membrane that enhances vapor permeability while restricting the salt passage. It has 1mm diameter pores and have porosity of 0.8 [12], this membrane is used to facilitates the movement of vapors while creating a layer where saline concentration gradients are crucial for inducing thermohaline convection, a natural circulation of fluid driven by temperature and salinity difference across membrane. Teflon tape is used to seal the interface between PTFE membrane and nylon frame to prevent any leakage or vapor loss. PTFE membrane specifications are taken as 100mm thick, 10x10cm 2 area [12]. Nylon Frame: The 3D printed, hallow nylon frame as shown Figure 18 used for structural support of the membrane, providing thermal insulation for minimization of heat loss. Its design allows only a defined portion of saline layer to contact to solar absorber, directing heat efficiently and promoting uniformity of temperature. The nylon material is chosen for its durability, low heat conductivity, and ease of fabrication, which collectively supports a consistent flow and prevent heat from dissipating excessively [12]. 30 Degree Tilt: Tilting device enhances natural circulation with confined layer to initiate thermohaline convection as shown in Figure 19. At this angle, the warm, less dense saline layer near the solar absorber flows towards membrane and denser, saline layer flows away from membrane side. This flow generates a buoyancy-driven circulation that forms clockwise vortices within the layer. These vortices increase heat transfer and distribute saline more evenly, reducing salt buildup near the evaporation interface, which helps maintain a high evaporation rate and overall efficiency [12]. Figure 18:(a) TSMD device, (b) TSMD stages, (c) single stage [12] For wick free, confined layer desalination, experiments for horizontal setup has also been performed. Identical conditions were given to both set up. Wick free horizontal setup has 80% solar to vapor efficiency for 3.5wt%, 10wt%, 20wt% saline. At 25% where brine was near its saturation efficiency reduced to 67% [11, 13]. But TSMD shows higher efficiency due to its tilted design which can initiate thermohaline convection in it. Comparison for horizontal and 30-degree tilt setup is discussed in Table 11. Table 11: Comparison of horizontal and tilted setup [12] Figure 19:(a) tilted and horizontal setup, (b) time and production rate, (c) Horizontal setup characteristics(d) thermohaline convection induced gradients [12] Using a single stage device induced with thermohaline convection on 20% saline under one sun illumination 1000 W/m 2 , stable temperatures across saline layer is observed indicating that this process supports effective mass transfer and heat transfer. TSMD has performed 180-hr continuous test for 20wt% salinity and rejected amount of salt of 229 days of desalination process [12]. The TSMD device produces more reliable results, more water production as compared to wicked based desalination (WBD), especially as salinity increases. TSMD device has a stable performance, producing 0.7kg/m 2 /h[12] for nearly saturated saline. TSMD is designed to prevent salt accumulation near the evaporating interface by directing salt to concentrated region, allowing it to flow back in bulk saline. Real world behavior can be examined by real world testing, which is outdoor performance of TSMD device. Outdoor experiments were performed at 20% [12] salinity, under natural sunlight conditions. Similar temperature fluctuations and results were found as in lab test. To optimize water production rates based on specific needs, TSMD device feature a modular design that allows for easy design of multistage (e.g. Three stages, ten stages) Table 12. Three stage devices gave results better than single stage but 10 stage devices performed best by giving highest water production rates. Table 12: Comparative of 3 and 10 stage TSMD device[12] The estimated cost of desalinated water from TSMD is up to $0.001-$0.003 per liters [12]. The device could produce 4–6 liters per hour if it is reduced down to the size of a small suitcase, potentially making it cheaper than tap water. The reduction in cost is due to the prolonged life span of TSMD device, unlike conventional devices that degrade under continuous salt exposure. TSMD’s design helps to avoid frequent replacements, making it more cost effective over time. Comparative Analysis of TMSS (thermally localized multistage solar still) and TSMD: By signifying major advancement in TMSS and TSMD in solar-driven desalination can handle salt management challenges and energy efficient differently. TMSS has a capillary wick system, i.e.- wicked based desalination, to extract water through small pores for evaporation, optimizing heat and mass transport through a multistage architecture. Comparative analysis of TMSS and TSMD is discussed in Table 13. Table 13:comparative analysis of TMSS[60] and TSMD [1] It can handle an impressive solar to vapor conversion efficiency of 385% driven by the recycling of latent heat between stages. Though, the salt management in TMSS is dependent on diffusion during inactive periods, such as night time, which limits continuous operation, particularly in high salinity environments. Salt tend to accumulate in areas of high diffusion resistance requiring periodic maintenance to ensure efficiency. The TSMD surpassed other traditional passive multistage solar distillation, highlighting its higher efficiency at varying salinities. Some wick-based desalination gives solar to vapor efficiencies up to 89% [13]. To compare water collection efficiency of TSMD and wicked based desalination, a setup was established, using identical conditions for both devices. Taking solution of varying salinities (3.5%, 10%, 20%)[12]. The TSMD device took slightly a longer time (about 10 minutes) to reach operational temperatures, due to its high thermal mass. For 5 hours performance of both TSMD device and WBD was observed that both produces more than 30g[12] of water, but TSMD outperformed, particularly for high salinities. In WBD, water production rate decreased with time and increasing salinity of water, from 1kg/m 2 /hr to 0.9kg/m 2 /hr for 3.5% salinity [12] similarly for 10% salinity it ranged from 0.85kg/m 2 /hr to 0.7kg/m 2 /hr[12], showing resisting to salt accumulation leading to consistent result and better water production rates. This was confirmed by 8 hour test on 10% NaCl solution where salinity increased only 10.5% after 4 hours and stabilizes to 10.6% after 8 hours [12]. Although, the WBD is outstanding in terms affordability, scalable application with moderate salt rejection capabilities, the TSMD offers outstanding performance in high salinity conditions with its continuous salt rejection and more advance thermal management. The TMSS provides an excellent solution for regions with moderate desalination needs, but the TSMD is better suited for harsher environments where continuous operation and salt management are critical [1,37] . Comparative Analysis of TSMD With the Highly Used SWRO (Sea Water Reverse Osmosis): Seawater reverse osmosis (RO) is a desalination technology that takes membranes to separate salts from seawater, with membrane selection based on permeability to salts. The points that are affecting RO plant design include feed salt concentration and operating pressure [63]. Seawater reverse osmosis (RO) requires pretreatment to remove fouling substances. This introduces softening and ballasted flocculation (SBF) to enhance desalination efficiency, achieving high settling velocities and reduced sludge generation, meeting RO membrane water quality standards [64]. Seawater Reverse Osmosis (SWRO) is a desalination technology that converts saline seawater into freshwater through a 14-stage process, including intake, ultrafiltration, and reverse osmosis, [65]. Traditional SWRO desalination has reached excellent energy efficiencies over the years, with seawater desalination energy consumption generally ranging between 3-4KWh/m3. Through different cases about SWRO systems have reported even lower energy consumptions value, close to 2KWh/m3 in certain studies. In contrast to SWRO, TSMD primarily harnesses solar energy as its energy resource, making it potentially less reliant on grid-based electricity or high-pressure pumps. TSMD can theoretically operate with near zero external electricity input when used in sunny, off-grid setting, which are integral to SWRO, positioning TSMD as a more energy efficient choice under the right environmental conditions. In regions with variable sunlight TSMD systems would require supplemental energy resources or thermal storage solutions to ensure steady performance. This is the region where SWRO has benefit, as it can run independently of solar energy, albeit with higher energy consumption. Membrane desalination, particularly RO have shaped the desalination landscape while offering an effective means to produce freshwater [57]. The brine to product water ratio in seawater recovery levels typically (30-50%)[58] . This concentrated brine when released into marine environments affect local ecosystem due to increased salinity, elevated temperatures, altered PH levels and presence of heavy metals and residual chemical[67]. TSMD presents a significant environmental advantage by addressing the brine disposal problem that has long challenged conventional desalination methods. TSMD gives promising solution to its capability to handle highly concentrated saline water, including brine solution up to 20%. This ability is critical for zero liquid discharge application where the objective is to recover maximum freshwater and minimize liquid waste. By using thermohaline convection to circulate and remove salts, TSMD prevents salt accumulation and fouling which is a common issue in brine other membrane distillation process Table 7. Environmental Aspects of TSMD: The study highlights that waste-water treatment using mushroom substrates can mitigate water pollution, benefiting ecosystems and agriculture. It emphasizes the importance of micro embolization and microfiltration processes in reducing environmental degradation caused by urbanization and industrialization.[68] The study identifies the production of the unspecific peroxygenase as the main environmental hotspot, contributing over 36% to impact categories. Key issues include enzyme production and immobilization, with significant effects on stratospheric ozone depletion and terrestrial eco-toxicity[69]. The environmental impacts of wastewater treatment include hazardous waste (53.4%), chemical consumption (38.3%), and electricity use (7.5%). Implementing heat recovery systems, can reduce hazardous waste impact by 12.2% for every cubic meter of treated water[70]. The paper assesses environmental impacts of hyper-crosslinked polymers (HCPs) in water treatment, revealing that flow synthesis of HCPs results in lower negative impacts on human health, ecosystem quality, climate change, and resource consumption compared to batch synthesis [71]. The environmental impacts of water treatment, primarily stem from energy consumption, which is the most significant contributor to various environmental impact categories. Strategies to reduce energy use and incorporate renewable sources are recommended [72]. Optimization in the resources is also a key aspect of TSMD systems are designed to maximum solar energy utilization, reducing the reliance on external, fossil fuel-based energy resources. By minimizing the carbon footprint of desalination, making it a more sustainable process compared to conventional methods. By recovering both freshwater management and contributes to reducing the environmental impact of industrial desalination processes. As Figure 20: Environmental aspects of TSMD process tells about the environment of water treatment processes. Figure 20: Environmental aspects of TSMD process Future Prospects and Challenges of TSMD: The necessity for fresh water is increasing worldwide and it is growing rapid due to population increase and economic expansion, challenging its accessibility for good life and exposing the global population to water security risks. Back in 2000, global water demand was about 4000 billion cubic meters, with expectation of 58% increase by 2030. Innovation wise developed countries have projected the increase in demand particularly pronounced reaching up to 93% higher than in developed nations. Standard potable water resources like surface and ground water are insufficient to meet this demand, necessitating alternative solutions like waste water treatment and seawater desalination. Desalination capacities projected to almost doubled by 2030 in Gulf cooperation council (GCC) countries and globally [73], technologies like Thermohaline convection membrane desalination (TSMD) offer viable response to these challenges. Traditional desalination system particularly seawater RO and thermally driven processes consume significant energy with SWRO at approximately 3.5 kWh/m 3 and thermal desalination process around 17kWh/m 3 .TSMD’s efficiency and lifespan are highly dependent on the stability of its components, especially the membrane[66]. Traditional desalination methods face fouling, scaling and degradation over time, particularly under extreme salinity conditions. TSMD system, which operates with confined saline layer to enhance thermohaline convection, uses hydrophobic membrane to resists long term salt exposure and high temperature. Researches on materials, like carbon based or Nano-composite membrane, can provide improved resistance to fouling and prolonged durability. TSMD is advancing scalability in response to water demand. Improvement multistage unit assembly and guaranteeing constant performance across different salinity levels are necessary for industrial scale applications. Developing an advanced system that can control to varying salinity and water demand will be essential for areas with changeable salinity or for industrial applications involving brine waste. The design and optimization of these modular systems for both low and high demand scenarios may be the main topics of future researches. Though TSMD utilizes solar energy efficiently, further improvements in solar to thermal conversion and minimizing heat loss are necessary to achieve maximum efficiency. Integrating nano-photonic materials that enhance sunlight absorption could be one path forward. TSMD is adaptable to real-world conditions, as the changing solar energy, temperatures and humidity. The outstanding design of TSMD that we are using now a days functions best in areas where solar energy from the sunlight is more but fluctuations due to cloudy weather or night time pose challenges. Recent power storing solutions or hybrid systems that are substituted between solar and another sustainable source, like waste heat, could help make TSMD feasible in diverse climates and continuous operation. The economic feasibility of TSMD is one of the issues, specifically in areas with limited access to freshwater. Even though TSMD has shown the talent to generate water at competitive pricing, it is difficult to scale up the technology to attain steady economic viability. To make TSMD more accessible and economical, future studies should concentrate on lowering material costs and streamlining system designs [50, 52]. Conclusion: Global water scarcity problem increases the need of new desalination technologies in which sea water is a major source of potable water. Desalination which is a global option to address the freshwater scarcity challenges presents diverse technologies that are unique in view of their prospects and drawbacks. Thermal driven methods like the Multi-Stage Flash (MSF) and Multi-Effect Distillation (MED) are proved to be very efficient at producing fresh water but are energy-intensive. Despite the high efficiency, easy scalability, and versatility. RO membranes and several related membrane technologies remain vulnerable to fouling. Forward Osmosis (FO). Natural Vacuum Desalination, and Thermohaline Convection are modern and fresh ways that explore ways to minimize energy utilization and environmental impacts. Although presented as viable, the methods are not always free of challenges. Thermal methods may be better to be adopted in the energy-endowed countries while membrane techniques would be suitable for countries demanding compact, efficient technology. In the long run then, improvements in the hybrid system and newer more efficient designs are the surest way to reduce the cost of desalination and make it more sustainable. Solar distillation is considered as cheaper way efficient way of water production. Solar absorber enhances the solar distillation efficiency by focusing heat at the liquid surface so that evaporation promotes. TSMD solve issue of heat localization and simultaneous salt rejection with the help of thermohaline convection process that naturally improves the salt rejection. Because of Multi stage TSMD devices, we can see the significant production rate, 4.74kg/m^3 pure water can be achieved and solar to water efficiency is 322%. TSMD operates at zero liquid discharge application. In short, TSMD is a cost effective and sustainable solution for the desalination of the saline water. Conflict of interest: The authors declare that there is no conflict of interest and that there was no funding received during, before or after the writing of this manuscript. All the data is available in the manuscript and files along with it. Supplementary Material File (natural sciences manuscript tables and figures.docx) Download 2.19 MB References 1. 1. Ahuja, S., Handbook of water purity and quality . 2021: Academic press.2. Rijsberman, F.R., Water scarcity: Fact or fiction? Agricultural Water Management, 2006. 80 (1): p. 5-22.3. Musie, W. and G. Gonfa, Fresh water resource, scarcity, water salinity challenges and possible remedies: A review. Heliyon, 2023. 9 (8): p. e18685.4. Bhagwati, A., M. Shah, and M. Prajapati, Emerging technologies to sustainability: A comprehensive study on solar desalination for sustainable development. 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Google Scholar Information & Authors Information Version history V1 Version 1 28 April 2025 Copyright This work is licensed under a Non Exclusive No Reuse License. Authors Affiliations Saad Nadeem 0000-0002-4138-8801 [email protected] NED University of Engineering & Technology View all articles by this author Fatima Qarsam NED University of Engineering & Technology View all articles by this author Dua Salman NED University of Engineering & Technology View all articles by this author Nazish Mustahssun NED University of Engineering & Technology View all articles by this author Sharmeen Chohan NED University of Engineering & Technology View all articles by this author Metrics & Citations Metrics Article Usage 612 views 207 downloads .FvxKWukQNSOunydq8rnd { width: 100px; } Citations Download citation Saad Nadeem, Fatima Qarsam, Dua Salman, et al. Advancing Sustainable Desalination: Thermohaline Solar Membrane Distillation for Enhanced Efficiency and Environmental Resilience. 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