Membrane distillation for seawater desalination applications in vietnam: potential and challenges - Duong Cong Hung

between two sides of the membrane, and is given as: . . ( ) m m f m pJ C P P= × − (10) where Cm is the membrane mass transfer coefficient, Pm.f and Pm.p are the water vapor pressures at the liquid-vapor interfaces on the feed and the permeate side of the membrane. The water vapor pressure of the process streams at temperature T is calculated as followed: ( ) 3816.4423.1964 46.13T water water P exp a T χ = − × ×  −  (11) where χwater is the molar fraction of water and awater is the water activity. For an aqueous saline solution, the water activity can be estimated by Eq. 12 [7]: 21 0.5 10water salt salta χ χ= − − (12) where χsalt is the molar fraction of salt in the solution. For an ideal dilute aqueous solution, Eq. (10) can be written as: 0 2 v m m P HJ C T R T ×∆ = × ×∆ × (13) where ∆Hv is the latent heat of vaporization, P0 and T are the average water vapor pressure and temperature within the membrane pores, ∆Tm is the temperate difference between the feed and permeate sides of the membrane. The calculation of Cm involves empirical correlations. The selection of the empirical correlation to calculate Cm is determined by mass transfer mechanisms occurring in the membrane pores. Employing the Dusty gas model to describe the mass transfer through the membrane, possible mass transfer mechanisms within membrane pores in MD are viscous flow, surface diffusion, Knudsen diffusion, and molecular diffusion. However, surface diffusion is often neglected in general MD applications [7]. Thus, depending on the structural properties of membrane, the properties of the transported vapor, and operating parameters, the predominant mass transfer mechanism can be viscous flow, Knudsen diffusion, molecular diffusion, or transition between them [24]. For seawater desalination by DCMD, Cm can be described as [8, 34, 36, 37]: 12/1 82 3 −         +      = M RT PD P M RT r C am ε τδpi ε τδ (14) where δ, ε, τ, and r are the membrane thickness, porosity, pore tortuosity, and pore radius, respectively, M is the molecular weight of water, R is the gas constant (i.e. 8.314 J/(mol.K)), T is the mean water vapor temperature (K) inside the membrane pore, P and Pa are the total pressure and the air partial pressure (Pa) inside the membrane pore, and D is the water diffusion coefficient. For seawater MD desalination, the transport of water vapor across the membrane from the feed to the distillate results in an increase in salt concentration in layers adjacent to the feed membrane surface, giving rise to a phenomenon termed concentration polarization. Membrane distillation for seawater desalination applications in Vietnam: potential and challenges 669 Concentration polarization renders the salt concentration at the feed membrane surface higher than that in the bulk feed solution (Figure 3), thus reducing water activity and hence water vapor pressure at the feed membrane surface. As a result, concentration polarization reduces water flux of the MD process. However, the influence of concentration polarization on water flux is negligible as compared to that of temperature polarization for MD desalination of seawater [8, 28, 31]. For the MD process of hyper saline feed waters, concentration polarization effect can greatly reduce water flux and increase the process propensity for membrane scaling [25]. The concentration polarization coefficient φ is used to quantify the concentration polarization. Given a nearly complete salt rejection of the MD membrane, φ of the seawater MD desalination process can be calculated as: . . m f b f x x φ = (15) where xb.f and xm.f are the salt concentration in the feed bulk solution and at the feed membrane surface, respectively. The calculation of the mass transfer (i.e. water flux) using the Eq. 10 involves the temperature and salt concentration at the membrane surfaces, hence it is impractical. Due to polarization effects, the temperature and salt concentration of the process solutions at the membrane surfaces differ from those in the bulk solutions, and it is unviable to measure them. Alternatively, water flux of the MD process can be calculated using properties of the bulk process streams as follow: . . ( ) m b f b pJ K P P= × − (16) where Km is the process mass transfer coefficient, Pb.f and Pb.p are respectively the water vapor pressure of the feed and permeate streams. Km depends on the membrane properties and operating conditions, and its value can be experimentally determined [38 – 40]. It is noteworthy that temperature and concentration polarization might be included in the experimental determination of Km. 3.3. Influences of operating conditions on MD water flux and thermal efficiency Main operating parameters of the MD process include feed temperature, permeate temperature, feed salinity, feed and permeate flow velocity, vacuum pressure, and air gap thickness. Feed temperature is the most influential MD operating parameter with respect to process water flux and thermal efficiency. Elevating feed temperature leads to an exponential increase in water vapor pressure at the feed membrane surface, thus exponentially increasing water flux in all MD configurations. Increasing feed temperature also enhances thermal efficiency of the MD process; therefore, it is beneficial to operate the process at high feed temperature [41 – 45]. However, the temperature and concentration polarization effects become more severe with increased feed temperature [28, 43, 46]. Exacerbated polarization effects might lead to the formation of scales on the membrane surface that consequently deteriorates water flux and distillate quality of the seawater MD process. Increasing permeate (or distillate) temperature on condition of constant feed temperature generally reduces the transmembrane vapor pressure difference, thus lowering water flux. However, the effect of permeate temperature on water flux varies for different MD configurations. In DCMD, an increase in water flux is observed when the permeate inlet Duong Cong Hung, Phan Duc Nhan, Nguyen Van Tinh, Pham Manh Thao, Nguyen Cong Nguyen 670 temperature is reduced [33, 47, 48]. Indeed, it is noteworthy that the effect of reducing permeate temperature on water flux enhancement in DCMD is about 2-fold lower than that of increasing feed temperature [49]. On the other hand, the effect of permeate inlet temperature on water flux is negligible in AGMD and SGMD [50]. As a result, increasing feed temperature is preferable to decreasing permeate temperature for water flux improvement the MD process [26]. Feed salinity affects the heat and mass transfer during the seawater MD desalination process at various extents depending on the process operating water recovery (i.e. the volumetric ratio of the obtained distillate over the seawater feed). At low process water recoveries (< 50 %), the influence of feed salinity on MD water flux and thermal efficiency is negligible [38, 51]. This is because the transfer of water in MD is driven by the water vapor pressure difference across the membrane, and is not affected by the osmotic pressure of the seawater feed as observed in reverse osmosis (RO). At high water recoveries, the seawater feed is concentrated several times. At this such high feed salinity, the effect of concentration polarization becomes noticeable. Increased feed salinity reduces water activity and increases the feed viscosity at the membrane surface, hence leading to a decline in water flux [33, 51, 43 – 45]. The thermal efficiency of the MD process also decreases at high feed salinity [41, 42]. Increasing the feed and permeate flow velocities improves the heat transfer coefficient in the feed and permeate channels, and reduces the concentration and temperature polarization effects, therefore increasing MD water flux. However, the effect of feed and permeate flow velocities on water flux is not as strong as that of feed temperature [49]. In addition, the feed flow velocity has a stronger impact on the water flux than the permeate flow velocity in the DCMD process. However, for the SGMD process, the influence of permeate flow (i.e. sweeping gas flow) velocity on water flux is more significant than that of the feed flow velocity. This is because in SGMD the mass flux is limited by the heat transfer through the sweep gas boundary layer [52] whereas in DCMD it is controlled by the heat transfer through the hot feed boundary layer [7]. The effect of permeate pressure (i.e. vacuum) on water flux and thermal efficiency is noticeable in VMD because the vapor transmembrane pressure difference is partially induced by applied vacuum in the permeate side. The permeate pressure in VMD might be the most effective parameter affecting the process water flux [53]. As the permeate pressure decreases, a higher driving force is induced; consequently, water flux increases linearly [32, 53-55]. However, decreasing permeate pressure to increase the flux also results in a reduction in selectivity in the VMD treatment of feed solution containing dissolved organics [55]. It is worth noting that decreasing permeate pressure also induces a higher transmembrane hydrostatic pressure, hence posing a higher risk of membrane pore wetting [36]. The air gap between the membrane and the condenser in AGMD mitigates the conductive heat loss through the membrane but increases the resistance to mass transfer in the permeate side of the membrane. Thus, the air gap thickness influences both water flux and thermal efficiency of the AGMD process. Lawson and Lloyd [7] observed a sharp decrease in water flux when the air gap thickness increased to 1 mm, then water flux slightly decreased as the air gap thickness reached 5 mm. The authors also reported a significant conductive heat loss with air gap thickness below 0.4 mm. Thus, optimal air gap thickness was recommended to balance water flux and thermal efficiency. Membrane distillation for seawater desalination applications in Vietnam: potential and challenges 671 3.4. Approaches to enhance water flux in MD Key factors affecting MD water flux are: (1) the membrane active surface for water evaporation; (2) the driving force induced by the transmembrane temperature difference; (3) the resistance to mass transfer [56]. The membrane active surface for water evaporation is a function of membrane porosity, and hence is difficult to be improved due to the required membrane mechanical strength. Thus, approaches to enhancing water flux are largely focused on maximizing the driving force and minimizing the mass transfer resistance. The difference between the temperatures at the liquid-vapor interfaces in the hot and cold side of the membrane is the driving force of the MD process. Temperature polarization lowers the driving force. As a result, to increase water flux the convective heat transfer coefficients in boundary layers need improved to mitigate the temperature polarization [57]. The convective heat transfer coefficients are inversely proportional to the boundary layer thickness. Thus, the convective heat transfer coefficients can be improved by promoting the stream turbulence and flow rate to reduce the boundary layer thickness. Employing spacers in MD channels to promote the stream turbulence is an effective method to increase water flux. Phattaranawik et al. [29, 30] employed spacers with various characteristics in the feed and distillate channels of a DCMD system. Temperature polarization was found to approach utility and the system water flux was increased by 60 % with the spacers used. Martinez-Diez et al. [58] also confirmed the effects of spacers on MD water flux. Turbulence caused by spacers led to the decreased temperature polarization and the enhanced mass flux. Moreover, Yun et al. [33] declared that the flux enhancement effect of the spacer in the feed channel was higher than that in the distillate channel. Gas bubbling was also incorporated into the MD process to enhance its performance. By introducing gas bubbles to the feed channel of a DCMD process, Chen et al. [46] observed an increase in the water flux up to 26 % in comparison with that of the non-gas bubbling assisted process. They attributed the water flux improvement to the lowered temperature and concentration polarization due to intensified local mixing and flow disturbance in the feed boundary layer [46]. Moreover, the positive impact of gas bubbling on the MD performance was found more significant at high feed temperature [46]. Using roughened-surface channel to increase MD water productivity in seawater desalination was proposed by Ho et al. [59]. By integrating a rough plate in the feed channel and spacers into the DCMD module, heat transfer in the feed channel was enhanced resulting in an increase of 37 % in water production. However, roughened-surface channel also led to the increase in energy consumption of the system. Thus, an optimum roughness of the feed channel surface was experimentally determined [59]. Employing microwave irradiation was also recommended for MD water flux enhancement. Ji et al. [32] investigated the performance of a VMD system equipped with a microwave source. They found that the mass transfer process of VMD was significantly improved because of applying microwave irradiation. Moreover, the effects of microwave irradiation on water flux enhancement were found to be more significant at low feed temperature, low feed velocity and low vacuum pressure [32]. However, the membrane scaling caused by the deposition of calcium was intensified by microwave irradiation [32]. MD water flux enhancement was also achieved by using fabricated polymeric membranes having higher hydrophobicity. Dumee et al. [60] investigated the performance of commercial membranes and the hydrophobicity-enhanced fabricated membranes with similar geometrical Duong Cong Hung, Phan Duc Nhan, Nguyen Van Tinh, Pham Manh Thao, Nguyen Cong Nguyen 672 properties. Higher flux was obtained with the fabricated membranes in comparison to the commercial membranes. The positive influence of increased membrane hydrophobicity to MD flux enhancement was also confirmed by Bonyadi and Chung [61]. Thus, employing fabricated membrane with high hydrophobicity might be a feasible approach to the MD water flux improvement. 4. POTENTIAL OF MD FOR SEAWATER DESALINATION APPLICATIONS IN VIETNAM MD embodies several prominent features that make it a promising candidate for seawater desalination applications, particularly for remote coastal areas and islands in Vietnam. As a thermally driven process, water flux in MD is negligibly affected by the feed osmotic pressure as compared with other pressure-driven membrane desalination processes (e.g. RO and nanofiltration (NF)). As a result, the MD process can concentrate the seawater feed up to the saturation limits of salts in the seawater feed. Given this capability, MD has been employed as a stand-alone seawater desalination process, or combined with a seawater RO desalination process to improve the process water recoveries and minimize the RO brine volume. MD can offer a cost-effective technology platform to seawater desalination application in Vietnam. The MD process does not involve high hydrostatic pressure to achieve salt-water separation as required in RO and NF; therefore, MD systems can be made from inexpensive non-corrosive materials (e.g. plastics and aluminium alloys) to reduce the process investment costs. The absence of high hydrostatic pressure together with the discontinuity of the liquid phase across the membrane renders the MD process much less prone to membrane fouling than RO and NF. As a result, the seawater MD desalination process can be sustainably operated with little seawater feed pre-treatment (e.g. sand filtration or cartridge filtration) without any membrane fouling issues. Indeed, Duong et al. [38] have demonstrated that the seawater MD desalination process could be sustainably operated at a water recovery of 70 % without any observable membrane fouling or scaling when actual raw seawater feed was pre-filtered by 0.45 µm filter paper. The MD process also inherits typical attributes of membrane processes, including modulation, compactness, and process efficiency; therefore, it requires significantly less physical and energy footprints as compared to conventional thermal distillation (e.g. multi- stage flash (MSF) and multi-effect distillation (MED)). Finally, the primary energy input to the MD process is heat at mild temperatures (i.e. ranging from 40 to 80 °C). Low-grade heat such as waste heat or solar thermal energy can be sourced to meet the energy demand of the MD process, leading to noticeable process energy cost savings [62]. As a result, MD can be an ideal replacement for RO or MSF and MED in the desalination applications which require a low-cost and maintenance-free process. Given the above-mentioned attributes, MD can be an ideal technology platform for small- scale seawater desalination applications in Vietnam. With more than 3000 km of coastline and great numbers of islands, Vietnam is in a great need for small-scale, de-centralized, stand-alone, and low maintenance or maintenance-free desalination systems that can provide drinking water at affordable cost directly from seawater. Given their low investment and operational costs, MD systems can be deployed to provide fresh water to people and military personnel in coastal areas or on islands, such as Spratly Islands. Small-scale MD systems can also be installed on fishing boats to meet drinking water demand of the fishermen on the boats. The waste heat from the boat engine can be utilized to supply the thermal energy demand to the MD system. With an MD system on boats, lack of fresh water will no longer be a concern for long-travelled fishermen. Membrane distillation for seawater desalination applications in Vietnam: potential and challenges 673 5. CHALLENGES TO SEAWATER MD DESALINATION 5.1. Membrane pore wetting One vital requirement for the seawater MD desalination process is the non-wettability of the membrane pores. To achieve a complete salt rejection, only water vapour is allowed to transfer through the membrane pores, and the pores must be in dry condition. Under certain conditions, liquid water can penetrate the membrane pores and render them wet. When the membrane pores are wetted, the membrane active surface area for water evaporation is reduced, leading to decline in the process water flux. In addition, the penetration of liquid saline water through the wetted membrane pores reduces the salt rejection of the membrane, and hence deteriorates the quality of the MD water product (Figure 4). Figure 4. Changes in water flux and distillate quality (i.e. distillate electrical conductivity) when the membrane pores are wetted due to membrane scaling in a AGMD process with actual seawater feed (from [39]). Factors that can lead to membrane pore wetting during the MD process are the deposition of contaminants in the feed water on the membrane surface and the resultant degradation of the membrane. As implied in the Eq. 2, a higher LEP value can be achieved when using a more hydrophobic membrane (i.e. θ > 90°) with the feed solution having a high surface tension (λL). Most membranes used in MD have water-membrane contact angle in the range from 120° to 130° [63], and fabricated surface-modified membranes with water-membrane contact angle as high as 160° and 178° have been proposed for the MD process for desalination applications [64, 65]. Contaminants depositing on the membrane surface can alter its hydrophobicity, thus reducing LEP and increasing the risk of membrane pore wetting. Moreover, organic contaminants such as surfactants and detergents can greatly reduce the surface tension of the feed water [66], leading to further reduction in LEP. Duong Cong Hung, Phan Duc Nhan, Nguyen Van Tinh, Pham Manh Thao, Nguyen Cong Nguyen 674 5.2. Membrane fouling and scaling Membrane fouling is a major hindrance to the commercialization of MD for water treatment and desalination [67, 68]. Fouling reduces permeability, shortens the lifetime of membranes, and increases energy consumption. Consequently, membrane fouling raises the operational costs of the MD process. The investment cost of the MD process is also increased because of additional pre-treatment facilities and chemicals required to prevent and control fouling [67, 69]. Membrane fouling in MD is defined as the accumulation of undesirable deposits onto the membrane surface or into the membrane pores leading to a decline of membrane efficiency [70, 71]. The formation of unwanted materials adds extra resistance to the total mass transfer resistance of the MD process. The undesirable deposits might be particulates, gels formed by organic substances, precipitated crystals of sparingly soluble salts, and biofilm formed by microorganisms. Membrane fouling is categorized into four types, namely colloidal fouling, organic fouling, scaling, and biofouling according to the nature of particles that induce fouling. Amongst these types, organic fouling and scaling are the most prevalent in seawater MD desalination applications [70, 71]. Organic fouling is a result of the adsorption of dissolved organic substances such as oil, macromolecules, proteins, humic acids onto the membrane surface. The accumulation of these organic matters on the membrane surface leads to a decline in membrane permeability. It is worth mentioning that despite their low concentration in the feed water, organic foulants often cause severe declines in MD water flux because they can form complexation with calcium scales in the feed water [72, 73]. Moreover, hydrophobic MD membranes are more prone to organic fouling due to hydrophobic adsorption of organic materials to the membrane surface [72, 74]. Scaling (or inorganic fouling) in the MD process is caused by the precipitation of sparingly soluble salts at their super-saturation state. The most likely scalants faced in MD desalination are calcium sulfate (CaSO4), calcium carbonate (CaCO3), and silicate [14, 39, 75]. These scalants have limited and temperature-inverse solubility (except silicate) in the MD operating temperature range [76]. During the MD process, when water is extracted from the feed solution, the concentrations of the sparingly soluble salts in the feed channel increase and might reach super-saturation, posing a high risk of scaling. The scale formation on the membrane can constrain the MD desalination process from achieving high water recovery ratios [51, 77]. MD operating parameters exert great effects on the scale formation rate and the scale morphology. Gryta [78] reported that increasing feed temperature resulted in a higher rate of the carbonate scale formation, and low feed flow velocity led to a more compact deposit layer on the membrane. A similar trend was observed in the study of Wang et al. [56]. Nghiem and Cath [68] observed more severe scale formation of CaSO4 than that of CaCO3 and silicate, and they also found that increased feed temperature and CaSO4 concentration led to a decrease in the induction time and an increase in the CaSO4 crystal size. He et al. [77] declared that the co-precipitation of CaCO3 and CaSO4 formed more adherent and tenacious deposit layers on the membrane than those consisted of single salts. Duong et al. [39] confirmed the uneven distribution of scale layers and salt crystal morphologies on the membrane surface due to the variation in stream temperatures along the channels of the AGMD module (Figure 5). The scale formation on the membrane in MD is also influenced by the temperature and concentration polarization effects. Due to the polarization effect, concentrations of the sparingly soluble salts in the boundary layer adjacent to the membrane are higher than those in the bulk feed solution, hence increasing the scale formation tendency [56. 76. 79]. In contrast, the Membrane distillation for seawater desalination applications in Vietnam: potential and challenges 675 temperature polarization effect reduces the temperature of the feed solution next to the membrane, and might increase the solubility of sparingly soluble calcium salts; therefore, it lowers potential for the scale formation. However, the influence of the temperature polarization effect on the scale formation is trivial in comparison with that of the concentration polarization effect [76, 77]. It is noteworthy that unlike sparingly soluble calcium salts, silica has solubility proportional to temperature, thus temperature polarization tends to raise the deposition of silica on the membrane surface [80]. Figure 5. Morphologies of scale layers during the AGMD process of seawater at feed and coolant temperatures of (A) 35/25 °C and (B) 60/50 °C (from [39]). 5.3. Thermal efficiency and energy consumption Together with membrane scaling, intensive energy consumption has been considered a hindrance to the realization of MD for seawater desalination applications. As a phase-change separation process, MD consumes huge amount of thermal energy (i.e. heating and cooling) to facilitate the phase conversion of water from liquid to vapor and vice versa. The transfer of the latent heat that is associated with the transfer of water coincides with the heat conduction through the membrane during the MD process. The heat conduction through the membrane, which is the heat loss, can account for up to 50 % of the total heat input of the MD process [7]. As a result, most MD processes reported in the literature demonstrate poor energy efficiency with specific energy consumption of several orders of magnitude higher than that of RO [9, 36, 81]. Specific thermal energy consumption (STEC) is commonly used to evaluate the performance of the seawater MD desalination process with respect to thermal efficiency. It is noteworthy that STEC of MD processes reported in the literature is widely dispersed as recently highlighted by Khayet [82]. The STEC of the MD process can differ in 3 orders of magnitude, ranging from as low as 1 up to 9,000 kWh/m3 [82]. The wide dispersion in STEC values is attributed to the variation in the configuration, membrane module geometry, and operating conditions of the MD process [82]. As a notable example, Carlsson [83] reported a very low STEC of 1.25 kWh/m3, but failed to provide any analytical details and operating parameters of the MD process used in his study. Koschikowski et al. [11] reported a STEC value of 117 kWh/m3 for an MD system with an 8 m2 spiral-wound AGMD membrane module at 75 °C evaporator inlet temperature and 350 L/h water flow rate. A larger AGMD system (i.e. with membrane area of 40 m2) exhibited a higher STEC value ranging from 200 to 300 kWh/m3 [84]. Much higher STEC values were reported for the MD processes using DCMD configuration. Of a particular note, Criscuoli et al. [85] demonstrated a DCMD process with really high STEC values ranging from 3500 to 4580 kWh/m3. (A) (B) Duong Cong Hung, Phan Duc Nhan, Nguyen Van Tinh, Pham Manh Thao, Nguyen Cong Nguyen 676 Thermal efficiency of the MD process can be significantly enhanced, and thus the process STEC can be reduced by recovering the latent heat associated with the water vapour transfer. In AGMD, the recovery of the latent heat can be achieved inside the membrane module. The feed water can be fed to the coolant channel to act as a coolant fluid, and in tandem to be preheated by the latent heat of water vapour condensation. Then, the preheated feed water can be additionally heated by an external heat source to reach a desired temperature prior to entering the feed channel of the AGMD membrane module. Thus, STEC of the AGMD process can be noticeably reduced. Operating conditions, including feed inlet temperature, feed salinity, and particularly water circulation rate, are expected to exert strong influences on the STEC of the AGMD process. Indeed, Duong et al. [13] have demonstrated a pilot single-pass seawater AGMD desalination process with a minimum STEC of 90 kWh/m3, and the process water circulation rate was found to be the most influential operating factor affecting the water flux and thermal efficiency of the pilot process. Unlike in AGMD, in DCMD the heat recovery can be achieved using an external heat exchanger [86]. The latent heat accumulated in the distillate stream is recovered to preheat the feed stream in the heat exchanger. When the heat exchanger is coupled with the DCMD membrane module, the relative flow rate between the feed and the distillate stream and the surface areas of the heat exchanger and the membrane module strongly determine the process STEC [86]. The DCMD process obtains minimum STEC at a critical relative flow rate and with infinite heat exchanger and membrane module surfaces [86]. In practice, however, it is unfeasible to have heat exchanger and membrane module with infinite surfaces. Thermal efficiency of the DCMD process can also be improved by brine recycling [38]. In the DCMD process, particularly for the small-scale system with short membrane channels, the warm brine leaving the membrane module contains a considerable amount of sensible heat. When the brine is recycled in the process, the brine sensible heat can be utilised, hence reducing the total heat demand and STEC of the process. Brine recycling also helps enhance the utilisation of the available membrane surface area to increase the water recovery ratio of the DCMD process. Indeed, Saffarini et al. [87] have suggested brine recycling for MD thermal efficiency improvement. A major challenge to brine recycling in seawater DCMD desalination is to manage the negative influence of membrane scaling and increased feed salinity on the water flux and salt rejection of the process. Thus, Duong et al. [38] have experimentally optimized the DCMD desalination process with an actual seawater feed under brine-recycling operation mode. The experimental results revealed an optimal process water recovery ranging from 30 % to 60 %. Within the optimal water recovery range, the influence of increased feed salinity on water flux was negligible, no membrane scaling occurred, and the process could obtain a virtually complete salt rejection. Most importantly, the STEC of the process under brine-recycling operation was reduced more than half when operated in the optimal water recovery range [38]. 4. CONCLUSIONS Seawater desalination using membrane distillation (MD) can be a pragmatic solution to fresh water scarcity in Vietnam. As a hybrid desalination process, MD inherits attributes of both pressure-driven membrane separation and thermal distillation. These attributes include process modularization, low susceptibility to feed osmotic pressure, low risk of membrane fouling and thus negligible feed water pre-treatment required, and low investment and operational costs. In this paper, a comprehensive review of the seawater MD desalination process was provided. 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