How to make membrane distillation greener: a review of environmentally friendly and sustainable aspects

Emilia Gontarek-Castro *^a and Roberto Castro-Muñoz ^b
aDepartment of Environmental Technology, Faculty of Chemistry, University of Gdansk, 80-308 Gdansk, Poland. E-mail: emilia.gontarek-castro@ug.edu.pl
^bFaculty of Civil and Environmental Engineering, Department of Sanitary Engineering, Gdansk University of Technology, 11/12 Narutowicza St., 80-233 Gdansk, Poland

First published on 16th November 2023

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1. Introduction

Over the past century, the population of the world has increased, as has the demand for water. This global challenge for water supply will become even more serious due to further expansion in the population and economic growth, followed by a continuing increase in demands on water resources. The World Water Council estimated that by 2030, a few billion people will suffer from water scarcity or poor water quality.¹ Thus, it is not surprising that governments and industry have tried to develop alternative water sources, water recycling, water imports, and desalination to face the concern of the world's growing demand for clean water. In general, desalination techniques can be divided into two groups: thermal-based and membrane-based. The former ones, thermal technologies, including multi-stage flash, multi-effect distillation, and vapor compression distillation,² require the supply of thermal energy to induce the evaporation of water molecules from seawater; the vapor is subsequently condensed to give drinking water. Despite its versatility and ease of operation, thermal desalination has one significant disadvantage, its high energy consumption, as it is a costly and environmentally unfriendly method. For this reason, alternative methods to thermal desalination are sought, mainly to reduce the costs of the process.

Currently, among the most dynamically developing desalination techniques are membrane-based technologies, which are becoming more popular due to their lower energy expenditure, compact modular construction, possibility of scaling-up, lower environmental footprint, and the possibility of spending investment recovery.³ Among membrane-based technologies, the dominant one used for desalination (in terms of the amount of treated water) is reverse osmosis, which, according to the literature, produces up to 60% of the world's desalted water;⁴ however, both costly installation and maintenance represent the main drawbacks of this technology. This has encouraged researchers to seek new membrane desalination alternatives, such as membrane distillation (MD), which is a promising technique for seawater desalination. The driving force of the process is based on a vapor pressure gradient across the membrane. This gradient is induced by the temperature difference between the feed and permeate solutions. The solutions are separated by a hydrophobic microporous membrane that allows the diffusion of vapor while preventing the permeation of the aqueous phase. Water evaporates at the solid/liquid interface on the heated feed side of the membrane, then diffuses through the air trapped in the membrane pores, and finally condenses at the cooler permeate side. Simultaneously, non-volatile substances remain in the aqueous feed solution. MD seems to be an effective method of seawater desalination as it successfully deals with high salinity content, high oil content, and high surfactant content feed. Despite the great progress in MD membrane preparation and performance improvement, their fabrication approaches and implementation procedures still need to be revised in terms of toxicity and pollution generation.^5,6 For instance, the typical materials used for membrane manufacture are perfluorocarbon-based non-biodegradable polymers, while toxic organic solvents are mostly applied at several membrane manufacturing stages, producing more than 50 billion liters of contaminated wastewater yearly and contributing to various health and environmental risks. Due to the fundamentals of the MD process, more and more emphasis is being placed on the fabrication of superhydrophobic or even omniphobic membranes; however, typical substances applied for the post-fabrication modification of membrane surface properties are toxic fluoroalkyl silanes (FAS). Another aspect is the energy expenditure; the low energy efficiency of the standalone MD system significantly reduces its potential application. Due to the constant growth of environmental pollution problems, improving the sustainability of MD and developing green manufacturing protocols are a must around all aspects. Fig. 1 illustrates a comparison of the current scenario of MD and the aim of pursuing the green and sustainable scenario soon. This latter panorama can be reached by following the updated “twelve principles of green chemistry”, developed by Anastas and Warner⁷ and proposed by the United States Environmental Protection Agency. The idea represents the design of safer chemical processes and products that reduce or eliminate the use or generation of hazardous chemicals.

In the light of green chemistry principles, the most often used definition of sustainable development is the one proposed by the Brundtland Commission in 1987 as “development that meets the needs of the present without compromising the ability of future generations to meet their own needs”.⁸ Thus, to be really “sustainable”, the MD process should not only be free of the use of hazardous substances in membrane fabrication but also in the operating step process. We believe that these aspects need to be reviewed and discussed exhaustively, and to the best of our knowledge, there has been no report discussing these aspects to date. Therefore, in this review, the latest advances related to the greener approaches of MD are described in detail. We highlight different ways to improve the sustainability of the membrane manufacturing process, such as the use of biodegradable polymers, green solvents and FAS-free modification. The strategies for the solvent-free fabrication are discussed together with the possibility of reducing the amount of solvents at various manufacturing stages. Finally, to reduce the energy consumption and make the MD process more competitive with reverse osmosis, the energy efficient routes offered by integrating MD systems are also addressed.

2. Polymer membrane fabrication

2.1. Biodegradable and perfluorocarbon-free polymers

For the MD process, only water-repellent membranes, such as the ones made of petroleum-based polymers (polytetrafluorethylene (PTFE) and polyvinylidene difluoride (PVDF)), or coated with perfluorosilanes, have been extensively investigated.^9,10 In the case of fluorocarbon membranes, they are expensive and prone to partial wetting during long-term use, due to the interactions between the membrane surface and contaminants.^11–13 These membranes owe their commercial interest to their suitable properties in terms of their high porosities, low thermal conductivities, stability and hydrophobic nature.¹⁴ Despite their many advantages, one of the main concerns of using and disposing of these polymer materials is their low biodegradability; this refers to materials which, once disposed of in nature, undergo degradation via microorganisms. Owing to their high thermal and chemical stability, they are resistant to microbial attack as their carbon linkages cannot be broken by enzymes and microorganisms. On the other hand, biodegradable polymers originate from three sources, plants (for example cellulose acetate and starch), animals (collagen and sericin), and sometimes synthetic resources (poly(butylene succinate) or poly(ε-caprolactone)). The application of these materials is rapidly growing, especially in the packaging industry; however, an increased interest is also observed in biodegradable membrane fabrication.¹⁵ Although efficient MD membranes made from biodegradable and low-cost materials are highly desired, they are still not commercially available.

Recently, a few studies have proved the possibility of using cellulose for MD membrane preparation.¹⁶ Cellulose, which is a renewable polymer and found abundantly on Earth, can be extracted from various sources, including underutilized biomass feedstocks such as agricultural residues, recycled cellulosic products, and industrial waste. These fibers possess unique structural and mechanical properties, and they are abundant, cost-effective, and environmentally friendly. Thus, it has been found attractive to explore the use of cellulose fibers as a material to create permeable MD membranes. The presence of hydroxyl groups on the cellulose surface makes it hydrophilic; therefore, a major concern in applying this material for MD membrane preparation is to create a hydrophobic surface with a green and sustainable approach. Several studies demonstrate different approaches for the creation of hydrophobic cellulosic surfaces, such as chemical modification with fluoropolymers or silanes, and processing methods, namely chemical vapor deposition, plasma treatment, and electrospinning (see Table 1).

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For instance, Leitch et al.¹⁷ fabricated a novel fibrous nanocellulose aerogel membrane from bacterial nanocellulose, which met the requirements of the MD process (e.g., high porosity >98%, and low thermal conductivity <0.03 W m⁻¹ K⁻¹). Due to their isotropic fibrous structure and easily tunable fiber diameter between 15 nm and 7 μm, these biomembranes may become a natural substitute for electrospun polymer membranes.^18,19 The authors compared the direct contact membrane distillation (DCMD) performance of nanocellulose aerogel membrane and commercial PVDF membrane. The high void fraction of nanocellulose aerogel membrane led to the reduction of conducted heat flux, thus improved membrane thermal efficiency and temperature polarization coefficient (see Fig. 2a and b). On the other hand, smaller average pore diameter and higher thickness, compared with the commercial PVDF membrane, increased the mass transport resistance of the nanocellulose aerogel membrane. Dizge et al.²⁶ fabricated a superhydrophobic and oleophobic membrane by a three-step process: the preparation of cellulose nanofibers by electrospinning, modification with silica and surface fluorination. The membrane system displayed exceptional resistance to wetting and maintained a high and stable performance. The water vapor permeability against 1 M NaCl feed solution was found to be as high as 46.3 L m⁻² h⁻¹, which was 1.7 and 1.6 higher when compared with the water vapor flux rate of commercial available PVDF and PTFE membranes, respectively.

Cellulose itself is the basic building block of plant fibers, proving mechanical support to specific growing parts of the plants. This molecule also provides mechanical integrity, which makes it an attractive material for membrane reinforcement.²⁰ Lalia et al.²¹ fabricated polyvinylidenefluoride-co-hexafluoropropylene membranes containing different loadings of nanocrystalline cellulose (NCC) by the electrospinning technique. The polymer membrane modified with NCC was found to have 30% higher tensile strength and 45% higher Young's modulus values than that of the nonmodified one. Additionally, the incorporation of NCC into the membrane resulted in enhancement of its liquid entry pressure (LEP) from 19 psi up to 27 psi. This high LEP led to the effective DCMD performance with a water flux of 11.5 L m⁻² h⁻¹ and salt rejection of 99%. Hou et al.²² have fabricated a cost-effective membrane directly from a sustainable wood material. In contrast to nanocellulose-based membranes, this membrane was made by directly removing lignin and hemicellulose from the wood samples via chemical treatment and freeze-drying. The prepared membrane had high porosity (∼90%) and hierarchical pore structure, while the presence of crystalline cellulose nanofibrils and xylem vessels and channels across the structure led to the facilitated water vapor transport. This innovative plant-based material demonstrated potential for MD water desalination, and the performance comparable to or better than that of commercially available membranes derived from fossil resources. For instance, the intrinsic permeability of nanowood membrane was 2.5 times higher than that of commercial polypropylene (PP) membrane, with exceptional thermal efficiency exceeding 70% (see Fig. 2c and d). Although nanowood membrane can be fabricated by a scalable top-down approach, it needed the FAS (perfluorodecyltriethoxysilane) treatment to become hydrophobic.

Recent studies proved the possibility of using cellulose for fabricating dual-layer membranes combining both hydrophobic and hydrophilic polymers. This type of membrane is supposed to eventually reduce the vapor transport path, thus improve the mass transport across the membrane. Owing to their surface properties, these membranes can be used for treating oily waste waters. For instance, Arumugham et al.²³ fabricated a dual-layered membrane made by coating the cellulose substrate with novel perfluorooctanoic acid-modified melamine nanofillers embedded in PVDF. The long perfluoro chain in hydrophobic fillers increased membrane surface roughness by randomly overlapping with the PVDF backbone, resulting in a higher water contact angle. Results indicated that the 1% PFOM membrane demonstrated high flux and rejection ratio, making it a promising candidate for seawater desalination.

Nassrullah et al.²⁴ used zeolite nanoparticles to open up the cellulose structure, and thus to increase the porosity in dual-layered membrane. Their process initially involves the preparation of casting solution using nano zeolite and microcrystalline cellulose, and the subsequent utilization of this solution to coat electrospun polymer membrane. Finally, each of the cellulose-coated membranes is physically stacked on the top of an unmodified electrospun membrane to form a double-layer membrane. The results showed that the addition of nano zeolite to the cellulose coating can effectively enhance the membrane performance without compromising the membrane selectivity. In the study of Joshi et al.,²⁵ superhydrophobic cellulose-based membranes containing a dual-layered structure have been fabricated using a straightforward method. The superhydrophobic properties have been achieved by the surface microstructure modification with inorganic filler, followed by a hydrophobic sizing agent treatment, which is a common additive in papermaking. Cellulosic membrane was tested for desalination of water using the DCMD configuration and exhibited high water flux (23.0 ± 0.06 kg L m⁻² h⁻¹) and high salt rejection (97.5%), comparable to the performance of commercial PTFE membranes.

Das et al. proposed the application of perfluorocarbon-free MD membranes derived from silica and poly(methyl methacrylate) (PMMA) that are both water-wet materials. Inspired by the cuticles of springtails and hairs of sea skaters, they developed a two-step drilling process to create biomimetic gas-entrapping membranes (GEMs). Interestingly, the resulting GEMs comprised vertically aligned cylindrical pores with re-entrant inlets and outlets, which effectively entrapped air on the membrane surface upon immersion in wetting liquids and gave it omniphobic character. To some extent, MD testing proved a robust separation of salt solutions from deionized water, yielding a stable and high flux of desalinated water permeation of 1 L m⁻² h⁻¹ for over 12 h.

Remark. A significant number of publications have been devoted to minimizing the environmental impacts of membrane manufacture, and to developing polymers with environmental sustainability and minor disposal concerns. Nevertheless, the work on bio-based and biodegradable membranes is much more widespread for membrane processes other than MD. Polylactide, poly(lactide-co-glycolide), polyhydroxyalkanoates, chitosan and chitin, starch and poly(vinyl alcohol) are common biodegradable membrane materials that have been extensively studied towards enhanced membrane separation via microfiltration, ultrafiltration, nanofiltration, electrodialysis, and reverse osmosis.¹⁵ Although the low cost of bio-based membranes may facilitate the dissemination of technology for water desalination in various areas, it is important to notice that for MD operation, one side of the membrane must be modified to become hydrophobic. So far, the chemical vapor deposition method, to cover the hydrophilic fibers with hydrophobic silane,⁹ and FAS (perfluorodecyltriethoxysilane) treatment²² have been proposed. Therefore, it is apparent that the abovementioned membrane preparation procedures are often not environmentally friendly. The development of more environmentally friendly methods for tailoring the structure of biodegradable membranes should be explored in detail.

2.2. Sustainable and green approaches for porous membrane preparation

Porous MD membranes can be prepared by various methods, including stretching, template leaching, track etching, phase inversion, and electrospinning, which are widely described and explained elsewhere.^19,27,28 Among them, phase inversion stands out as the most applied for the manufacture of commercially available membranes, and it attracts continuously growing attention from researchers, as reflected in the growing number of publications (see Fig. 3a). The basis of the phase inversion process is the preparation of a thermodynamically stable polymer solution which is subsequently transformed from a liquid into a solid state (coagulation) in a controlled manner. This coagulation is preceded by a so-called liquid–liquid demixing. In general, a polymeric solution is used to form the shape of the membrane, which is followed by membrane immersion into a liquid coagulant also called nonsolvent. Then the demixing starts, separating the polymer solution into a polymer-rich and a polymer-lean phase. The higher polymer concentration phase starts to coagulate and leads to the solid membrane matrix, while the polymer-lean phase leads to pore formation. The membrane immersion in a nonsolvent bath is called a non-solvent induced phase separation (NIPS); however, the demixing process may also be induced by controlled evaporation of the volatile solvent from the polymer solution (evaporation induced phase separation, EIPS), thermally induced phase separation (TIPS) or by placing the cast film in a vapor phase consisting of nonsolvent – vapor induced phase separation (VIPS).

The other membrane preparation method is electrospinning. It is known as a simple and reliable technique for nanofiber preparation from a variety of polymers and is extensively used by the research community for the design of nanostructured matrices for different applications (see Fig. 3b). It involves the application of a high electric field to create nanofibers from a charged polymer solution or melt. By varying electrospinning parameters and polymer solution properties, different morphologies of the membrane can be produced. Despite the universality of phase inversion and electrospinning methods, their ease and the feasibility of producing membranes with the intended properties, they require the preparation of a polymer dope solution in order to form a membrane shape. Usually, hydrophobic polymers, such as PTFE, PVDF, PP and/or polyethylene (PE),²⁹ exhibit low polarity, requiring them to be dissolved in conventional non-polar (or nearly non-polar) solvents, such as N-methyl-2-pyrrolidone (NMP), N,N-dimethylformamide (DMF) and N,N-dimethylacetamide (DMAc). Although these chemicals are considered as fine universal solvents, with great abilities to dissolve both amorphous and semi-crystalline polymeric materials, they have an adverse effect on living organisms due to their serious toxicity according to multiple reports (see Table 2).³⁰ For instance, DMF is considered as a carcinogenic compound that, according to the International Agency for Research on Cancer, gives rise to mutations in mammalian somatic cells. On the other hand, DMAc is deemed to be a human reproductive toxicant and responsible for the promotion of congenital malformation in the fetus, while NMP brings an adverse effect on human fertility or on the unborn child.

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Therefore, to fabricate sustainable membranes, one must refrain from using these toxic and dangerous solvents by applying one of two common approaches. One of them is to employ green solvents for polymer solution preparation (as discussed in the next subsection), and the second requires the use of a solvent-free method.

3. Membrane modification – substitution of FAS

Fluoroalkyl silanes (FASs) are a group of substances commonly applied in the fabrication of MD membranes in order to create a superhydrophobic or omniphobic surface.⁸⁵ FASs are composed of long hydrophobic carbon and fluorine chains and functional groups that may form strong covalent bonds with the membrane material.⁸⁶ Since the omniphobic surface can repel almost any liquid that it comes into contact with, membrane modification has been studied extensively in order to achieve this property,⁸⁷ as reported in Table 5. It is believed that combination of the multi-scale re-entrant topography and low surface energy material is the best solution for the creation of omniphobicity.⁸⁸ In recent years, several studies have been carried out on omniphobic surfaces for MD application. For instance, various studies reported the use of FASs in order to lower the surface energy of membrane material.^89,90 Lee et al.⁸⁸ have fabricated omniphobic electrospun PVDF-HFP membrane for DCMD operation. To create multi-scale re-entrant structures, silica nanoparticles were attached to the polymer via electrostatic interaction. Then, low surface energy (heptadecafluoro-1,1,2,2-tetrahydrodecyl)trichlorosilane (FDTS) was grafted on the silica-modified PVDF-HFP nanofibers, which provided excellent omniphobic properties. Recently, Deng et al.⁹¹ fabricated a nanofibrous PVDF membrane with a self-roughened fluorosilane omniphobic coating, without the use of any auxiliary nanoparticles. The preparation method included the electrospinning of PVDF nanofibers, followed by fiber functionalization with long-chained fluorododecyltrichlorosilane (FTCS) via simple solution immersion. The morphologies of these materials revealed the presence of hierarchically re-entrant structures that exhibited the characteristics of high omniphobicity.

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An important aspect in the application of FASs is their cost; these chemical reagents are typically expensive feedstock. For example, 1 g of commonly used 1H,1H,2H,2H-perfluorodecyltriethoxysilane can cost 28–40$.⁹⁹ Furthermore, FASs may pose a negative environmental impact. Because of the difficulty of breaking the carbon–fluorine bond, per- and polyfluoroalkyl substances are persistent in the environment. Based on their chemical properties and various studies suggesting potential human toxicity and emerging water pollutant concerns,^100,101 researchers began to look for replacements for long-chain per-fluorinated additives in lowering the membrane surface energy.

A CF₄ plasma treatment has been proven to be an effective method to enhance membrane hydrophobicity (Yang et al.,^102,103). It provides a suitable membrane etching that increases surface roughness, and the formation of CF₂–CF₂ and CF₃ bonds that decrease the membrane surface energy. While this treatment imparted the omniphobic characteristic to the membrane and significant improvement in transport properties,¹⁰⁴ it still required the use of a fluorinated additive, that is CF₄ gas, with the potential negative impact on the environment and the human body. Lee et al.¹⁰⁵ proposed a two-step electrospinning–electrospray method to fabricate a superhydrophobic surface without fluorinated additives. They created a polymeric microsphere coating through electrospraying a polymer mixture of PVDF and poly(dimethylsiloxane). Although the pure microsphere-coated membrane showed superhydrophobic properties with a 156.7° contact angle, the investigators needed to add silica-based aerogel particles to strive toward omniphobicity. The 30% aerogel-assisted microsphere-coated membrane presented a high and stable resistance to wetting by a relatively low surface tension solution of 3.5% NaCl containing 0.1 mM sodium dodecyl sulfate. Nevertheless, such a surface coating method requires a more complicated two-step process, which leaves internal pores unmodified and poses a risk of inhomogeneous surface coverage. Guo et al.¹⁰⁶ adopted a one-step biaxial electrospinning and electrospray method to fabricate a hybrid monolayer nanofiber and nanosphere superhydrophobic PVDF-HFP membrane without the use of any long-chain perfluorinated additive. The surface roughness of the nanosphere–nanofiber membrane increased 3.58 times, while good surface stability was achieved thanks to its single-layer structure. The fabricated membrane exhibited excellent resistance to wetting and fouling when operated with saline feed water, which consisted of dissolved low surface tension surfactants and organic matters, confirmed with a 7-day MD test (rejection rate of 99.8% and 30 LHM flux). Zheng et al.¹⁰⁷ fabricated a hierarchically structured membrane by grafting octavinyl-polyhedral oligomeric silsesquioxane on PVDF membrane via UV-induced thiol–ene click reaction. Although this method requires chemical preparation of PVDF support (the hydroxylation and sulfhydrylation pretreatment), it offers a fluoride-free, green and cost-effective strategy for superhydrophobic membrane preparation. The fouling and anti-wetting properties of the as-prepared membranes were found to be superior compared with the peer membrane reported in the previous literature. In the study of Zhang et al.,¹⁰⁸ fatty acids, as naturally occurring compounds, have been proposed as a cost-effective and more environmentally friendly alternative for hydrophobic modification. The carboxylic groups of the fatty acids can be involved in the creation of chemical bonds with the membrane surface, while the long carbon chains are supposed to contribute to the hydrophobicity enhancement. The authors studied two different fatty acid chlorides of palmitoyl chloride and stearoyl chloride with different carbon chain lengths. Due to the presence of an acyl chloride group, these chemicals are supposed to easily react with functional groups on ceramic membranes. Both fatty acid chlorides with different carbon chain lengths formed strong covalent bonds on the membranes with high water contact angle and LEP values, comparable to the hydrophobic membranes prepared through silanization. The comparison of as-reported FAS-free MD membranes is included in Table 6.

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Remark. The elimination of fluoroalkyl silanes from the procedure for obtaining omniphobic membranes is in fact problematic due to their extremely low surface energy, which is unprecedented in other substances. The abovementioned studies on FAS-free synthesis of omniphobic membranes applied other fluorine materials instead, e.g., CF₄ or fluoropolymers, to achieve low surface energy membranes. Therefore, these methods for obtaining omniphobic surfaces cannot be described as environmentally friendly. On the other hand, attempts have been made to replace FASs with naturally occurring compounds, e.g., fatty acids, giving promising results. However, in the field of MD membranes, this topic is still in its infancy.

4. Design for energy efficiency

According to the 6^th principle of green chemistry, the energy requirements of chemical processes should be recognized for their environmental and economic impacts and should be minimized. If possible, synthetic methods should be conducted at ambient temperature and pressure. The MD process meets this regulation in two ways. First of all, it operates at low temperatures as compared with the conventional thermally driven desalination processes, namely multi-stage flash and multiple-effect distillation. Unlike conventional pressure-driven membrane processes, such as reverse osmosis, MD can operate at atmospheric pressure. On the other hand, since MD is a thermally driven process, it remains an energy-demanding technology. The overall estimations of the cost of energy in water production clearly emphasise the economic weakness of the standalone MD systems. Energy consumption accounts for 30% of the total cost of water price using MD (see Fig. 6), and at the same time, it is the highest contributing factor among others, such as the cost of heat exchangers, storage and membrane modules, and other devices. The high energy consumption in MD systems is due to the need to heat the feed to high temperatures (even up to 70 °C) to provide high driving force for vapor permeation through the membrane. Moreover, MD systems are prone to heat loss by the conduction that occurs in the membrane during the mass transport of vapor.¹¹⁰ However, based on the analysis and economic evaluation made by Usman et al.,¹¹¹ MD systems may show better economic potential than reverse osmosis systems (Fig. 6). Due to relatively low operating temperatures, alternative energy sources can be applied to run the MD process. For instance, the integration of renewable energies into the MD process significantly reduces the contribution of energy consumption in the total cost of water price and makes the process environmentally friendly and sustainable. As shown in Fig. 6b, the implementation of waste heat recovery into MD systems decreases the water price from 6.78 $ per m³ to 4.63 $ per m³. In addition, as shown in Fig. 6c, the effect of using solar energy further reduces the price of the water to 1.60 $ per m³. The cost breakdown is a result of the reduction of energy impact on the total cost of water price to only 2%, which is a significant decrease when compared with conventional standalone MD systems (30%).

The utilization of low-grade waste heat as a driving force has gained considerable attention, as it significantly reduces operating costs,¹¹² giving MD promising potential as a viable technology for future desalination.¹¹³ Recent studies examined the suitability of the MD systems for integration with different types of low-grade heat sources,¹¹⁴ such as solar energy, geothermal energy¹¹⁵ and waste heat.¹¹⁶ Among all low-grade types of heat applied in MD systems, solar energy seems to be the most studied so far. However, the use of solar energy requires advanced control strategies.^117,118 Furthermore, the instability of solar radiation and its limited supply time needs to be overcome using technically and economically feasible systems.¹¹⁹ Nevertheless, various studies confirm the possibility of implementing integrated modules in different MD configurations. For instance, Ma et al.¹²⁰ recently built a small-scale VMD unit to provide drinking water in remote areas and provide electricity via direct solar heating. Concurrently, Soomro et al.¹²¹ investigated the performance and economic comparison of solar power plants integrated with DCMD. Interestingly, the water production cost was found to be 0.314 USD per m³ for the solar-powered plant integrated with the DCMD system. Moore et al.¹²² developed a comprehensive process model and designed an economically optimal system. Thermal energy for distillation was provided by solar thermal collectors and electricity was provided using a photovoltaic collector. Recently reported studies on MD systems using low-grade heat energy and their desalination performance are summarized in Table 7.

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Baghbanzadeh et al.¹²⁷ have demonstrated the innovative concept of a zero-waste and energy efficient, thus sustainable MD strategy. Since the main energy concern of conventional MD desalination is its requirement for large quantities of thermal energy to heat up the feed, these authors have established a zero thermal energy input MD. The proposed process implies the use of the naturally occurring temperature difference between the surface of seawater (at 30 °C) and the bottom (at 10 °C) as the process driving force. The results show the feasibility of producing pure water with a flux reaching 11.3 L m⁻² h⁻¹ and a cost equal to $0.28 per m³, which is significantly lower than that offered by the currently dominating membrane desalination technology, reverse osmosis ($0.45–2.00 per m³).

Although the abovementioned studies are praised for the significant reduction of water cost production, the MD process still cannot withstand large production rates while maintaining energy efficiency. Various hybrid systems have already been thoroughly discussed in terms of water production and energy efficiency.^128,129 For instance, MD integration with conventional desalination processes, such as reverse osmosis, multi-stage flash and multi-effect distillation may result in nearly zero liquid discharge and higher performance. The integration of a reverse osmosis system with MD led to a reported water recovery higher than 80%,¹³⁰ which is twice as much as the value for the conventional reverse osmosis. Various integrated desalination systems have been evaluated by González-Bravo et al.¹³¹ It has been shown that MD desalination systems coupled with multi-stage flash or multi-effect distillation techniques achieved the maximum economic and environmental advantages among other MD integrated systems. Interest in integrated systems is also growing within emerging processes. The combination of forward osmosis with MD enables the simultaneous production of clean water and regeneration of the draw solution, ensuring process sustainability. The study of Wang et al.¹³² demonstrated the prospect of employing the forward osmosis–MD hybrid systems for seawater desalination with a high vapour flux of 6/32 LMH (FO/MD). Kim et al.¹³³ used the low vacuum pressure naturally created by adsorbents during the adsorption desalination cycle to drive a VMD system without the need for a vacuum pump. The authors noted a 23% increase in the water recovery ratio and a 21% increase in water production compared with a standalone VMD. More recently, MD hybrids in which the electrodialysis¹³³ or electrocoagulation¹³⁴ modules were used for wastewater pretreatment have emerged. Pretreatment of the feed streams is essential when developing large-scale MD processes, as it significantly reduces the susceptibility of the membrane to fouling and wetting and removes chemicals potentially harmful to the membrane.

Remark. By using low-grade heat energy, it is possible to implement and improve MD for the water supply situation. The current literature suggests that inexpensive energy sources, such as solar energy, geothermal energy, and waste heat, play a crucial role in the water–energy nexus, making the MD process more sustainable and cheaper. Although the techno-economic analysis estimates that the utilization of energy from low-grade heat sources makes the MD process competitive with conventional desalination techniques, an important factor that has a significant impact on MD economics is its ability to create hybrid systems using the strengths of two or more processes. An extensive evaluation of each hybrid energy demand is needed in order to minimize their specific energy consumption.

5. Conclusions

In the future, the definition of the green MD membrane process should be expanded, including sustainability considerations. This transformation requires the innovation of science and technology coupled with new emerging systems design, resulting in a positive impact on a global scale. In the past decade, most research has mainly focused on treating wastewater using membrane technology but has ignored wastewater production during the membrane preparation process itself. According to the literature, the wastewater contributed to more than 95% of the total waste produced during the membrane fabrication process. To achieve a completely green and sustainable development of membrane technology, more attention should be paid to research on the green preparation process. Designing for a circular economy, which requires the establishment of a series of interconnected closed-loop processes, is crucial. This latter idea states that no waste streams are generated while being recycled or repurposed to retain or increase their value. This transition, applicable to membrane science and engineering as well as other domains, calls for innovative engineering solutions that enable new ways of designing. In general, some unsolved scientific and technical problems mentioned in the remark at the end of each section are pending and still need to be investigated.

Abbreviations

AGMD	Air gap membrane distillation
ATBC	Acetyl tributyl citrate
DCMD	Direct contact membrane distillation
DAMc	N,N-Dimethylacetamide
DMF	N,N-Dimethylformamide
DMSO	Dimethyl sulfoxide
ECTFE	Poly(ethene-co-chlorotrifluoroethene)
EIPS	Evaporation induced phase separation
FASs	Fluoroalkyl silanes
FEP	Poly(tetrafluoroethylene-co-hexafluoropropylene)
FDTS	(Heptadecafluoro-1,1,2,2-tetrahydrodecyl)trichlorosilane
FTCS	Fluorododecyltrichlorosilane
GEMs	Gas-entrapping membranes
LEP	Liquid entry pressure
MD	Membrane distillation
MS-S	Melt-spinning and stretching
NCC	Nanocrystalline cellulose
NIPS	Non-solvent induced phase separation
NMP	N-Methyl-2-pyrrolidone
PE	Polyethylene
PEG	Polyethylene glycol
PES	Polyethersulfone
PMMA	Poly(methyl methacrylate)
PP	Polypropylene
PTFE	Polytetrafluorethylene
PVDF	Polyvinylidene difluoride
PVDF-HFP	Poly-(vinylidene fluoride-hexafluoropropylene)
SANIPS	Spray-assisted nonsolvent-induced phase separation
TEP	Triethyl phosphate
TIPS	Thermally induced phase separation
TOTM	Trioctyl trimellitate
VIPS	Vapor-induced phase separation
VMD	Vacuum membrane distillation
WGMD	Water gap membrane distillation

Conflicts of interest

There are no conflicts to declare.

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