Solar-assisted membrane technology for water purification: a review

A shortage of safe drinking water is one of the leading problems in the world. Even in developed countries where water treatment systems are present, safe drinking water may not be always available due to the limitations of advanced water treatment techniques and high energy costs. On the other hand, many rural communities in Asia and Africa situated in semi-arid to arid regions are without reliable access to clean drinking water. It is, therefore, important to explore how solar energy can be linked to water treatment systems for clean drinking water production. Membrane-based water purification technologies play a major role in water purification by utilization of low-cost heat sources to make the process economically and technically viable for small, medium, and large-scale applications. Solar energy can be a viable source of power for water purification facilities in the coming years. Photovoltaic panels and solar thermal collectors are appropriate solar energy collectors for making a solar-powered water treatment system. Solar-assisted membrane-based water purification techniques could have a viable solution to the existing problems in semi-arid and arid regions. Due to the high quality of potable water demand, studies have been carried out on solar-assisted membrane-based technologies in water purification. This review considers basic concepts, specific energy consumption, water production cost, and applications of solar-driven membrane-based water purification technologies such as reverse osmosis, forward osmosis, electrodialysis, membrane distillation, and hybrid membrane systems. This review will allow the researchers to have a wider overview of the effort made by several investigators in the area of solar-assisted membrane-based water purification technology.


GRAPHICAL ABSTRACT INTRODUCTION
Water is one of the most abundant resources on earth, covering three-quarters of the planet's surface. On the other hand, the abstraction of water by human activities is far above the water available. The amount of available freshwater in natural occurrence is constant. In recent years, freshwater demand across the globe is increasing at an alarming rate due to rapid industrial growth, population growth, and higher standards of living and climate change (Kummu et al. ). Therefore, the provision of freshwater is becoming an increasingly important issue in many areas of the world. Water purification is the process of removing undesirable chemicals, biological poisons, suspended solids, and gases from contaminated water. To provide fresh water in an adequate quantity, people are forced to rely on the available water resources treated by thermalbased and membrane-based processes.
The most common thermal-based water purification plants for desalination are multi-stage flash distillation (MSF), multi-effect distillation (MED), and vapor compression (VC). Those conventional water purification methods are energy-intensive, in addition to their complexity and different related operational problems. These shortcomings have forced researchers to search for advanced alternative technologies. One of these alternative technologies is membrane-based water purification technology, which can be coupled with solar energy. Solar water purification can be broadly categorized into two methods.
The first one is the direct use of energy collected in a solar still. Direct methods are suitable for small-scale systems.
The second one is the indirect usage. Indirect methods can convert solar energy into heat or electricity. These indirect methods are preferable for medium and large-scale desalination systems (Sharon & Reddy ). These outstanding properties made inorganic microporous membranes a primary candidate to be used for water purification and desalination applications. Among the membrane-based water purification technologies, MD is an emerging and promising technology for sustainable water purification. This technology can be utilized with lowgrade waste heat or solar energy. Therefore, the coupling of MD with solar energy for water purification is an attractive solution for saline water purification (Li et al. a, b).
Thus, compared with other solar-powered membranebased water purification technologies: solar-powered membrane distillation (SPMD) is an innovative and promising approach for an energy-efficient, cost-effective, robust, and popular solution. MD has received attention in recent years due to its potential advantages regarding energy consumption, simplicity, low maintenance, and its ability to be This paper reviews solar-driven membrane technologies for water purification such as RO, FO, ED, and MD, as standalone processes or as hybrid systems, evaluating energy consumption and water production costs. This review intends to suggest the appropriate solar-assisted membrane-based water purification technology for any given application.

TECHNIQUES AND PROCESSES OF WATER PURIFICATION
Water purification is a process of removing undesirable chemical compounds, organic and inorganic materials (suspended solids and gases), and biological contaminants from raw water to produce safe and clean water for household, agricultural, and industrial use (Othmer ). The conventional water treatment process includes physical, chemical, and biological processes. A physical process physically removes unwanted impurities from raw water. The most commonly used physical processes are screening, centrifugal separation, and sedimentation (Rao ). A chemical process chemically removes undesirable contaminants from raw water. The most common chemical processes used for water purification are flocculation, coagulation, chlorination, and distillation. Adsorption and filtration are the most common physicochemical process used in conventional water purification technologies.
A biological process effectively removes undesirable contaminants by three main mechanisms: biodegradation, adsorption, and filtration. A biological process is followed by physicochemical processes (adsorption and filtration).
The most widely used biological processes are slow sand filters and biologically active carbon. A combination of biologically active carbon units with slow sand filtration is a viable technology for the removal of contaminants from raw water (Dignac et al. ). Membrane technology is a category of separation technologies that can be used for separation, concentration, and purification of various mixtures.
Membrane technology is the most suitable water purification technologies with wide industrial and commercial applications, due to the following attractive features: some processes can be easily coupled with low-grade waste heat or renewable energy sources, they represent a clean technology with operational ease, yielding high-quality products and a greater flexibility in designing systems (Chen et al.

SOLAR-POWERED MEMBRANE-BASED WATER PURIFICATION TECHNOLOGIES
Solar energy is an affordable or viable energy source to be utilized in producing freshwater from contaminated water (Al-Kharabsheh ). The solar water purification system is a water decontamination system at the household and industrial level based on the direct use of solar energy and indirect use of solar energy to convert it into heat or electri- The application of the solar water purification process has a long history. For instance, Arab alchemists introduced the first solar distillation system in 1,551 to produce fresh drinking water from saline water or contaminated water (Malik et al. ).
Among solar-powered water purification technologies, membrane-based water purification technology is the most promising technology due to its environment-friendly nature and economic viability (Ali et al. ; El-Sebaii & El-Bialy ). Figure 1 provides an outlook, based on the studies reviewed in this contribution, about main membranebased water purification technologies where such solarpowered reverse osmosis (SPRO), solar-powered forward osmosis (SPFO), solar-powered electrodialysis (SPED), SPMD, and solar-powered hybrid membrane system (SPHMS) have been tested. The figure shows that SPRO technology (38%) has been the most applied technology.

Reverse osmosis
RO is a continuously operating membrane-based separation technology that uses pressure to pass source water through a semipermeable membrane and thereby produce purified water out of contaminated water or seawater (Mona ).
RO membrane technology is universally adopted and recognized as the leading and the most optimized membrane technology of seawater desalination, drinking water production, brackish water treatment, and wastewater treatment. RO is currently the most mature and commercially available pressure-driven membrane-based separation technology, which can remove suspended solids, dissolved matter, ions, bacteria, all dissolved salts, and organic matter from drinking water using a semipermeable membrane and a high operating pressure.
RO can use low-grade waste heat (Tidball & Kadaj ; Li et al. a, b)

).
In the last two decades, a total of 37,403 research papers (review articles, research articles, encyclopedia, book chapters, book reviews, conference papers, mini-review, and patents reports) on the focus above-mentioned of the RO has been published since 2000 until the second quarter of 2020 (retrieved from Web of Science database using the search keyword 'reverse osmosis', accessed on 29 April 2020).

History and evolution of RO
The concept of the RO process was first described by French physicist Jean-Antoine Nollet in 1748. He carried out experiments by using a pig bladder to study the osmosis process. In 1981, the first SPRO pilot plant was built in Saudi Arabia with a total capacity of 100-400 m 3 /day of desalted water from seawater or brackish water (Boesch

Solar-powered reverse osmosis
RO is a commercialized membrane-based water purification technology that can be coupled with solar energy systems for economical and energy-efficient desalination of brackish and seawater and water/wastewater treatments to produce freshwater. Solar energy can be used to pump water either through direct conversion or by using the indirect thermodynamic power generation method. The main parts of the SPRO hybrid systems are a solar thermal collector/photovoltaic module, high-pressure pump, feed and permeate tank, and RO membrane module. Solar thermal collectors have a working fluid that absorbs solar radiation, e.g., oil, water, the refrigerant transfers thermal energy to mechanical work for the generation of mechanical power required by the RO. The general principle scheme of solar thermal driven of RO desalination systems is given in Figure 2.
SPRO has been studied intensively over the last two decades, and more than 4,120 academic documents (review articles, research articles, encyclopedia, book chapters, book reviews, conference papers, mini-review, and patents reports) have been published on the topic since 2000 until the second quarter of 2020 (retrieved from Web of Science database using the search keyword 'solar-powered reverse osmosis', accessed on 29 April 2020. Davies () developed a new system that uses a solar-Rankine cycle to drive RO for high recovery fresh water from saline groundwater. The system used a spiral-wound membrane module with an effective area 2.4 m 2 and linear Fresnel collectors with a steam turbine (solar collector area 1,000 m 2 ). The final results of the study show that the steam cycle is operated without a vacuum condenser with an output of 350 L/m 2 day and predicted overall water output of 500 L/m 2 day. Thus, the proposed system could desalinate 350 m 3 from saline water containing 5,000 ppm of sodium chloride with a recovery ratio of 0.7.
Peñate & García-Rodríguez () designed an optimum solar desalination system for nominal capacities of 1,000-5,000 m 3 /day. The system was used as a parabolic trough collector with a specific energy consumption of 2.14 kWh/m 3 .
Results show that the proposed design would be suitable for a standalone operation because all the energy requirements are supplied by the solar system. Another study on SPRO conducted by Nihill et al. () showed that the recovery ratio was low, i.e., 26%. The authors briefly described the working principle of the new thermal water pump with the help of schematics and thermodynamic curves (P-h and P-h). The pump was a highly compacted heat engine that converted thermal energy directly to pressurized fluid flow. Furthermore, the authors presented the design and experimental analysis of a thermal water pump coupled with an RO desalination system with feed water at a salt concentration of 1,184 ppm, a heat source temperature of 86 C, and the product water salinity of 111 ppm. The recovery ratio obtained is shown to be 26%. The results indicate that the proposed design has the immediate potential to compete with conventional thermal desalination systems.
In the work of Wu et al. (), a multi-objective optimization was used in the design and solar energy of standalone RO desalination driven by a photovoltaic and diesel generator hybrid system in Iran. Furthermore, the effects of varying fuel cost, interest rate, photovoltaic initial cost, and battery initial cost on the economic parameters of the hybrid system are also discussed. The RO units utilized a A summary of selected studies carried out on solar thermal/photovoltaic-driven RO over the last two decades is given in Tables 1 and 2. This list describes information about the study year, application, feed water type, energy source, system description, daily production, specific energy consumption, and corresponding reference.

Forward osmosis
FO is an emerging and promising membrane-based separation technology that uses natural osmotic pressure to transport a solvent (normally water) across a selective permeable FO membrane, as opposed to pressure-driven membrane-based separation technologies like RO,  Among the concentration-driven membrane-based water purification technologies, FO has multiple benefits: it is energy-friendly and has a lower energy requirement, higher water recovery, and lower membrane fouling tendency. However, this technology has a few critical

Solar-powered forward osmosis
A schematic diagram of a solar-driven FO system is shown in Figure 3. The main parts of the solar-driven FO system are the solar thermal collector/photovoltaic module, feed, permeate and brine tank, FO membrane module, and pumps. The energy required for an FO system is lower than for an RO system of the same capacity; therefore, FO is competitive in energy terms (Zheng ).
SPFO has been explored intensively over the last two decades, and more than 1,186 research papers (review articles, research articles, encyclopedia, book chapters, book reviews, conference papers, mini-review, and patents reports) have been published on the topic since 2000 until the second quarter of 2020 (retrieved from Web of Science database using the search keyword 'solar-powered forward osmosis', accessed on 29 April 2020).

Schrier () developed a simple, low-cost, and scalable
alternative method for ethanol concentration by FO with solar-regenerated draw solution. The draw solution was an aqueous brine that was regenerated by solar evaporation.
The author found that the proposed system produced 95, 50, and 30% (w/w) ethanol solutions. The mean production rate in Ethiopia was 0.69 and 1.41 kg/m 2 day for 50 and 30% product, respectively. In comparison, the main production rate in Brazil was found to be 0.74 and 1.54 kg/m 2 day for 50 and 30% product, respectively.
Razmjou et al. () studied the thermodynamic feasibility of bilayer polymer hydrogels as draw agents for the continuous production of fresh drinking water using SPFO. Thermodynamic analysis results show that the bilayer hydrogel-driven FO process requires a specific heat The specific solute flux of this work was 0.031 g/L. The quality of produced water was superior compared with potable water standards. Some selected references based on their study year, application, feed water type, types of draw solute(s), energy source, system description, specific energy consumption, and corresponding reference are presented in Table 3.

Electrodialysis
ED is one of the most commonly used membrane-based water purification technologies using semipermeable membranes to remove an undesirable ionic substance from an aqueous solution by applying an electric field. The process uses a driving force to transfer ionic species from the source water through anion exchange membranes and cation exchange membranes to a concentrated wastewater stream, creating a more dilute stream (Valero et al. ). The first reverse ED pilot-scale plant was installed and tested in 2014 in Trapani, Italy, for the production of fresh water from real brackish water and brine (Karabelas et al. design of the ED process for domestic use.

Solar-powered electrodialysis
A schematic diagram of a solar-driven ED system is shown in Figure 4. The main parts of the solar-driven ED are a photovoltaic module, feed, permeate and brine tank, ED membrane module, and circulating pumps. The solar radiation on the surface of the solar panel is transformed into electric energy, which is required by the ED.  The results showed that a salt removal efficiency of 95% and an arsenic removal efficiency of >99.9% was reported.
The system required the lowest specific electricity consump- Some selected references based on the study year, application, feed water type, energy source, system description, daily production, specific energy consumption, and corresponding reference are listed in Table 4.

Membrane distillation
MD is a membrane-based water purification technology in which the separation process takes place by a difference in

Solar-powered membrane distillation
MD is a membrane-based desalination technology that can be coupled with solar energy systems (Al-Obaidani et al. ). The SPMD system is a hybrid and a standalone system. The system is an off-the-grid electricity system. A schematic diagram of the solar standalone DCMD system is shown in Figure 6. The main parts of the solar standalone DCMD system are a solar thermal collector/photovoltaic module, feed and distillate tank, MD module, and circulating pumps. The hybrid system here is defined as an off-the-     concentration. The performance ratio of a compact prototype was never larger than 0.53, while for multi-stage, the performance ratio reached a maximum of 1.96. Finally, the multi-stage analysis showed that by increasing the number of modules not only the heat recovery is greater but also the distillate throughput is to some extent better; therefore, multi-stage concept shows better features for scaling of the system. The authors conclude that the best specific distillate production and heat consumption results were as high as 5.09 L/m 2 h and as low as 294 kWh/m 3 , respectively.
Selected studies relevant to MD showing the study year, system scale, energy source, system configuration (flux and rejection), daily production, specific energy consumption, water production cost, and corresponding reference are presented in Table 6.
As can be seen in Table 6

Hybrid membrane systems
A membrane-based hybrid water purification technology is a combination of two or more membrane-based separation techniques that results in a better technique compared with these processes used alone. By using hybrid membrane systems, benefits in product quality, environmental and energy impacts, and plant footprint can be obtained (Drioli & Fontanova ). By using hybrid membrane processes, the combinations of water purification techniques are considered to provide an opportunity to exceed the limitations of conventional processes (Buonomenna ). Such systems often target reducing weaknesses of a certain conventional process. They are characterized by flexibility in operation, less specific energy consumption, high water productivity, low construction cost, high plant availability, and better power and water matching.
Hybrid membrane processes are expected to offer new innovative solutions and they offer new possibilities for sustainable industrial growth (Drioli et al. ). In the last two decades, a great deal of research has been conducted on the hybrid membrane system. For example, more than 5,143 academic documents (review articles, research articles, encyclopedia, book chapters, book reviews, conference papers, mini-review, and patents reports) have been published on hybrid membrane system since 2000 until the first quarter of 2020 (retrieved from Web of Science database using the search keyword 'solar-powered hybrid membrane system', accessed on 29 April 2020). The number of publications on the hybrid membrane system was limited to three journals, i.e., Journal of Membrane Science, Desalination, and Chemical Engineering Journal. The above-mentioned three journals were the core journals in a hybrid membrane system.

History and evolution of hybrid membrane systems
The concept of hybrid membrane processes was first intro-  A summary of selected studies carried out on integrating/hybrid membrane-based/non-membrane-based water purification technologies is given in Table 7.

ECONOMIC EVALUATION OF SOLAR-POWERED MEMBRANE-BASED SEPARATION TECHNOLOGIES
Techno-economic assessment is necessary to evaluate the A summary of selected studies carried out on membrane-based separation technologies and their specific energy consumption and water production cost is shown in Table 8. This list describes available documented information about the study year, system scale, energy source, system description, daily production, specific energy consumption, and water production cost.
As can be seen in