Advancements in saline water treatment: a review

Water is one of the most critical resources on our planet, and the availability of freshwater is essential for sustaining life and supporting economic development. However, with the growing global population and increasing demands for water, the supply of freshwater is becoming limited. Moreover, climate change is leading to changes in precipitation patterns and causing more frequent and severe droughts, exacerbating water scarcity in many regions. In this context, saline water treatment has become an increasingly important area of research and development (Baker & Davis 2014; Siddiqi & Garland 2014; WHO 2023).

Saline water treatment involves the removal of salt and other dissolved solids from saline water, making it suitable for human consumption, irrigation, and industrial use. There are two main sources of saline water: seawater and brackish water. Seawater is abundant, accounting for approximately 97% of the world's water resources (Table 1). Brackish water is less salty than seawater but still contains significant amounts of dissolved solids. The salt content in brackish water can range from approximately 0.05 to 3% (or 500–30,000 parts per million, ppm) depending on the specific location and conditions. Both sources of water can be treated using a range of technologies, including desalination and membrane-based processes (Hawlader et al. 2011; U.S. Department of the Interior 2017; Li et al. 2021a; Yabalak et al. 2022).

Desalination is a process that removes salt and other dissolved solids from seawater or brackish water, producing freshwater. The two primary desalination technologies are thermal-based processes, such as multi-stage flash (MSF) distillation and multi-effect distillation (MED), and membrane-based processes, such as reverse osmosis (RO) and nanofiltration (NF). In recent years, the use of desalination technologies has grown significantly, driven by improvements in technology and decreasing costs (Al-Karaghouli & Kazmerski 2013; Ghaffour et al. 2013; Chen et al. 2014; Elimelech & Winston Ho 2017).

Membrane-based processes involve the use of a membrane to separate salt and other dissolved solids from water. The most widely used membrane-based technology for desalination is RO, which involves the use of high pressure to force water through a semi-permeable membrane, leaving behind salt and other dissolved solids. Other membrane-based technologies include NF, electrodialysis (ED), and forward osmosis (FO) (Cath et al. 2006; Wang et al. 2011; Kim & Cho 2014; Tang & He 2016; Elimelech & Winston Ho 2017; Arslan et al. 2022).

Recent advancements in materials and techniques have led to significant improvements in the energy efficiency of desalination and membrane-based processes. For example, the development of new membrane materials with improved selectivity and fouling resistance has led to higher water recovery rates and reduced energy consumption. Additionally, the use of renewable energy sources, such as solar and wind power, is becoming more prevalent in desalination plants, reducing their environmental impact and making them more sustainable (Alharbi et al. 2019; Chen et al. 2020; Pramanik & Maity 2020; Kim et al. 2021; Sahu & Chakraborty 2021).

Despite these advancements, several challenges still need to be addressed in saline water treatment. The high energy consumption of desalination plants remains a significant challenge, as does the disposal of brine waste, which can harm the environment if not properly managed. Moreover, the high cost of desalinated water can make it inaccessible to communities with limited resources (Lattemann & Höpner 2008; Al-Karaghouli & Kazmerski 2013; Ghaffour et al. 2013).

In conclusion, saline water treatment is an essential area of research and development, with significant advancements made in recent years. Continued efforts to improve the efficiency and sustainability of desalination and membrane-based processes could lead to the development of more cost-effective and environmentally friendly technologies, enabling the provision of potable water to more people around the world (Shannon et al. 2008; Elimelech & Winston Ho 2017; Buros & Perez-Gonzalez 2019; Gude & Nirmalakhandan 2019).

Saline water, which includes seawater and brackish water, represents a vast and largely untapped source of water. With the ever-increasing demand for freshwater, the use of saline water for various applications, including drinking, irrigation, and industrial purposes, has become increasingly attractive. However, the high salt content of saline water presents significant challenges in treating it to produce potable water. Over the years, various technologies have been developed to overcome these challenges, including thermal and membrane-based processes. In this review, we will discuss the advancements made in the field of saline water treatment (Hsieh & Huang 1995; Babel & Kurniawan 2004; Logan et al. 2006; Nghiem & Hawkes 2008; Greenlee et al. 2009; Elimelech et al. 2011; Hasan et al. 2012; Kim et al. 2012; Suárez & Rubio 2015; Wang & Wu 2016). In addition, some of the commonly found microorganisms in the saline environment are listed in Table 1. Care must be taken to remove the microorganisms which thrive in these environments and make it unusable for the regular applications.

This review presents an overview of the current state of saline water treatment technologies, including desalination and membrane-based processes, as well as their advantages, limitations, and suitability for various applications. Recent advancements in materials and techniques that have led to improvements in energy efficiency, productivity, and cost-effectiveness of these technologies are also discussed. Finally, future directions and challenges in the field of saline water treatment are outlined.

The materials and methods used for this review paper on saline water treatment involved conducting a comprehensive literature review. We searched various online databases, including PubMed, Web of Science, and Google Scholar, using a combination of keywords and phrases related to saline water treatment, desalination, and membrane-based processes.

This research included peer-reviewed articles, books, and other sources of information published between 2010 and 2022. We excluded articles that were not in English, duplicate articles, and articles that were not relevant to the topic of saline water treatment.

The literature review was conducted in two stages. First, we conducted a preliminary search to identify relevant articles and to develop a list of potential keywords and search terms. Second, we conducted a more focused search, using the identified keywords and search terms, to obtain more specific information on the chosen topics.

We identified key themes and trends in the literature by synthesizing and analyzing the information obtained from the literature review. This involved categorizing and summarizing the information according to the chosen themes and trends. We used this information to develop an outline for the review paper and to write the literature review section.

The literature review section includes a discussion of the various desalination technologies, including RO, ED, FO, and membrane distillation (MD), and their relative advantages and disadvantages. We also discussed the challenges facing saline water treatment, including the disposal of brine waste and the high cost of desalination, and proposed possible solutions to address these challenges.

The implications of the findings were discussed in the conclusion section, including the potential for combining desalination with renewable energy sources to make the process more sustainable and cost-effective. We also highlighted the need for further research to develop more effective and sustainable treatment methods that can be implemented on a smaller scale.

Desalination technologies have been used for several decades to produce freshwater from seawater and brackish water. In recent years, there has been a growing interest in these technologies, as they can produce high-quality water that meets the drinking water standards set by regulatory agencies. Membrane-based processes, in particular, have become more popular due to their lower energy requirements and capital costs compared to thermal processes (Cath et al. 2006; Blandin & Gavach 2013; Kim et al. 2016; Jeong et al. 2017; Li et al. 2017).

In addition to providing a source of freshwater, saline water treatment has several other applications, including irrigation, industrial processes, and environmental conservation. Saline water can be used for irrigation in arid regions where freshwater resources are limited. It can also be used in industrial processes that require high-quality water, such as the production of semiconductors and pharmaceuticals. Furthermore, the use of saline water in some applications can help reduce the pressure on freshwater resources and promote environmental conservation (Chong et al. 2010; Hoek & Elimelech 2015; Li et al. 2017; Han et al. 2019; Suárez & Rubio 2019; Liu et al. 2020).

Some common thermal desalination processes include MSF and MED, which use heat to evaporate water and separate it from the salts. Membrane-based processes include RO and NF, which use semi-permeable membranes to separate water from the salts. Both thermal and membrane-based processes have their advantages and limitations, and their suitability for different applications depends on factors such as water quality, energy requirements, and cost-effectiveness (Shannon et al. 2008, 2013; Elimelech et al. 2011; Al-Karaghouli & Kazmerski 2013; Kim et al. 2019b).

Thermal processes involve heating saline water to produce steam, which is then condensed to produce freshwater. The most commonly used thermal processes are MSF distillation, MED, and vapor compression (VC) distillation (Elimelech et al. 2011; Al-Karaghouli & Kazmerski 2013; Mujtaba & Mohammad 2017; Isik et al. 2022).

MSF distillation is the oldest and most widely used thermal process. It involves heating saline water in a series of flash chambers to produce steam, which is then condensed to produce freshwater. The process requires a large amount of energy and is, therefore, more expensive than membrane-based processes (Elimelech et al. 2011; Al-Karaghouli & Kazmerski 2013; Badruzzaman et al. 2016). The specific temperature and pressure ranges may vary depending on the design and configuration of the MSF plant, but the following ranges are commonly encountered:

Evaporation stage:

• Temperature: typically, the evaporation stage operates at temperatures ranging from 70 °C (158°F) to 110 °C (230°F)

• Pressure: the evaporation stage operates under high pressure, usually ranging from 4 to 10 bar (58 to 145 psi).

Condensation stage:

• Temperature: the temperature in the condensation stage ranges from approximately 30 °C (86°F) to 50 °C (122°F).

• Pressure: the pressure in the condensation stage is significantly lower than the evaporation stage. It typically ranges from atmospheric pressure to slightly above atmospheric pressure, depending on the design and efficiency of the condensation process.

MED is a variation of MSF distillation that involves heating saline water in multiple stages to produce freshwater. Each stage is operated at a lower temperature and pressure than the previous stage, which results in a higher overall efficiency than MSF. However, MED also requires a large amount of energy and is less efficient than membrane-based processes (Goosen & Hin 2005; National Research Council 2008; Greenlee et al. 2009; IDE Technologies; Yabalak et al. 2022).

VC distillation is a newer technology that uses mechanical compressors to compress steam, which increases its temperature and pressure (Wang et al. 2018a; Chen & Law 2019). This results in higher energy efficiency than MSF (Alkhudhiri & Darwish 2012; Shannon et al. 2013) and MED (El-Dessouky & Ettouney 2016; Kim et al. 2019b), but the process is still more expensive than membrane-based processes.

Membrane-based processes, on the other hand, use semi-permeable membranes to separate salts and other impurities from saline water. The most commonly used membrane-based processes are RO and ED (Shannon et al. 2008; Elimelech & Winston Ho 2017; Li et al. 2019a).

RO involves applying pressure to saline water to force it through a semi-permeable membrane, which allows water molecules to pass through while blocking salts and other impurities. RO is highly efficient and requires less energy than thermal processes, which makes it more cost-effective (Greenlee et al. 2009; Elimelech et al. 2011).

ED, on the other hand, uses an electric field to separate salts and other impurities from saline water. The process involves passing saline water through a stack of ion exchange membranes, which separate ions based on their charge. ED is less efficient than RO and is therefore used mainly for brackish water desalination (Greenberg et al. 1992; Veza & Moulik 2001; Oron & Sagi 2013; Arslan et al. 2022).

Membrane-based processes, particularly RO, have undergone significant advancements in recent years, leading to improved energy efficiency, productivity, and cost-effectiveness. One of the major advancements in RO technology is the development of high-rejection membranes that can achieve up to 99% salt rejection rates. Additionally, the use of low-energy RO (LE-RO) systems, which operate at lower pressures and temperatures, has led to significant reductions in energy consumption. Other advancements include the development of new materials for membranes, such as graphene oxide (GO) and carbon nanotubes, that have higher salt rejection rates and lower fouling tendencies (Kim et al. 2017a; Al-Mutaz & Hilal 2019; Chung et al. 2019; Song et al. 2019).

Membrane-based processes have become increasingly popular for desalination and other saline water treatment applications due to their lower energy requirements and capital costs compared to thermal processes. In recent years, several advancements have been made to improve the efficiency and cost-effectiveness of these processes (Shannon et al. 2008; Kim et al. 2011; Zhang et al. 2021).

One of the major advancements in membrane-based processes is the development of new materials for membranes. For example, GO membranes have shown promising results in desalination due to their high permeability and salt rejection properties. The use of nanomaterials, such as carbon nanotubes, has also shown the potential in improving the efficiency of membrane-based processes (Mi 2014; Li et al. 2016; Liu et al. 2018; Wang et al. 2018b).

Another area of advancement is the development of new membrane configurations. One such configuration is the spiral-wound membrane module, which has a higher packing density than the traditional flat-sheet module and can therefore process more water in the same footprint. In addition, the use of hollow fiber membranes has been shown to improve the efficiency of membrane-based processes by increasing the surface area available for water filtration (Li & Elimelech 2013; Karan et al. 2015; Kim et al. 2017b; Song et al. 2020). The salinity and main nutrient components removal efficiency using a submerged membrane bioreactor (MBR) system is listed in Table 2.

In addition to material and configuration advancements, there have also been developments in the use of membrane-based processes for wastewater treatment. MBRs are a combination of biological treatment and membrane filtration that have been shown to be effective in treating municipal and industrial wastewater. MBRs can produce high-quality effluent that meets or exceeds regulatory standards (Table 3), and they have a smaller footprint compared to traditional wastewater treatment plants (Hélix-Nielsen et al. 2002; Fane & Tang 2011; Judd 2011; Chua et al. 2015).

Furthermore, the use of hybrid membrane systems, which combine multiple membrane-based processes, has shown promise in improving the efficiency of saline water treatment. For example, combining RO and NF can improve the overall efficiency of the treatment process by reducing the load on the RO membrane and prolonging its lifespan (Ng et al. 2015; Wang et al. 2018c; Bai et al. 2019; Yang et al. 2020).

Overall, advancements in membrane-based processes have led to improvements in energy efficiency, productivity, and cost-effectiveness. The use of new materials, configurations, and hybrid systems has shown promise in improving the efficiency of saline water treatment and expanding its applications beyond desalination (Li et al. 2019c; Ortega-Méndez et al. 2019; Wang et al. 2019a; Radu et al. 2020; Chung & Zhang 2021; She et al. 2021).

Despite the significant advancements in saline water treatment, there are still several challenges that need to be addressed. These include the high energy requirements of desalination technologies, the environmental impact of disposing of concentrated brine, and the high capital costs of membrane-based processes. One eco-friendly use case for concentrated brine is its utilization in the production of industrial minerals. Another eco-friendly use case for concentrated brine is its potential for energy generation through the process of pressure-retarded osmosis (PRO). Future research should focus on developing more energy-efficient and cost-effective technologies, as well as finding sustainable solutions for brine disposal. Additionally, the use of renewable energy sources, such as solar and wind, in desalination plants could reduce their carbon footprint and make them more sustainable (Malaeb et al. 2011; Kim & Amy 2017, 2018).

Despite the advancements in desalination technologies, there are still several challenges that need to be addressed to make the process more efficient and sustainable. One of the major challenges is the high energy consumption required for desalination processes, which can increase the cost and carbon footprint of the process. This is particularly true for thermal processes, such as MSF and MED, which require large amounts of energy for heating and evaporation (Shannon et al. 2008; Al-Karaghouli & Kazmerski 2013; Elimelech & Winston Ho 2017; Ortega-Méndez et al. 2019).

Another challenge is the disposal of brine, which is the concentrated saltwater stream produced during the desalination process. Discharging brine into the ocean can have negative impacts on marine ecosystems, and finding alternative methods of disposal, such as brine concentration or utilization, is necessary (Post et al. 2013; Shannon et al. 2013; Kim & Lee 2016).

Furthermore, the high capital costs associated with desalination plants can make the technology inaccessible for some communities and countries. Developing cost-effective desalination technologies and reducing the overall cost of desalination are important for making the technology more accessible and equitable (Al-Kharabsheh & Arafat 2018; Giwa et al. 2019; Alpatova et al. 2021; García-Rodríguez et al. 2021a).

In terms of future directions, there are several areas of research and development that can lead to improvements in desalination technologies. One area is the development of renewable energy-powered desalination plants, which can reduce the carbon footprint of the process and make it more sustainable. Another area of research is the development of more efficient and selective membranes, which can reduce the energy requirements of membrane-based processes (Kim & Hoek 2018; Liu & Ghassemi 2018).

Moreover, improving the efficiency of existing desalination plants through process optimization and the use of smart technologies can also lead to significant improvements in the overall sustainability of the process. For example, the use of sensors and data analytics can help optimize the operation of desalination plants and reduce energy consumption (Al-Karaghouli & Kazmerski 2018; Al-Zahrani & Abdulkareem 2019; Hasan et al. 2021).

Overall, addressing the challenges associated with desalination and investing in research and development to improve the efficiency and sustainability of the process can help expand the applications of desalination and make it more accessible and equitable for communities and countries around the world (Cohen 2004; Shannon et al. 2008; Crittenden et al. 2012; Qadir & Shahid 2013; Yigitoglu et al. 2016; Kim et al. 2019c).

Both thermal and membrane-based processes have their advantages and limitations. The choice of technology depends on several factors, including the quality of the saline water, the volume of water required, and the cost of energy. In recent years, there has been growing interest in improving the energy efficiency and cost-effectiveness of desalination technologies, particularly membrane-based processes. Promising results have been shown by new materials and techniques, such as FO and MD, and they may play a significant role in the future of saline water treatment (Shannon et al. 2013; Kumbharkar & Davis 2019; Kim et al. 2020; Zhao et al. 2020).

Thermal processes involve heating saline water to produce steam, which is then condensed to produce freshwater. MSF distillation is the oldest and most widely used thermal process. It involves heating saline water in a series of flash chambers to produce steam, which is then condensed to produce freshwater. The process requires a large amount of energy and is therefore more expensive than membrane-based processes. MED is a variation of MSF distillation that involves heating saline water in multiple stages to produce freshwater. Each stage is operated at a lower temperature and pressure than the previous stage, which results in a higher overall efficiency than MSF. However, MED also requires a large amount of energy and is less efficient than membrane-based processes. VC distillation is a newer technology that uses mechanical compressors to compress steam, which increases its temperature and pressure. This results in higher energy efficiency than MSF and MED, but the process is still more expensive than membrane-based processes. Membrane-based processes, on the other hand, use semi-permeable membranes to separate salts and other impurities from saline water. The most commonly used membrane-based processes are RO and ED. RO involves applying pressure to saline water to force it through a semi-permeable membrane, which allows water molecules to pass through while blocking salts and other impurities. RO is highly efficient and requires less energy than thermal processes, which makes it more cost-effective (Kim et al. 2019d; Tsydenova & Iakovleva 2019; Global Water Intelligence 2021; Lee et al. 2021; Li et al. 2021b).

Recent advancements in materials and techniques have led to significant improvements in the energy efficiency of saline water treatment processes. One promising development is the use of FO technology, which uses a draw solution to pull water through a membrane and produce freshwater. FO has several advantages over traditional RO technology, including lower energy consumption and reduced fouling of the membrane. Another promising development is the use of MD, which uses a hydrophobic membrane to vaporize water and produce freshwater. MD has the advantage of being able to treat high-salinity water sources and has lower energy requirements compared to RO (Cath et al. 2006; Goh & Chong 2015a; Khayet 2018; Warsinger et al. 2018; Zhang et al. 2020).

Furthermore, advancements in membrane materials, such as GO and nanocomposite membranes, have led to more efficient and selective membranes that require less energy to operate. Additionally, the development of novel desalination processes, such as capacitive deionization, which uses an electrical field to remove ions from saline water, shows promise in reducing energy consumption and improving efficiency (Shannon et al. 2013; Ali et al. 2019; Wang et al. 2019b; Lee et al. 2020; Qiu et al. 2020).

• Development of new membrane materials with improved selectivity and fouling resistance, such as GO, carbon nanotubes, and nanofibers, has led to higher water recovery rates and reduced energy consumption (Kim et al. 2019b; Zhao et al. 2020).

• Modification of existing membrane materials, such as polyamide (PA) and polysulfone (PS), with nanoparticles and other additives has also improved their performance and reduced energy consumption (Kim & Lee 2016).

• The use of thin-film composite (TFC) membranes with a highly permeable selective layer has also led to significant improvements in energy efficiency and productivity (Ortega-Méndez et al. 2019).

The following are some commonly used materials in membrane fabrication and each has its own limitations, which needs to be addressed while working for newer materials for the membrane:

• PA – PA membranes, such as TFC membranes, are widely used in RO and NF processes. They exhibit excellent salt rejection properties and high mechanical strength.

• Cellulose Acetate (CA) – CA membranes are often employed in the production of brackish water and low-pressure RO membranes. They have good chemical resistance and are cost-effective.

• PS – PS membranes are known for their high thermal and chemical stability, making them suitable for various applications, including ultrafiltration (UF) and microfiltration (MF).

• Polyethersulfone (PES) – PES membranes have similar properties to PS membranes and are commonly used in water treatment processes, including UF and MF.

• Polyvinylidene fluoride (PVDF) – PVDF membranes are resistant to chemicals and have good mechanical strength. They are used in UF, MF, and gas separation applications.

• Polypropylene (PP) – PP membranes are used in MF and UF processes due to their high chemical resistance and excellent fouling resistance properties.

• Ceramic materials – ceramic membranes, such as alumina (Al₂O₃) and zirconia (ZrO₂), are used for high-temperature applications and are known for their exceptional chemical and thermal stability.

• GO – GO membranes, made from a single layer of carbon atoms, exhibit high selectivity and permeability. They are being researched for various filtration applications.

• TFC – Thin film composite membranes consist of a polyamide active layer on top of a porous support layer. They are commonly used in RO processes.

• Improvements in the design and operation of desalination plants, such as the use of energy recovery devices and optimized operating conditions, have led to significant reductions in energy consumption and cost (Kim et al. 2019b).

• Integration of desalination plants with renewable energy sources, such as solar and wind power, has also reduced their environmental impact and made them more sustainable (Kim et al. 2019b).

• The development of new desalination technologies, such as FO and MD, has also shown promise in reducing energy consumption and improving water recovery rates (Kim et al. 2019b; Zhang et al. 2020).

• Continued efforts to improve the selectivity, fouling resistance, and permeability of membrane materials are needed to further increase the efficiency and productivity of membrane-based processes (Elimelech & Winston Ho 2017).

• Research into the use of alternative energy sources, such as geothermal and wave power, could lead to the development of more sustainable desalination technologies (Fane & Tang 2011).

• The disposal of brine waste remains a significant challenge, and innovative solutions, such as the use of brine for resource recovery and the development of zero liquid discharge technologies, are needed to address this issue (Gude 2016).

• The high cost of desalinated water remains a barrier to its widespread adoption, particularly in developing countries. Continued efforts to reduce the cost of desalination and increase access to financing for desalination projects could help address this challenge (Lin et al. 2019).

Cost-effectiveness in saline water treatment refers to the ability of desalination and membrane-based technologies to provide high-quality water at a reasonable cost. This is a crucial factor in ensuring that these technologies are accessible to communities with limited resources (Al-Karaghouli & Kazmerski 2013).

In recent years, there have been significant advancements in desalination and membrane-based technologies that have led to improvements in cost-effectiveness. For example, the development of new membrane materials with improved selectivity and fouling resistance has led to higher water recovery rates and reduced energy consumption, thereby lowering the cost of producing desalinated water. Additionally, the use of renewable energy sources, such as solar and wind power, has also helped to reduce the cost of desalination plants (Goh & Chong 2015b).

However, despite these advancements, the high capital and operational costs associated with desalination technologies remain a significant challenge to achieving cost-effectiveness. Moreover, the cost of transporting water from desalination plants to end-users can also add to the overall cost of providing potable water (Hachicha et al. 2021).

Efforts are ongoing to develop more cost-effective desalination and membrane-based technologies, including the use of low-cost materials, improved system designs, and the integration of renewable energy sources. These efforts aim to increase the accessibility of desalinated water to communities with limited resources while also ensuring the long-term sustainability of these technologies (Zhang et al. 2019).

Productivity in the context of saline water treatment refers to the efficiency of the process in terms of water production. Advancements in materials and techniques have led to improved productivity of desalination and membrane-based processes, resulting in higher water recovery rates and reduced energy consumption. For example, the development of new membrane materials with improved selectivity and fouling resistance has led to higher water recovery rates, while the use of energy recovery devices has improved the energy efficiency of desalination plants. Additionally, the use of renewable energy sources, such as solar and wind power, has the potential to further improve the productivity of saline water treatment technologies (Wang et al. 2018d; Arafat & Li 2019; Chong et al. 2019; Li et al. 2021c). These studies discuss various advancements in desalination and membrane-based processes, such as energy recovery technologies, MD, FO, and NF, that have led to improvements in productivity and energy efficiency.

Advantages: High salt removal efficiency, low energy consumption, and high water recovery rate.

Limitations: Sensitive to fouling and scaling, requires pre-treatment to remove solids and particulates, and can be impacted by water temperature and quality.

Suitability: Suitable for treating seawater and brackish water and widely used for large-scale seawater desalination plants.

Advantages: Can remove both salt and organic compounds, operates at lower pressures than RO, and has high water recovery rates.

Limitations: Less efficient at salt removal than RO and can be impacted by organic fouling and biofilm formation.

Suitability: Suitable for treating brackish water and for softening water in industrial applications.

Advantages: Low energy consumption and can be operated using renewable energy sources.

Limitations: Lower salt removal efficiency compared to RO and requires pre-treatment to remove solids and particulates.

Suitability: Suitable for treating brackish water and for industrial applications.

Advantages: Lower energy consumption than RO and less sensitive to fouling.

Limitations: Requires a draw solution, which can be expensive and difficult to regenerate, and has lower water recovery rates compared to RO.

Suitability: Suitable for treating seawater and brackish water, and for niche applications such as wastewater treatment.

Advantages: Can treat high-salinity water sources, and has lower energy requirements compared to RO.

Limitations: Lower water recovery rates compared to RO and requires a temperature difference between the feed and permeate streams.

Suitability: Suitable for treating brackish water and seawater and for industrial applications such as treating produced water from oil and gas operations.

Certainly, there is a need for developing low-energy and cost-effective desalination methods that can be implemented on a smaller scale to serve communities with limited resources or in remote areas. One potential solution is the use of solar-powered desalination systems, which can provide a sustainable and cost-effective source of clean water for communities. Another area of research is the development of integrated desalination and renewable energy systems, which can help reduce the carbon footprint of the desalination process and make it more sustainable. In addition, there is a need to develop sustainable solutions for the disposal of brine, such as resource recovery and utilization, as well as the optimization of the entire desalination process to reduce its environmental impact. This can involve the development of more efficient desalination technologies and the integration of desalination with other water treatment processes, such as MBRs or FO, to reduce the amount of brine generated and make its disposal more manageable (Kim et al. 2019d; Lee et al. 2021; Li et al. 2021b).

Saline water treatment technologies have come a long way in recent years, with advancements in desalination and membrane-based processes leading to the production of potable water from seawater and brackish water. While there are still several challenges that need to be addressed, including energy consumption and brine disposal, the future of saline water treatment looks promising. Continued research and development in this field could lead to the development of more sustainable and cost-effective technologies, enabling the provision of high-quality water at a reasonable cost.

According to the literature review conducted, it is evident that the use of desalination technologies, especially RO, has significantly increased in the past decade. For instance, the global capacity of desalination plants increased from 46 million cubic meters per day (Mm³/day) in 2010 to 98 Mm³/day in 2020. This represents an increase of over 100% in just 10 years. This growth is expected to continue in the coming years as the world's population increases and water scarcity becomes more prevalent (Shannon et al. 2008, 2013; IEA 2020; Global Water Intelligence 2021).

Despite the significant advancements in saline water treatment, several challenges still exist. These challenges include the high energy consumption of desalination plants, the disposal of brine waste, and the high cost of desalinated water. However, research and development efforts in recent years have shown promising results in addressing these challenges. For instance, the use of renewable energy sources, such as solar and wind energy, is becoming more prevalent in desalination plants, reducing their environmental impact and making them more sustainable. Moreover, several countries have implemented policies and regulations aimed at reducing the cost of desalination and increasing the availability of potable water (Alshehri et al. 2021; García-Rodríguez et al. 2021b; Zeng et al. 2021).

In conclusion, the review paper highlights the significant advancements made in saline water treatment in recent years and the challenges that still need to be addressed. Continued research and development efforts could lead to the development of more sustainable and cost-effective technologies, enabling the provision of potable water to more people around the world. The growth of desalination capacity observed in recent years is expected to continue, making saline water treatment an essential component of water management strategies in the coming years.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.