Table of Contents
Clean-water technologies are becoming increasingly crucial for consumable water as climate change introduces new water distribution patterns worldwide. Desalination techniques are advancing with time because of changing requirements and initiatives to enhance sustainability. Desalination may be complicated, expensive, and energy-intensive if the relevant factors or variables are not thoroughly investigated.
According to a report by the International Desalination Association (IDA), 150 countries use desalination to support the everyday activities of more than 3 billion people globally. Desalination plant numbers and daily water output rose between 2016 to 2019 by 12.4% to 41.2%, respectively, demonstrating technology’s rapid development . Water scarcity will impact the lives of 5 billion people by the year 2050, according to the 2022 report . Reverse osmosis (RO) and thermal desalination are the two primary desalination technologies, which also include multi-effect distillation (MED) and multi-stage flash distillation (MSF). The desalination technique applies to both sea and brackish water.
Brackish water contains less salt as compared to seawater. It may be a waste stream or flows out of the ground. Brackish desalination provides high freshwater recovery and brine reduction since brine disposal can be highly expensive and environmentally problematic. The most considerable cost portion of brackish and industrial water desalination is due to brine disposal or treatment.
There were around 5,300 seawater desalination facilities in 2018, with a daily desalination capacity of about 58 million cubic meters. Although seawater reverse osmosis (SWRO) is the most commonly used desalination method, there are different disadvantages, including the generation of a brine stream, high energy needs, etc.
Rising energy costs are the main factor driving desalination’s shift from coastal applications to industrial and inland brackish sources. Desalination is crucial for industrial processes and numerous inland utilities that depend on it. Like other treatment technologies, water prosperity and security depend on better efficiency and future improvements.
In order to overcome physical and financial obstacles, technological advancements have always been essential to the development and expansion of the desalination industry. For instance, innovation from the 1990s to 2000s concentrated on lowering energy consumption in seawater desalination. This work led to the improvement of reverse osmosis (RO) membrane performance and reliability as well as the invention of energy recovery devices, among other ground-breaking developments. Large membrane desalination facilities also became feasible, and the amount of energy needed for saltwater desalination has been reduced to half.
Today’s RO plants function at about the lowest energy usage feasible due to the excellent work done by several researchers, inventors, and businesses, and they have gained broad adoption. We observe a lot of R&D and innovation in this area, and we anticipate it will continue over the coming decades.
The engineering of deployable systems, which encounter many practical difficulties, should be a key consideration while considering the major desalination issues. Although the academic literature is frequently exciting, it might take a long for those ideas to reach the market as scalability and practical problems are resolved.
1 Challenges in Desalination Process
Some of the most major problems regarding the desalination process are summarized here:
1.1 Membrane Performance
The maximum salt rejection at the minimum working pressures is what engineers strive for. In general, as the pressure is decreased, less salt is extracted from the processed water. SWRO normally runs at pressures between 60 and 70 bar. We can reduce the energy required if our membranes can continue to reject salt at pressures of 50 to 55 bar. Making membrane materials that can function under low pressures with high salt extraction is a current research goal.
1.2 Variations in Seawater Conditions
The input pre-treatment system for saltwater desalination facilities can get overloaded by sudden, unpredictable seawater conditions and hazardous algal blooms. In some circumstances, dissolved-air-flotation (DAF) systems can be established; however, doing so would require a significant financial investment for a system that might not be used frequently.
1.3 Brine Disposal
Ecosystems may disturb if RO brine (concentrate left over from desalinating water) is discharged into the ocean. Due to RO brine’s high density, toxins can transport to the ocean bottom, where there is less wave propagation for dilution and potentially harmful effects on marine creatures. Dilution and/or diffusers may be necessary for some countries before or while RO brine is released into the ocean. Brine disposal can be particularly problematic for inland desalination since it cannot be dumped into surface water bodies or municipal sewage systems, necessitating reinjection wells or evaporation ponds.
1.4 Membrane Fouling
Inorganic solutes, biofilms, and organic debris that pollutes feed water slowly build up on membranes as they are used. The total result is a drop in permeability, often countered by a modest increase in pressure to preserve water output. Over the course of the plant’s life, these increase energy and water costs. However, routine membrane cleaning shuts down the facility and creates liquid waste that must be disposed of.
1.5 Energy Recovery
SWRO consumes a lot of energy even though it is considered energy-efficient commercial saltwater desalination technology. Most systems use electrical power accounts for 35% to 40% of overall running expenses. Up to 60% less energy is used when energy recovery devices (ERDs) are used.
1.6 Increased Freshwater Recovery
Creating process designs with transient or batch activities is one strategy for enhancing RO output. A flushing phase comes after recirculating the brine back to the membrane feed for a while. Recent results have been encouraging with estimated brackish RO recovery rates rising from the historical average of 75% to as high as 98%.
Higher brine intake causes salts to accumulate on equipment surfaces or piping systems and halt or sluggish the desalination process. One solution to this issue is to precipitate and remove this salty content before desalination. It takes a lot of work to incorporate precipitation and thermal desalination processes, and we anticipate more advancements in this area.
1.8 Marine Life Protection
A significant environmental problem is a harm that seawater intake systems cause to marine species. It is possible to reduce the negative impact of intake systems on aquatic organisms using subsurface intakes. Subsurface intakes can be useful in small systems since they offer some pre-treatment. For large-scale plants, their installation dramatically raises construction costs and time. Additionally, suitable geology and sediment properties, such as gravel and sand with enough high porosity and transmissivity, are necessary to install subsurface intakes.
2 Total Dissolved Solids and Energy Requirements for Desalination
The limits for the minimal amount of energy required to remove these solutes from water are set by Total Dissolved Solids (TDS) levels (or to move water away from these solutes). Energy is required to separate solute from solvent and is based on the concentration of the solute. Table 1 demonstrates that water from low-salinity streams, such as those from wastewater reuse, may require significantly less energy for desalination than water from higher-salinity streams, like seawater.
The global attention is now primarily on saltwater and water recovery from marginal sources, including brackish water, surface water, and recycled wastewater, due to the increasing demand for freshwater sources. Additionally, it has increased understanding and sped up the adoption of wastewater reuse, in which wastewater is highly processed and occasionally utilized for either indirect or direct re-utilization. Therefore, desalination is an important technique for humanity to enable sustainable growth.
Table 1: Common sources of desalination-related water with the TDS ranges for each and the estimated minimum energy required for separation.
3 Desalination Conventional and Emerging Technologies
Desalination technologies are categorized as traditional and emergent based on their level of scientific and technological advancement and market penetration. Scientific breakthroughs known as emerging desalination technologies encourage financial investment in desalination. These advancements are based on new technology that enhances desalination (lower energy use, less rejection, and higher quality water). Additionally, limiting the impact of the local rise in sea salinity caused by the reject stream should be considered a significant problem for the desalination business.
Conventional and cutting-edge technologies are further divided into groups based on the physicochemical process and the gradient type (electric, pressure, thermal, and chemical) (Table 2).
Table 2: Summary of conventional and emerging technologies for desalination
4 Innovations in Membrane Processes
The most significant technological advancements relate to developing polymeric materials with the enhanced selectivity and productivity of desalination operations. Similarly, advances in traditional technologies were made by playing with separation gradients, mass transportation systems, and renewable energy sources. The goal is always to find the most affordable, dependable, desalinated water production system.
4.1 Nanomembranes (NMs)
In order to increase hydrophilicity, production, and salt rejection, NMs incorporate nanoparticles (metal oxide or zeolitic type) in the active polymer matrix layer .
The phase inversion technique creates NMs, often thin film nanocomposite membranes (inorganic-organic mixed matrices). Due to its good chemical stability, fouling resistance, low toxicity, and available photocatalytic capabilities, TiO2 is a typical nanomaterial employed in NM synthesis . Since it is flexible, has a high density of spiral wound packing, is simple to manufacture, and has superior selectivity and permeability, organic NM allows for various geometries. In turn, the inorganic component of the NMs enables high rejection because of high charge density per unit surface, negative zeta potential, ion exchange capacity, and boosts permeability because of greater hydrophilicity and salt selectivity lessens bio-fouling because of biocidal and antimicrobial ability.
At the same salinity, pressure, flow rate, and temperature, several scientists compared NMs with traditional reverse osmosis membranes. They found that NMs are more productive (10–20%), have consistent selectivity (99.5–99.8%), less fouling, and use less energy  (see Figure 2).
4.2 Membrane Distillation (MD)
MD employs hydrophobic polymeric membranes with porosities between 0.01 and 0.5 micrometers, such as PTFE, PP, and PVDF. Gradients in temperature, vacuum or partial pressure can form the vapor distillate in MD. Different MD designs that have expanded their uses include direct contact membrane distillation (DCMD), vacuum membrane distillation (VMD), air-gap membrane distillation (AGMD), and sweep gas membrane distillation (SGMD).
The MD designs studied the most are DCMD and VMD . On opposite sides of a microporous membrane, two liquid phases in direct contact form DCMD. A gas phase exists in stationary forms inside the pore. The temperature differential causes surface-level evaporation and condensation on one side of the membrane. Furthermore, the partial pressure gradient of evaporated components contributes to mass diffusion. For heat gradients between 20 to 40°C, several researchers observed a permeate (evaporate) flow between 5 and 30 (L/hm2) . Because of its ability to operate at pressures lower than 5 kPa and produce large evaporate fluxes, VMD has higher productivity than DCMD .
4.3 Forward Osmosis (FO)
This developing technology has been discussed as a viable and affordable replacement for traditional membrane-based separation techniques, including membrane distillation and RO .
The draw solution (DS) is osmotically diluted, and clean water is produced from the diluted DS to desalinate seawater using FO methods . Instead of a hydraulic pressure difference (as in RO), an osmotic pressure differential across the membrane is what drives water through the membrane in FO. Without the need for externally provided hydraulic pressure, FO can be installed with low-pressure equipment that is easy to use and economical, which lowers the upfront expenses of pumping and system installation . FO connection to a traditional desalination process is shown in Figure 3. Pure water can flow from feed water because of re-concentrated DS flux. Desalination is made easy, strong, and dependable by the coordinated functioning of the two processes, which is a crucial component in the design of the entire process.
The DS characteristics have a significant impact on the FO membrane’s performance. A good DS should have high osmotic pressure, be nontoxic, stable and cheap, easily recover solutes and lessen inner concentration polarization .
Recently, the FO and DS membranes have been modified to increase the FO membrane’s desalination performance . In Al Najdah (Oman), Modern Water PLC constructed “Manipulated Osmosis Desalination (MOD)”; the first FO plant that produced 200 m3/d .
4.4 Reverse and Shock Electrodialysis
Reverse Electrodialysis (RED) works similarly to Shock Electrodialysis (SED); however, the voltage is reversed three to four times per hour, resulting in an overall water recovery of 97% . The polarity of an electrode is regularly reversed to prevent membranes from fouling, eliminating the necessity of pre-treatments and membrane cleaning. The rejection rate is between 75 to 90 %, depending on the ion type, electrical potential, valence, and feeding rate . RED is used in pilot scale desalination of sea and brackish water. Using this process in higher salinity waters can be justifiable because it can be associated with a renewable energy source.
The life cycle assessment of salinity gradient energy capture by reverse electrodialysis (SGE-RED) was examined . The findings have shown that SGE-RED is environmentally competitive with other renewable sources such as photovoltaics and wind. The high salinity solution treated with RED has a decreased salt content that reduces pump work when fed to RO plant.
SED is an under-developing technology that uses polarization zones to purify water by increasing the ion saturation in porous media close to an ion-selective membrane. A SED cell comprises ion exchange membranes or two electrodes that allow feedwater to pass through a charged porous region with thin double layers that principally play the role of a “leaky membrane” . In the SED cell, current causes an ion-depleted zone to develop across an ion-selective element (the cathode). SED operates on a small scale, making it a decentralized, point-of-use desalination system with promise. SED can be used as a pre-desalination step in the RO procedure, enhancing water recovery, lowering energy use, and offering a cost-effective solution.
4.5 Graphene Oxide Membrane (GM)
Among the many types of nanomaterials, graphene oxide (GO) stands out for its good absorption, hydrophilicity, chemical stability, porous structure, and good anti-fouling capabilities . GO has been applied as an absorber in solar desalination and a filter film in membrane desalination . Salt rejection and water flux are typically traded off in GMs. However, this limitation can be overcome by intercalation, altering the way GO film is deposited or employing electrostatic interaction between ions and GO. GMs are now being researched in laboratories . GMs needed lower operating pressures and had a greater mass transfer capability, producing permeates with total dissolved solids below or equivalent to 500 ppm. The study concluded that the energy usage of GMs is lower compared to commercialized membranes .
4.6 Aquaporin-Based Biomimetic Membrane (ABM)
Aquaporins (AQPs) (or vesicles) are special pore-membrane in biological cells. The cell membrane has a bundle of six transmembrane helices that make up these structures. Under the proper circumstances, aquaporins can create a water channel that only carries water molecules and exclude ionic species or other polar molecules. Aquaporins are an excellent model for creating a low-energy purification system in saltwater desalination because of their unique characteristic . Research has shown that ABMs are being developed as ultrahigh permeability (UHP) RO membranes . Aquaporins can provide water channeling/gating by being impregnated into a polymeric matrix, which results in regulated water permeability and ion selectivity.
In many cases, small-area membranes have been produced due to the extremely specialized synthesis procedures; the main challenge for ABMs is scaling up for industrial uses. The ABM showed steady flow, rejection, and strong mechanical stability over weeks to months. Its permeability is around 40% more than commercial brackish water RO membrane (BW30) and orders of magnitude greater than saltwater RO membrane (SW30HR), which shows the enormous potential of ABMs for desalination applications . ABMs can lower the energy expenses associated with treating water. Even now, these membranes are only being used on a laboratory scale; thus, additional development is required to increase their mechanical and chemical resistance.
4.7 Hybrid and Integrated Systems
Hybrid membrane techniques have recently achieved better indications of the desalination process. For instance, research shows the most advanced MD hybrids that use a variety of separation techniques, including PRO, RO, FO, electrocoagulation, MVC, ED, MED, MSF, crystallization, and adsorption, to increase water output and energy efficiency . Each of these procedures offers benefits at the expense of moderate to severe downsides, and their connection to MD presents the potential for improvement. Research has developed a revolutionary module architecture to merge FO and MD . The system is small and appropriate for various applications since the two operations are carried out in one module concurrently. According to the results, initial draw solution (MD feed) flow rate and concentration are the most crucial variables for the integrated module to operate steadily.
5 Thermal Desalination Innovation
Using renewable energy sources in conjunction with the desalination process is the major novelty in newly developed desalination technology. Solar and geothermal energy are the routes most often used as renewable energies. Researchers are investigating renewable energy sources that could be utilized to run desalination systems and the potential match between renewable energy sources and desalination. They also focus on needed improvements in solar energy and geothermal-driven desalination systems.
5.1 Solar Desalination (SD)
SD can be powered by direct and indirect solar energy. Solar collectors that evaporate water and make distillates are examples of direct solar energy. The architecture of a desalination plant that employs two sub-systems: a solar collector (photovoltaic or thermal) and a desalination unit (e.g., RO), is called indirect solar energy.
A summary of solar energy capture is presented in Figure 4. The highest temperature level (Tm) in the evaporation-condensation phenomenon determines the solar collector technology for thermal distillation operations. Tm less than or equal to 130 °C is considered low-temperature sun energy (LTSE), while Tm > 130 °C is considered high-temperature sun energy (HTSE). LTSE is produced in non-concentrated or low-concentration collectors and employs simple solar fields, meaning there are no moving components and little up-front and ongoing expenditures . HTSEs use mirrors to direct solar energy from vast surfaces (the aperture area) onto the receiver’s small area. The desalination technologies, including MED, MSF, low-temperature MED, humidification-dehumidification, membrane distillation, etc., use thermal energy and thus need thermal energy storage (TES) for continuous operation . Thermoelectric storage heaters, pressurized water, molten salts, and heated oil are the most often utilized TES techniques with maximum working temperatures of 550C, 150C, 250–550C, and 395C, respectively [28,29].
A desalination facility using a solar thermal energy concentrating device (CSP) and RO is shown in Figure 5(a). The CSP uses thermal energy that is captured to create superheated steam. Several CSPs permit heat storage in a subsidiary process based on molten salts . In order to turn a turbine and generate electricity, superheated steam is used. The electrical energy produced is used to power the high-pressure pump. A photovoltaic and RO (PV/RO) desalination facility is shown in Figure 5(b). This method is helpful for brackish water that requires minimal pumping power and small desalination equipment (0.2 L/s) . Batteries are needed for PV/RO to continue operating continuously.
Since of this, indirect methods are more suitable for industrial-scale manufacturing because they are more effective.
5.2 Geothermal Desalination (GD)
Seasonal and climatic fluctuations can have little impact on geothermal energy extraction. Geothermal energy can be utilized in membrane and thermal desalination operations, depending on the water’s location and physical and chemical properties.
GES-integrated membrane processes are still being developed. A study has evaluated the possibility of using geothermal energy for MD . It concluded that process waste heat sources might be used to increase permeate flow rate by around 6.1% for every 2-degree temperature variation (in feed water). These technologies are attractive, particularly in the Gulf area, where geothermal energy is abundantly accessible.
6 Improvements in Energy Recuperation
Biochemical systems for recovering brine energy can be broadly divided into pumps to convert rejection pressure energy to mechanical power and pressure exchangers to directly convert brine pressure to feed flow and turbines . RO systems that produce more than 3000 m3/d use energy recovery technologies, including Pelton turbines, turbochargers, and pressure exchangers (PX devices). These devices use the same process to recover energy in desalination.
The rejection pressure is converted into kinetic energy via the Pelton turbine (Figure 6). A wheel with vanes connected to a high-pressure pump motor receives the pressurized liquid. The turbocharger is a small energy recovery device with a single shaft and inversely coupled pumps and turbines. The turbocharger turbine increases fluid pressure, transforming hydraulic pressure into mechanical energy that the pump can use. The PX device does not transform the high pressure of the reject brine into mechanical rotation energy before transferring it directly to the saltwater. The system makes use of isobaric chambers and the positive displacement concept. These three technologies perform with great efficiency (up to 97%) and can save up to 40% of the energy needed .
Nearly half of the world’s desalination plants are located in the Middle East, and the need for desalination in the area is still increasing. There is still a need to ensure convenient access to clean water for millions of people. Because of how quickly technology has advanced in this sector and continues to do so, it is plausible to anticipate considerable advancements in the near term.
A major factor contributing to this increased tendency is the growing population and high water use per capita. The continual reduction in expenses has also contributed to the expansion of capacity. Desalinated water can cost 0.5 to 1 USD/m3, which is comparable to many traditional water sources.
The ongoing drive to cut prices has brought a breakthrough in desalination technology. RO, an electrically-driven operation, has replaced the historical thermally-driven MSF and MED processes. RO utilizes less energy than MSF and MED, which results in lower costs and emissions. However, RO continues to have a considerable negative impact on the environment, not only because of the CO2 released into the environment but also because of the huge amounts of brine dumped into the ocean.
7.1 Desalination Future
Despite successfully cutting energy use, the desalination industry has so far given very little attention to brine outflow. A whole new sector might be developed around “mining” because of the availability of many minerals in seawater, which also lessens the impact on marine ecosystems. The prospect is under the notice of research funding organizations worldwide.
RO is often unable to manage the severe salinities encountered. Hence, thermal technologies typically handle the latter phases of brine concentration and mineral recovery. However, when given a feed that has already been preconcentrated, thermal methods need less effort (and use less energy). This notion is now driving several advancements in high-recovery RO, from the merely conceptual to the operating industrial plant and spanning technological readiness stages. For instance, “Low Salt Rejection Reverse Osmosis” (LSRRO) has been proposed, which calls for RO membranes to be adjusted to have imperfect salt rejection. The membrane’s differential osmotic pressure, which may surpass 200 atmospheres, is reduced by little salt leakage (enough to crush even the most robust membranes available). Theoretically, employing several such membranes in succession is projected to give rejection rates close to 100% while using energy at a fraction of the cost of conventional brine concentration techniques. Counter-flow RO is a more established method that dilutes the feed by using water that is moved from brine that leaves the system to the point where a standard RO device can handle it. The difficulty with high salinity RO, as in LSRRO, is to avoid a significant concentration differential across the membrane while maintaining high absolute concentrations to prevent the damaging pressures often associated with high salinity RO.
RO technique considers various phases, energy recovery techniques, and possibilities available throughout the process. Additionally, choosing the most recent synthesized membranes is crucial for maximizing efficiency.
Seawater and brine can be concentrated by sun evaporation, as is the case in conventional salt works. However, this method uses a large area and is expensive to maintain. However, using wind-driven evaporation can also save a lot of money and space. Seawater Greenhouse technology in Africa proves that a wind-driven evaporator can provide cooling advantages, bringing the cost to zero. A winning solution at the intersection of water, energy, and food might come from an efficient system integrating seawater desalination, greenhouse farming, brine concentration, and mineral recovery.
When it comes to RO systems, increased salinity generally translates into higher pressures, but how this affects energy use also depends on whether you are talking about normal or peak pressure. The average pressure required for batch and semi-batch RO processes is significantly less than the peak pressure. Significant energy savings are possible when used in conjunction with positive displacement pumps that can easily tolerate pressure changes. At a recovery rate of less than 50%, savings are minimal. Compared to semi-batch RO, batch RO is more energy-efficient but requires more pressure vessels, which might increase the initial investment. This flaw may be fixed by merging batch and semi-batch techniques (hybrid version), which has nearly perfect efficiency and achieves >90% recovery. The residual brine is diluted to a capacity of one-tenth its original size while 90% of the input water is turned to freshwater.
Significant cost reductions in RO are anticipated if the working pressure can be reduced without impacting the performance of the membranes.
The technique discussed here, which incorporates all the efficiency-improving methods, offers the safest method for desalination in terms of the consumption and production of the energy required for the process.
Even though it is ideal to imagine a plant today processing hundreds or thousands of cubic meters of saline water per day through the sole supply of renewable energies. It is recommended to partially associate these large desalination plants with renewable energy, primarily solar or wind.
7.3 Desalination Benefits have to Reveal
Desalination technology has grown to a certain level of maturity, but it is also about to undergo a significant transformation. Desalinated water has largely been used for municipal purposes up to this point, but it is now becoming affordable enough to be used for irrigation. Given that irrigation accounts for 90% of all water use in the Middle East, this is expected to result in a new growth phase that will assist fulfill the region’s rising food demand. With the potential to quickly advance the several developing technologies that will make desalination more accessible and sustainable, this is anticipated to draw further investments into the sector.
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