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| Fig. 1: Reverse Osmosis Process. The main step of reverse osmosis is detailed in the green box, where a pump pushes water through membranes to filter out solids. The rest of the process addresses challenges described below. (Source: D. Spulber, after Almasoudi and Jamououssi. [6]) |
2.5% of the water on Earth is freshwater. Most of the freshwater is trapped inside of ice caps, so only 0.8% of the water on Earth is accessible freshwater. [1] Accessibility does not mean it is equally distributed; the freshwater is concentrated in regions with climates that allow for it, leaving certain parts of the world in water crises. Innovations in water technologies were driven by the need for freshwater for drinking, agriculture, and daily use. One such of these technologies is desalination, which is beneficial for areas saturated with saltwater.
One region for which this is a pressing issue is the Middle East an extremely arid area lacking freshwater sources. It only holds 1.4% of the global freshwater. [2] Access to the Arabian Gulf, massive wealth, and an abundance of fossil fuels in countries such as Saudi Arabia and the UAE have propelled their desalination industries. Saudi Arabia is the largest producer of desalinated water with over a dozen plants in the state. [3] It is also the third largest user of desalinated water in the world behind the United States and Canada. [4] 70% of the drinking water in Saudi Arabia comes from desalination with the rest coming from depleting groundwater reserves. [4] It is undergoing a project called Vision 30 with goals of producing 90% of its drinking water from desalination. [5]
One of the largest desalination plants in Saudi Arabia is the Ras Al-Khair Power and Desalination Plant. It is the second largest plant in the world and is partially powered by wind energy. [5] It has a production capacity of about 1,000,000 cubic meters desalinated water per day, primarily coming from reverse osmosis and multi-stage flash distillation. Saudi Arabia is also modernizing a plant which will be its largest plant and a reverse osmosis plant, is called Al-Jubail, and will have a similar capacity of 1,000,000 cubic meters desalinated water per day. [5] Saudi Arabian desalination plants and their capacities are detailed in Fig. 1.
Desalination techniques are split into two groups: thermal desalination and membrane desalination. In Saudi Arabia, desalination is split in half between these two groups. [2] Thermal desalination relies on principles of distillation; it involves heating up the water, which will cause the water to evaporate while leaving the solids consisting of pollutants, salts, and microbes. The evaporated water will then cool and condense under pressure. [6] Multi-stage flash distillation is the main example of thermal desalination. As of 2019, it still comprised 23% of desalination techniques. It involves up to 25 stages of the process described above, where each stage is at a slightly lower temperature until the water is room temperature. The recovery percentage of this technique is only 25% on average but can be up to 50%. [1]
The main membrane technique used for desalination is reverse osmosis. As of 2019, reverse osmosis comprised 65% of desalination techniques. [1] In reverse osmosis, water is pushed via pressure through a semipermeable membrane. The membrane pore size is small enough to allow water to pass without the accompanying solids under a large enough pressure. The applied pressure must be larger than the osmotic pressure to prevent the backflow of desalinated water (low solute concentration) to a higher concentration water. [6]
Compared to previous techniques, reverse osmosis is more efficient in terms of water volume, energy, and cost, which allows for great scalability. Reverse osmosis can also be used to purify water of any reasonable salinity content. However, the salinity of the water affects parameters such as the recovery percentage, energy usage, and water production cost. For example, take water with low salinity, known as brackish water. This is defined as water with a concentration of 0.5-15 g/L total dissolved solids. The recovery percentage of desalinated water is 60-75%. In contrast, seawater, which has a higher salinity at a concentration of 15-50 g/L, only has a recovery percentage of 40-60%. This is due in part to the membrane efficiency, especially considering how chemicals used in the process react with the materials in the membrane. Additionally, higher salinity means more membranes and stronger pumps are required to filter out the solids; this is reflected in the energy usage (2.5-4 kWh/m3 for seawater vs. 0.3-2.8 kWh/m3 for brackish water) and production cost (0.5-3.0 $/m3 desalinated seawater vs. 0.2 - 1.8 $/m3 desalinated brackish water). Of course, which type of water is accessible geographically will dictate these parameters. [7]
The biggest challenge with reverse osmosis is the durability of the membrane. To prevent buildup on the membrane, the water is treated prior to filtration with chemicals designed to remove anything that would react with particles and create salts. The downside is that these chemicals could also react with the materials in the membrane, reducing its effectiveness. Additionally, post-treatment of the water is required to make up for deficiencies in the reverse osmosis process. One problem extremely relevant to the Middle East is related to oil extraction. The extraction process can result in organic byproducts leaking into the supply water, which impacts water quality and can be unsafe to drink even in low quantities. [1] They can even speed up the decay of the membranes. A thermal desalination technique is implemented before reverse osmosis to limit these organics, but this involves increased costs to design this infrastructure and recheck the water constantly for organics.
Post-treatment is also necessary to remedy the filtering out of too many solids. This results in the water being slightly acidic, which will damage the desalination infrastructure and can be too acidic for potability. Minerals such as magnesium and calcium are added into the filtered water to neutralize it. [1] This adds another stage to the desalination process, reducing capacity and increasing costs. Despite this, reverse osmosis has proven to be the best desalination technique to date. The full reverse osmosis process is shown in Fig. 1.
To get a sense of these numbers, let us look at the minimum energy required for a desalination process using reverse osmosis. The osmotic pressure defines the energy required to push water through a membrane into a lower concentration region. Modeling this as a piston moving adiabatically, W = P dV where P is pressure and dV is the change in volume, the work for this reversible process is:
where n is the number of moles of solute, R is the ideal gas constant, T is the temperature, and C'/C is the ratio of the concentration of the water before and after the piston motion. Ideally, the number of solutes moving across the membrane is not changing, while only the volume of freshwater is being pushed across the membrane. This volume ratio is defined as the recovery rate r of the desalination process, or the volume of produced water over the volume of feedwater. Thus the expression for the work per volume produced water can be approximated as (and converting the number of moles of solute in the feedwater to concentration of equal amounts Na and Cl ions in produced water)
Taking seawater with average salinity C = 30,000 mg/L at room temperature with a recovery rate of 50%, [7]
| W | = | - | 30,000 g m-3 0.5 × 22.9 g mol-1 + 0.5 × 35.45 g mol-1 |
× 8.314 J mol-1 K-1 × 298 K × | ln(0.5) 0.5 |
| = | 3.53 MJ m-3 = 0.98 kWh m-3 | ||||
This is an approximation assuming a perfect membrane. Real values for energy costs are about 3 times larger. [7] These energy costs include running the pumps, transporting the brine, treating water to remove organics before filtering, and treating the water by adding minerals and homogenizing the mixture after membrane filtering.
The waste from desalination is known as brine. It includes any chemicals used throughout the process and the solids that were filtered out of the water. For reverse osmosis, the brine from seawater desalination has twice the original salinity, and the brine from brackish water desalination has four times the original salinity. [1] The disposal of this waste is simple dump it back into the water supply. In this way the Arabian Gulf has increased in salinity and wildlife has been affected. [8]
This is a cycle where even more energy is required to desalinate the water, resulting in even more greenhouse gases being released. Renewable energy has increasingly been considered for powering desalination plants. The renewable energy capacity in Saudi Arabia quadrupled just from 2020 to 2021. [1] In Middle Eastern countries, there is great potential for the use of solar and wind energy. Saudi Arabia also has large geothermal reserves it is not currently using for energy production. In addition to these renewable energy sources providing energy for pumps to push water through membranes in reverse osmosis, solar panels can also source heat used for thermal desalination. [1] Saudi Arabia is currently developing the worlds largest fully solar-powered desalination plant. [4] Despite this, oil is still the dominant energy source used in Saudi Arabias desalination plants.
Without diving too deep into this, the issue of desalination cannot be untangled from geopolitics. It is important to remember that water is a nonrenewable resource essential to all life, and tensions arise when water becomes a state-controlled resource. Hierarchies arise between states with water resources, states with financial resources, and others. This issue will become only more pressing as climate change further depletes freshwater sources and the global population increases.
Desalination will remain a crucial part of the water industry in Saudi Arabia for as long as it exists. The energy required to power it will move increasingly towards renewable resources. Despite all the challenges and limitations described above, reverse osmosis remains the dominant desalination technique and the main source of freshwater in countries such as Saudi Arabia. Technological advancements will leader to plants that are more energy efficient, have a larger capacity for desalinated water production, and are hopefully more environmentally friendly. It will be interesting to see the changes that come about because of Vision 2030, and how evolving geopolitics impacts development in this field.
© Diana Spulber. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] N. Kress, Marine Impacts of Seawater Desalination (Elsevier, 2019).
[2] A. Mahmoudi et al., "Challenges and Opportunities of Desalination With Renewable Energy Resources in Middle East Countries," Renew. Sustain. Energy Rev. 184, 113543 (2023).
[3] E. Jones et al., "The State of Desalination and Brine Production: A Global Outlook," Sci. Total Environ. 657, 133 (2019).
[4] E. DeNicola et al., Climate Change and Water Scarcity: The Case of Saudi Arabia," Ann. Glob. Health 81, 342 (2015).
[5] A. Allouhi and K. M. Almohammadi, "Towards Green Desalination: A Multi-Site Analysis of Hybrid Renewable Energy Integration in Saudi Arabian RO Plants," Desalination 592, 118087 (2024).
[6] S. Almasoudi and B. Jamououssi, "Desalination Technologies and Their Environmental Impacts: A Review," Sust. Chem. One World 1, 100002 (2024).
[7] S. M. Alawad et al., "Renewable Energy Systems For Water Desalination Applications: A Comprehensive Review," Energy Convers. Manag. 286, 117035 (2023).
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