A desalination plant, hailed as being the world’s largest, was completed in May 2009 in the new Jubail II Industrial Zone in the eastern province of the Kingdom of Saudi Arabia. The plant produces 2,750 megawatts of electricity and 800,000 cubic meters of water per day. Photo: Jubail Water and Electricity Co.
Seawater desalination facilities are already vital for economic development in many arid areas of the world, but there are real examples of desalination plants that have been
- overly expensive,
- inaccurately promoted,
- poorly designed, or
- inappropriately located.
Any of these may lead to a project ultimately becoming useless. It is important, therefore, to consider both the advantages and the disadvantages of seawater desalination in order to make informed choices about the appropriateness of this technology for an area.
How does desalination work?
The Earth’s hydrologic cycle naturally desalinates water using solar energy. Water evaporates from oceans, lakes, and land surfaces leaving salts behind. The resulting freshwater vapor forms clouds that produce precipitation, which falls to earth as rain and snow and moves through soils, dissolving minerals and becoming increasingly salty. The oceans are salty because the natural process of evaporation, precipitation, and runoff is constantly moving salt from the land to the sea, where it builds up over time.
Distillation is one of humanity’s earliest forms of water treatment, and it is becoming more popular today as a solution to water shortages. Diagram from Desware: The Encyclopedia of Desalination and Water Resources.
A wide variety of desalination technologies effectively remove salts from salty water (or extract fresh water from salty water), producing a water stream with a low concentration of salt, called the product stream, and another with a high concentration of remaining salts, referred to as brine or concentrate. The majority of technologies in use today rely on either thermal distillation or membranes to separate salts from the product.2,3,4,5 Ultimately, the selection of a desalination process depends on site-specific conditions, including the salt content of the water, economics, quality of water needed by the end user, and local engineering experience and skills.
Desalination process options include:
- Membrane and filtration processes: Membranes and filters can permit or prohibit the passage of certain ions selectively, and desalination technologies have been designed around these capabilities. These natural principles have been adapted in two commercially important desalting processes: electrodialysis (ED) and reverse osmosis (RO). Both of these concepts have been understood for a century; however, commercialization has lagged until the technology for creating and maintaining membranes improved. Although they have typically been used to desalinate brackish water, versions are being applied to seawater increasingly, and these two approaches now account for more than half of all desalination capacity. A growing number of desalination systems are also adding filtration units prior to the membranes in order to remove contaminants that affect long-term filter operation.
Thermal distillation: Approximately 40% of the world’s desalted water is produced with processes that use heat to distill fresh water from seawater or brackish water. The distillation process mimics the natural water cycle by producing water vapor that is then condensed into fresh water. In the simplest approach, water is heated to the boiling point to produce the maximum amount of water vapor. Water will boil at 100¡ãC under atmospheric pressure. By decreasing pressure, however, the boiling point can be reduced. At one-quarter of normal pressure, water will boil at 65¡ãC, and at one-tenth of normal pressure, it will boil at only 45¡ãC. To take advantage of this principle, systems have been designed to allow, “multiple boiling” in a series of vessels that operate at successively lower temperatures and pressures. The concept of distilling water with a vessel operating at a reduced pressure has been applied for well over a century.
Other desalination processes: Water can be desalted though many other processes including small-scale ion-exchange resins, freezing, and membrane distillation. None has achieved much commercial success, and together, they account for less than 1% of total desalination capacity.5 Nevertheless, some of these approaches can be effective, and even preferable, under special circumstances.
Where is seawater desalination practiced today?
The idea of separating salt from water is an ancient one, dating from the time when salt—not water—was a precious commodity. As populations and demands for fresh water expanded, however, entrepreneurs began to look for ways of producing fresh water in remote locations, and especially, on naval ships at sea.
The Tampa Bay Seawater Desalination facility on Florida’s Gulf Coast provides up to 25 million gallons per day of drinking water to the region by reverse osmosis. Photo: Tampa Bay Water agency.
Desalination technology is in use throughout the world for a wide range of purposes, including
- providing potable fresh water for domestic and municipal purposes
- treating water for industrial processes
- obtaining emergency water for refugees or military operations
Desalination facilities are found in many arid and water-short areas of the world, and they are vital for economic development. In particular, desalination is an important water source in parts of the arid Middle East, Persian Gulf,6 North Africa, Caribbean islands, and other locations, where the natural availability of fresh water is insufficient to meet demand, and where traditional water-supply options or transfers from elsewhere are implausible or uneconomical. Increasingly, other regions are exploring the use of desalination as a potential mainstream source of reliable, high-quality water as the prices slowly drop toward the cost of more traditional alternatives.
What are the advantages of seawater desalination?
Water supply reliability Proponents of desalination argue that one of the important benefits of desalination is its supply reliability. The production of desalinated water is largely independent of weather; instead, it depends on ensuring the continued operation of desalination infrastructure. This benefit is particularly valuable in arid and semi-arid climates where weather variability is high. There is also a value to new supply under local control and to increased diversity of supply as a way to increase resilience to natural disasters or other threats to water systems.
High-quality water One of the advantages of desalination is the potential to produce high-quality water. Desalination facilities are designed to remove numerous impurities and produce water that may be a large improvement over existing water sources. The desalination process, nonetheless, can also run the risk of introducing harmful chemicals and metals into the water it produces, or leaching them out of the distribution system on the way to users. These concerns, however, can be mitigated through adequate monitoring and appropriate regulation of all desalination facilities.
Local control of water resources In many regions of the world, water resources are increasingly transferred from one place to another—especially from rural to urban communities, from water-rich to water-poor regions, and toward economic interests willing and able to pay for water. These transfers raise two separate issues of local control of resources:
- Exploitation of local resources. The first concern is the worry of rural—often agricultural—areas that distant urban or economic powers will steal local resources. The classic example is the effort of the City of Los Angeles in the early part of the 20th century to obtain water from farming communities hundreds of miles away, which has colored California water politics ever since.7
- Dependence on potentially unreliable, distant resources. The second concern is that urban centers will become dependent on distant resources and increase their vulnerability to supply disruptions over which they have limited control.
Desalination may offer a solution to both of these political problems by providing reliable, high-quality sources of water under direct local control, reducing the need for imported water, and reducing the vulnerability to outside disruptions at the same time.
What are the disadvantages of seawater desalination?
Cost The cost of desalination has fallen in recent years, but it remains an expensive water-supply option. Typical costs for water produced through desalination range from $1,200-2,000 per acre-foot—substantially more expensive than most other water supply and demand management options. The assumption that desalination costs will continue to fall may be false. Further cost reductions may be limited, and actually, future costs may increase.
Energy use Seawater desalination is an energy-intensive process. Energy is the largest single variable cost for a desalination plant, varying from one-third to more than one-half the cost of produced water.8,9 Additionally, more energy is required to produce water from desalination than from most other water supply or demand-management options. Because of its energy intensity, desalinated water is more sensitive to changes in energy prices than other sources of water. Efforts to reuse energy or minimize energy demands will help reduce overall costs. Although opportunities for reducing energy use certainly exist, there are ultimate limits beyond which energy-efficiency improvements cannot be made.10
Environmental impacts Desalination, like any other major industrial process, has environmental impacts that must be understood and mitigated (see review by Einav et al.11 and The National Academy of Sciences9). These include the effects associated with the construction of desalination plants, and especially the long-term operation of these plants—including the consequences of withdrawing large volumes of brackish water from an aquifer or seawater from the ocean. Large marine organisms, such as adult fish, invertebrates, birds, and even mammals, are killed on the intake screen (impingement); organisms small enough to pass through the intake screens, such as plankton, eggs, larvae, and some fish, are killed during processing of the salt water (entrainment). Impingement and entrainment effects are species and site specific, and only limited research has been completed on the impacts of desalination facilities on the marine environment.9
Adequate and safe disposal of the concentrated brine produced by the plant presents a significant environmental challenge. Typical brines contain twice the salt as the feedwater. Brine from seawater desalination facilities can contain concentrations of constituents typically found in seawater, such as manganese, lead, and iodine, as well as chemicals introduced via urban and agricultural runoff, such as nitrates.12 Further, the brine can contain impinged and entrained marine organisms killed during the desalination process.
Does climate change impact seawater desalination?
The “Sustainable Energy and Desalination on Vessel” ship is an Australian invention by Seadov Pty Ltd., which hopes to solve water shortages cost-effectively using “green marine technology.” The ship is in the planning stage and not yet under construction. Photo: Still image from video produced by Seadov Pty Ltd., Australia.
Desalination offers both advantages and disadvantages in the face of climatic extremes and human-induced climate changes. Desalination facilities may help reduce the dependence of local water agencies on climate-sensitive sources of supply. Extensive development of desalination, however, can lead to a greater dependence on fossil fuels, an increase in greenhouse gas emissions, and a worsening of climate change. Furthermore, coastal desalination facilities will be vulnerable to the effects of climate change, including rising sea levels, storm surges, and extreme weather events.
The potential benefits of ocean desalination are great for human needs, but the economic, cultural, and environmental costs of worldwide commercialization remain high. In many parts of the world, alternatives can provide the same freshwater benefits of seawater desalination at far lower economic and environmental costs. These alternatives include treating low-quality local water sources, encouraging regional water transfers, improving conservation and efficiency, accelerating wastewater recycling and reuse, and implementing smart land-use planning.
Desalination may provide environmental benefits if it can reduce pressures on rivers and streams. Typically, however, the link between desalination (or other new sources) and more water for environmental purposes is weak. Unless a water rights order, or potential order makes water for the environment mandatory, or more water taken from the environment problematic, there is usually no explicit mechanism to link desalination project approval with environmental water. Without a mechanism, there is no guarantee that water will be used for ecosystem restoration.
Is seawater desalination the ultimate solution to our water problems? No. Is it likely to be a piece of our water management puzzle? Yes. In the end, decisions about desalination developments will revolve around complex evaluations of local circumstances and needs, economics, financing, environmental, and social impacts, and available alternatives.
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