Desalination (also desalinization) converts salt-containing waters such as seawater into
freshwater. Desalination has the potential to be particularly valuable in places suffering drought, countries in severe water stress, or in areas of expanding desertification.
Two common methods for removing salts from water are distillation and reverse osmosis. Distillation is an inexpensive process in which freshwater evaporates out of heated salt water. Reverse osmosis (RO) requires more expensive filtration equipment than distillation. In RO pressure forces seawater through a filter, called a membrane, containing very small diameter pores. The pores let water pass through but remove about half of the dissolved salts. The concentrated salt water can be returned to the ocean and the freshwater used for irrigation. More advanced RO systems contain membrane pores in the range of 1 micrometer (μm) that make the treated water safe to drink. Microfiltration uses smaller pores of 0.05 to 0.5 μm diameter, and ultrafiltration uses pores of 0.001 to 0.01 μm diameter. Both of these filtration techniques ensure that water is safe to drink because they remove even very tiny contaminants such as viruses. RO plants usually employ a pre-RO filtration step called coarse screening that catches large insoluble materials on a screen to make the membrane filtration more efficient.
At least 7,500 desalination plants operate worldwide with about 60 percent of them in the
Middle East. Saudi Arabia, Kuwait, and Israel depend on desalination for a major portion of their clean freshwater. Saudi Arabia owns the world’s largest plant, which produces 130 million gallons (492 million l) of freshwater daily. North Africa, the Caribbean, and countries in the Mediterranean region have also explored desalination; Mexico and the United States use it on a small scale.
Though desalination technology can supply water to thirsty areas of the world, it currently produces less than 1 percent of the world’s water needs. Three disadvantages contribute to desalination’s slow acceptance. First, the treatment plants, especially RO, are expensive to build. RO requires costly equipment, and both distillation and RO consume large amounts of energy, so desalination’s costs create too great a burden for drought-stricken developing countries. Even in developed nations, desalination costs more than other water treatment methods. Second, the desalination process creates a large quantity of salt, which must be cleaned from equipment on a regular maintenance schedule. Third, the excess salt and high-salt wastewater must be returned to the environment. Dumping the high-salt wastes into the ocean harms aquatic ecosystems in the area; dumping it on land has the potential to contaminate surface waters and groundwaters.
In order for desalination to lessen world water shortages, technology will need to design
more efficient, inexpensive filters. The Canadian author and water treatment expert Maude Barlow said in 2008, “Even with current plans to triple global production, including nuclear-powered desalination plants, this technology cannot meet the demand for freshwater in the world.” Desalination adds to the world’s water supply, but it does not appear that it will be part of sustainable water use in the near future.
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