Desalination is a process that takes away mineral components from saline water. More generally, desalination refers to the removal of salts and minerals from a target substance, as in soil desalination, which is an issue for agriculture. Saltwater (especially sea water) is desalinated to produce water suitable for human consumption or irrigation. The by-product of the desalination process is brine. Desalination is used on many seagoing ships and submarines. Most of the modern interest in desalination is focused on cost-effective provision of fresh water for human use. Along with recycled wastewater, it is one of the few rainfall-independent water resources.
Due to its energy consumption, desalinating sea water is generally more costly than fresh water from surface water or groundwater, water recycling and water conservation. However, these alternatives are not always available and depletion of reserves is a critical problem worldwide. Desalination processes are using either thermal methods (in the case of distillation) or membrane-based methods (e.g. in the case of reverse osmosis) energy types.
An estimate in 2018 found that "18,426 desalination plants are in operation in over 150 countries. They produce 87 million cubic meters of clean water each day and supply over 300 million people." The energy intensity has improved: It is now about 3 kWh/m3 (in 2018), down by a factor of 10 from 20-30 kWh/m3 in 1970. Nevertheless, desalination represented about 25% of the energy consumed by the water sector in 2016.
History
Desalination has been known to history for millennia as both a concept, and later practice, though in a limited form. The ancient Greek philosopher Aristotle observed in his work Meteorology that "salt water, when it turns into vapour, becomes sweet and the vapour does not form salt water again when it condenses," and also noticed that a fine wax vessel would hold potable water after being submerged long enough in seawater, having acted as a membrane to filter the salt. There are numerous other examples of experimentation in desalination throughout Antiquity and the Middle Ages, but desalination was never feasible on a large scale until the modern era. A good example of this experimentation are the observations by Leonardo da Vinci (Florence, 1452), who realized that distilled water could be made cheaply in large quantities by adapting a still to a cookstove. During the Middle Ages elsewhere in Central Europe, work continued on refinements in distillation, although not necessarily directed towards desalination.
However, it is possible that the first major land-based desalination plant may have been installed under emergency conditions on an island off the coast of Tunisia in 1560. It is believed that a garrison of 700 Spanish soldiers was besieged by a large number of Turks and that, during the siege, the captain in charge fabricated a still capable of producing 40 barrels of fresh water per day, though details of the device have not been reported.
Before the Industrial Revolution, desalination was primarily of concern to oceangoing ships, which otherwise needed to keep on board supplies of fresh water. Sir Richard Hawkins (1562-1622), who made extensive travels in the South Seas, reported in his return that he had been able to supply his men with fresh water by means of shipboard distillation. Additionally, during the early 1600s, several prominent figures of the era such as Francis Bacon or Walter Raleigh published reports on water desalination. These reports and others, set the climate for the first patent dispute concerning desalination apparatus. The two first patents regarding water desalination date back to 1675 and 1683 (patents No.184 and No. 226, published by Mr. William Walcot and Mr. Robert Fitzgerald (and others), respectively). Nevertheless, neither of the two inventions was really put into service as a consequence of technical problems derived from scale-up difficulties. No significant improvements to the basic seawater distillation process were made for some time during the 150 years from the mid-1600s until 1800.
When the frigate Protector was sold to Denmark in the 1780s (as the ship Hussaren) the desalination plant was studied and recorded in great detail. In the newly formed United States, Thomas Jefferson catalogued heat-based methods going back to the 1500s, and formulated practical advice that was publicized to all U.S. ships on the backs of sailing clearance permits.