Desalination
Desalination is the artificial process by which salt water (generally sea water) is converted to fresh water. More generally, desalination is the removal of salts and minerals from a substance.[1] It is possible to desalinate saltwater, especially sea water, to produce water for human consumption or irrigation, producing brine as a by-product.[2]
Interest in desalination mostly focuses on cost-effective provision of fresh water for human use. Along with recycled wastewater, it is one of the few water resources independent of rainfall.[3] As stress on the need for freshwater intensifies globally, desalination has become a key part of strategies for global water security. According to a 2019 review in Science of the Total Environment, around 95 million cubic meters per day of desalinated water is produced worldwide, and the demand for desalinated water is expected to grow significantly to help close the global water supply gap.
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.[4][5][6] Desalination processes use either thermal methods (in the case of distillation) or membrane-based methods (e.g. in the case of reverse osmosis).[7][8]: 24
As of 2020, global desalination capacity stood at roughly 97 million m3/day from over 16,800 operating plants, with contracted projects pushing total potential capacity beyond 114 million m3 /day worldwide.[9] In 2018, the global energy intensity of desalination was about 3 kWh/m3 (in 2018), improved by a factor of 10 from 20–30 kWh/m3 in 1970.[8]: 24 Nevertheless, desalination represented about 25% of the energy consumed by the water sector in 2016.[8]: 24 Key companies in the desalination industry include Acciona, Dow, Evoqua Water Technologies, Siemens AG, DuPont, Doosan Enerbility, Toray Industries Inc., and Xylem.[10]
History
[edit]Ancient times
[edit]Early concepts related to desalination can be traced to Aristotle, who observed in Meteorology that when seawater evaporates, the resulting vapor condenses as fresh water rather than salt water:
Salt water when it turns into vapour becomes sweet, and the vapour does not form salt water when it condenses again. This I know by experiment.[11]
This reflects an early understanding of evaporation and condensation. He also described how a wax vessel could retain potable water after immersion in seawater, suggesting a primitive form of filtration.[12]
References to seawater desalination also appeared in ancient China. Texts associated with the Period of the Warring States and the Eastern Han dynasty describe how bamboo mats used in rice steaming developed a thin outer layer after prolonged use, which was believed to have salt-absorbing properties.[13]
More generally, examples of experimentation in desalination appeared throughout Antiquity and the Middle Ages, although these remained limited in scale and application.[14] Large-scale desalination, however, did not become feasible until the modern period.[15]
Middle Ages and Renaissance (5th–17th century)
[edit]During the Middle Ages, desalination remained limited and was primarily based on distillation, in which seawater was heated to produce vapor that was then condensed into fresh water. These methods were generally small-scale and used in specific circumstances, particularly in maritime settings.[16]
A notable Renaissance contribution came from Leonardo da Vinci, who proposed that distilled water could be produced more efficiently by adapting a still to a cookstove.[17] In Central Europe, distillation techniques also continued to develop during this period, although not always specifically for desalination.[16]
Historical accounts from the 16th century describe the use of desalination devices under emergency conditions. One example may have occurred in 1560 on an island off the coast of Tunisia, where a besieged Spanish garrison reportedly constructed a still to produce fresh water, although technical details of the apparatus are not known.[16][18]
Pre-industrial and early modern period (16th–18th century)
[edit]Before the Industrial Revolution, desalination was mainly used aboard oceangoing ships, where access to fresh water was essential for long voyages. Shipboard distillation was used to supplement stored supplies when necessary.[19]
Interest in desalination increased during the 17th century. Figures such as Francis Bacon and Walter Raleigh discussed methods of water purification, while Sir Richard Hawkins reported supplying his crew with fresh water by distilling seawater aboard ship.[18][20][19]
This growing interest was reflected in the first patents for desalination apparatus. Patents were granted in 1675 and 1683 to William Walcot and Robert Fitzgerald and others, respectively, although neither invention entered widespread service because of difficulties in scaling the technology.[21][22][16]
Throughout this period, progress remained gradual, and no major improvements to the basic distillation process were recorded between the mid-17th century and the end of the 18th century.[16]
When the frigate Protector was sold to Denmark in the 1780s and renamed Hussaren, its distillation apparatus was studied and documented in detail.[23] In the United States, Thomas Jefferson later compiled earlier heat-based desalination methods and circulated practical guidance for their maritime use.[24][25]
Industrial era (19th century)
[edit]Desalination methods changed significantly from the beginning of the 19th century with the spread of the steam engine and advances in thermodynamics. The growing demand for pure water for steam boilers, together with the expansion of European colonialism into arid regions, created favorable conditions for the further development of desalination technologies.[16][26][27]
More efficient thermal systems, including multiple-effect evaporators, were introduced, allowing heat to be reused across several stages.[16] In 1852, Alphonse René le Mire de Normandy patented a vertical-tube seawater distillation unit that became widely used, particularly in maritime applications, because of its relatively simple design and ease of construction.[16]
By the second half of the 19th century, land-based desalination plants had begun to appear more frequently.[28] In the 1860s, the U.S. Army installed Normandy evaporators in Key West and Dry Tortugas.[16][28][29] In the 1880s, another plant was installed at Suakin to provide fresh water for British troops, using six-effect distillation.[16][28]
20th century
[edit]Desalination technology advanced significantly after World War II. Thermal methods such as multi-effect flash and multi-stage flash desalination were developed and widely implemented, particularly in regions with limited freshwater resources. Freeze–thaw desalination also emerged as an alternative approach based on crystallization processes.[30][31]
In 1955, the Office of Saline Water was established within the United States Department of the Interior to promote research and development in desalination.[5][32][33] These efforts contributed to the construction of distillation and electrodialysis plants and encouraged international cooperation in the field.[34]
One of the most important developments of the 20th century was reverse osmosis (RO), a membrane-based process in which pressure forces saline water through a semi-permeable membrane that retains dissolved salts. By the late 1960s and early 1970s, RO had emerged as a promising alternative to thermal desalination. Research was carried out at universities in California and by companies including Dow Chemical Company and DuPont.[35]
The first industrial desalination plant in the United States opened in Freeport, Texas, in 1961 after a prolonged regional drought.[5] The first commercial RO plant for brackish water was inaugurated in California in 1965.[36] Sidney Loeb and colleagues at the University of California, Los Angeles helped develop large pilot systems that demonstrated the feasibility of RO for municipal water supply.[37] In 1975, the first seawater reverse osmosis desalination plant came into operation.
21st century and modern developments
[edit]Since the early 21st century, desalination capacity has expanded rapidly, with thousands of plants operating worldwide, particularly in the Middle East.[38] Reverse osmosis has become the dominant desalination technology because of its scalability and energy performance.[38]
Research in the 21st century has focused increasingly on efficiency and sustainability. This has included the integration of renewable energy sources such as solar and wind power into desalination systems, as well as the development of advanced membranes including graphene-based, biomimetic, ceramic, and nanocomposite materials.[39][40]
As of 2021, about 22,000 desalination plants were reported to be in operation worldwide.[38]
Applications
[edit]| External audio | |
|---|---|
 "Making the Deserts Bloom: Harnessing nature to deliver us from drought", Distillations Podcast and transcript, Episode 239, March 19, 2019, Science History Institute |
A – steam in B – seawater in C – potable water out
D – brine out (waste) E – condensate out F – heat exchange G – condensation collection (desalinated water)
H – brine heater
The pressure vessel acts as a countercurrent heat exchanger. A vacuum pump lowers the pressure in the vessel to facilitate the evaporation of the heated seawater (brine) which enters the vessel from the right side (darker shades indicate lower temperature). The steam condenses on the pipes on top of the vessel in which the fresh sea water moves from the left to the right.
There are now about 21,000 desalination plants in operation around the globe. The biggest ones are in the United Arab Emirates, Saudi Arabia, and Israel. The world's largest desalination plant is located in Saudi Arabia (Ras Al-Khair Power and Desalination Plant) with a capacity of 1,401,000 cubic meters per day.[41]
Desalination is currently expensive compared to most alternative sources of water, and only a very small fraction of total human use is satisfied by desalination.[42] It is usually only economically practical for high-valued uses (such as household and industrial uses) in arid areas. However, there is growth in desalination for agricultural use for soil desalination. Many seagoing ships and submarines also use desalination. The most extensive use is in highly populated areas such as Singapore[43] or California.[44][45] and especially in the Persian Gulf.[46]
Energy cost in desalination processes varies considerably depending on water salinity, plant size and process type. At present the cost of seawater desalination, for example, is higher than traditional water sources, but it is expected that costs will continue to decrease with technology improvements that include, but are not limited to, improved efficiency,[47] reduction in plant footprint, improvements to plant operation and optimization, more effective feed pre-treatment, and lower cost energy sources.[48]
While noting costs are falling, and generally positive about the technology for affluent areas in proximity to oceans, a 2005 study argued, "Desalinated water may be a solution for some water-stress regions, but not for places that are poor, deep in the interior of a continent, or at high elevation. Unfortunately, that includes some of the places with the biggest water problems.", and, "Indeed, one needs to lift the water by 2000 m, or transport it over more than 1600 km to get transport costs equal to the desalination costs."[49]
Thus, it may be more economical to transport fresh water from somewhere else than to desalinate it. In places far from the sea, like New Delhi, or in high places, like Mexico City, transport costs could match desalination costs. Desalinated water is also expensive in places that are both somewhat far from the sea and somewhat high, such as Riyadh and Harare. By contrast in other locations transport costs are much less, such as Beijing, Bangkok,