An old idea with a long delay
The basic principle behind desalination plants is ancient: heat salt water, collect the steam, and let it condense as fresh water. The salt stays behind. Greek writers such as Aristotle noted that vapor rising from seawater leaves its salt behind, and sailors in the age of exploration experimented with stills to stretch fresh water on long voyages. Reports of such stills appear in accounts from the sixteenth century onward, though they were small and often unreliable.
What took centuries was scale and economy. Boiling seawater takes a great deal of heat, and until cheap fuel and efficient equipment were available, distillation was a last resort for ships and isolated settlements.
Ships lead the way
The nineteenth century brought a new need. Sailing ships carried barrels of water and could tolerate rationing, but steamships had boilers that consumed large quantities of fresh water. Boilers fed with salt water built up deposits that damaged them. Navies and shipping lines therefore installed distillers on board to make boiler feed water and, increasingly, drinking water. Thomas Jefferson had already written a report to Congress in 1791 on ways of obtaining fresh water from the sea, an indication of how seriously the question was taken even in the age of sail.
Ship distillers taught engineers practical lessons. Scale, which is a crust of minerals that forms on heated surfaces, reduces efficiency and had to be controlled. Materials that resisted corrosion, such as certain alloys, mattered. Some of these lessons later carried over to land.
Land plants for dry places
Shore-based plants were first built where fresh water was scarce and no alternative was available, such as arid islands and military or colonial outposts. Distilling plants operated at some Caribbean and Red Sea locations in the early twentieth century, though sources vary on exact dates. They were expensive to run and served small populations.
The real expansion came after the Second World War. Several developments combined:
- Growing populations in dry regions, especially around the Persian Gulf, where there were no major rivers.
- Cheap local fuel. The Gulf states had abundant oil and gas, and thermal plants consume a lot of energy. Revenue from petroleum refining and exports provided the capital.
- Better engineering. Researchers in the 1950s developed the multi-stage flash process, often associated with the British engineer Robert Silver, in which heated seawater is released into a series of chambers kept at lower and lower pressure, so that part of it “flashes” to steam in each stage. Reusing the heat from one stage in the next improved efficiency considerably compared with simple boiling.
Kuwait began desalinating seawater at scale in the early 1950s and became one of the first countries to depend heavily on it. Saudi Arabia, the United Arab Emirates and other neighbors followed, building ever larger plants, often combined with power stations. Using the waste heat from electricity generation to run distillation improved the economics, a pairing known as cogeneration. Plants in these places have used stainless steel and other resistant materials to cope with hot, salty, corrosive conditions.
Impact on water supplies
For arid countries, desalination changed what was possible. Cities such as Kuwait City and Dubai grew in places where local fresh water had always been extremely limited. In some of these countries, desalinated water is now the main source for municipal supplies, though exact shares vary and are best checked in current official data.
The phrase “water preservation” in the title needs careful reading. Desalination does not preserve existing water; it adds a new supply that does not depend on rainfall, which can reduce pressure on groundwater and rivers. That is a real benefit where aquifers are being drawn down. But it does not make water use costless, and it can encourage consumption to rise if the water seems unlimited.
Costs and problems
Thermal desalination is energy hungry. Even with heat recovery, large plants burn substantial amounts of fuel, which raises costs and emissions. Other concerns include:
- Brine discharge. The leftover concentrated salt water, often warmer than the sea, is usually returned to the coast and can affect local marine life, especially in enclosed bodies of water.
- Chemicals. Plants use anti-scaling and cleaning chemicals that must be managed.
- Dependency. A city that relies on a few large plants is vulnerable if one fails.
Some of these issues are under study, and the evidence on environmental impact varies by site.
Later direction
From the 1970s and 1980s onward, membrane systems started to take over new construction in many places because they use less energy than thermal plants. That shift is covered in the article on reverse osmosis. Thermal plants remain important in the Gulf, however, and a few use solar panels or other renewable sources for part of their energy needs. The story of large plants shows how a very old principle became practical only when industrial energy and engineering caught up with it.