Desalination—the removal of salt from seawater to make it drinkable —has long been a possible answer to the world's water shortages. Can technological advances bring it into the mainstream, or will it remain too expensive and energy-inefficient?by Fred Pearce / May 20, 2006 / Leave a comment
Published in May 2006 issue of Prospect Magazine
Is desalination of seawater the answer to the world’s water troubles? Some people think so—not least the public. While touring British radio studios to promote my recent book, When the Rivers Run Dry, I found that the question most frequently asked by callers to phone-in programmes was: “Why should we be short of water when we are surrounded by sea? Surely desalination is the answer?”
And it’s a good question, especially as the cost of desalination is falling fast. It is often claimed that the 21st century will witness the first water wars. But it could turn out instead to be the century when our water shortages are solved forever as we tap into the most abundant source of water on the planet—the oceans. Countries that now suffer desperate water shortages could soon be awash with the stuff. But like other technological holy grails, such as nuclear fusion, desalination may be destined to stay tantalisingly over the horizon.
There are two technologies for removing salt from seawater to make it drinkable. Distilling seawater by boiling it and collecting the water vapour is an age-old activity. Boiling removes most impurities from water, including salts, which are left behind as the steam is given off. Ancient Egyptians, Persians and Greeks knew this.
The Royal Navy built distillation stills into its warships. But today’s distillation technology was first developed by the US navy to provide water for its operations on remote Pacific islands during the second world war. Then, in the 1950s, large-scale distillation took off in the arid Gulf states, which have plenty of oil to provide the necessary energy.
In typical modern distillation systems, the salt water is heated by passing it through tubes inside a chamber containing waste steam from a power plant—a kind of radiator in reverse. The hot salty water then enters a depressurised chamber that reduces the temperature at which the water boils. It “flashes” to steam.
The second desalination technology, reverse osmosis, has grown in popularity since the 1970s. This is, in essence, a filtering system. Pumps blast water at high pressure through a membrane that catches the larger salt molecules and lets through the smaller molecules of clean water. The filters are only partially effective, however, and the water has to be pressurised and passed through filters several times before it is cleaned up.