Fresh Water from the Sea
A About 97 percent of the water on the Earth is salty, and most of the fresh water that remains is locked in ice or deep underground. For people who live in dry coastal regions, the sea is therefore the largest water supply that is within reach. The idea of turning sea water into drinking water is old. Sailors boiled sea water on ships and collected the steam, and the same principle, called distillation, was used on a larger scale in the twentieth century, when towns in desert countries began to build plants beside the sea.
B The first large plants were thermal. They heated sea water, collected the steam and cooled it to make fresh water, leaving the salt behind. The method works with almost any sea water, but it needs a great deal of heat, and heat is expensive. For this reason thermal plants were built mainly in countries with cheap fuel, and in many of them they were combined with power stations so that the waste heat from the electricity could be used for making water.
C In recent decades a second method has taken the lead. In reverse osmosis a pump pushes sea water at high pressure against a thin membrane. Water passes through the membrane, but most of the salt cannot. Because the water is not heated, the process uses much less energy than distillation. Improvements in the membranes and in the devices that recover energy from the pressurised waste stream have lowered the cost of fresh water by a large amount since the 1990s, and reverse osmosis now supplies most of the new capacity in the world.
D Cheaper desalination does not mean cheap. A plant is expensive to build, and its running costs depend on the price of electricity. Water from the sea usually costs more than water from rivers, wells or reservoirs, so it tends to be chosen only where these sources are scarce or are shrinking. Some cities have built plants as a form of insurance, which they run at a low level in normal years and expand during a drought. Critics say that this money might be better spent on repairing leaking pipes, which waste a large share of the water in many cities.
E There are also concerns about the environment. A plant takes in sea water and returns a stream of very salty water, called brine, to the sea. If the brine is released in one place without care, it can sink to the sea bed and harm animals that cannot tolerate high salt levels. Engineers can reduce the harm by mixing the brine with other water and by spreading it through long pipes with many small outlets. Some researchers are also studying ways to extract useful minerals from the brine, so that it becomes a resource rather than a waste.
F The energy used by desalination is the other question. If the electricity comes from fossil fuels, making fresh water adds to greenhouse gas emissions. A growing number of plants therefore use solar or wind power for part of their needs. Many experts expect that desalination will continue to grow, but they argue that it should be one part of a wider plan that also includes saving water, reusing waste water and protecting the sources that already exist.