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
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.
Both technologies require large amounts of energy. Until recently, it cost several dollars to produce a cubic metre (1,000 litres) of unsalty water—about 100 times the cost of conventional water supplies. But better filters are cutting the cost of producing acceptable drinking water. And as shortages push up the price of delivering conventional supplies of fresh water, desalination has become increasingly attractive.
Today, global desalination capacity is approaching 10 cubic kilometres a year—roughly 3 per cent of the global domestic tap water supply. Two thirds of this is devoted to processing seawater and the rest to cleaning up brackish underground water. But water for homes is a minor demand in most countries (Britain is an exception here). Two thirds of the world’s water is used for irrigating crops. So desalinated water accounts for only a tenth of 1 per cent of total water use.
Four fifths of the world’s desalination capacity still comprises distillation works, and most of it is in the Gulf. The Saudi capital, Riyadh, which has virtually no rain and no rivers or surface lakes, alone accounts for one tenth of world output of desalinated water. In 2004, the Saudis announced plans for six more plants, costing $5bn in total.
But desalination technology is spreading fast to countries with shrinking rivers and soaring demand. Holiday islands where tourists have overwhelmed local supplies have been in the forefront. Malta now gets two thirds of its drinking water from desalination. Greek islands like Mykonos have been desalinating for years, as have the Caymans, Antigua and the Virgin Islands in the Caribbean. And Cyprus has so overpumped its underground freshwater supplies that seawater has rushed into the vacated pores in the rocks—so now the country has to desalinate its underground water too.
In the past five years, continental cities in arid zones have also been embracing the technology. Tampa Bay in Florida and Santa Cruz in California have both taken the plunge, and more reverse osmosis plants are planned for Houston and Cape Town. In Perth drought has cut run-off into the dams that supply the city by two thirds since the 1970s; so to keep the taps running, the city is building a $278m desalination plant.
In Spain, the Zapatero government, elected in 2004, abandoned its predecessor’s plans to relieve parched fields and empty swimming pools in the south by pumping water from the wetter north. They decided instead to build 20 reverse osmosis plants along the Costas, which are expected to meet slightly over 1 per cent of Spain’s total water needs.
The falling costs of desalination The cheapest desalinated seawater is found in Israel, where the world’s largest reverse osmosis plant has been built on the Mediterranean coast at Ashkelon. It produces 270,000 cubic metres of water a day. Israeli water economics are notoriously opaque but the government claims to be able to deliver water at around 50 US cents per cubic metre. This is around a third of the production cost in Saudi Arabia, and a sixth of the typical cost of desalination 20 years ago.
More pertinently for Israelis, it bears comparison with the 30 cents it costs to pump fresh water from the sea of Galilee to coastal cities such as Tel Aviv, and the $2 to buy and ship water from Turkey. A national plan agreed in mid-2004 will raise production of desalinated seawater to meet half the country’s current water demand by the end of the decade.
This dramatic fall in cost is encouraging cities in less extreme circumstances, and in cooler and wetter climates, to join the reverse osmosis revolution. In 2004, China announced plans for a giant desalination plant for Tianjin, the country’s third largest city, whose 10m inhabitants face endemic water shortages as water tables plunge and the Yellow river dries up. It will produce 100,000 cubic metres of desalinated seawater a day—a large output, but only a fraction of the city’s water needs.
More surprisingly, in 2004 Thames Water announced that it wanted to build a £200m reverse osmosis plant, taking water from the Thames estuary in east London. The mix of seawater and freshwater in the estuary means that it will be less salty, and so cheaper to process, than water from the North sea.The plant would be kept in reserve for droughts, when it would be able to meet the domestic needs of almost 1m Londoners. So far the scheme has been vetoed by London planning authorities, and the Environment Agency thinks there are better ways of securing the capital’s water supplies, such as plugging leaks in water mains. But Londoners may eventually be drinking desalinated seawater.
The boom in desalination is beginning to alarm environmentalists. One problem is what to do with the briny waste, which can amount to half or more of the total volume of water processed. Most plants, naturally enough, dump it back into the sea. But this salty waste water also contains the products of corrosion during the desalination process, as well as chemicals added to reduce both the corrosion and the build-up of scale in the plants.
Of equal concern is the huge energy demand of desalination. A typical modern reverse osmosis plant consumes six kilowatt-hours of electricity for every cubic metre of water it produces. Most of the power, inevitably, comes from burning coal, oil and other fossil fuels. So while desalination could conceivably become a viable source of drinking water in coastal regions, it would be at the expense of extra carbon emissions.
Desalinated water also often costs more than normal water to distribute. It is produced, unavoidably, at sea level. Although much of the world’s population lives in coastal regions, water will still need pumping further up, which costs money and energy. Most water supply systems are designed to catch water as high as possible and deliver it using gravity. That is not an option for desalinated seawater.
While distillation technology is fairly mature, with little potential for further big advances, reverse osmosis technology still offers the prospect of real breakthroughs that could transform the disadvantages of both price and pollution. Early nylon and cellulose acetate membranes easily became clogged. This reduced their efficiency and required expensive pre-treatment of the water with chemicals that remained in the plant effluent. But a new generation of tough-but-thin composite membranes made from polyamide films can remove up to three quarters of the salt in a single pass, as well as lasting much longer than the old ones. The water still has to pass through the filter several times, but the goal of a single pass may finally be within reach.
Another desirable innovation would be to find ways of capturing and recycling the energy used in the process. One idea is to install turbines in the chamber where the pressurised water bursts through the filter. Those turbines, turned by the turbulent water as it depressurises, would help power the pressurisation of the next body of water to pass through the filter.
Where does this leave us? It is hard to see desalination penetrating the agricultural market, where most of the world’s water is used. The costs are at least an order of magnitude too high. For the foreseeable future it will be cheaper to import food than to go to the trouble of desalinating seawater for irrigating crops.
One testimony to the futility of trying to irrigate with desalinated water sits outside Yuma in Arizona, on the Colorado river just upstream of the border with Mexico. A giant desalination plant was built by the US government two decades ago at a cost of $280m to help fulfil its treaty obligation to provide usable water to Mexico. The idea was to desalinate salty drainage water from local irrigated fields and transport it down the dried-up bed of the Colorado river. But it turned out to be cheaper to abandon those fields and allow some water to remain in the river. The Yuma plant has never been used.
Many regard desalination as a prohibitively expensive high-tech solution to a global water problem that is mainly caused by wasteful use. It is a supply-side solution to a demand-side problem. But for many coastal regions—and even some inland areas where the circumstances are exceptional, such as the West Bank—it may become the technology of choice for both domestic and industrial use. If California can no longer rely on cheap water from the Colorado river, and as rivers run dry around the Mediterranean, and even perhaps as droughts become endemic in southern England—desalination could provide one way of keeping the water flowing.
Water conflict in the middle east
In his memoirs, Ariel Sharon said the six day war of 1967 was fought less for land and more for control of the Jordan river, which now largely lies within Israeli-controlled territory and supplies much of the country with water. And one of the main daily grievances of Palestinians on the West Bank is that Israeli authorities prevent them from sinking new wells and boreholes into the water-bearing rocks beneath their feet. So could desalination of seawater be a key to hydrological peace in the region?
The Israeli government thinks so. It has drawn up a plan for a giant desalination plant on the Mediterranean coast that would distribute water to a network of pipes across the West Bank. It wants the US, as part of any final peace deal for the region, to pay for the plant and pipes, which could, it says, supply an area from Jenin in the north of the territory to Ramallah in the south, including many of the 250 villages that currently rely on ancient local springs and small wells. But even before the election of the new Hamas government, Palestinian negotiators were uneasy about the plans, believing they would leave Palestinians dependent on expensive water desalinated with western aid, while Israelis took the water from beneath the West Bank.