FOR almost half a century, making drinking water from the large-scale desalination of seawater has been the preserve of a few desert states with pots of cash and precious little fresh water. But that鈥檚 changing fast. In recent weeks, two cities with temperate climates 鈥� London and the Chinese city of Tianjin 鈥� have announced plans to build large desalination works.
And desalination is gaining popularity in dozens of other cooler countries with shrinking rivers and dwindling water reserves. Could the 21st century, often billed as the era in which wars will be fought over water, instead see our water woes solved?
Distilling seawater for drinking is an age-old activity. But large scale desalination 鈥� by heating seawater and collecting the water vapour 鈥� took off in the oil-rich Gulf states in the 1950s where the necessary energy is plentiful. The main alternative technology is reverse osmosis (RO), developed in the 1970s, which forces water through a membrane that filters out the salt.
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Today, global desalination capacity is approaching 30 million cubic metres a day 鈥� 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.
Most of this capacity is still in the Gulf states, and 80 per cent still uses evaporation, according to desalination industry consultant Klaus Wangnick of Gnarrenburg in Germany. Saudi Arabia alone accounts for one-tenth of world output. In Mediterranean Europe, desalination by reverse osmosis also plays a significant role. Malta, for example, gets two-thirds of its drinking water this way.
Both technologies require large amounts of energy to evaporate the salty water or force it under pressure through the filters. Until recently, it cost several dollars to produce a cubic metre of unsalty water. But costs for reverse osmosis have come down in the past five years as tough composite filter membranes have replaced flimsier nylon and cellulose acetate ones. The new membranes remove up to 75 per cent of the salt in a single pass, and also last longer. To remove the salt, water is filtered several times.
At Tampa Bay in Florida, a reverse osmosis desalination plant ekes out underground reserves during drought years. Santa Cruz in California is building a similar plant. Plans are also being discussed in Houston in Texas, Cape Town in South Africa, and Perth in Australia. And in Spain the newly elected government has abandoned its predecessor鈥檚 plans to pump water from the river Ebro in the north to the arid south. Instead it is building 20 RO plants in the south-east of the country. The plants are expected to meet slightly over 1 per cent of Spain鈥檚 total water demand.
The cheapest desalinated seawater is now in Israel, where one of the world鈥檚 largest RO plants delivers water at around 50 US cents per cubic metre. This compares with the 30 cents per cubic metre it costs to pump fresh water from the Sea of Galilee to Tel Aviv. Around 10 per cent of Israel鈥檚 water is desalinated.
These prices are encouraging cities in temperate lands to join in the RO revolution. Last month China announced plans for a giant RO plant for Tianjin, the country鈥檚 third-largest city. It will produce 100,000 cubic metres of fresh water a day. The water deficit in the area around Tianjin now exceeds 100 million cubic metres a day, says Olivia Lum of Hyflux, the Singapore manufacturer that will supply the plant.
And the UK鈥檚 Thames Water has said it will be building a 拢200 million RO plant on the river Thames in east London that will produce up to 150,000 cubic metres a day, enough to meet the domestic needs of almost a million people. The plant, which will take estuary water that is only a quarter as salty as the sea, will be an emergency back-up in case of drought, rather than a regular source of supply.
Wangnick warns that the boom in desalination could have an impact on the environment. The salt from the desalinated water emerges in a concentrated brine, which most plants dump back into the sea. This salty waste water also contains the products of corrosion, and chemical additives designed to prevent corrosion and the build-up of scale in the plants.
And the energy demands of desalination remain a big problem too. A typical RO plant consumes 6 kilowatt-hours of electricity for every cubic metre of water it produces. There are now some pilot projects harnessing nuclear power plants to desalinate seawater, the latest being in Chennai, India.
Today around 1 per cent of the world鈥檚 drinking water comes from desalination. Governments worldwide are looking at increasing that proportion substantially. If these plants prove successful, they could help alleviate water shortages around the world. But without careful management of waste and use of renewable fuels to power the plants, the very technology that could save us from water wars might contribute to climate change. And that could put the remaining natural supplies of fresh water under even greater pressure.
