Tapping the oceans

THERE are vast amounts of water on earth. Unfortunately, over 97% of it is too salty for human consumption and only a fraction of the remainder is easily accessible in rivers, lakes or groundwater. Climate change, droughts, growing population and increasing industrial demand are straining the available supplies of fresh water. More than 1 billion people live in areas where water is scarce, according to the United Nations, and that number could increase to 1.8 billion by 2025.

One time-tested but expensive way to produce drinking water is desalination: removing dissolved salts from sea and brackish water. Its appeal is obvious. The world’s oceans, in particular, present a virtually limitless and drought-proof supply of water. “If we could ever competitively—at a cheap rate—get fresh water from salt water,” observed President John Kennedy nearly 50 years ago, “that would be in the long-range interest of humanity, and would really dwarf any other scientific accomplishment.”

According to the latest figures from the International Desalination Association, there are now 13,080 desalination plants in operation around the world. Together they have the capacity to produce up to 55.6m cubic metres of drinkable water a day—a mere 0.5% of global water use. About half of the capacity is in the Middle East. Because desalination requires large amounts of energy and can cost several times as much as treating river or groundwater, its use in the past was largely confined to wealthy oil-rich nations, where energy is cheap and water is scarce.

But now things are changing. As more parts of the world face prolonged droughts or water shortages, desalination is on the rise. In California alone some 20 seawater-desalination plants have been proposed, including a $300m facility near San Diego. Several Australian cities are planning or constructing huge desalination plants, with the biggest, near Melbourne, expected to cost about $2.9 billion. Even London is building one. According to projections from Global Water Intelligence, a market-research firm, worldwide desalination capacity will nearly double between now and 2015.

Not everyone is happy about this. Some environmental groups are concerned about the energy the plants will use, and the greenhouse gases they will spew out. A large desalination plant can suck up enough electricity in one year to power more than 30,000 homes.

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