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Dash of salt

india Updated: May 07, 2007 05:16 IST
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It’s one of the greatest ironies of nature: three-quarters of Earth’s surface is covered by water, yet little of it is drinkable. Water that contains more than 500 parts per million (ppm) of dissolved minerals is not potable. Many brackish lakes and water bodies have more than 5,000 ppm of minerals. But this is nothing compared to the 35,000 ppm of minerals in seawater, which makes up most of the water on the planet.

Making seawater drinkable has always been a challenge for scientists. Before World War II, heat from a ship’s engines was used to boil seawater and the steam was condensed into fresh water. In water-starved West Asia, where the sea is never far away, Israel built its first seawater distillation plant in 1965, and Arab countries followed suit. Those early plants used ‘flash evaporation’, in which seawater is heated and pumped into a low-pressure tank. The decrease in air pressure brings down the boiling point of water and it vaporises — or ‘flashes’ — into steam, which is condensed into pure water.

You could also make fresh water by freezing, instead of boiling, seawater: it’s all a matter of degrees! Seawater has a freezing point lower than fresh water, so if you lower its temperature below the freezing point of fresh water (but above that of seawater), fresh water crystals are produced. Wash these crystals free of salt, melt them, and you have water purer than that from any city tap! In electro-dialysis, salt is dissolved in water so that it splits into positive and negative ions. An electrical current passed through the salt-water draws out these ions, leaving pure water behind.

The most popular technology, however, is reverse osmosis. Water is forced under pressure to leave the salty side of a semi-permeable membrane (that only allows pure water molecules to pass) and flow into the ‘unsalted’ side. Salt, along with dirt, bacteria, and other contaminants, is left behind. India recently developed its own cheap method of getting fresh water from the sea. On a barge floating off the Chennai coast, cool deep-sea water from 600 metres below the surface helps condense warm surface seawater. It costs just six paise per litre! The downside of desalination is its environmental cost. Filtering two litres of salt water, for instance, not only produces one litre of drinking water, but also an outflow of extra-salty brine, which is dumped back into the ocean. This kills small sea creatures. Still, with water shortages looming, desalination remains the best bet for slaking thirst.