The world faces a growing water shortage. In some areas of low rainfall, such as the Middle East, the natural water supply is inadequate. One answer is desalination – removing the salt from sea water.
One method of desalination has been known since the 4th century BC, when the Greek philosopher Aristotle noted that when salty water is boiled the steam that rises leaves the salt behind. Condensed into water again, it is pure.
The simplest desalination plant is a still in which water is boiled and the steam condensed. A simple solar still can be made with a glass done over a pool of salt water. The water is heated by the sun, vaporizes, and then condenses on the glass and runs down it to gather in channels around the edge. A still one yard square in area should produce a gallon (4-5 litres) of fresh water a day.
To produce really useful amounts of water a much larger still is needed. The water is first heated to above its atmospheric boiling point, but in a vessel that is under flows into a separate chamber at a lower pressure, where some of it instantly ‘flashes’ into vapour. The vapour is then condensed by contact with tubes carrying the incoming supply of cool sea water. The hot salt water that did not boil in the first chamber moves on to a second, at slightly lower pressure again, where more of it flashes into vapour and is condensed.
A more modern desalination system, called reverse osmosis, is more cost-effective than fish distillation. It uses membranes made of plastic which have tiny holes in them big enough for water molecules to pass through, but too small for salt molecules. The membranes are formed intro a tube and salt water is pumped into them under pressure. Pure water drips from the outside of the tube.
One of the world’s largest reverse osmosis plants has been built in Bahrain. The plant produces over 12 million gallons (54 million litres) of freshwater every day.
As more arid areas are opened up for habitation and cultivation, the demand for desalination increases. By 1988 there were more than 2200 plants operating. But the cost of the water is high, which means it can only be justified for drinking, industry, or high-value crops.
Picture Credit : Google
