A one million litre per day desalination plant mounted on a barge 'Sagar Shakti’ 40 km off the coast of Chennai has started functioning.
Announcing this Earth Sciences Minister Kapil Sibal said that the demonstration plant set up at a cost of Rs 22 crore has proved the success of the technology and now a higher capacity plant of 10 million litres per day would be set up with public private partnership.
He said the cost per litre of fresh water from the higher capacity plant is expected to be around 3 paise per litre compared to 6 paise per litre for reverse osmosis technology.
A totally indigenous plant it has been developed by the National Institute of Ocean Technology. The low temperature desalination plant works on the basic principle of utilizing the difference of temperature between water at sea-surface and at the depth of sea.
While the surface water is at 28-30 0C the deep sea water is cold at 9-12 0C. The sea surface water is put into low pressure chambers with pressure of 25 millibar (the normal atmospheric pressure is 1000 millibars) resulting in a vaccum causing water to boil and give off steam. The steam is passed into another chamber cooled by pipes through which cold water drawn from deep sea is passing which condenses the water vapour producing potable water which is collected. The total dissolved solids from 35,000 parts per million in sea water came down to 10 ppm in fresh water, 500 ppm of total dissolved solids is acceptable standard internationally for drinking water.
Mr Sibal informed that the first successful land based plant following this technology commissioned at Kavarati in Lakshadweep since May 2005 has so far produced more than 5 crore litres of fresh water. Water borne diseases had been reduced by a factor of 20. Eight such units are being set up on other islands of Lakshadweep, he added.
For the Chennai plant one the biggest challenge was to make the 600 m long pipe of 1 m diameter of High Density Polyethylene weighing around 100 tonnes to withdraw cold water from the ocean.
Heavy weights had to be attached to the pipe to place it vertically from the barge which itself is tied at 1000 metres - the deepest single point mooring in Asia,he said.
Water bags of special material have been designed which can hold and carry 2 lakhs litres of fresh water. Since fresh water is lighter than the sea water, these bags could be towed by small boats to shore,he added.
He said the technology could be used by Chennai’s power plants which used sea water for cooling purposes to provide 25 per cent of the city’s requirment of fresh water.