Population growth along with limited fresh water sources has made good quality water a precious commodity today. Towards the alleviation of acute shortage of fresh water in the costal areas and Island territories of India, National Institute of Ocean Technology (NIOT), Chennai, an autonomous body under the Ministry of Earth Sciences has undertaken to establish low temperature thermal desalination plants to generate the fresh water from the sea surface water (28-30 ⁰C) and cold deep sea water (9-12 ⁰C). By flash evaporating the warm sea water under low pressure and condensing the water vapour with the cold sea water potable water   is produced. This was revealed by Shri Kapil Sibal, Union Minister for Science and Technology & Earth Sciences at a press conference in New Delhi today.

After successful trials in the laboratory and an experiment on a moored barge off Tuticorin in 400 m water depth, a land based plant was commissioned at Kavarati in Lakshadweep which is continuously generating good quality water since May 2005. So far more than 5 crore litres of water has been produced. The availability of high  quality fresh water has drastically reduced dysentery  and other such diseases. Buoyed with this success, eight more such units are being set up on other islands of Lakshadweep.  Towards achieving large capacity plants suitable for mainland requirements an experimental floating desalination plant of one million litre per day capacity working on low temperature thermaldesalination plant has been commissioned for the first time ever in the world at a location 40 km offshore Chennai. 

The most complex part of the process is the drawal of cold water from the ocean which requires a long pipe of 1 m diameter made up of HDPE. HDPE pipes of such diameter are manufactured in the length of 12 m and these have to be fused together to make a 600 m long pipe weighing 100 tonnes.  Since the density of the pipe is almost equal to saline water, it floats in the water and hence, heavy weight has to be attached at one end of the pipe to make it straight to reach the depth of 600 m where cold temperature of 9-12 ⁰C is available.  The deployment and connection of this pipe to the barge is a very challenging work,

especially with limited offshore facility available in the country. The 600 m pipe  was assembled at Ennore port, then towed to the site and connected vertically below the barge ‘Sagar Shakti’. The barge is moored to a single point mooring in deep waters. 

Having generated that quantity of water, the next difficult task is its storage and transportation from 40 km offshore point to the shore. For this purpose, 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, it floats and very little power is required to tow it to the shore.

Achievements of the Project

§         Single point mooring of 1000 m which is of its first kind in the world.

§         For the first time ever a long pipe was suspended vertically over the barge in deep waters and cold water of temperature as low as 9⁰C was pumped up.

§         Surface sea water has been flash evaporated in low pressure chamber of 25 millibar (the normal atmospheric pressure is 1000 millibars).

§         Water of TDS less than 10 ppm has been generated.

§         Institute is now planning to scale up the capacity of 10 million litres per day where the cost of water is expected to be below 4 paise per litre. In this endeavour possibility of collaboration with private industries will also be explored.

^§          The technology developed and demonstrated successfully can readily be adapted for shore based power plants using sea water for cooling. The sea water used for cooling purposes by thermal power plants attains temperature of 42-44⁰C and can be used as warm water for evaporation in the flash chamber of desalination plant. Sea surface water at 28-30⁰C can be used for cooling the vapour in the condensation plant. It is estimated that the power plants in Chennai can be configured to provide over 25 per cent of the city’s requirement of fresh water in this fashion.  

   PRA:NC