SEA WATER DESALINATION

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TOPICS: Drought prone area, water scarcity, salinity.

Key words- 

Feed solution: It is low concentrated solution used in Forward Osmosis for purification purposes to separate its dissolved matters. For example, sea water which is saline ( meant salt dissolved in water).

Draw solution : It is high concentrated solution used in osmosis to be diluted by the filtered water.

Osmotic pressure : It is a pressure difference between draw solution and feed solution which is used in Forward Osmosis. For example, Ammonium bicarbonate.

Hydraulic Pressure: It is an applied pressure on solution hydraulically which is used in Reverse Osmosis.


 
Problem: Ramanathapuram  district  in south east coast of Tamilnadu state of India is severely affected by scarcity of potable water due to salinity, brackishness and also poor ground water source availability. The district of 423000 hectares has a long coast line measuring about 265 km accounting for nearly 1/4 of the total length of the coastal line of the state.

WHAT IS FORWRD OSMOSIS?
Osmosis 

"Forward osmosis is also known as osmosis. Osmosis is a process in which low concentrated solution pass through a semi-permeable membrane to high concentrated solution". In this process water is separated from the dissolve solutes. 

SEA WATER DESALINATION THROUGH SOLAR THERMAL FORWARD OSMOSIS 

During the forward osmosis process, the membrane rejects the salts from the feed solution and makes it concentrated and draw solution is diluted with time. Pure water has to be recovered from the diluted draw solution by using an energy efficient such as solar thermal separation technique. Unlike R.O., F.O. utilises natural osmotic pressure to transport the water molecules and does not require hydraulic pressure to overcome the osmotic pressure of the feed solution.
In this process Forward osmosis is completed in two steps
  1. water molecule is permeated from feed to the draw solution across the semi-permeable membrane consequently the feed solution is concentrated and draw solution is diluted and it has  become diluted draw solution.
  2. In this step, the draw solution is recovered back from the diluted draw solution, that was used for water purification through thermal heating. Here ammonium bicarbonate is used as a  draw solution  which generated best osmotic pressure gradient, which is easily decomposed on heating up to 60⁰c into ammonia and carbon dioxide gases by solar thermal heating. The gases escape draw solution and give the pure hot water.

Forward osmosis system has been installed at Naripaiyur village, Ramnathapuram district, Tamilnadu that is based on solar thermal hot water system. The production of pure water / fresh water is about 20,000 litres per day which is sufficient to distribute 2litres water per day per person for 10,000 village community.

Features: The sea water forward  osmosis technology is operated at a near 2bar pressure unlike sea water R.O. that operates at 50 bar pressure. It versatile, high energy efficiency, low operation and maintenance costs compared to other technology. F.O. facilitates high recovery rate, low energy consumption, less fouling of membrane  because of low pressure operation, effective cleaning of membrane and long life.

Reference 

https://pib.gov.in/PressReleasePage.aspx?PRID=1728530   

https://www.slideshare.net/ccharisiadis/forward-osmosis-fundamentals-hybrids

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