Desalination

 

Purified sea water can be a valuable source of clean water in water scarce regions. Sea water contains salts such as chloride, sodium, sulphate, magnesium, calcium and potassium as major constituents.  Distillation, Electrodialysis and reverse osmosis are the methods used to purify sea water. Multi stage flash, multi effect distillation, reverse osmosis, electrodialysis and vapour compress are desalination techniques.

Distillation comprises the evaporation of water and separation from salt. Water is collected from condensation of desalinated vapour.  Salt can be filtered out using electro dialysis method. Reverse osmosis method is an efficient method which removes 95 to 99 percentage of salt from sea water.

Reverse Osmosis involves four methods. Pre-treatment removes suspended solids and adjust pH. Pressurisation raises the pressure of pre –treated water appropriate for the membrane. Separation is completed through permeable membrane by blocking soluble salts and producing desalinated water. Stabilisation involves the pH adjustment and degasification of the product water and ready for potable use. The product water contains less than 500 mg/l dissolved solids.

Brine solution has concentrated salts filtered out from the process.  It usually contains higher concentration salts and has adverse effects on the aquatic environment. The brine solution therefore needs to be treated before its discharge to the sea water.