Desalination of Brackish Water

DESALINATION OF BRACKISH WATER

The process of removing common salt (sodium chloride) from the water is known as desalination. The water containing dissolved salts with a peculiar salty (or) brackish taste is called brackish water.

Depending upon the quantity of dissolved solids, water is graded as

1. Fresh water - Contains < 1000 ppm of dissolved solids.

2. Brackish water - Contains > 1000 but < 35,000 ppm of dissolved solids.

3. Sea water - Contains > 35,000 ppm of dissolved solids.

Sea water and brackish water can be made available as drinking water through desalination process. Desalination is carried out by reverse osmosis.

1. Reverse Osmosis (RO) Com

When two solutions of different concentrations are separated by a semi-permeable membrane, solvent (water) flows from a region of lower concentration to higher concentration. This process is called osmosis. The driving force in this phenomenon is called osmotic pressure.

If a hydrostatic pressure in excess of osmotic pressure is applied on the higher concentration side, the solvent flow is reversed i.e., solvent flows from higher concentration to lower concentration. This process is called reverse osmosis (Fig. 1.3) Thus, in the process of reverse osmosis pure water is separated from salt water. This process is also known as super-filtration. The membranes used are cellulose acetate, cellulose butyrate, etc.

Advantages

(i) The life time of the membrane is high, and it can be replaced within few minutes.

(ii) It removes ionic as well as non-ionic, colloidal impurities.

(iii) Due to low capital cost, simplicity, low operating, this process is used for converting sea water into drinking water.