The ocean contributes about 97% of the total water on Earth. The sea water contains about 2.5% of salts and it has peculiar salty taste and is known as brackish water. This water is totally unfit for drinking.
The process of removing common salt from sea (brackish) water is called desalination.
Based on the quantity of dissolved solids, the water quality is graded as:
- Fresh water (<1000 ppm of dissolved solids);
- Brackish water (1000-35,000 ppm of dissolved solids);
- Sea water (>35,000 ppm of dissolved solids).
The common method used for the desalination of brackish water is reverse osmosis.
Reverse osmosis
When a semi-permeable membrane separates two solutions of different concentrations, the flow of solvent (water) takes place from a region of lower concentration (dilute solution) to higher concentration (concentrated solution). This process is called osmosis.
On applying hydrostatic pressure greater than osmotic pressure, the solvent flow can be reversed, the flow of solvent takes place from concentrated solution to diluted solution. This process is known as reverse osmosis.
In this method water is squeezed out of the waste, instead of taking the waste out of water.
Process
The equipment consists of two compartment separated by a semi-permeable membrane made up of polystyrene, polyvinyl chloride or ethyl cellulose etc. The salty water (sea water) is pumped into the top of the reverse osmosis cell as shown in the fig 1.
When high pressure is applied on the sea water, pure water passes through the membrane to the other compartment. The fresh water is thus separated from the saline water coming out of the outlet pipe as shown in figure.
Advantages and Disadvantages of Reverse osmosis Process
| Advantages | Disadvantages |
| Highly reliable method | In reverse osmosis, wastage of water is more. |
| Low capital and low operating cost | It requires periodic maintenance |
| Reverse osmosis removes ionic, non-ionic, bacteria, sugar, salts and other molecular constituents easily. | Pesticides and other volatile chemicals are not completely removed. |
| Life time of membrane is more. | |
| The membrane can be replaced with in a few minutes. | |
| The water obtained by this process is used for high pressure boilers. |
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