In a system that consists of water separated by a semi permeable membrane, with salt added to one side of the system, water will start flowing through the membrane from the pure water side into the salt solution. Flow will continue until the pressure is equal on both sides of the membrane.  The water level will now be higher on the side containing the salt solution. The difference in water level, caused by the addition of a specific amount of salt is called the osmotic pressure.  The osmotic pressure of sea water is around 26 bar (260 metres head of water).

REVERSE OSMOSIS EXPLAINED :

To desalinate water we must first create a flow through the membrane that causes the water to leave the salty side of the membrane and flows into the unsalted side. To achieve this, pressure must be exerted upon the water column on the salt side of the membrane to overcome the natural osmotic pressure and secondly, to exert additional pressure on the water column. This in turn pushed the water through the membrane. For the desalination of sea water, the pressure must be approximately 50-60 bars.

There are several different techniques that can be applied for water desalination including Reverse Osmosis, Electro Dialysis, Distillation and Ion Exchange.

Reverse Osmosis is the most economic process for the desalination of brackish water and sea water. When we compare this process to the traditional thermic process of distillation, the capital investments and the energy used are much lower.

DESALINATION :

A desalination installation consists of a pretreatment system, a desalination unit and post treatment to partially re-mineralise the water if necessary. A high pressure pump increases the feed water pressure to the required level.  The working pressure is considerably higher that the osmotic pressure in order to produce a suitable flow through the membrane. A typical desalination installation includes flow and pressure monitoring for process control.  There are also a number of other operational features for the safe and efficient operation of the plant which include low & high pressure switching and conductivity measurement to monitor the quality of permeate produced.