Reverse Osmosis

Reverse osmosis is a phenomena used to get pure water. It is based on the reversal of the phenomena of osmosis. When a semi-permeable membrane (permeable for water) is placed between salt water and fresh water, water will start moving in direction of salt water, trying to equalize the concentration on both sides. This a natural phenomena, called osmosis resulting in dilution of concentrated solution. This process continues until the chemical potential becomes equal on both sides. The chemical potential of water in pure form is higher than that on the salt water side at same pressure. As water moves towards salted water, the pressure increases, which increases the chemical potential on that side. After some time the pressure reaches at a level at which the chemical potential on both sides becomes equal and the osmosis stops. Now the system is at equilibrium.

At osmotic equilibrium, at the same level, the pressure becomes P1 in the left and reaches P2 in the right. At this condition the chemical potential becomes equal. The difference in pressure is called osmotic pressure, and in order to get pure water (left side) a pressure greater than P2 has to be applied on the right (salt) side.