Principle of reverse osmosis (RO) :

• The reversal of solvent flow, from higher concentration solution to lower concentration solution through a semipermeable membrane, by applying an external pressure slightly higher than the osmotic pressure of higher-concentration solution, is known as reverse osmosis.
• Normal osmosis process, is shown in Fig. 1, where the solvent flows from low concentration solution to higher concentration solution, through the semipermeable membrane, until difference in water levels creates a sufficient pressure to counteract the original flow. The difference in levels represents osmotic pressure of the solution.
• In the reverse osmosis, we apply external pressure on the higher concentration solution slightly higher than its osmotic pressure.
• The flow of solvent takes place in reverse direction i.e. from higher concentration solution to lower concentration solution, through the SPM. Thus in RO, we separate water from its contaminants rather than contaminants from water.

Fig. 1 Reverse osmosis proces

Method :

• Sea water or water polluted by ionic pollutants, is filled in reverse osmosis cell.
• A pressure of 200 - 800 psi is applied on it to force the solvent to pass through SPM. (SPM has such porosity that it allows only H2O molecules to pass through and higher sized ions / molecules are prohibited from passing).
• Membrane consists of a polymeric material film made of proper porosity, from materials like acrylics, polyamides, aramids etc.