Reverse Osmosis (RO) membranes are designed to remove dissolved salts from water. The water passes readily through the Reverse Osmosis (RO) membranes, dissolved salt passes through very slowly. Under natural conditions of osmosis, water will diffuse through a semipermeable membrane toward a region of higher salt concentrations to equalise solution strength on both sides of the membrane. To overcome and reverse this osmotic tendency, the pressure is applied to the feedwater, thereby producing a purified permeate stream.
In the normal osmosis process, the solvent naturally moves from an area of low solute concentration (high water potential), through a membrane, to an area of high solute concentration (low water potential). The driving force for the movement of the solvent is the reduction in the free energy of the system when the difference in solvent concentration on either side of a membrane is reduced, generating osmotic pressure due to the solvent moving into the more concentrated solution. Applying an external pressure to reverse the natural flow of pure solvent, thus, is reverse osmosis.
The process is like other membrane technology applications. However, key differences are found between reverse osmosis and filtration. The predominant removal mechanism in membrane filtration is straining, or size exclusion, so the process can theoretically achieve perfect efficiency regardless of parameters such as the solution’s pressure and concentration. Reverse osmosis also involves diffusion, making the process dependent on pressure, flow rate, and other conditions. Reverse osmosis is most commonly known for its use in drinking water purification from seawater, removing the salt and other effluent materials from the water molecules.
The dissolved salts are disposed of through the reject portion of the membrane, and referred to as concentrate, as this is the cycled-up salts removed by the process and discharged to waste.
