Reverse osmosis is a water purification technology which uses a semipermeable membrane to eliminate ions, molecules and larger particles from drinking water. In reverse osmosis, an applied pressure is used to overcome osmotic pressure, which is a colligative property that is driven by chemical potential differences of the solvent, a thermodynamic parameter. Reverse osmosis can remove many types of suspended species from water, including bacteria, and is used both in industrial processes and in the production of potable water. The result is that the solute is retained on the pressurized side of the membrane and the pure solvent is allowed to pass to the other side. In order to be “selective”, this membrane should not allow large ions or molecules through the pores (holes), but should allow smaller components of the solution (such as solvent molecules) to pass freely.
In the usual osmosis process, the solvent naturally moves from an area of low solute concentration (high water potential) to an area of high solute concentration (low water potential) through a membrane. The force that triggers 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 as a result of the solvent moving into the more concentrated solution. Reverse osmosis is, therefore, applying an external pressure to reverse the natural flow of the solvent. The process is similar to other membrane technology applications. However, there are key differences between reverse osmosis and filtration. The predominant removal mechanism in membrane filtration is size exclusion (straining), which is why the process can theoretically achieve perfect efficiency regardless of the pressure and the concentration of the solution. Reverse osmosis involves diffusion, making the process dependent on pressure, flow rate and other conditions.
If pressure is applied on the area of the concentrated solution, its flow in the membrane will be reduced as a consequence, but if said pressure is increased, the water flow will gradually decrease. This process is called osmotic pressure. If more pressure than expected is applied by the osmotic pressure the water will become an even less concentrated solution. Hence it will manage to cross the membrane for being more diluted.
Reverse osmosis is generally used to purify drinking water from seawater, removing the salt and other effluent materials from the water molecules.