Reverse Osmosis (RO) was developed in the late 1950’s under U.S. Government funding, as a method of desalinating seawater. Reverse osmosis (RO) is a separation process that uses pressure to force a solvent (water) through a membrane, which retains the solute (contaminant) on one side and allows the pure solvent (water) to pass to the other side. More formally, it is the process of forcing a solvent from a region of high solute concentration through a membrane to a region of low solute concentrate by applying pressure in excess of osmotic pressure. This process is the reverse of the normal osmosis process, which is the natural movement of solvent from an area of low solute concentration, through a membrane, to an area of high solute concentration when no external pressure is applied. The membrane here is semi-permeable, meaning it allows the passage of solvent (water) but not of solute (contaminant).