Reverse Osmosis (RO) is the most commonly used membrane technology and is the reverse of osmosis. It is mainly used to desalinate low salinity water, especially brackish water and waste water.
By definition, RO is the movement of water from an area of low to high water concentration across a semi-permeable permeable membrane by applying a pressure greater than osmotic pressure. The osmotic pressure is the pressure created between two solutions on either side of a semi-permeable membrane, where one has a higher concentration of dissolved salt than the other. If a pressure equal or less than the osmotic pressure is applied to the solutions, water moves from an area of low to high water concentration. The movement of water can only be reversed if the pressure applied is greater than the osmotic pressure – about 420 psi, depending upon the salinity of the feed water.
Semi-permeable membranes are not completely impervious to anything that is not water, but other substances will flow through the membrane at a much slower rate. The materials used in a membrane are usually cellulose acetate (CAB); cellulose triacetate; a blend of cellulose acetate (CAB) and cellulose triacetate; composite polyamide (CPA) or thin film composite (TFC) membranes. Composite polyamide membranes consist of a support layer made of polysulfone which separates water from dissolved ions and a semi-permeable membrane skin that is water porous. Similar to this are thin film composite membranes which also consist of two layers but the first layer is extremely thin surface layer with a diameter of 0.01 to 0.1 ?m. Another membrane material that is attracting a lot of interest is ceramic.
A typical reverse osmosis system consist of both a low and high pressure pump, membrane, pre and post treatment and an energy recovery device such as turbine, discussed in further detail below. The pressure exerted in the system depends largely on the salinity and quality of the source water. For brackish and fresh water a pressure range of 30 to 250 pounds per square inch (psi) is required compared to 600 to 1,000 psi for seawater, making seawater desalination usually more expensive.