Reverse osmosis is the process of forcing a solvent from a region of high solute concentration through a membrane to a region of low solute concentration by applying a pressure in excess of the osmotic pressure. This 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 semipermeable, meaning it allows the passage of solvent but not of solute.

the pretreatment process for RO Plants may consist of all or some of the following treatment steps:

• Clarification followed by Sand Filtration for Turbidity removal
• Water disinfection with chlorine
• Hardness reduction by Softening
• Addition of scale inhibitor
• Reduction of free chlorine using sodium bisulfite/ Activated carbon filters
• Final removal of suspended particles using cartridge filters