In simple terms, reverse osmosis is the process of pushing water through a filter that traps the impurities ( bacteria, virus, organic & inorganic impurities and other contaminats ) on one side and allows the pure water to be obtained form the other side. More formally,it is the process of forcing a solvent (water) from a region of high solute concentration (impurities) through a Thin film composite (TFC) with a size pore of 0.0001 micron membrance to a region of low solute concentration by applying a pressure. The membrance here is semipermeable, meaning it allows the passage of solvent (water) but not of solute (impurities). The membranes used for reverse osmosis have no pores, the separation takes place in a dense polymer layer of only microscopic thikness. In most cases the membranes is designed to only allow water to pass through.