This is reversing the natural process of movement of liquids from a lower concentration level to a higher concentration. It is usually done through a semi –permeable membrane and is scientifically called Osmosis. The pressure difference that exists between the liquids is called osmotic pressure. In the water purification, this process is mechanically reversed by applying an external hydrostatic pressure beyond the osmotic pressure of the liquid with higher concentration. This results in a reverse process, and hence called reverse osmosis.
Reverse osmosis is a common method applied to treat household drinking water supplies. It is quite effective in removing naturally occurring contaminations like salinity, foul taste, smell and odour.