The reverse osmosis process forces liquid at high pressure through the semipermeable membrane. The semipermeable membrane is made of materials such as PTFE or polyamide and has fine pores with diameters of only a few nanometers. Thanks to the high pressure, the fluid can overcome the natural osmotic pressure and penetrate the membrane. The membrane acts as a kind of filter and holds certain molecules or ions.
After penetrating the membrane, the liquid is "purified" and has a lower concentration of solutes. On the other side of the membrane, however, the concentration increases. The reverse osmosis and the "membrane filter" remove, for example, viruses, bacteria, salts, lime, toxins or heavy metals from the liquids.