This is a natural and fundamental process in which a liquid is flown through a semi permeable membrane, which blocks the transfusion of salts/solutes through this membrane.
Reverse Osmosis :
“Reverse Osmosis (R.O.) is the most economical method of removing 95% to 99% of all contaminants. The pore structure of R.O. Membranes is much tighter than that of UF membranes. RO membranes are capable of rejecting practically all particles, bacteria and organics >200 Dalton molecular weight (including pyrogens) at a rate close to 99%. Natural osmosis occurs when solutions with two different concentrations are separated by a semi-permeable membrane. Osmotic pressure drives water through the membrane; the water dilutes the more concentrated solution, and the end result is equilibrium.
In water purification systems, hydraulic pressure is applied to the concentrated solution to counteract the osmotic pressure. Pure water is driven from the concentrated solution at a flow rate proportional to applied pressure and colleted downstream of the membrane.
RO also involves an ionic exclusion process. Only solvent (i.e. water molecules) is allowed to pass through the semi-permeable RO membrane, while virtually all ions and dissolved molecules are retained (including salts and organic molecules such as sugars). The semi-permeable membrane rejects salts (ions) by a charge phenomenon action: the greater the charge, the greater the rejection. Therefore, the membrane rejects nearly all (>99%) strongly ionized polyvalent ions but only 95% of the weakly ionized monovalent ions like sodium. Salt rejection increases significantly with applied pressure up to 5 bar.