Reverse Osmosis/Nanofiltration

Osmosis is the natural flow of water through a semipermeable membrane from a low solute concentration solution into a solution with a higher solute concentration, in order to equate the concentrations. 

Reverse osmosis (RO) is achieved by applying pressure greater than the osmotic pressure to the concentrated solution in order to induce a flow towards the dilute solution. 

The RO module utilizes this principle of reverse osmosis with its separation membranes. The NF membrane is a variant of the RO membrane. Univalent ions such as Na+ and Cl- are selectively filtered, but multivalent ions such as SO42- and polyvalent substances such as pigments are blocked.