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. (see Figure 1.)
This phenomenon is called “osmosis.”
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. (see Figure 2)
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.