Nanofiltration, in concept and operation, is much the same as reverse osmosis. The key difference is the degree of removal of monovalent ions such as chlorides. Reverse osmosis removes the monovalent ions at 98-99% level at 200 psi. Nanofiltration membranes’ removal of monovalent ions varies between 50% to 90% depending on the material and manufacture of the membrane. For this reason, there is a variety of Nanofiltration membranes available. Each type is particularly suited to a certain application and may not be acceptable to a different application. Nanofiltration membranes and systems are used for water softening, food and pharmaceutical applications.

Nanofiltration is a relatively recent membrane process used most often with low total dissolved solids water such as surface water and fresh groundwater, with the purpose of softening (polyvalent cation removal) and removal of disinfection by-product precursors such as natural organic matter and synthetic organic matter.

Nanofiltration is also becoming more widely used in food processing applications such as dairy for simultaneous concentration and partial (monovalent ion) demineralisation.

While nanofiltration is used for the removal of other substances from a water source, it is also commonly used for the desalination of water. As seen in a recent study in South Africa, tests were run using polymeric nanofiltration in conjunction with reverse osmosis to treat brackish groundwater. These tests produced potable water, but reverse osmosis removed a large majority of solutes. This left the water void of any essential nutrients (calcium, magnesium ions, etc.), placing the nutrient levels below that of the required World Health Organization standards. This means nutrients should be added back to bring te water back to the standards levels for drinking water. Nanofiltration methods remove fewr solutes, so may avoid the need to remineralize the water.