Nanofiltration is a membrane separation process whose cut-off lies between that of reverse osmosis and ultrafiltration. The commonly used operating pressure is in the range of 3 – 40Bar. Cut-offs are specified as being between 180 and 2000 Dalton.

The selectivity of a nanofiltration membrane is determined by two different parameters. Firstly, retention depends on the compound size i.e. from molecular weight. As far as nanofiltration membranes are concerned, retention and permeability are also a function of electric charge and the valence of the salts and compounds in the solution. Diluted solutions of monovalent ions can mostly pass through a nanofiltration membrane unhindered while most of the multivalent ions (such as sulphate and carbonate) are retained. As for anions, retention by a nanofiltration membrane increases by order.

Similar to reverse osmosis membranes, nanofiltration membranes are composite membranes consisting of a carrier structure as well as a thin selective layer. 

Membranes with good chemical resistance are now available for the treatment of acids, bases and oxidative media, therefor making the process suitable for several separation operations:

• Softening of process, product or drinking water.
• Separation of organic impurities in water treatment.
• Filtration of acids and bases.
• Desalination of organic materials.