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Nanofiltration

Nanofiltration (NF), in concept and operation, is much the same as reverse osmosis (RO). The key difference is the degree of removal of monovalent ions such as chlorides. NF removal efficiencies for monovalent ions varies between 50% to 90% depending on the material and manufacture of the membrane, whereas RO can reject monovalent ions at >99%.

NF membranes are usually classified by their ability to reject divalent ions, e.g. magnesium sulphate at 98%. For this reason, they are sometimes referred to as "softening" membranes due to their ability to reject calcium and magnesium. Removal of sulphate ions from surface water is one of the main areas of use for NF membranes.

NF membranes are also widely used for desalting (i.e. the removal of chloride) of cheese whey and dyestuffs, utilising the the capability of the membranes to pass chlorides to the permeate whilst retaining other fractions (such as dye).

NF membranes will also remove bacteria and viruses as well as organic-related color without generating undesirable chlorinated hydrocarbons and trihalomethanes (THM). Because NF operates at a much lower pressure than RO it is, by comparison, a very energy efficient process.