Nanofiltration works on a similar principle as reverse osmosis, but it uses a porous membrane having a pore size from 0.01 to 0.001 µm, which usually carries a functional groups with a negative charge (e.g. sulfonic acid groups). The nanofiltration membranes work on the the sieve effect principle -(molecules larger than the membrane pores can not pass through the membrane) and besides that, a part of the molecules dissolves in the membrane, which is followed by their diffussion through the membrane and desorption on the other side. Also the effects resulting from the presence of an electrical charge on the membrane polymer are observed . Nanofiltration is operated at lower operating pressures from 0,5 to 1,5 MPa.
Nanofiltration membranes have the ability to selectively capture ions with a higher charge. This fact very well applies in the process of water softening, as the nanofiltration technology captures significantly more bivalent ions (98%) than monovalent ions (60%).
The most commonly used modules used in nanofiltration are spiral wound, plate, tubular, hollow fiber, ceramic, or capillary. To select a suitable membrane, we frequently take into account the energy demands, flux, fouling control (clogging by sediment of biological origin) and scaling (clogging by sediments of inorganic origin - mainly calcium and magnesium salts in combination with carbonates).
Another field of nanofiltration application is the removal of pathogenic substances, precursors of chlorinated derivatives of the organic substances which are formed during the water disinfection, organic substances with molecular weight above 200, pesticides etc.