Nanofiltration is a technological process that is used for the partial desalination of water. It is mainly used to remove divalent cations (Ca2+ and Mg2+) and sulfate ions from water. That is, it can be argued that the nanofiltration process can be used to soften water.
Nanofiltration is a technological process very similar to the reverse osmosis process both in terms of the mechanism for removing salts from water, and in the design of membranes and equipment used, the principle of organization and architecture.
The main difference between the nanofiltration process and the reverse osmosis process is the type of membranes used. Nanofiltration membranes have larger pores than reverse osmosis membranes and are therefore less selective and have a higher specific productivity (permeability). Due to this, the nanofiltration process is characterized by a lower operating pressure at the same performance.
The permeability of a nanofiltration membrane (as well as a reverse osmosis membrane) at constant pressure is characterized by the volume of the passed filtrate per unit of time through a unit of surface and is expressed in l/m2*h.
Nanofiltration plants are advisable to use when it is necessary to carry out partial desalination of water, mainly due to the removal of hardness ions and sulfate ions. During the course of the process, in the case of a non-equivalent content of hardness salts and sulfates in the source water, a partial decrease in alkalinity and chloride content also occurs.
A clear advantage of the nanofiltration process in relation to the softening process on ion-exchange materials is the almost complete removal of organic matter from water.
However, the process of nanofiltration, as well as the process of reverse osmosis, requires the correct selection of equipment at the stage of preliminary water treatment.