Nanofiltration (NF) is a low to moderately high pressure (typically 50-450 psig) process in which monovalent ions will pass freely through the membrane but highly charged, multivalent salts and low molecular weight organic will be rejected to a much greater degree. Typical NF application include water softening, desalination of dyestuffs, acid and caustic recovery and color removal.

Crossflow membrane filtration controls the effect of concentration polarization and the gel layer. It provides the most rapid, and hence economic, continuous membrane filtration. NF is a pressure driven process for separating larger size solutes from aqueous solutions by means of a semi-permeable membrane. This process is carried out by having a process solution flow along a membrane surface under pressure. Crossflow membrane filtration uses a high cross flow rate to enhance permeate passage and reduce membrane fouling. Retained solutes (such as dissolved salts) leave with the flowing process stream and do not accumulate on the membrane surface. Pores have not been observed in NF membranes under any microscope, however, water can still pass through the membrane and multivalent salts and low molecular weight organics are rejected.

It is difficult to predict the performance of NF membranes, especially if more than three solutes are present in the solution, since membrane rejection is influenced by the size, structure and charge of the components in solution. As a result, piloting is highly recommended for NF application, even if a detailed feed water analysis is available.