Nano filtration (NF) uses partially permeable membranes to preferentially separate different fluids or ions, and will remove particles from approximately 0.0005 to 0.005 microns in size. NF membranes are usually used to reject high percentages of multivalent ions and divalent cations while allowing monovalent ions to pass. Removal includes sugars, dyes, surfactants, minerals, divalent salts, bacteria, proteins, particles, dyes, and other constituents that have a molecular weight greater than 1000 daltons.

Nano filtration, like reverse osmosis (RO), is affected by the charge of the particles being rejected. Thus, particles with larger charges are more likely to be rejected than others particles.

Nano filtration is not effective on small molecular weight organics, such as methanol. Nano filtration has often been wrongly thought of as "loose reverse osmosis" filtration. NF is not as fine a filtration process as RO, since the membrane pores are typically much larger. However, NF does not require the same energy to perform the separation. The differences between the two types of membrane are subtle, but distinct. Most notably, NF can reject only ions with more than one negative charge, such as sulphate or phosphate, while passing single charged ions.