Nano-filtration belongs among the latest pressure membrane processes. Nano-filtration membranes reach the limit of discernible pores with the dimension of 1–3 nm and usually bear functional groups with negative charge. The selectivity of nano-filtration is caused by different speeds of dissolution of substances in polymer and by different speeds of diffusion of substances in polymer. Nano-filtration separates low-molecular organic substances and the dimension of separated substances is stated rather in units of molecular weight that in their size. Nano-filtration separates substances in the dimension up to approximately 102–104 Da, corresponding e.g. to molecules of saccharides, paints, pesticides and herbicides.
Requirements for pressure are much higher than those of ultra-filtration, due to the fact that the osmotic pressure begins to manifest itself, and they are in the order of tens of bars.
Due to the ability to retain selectively ions with higher charge, the most important applications are in water softening, removing of pesticides in production of drinking water, purification of waste water from organic syntheses, desalination of paints after azo coupling or cleaning of water in waste dumps.
Modules are almost exclusively spirally wound, exceptionally also desks or hollow fibres, almost always made of polymeric material and containing a functional group.
The company MemBrain used nano-filtration in combination with reverse osmosis with some pilot units to process whey and mine or waste dump water.