Nanofiltration and reverse osmosis
In drinking water, we mostly use osmosis to filter dissolved solids like salt, nitrates and certain organic materials. The osmosis membranes are not porous like the ultrafiltration but semi-permeable. They are not qualified the same way as are the ultrafiltration membranes.
The osmosis membrane is not qualified by the size of its pores but by the molecular weight of the particles it can remove. Even if reverse osmosis can remove microorganisms that have a very small molecular weight, it is not recognized as an absolute barrier to microorganisms. It does not have any pores and can have slight imperfections that can have a minor impact with water desalination. With water disinfection, it might not catch all the microorganisms.