Reverse osmosis is the finest type of filtration with a pore size of less than 1nm and a rejection of ions of more than 98%. The typical operating pressure of a reverse osmosis unit operated on tap water is 12-16 bar.
Nanofiltration is also a very fine filtration but the rejection of ions is lower compared to reverse osmosis. The typical operating pressure of a Nanofiltration unit is 4-7 bar.