From Membrane Technology
Reverse osmosis plants use the membrane selectivity to obtain the water demineralization. This process doesn’t consist only of a physical obstacle (produced by the membrane pores dimensions) to the passage of the molecules, but it use the different chemical affinity of the solutions with the membrane, permitting the hydrophile molecules (or water-like) passage. This method uses the tangential filtration principle, like other separation techniques by the use of membranes as the ultrafiltration, microfiltration and the MBR membrane bioreactor (see Membrane technology).
Nanofiltration plants are usually used for softening. The installed membrane type installed make the nanofiltration plants different from the reverse osmosis units. The nanofiltartion membrane has bigger pores and it doesn’t stop all the salts but only the molecules with bigger dimensions. The nanofiltration is mostly use to remove organic solutions (colored substances, nitrates, etc.) and multivalent ions.
Reverse osmosis and nanofiltration plants work at high pressures.