being filtered off as purified water an important feature of this invention is that nanofilter elements having increasing salt rejection functions are used in addition to decreasing size features as the reject water increases in salt concentration tighter membranes that have a higher salt rejection capability are used the desal company 39 s sh 74 product has a higher salt rejection feature than the desal dk 73 product similarly the desal product sg 75 has an even higher salt rejection than its sh 74 product for example the eight inch nanofilter 71 receiving water through the 5 micron filter 30a at the rate of twelve gallons per minute discharges 3 54 gallons per minute of purified water with filter 72 discharging 7 08 gallons per minute filters 73 74 and 75 discharge 10 61 gallons per minute 11 31 gallons per minute and 11 60 gallons per minute respectively decreasing sizes are used as the flow diminishes due to removal of product water to maintain appropriate flow velocity thus nanofilter 74 is smaller in diameter than nanofilter 73 and nanofilter 75 is smaller in diameter than nanofilter 74 these decreasing sizes can also be achieved using a decreasing number of vessels in each stage the volume of water 28 flowing through the series connection of the filters drops through each of the filters as indicated by the drawing finally discharging through nozzle 76 to a reject site at about 0 46 gallons per minute fig6 is a cutaway view of a typical nanofilter 71 comprising a housing 72 enclosing a cartridge 73 although these filters are manufactured in various.