Due to their many benefits ultrafiltration systems are becoming more and more popular for pool water treatment in public pools than the previously used sand filter screens. Through the very high retention rate of the ultrafiltration membranes a reduction of the chlorine is achieved because the biomass that is usually oxidized by the chlorine is retended by the ultrafiltration membranes. In addition, operating costs are reduced, because the back flush water amount for cleaning is minimized and a reduced circulation feed is necessary.