A large-scale nanofiltration plant will produce 140,00 m(3)/day of permeate in the Paris Area from river water. The river water must undergo pretreatment before nanofiltration. The water varying temperature is also a specificity. A nanofiltration prototype since 1993 distributes water to 6,000 residents of Auvers-sur-Oise. It allowed to design a large-scale plant at minimum energy and chemicals costs. A new membrane has been developed which is able to retain pesticides (atrazine) and allow the passage of calcium and bicarbonate.