The water to treat is received in a chamber (1) with a rapid mixer and from there it passes to a second chamber for coagulation (3) and from that to a third chamber for flocculation (4) where it is mixed with certain coagulating reagents. It then enters a separator (7) provided with some filtering membranes (5) where it is clarified. In the separator (7) there are some air nozzles (6) which sweep the surface of the said filtering membranes (5), some inlet nozzles (8) for floccules coming from the coagulation-flocculation and other air outlet nozzles (6) for distribution of said floccules underneath the membrane (5).