(1) Polluted water is pretreated by adding metal salt adjusting the pH-value and adding a flocculation aid into separate reactors for flocculation. Electrolytically produced gas bubbles float the flocculated matter to the top of the tank (2) where the treated clean water leaves the tank over a weir for discharge (3). The floated floc (4) is then vacuum skimmed into the cyclone tank (5) and separated from air. The resulting sludge is collected in a tank (6) and pumped into a reactor for chemical treatment or to a filter press for dewatering. The treated sludge or the 35% solids filtercake is then disposed of as required.