Abstract: The invention disclosed concerns a procedure for purifying waste wter containing chlorinated phenolic compounds, such as the bleaching effluent of a sulphate cellulose pulp mill. The purification is carried out biologically in a floating bed reactor which has been inoculated with a bacterial population decomposing chlorinated phenolic compounds and wherein the bacteria use the floating particles for affixing substrate. The purification takes place in the reactor under anaerobic conditions, and the water removed from the reactor may be oxygenated or aerated and conducted to a supplementary aerobic purifying step, where another floating bed reactor or a bark filter is used.