The invention relates to a method and apparatus for removing impurities from waste water by electroflotation. The waste water to be cleaned is conducted through an electrolytic cell. Electrolysis is performed between two electrodes (1, 2) of different electronegativities, such that the more electronegative electrode (1), which is non-wearing in a cleaning process, is used for producing hydrogen gas and hydroxyl ions from water. The less electronegative electrode (2), which is an active, wearing electrode in a cleaning process, is used for producing metal ions in a solution to be cleaned. In addition to this basic reaction, a desired oxidation-reduction reaction is initiated in the cell in a strictly controlled electric field for removing one or more designated impurities from cleaned water.