Abstract: Purifying waste waters contaminated with tensides, from nuclear power plants and other plants in which radioactive substances are processed wherein before return of the water to the plant, the water is passed through an evaporator and a mixed-bed filter. Contaminant content of the water is materially reduced by first acidifying the waste water to a pH of 2.5-3, then treating with activated carbon, KMnO.sub.4, MnSO.sub.4 and CaCO.sub.3, and thereafter raising the pH to 8.5-9. The mixture is permitted to form a lower sludge layer and a supernatant water layer. The sludge layer is sent to waste disposal and the supernatant layer is directed to the evaporator.