Abstract: A process to purify waste waters using chemical oxidation and Fenton's reagent by the action in the acid range of hydrogen peroxide and iron(II) compounds and subsequent precipitation in the weakly acid to alkaline range of the iron(III) compounds, said process being characterized in that the sludge containing iron(III) accumulating in addition to the purified waste water is reduced electrolytically back into iron(II) compounds and in that the sludge so reduced is fed back into the process. The process of the invention allows extensive purification of highly concentrated waste waters and furthermore problems arising with the treatment and elimination of the Fenton sludge can be circumvented.

Abstract: In an activated sludge process for the biological purification of wastewater wherein undesirable bulking sludge is formed in addition to normal activated sludge, the improvement which comprises mechanically destroying the microorganisms causing the bulking sludge without substantially destroying the microorganisms required for the normal activated sludge.