Abstract: A method for operating an at least approximately round reactivating tank with a vertical axis for a purifying plant, according to which the sludge-water mixture to be purified is revolved in the tank and compressed air is introduced into the mixture to supply the reactivating sludge flakes of the sludge-water mixture in the tank with oxygen whereupon the mixture in the tank is forced to carry out and to maintain for a desired time a circular flow of the mixture to be purified which flow is substantially coaxial with the vertical axis of the tank. The invention also includes a reactivating tank for purifying a sludge-water mixture therein which tank has arranged therein adjacent the outer wall thereof turbine means having the axis thereof extending parallel to a tangent to the outer wall of the tank which outer wall is at least approximately round.