A method for treatment of waste water in an activated sludge plant, the plant comprising a plurality of process tanks comprising activated sludge with a treatment rate being variable, each process tank having an inlet for receiving waste water and an outlet for supplying treated water to at least one clarification tank for separation into a water fraction and a sludge fraction, an aeration structure configured for aeration of at least one of the process tanks, and a control unit, the method comprising the steps of: - measuring a content of a substance/parameter characterising the process tanks by measurements in the process tanks, the substance/parameter one or more of: COD (Chemical Oxygen Demand), BOD (Biochemical Oxygen Demand), TOC (Total Organic Carbon), O 2, N, NH 4 -N, NH 4 -N + NO 3 -N, and OUR (Oxygen Uptake Rate) by use of at least one sensor configured to measure said content of said substance/parameter, - determining an actual spare capacity indicating an amount of waste water which can be added to and treated in the activated sludge plant, wherein the actual spare capacity is determined based on the measured content of the substance/parameter characterising the process tanks, the actual spare capacity being determined by the control unit, - supplying waste water only to a first group of process tanks, the first group of process tanks being a subset of the plurality of process tanks, wherein the number of process tanks in the first group is determined based on the actual spare capacity, the supply being controlled by the control unit, and - automatically temporarily interrupting supply to a second group of process tanks based on the actual spare capacity without emptying the tanks, the supply being controlled by the control unit, the method further comprising a step of interruption aeration to at least one of the process tanks in the second group, and a step of initiating aeration of the at least one process tank before supply of waste water to at least one of the process tanks of the second group of process tanks is initiated. - measuring a content of a substance/parameter characterising the process tanks by measurements in the process tanks, the substance/parameter one or more of: COD (Chemical Oxygen Demand), BOD (Biochemical Oxygen Demand), TOC (Total Organic Carbon), O 2, N, NH 4 -N, NH 4 -N + NO 3 -N, and OUR (Oxygen Uptake Rate) by use of at least one sensor configured to measure said content of said substance/parameter, - determining an actual spare capacity indicating an amount of waste water which can be added to and treated in the activated sludge plant, wherein the actual spare capacity is determined based on the measured content of the substance/parameter characterising the process tanks, the actual spare capacity being determined by the control unit, - supplying waste water only to a first group of process tanks, the first group of process tanks being a subset of the plurality of process tanks, wherein the number of process tanks in the first group is determined based on the actual spare capacity, the supply being controlled by the control unit, and - automatically temporarily interrupting supply to a second group of process tanks based on the actual spare capacity without emptying the tanks, the supply being controlled by the control unit, the method further comprising a step of interruption aeration to at least one of the process tanks in the second group, and a step of initiating aeration of the at least one process tank before supply of waste water to at least one of the process tanks of the second group of process tanks is initiated.