A method for controlling a plant for treatment of waste water, wherein the plant comprises: - a basin constituted by a circulation channel and configured to house a liquid, - at least one flow generating machine constituted by a submergible mixer machine and arranged in the basin and configured to generate a liquid flow along the circulation channel, wherein the operational speed N of the flow generating machine is higher than a predetermined lowest allowable operational speed N min, preventing solid matter from accumulating on the bottom of the basin, and is lower than a predetermined highest allowable operational speed N max, preventing the flow generating machine from being overloaded, - at least one equipment that is arranged in the basin and that effects the momentum of the liquid flowing in the basin, and - a control unit that is operatively connected to said at least one flow generating machine, the method being characterized by the steps of: - in the control unit storing a predetermined relationship between the operational speed N of the flow generating machine and an operational parameter P from which the torque M of the flow generating machine may be derived, which relationship depends on a predetermined liquid flow speed V in the basin at the flow generating machine, - determining the operational speed N of the flow generating machine, - from the determined operational speed N determining a set value of the operational parameter P of the flow generating machine based on said relationship between the operational speed N of the flow generating machine and the operational parameter P of the flow generating machine, - by means of the control unit determining a real value of the operational parameter P of the flow generating machine, and - by means of the control unit adjusting the operational speed N of the flow generating machine if the real value of the operational parameter P of the flow generating machine is different than the set value of the operational parameter P of the flow generating machine. - a basin constituted by a circulation channel and configured to house a liquid, - at least one flow generating machine constituted by a submergible mixer machine and arranged in the basin and configured to generate a liquid flow along the circulation channel, wherein the operational speed N of the flow generating machine is higher than a predetermined lowest allowable operational speed N min, preventing solid matter from accumulating on the bottom of the basin, and is lower than a predetermined highest allowable operational speed N max, preventing the flow generating machine from being overloaded, - at least one equipment that is arranged in the basin and that effects the momentum of the liquid flowing in the basin, and - a control unit that is operatively connected to said at least one flow generating machine, the method being characterized by the steps of: - in the control unit storing a predetermined relationship between the operational speed N of the flow generating machine and an operational parameter P from which the torque M of the flow generating machine may be derived, which relationship depends on a predetermined liquid flow speed V in the basin at the flow generating machine, - determining the operational speed N of the flow generating machine, - from the determined operational speed N determining a set value of the operational parameter P of the flow generating machine based on said relationship between the operational speed N of the flow generating machine and the operational parameter P of the flow generating machine, - by means of the control unit determining a real value of the operational parameter P of the flow generating machine, and - by means of the control unit adjusting the operational speed N of the flow generating machine if the real value of the operational parameter P of the flow generating machine is different than the set value of the operational parameter P of the flow generating machine. - in the control unit storing a predetermined relationship between the operational speed N of the flow generating machine and an operational parameter P from which the torque M of the flow generating machine may be derived, which relationship depends on a predetermined liquid flow speed V in the basin at the flow generating machine, - determining the operational speed N of the flow generating machine, - from the determined operational speed N determining a set value of the operational parameter P of the flow generating machine based on said relationship between the operational speed N of the flow generating machine and the operational parameter P of the flow generating machine, - by means of the control unit determining a real value of the operational parameter P of the flow generating machine, and - by means of the control unit adjusting the operational speed N of the flow generating machine if the real value of the operational parameter P of the flow generating machine is different than the set value of the operational parameter P of the flow generating machine.