A plant and method for controlling such a plant suitable for treatment of waste water. The plant includes a basin, a flow generating machine adapted to generate a liquid flow in the basin, equipment in the basin that effects the momentum of the liquid flow, and a control unit. The method includes the steps of: 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, determining the operational speed N, determining a set value of the operational parameter P of the flow generating machine, determining a real value of the operational parameter P of the flow generating machine, and adjusting the operational speed N if the real value of the operational parameter P is different than the set value of the operational parameter P.