A device for decanting a supernatant includes a decanter which is pivotally attached to a clarifier and which includes a decanter head that has a water inlet opening, a pivoting device which is connected to the decanter via an actuator with an adjustable length for raising and lowering the water inlet opening relative to a floor of the clarifier, a first measuring device for measuring a fill level of the wastewater received in the clarifier, a device for determining a length of the actuator, an evaluating device with which a distance of the water inlet opening of the decanter to the floor of the clarifier can be determined from the length of the actuator, and a controller for controlling the pivoting device such that a water inlet opening immersion depth, which results from the difference between the fill level and the distance, is kept in a specified range.