A process and a device for biological treatment of a suspension in a bioreactor, wherein the suspension is circulated, at least some of the suspension is routed through a vertically aligned guide zone in the bioreactor so that a vertical flow of the suspension is produced. The top scum in the area of the suspension fill level is controlled by feeding a fluid, especially in the form of a free liquid jet, into the bioreactor via at least one nozzle at the fill level, such that the surface of the suspension and/or the top scum floating on the surface of the suspension is forced into rotary flow.