A process and a device for biological treatment of a suspension in a bioreactor (2) are described. To circulate the suspension, at least some of the suspension is routed through a vertically aligned guide zone (5) so that a vertical flow of the suspension is produced. To control sediment problems, it is suggested that by feeding a fluid, especially a free liquid jet, via at least one nozzle (11), a horizontal flow is superimposed on the vertical flow in the vicinity of the bottom of the bioreactor (2), by which a spiral flow to the central outlet area of the bioreactor (2) is produced (FIG. 1).