A process and a device for biological treatment of a suspension in a bioreactor is described, which is made especially as a loop reactor with a central guide pipe (2) and gas injection (3). In order to be able to use the bioreactor for treatment of renewable raw materials as well, it is proposed that the treated suspension be removed from a zone of the bioreactor with a small solid concentration and the removed suspension be routed out of the bioreactor by way of a tilted discharge (4). In the tilted discharge (4), separation of gas and solids is produced, and at least a portion of the solids is returned against the main suspension flow in the discharge (4) to the bioreactor. In this way, even for treatment media with a high concentration of dissolved compounds and few suspended solids, the biomass is concentrated in the bioreactor.