A device for fermenting low-viscosity organic materials, having a bioreactor and at least one overstowed solid separator, wherein the bioreactor consists of: - a reactor container with a conical reactor bottom with solid removal means for removing the fermentation residue, side walls and a reactor cover, wherein at least one receptacle for fastening a multiplicity of growth supports is arranged in the interior of the reactor container essentially parallel to the reactor cover and in the central third of the reactor container, and wherein the region from the conical bottom to the at least one receptacle forms a first reaction space and the region from the at least one receptacle to the reactor cover forms a second reaction space, - a multiplicity of growth supports, which are stretched from the reactor cover to at least one receptacle, - at least one substrate supply in the region of the first reaction space, wherein the substrate supply has a first heat exchanger outside of the reactor container for temperature control of the substrate located in the substrate supply, - at least one recirculation means, which is suitable for transferring recirculated material from the second reaction space to the first reaction space, wherein the recirculation means has a means for accommodating the recirculated material and a means for outputting the recirculated material and also a second heat exchanger for the temperature control of the recirculated material located in the recirculation means, wherein recirculation means and heat exchanger are arranged outside of the reactor container, - at least one overflow, which is arranged in the region of the second reaction space between the reactor cover and the recirculation means, wherein the part of the second reaction space located between reactor cover and overflow is a gas collection space, in the region of which, a gas removal means is provided, and wherein the at least one overstowed solid separator - has a conical bottom, wherein a solid removal means is arranged at the tip of the cone, which is connected to the substrate supply of the bioreactor, and - has at least one feed, which is connected to the overflow of the bioreactor, and wherein the overstowed solid separator is arranged at the height of or below the reactor bottom next to the bioreactor. - a reactor container with a conical reactor bottom with solid removal means for removing the fermentation residue, side walls and a reactor cover, wherein at least one receptacle for fastening a multiplicity of growth supports is arranged in the interior of the reactor container essentially parallel to the reactor cover and in the central third of the reactor container, and wherein the region from the conical bottom to the at least one receptacle forms a first reaction space and the region from the at least one receptacle to the reactor cover forms a second reaction space, - a multiplicity of growth supports, which are stretched from the reactor cover to at least one receptacle, - at least one substrate supply in the region of the first reaction space, wherein the substrate supply has a first heat exchanger outside of the reactor container for temperature control of the substrate located in the substrate supply, - at least one recirculation means, which is suitable for transferring recirculated material from the second reaction space to the first reaction space, wherein the recirculation means has a means for accommodating the recirculated material and a means for outputting the recirculated material and also a second heat exchanger for the temperature control of the recirculated material located in the recirculation means, wherein recirculation means and heat exchanger are arranged outside of the reactor container, - at least one overflow, which is arranged in the region of the second reaction space between the reactor cover and the recirculation means, wherein the part of the second reaction space located between reactor cover and overflow is a gas collection space, in the region of which, a gas removal means is provided, and - has a conical bottom, wherein a solid removal means is arranged at the tip of the cone, which is connected to the substrate supply of the bioreactor, and - has at least one feed, which is connected to the overflow of the bioreactor,