Abstract: A flat plate vessel comprising an extraction device having a sliding frame on the flat plate, the frame being associated at least with one upwardly open channel, which is parallel to the flat plate and is provided with a transport screw, in which the vessel is first of all closed by a cover having an opening communicating with an inlet duct for material to be subjected to anaerobic fermentation and an opening communicating with a discharge duct for gas produced during fermentation; secondly, at least one activating shaft for the sliding frame and the drive shafts for the transport screws are surrounded by sealing gland devices; and finally, the channels provided with transport screws end at at least one discharge channel, the cross-section of which reduces towards the outlet opening, and in which there is located a transport screw extending towards the outlet opening, without however reaching this opening, and this screw forms, in the region of the discharge channel the cross-section of which narrows, a plug