Abstract: Apparatus for gas-producing treatment, particularly fermentation and digestion, of organic material contained in a suspension comprises a horizontal, rotatably mounted treatment drum having an inlet for the suspension at one end and an outlet for the treated suspension at the other end. A plurality of baffles extending in axial planes throughout the length of the drum are secured to the interior side of the drum and extend from there towards the central region of the drum. In each of a number of angular drum positions, the number being equal to the number of baffles, one of the baffles defines jointly with the wall of the drum a downwardly open gas collection space which is spaced from the vertical plane containing the axis of rotation of the drum. The gas collected therein causes an uneven distribution of the suspension within the drum and thereby causes a turning moment acting on the drum.