NOVELTY - In a method for operating a plug-flow fermenter for anerobic fermentation of biogenic waste, with a stirrer consisting of a shaft (10) carrying stirring arms (12) and passing longitudinally through the fermenter, the stirrer is successively (a) rotated in one direction to give plowing action, (b) rotated in the opposite direction to give transporting action and (c) kept stationary (after which the cycle is repeated).    USE - For anerobic fermentation of biogenic waste (claimed), specifically to produce biogas.    ADVANTAGE - The two-phase stirring arrangement can both thoroughly mix the methane-producing bacteria and transport the sedimented solids through the fermenter, so that an extremely high torque is not required to restart the stirrer after a shut-down period, even in the case of a large fermenter creating a large amount of heavy sediment.    DETAILED DESCRIPTION - In a method for operating a plug-flow fermenter for anerobic fermentation of biogenic waste, with an inlet, an outlet and a stirrer consisting of a shaft (10) passing longitudinally through the fermenter, carrying stirring arms (12) and mounted in the fermenter end regions, the stirrer is operated in the following cycle of stages: (a) rotating in one direction to give plowing action, for a period t1; (b) rotating in the opposite direction to give transporting action for a period t2; and (c) stationary for a period (t3) (after which the cycle is repeated).    An INDEPENDENT CLAIM is included for apparatus for carrying out the process, consisting of horizontal fermenter consisting of tank through which a driven shaft (10) carrying stirring arms (12) with paddles (13), where the paddles consist of two share plates (15) arranged in a V-shape and act as a paddle on rotation in one direction and as a plow-share on rotation in the opposite direction.    DESCRIPTION OF DRAWING(S) - The figure shows a section through a stirrer arm.    Shaft (10)    Shaft body (11)    Stirrer arm (12)    Paddle (13)    Flange plates (14)    Share plates (15)    Common edge (16)    Paddle plate (18)    Scraper plate (19)    H-Beam (20)    Parallel longitudinal flanges (21)    Connecting bridge (22)    Connecting bridge extension (23)    Bent extension (30)