NOVELTY - In a stirrer for a fermenter comprising a vertical stirrer shaft (12) carrying paddle(s) (14) and rotated by a drive mechanism (13) acting on its upper end, the lower end of the shaft is centered by insertion into a centering bearing (16), in which the shaft is releasably held solely by gravity.    USE - The stirrer and the fermenter are useful in dry fermentations the fermentation of organic materials (e.g. solid or liquid manure, plant residues, plants, brewing residues, organic sludges, industrial effluents or communal waste disposal residues) to produce biogas (mainly methane) for use as an energy source, e.g. by burning for current generation. The biomass remaining after the biogas fermentations may be used as a biological fertilizer.    ADVANTAGE - The stirrer shaft can be exchanged easily and rapidly, without interruption of the operation of the fermenter, by withdrawal of one shaft and insertion of another shaft from the top of the fermenter. The fermenter, with downward flow of material, is suitable for continuous operation, specifically by formation of an uppermost, lowest density fluidization zone (43), a central, compressed medium density methanization zone (44) (in which most of the methane is formed) and a lowest, highest density removal zone (45) of (almost) completely degraded substrate. The downward flow provides hydraulic coupling of the degradation stages, so that the biomass yield is optimized.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for:    (1) a fermenter (1) comprising a housing (2) base plate(s), side wall(s) (5) enclosing the base plate, a stirrer (11) (preferably of the type described above), an input device (36) for material to be fermented and an outlet opening (41), where the input device is in the upper region of the fermenter and the outlet opening is in the lower region; and    (2) a method for operating the fermenter, involving supplying the material to be fermented to the upper region of the fermenter and withdrawing the fermented substrate from the lower region.    DESCRIPTION OF DRAWING(S) - The figure shows a sectioned view of a fermenter, together with an input device.    Fermenter (1)    Fermenter body (2)    Substrate (3)    Base plate (4)    Side wall (5)    Lid (6)    Stirrer (11)    Stirrer shaft (12)    Drive mechanism (13)    Centering bearing (16)    Input device (36)    Storage container (37)    Conveyor screw (38)    Conveyor channel (39)    Inlet opening (40)    Outlet opening (41)    Fluidization zone (43)    Methanization zone (44)    Removal zone (45)    Opening (46)