NOVELTY - The reactor, a thermally-insulated (2) cylindrical tank (1) closed by a cover (3) includes a rotary, central mixing tube (5) carrying a stirrer (7) fixed to its base plate (6). An opening (9) corresponding with the impeller (8) in diameter and location, provides free access. At the upper end of the tube, a cut-out (10) directly over the opening (9) can be closed by a slide plate (11). A drive tube (12) connects to, and drives the mixing tube. Total external length of the mixing tube, contacts the lip seal of a radial intermediate wall reaching to the reactor casing. Nearby, a further, radial wall from the casing, does not reach the mixing tube. The sector enclosed by these walls includes a radially-outer feed line (16).    USE - A reactor producing biogas from e.g. farm slurry.    ADVANTAGE - The design avoids conventional use of a stirring arm. Thorough mixing is provided, the blades of the impeller being sharpened to cause comminution simultaneously. Larger items can be introduced through a tube (12). Design and dimensioning for flow, avoids dead spots, rotation of the mixing tube carrying the impeller, causing all-round sweeping agitation. The slide allows for optimal adjustment of the recirculation flow. Floating layers are continuously broken up and circulated. Arrangements are suggested for heavy solids removal via the outlet line. A residence time of 14-20 days is cited. A line (18) provides for optional aerobic operation and/or process control additives. Construction is simple, comparatively robust. The reactor is suitable for co-fermentation. Operation is mesophilic and aerobic, or anaerobic.    DETAILED DESCRIPTION - A further inlet line (25) discharges outside the sector, near the shorter intermediate wall. The reactor outlet line leaves through the base (4), outside the sector, near the longer intermediate wall. A line (18) through the drive tube leads to the agitator. The biogas line (19) is mounted in the top cover. Preferred features: Reactor base (4) and casing have heating provision. Upper edges of mixing tube (5) and intermediate walls are at the same height, above slurry level (20). Sector chamber width between radially-inner edges of the walls, is approximately equal to the diameter of the impeller (8). Impeller rotor blades are sharpened.    DESCRIPTION OF DRAWING(S) - The reactor is shown in vertical cross section. A useful plan view is also included, in the disclosure.    Cylindrical tank (1)    Thermal insulation (2)    Cover (3)    Base (4)    Rotary, central mixing tube (5)    Base plate (6)    Stirrer (7)    Impeller (8)    Opening (9)    Cut-out (10)    Slide plate (11)    Drive tube (12)    Feed line (16)    Further inlet line (25)    Biogas line (19)    Slurry level (20)