NOVELTY - The biogas plant for fermentation of organic materials, has a service duct (14) formed as a gas-sealed and dome-shaped enclosure is mounted over a fermentation tank opening in a cover of the fermentator (3). The service duct exhibits a duct hole in the duct walls for maintenance purposes. An agitator (11); is movable from a function position in the fermentation tank (2) upwards to a service position through the opening of the fermentor by a control unit as a swivel arm; is fixable in the service position; and exhibit same form and length in a four-blade arrangement.    USE - Used for the fermentation of organic materials.    ADVANTAGE - Simple assembly and cost effectiveness of the biogas plant. Guarantees high industrial safety.    DETAILED DESCRIPTION - The biogas plant for fermentation of organic materials, has a service duct (14) formed as a gas-sealed and dome-shaped enclosure is mounted over a fermentation tank opening in a cover of the fermentator (3). The service duct exhibits a duct hole in the duct walls for maintenance purposes. An agitator (11); is movable from a function position in the fermentation tank (2) upwards to a service position through the opening of the fermentor by a control unit as a swivel arm; is fixable in the service position; and exhibit same form and length in a four-blade arrangement. The agitator exhibits a driving motor with horizontally arranged drive shaft, to which an agitator blade member transmitting the rotational motion is attached with agitating blades angularly disaligned at a hub element. An additional agitator blade member is connected with the drive shaft with a detachable agitating blades angularly disaligned to the agitating blades of the other agitator blade member. The agitating blade of the additional blade member is attached to a hub ring, which is non-turnably fixed on the end of the drive shaft projecting from the hub element. The all-agitating blades in the four-blade arrangement are disaligned in each case with an angle shift of 90degrees to one another. The all-agitating blades in the three-blade arrangement are disaligned in each case at an angle of 120degrees. The drive shaft, the hub element, the hub ring exhibit an axial spline connection as anti-twist plate for the drive shaft, and interposition elements are arranged at the hub element and hub ring in such a way that the desired angular position of the agitating blades to one another is predetermined. The hub element and the hub ring are attached on the drive shaft and fastened against a stop ring of the drive shaft by a tightening screw. The hub element and the hub ring are screwed onto the stop ring and onto the hub element respectively. The control unit is a vertically held supporting rod (12) in the fermentation tank, with an upper end of the supporting rod, which projects into the interior of the service duct, and the agitator is led along the supporting rod.    DESCRIPTION OF DRAWING(S) - The figure shows a biogas plant for fermentation of organic materials:    Biogas plant; (1)    Fermentation tank; (2)    Fermentor cover; (3)    Biomaterial; (5)    Conveyor belt; (6)    Agitator; (11)    Supporting rod; (12)    Fermentor opening; (13)    Service duct. (14)