A biogas plant contains a fermenter vessel, in which an assembly support is mounted, preferably vertically oriented. An assembly, in particular a submersible motor driven stirrer, is retained by a height adjustment device so as to be vertically adjustable on the assembly support. Accordingly, the height adjustment device is formed by a belt drive of which a traction device is guided around at least two deflecting elements spaced apart from one another in the longitudinal direction of the assembly support. At least one of the deflecting elements can be driven in rotation by a driving device and the assembly is coupled to the traction device in such a way that when the deflecting element is rotationally actuated in a first direction the assembly is raised and when the deflecting element is rotationally actuated in a second, opposite direction, the assembly is lowered.