NOVELTY - The combined plant comprises fermenters (2) working according to the principle of the dry fermentation for producing biogas in the batch-operation with a biogas outlet (8) and a rinsing gas inlet (24), a biogas line (12) connectable with the outlet, an exhaust gas line (26) to supply carbon dioxide-containing exhaust gas to the inlet, an exhaust gas stack (20) connected with the outlet via a first exhaust gas line, a surplus gas torch (22) connected with the outlet via a second exhaust gas line, a fresh air line connected with the rinsing gas inlet, a control device, and measuring device.    USE - Combined plant for producing biogas and compost from biomass obtained from agriculture, bio-waste and municipal maintenance areas.    ADVANTAGE - The combined plant provides safe switching of a fermenter between biogasification and composting thus avoiding the formation of explosive biogas/air mixture.    DETAILED DESCRIPTION - The combined plant comprises fermenters (2) working according to the principle of the dry fermentation for producing biogas in the batch-operation with a biogas outlet (8) and a rinsing gas inlet (24), a biogas line (12) connectable with the outlet, an exhaust gas line (26) to supply carbon dioxide-containing exhaust gas to the inlet, an exhaust gas stack (20) connected with the outlet via a first exhaust gas line, a surplus gas torch (22) connected with the outlet via a second exhaust gas line, a fresh air line connected with the rinsing gas inlet, a control device, and measuring device connected with the control device. The control device is intended for connecting the biogas outlet with the biogas line or with the exhaust gas stack over the first exhaust gas line or with the surplus gas torch over the second exhaust gas line and for connecting the rinsing gas inlet with the exhaust gas line or with the fresh air line. The measuring device comprises a first sensor for detecting a methane concentration in the common biogas line and a second sensor for detecting a carbon dioxide concentration in the gas mixture extracted from the fermenter. The measuring device is arranged in the biogas outlet. The biogas outlets are selectively connected over a common exhaust gas line with the exhaust gas stack or the surplus gas torch and the second sensor is arranged in the common exhaust gas line. The exhaust gas line supplies exhaust gas from an internal combustion engine. The biogas line makes the connection to a recycling device for biogas, which produces carbon dioxide exhaust gas and comprises a cogeneration power plant, a fuel cell and a gas processing device. A first rinsing gas inlet is present in the fermenter within the area above the biomass. The fermenter comprises a base plate. Rinsing gas channels are intended in the base plate of the fermenter and is connected with a second rinsing gas inlet. INDEPENDENT CLAIMS are included for:    (1) a method for switching a fermenter between biogasification and composting in a combined plant; and    (2) a method for starting a fresh fermenter loaded with biomass.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic view of a combined plant for producing biogas and compost.    Fermenters (2)    Biogas outlet (8)    Biogas line (12)    Exhaust gas stack (20)    Surplus gas torch (22)    Rinsing gas inlet (24)    Exhaust gas line. (26)