NOVELTY - Operating a fermentation plant comprising at least one digester (1) that supplied a substrate (1a), where the substrate is percolated with a percolate (2a) involves withdrawing biogas that is generated in the digester and/or in the percolate container; treating the withdrawn biogas in a biogas treatment device (3) and where carbon dioxide (CO2)-containing exhaust gas that is generated during treatment in the treatment device (3); and introducing CO2-containing exhaust gas into the digester using a purge line (4) for removing any biogas that was generated in the digester.    USE - For operating a fermentation plant (claimed).    ADVANTAGE - The method reduces the methane loss to a minimum.    DETAILED DESCRIPTION - Operating a fermentation plant comprising at least one digester (1) that supplied a substrate, where the substrate (1a) is percolated with a percolate (2a) from a percolate container (2) involves withdrawing biogas that is generated in the digester and/or in the percolate container; treating the withdrawn biogas in a biogas treatment device (3) and where carbon dioxide (CO2)-containing exhaust gas that is generated during treatment in the treatment device; introducing CO2-containing exhaust gas into the digester using a purge line (4) for removing any biogas that was generated in the digester, where the CO2-containing purge gas is introduced into the digester such that it passes through the substrate, where a treatment device for treating biogas and/or at least one additional digester is provided, where the treatment device for the purpose of supplying CO2-containing exhaust gas generated during the treatment process via an exhaust gas line and/or the additional digester for supplying the CO2-containing biogas that was generated during the fermentation process in the additional digester can be connected with the purge line, where a port on the digester for the purge line is arranged such that during the purging process, CO2-containing gas flows through the substrate that is located in the digester.    DESCRIPTION OF DRAWING(S) - The figure shows block diagram of fermentation plant.    Digester (1)    Substrate (1a)    Percolate container (2)    Biogas treatment device (3)    Purge line (4)