A novel method reduces methane slack when operating a biogas fermenter. When starting up a freshly charged fermenter, the methane portion of the produced biogas is initially so low and the portions of carbon dioxide and nitrogen are so high that the biogas cannot be directly used in a combined heat and power plant. Conventionally, the biogas generated during the startup phase has a small portion of methane that is discharged directly into the atmosphere or is flared off if the methane fraction is larger. The initially produced methane is consequently not used and becomes methane slack. To reduce methane slack, the biogas with the low methane fraction is fed to a gas treatment unit in which non-methane components of the gas mixture are partially separated, and the remaining gas mixture with a higher methane content is returned to the biogas fermenter until the methane fraction is sufficiently high.