Abstract: The invention relates to a method and apparatus for the production and purification of biogas. This involves in principle the production of biogas from biomass in a fermenter. The biogas is divided by means of a separation stage into a methane gas flow and a lean gas flow, and the lean gas flow is converted into heat and electrical power in a combined heat and power plant. The invention is characterised in that, by means of a bypass line which circumvents the separation stage, a variable proportion of the crude gas flow may be fed directly to the combined heat and power plant.