An enclosed structure where the anaerobic break down of biomass takes place and usable biogas is generated. The digester has usually the form of a large, insulated, concrete, water-proof, and air-tight cylinder. The roof consists of a polyester fabric under which the generated biogas, a mixture of mainly methane and carbon dioxide, is stored. The fuel is usually organic agriculture and municipal waste and manure. The mixture is broken down by microorganisms at temperatures of around 35—60 °C, so the digester needs heating for optimal functioning. The content of the digester is agitated by agitators to prevent the formation of layers at the top and bottom of the tank and to let biogas bubble out freely. The biogas from the digester is used to generate heat and electricity in the cogeneration plant (part of the heat goes to heating the digester), while the processed organic matter is used as fertilizer.