In a heated and closed digester, organic matter in the sludge is degraded in the absence of oxygen. The degradation process produces a combustible digestive gas (biogas), mainly methane, which is collected in biogas tanks and converted into electricity and heat by cogeneration units. The level gauge can be used to control the feeding process of the digester, and the limit switch can be used to prevent foam from entering the gas storage equipment along with the collected methane.