This type is constructed underground based on standard construction designs using local materials including quality bricks, sand and cement among others. The plant comprises of a mixing tank through which feedstock is introduced into the digester. A digester is an air-tight chamber where the biogas formation process takes place under anaerobic conditions. The biogas is pipped off to points of application. The residue, a high-quality organic manure, is expelled through the expansion chamber ready for use.