Development of an anaerobic digestion unit for biogas production from cow dung substrate

An anaerobic digestion unit for producing biogas from cow dung in the rural communities was designed, fabricated and tested for performance, durability and throughput. The major components of the digester included the substrate holding tank, tank cover, agitator, debris collector, inlet and outlet pipes, gas reception tank, hose and heat source. The digester is a vertical cylindrical tank with an inlet pipe for the introduction of substrate and an outlet pipe to collect the digested substrate. An agitator is incorporated inside the digester to break scum on the substrate and create uniform temperature profile in the digester while a pressure gauge was fitted to the gas outlet valve to measure the gas pressure in the tank. The agitator shaft is extended outside to be driven by an electric motor through belt and pulley system. At the bottom of the digester are two collectors which serve as a reservoir for stones and other inorganic materials that might be present in the substrate. The criteria considered in the design of the digester included air tightness of the system, mesophilic and thermophilic temperature, nature and type of substrate used, substrate retention period, number of crank turns per minute, and volumetric capacity of the digestion tank. Other considerations included the desire to make the digestion tank and gas reception tank of galvanized steel to ensure good quality of the product and the need for a strong structural support to ensure structural stability of the system. After construction and assembly, the biogas digestion unit was tested with 40 kg of cow dung diluted with 80 kg of water and subjected to a retention period to make a substrate (slurry) of 10 % total solid (TS). Daily gas yield was determined; gas pressure in the tank was measured the pressure gauge, while the ambient temperature was taken at 5 hours interval. Results showed that a cumulative gas yield of 0.415 litres after 22 days retention period at average substrate temperature and pH of 29 degrees C and 6.2 respectively. The digester has a substrate holding capacity of 330.8 litres and a production cost of $375 with all the construction materials being available locally.