Design, Fabrication and Assembly of a Biogas Digester for Sustainable Bioenergy and Bio-Fertilizer Production from Cow Dung.

Abstract:

The demand for renewable energy is on the increase in the sub-Saharan Africa due to the escalating cost of fossil fuel and unreliable conventional power supply from national grid. The technology is gaining recognition and adoption but the process of biogas production from this waste resources is still at its experimental stage in most developing countries. This study therefore presents a comprehensive engineering design, fabrication, and assembly of a biogas digester for bioenergy and bio-fertilizer production. This system consisted of a low-cost, scalable biodigester suitable for decentralized bioenergy generation and high-quality biofertilizer production. A 1000-litre capacity Intermediate Bulk Container (IBC) was made of High density polyethylene (HDPE) designed for use in the anaerobic digestion of biological wastes. The IBC container was repurposed as the main digestion chamber due to its structural durability, chemical resistance, and availability as recycled industrial packaging. The engineering design process incorporated a range of analytical calculations, including mechanical, hydrostatic, and biochemical equations, which were applied to determine the biodigester's structural feasibility and performance potential. The IBC container was repurposed as the main digestion chamber due to its structural durability, chemical resistance, and availability as recycled industrial packaging material. The engineering design process incorporated a range of analytical calculations, including mechanical, hydrostatic, and biochemical equations, which were applied to determine the biodigester's structural feasibility and performance potential. The digester system includes polyvinyl chloride (PVC) piping as flow channels, gas collection ports, and a water-based bubbler (scrubbers, gas storage, and mixing component). This system design was guided by multidisciplinary principles and integration of concepts from fluid mechanics, material science, biochemical kinetics, and structural engineering to ensure both technical efficiency and structural stability. The biodigester system was designed as a continuous-feed type, above-ground anaerobic bioreactor. The approach incorporates mechanical stress evaluations, hydrostatic calculations, and gas pressure considerations in order to deliver a robust and efficient system suitable for decentralized, rural, or small-scale energy production. The biodigester inlet pipe area was 0.00196m2 and average volumetric flow of cow dung slurry was 4.92×10-7 m3 /s. The slurry was formed using a 1:2 mixture of cow dung and water, resulting in a dilution of approximately 8–10% Total Solids (TS). The digester was filled to a volume of 850 litre mark, with 1:2 cow dung-water slurry, while the 150 litre space was meant for gas collection. The slurry was left inside of the biodigester for a 20 days retention time. The average fluid velocity when total flow rate is spread evenly across the entire cross-sectional area of the tank was 4.1×10-7 m/s. The inlet pipe was inserted vertically into the tank, up to a minimum insertion depth of 70 cm to facilitate smooth flow, while the outlet pipe was installed at the bottom corner (with a valve) to facilitate efficient removal of digestate.