Performance Evaluation of a Repurposed LPG Gas Cylinder as a Compact Household Anaerobic Digester

Abstract

Access to clean and affordable household energy remains a pressing challenge in many low-income regions, where dependence on firewood and fossil fuels contributes to deforestation, indoor air pollution, and greenhouse gas emissions. Biogas technology offers a renewable and decentralized alternative, yet the affordability, durability, and safety of small-scale digesters remain barriers to widespread adoption. This study presents the design, construction, and performance evaluation of a small-scale biogas digester repurposed from a decommissioned 50 L Liquefied Petroleum Gas (LPG) steel cylinder. The conceptual design incorporated an airtight slurry inlet, digestate outlet, and gas outlet fitted with a pressure relief valve, non-return valve, and gas purification system (moisture trap and H₂S scrubber). To enhance durability, the interior was coated with food-grade epoxy resin, and the vessel was insulated with polyurethane foam to maintain mesophilic conditions (30-40 °C). Engineering analyses guided reactor volume sizing, retention time (20-30 days), biogas production estimation, thermal insulation design, and pressure safety limits. The construction process emphasized leak prevention and corrosion resistance, while experimental testing was conducted over 30 days using cow dung and kitchen waste at a 1 : 1 feedstock-to-water ratio. Daily monitoring recorded slurry temperature, biogas yield, and methane concentration. Results showed cumulative biogas production of 268.6 L (0.537 m³·kg⁻¹ Volatile Solids (VS)) and methane yield of 163.3 L (0.327 m³·kg⁻¹ VS), corresponding to an energy output of ~11.7 MJ·kg⁻¹ VS (3.25 kWh·kg⁻¹ VS). Methane concentration increased steadily from 54% to 66% during the first 15 days, stabilizing thereafter before declining in the final phase due to substrate depletion. Statistical analysis indicated a positive correlation between slurry temperature and daily gas yield, confirming the importance of thermal regulation. The findings demonstrate that repurposed gas cylinders can provide a low-cost, portable, and pressure-rated solution for decentralized biogas production.