Design, Fabrication and Evaluation of a Mini-Biogas Plant with Automatic Pressure Relief Control for Power Generation

Abstract

The growing demand for decentralised, sustainable power in Nigeria, combined with the poor management of organic waste in rural and peri-urban areas, motivates the development of small-scale, waste-to-energy technologies. This paper reports the design, fabrication and performance evaluation of a mini-biogas plant that converts cow dung and kitchen waste into electricity through anaerobic digestion, gas conditioning and a modified petrol generator, while a fully passive, electronics-free automatic pressure relief system based on an adjustable water seal protects the digester from over-pressurisation. The 1000 L high-density polyethylene (HDPE) digester was filled with 70% cow dung and 30% kitchen waste in the ratio of 1:1 of dung to water and kept under observation for 5 weeks of retention time.The pH of digester was maintained in the desired range of 6.8 to 7.5 from Week 2 onwards whereas the average digestion temperature ranged from 28.1 °C to 30.8 °C. Average biogas production was 0.92 m³/day (design target 0.5-1.5 m³/day), with a peak of 1.18 m³/day. Gas conditioning raised methane content from 58.4% in the raw biogas to 67.2% after CO₂ scrubbing, within the 60-70% target band. The water-seal pressure relief system activated within 3-4% of its calculated set point at all three tested submerged depths (8, 10 and 12 cm) and re-sealed within 4-6 seconds, confirming that safe, automatic pressure regulation can be achieved without sensors, controllers or external power. These results demonstrate that a low-cost, locally fabricated mini-biogas system with a passive safety mechanism is a technically viable route to decentralised electricity generation and organic-waste management in resource-limited settings.