Development of an Underground Digester for the Production of Biogas from Organic Waste Materials

ABSTRACT The global energy crisis and environmental degradation due to fossil fuel dependency necessitate the exploration of sustainable alternatives. Nigeria, with its significant organic waste generation, faces both energy shortages and waste management challenges. This study addresses these problems by developing an underground fixed-dome biogas digester using locally sourced materials to convert cow dung and millet cob into biogas through anaerobic digestion with which the digester was designed, constructed, and performance of a low-cost was evaluated for a household-scale. Specific goals included assessing its operational efficiency and conducting an economic feasibility analysis. Methodologically, a 1.5 m³ underground fixed-dome digester was constructed, using a 1:3 ratio of cow dung and millet cob mixed with water. Key operational parameters like pH, temperature, hydraulic retention time (24 days), and organic loading rate were optimized. Daily gas readings and volume outputs were monitored over a production cycle. The system achieved a cumulative biogas yield of approximately 88.6 liters within six days, with daily outputs increasing steadily from 6.24 L to 19.4 L. Despite the absence of full gas composition analysis, physical characteristics such as odor and density indicated successful methane-rich gas generation. The digester also produced a nutrient-rich slurry suitable for use as organic fertilizer.