Development of a biomass digester for domestic use

Renewable energy systems

This study presents the development of a 250-litre high-density polyethylene biogas digester integrated with a multi-stage purification system, designed for decentralised household energy applications. The system utilised 92.5 kg of fresh cattle dung mixed with 49.5 kg of water (1.87:1 ratio), maintaining a 165-litre working volume (66% capacity) under mesophilic conditions (26-32°C). A comprehensive monitoring protocol was implemented, recording temperature, pressure, and daily biogas production using a calibrated pressure gauge and measuring the volume of the flexible storage chamber. The digestion process exhibited characteristic phases of microbial adaptation, with biogas production reaching 140 litres on day 26, representing the system's peak daily output. The three-stage purification unit effectively treated the raw biogas by using iron fillings for the removal of hydrogen sulphide, water scrubber for ammonia and silica gel for absorption of moisture, producing fuel suitable for domestic applications. System performance was enhanced through several design innovations: (1) a reinforced gas holder capable of sustaining pressures up to 236 kPa, (2) optimised slurry mixing through periodic manual agitation, (3) the addition of calcium carbide as a catalyst increases the methane content of the produced gas, and (4) integrated safety features including pressure relief valves and a separate gas storage system from the digester. The purified biogas demonstrated consistent combustion characteristics in standard cooking tests, with a rich blue flame indicating a high burning temperature. These results validate the technical feasibility of small-scale anaerobic digestion systems using locally available materials. The study provides empirical data on operational parameters and production time lines that can inform future designs. While demonstrating successful biogas generation, the work also identifies opportunities for improvement through enhanced mixing mechanisms and temperature control. This research contributes to the growing body of knowledge on appropriate renewable energy technologies for rural communities, offering a model for sustainable waste-to-energy conversion.

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