A Prototype of Anaerobic Biomass Digester for Biogas Production

Abstract

In this work, an anaerobic digester with semi-automated control was built for the purpose of understanding the production of biogas. Classical thermodynamics concepts were applied to the study of microbial behaviour in anaerobic digestion of an organic fraction of waste (OFW) for biogas (CH4) generation. A 1000-litre plastic tank was used to design and assemble a prototype single-stage batch anaerobic digestion system which was operated and controlled by a computer via instrumentation subsystems. The factors considered in the design and operation include the type of organic material used as feedstock, temperature of operation of the digester, ease of construction and digester portability. The assembled digester was operated as a bench system for a cycle of 24 days under mesophilic temperature conditions. The maximum and minimum temperatures recorded during the operation of the digester were 38.5 oC and 32.4 oC respectively. The average operating temperature was 36.1 ± 1.5 oC. A slurry of fresh cow dung with calorific value of 26.45 MJ kg-1 was used as substrate for the study. A motorized stirrer and a thermostatically operated heat exchanger system were used to ensure that there were no temperature and concentration gradients during the operation of the digester. The final composition of the biogas produced was 60.2 % CH4 and 39.7 % CO2, with the highest daily gas production of 0.474 m3, which was recorded on the 18th day. . The performance of the digester is deemed satisfactory and confirms that this pilot-scale prototype is a valid proof of concept and can therefore be used as the basis for the design and assembly of production-scale digester systems.