Optimization of Biogas Production using Some Process Parameters in a Fixed Dome Laboratory Bioreactor

Biogas is a renewable energy that has many applications including cooking, lighting households among others. It is produced through the breakdown of organic matter in an air tight compartment through a biochemical process which is generally termed digestion. This technology involves various techniques through use of digesters or bioreactors and operational parameters which could be predicted and must be optimised. A 0.15m3 capacity fixed dome laboratory bioreactor was used to determine the effect of total solids, temperature, and substrate retention time on biogas production rate. The feedstock was cow dung from dairy cows managed under a free-range system during the day but held under a shed overnight at Egerton University, Kenya. Three different experiments were conducted in a batch feeding regime of the bioreactor. In the first one, the substrate at total solids of 6%, 7%, 8%, 9%, and 10% was digested at a constant temperature of 350C (using auto control system). The second experiment was conducted at mesophilic temperatures of 250C, 300C, 350C, 400C, and 450C using a cow dung substrate at total 8% solids. An evaluation of existing biogas production prediction models was done. A third model (named the fixed dome temperature model) was developed and tested. Biogas production rate was optimised with the help of response surface methodology – in which a central composite design was applied. An interaction of three variables namely total solids, temperature, and substrate retention time were tested at five different levels. The highest average biogas production rate was 0.48 m3of biogas per m3 of digester volume per day (m3/m3d) at 8% total solids. The highest average result of 0.52 m3/m3d occurred at 400C. Lastly substrate retention time was observed while the cow dung was at 8% total solids and 350C; and the highest average output was 0.68 m3/m3d at 11 days. Low Temperature Lagoon model and Toprak model suited the results obtained in this research. The optimum output of 0.50 m3/m3d was achieved at a level of 8% total solids, 43.410C, and 15 days. The optimal values were verified and found to be in agreement with experimental results at an admissible tolerance of 6.6-10.7%. The above conclusions can be transferred for adoption for field and industrial fixed dome digesters for biogas production into operational guidelines for biogas stakeholders including designers an operators.