An assessment of the performance of a biogas digester when insulated with sawdust


Patrick Mukumba*, Golden Makaka, Chipo Shonhiwa

Physics Department, University of Fort Hare, P. Bag X1314, King Williams Road, Alice, 5700, South Africa

Abstract: Biogas is mixture of gases, predominantly methane and carbon dioxide, produced by anaerobic digestion. In biogas production temperature of slurry is an important parameter that affects the rate of biogas production. The biogas digester was fed with cow dung before it was insulated. Ambient, biogas and slurry temperatures were measured. The temperature sensors were connected to the CR1000 data logger. Results were collected and compared before the construction of the second wall of the digester for the insulation. There was a strong positive relationship between slurry and ambient temperatures (R2= 0.882). The results showed the interdependence of slurry, biogas and ambient temperatures. The magnitude of biogas temperatures was slightly different from slurry temperatures because of the differences in the thermal conductivity properties of concrete and brick wall. The biogas was in contact with concrete dome with a thermal conductivity of 0.2 W/ (m.K) while the digester slurry was in contact with the brick wall with a thermal conductivity of 0.8 W/ (m.K). The biogas digester that was not insulated produced biogas of average methane yield of 38%. The second wall for the biogas digester was constructed and sawdust with a thermal conductivity of 0.08 W/ (m.K) was put in the gap between the inner and outer walls of the biogas digester. The sawdust insulated digester produced average methane yield of 50% when fed with cow dung. The results showed that insulation of the surface digester improves methane yield. However, insulation does not give 100% slurry temperature stability even if the digester is built underground.

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Keywords: Digester, Cow Dung, Temperature, Biogas, Insulation, Methane

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