Simplified Mathematical Model for Quantitative Analysis of Biogas Production Rate in a Continuous Digester

Experiments were conducted in a horizontal continuous biodigester constructed of 12" ID PVC pipe of 4m, 5m, and 6m length to produce biogas from cow manure slurry. Mathematical models were developed based on a simplified mechanism of anaerobic digestion. The purpose of this attempt was to analyze the digestion phenomena quantitatively and objectively in order to make quick decisions in the optimization of the installed biodigesters in the field. The data from field measurements were used to fit the mathematical model for predicting the rate of biogas production and the selectivity of methane production over carbon dioxide formation. The mathematical models described in this paper were reasonably good to fit the experimental data. Simulation using the model led to more systematic field trials to improve the biodigester performance. The analysis resulted in two useful hints for the practical improvement of the biodigesters. Firstly, the selectivity of methane over carbon dioxide was significantly affected by the ratio of water and manure in the slurry. Secondly, the conversion of the organic matters into biogas could be increased by recycling a portion of the biodigester effluent.