A simple model was developed linking biogas production to digester volume, retention time,
feedstock addition rate and composition and air temperature. The retention time depends on the
required rate of biogas production, which strongly depends on the ambient temperature. The model
was constructed in simple terms to limit the data requirements because little information is available in
rural areas of developing countries.
Calculations were carried out using Mathematica® software [Wolfram Research, Champaign, IL
(worldwide headquarters); Oxfordshire, UK (European headquarters)] with a dome digester in the province
of Hanoi in northern Vietnam used as a case study. The aim was to achieve constant biogas production
throughout the year at the level achieved in the summer. The calculations made the following assumptions:
1. Animal slurry is a mixture of manure and water mixture. The composition is determined by the
manure:water ratio.
2. The heat capacity of the slurry is equal to that of water.
3. The density of the slurry is equal to that of water.
4. The temperature of the input slurry is equal to the ambient air temperature [19].