The elemental balance of microbial growth was used to det. the biogas yield and the CH4 concn. in biogas under anaerobic digestion. The CH4 yield is directly related to the deg. of reduction of substrate(s), 50% (from carbohydrates) to 75% (from lipids), and to biomass yield. The av. biomass yield was estd. at 0.17C-mol/C-mol. The anaerobic digestion model No. 1 was used to exam. the effect of feedstock compn. on the kinetics of methane formation. Simulations showed carbohydrates to degrade faster and lipids to make most methane. 22 refs.