Household Biogas from the Waste a Family Actually Has: An Honest Energy Budget and a Digester Design for a Six-Person Household with Three Cattle

A fixed-dome household biogas digester (2.48 cubic metres, 40-day retention) is designed from the waste a stated six-person household with three grazing cattle actually produces: 18.0 kg collectable dung and 1.5 kg kitchen waste per day, 2.93 kg volatile solids per day. Calculated production is 213 cubic metres of biogas per year, covering 53% of cooking demand averaged over the year, 61% in the warmest months and 38% in a cold spell. Full coverage would require about six grazing cattle in summer and about ten in winter, or an extra 4.5 kg per day of co-digested waste in summer. Simple payback against LPG is about 5.1 years before family labour is valued. Design status: SIMULATED, NOT MEASURED. All numbers are reproduced by the accompanying calculation script and its saved output.