Sizing a Community Digester for Food Waste: Methane Yield, Reactor Volume, Cooking Duty and the Contamination Limit

Anaerobic digestion is one of the few waste treatments that returns useful energy, but community digesters fail in a predictable way: they are sized on clean-feedstock gas yields and then fed contaminated waste. This paper models methane yield by feedstock class with an explicit contamination derating, and sizes a complete community plant for a stated stream of 500 kg per day of collected food waste. The clean mixed feedstock yields 112.95 cubic metres of methane per tonne wet weight (188.24 cubic metres of biogas at 60% methane), from a weighted volatile-solids fraction of 0.2585 and a weighted methane potential of 436.9 cubic metres per tonne of volatile solids. At the design contamination of 10%, the plant produces 47.78 cubic metres of methane per day (79.63 cubic metres of biogas), or 390.6 kWh of net thermal energy per day after an 18% parasitic heating load, covering the full cooking need of about 119 households. The reactor requires 36.7 cubic metres working volume (43.1 cubic metres total) at a 28-day retention time and 10% slurry solids, with 47.8 cubic metres of gas storage to cover the evening cooking peak, at an organic loading rate of 3.17 kg of volatile solids per cubic metre per day. Digestate provides about 1,138 kg of nitrogen, 500 kg of phosphate and 727 kg of potash per year, worth $2,484 per year at stated fertiliser prices. Annualised economics give a net benefit of $4,291 per year at design contamination, falling to $539 per year at 20% contamination and turning negative beyond a break-even contamination of 21.6%. Above that level the plant stops being worth running at the stated prices and should be fixed at the collection point or stopped, not subsidised to digest plastic and grit. SIMULATED, NOT MEASURED: all yields, volumes and values are model outputs at stated parameters.