Anaerobic Digester

An anaerobic digester is a sealed vessel where livestock manure is broken down by bacteria that work without oxygen, which is what anaerobic means. Two things come out: biogas, a methane-rich fuel that can be burned for heat, light or power, and digestate, the spent manure, which still holds the plant nutrients that went in. It also removes most of the odour and most disease organisms.

What is really kept alive is the culture of bacteria; the vessel is only its housing, so temperature defines the practice. A mesophilic digester runs at 35 to 41 C (95 to 105 F) and holds each batch about 20 days; a thermophilic one runs near 57 C (135 F) and needs 5. It is charged with manure and water mixed to keep solids near 5 percent; measured output on such units averaged about 223 litres of gas per person per day at 56 percent methane.

Gas is not fit to burn as it comes off: hydrogen sulphide on working farms ran several times above the 100 to 500 parts per million an engine tolerates, and it forms an acid that eats copper, brass and steel, so a cleaning stage stands between vessel and appliance. The chemistry fails before the hardware: overfeeding easily fermented material lets acids build faster than the methane-making bacteria consume them, the pH falls, and gas can stop weeks after the overload, not at the time of it. Most recorded failures are managerial rather than technical: over- or under-feeding, no supervision. Upkeep is daily charging, weekly checks of the safety valve and outlet tubes, shade against sunlight that degrades plastic and a fence against stock; regulators, valves and sensors are consumables on a two-year cycle in a vessel built to last 25. The digestate holds the same nitrogen, phosphorus and potassium that went in, but more of the nitrogen is now ammonium: readier for plants, and readier to escape into the air unless worked into the soil.