Septic Digester

A septic tank holds a house's wastewater, toilet water included, in a sealed vessel where bacteria that live without oxygen break the solids down. That process gives off methane, the main gas in natural gas. A septic digester is such a tank built or fitted so the methane is collected and piped away to burn rather than vented and lost.

The gas is the part that disappoints: human waste alone does not feed the bacteria well enough to yield a useful amount, so every documented version takes a second organic feedstock into the same vessel, classically manure from an animal pen beside the latrine, in a vessel of 6 to 8 cubic metres that holds the slurry more than 30 days. Raw biogas is not ready to burn indoors: it can carry hydrogen sulphide at 0.1 to 5.0 percent by volume, corrosive and poisonous, so it is stored and cleaned before reaching an appliance. The liquid half still depends on the ground. It drains to a buried field of pipes that wants gently sloping permeable soil, neither strongly sandy nor clayey, with deep groundwater; rock at the surface, sticky clay or a hard layer rules a site out.

Treatment and gas fail together, because both rest on the same bacteria. Grease, paints, solvents and pesticides poured down a drain kill the culture; solids left to build up wash out and clog the drainfield until sewage surfaces or backs up indoors. Pumping runs on a schedule set by tank size and the number of people using it, in years rather than months, and the outlet filter is cleaned every two or three. Done properly, the effluent leaves with most parasite eggs destroyed. The hazard that kills people is separate. This is a confined space holding methane, explosive at small concentrations near a flame, and hydrogen sulphide, dangerous to life at 50 parts per million and stirred out of the liquid by any disturbance. Building or servicing one is licensed, permitted work.