When I was a kid, my dad’s euphemism for fart was ‘let wind’; as in, ‘Did you let wind?’
Just had to get that out.
Most large sewage treatment plants use a two-stage digestion system in which organics are metabolized by bacteria anaerobically (in the absence of oxygen). In the first stage, the sludge, thickened to a dry solids (DS) content of about 5 percent, is heated and mixed in a closed tank for several days. Acid-forming bacteria hydrolyze large molecules such as proteins and lipids, breaking them into smaller water-soluble molecules, and then ferment those smaller molecules into various fatty acids. The sludge then flows into a second tank, where the dissolved matter is converted by other bacteria into biogas, a mixture of carbon dioxide and methane. Methane is combustible and is used as a fuel to heat the first digestion tank as well as to generate electricity for the plant.
Pardon the bump, but I’m staring at a BloombergNEF report (no link, sorry) pegging wastewater gas at $8-26/MMBTU, compared to ~$4 (citygate).