When anaerobic digestion takes place, organic materials – think food waste – are broken down by microorganisms in an oxygen free environment, producing biogas.
Described by the U.S. Department of Energy as a proven energy source both in the U.S. and around the world, biogas can in turn be processed, purified and turned into biomethane.
The DOE describes biomethane as being “pipeline-quality” and “fully interchangeable with conventional natural gas.”
read more at: https://www.cnbc.com/2017/05/11/the-business-turning-sewage-sludge-into-fuel.html