Biogas
Biogas is a renewable energy source that's produced by breaking down organic matter like food or animal waste in an oxygen-free environment. It's made up of methane and carbon dioxide, along with other gases.
Animal manure, food scraps, wastewater, and sewage are examples of organic matter that can generate biogas through anaerobic digestion. Due to the high methane content (typically 50-75%), biogas is flammable, and therefore produces a deep blue flame and can be used as an energy source.