i don't think they mean gas as in gasoline, but Biogas from a digester tank

biogas=Biogas a renewable fuel that's produced when organic matter, such as food or animal waste, is broken down by microorganisms in the absence of oxygen. This process is called anaerobic digestion. For this to take place, the waste material needs to be enclosed in an environment where there is no oxygen.

digester tank=A digester is a large tank where various biological reactions occur, usually to promote decomposition or to extract essential components. Digesters are used in different types of industrial processes such as anaerobic digestion, soda process, and sulfite process.