BioCycle, Feb. 2017, by Craig Coker and Gene Smith.
https://www.biocycle.net/2017/02/14/digester-mixing-fundamentals-2/

"Anaerobic digestion (AD) is a biological manufacturing process where waste materials such as food wastes, animal manures and sewage sludges are converted, in the absence of oxygen, to products such as biogas and digestate. AD is traditionally a liquid process; that is, it often occurs in sealed reactors processing a thin liquid with a 93 to 99 percent moisture content.... Mixing maintains integrity of the decomposition process by improving circulation and consistency, and minimizing volumes of the scum and grit layers."