Raw sludge from the primary or secondary clarification steps is very wet and becomes putrid on holding. It would be expensive to haul away with so much water present. Anaerobic digestion results in loss of roughly half of the solids, and digested sludge is more dense than raw sludge. Little mixing and no aeration are required during digestion. While this suggests that an anaerobic step for waste treatment should be inexpensive, very large vessels are required because digestion is slow. Methane evolved during digestion is collected and burned for heating; the digester must either be a pressure vessel or have a floating lid that holds gas at a pressure sufficient to compensate for the weight of the lid.
Holding sludge under anaerobic conditions results in a succession of microbial growth and death cycles where the decay of cells provides nutrients for the next generation.
The sketch on the right shows more gas. Note that the green color in the seal that represents water is depressed on the higher pressure side. Note also that the digester is stratified into the layers labeled on the left sketch. Adding or removing sludge through the pipe labeled sludge I/O provides a small amount of mixing.