The residence time in a digester varies with the amount and type of feed material, the configuration of the digestion system and whether it be one-stage or two-stage. In the case of single-stage thermophilic digestion residence times may be in the region of 14 days, which compared to mesophilic digestion is relatively fast. The plug-flow nature of some of these systems will mean that the full degradation of the material may not have been realized in this timescale. In two-stage mesophilic digestion, residence time may vary between 15 and 40 days. In the case of mes­ophilic UASB digestion hydraulic residence times can be (1 h-1 day) and solid retention times can be up to 90 days. In this manner the UASB system is able to separate solid and hydraulic retention times with the utilization of a sludge blanket.

Continuous digesters have mechanical or hydraulic devices, depending on the level of solids in the material, to mix the contents enabling the bacteria and the food to be in contact. They also allow excess material to be continuously extracted to maintain a reasonably constant volume within the digestion tanks.