4U3/6U3. 2. Schematic of WWTP. The Primary Sludge and Thickened WAS are mixed and directed to an anaerobic digester.. 4U3/6U3 - Class 20. From: Grady, Daigger and Lim. 3. What is An Anaerobic Digester?. Essentially a chemostat, or a bioreactor where the HRT is equal to the SRT. This is one of the reasons to thicken the solids previous to digestion. Minimize required volume.