Sludge digestion is accomplished by the anaerobic process. This anaerobic digestion occurs in the three covered digesters located southeast of the operations building. Sewage solids (sludge and scum) are collected in the primary sedimentation tanks and pumped to the digesters. Similarly, waste activated sludge and scum collected in the secondary sedimentation tanks and thickened in the sludge thickener is also pumped to the digesters. Waste activated sludge and scum from the secondary sedimentation tanks may be directed to the headworks or the digesters when the sludge thickener is not in operation.
After the solids have been stabilized sufficiently by the digestion process and are no longer offensive in odor, the solids are held in a sludge holding tank until subsequent dewatering by the belt press.