The Sacramento Regional Wastewater Treatment Plant (SRWTP) treats primary and thickened waste activated sludge in seven conventional shaped primary anaerobic digesters as part of a 181 mgd high-purity oxygen activated sludge plant. In 1990, the current digestion system reached its theoretical design capacity. Historically, the solids processing train has been troubled by uncontrolled Nocardia growth resulting in severe foaming problems at the digesters. The project consists of upgrading six existing digesters and the construction of three new digesters. The design is intended to achieve foam control and optimization of the anaerobic digestion process. Upgrades to existing digesters include new feed and withdrawal systems and retrofitting existing floating covers to submerged fixed covers. The new digester covers are of the submerged fixed type. The new sludge mixing system uses mechanical draft tube mixers. Continuous feed and withdrawal will be used to operate the new and existing digesters.