Existing plant capacity at Plant No. 2 of Orange County Sanitation District (OCSD) is limited by sludge bulking, which in turn limits the solids and hydraulic loads that can be applied to the secondary clarifiers. To address this issue, Orange County Sanitation District (OCSD) conducted an aeration basin selector zone feasibility study to evaluate the potential performance improvements and associated costs of implementing anaerobic selectors at the Plant 2 HPO secondary treatment system. An evaluation of alternatives for improving the performance and efficiency of the high-purity oxygen (HPO) activated sludge secondary treatment system was completed. Subsequently, the plant operations staff has made operational changes which resulted in significant improvements in sludge settling quality. These operational changes places the plant at a unique position where low solids retention time (SRT) is used as the means for controlling sludge bulking without impacting the plant performance. INTRODUCTIONPlant No. 2 uses a high-purity oxygen (HPO), activated sludge system for secondary treatment. The plant is divided into two trains, the East Train and the West Train. Each train includes four aeration tanks and six secondary clarifiers. Each train also has a separate secondary sludge pump station. The aeration tanks in the West Train are currently in operation, and the secondary clarifiers in both trains are operating.