The City of Portland, Oregon is installing two large CSO interceptors that will increase significantly the peak flow to the City’s Columbia Boulevard Wastewater Treatment Plant (CBWTP). The capacity of the secondary treatment plant is limited to 380-455 ML/day (100-120 mgd). Flows in excess of this are directed to 50-year old primary clarifiers referred to as the wet weather primary clarifiers (WWPC). The peak flow is anticipated to rise from 1,325 to 1,700 ML/day (350 to 450 mgd). The historical performance of the WWPCs is poor at high surface overflow rates (SORs). Chemically enhanced primary treatment using ferric chloride and anionic polymer was investigated to improve the performance at high storm-flow induced flows. TSS removals of up to 90 percent were achieved at surface overflow rates (SORs) up to 2,000 gpd/ft2. The benefit of CEPT is that capital expenditures on new infrastructure are minimized.