The design of King County’s new 38-mgd Brightwater Treatment Plant calls for using chemically enhanced primary treatment (CEPT) of high flows. A full-scale CEPT simulation was conducted at King County’s South Plant during the winter of 2004/5 to evaluate the potential of this technology to achieve the levels of TSS and BOD removal required to meet permitted discharge conditions. Dosing with the dual coagulants, ferric chloride and a polyaluminum chloride compound, followed by the addition of an anionic polymer resulted in primary effluent TSS and BOD concentrations that exceeded the target 70 and 50 percent removal efficiencies, respectively. These were achieved at the projected peak hour surface overflow rate (SOR) of 3,600 gpd/ft2. At higher SORs of 5,000 gpd/ft2, primary clarifier TSS and BOD removal efficiencies of greater than 65 and 40 percent were achieved.