King County, Washington is currently designing their new 38 million-gallon-per-day (mgd) Brightwater Wastewater Treatment Plant to include membrane bioreactors (MBR) for secondary treatment, combined with enhanced primary treatment for peak flow management. Enhanced primary treatment was of interest as a method of reducing MBR size for Brightwater. The County initially selected ballasted sedimentation for the peak flow primary treatment role. Chemically enhanced primary treatment (CEPT) using conventional primary clarifiers was not initially considered a viable alternative due to low surface overflow rates assumed necessary for adequate performance. A review of reported results from CEPT operation at several plants on the west coast and King County’s own past pilot efforts yielded little useful data at high SORs. However, substantial capital cost savings would be realized if CEPT proved successful at SORS of 3,500 to 5,000 gallons per day per square foot (gpd/ft2 and gpd/sf). In a separate investigation, the County was interested in the potential of CEPT to contribute to additional capacity at their East Section Reclamation Plant (South Plant). Recognizing the limited data for CEPT at high overflow rates, the County undertook further full scale pilot testing to understand both potential performance at high overflows and test new chemical blends that had been reported to significantly improve CEPT performance. Full scale testing conducted at the South Plant over the winter of 2004/2005 demonstrated BOD removals above 40 percent at surface overflow rates (SORs) of 5,000 gallons per day per square foot and above 60 percent at SORs of 3,600 gpd/ft2. Based on this outstanding performance at high surface overflow rates, the County found that CEPT was cost effective for its Brightwater Plant and selected it in lieu of ballasted flocculation. The County is currently considering the potential to re-rate South Plant to recognize significant capacity through use of CEPT for peak flow treatment. CEPT, with use of modern chemical mixtures and proper chemical dosing protocols, makes sense for intermittent usage during high flow events for a number of potential applications. Plant re-rating using the technology could avoid the construction of new treatment units in many instances.