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Combined sewer overflows (CSOs) are gaining the attention of environmentalists and regulators. Communities are taking steps to decrease the number of overflow events and to reduce the amount of pollutants discharged in these events. Like most communities in the Northwest, Mount Vernon, Washington, has a mandate to reduce CSOs to less than one event per year. Engineers, regulators, and City staff members investigated cost-effective means to implement CSO reduction programs. This paper summarizes the cost-effective CSO reduction program implemented by the City of Mount Vernon. The program involves storage, treatment, and blending as methods to achieve the desired level of less than one CSO event on average per year. KEYWORDS Combined sewer overflow (CSO), Modified Ludzack-Ettinger Process (MLE), blending PROBLEMMount Vernon, Washington, like many Northwest communities, is faced with a population explosion. The population increase is due to an influx of people desiring affordable housing, cleaner air, less traffic, and a slower-paced life than is offered by the large West Coast cities. Preserving the pristine waters that have attracted these environmentally friendly people presents challenges for the communities that are experiencing this growth. The challenge shared by all communities has been to reduce the number of combined sewer overflows (CSOs) that discharge into streams, rivers, and Puget Sound.Mount Vernon's original sewer conveyance system was developed as a combined system that discharged into the Skagit River. Mount Vernon is unique in that the older portion of the city is below the flood level of the river. As a result, all flows are pumped either to the wastewater treatment plant or are diverted to stormwater pumping stations that ultimately discharge to the Skagit River.The Washington State Department of Ecology's (Ecology's) guidelines for CSO facilities are to reduce the number of untreated events to one or less per year. Along with the CSO reduction requirements, many communities are also required to reduce the amount of ammonia discharged. Since Mount Vernon discharges to a pristine salmon stream, it has elected to reduce the ammonia in its discharge in addition to reducing its CSO events. An aging facility originally designed for 6782 WEFTEC®.06