The City of Seattle sewer system was originally built to manage both sanitary sewage and stormwater – it was a combined system. These systems have overflow points that discharge excess flows during larger storms at sites called Combined Sewer Overflows (CSOs). Responsibility for
this system and its CSOs in Seattle is divided between two agencies – the City of Seattle, the local service provider, with its 113 CSO locations, and King County, the “wholesaler” of conveyance and wastewater treatment, with its 37 CSO sites. While these agencies have coordinated
their CSO correction efforts since they began this work in the late 1970s, they have tended to work independently, sometimes with competing priorities. But in 1991 the City of Seattle proposed a major collaboration. Seeing the linkage between the City's correction needs around Lake Union,
the County's need to correct their largest CSO, Denny Way, and the opportunity for federal construction grants, the City of Seattle proposed that the two agencies pursue a joint project. Nine years of planning, design and construction have occurred since that time and the County is now
ready to “dig dirt” on it's part of the project – a 6,200 ft. long, 14 ft 8 in. diameter storage⊘treatment tunnel, further CSO treatment facilities, and two new outfalls. Many challenges were faced during the design phase, many of which pitted competing community
values against each other. This project resulted in a new approach to the provision of primary treatment as required by Washington State regulations, initiating a “new generation” of CSO control in the State. This presentation will discuss the project and the resolution of these
challenges from King County's perspective.
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