Seattle Public Utility’s (SPU’s) wastewater collection system is one of the largest in the State of Washington. It includes both sanitary sewers and combined sewers servicing more than 1.3 million people, including residents of most of Seattle and parts of Northern King County. Sanitary sewage and wastewater combined with stormwater collected from within the SPU service area are directed to large-scale conveyance and treatment facilities operated by King County’s Wastewater Treatment Division. These treatment facilities are designed to provide treatment for flows up to 440 million gallons per day (mgd). However, during large storm events, the contribution from the combined sewer system sometimes exceeds the capacity of the sewer facilities to convey the stormwater and sewage to King County’s conveyance and treatment facilities. During these events, combined sewer overflows (CSOs) can occur. SPU currently maintains around 90 CSO facilities while King County maintains around 37 CSO facilities.
King County has already been using DHI’s MOUSE model for several years to simulate the hydraulics in its separated system. The King County systems include over 353 miles of pipes and 61 pump and regulator stations conveying wastewater from the 34 cities and sewer districts that it serves to the appropriate treatment facilities. In prior modeling by King County, wet weather response models were prepared and calibrated for 146 modeling basins in the separated system to assess and characterize inflow and infiltration contributions and have been used to prioritize capital improvements and evaluate wet weather management alternatives to reduce CSOs. The current modeling effort is in the combined sewer system.
Since SPU’s combined sewers are connected to King County’s large-capacity conveyance facilities, the existing and proposed CSO controls considered by either agency can have impacts reaching into both agencies systems. The model being built should allow for a shared understanding and technical analysis of the system, the impacts of proposed projects, and also for exploration of projects benefiting both agencies.
In an effort to coordinate activities, SPU has recently purchased software licenses of MIKE URBAN for use on a joint project with King County. This project involves the development of a system-wide model of the King County trunk network which serves as the backbone of the City of Seattle system. The objective is a better understanding of both agencies’ system operations aimed at CSO reduction as mandated by their state and federal regulators.
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