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Since the King County Council adoption of the Regional Wastewater Services Plan (RWSP) in 1999, the Conveyance System Improvement (CSI) program has focused on implementing RWSP conveyance policies. The CSI program works to:

  • Use the 20-year peak flow storm as the design standard for its separated wastewater system to avoid sanitary sewer overflows
  • Ensure sufficient capacity in the regional conveyance system to accommodate planned growth
  • Include future population growth when sizing facilities

2017 CSI program Update

The 2017 CSI Program Update was based on data collected during WTD's Decennial Flow Monitoring project in the 2009-2010 and 2010-2011 wet seasons. This update culminates in a prioritized list of projects to improve capacity in the separated portion of the regional wastewater treatment system. The prioritized list provides WTD direction on near term projects for implementation and long term program cost projections for financial planning. The report also contains a summary of the entire process that led to the prioritized list of projects.

Update process and reports

Stage 1: Update planning assumptions (completed in Dec 2013)

Planning assumptions are necessary to forecast future peak flows in the sewer system. The CSI program uses planning assumptions to forecast future 20-year peak flow rates and determine where capacity improvement is needed to meet projected demands.

WTD works with the Engineering and Planning subcommittee of the Metropolitan Water Pollution Abatement and Advisory Committee (MWPAAC) and utilizes the Puget Sound Regional Council’s (PSRC) population and employment forecasts in the planning assumptions. The forecast data is reviewed with local agencies for input during the regional needs assessment discussions.

Stage 2: Complete a regional needs assessment

The regional needs assessment, or RNA, assesses the separated conveyance system to identify portions that need to be expanded to allow for 20-year peak flow in 2060. It provides the basis for identifying and evaluating alternative approaches to making capital investments to address capacity needs.

The regional needs assessment is complete and the results are in the report at the link below. As is the case with other CSI Program Update activities, the RNA includes review and input from the MWPAAC Engineering and Planning Subcommittee and the local agencies that contract with WTD for wastewater treatment services.

Stage 3: Develop conceptual projects and planning level cost estimates

The Conceptual Projects to Meet Identified Capacity Needs report is complete and available at the link below.

Stage 4: Prioritize conceptual projects

The CSI Program Update culminates in a prioritized list of projects to improve capacity in the separated portion of the regional wastewater treatment system. The prioritized list provides WTD direction on near term projects for implementation and long term program cost projections for financial planning.

The CSI Program Update report presents the prioritized list of conveyance system projects. The report also contains a summary of the entire process that led to the prioritized list of projects. The report is available at the link below.

Consistent with other CSI Program Update activities, the prioritized list of projects was shared with the MWPAAC Engineering and Planning Subcommittee and the local agencies that contract with WTD for wastewater treatment services for input.

2007 CSI Program Update

The 2007 CSI Program Update extended the planning horizon to 2050 and identified 33 projects needed for increasing capacity in the conveyance system to accommodate population growth. It also provided a status on the County's asset management program for replacing or substantially rehabilitating deteriorated portions of the system.

2004 CSI Program Update and Basin Planning

In 1999, the Conveyance System Improvement (CSI) program was charged with developing a system to identify and prioritize twelve planning areas in the wastewater service area. This enabled the CSI team to target system improvements in the basins with the most critical need and to integrate conveyance planning with 34 local agency plans and with other Regional Wastewater Services Plan (RWSP) programs. 20 projects were identified to meet capacity needs through the planning horizon of 2030.

Reports for the following basins are available upon request: City of Carnation, Green River North, Green River South, Hidden Lake, Lake Sammamish North, Lake Sammamish South, Lake Washington North, Lake Washington Northeast, Lake Washington Northwest, Lake Washington South, and Lake Washington Southeast.

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