Projects & Programs
Design of Combined Sewer Outfall Solids Removal System, Washington
Intermittent but persistent loading from combined sewer overflows (CSOs) and stormwater drains has been identified as a primary cause of sediment contamination in the waterways around Puget Sound. The objective of the Henderson/M.L. King project was to completely eliminate CSO loading to Lake Washington by collecting and rerouting the CSOs to treatment plants. However, due to limitations in the available conveyance capacity, there would be occurrences of CSO discharges to the Duwamish River during peak storms. To ensure that these discharges would not cause recontamination of the Duwamish River sediments, King County proposed treating the overflows into the Duwamish River using specially designed storage devices. Two designs were considered. One was a large rectangular basin that functioned similarly to a conventional primary clarifier, and the other was a tunnel with overflow weirs near the outlet allowing it to function as a storage and clarification device. PNNL staff, conducted performance modeling of the selected alternative to optimize the design and ensure that the proposed design would function through various CSO loading conditions. The computational fluid dynamics (CFD) code PHOENICS was used to generate hydrodynamic response of the clarifiers during storm events. Motion of the CSO solids through the rectangular clarifier and the tunnel configuration was simulated using the particle-tracking model PARTRAK. Impact of the resulting effluent discharge to the Duwamish River was then modeled using the SEDCAM/METSED model. The selected final design ensures that sufficient sediment and associated contaminants are removed from the CSOs such that long-term accumulation of contaminants in the sediments will not occur in the vicinity of the outfall in the Duwamish River.
Project Highlights:
- A three-dimensional CFD model was set up to simulate performance of candidate designs of the proposed clarifiers
- The model allowed dynamic evaluation of clarifier efficiency (performance during a storm event) and helped support selection of an optimum design
- The selected design ensured that sufficient sediment and associated contaminants are removed from CSO such that sediments near the outfall will not be recontaminated.