Henderson/M.L. King CSO ProjectInside the combined sewer storage tunnel
The CSO tunnel runs 3,100 feet under Beacon Hill and is King County 's second largest storage facility.
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The outlet regulator is designed with an environmentally friendly "green roof" that collects and reuses stormwater runoff and keeps it from entering the combined sewer system. | The underground storage tunnel has an inside diameter of about 15 feet. It runs two-thirds of a mile to the south under Beacon Hill at depths of 30 to 100 feet beneath 42nd Avenue South. 
View project alignment.
The tunnel can hold and treat up to 4 million gallons of combined wastewater and stormwater. Temporarily storing combined sewage in the tunnel provides time for storms to pass and flow levels to subside in pipelines throughout the King County wastewater system. Similar underground storage facilities are also located under Denny Way just north of downtown Seattle and at Alki Point in West Seattle. After flows have decreased and the system's regional wastewater treatment facilities have enough capacity, all flows that have been stored and treated in the new Beacon Hill tunnel are drained through an outlet regulator at the south end of the tunnel. The regulator is in the Norfolk Industrial Park at 42nd Avenue South and South Norfolk Street. As the flows are drained from the tunnel, they are conveyed west in 72-inch pipelines under Interstate 5 and rail lines to Airport Way South. From there, flows can be sent to either the West Point Treatment Plant in Seattle's Magnolia area or the South Treatment Plant in Renton, depending on existing capacities and system needs. In rare occasions after severe storms, and if the region's two treatment facilities are at capacity, treated flows could also be safely released to an outfall in the Duwamish Waterway.
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