Henderson / MLK Combined Sewage Overflow (CSO) Tunnel

This is a combined sewage overflow "CSO" facility that includes two multi-story cast-in-place concrete structures (the Inlet Regulator Structure "IRS" and the Outlet Regulator Structure "ORS") with a 3,100 lineal foot tunnel below south Beacon Hill connecting them. During storm conditions the tunnel is designed to hold up to 3.5 million gallons of combined storm water and sewage. This prevents the untreated wastewater from discharging directly into Lake Washington. Once storm conditions have subsided the wastewater is discharged into King County's sewage conveyance system for final treatment.