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The King County Department of Natural Resources' East Section wastewater service area is comprised of two major interceptors, the Eastside Interceptor (ESI) and the South Interceptor (SI). The ESI and SI flow to King County's South Plant and are both large diameter pipe, ranging in size from 30 to 120 inches (0.762 to 3.048 meters). The approximate length of the ESI is 19.5 miles (31.4 kilometers). King County staff observed periodic odors that appeared to originate from the ESI in an area immediately east of the South Plant. King County believed that the source of the ESI odors was an air pocket that formed upstream of the low-head conduit under Springbrook Creek (P-1 channel) along the east boundary of the South Plant. The air pocket is created as the wastewater level in the pipe rises and completely fills the low head conduit but does not completely fill the 108-inch (2.74 meters) diameter pipe upstream of the conduit. It is estimated that an air pocket forms as flows exceed 30 mgd (1,314 L/s).As air moves down the sewer, pressure builds up in the air pocket, forcing foul air out through the upstream manholes and the gravity laterals that discharge into the ESI. Several odor control alternatives, including activated carbon, chemical scrubbing, and biofiltration treatment located adjacent to the air pocket and across Springbrook Creek were evaluated. A foul air collection and treatment system, consisting of a foul air pipe that connects an ESI manhole upstream of Springbrook Creek crossing to a biofiltration system located on the South Plant site was selected because of lower life-cycle costs and superior aesthetics near the plant property boundary. The biofilter media selection was based on pilot testing at the South Plant. The biofilter uses recycled wastewater for irrigation. Construction of the foul air collection and treatment system was completed in the fall of 2001. Testing is being performed to confirm that this system can achieve its design capacity and prevent formation of a stagnant air pocket. Data including air velocities in sewer, hydrogen sulfide removal efficiency, air test results, and dispersion modeling results will be available for presentation at the conference. KEY WORDSLarge diameter sewer, corrosion control, biofilter, sewer ventilation.

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