High Purity Oxygen with Flexible Feed Distribution Provides Enhanced Treatment Capability for Combined Sewer Flows

The King County Wastewater Treatment Division's West Point Treatment Plant serves a combined sewer system located primarily within the City of Seattle. The plant was originally constructed as a primary treatment plant in the early 60's. Design to upgrade the facility to secondary was begun in the mid 80's. Due to a number of design constraints, chiefly the limited footprint available and the need to keep odors to an absolute minimum, it was decided that a high purity oxygen activated sludge process should be used. Although the inclusion of flexible feed distribution was an accepted operational strategy for conventional activated sludge treatment plant design (WEF MOP OM-9), it had not been a common strategy for high purity oxygen facilities. However, based on input from County operational staff, this operational flexibility was ultimately provided.Normal operation involves running the secondary in plug flow mode. Operations staff carefully monitor secondary effluent turbidity for signs of solids washout. When a combination of rising plant flow and a slow-settling secondary sludge occurs (SVIs commonly exceed 150 ml/g), the primary effluent feed points are moved further downstream in the aeration trains. This change creates an area of reaeration (RAS only), sheltering a portion of the secondary biomass and temporarily decreasing solids loading to the secondary clarifiers. When the event passes, crews switch back to plug flow mode.This paper will fully discuss the use of this feed flexibility and other control strategies for dealing with CSO flows and the resultant plant performance.