Maximizing Secondary Wet Weather Treatment Capacity at a Northwest CSO Facility

The Columbia Boulevard Wastewater Treatment Plant (CBWTP) is the largest treatment plant in the State of Oregon, treating an average day flow of 66 mgd. The collection system includes both a combined system and separated sanitary system. Portland's combined storm and sanitary collection system has necessitated the City to explore strategies to reduce combined sewer overflows (CSOs) including the installation of 13 miles of large diameter pipe to collect and store combined sewage during rain events. Captured CSO1 and combined sewer system flows as high as 450 million gallons per day can be conveyed to the CBWTP for treatment and the City aims to maximize wet weather flows receiving secondary treatment. However, high sludge volume index (SVI) values, lack of wet weather operating modes and inadequate process control functionality limit the capacity of the secondary process. Secondary treatment is further limited by the existing shallow, peripheral feed secondary clarifiers. A project is currently under construction to address the short-comings of the existing secondary system and provide up-todate process control technology. The process design includes multiple wet weather operating modes to address the rapidly changing, diverse character of the wet weather flow conditions experienced at the CBWTP. This paper will summarize the process design and control system functionality that will maximize secondary capacity while maintaining sludge settleability during wet weather events.