Water Environment Federation (WEF)

A Site-Specific Tool for Optimizing Final Clarifier Design and Operation

The final clarifier is one of the most important unit processes and often determines the capacity of a treatment plant. Clarification is a solids separation process, which results in the removal of 99 percent or more of the suspended solids (biological floc) received from the activated sludge system. The separated solids are transported to the bottom of the clarifier and removed as return activated sludge (RAS). The clarifier failure can lead to solids carryover resulting in a reduction in solids retention time (SRT) below that required for meeting process goals. The State Point Analysis (SPA) is a practical tool that can be used to perform a 'what if' analysis based on sitespecific data. Consequently, it enables engineers to examine clarifier behavior under various operating scenarios during the design phase, lower design safety factors and avoid over design, size clarifiers in conjunction with the biological process, and achieve cost savings. Likewise, using the SPA approach operators can predict impending problems early, implement corrective measures in a timely fashion, and adapt to upstream changes in the biological process.

The Valrico Advanced Wastewater Treatment Plant (AWTP), is a biological nutrient removal (BNR) facility owned and operated by Hillsborough County, FL. The plant is being expanded from 6.0 to 12.0 mgd (AADF) with provision for the ultimate 18 mgd facility. Included in the expansion are new oxidation ditches and final clarifiers. In an effort to realize capital cost savings without compromising flexibility and functionality, the SPA was used as a design tool to assess clarifier performance under various operating scenarios resulting in optimum clarifier size for the expanded plant. This paper outlines the SPA approach and its role in optimizing the design and operation of final clarifiers based on site specific data.

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