Water Environment Federation (WEF)

The Operator’s Guide to Monitoring Secondary Clarifier Performance

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Courtesy of Courtesy of Water Environment Federation (WEF)


The State Point Analysis (SPA) procedure, formalized by Keinath, et. al. (1977), provides exceptional insight into the expected performance of activated sludge secondary sedimentation basins. Yet, the operating guidance offered by the SPA is not employed regularly and with confidence - particularly by operations personnel. Such lack of use may stem from the perceived complexity of the procedure, including the (unnecessary) need to perform graphical
constructions. The paper seeks to encourage the application of SPA principles by presenting straightforward algebraic and spreadsheet calculations for assessing the risks of clarification or thickening failure in clarifiers. Clarifier performance is monitored by evaluating a Clarification Safety Factor, based on comparing the observed settling velocity of the sludge with the overflow rate in the clarifiers. Similarly, a calculated Thickening Safety Factor provides warning of impending process limitations due to deteriorating settleability or limited return sludge pumping capacity. The
mathematical background is summarized and step-by-step calculations are detailed in the paper.

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