The causes of high effluent suspended solids (ESS) from secondary clarifiers are often incorrectly identified resulting in improperly focused remedial actions that do not yield the desired results. In this paper we demonstrate a structured diagnostic approach that allows relatively rapid problem isolation and resolution. A sequence of tests, procedures or observations are described that allow problems to be classified into one or more of the following categories: (1) denitrification or rising sludge, (2) high sludge blankets, (3) flocculation or breakup problems, and (4) hydraulic problems. The following tools are used in the approach: (1) direct observation, (2) blanket detection, (3) measurement of sludge thickening properties, (4) solids flux or state point analysis, (5) dispersed suspended solids (DSS) test, (6) flocculated suspended solids (FSS) test, (7) dye testing, and (8) hydrodynamic modeling. The structured diagnostic approach has been applied and refined by the authors over nearly three decades and has proven extremely useful in rapidly identifying the least-cost technology for reducing ESS. Case examples are presented where the structured diagnostic approach is used to isolate the problems and identify the most appropriate remedy.

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