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Historically, secondary clarifiers have been designed using established design criteria for hydraulic loading rate (HLR) and solids loading rate (SLR) (Tchobanoglous and Burton, 1991). Although unstated, these criteria correspond to typical mixed liquor suspended solids (MLSS) concentrations and settling characteristics. For design conditions other than those within typical ranges, the established design criteria may be inappropriate. The City of Atlanta wished to determine whether the secondary clarifiers at its three largest Water Reclamation Centers (WRCs) would perform successfully at higher than typical design SLRs. To determine this, batch-settling tests were conducted at each plant to obtain site-specific Vesilind parameters that describe sludge settleability. Then, one full-scale secondary clarifier at each plant was tested to determine whether a state point analysis performed using these Vesilind parameters would accurately predict overloaded conditions.

Results show that activated sludge process suspended solids settling characteristics at the three plants were quite different from “typical,” as well as from one another. Settleability at one of the plants differed from earlier measurements at the same plant.

State point analysis was successful at predicting full-scale clarifier failure, with failure actually occurring at solids loadings 75 to 85 percent of those predicted. This difference is consistent with the findings of other investigators and can be attributed to hydrodynamic factors (Walberg, 1998). Given the success of these predictions, the City will use the site-specific Vesilind parameters in a settling model as part of a larger project to assess the capacity of its plants.

Document Type: Research Article


Publication date: 2004-01-01

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