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Conventional principles of secondary clarifier operation based on solids flux theory recommend the use of increased return activated sludge (RAS) rates as a method of alleviating sludge blanket buildups resulting from solids flux limitations. In apparent violation of these principles, at the 88-mgd Clark County Sanitation District Wastewater Treatment Plant in Las Vegas, Nevada, increased RAS rates were found to actually exacerbate clarifier problems, and it was found that clarifier operation could only be stabilized by operating at low RAS rates. A study of the CCSD secondary clarifiers revealed that this anomalous behavior was due to limitations in the transport capacity of the sludge removal mechanism. Sludge withdrawal is a two-step process consisting of sludge transport to the withdrawal point (by the removal mechanism) and pumping from the withdrawal point (by the RAS pumps). Conventional solids flux theory neglects the transport step and assumes that the overall withdrawal process is controlled by the RAS pumps. This assumption was not true at the CCSD WWTP where the 140-feet diameter secondary clarifiers are equipped with segmented scraper-type sludge removal mechanisms. In these clarifiers, the transport capacity of the mechanisms was found to be controlling the overall sludge withdrawal process. Because of their limited transport capacity, the mechanisms were unable to keep up with the RAS pumping demand at high RAS rates. This limitation was established using a combination of theoretical calculations and actual field observations. A desktop model of sludge transport using segmented-scraper mechanisms was developed, and this model was found to qualitatively predict operational trends at the CCSD plant. This paper includes details of the model along with actual operational data from plant operation.
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Document Type: Research Article

Publication date: 01 January 2000

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