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When first contemplated, it seems elementary that a primary treatment system that settles and removes solids only once would be advantageous to a system that settles and removes solids twice. In the latter system, the underflow from the first phase of settling becomes the influent to the second phase. It is obvious that there are additional chances to lose solids to the effluent in the latter system and effective removal efficiencies are the product of removal efficiencies in each removal step. Therefore, common sense may lead us to the premature conclusion that a primary system that must settle and remove solids twice would not be advantageous.

However, the Town of Speedway, Indiana Wastewater Treatment Plant operates successfully in a mode that requires settling of solids twice to be effectively removed in primary treatment. The rectangular primary settling that receive process flow are operated in a “zero-blanket” mode. Sludge withdrawal from the five process tanks is continually sent to an off-line gravity thickener to be settled and removed a second time. While overall, average primary removal efficiencies remain comparable to that of conventional primary treatment, primary treatment performance is significantly enhanced under peak, wet-weather flow scenarios. This is due to the absence of a sludge blanket in the tank's receiving peak process flows that would otherwise be subject to effluent washout under peak flow conditions. Superior performance under peak conditions is now achievable without impacting primary sludge solids content. Elevated sludge blanket levels necessary for optimizing underflow solids content are maintained in the primary sludge gravity thickener. Sludge blanket management has historically been a “dual edged sword” where the ability to process peak flows without solids washout was only possible at the expense of underflow solids content. With process modifications to allow zero-blanket operation with a primary sludge gravity thickener, the dual objectives of effectively handling peak flows and maximizing sludge solids content are achievable in a simple-to-operate primary treatment system.

Document Type: Research Article


Publication date: January 1, 2003

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