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The Wards Island water pollution control plant (WPCP) in New York City provides secondary treatment using the step feed activated sludge process. In a recent expansion, aeration tank 13 (AT-13) of Battery “E” was placed in operation as a step-feed biological nitrogen removal (BNR) process.

The most important mechanisms active in the removal of SS in final settling tanks, (FSTs), are the settling velocities of the solids, flocculation, and the hydraulic profiles within the FSTs. The New York City Department of Environmental protection, (DEP), working with The City College of New York, (CCNY), is developing a three dimensional computer model of the Gould II type FSTs as currently operated in Battery “E”. The model, once calibrated and validated, will be used in part to assess proposed structural modifications and to estimate the maximum hydraulic and solids loading rates.

This paper presents the experimental work carried out in situ at Battery “E”, as well as bench scale work to determine the site specific relationships of settling velocities, impact of flocculation, and hydraulic profiles expected in the existing FSTs. Settling experiments to quantify the discrete settling occurring in the upper regions of the tank, as well as zone and compression settling were carried out over a 14 month period. Thus settling velocities were measured during winter and summer months and at different SVI values. The data showed that zone and compression settling can be expressed by a single Vesilind equation where V0 and k can be estimated by the following equation:

V0= 0.27(T, °C)-0.02(SVI)+6

k is a constant and the average value of k equaled 0.43 L/g

In addition, in-situ test for DSS using the Kemmerer sampler and batch experiments using the Phipps and Bird jar test were conducted to document the importance of flocculation in the performance of the FSTs. The traditional way used to calculate “G” cannot be applied to FSTs. Therefore the jar tests were modeled similarly to the FSTs so that “G” would be calculated in an identical manner. The new definition of “G” was used to calculate the coefficients of KA and KB in Parker's formula for flocculation. Tracer studies using the Rhodamine WT dye and a modification of Crosby's “multi-point dispersion test procedure” were conducted to demonstrate the cross three dimensional movement of flow within the FSTs.
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Document Type: Research Article

Publication date: 2007-01-01

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