Water Environment Federation (WEF)

Application of a CFD Model to Improve the Performance of Rectangular Clarifiers

There has been a long standing debate about the relative merits of circular and rectangular clarifiers. Both have been shown to be capable of excellent performance and both have experienced problems. The hydrodynamics of each configuration under similar hydraulic loading differ in fundamental aspects that determine what types of geometric modifications are likely to improve the clarifier performance. Griborio (2004), Griborio and McCorquodale (2005) and
McCorquodale et al. (2005) presented the development and application of a computational fluid dynamics (CFD) model for predicting the performance of circular clarifiers under different geometric configurations. This paper uses a 2-D CFD model to investigate the response of a secondary rectangular settling tank to various internal modifications. Among the salient features of this model are: (1) discrete, zone and compression settling; (2) flocculation; (3) non-
Newtonian flow; (4) floatable particles; (5) variable internal tank options including skirts and perforated baffles.

A simple rectangular tank with no special treatment was selected to demonstrate the response of rectangular tanks to different internal geometries. The selected clarifier is the Tomelilla secondary settling tank, which is describe by Larsen (1977). This tank was selected because performance data are available for model calibration, and because it represents a marginal performance case. After calibration, the model was used to evaluate different internal
configurations. For the simulated trials, the best combination was an inlet skirt and an extended inboard launder with a perforated baffle.

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