Optimum Design of Your Center Well: Use of a CFD Model to Understand the Balance Between Flocculation and Improved Hydrodynamics

The use of center wells (CWs) in circular secondary settling tanks (SST) has become a common practice. According to Parker et al. (1996) the main purpose of the CW is to promote the aggregation of remaining dispersed particles into settleable flocs, i.e., to promote the flocculation process inside the settling tank. Parker et al. (1996) and Parker and Stenquist (1986) showed strong evidence that, under similar operating conditions, circular tanks equipped with CWs can achieve lower effluent suspended solids and tend to perform better than tanks without a CW. A large study presented by Wahlberg et al. (1994) concluded that a residence time of about 20 minutes in a flocculation zone inside a SST can lead to more than 90% of the achievable flocculation. This 20 minute residence time, in conjunction with the average dry weather flow and a 50% return activated sludge, has been use as rule of thumb to determine the volume of the center well (WEF, 2005).

Merrill et al. (1992) used a two-dimensional (2-D) hydrodynamic clarifier model to optimize the geometry of the CW in a circular SST. The numerical model did not include flocculation in the CW; however, Merrill and co-workers found that the optimum placement coincided with the optimum diameter recommended for flocculation. These results open the discussion about the main role of the CW. Is it to promote flocculation or to improve the tank hydrodynamics? This
paper uses a computational fluid dynamics model to help answer this question. Furthermore, this numerical model is used to better understand the effects that the geometry of the CW has in the hydrodynamics of the SST, and how this geometry affects the flocculation process.

Results demonstrate that the CW promotes the aggregation of unflocculated particles and this effect has a major effect on the clarifier performance. However, an even greater benefit of the CW is the improvement of the tank hydrodynamics. The CW helps to control the re-entrainment of clarified fluid with the influent flow, reducing the strength of the upflow current close to the launder. Sensitivity analyses demonstrate that the optimum dimension of the CW is about 20 to 35% of the total clarifier diameter. The incoming MLSS does not seem to have a major effect on the optimum dimension, while it appears that the optimum dimension tends to increase as the surface overflow rate increases.

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