ESEL Paper Review_20130624

By Hong Guo

Mail: hongguo@gist.ac.kr

Phone: (+82) (0)10 82276568

1, Title and Author

Title: Secondary settling tank modelling and design Part 1: Review of theoretical and practical developments

Journal: Water SA

Authors:

AE Ozinsky and GA Ekama*

Department of Vicil Engineering, University of Cape Town, Rondebosch 7700, Cape Twon, South Africa

2. Summary of Paper

In this paper, the 3-D velocity and concentration distributions in a circular clarifier was investigated, and the influence of inlet baffle length and baffle location on the SS, the dynamic process of sludge settling and the density current are discussed

3. Results

? A grid dependency study was performed to eliminate errors caused by the coarseness of the grid and to determine the best compromise between the simulation accuracy, numerical stability, convergence, and computational time

? In addition, the mesh density was chosen such that the grid was finest where the velocity gradients were expected to be largest.

? After impinging on the baffle, the influent was deflected downwards towards the bottom of the tank. This impingement with the baffle and the sudden decrease in velocity resulted in a large loss of kinetic energy. The flow splits at the bottom near the sludge hopper, which created a recirculation eddy and an underflow. The surface return flow was found in the tank.

? In the inlet zone, the turbulence intensity was larger due to the high velocity of the inlet jet and the inlet baffle. Because the velocity suddenly decreased (see Fig. 8), the turbulent kinetic energy decreased accordingly.

? Although the density flow was detrimental to the sludge settling, the formation of a relatively stationary water layer overhead can inhibit the upward diffusion of sludge to a certain extent.

? The sharp drop of the contours near the inlet is related to the downward flow of the return current as it circulates back to meet the bottom current. When the baffle was closer to the centre, the evolution of the solids concentration became more complicated.

? The reflux of the low solids concentration surface joined the feed flow of the high-solids concentration, which is detrimental to sludge settling.

4. Contribution:

The authors study the effect of the length and location of baffle to the sedimentation tank in this paper. In addition, the solid phase model is also other research point , which could be applied the our future research.