Page History

...

Effect of length of the flocculator on residual turbidity:
The length of the flocculator has different effects on the residual turbidity depending on the initial effluent turbidity. In the next figures (fugures 8 and 9), we are comparing the mean residual turbidity as a function of the alum dose at 2 different lengths of flocculator, 2796 cm and 8388 cm (3*2796 cm) and for different influent turbidity, 5NTU (figure 8) and 500 NTU (figure 9) .
As shown in figure 8, at 5 NTU the length has a real effect on flocculation. Low turbidity water is more difficult to flocculate than water with high turbidity because the collision potential required to make large flocs has to be higher. There are few colloidal particles in the water so the average volume occupied by a floc or a particle is bigger so that the average time between collisions is greater leading to a higher collision potential. (Mettre la formule) With a longer flocculator, the residence time is longer and the colloidal particles have more time to collide and form big flocs with a higher terminal velocity than the one at the end of the shorter flocculator. If the terminal velocity of the flocs is bigger, it means that more flocs are going to settle and it will reduce the residual velocity. As shown on figure 8, the residual turbidity is smaller for the longer flocculator.
With an effluent water of 500 NTU, the difference between the two lengths of flocculator can't really be seen in figure 9. The mean residual turbidity is even a little bit higher with the longer flocculator at high alum dose. This could be explained by the fact that we break up flocs. A high turbidity water is easier and faster to flocculate so we are making really big flocs even with the smaller flocculator and as the flocs grow bigger and bigger, they are easier to break. The hypothesis is that a sort of equilibrium is established in the flocculator. Flocs are growing but at one point they are too big and fluid shear stresses break the flocs into smaller fragments. But then these smaller fragments can flocculate again. After some time, a steady state is reached between flocculation and fragmentation and the floc size distribution stay the same. That's why the same residual turbidity is observed for the two lengths of flocculator: the steady state is reached in the two flocculators.

Image Removed
Figure 8: Plot of the residual turbidity as a function of alum dose at two different flocculator length, 796 cm and 8388 cm, for an effluent water of 5 NTU

...