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Mean sedimentation velocities

Mean sedimentation velocities have a different evolution when increasing alum dose depending on the influent turbidity of the raw water (100 NTU and 500 NTU in the case of the experiments).

For a 100 NTU influent water, (figure 9 ), at a low alum dose, the mean sedimentation velocities of the flocs increase up to a maximum. Further increase in alum dose shows a decrease in the mean sedimentation velocity of the flocs. This could be explained by the fact that alum is less dense than clay. At low alum dose, alum precipitation allows the clay particles to stick to each other and to grow larger but the majority of the floc is clay. After a certain alum dose, the hypothesis is that flocs grow so big that they are vulnerable to shear stress and break up in the flocculator and then reflocculate. Equilibrium is then reached between floc break up and flocculation producing a similar distribution of particles sizes when increasing alum dose. It is hypothesized that when alum dose relative to clay increases, the floc density decreases and the mean sedimentation velocity decreases.

Figure 9: Plot of the mean sedimentation velocities as a function of alum dose for an effluent water of 500 NTU and a flocculator 2787 long

For an influent water of 500 NTU, the evolution of mean sedimentation velocities is different. Figure 10 below is a plot of the mean sedimentation velocity as a function of alum dose for an influent water of 500 NTU and a flocculator 2787 cm long. At low alum dose, mean sedimentation velocities increase with the alum dose. However, after a certain alum dose, mean sedimentation velocities reach a threshold. This could be explained by the fact that high turbidity water is flocculated quite quickly because the average time between two collisions of flocs is short and we hypothesize that flocs grow only to a certain point before they become too big and vulnerable to fluid shear stresses that break them. Like the 100 NTU water, after a characteristic time, a steady state is reached between flocculation and fragmentation and the floc size distributions stay constant. However, in the case of 500 NTU water, there is so much clay that increasing alum dose doesn't make the flocs less dense.

Figure 10: Plot of the mean sedimentation velocities as a function of alum dose for an effluent water of 100 NTU and a flocculator 2787 long

Mean sedimentation velocities are also influenced by the length of the flocculator. Figure 11 and 12, show mean sedimentation as a function of alum dose for three different lengths of flocculator. In figure 11, experiments were conducted with an influent water of 100 NTU and for figure 12, the experiments were conducted with an influent water of 500 NTU. On these two graphs the same trend can be observed when varying flocculator length. When the increasing flocculator length, the mean sedimentation velocities follow the same trend as alum dose increases but their corresponding magnitude decreases.

Figure 11: Plot of the mean sedimentation velocities as a function of alum dose for an effluent water of 100 NTU at different length of the flocculator (2787 cm, 5592 cm, 8388 cm)

Figure 12: _Plot of the mean sedimentation velocities as a function of alum dose for an effluent water of 500 NTU at different length of the flocculator (2787 cm, 5592 cm, 8388 cm)_

A few hypotheses seem plausible explaining floc break up in the flocculator. Flocs could break up because of interactions between particles and the wall or particles and the fluid or particles and particles. Comparing mean sedimentation velocities found for an influent water of 100 NTU and 500 NTU for the same length of flocculator (figure ?13), it appears that the maximum mean sedimentation velocities are lower for a 500 NTU water than for a 100 NTU water. This result seems to validate the hypothesis that floc can be broken up because of particles-particles interactions. However, further investigation should be done in order to confirm the existence of this kind of interactions.

figure out what interactions
Figure 13: Plot of the mean sedimentation velocities as a function of alum dose, flocculator 2787 cm long, for an influent water of 100 NTU and 500 NTU

Reliability of the data

Replicability:

Every experiment was replicated to test the reliability of the results given by FReTA. The comparison between the mean residual turbidities of the experiments and their replicates has been explained in a revious paragraph. Mean sedimentation velocities were also compared. The figure 14,15 and 16 show that our experiments were quite accurate when analyzing mean sedimentation velocities.

Figure 14: Plot of the mean sedimentation velocities as a function of alum dose for an effluent water of 100 NTU at the smallest length of the flocculator (2787 cm)

Figure 15: Plot of the mean sedimentation velocities as a function of alum dose for an effluent water of 100 NTU at the medium length of the flocculator (5592 cm)

Figure 16: Plot of the mean sedimentation velocities as a function of alum dose for an effluent water of 500 NTU at the smallest length of the flocculator (2787 cm)

Mean sedimentation velocities :
The data processor retrieves mean sedimentation velocities of the particles at each alum dose during an experiment from our data. They are calculated when the data is fitted to a gamma distribution.

Since the mean sedimentation velocities are calculated when we fit the data, the mean sedimentation velocities given for a 5 NTU influent water are not accurate. Indeed, even if the data processor tries to fit the data, shape of the evolution of turbidity as a function of sedimentation velocity (figure 17) does not look like the fit function.

Figure 17: Plot of the turbidity vs sedimentation velocity for an influent water of 5 NTU. Flocculator 2787 cm long

For 100 NTU and 500 NTU, the fitted data seem accurate enough that we can trust the mean sedimentation velocities or the coefficient of variation retrieved. Data retrieved at low alum dosage seems to be really well fitted (figure 18). However, at higher alum concentrations, at some points, raw data seems to have a behavior slightly different than the gamma function (figure 19,20 and 21)

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Figure 18: Plot of the normalized turbidity as a function of sedimentation velocity for the ram data and the fitted data. 500 NTU water, alum dose of 10 mg/L, flocculator 5592 cm long

Figure 19*: Plot of the normalized turbidity as a function of sedimentation velocity for the ram data and the fitted data. 500 NTU water, alum dose of 50 mg/L, flocculator 5592 cm long

Figure 20*: Plot of the normalized turbidity as a function of sedimentation velocity for the ram data and the fitted data. 500 NTU water, alum dose of 70 mg/L, flocculator 5592 cm long

Figure 21*: Plot of the normalized turbidity as a function of sedimentation velocity for the ram data and the fitted data. 100 NTU water, alum dose of 100 mg/L, flocculator 5592 cm long