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The goal of this experiment is to study the effects of different alum doses and flocculator lengths in order to find the most effective way to improve the performance of the actual AguaClara flocculator. A well performing flocculator produces flocs of certain sizes that can settle in the sedimentation tank but do not prevent or aid in the formation of a floc blanket and resulting in a low resulting effluent turbidity indicating the production of clean water.
The use of alum facilitates this process; however, too much alum can create oversized flocs that settle out in the flocculator before reaching the sedimentation tank. In addition, is necessary to this process and optimizing alum dosage will be more cost effective in running the AguaClara plant. To find an ideal alum dosage for a particular influent turbidity, in our experiments, we have varied the alum doses within certain ranges and are going to see verify which dosage works best for each situation. In addition to the use of alum, the optimization of the flocculator's length is critical. The length of the flocculator seems to have an influence on the optimum alum dose.
Long flocculators can produce large flocs that settle out in the flocculator before reaching the sedimentation tank. Conversely, in short flocculators flocs will not have a long enough collision time to reach a large floc size. Through experiments, we aim to find the best combination of alum dosage and flocculator length that performs most efficiently.
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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, see (figure 9 below (plot of the mean sedimentation velocity as a function of alum dose for an influent water of 100 NTU and a flocculator 2787 cm long ), at a really low alum dose, the mean sedimentation velocities of the flocs increase up to a maximum and then a further . Further increase in alum dose will make shows a decrease in the mean sedimentation velocity of the flocs decrease. This could be explained by the fact that alum is less dense than clay. At low alum dose, alum precipitating precipitation allows the clay particles to stick to each other and to grow bigger 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. However, when increasing the It is hypothesized that when alum dose the ratio of clay to alum inside a floc increases and because alum is less dense than clay, flocs are more buoyant making their relative to clay increases, the floc density decreases and the mean sedimentation velocity decreasedecreases.
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) It is interesting to observe the evolution of the coefficient of variation of sedimentation velocities as alum dose is increased. The coefficient of variation is the ratio of the standard deviation of the distribution of velocities divided by the mean of the sedimentation velocities. Figure 13 shows the evolution of the coefficient of variation as a function alum dose
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 ?), 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 figure out what interactions
Reliability of the data
NTU. It can be observed that the coefficient of variation decreases when we increase alum dose at a low alum dose. A decreasing coefficient of variation means that the distribution of particle size is getting narrower. However, after a certain alum dose, increasing the alum concentration makes the coefficient of variation increase. The spread of the sedimentation velocities is slightly getting larger with increasing alum dose.
Figure 13: _Plot of the coefficient of variation 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 15 shows the evolution of the coefficient of variation as a function alum dose for an influent water of 500 NTU. In this case, the coefficient of variation decreases with increasing alum dose at low alum concentrations up to a point where the coefficient of variation stays constant with increasing alum dose. This would mean that after a certain alum dose, increasing the dosage doesn't affect the distribution of the particle size. This is consistent with the hypothesis of a steady state in the flocculator.
Figure 14: Plot of the coefficient of variation as a function of alum dose for an effluent water of 500 NTU at different length of the flocculator (2787 cm, 5592 cm)Reliability of the dataReplicability:
Every experiment was replicated to test the reliability of the results given by FReTA. The results were quite good for the 5 NTU water as you can see on figure 15 A and B. For experiments at 100 and 500 NTU, looking at residual turbidity don't really seem accurate, figure 16 and 17. This could be explained by the fact that alum clogged the connector where alum and water are mixing and this problem was found later. However, we felt confident using the data since the experiments and their replicate 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 ? ? show that our experiments were quite accurate when analyzing mean sedimentation velocities.
were really close on the mean sedimentation velocities graphs.
A
B
Figure 15: Plot of the residual turbidity as a function of alum dose for an effluent water of 5 NTU and it's replicate A: For a flocculator 2787 cm long. B: For a flocculator 8388 cm long.
Figure 16: Plot of the residual turbidity as a function of alum dose for an effluent water of 100 NTU and it's replicate for a flocculator 5592 cm long
Figure 17: Plot of the residual turbidity as a function of alum dose for an effluent water of 500 NTU and it's replicate for a flocculator 2787 cm longMean 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, the shape of the evolution of turbidity as a function of sedimentation velocity (figure ?) does not look like the fit function.
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. Figure 18 below shows a plot of the raw data and the fitted data for 500 NTU water and an alum concentration of 70 mg/L.
Data retrieved at low alum dosage seems to be really well fitted. However, at higher alum concentrations, at some points, raw data seems to have a behavior slightly different than the gamma function (figure ??)