Analysis of the effect of alum dose and flocculator length on flocculation

Introduction

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 the formation of a floc blanket and a low resulting 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, 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 which dosage works best for each situation.

In addition to the use of alum, the optimization of the flocculator's length is critical. 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.

Results and Discussion

Residual Turbidity Analysis

Results from 5 NTU experiments:
Both of our replicates (Fig. 1) for a flocculator of 2796cm showed close agreement. Starting at a residual turbidity close to 5 NTU with an alum dose of 10mg/L, the residual turbidity dropped with increased alum dose until it appears to hit a limiting value of about 0.8 NTU with an alum dose of 50 mg/L. After this point, increasing alum dose no longer appears to have an effect on residual turbidity. We hypothesize that initially, floc buildup is the dominiant process occurring. However, as the flocs grow in size, shearing forces increase causing the flocs to breakup. Eventually this breakup balances the floc growth due to alum. As a result, flocs cannot continue to grow larger, and as a consequence residual turbidity stops decreasing.

Figure 1:  Residual turbidity vs. alum dose for 5 NTU influent water for a flocculator length of 2796cm.

For a flocculator length of 8388cm, we focused on the lower alum dosages to see how then residual turbidity plot developed; experiments with higher alum dosages tended to show relatively constant residual turbidities around the minimum value achieved with low dosages (Fig.2). Here we a similar trend to what was observed at 2796 cm, although the two replicates do not match as closely at the low end. We suspect this is due to limits in the calibration of the pumps, combined with the possibility of blockages in the alum line and variations in influent turbidity, making it difficult to achieve consistent results at such low alum dosages.


Figure 2: _Residual turbidity vs. alum dose for 5 NTU influent water for a flocculator length of 8388cm, focusing on lower alum dosages._It appears that both flocculator lengths achieve similar minimum residual turbidity values of around 0.7 NTU (Fig. 3). However, the longer flocculator is able to achieve this with much lower alum dosages than the shorter one. Since shear rate was held constant, we hypothesize that the increased residence time of the longer flocculator allowed the flocs to grow larger with less alum since they had more time to collide before reaching the settling column. Although the minimum residual turbidity was achieved with lower alum dosages in the longer flocculator, it appears that increasing the length of the flocculator did not result in a lower residual turbidity than could otherwise have been achieved with the shorter flocculator using higher alum dosages.


Figure 3: Comparison of residual turbidity vs. alum dose for two different flocculator lengths of 2796cm and 8388cm.Results from 100 NTU experiments:

With 100NTU water and a flocculator length of 2796cm, we see a similar behavior to what was observed with 5 NTU influent water (Fig. 4). The residual turbidity starts out high before approaching a minimum value. However, this minimum residual turbidity is higher than what was observed with 5 NTU water, being around 0.9-1 NTU.

Figure 4: Residual turbidity vs. alum dose for 100 NTU influent water for a flocculator length of 2796cm. A logarithmic scale is used on the residual turbidity axis to show details of the minimum residual turbidity achieved.

Both replicates at 5592cm (Fig. 5) show the characteristic decay of residual turbidity to a limiting value. However, they do not agree at the low alum dosages; the second repliucate shows a much lower residual turbidty for comparable alum doses at the low end of the curve. We suspect that there may have been a blockage in the alum line during the first experiment resulting in this irregularity. This is because alum doses as high as 20 mg/L produced a residual turbidity of around 80 NTU, much higher than would be expected. We then hypothesize that the blockage cleared by the fourth data point in the second replicate, and as a result both replicates achieved similar limiting residual turbidities of around 0.8 NTU.


Figure 5: _Residual turbidity vs. alum dose for 100 NTU influent water for a flocculator length of 5592cm. A logarithmic scale is used on the residual turbidity axis to show details of the minimum residual turbidity achieved._The three replicates of a flocculator length of 8388 cm at 100 NTU show reasonable agreement, as well as the same characteristic decay of residual turbidity observed in previous experiments (Fig. 6). However, the minimum residual turbidity achieved averaged around 1.6-2 NTU, significantly higher than observed at the previous two lengths. It appears that for a flocculator length of 8388cm, the floc breakup behavior becomes more dominant, resulting in elevated residual turbdities.


Figure 6: Residual turbidity vs. alum dose for 100 NTU influent water for a flocculator length of 5592cm. A logarithmic scale is used on the residual turbidity axis to show details of the minimum residual turbidity achieved.
Comparing characteristic replicates from each of the three flocculator lengths, the same trend as seen with 5 NTU influent water is observed (Fig. 7). The longer the flocculator, the lower the alum dose needed to achieve the minimum residual turbidity. However, when the flocculator is increased from 2796cm to 5592cm, the minimum residual turbidity decreases slightly from about 0.9 NTU to about 0.8 NTU. However, when it is further increased to 8388cm, the residual turbidity jumps up to around 1.8 NTU. Therefore, it appears that while increasing flocculator length can produce favorable results as seen in the decrease in alum dose needed to achieve similar residual when comparing the two shorter lengths, further increases can be detrimental to flocculation performance. This implies that increasing flocculator length too much causes the flocs to breakup and reform smaller flocs that do not settle out as effectively.


Figure 7: Comparison of residual turbidity vs. alum dose for three different flocculator lengths of 2796cm, 5592cm, and 8388cm with influent water at 100 NTU. A logarithmic scale is used on the residual turbidity axis to show details of the minimum residual turbidity achieved.Results from 500 NTU experiments:

For the 500 NTU experiments, similar minimal residual turbidities were achieved using flocculator lengths of 2796cm and 8388cm of around 2 NTU. Additionally both showed similar behaviors to what was observed before. It is unclear if the 5592cm flocculator achieved its minimum residual turbidity faster than the 2796cm flocculator since it is bounded on both sides by a 2796cm replicate (Fig. 8). Further data is needed to draw conclusions regarding how flocculator length impacts residual turbidity with 500 NTU water.
The following results are from the experiments conducted with varying influent turbidity and flocculator length.


Figure 8: Comparison of residual turbidity vs. alum dose for two different flocculator lengths of 2796cm and 5592cm with influent water at 500 NTU.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 increase in alum dose will make 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 allows the clay particles to stick to each other and to grow bigger 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 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 mean sedimentation velocity decrease.
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 longFor 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 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 for an influent water of 100 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 data

Replicability:
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 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. 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 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.