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 oversize 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. Overly long flocculators cause flocs to settle out in the flocculator before reaching the sedimentation tank. And inversely, short flocculators will prevent colloidal particles to reach their maximum floc size as they won't be given enough time to collide. Through experiments, we aim to find the best combination of alum dosage and flocculator length that performs most efficiently.

Results and Discussion

The following results are from the experiments conducted with varying influent turbidity and flocculator length. See detailed analysis of each experiment here.

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 influent turbidity. In figures 8 and 9, 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 influent turbidities, 5NTU (figure 8) and 500 NTU (figure 9) are compared.
As shown in figure 8, at 5 NTU the length has a huge impact on the flocculation. Water with low turbidity is more difficult to flocculate compared to water with higher turbidity since the latter will require a higher collision potential to create large flocs. For water with a low turbidity, few colloidal particles are present and the average volume occupied by a floc or particle is smaller. Thus, the average time it takes between collisions is greater and will require a higher collision potential.

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Then, a longer residence time enables longer collision times and produces larger flocs. The residual turbidity is lower for the longer flocculator (Figure 8). Flocs produced with more collisions tend to have a higher terminal velocity than the ones resulting from shorter flocculators.

With an effluent turbidity of 500 NTU, there is no noticeable difference between the two lengths of flocculator as seen in figure 9. The mean residual turbidity seems to even be bit higher with the longer flocculator at high alum dose. High turbidity water is flocculated more quickly than low turbidity water because the average time between two collisions is less but, from the result, we hypothesize that flocs grow only to a certain point where they become too big and vulnerable to fluid shear stresses that break the flocs. Floc can then grow again but because flocs originally break at their weakest point, they tend to grow again more compact. After a characteristic time, a steady state is reached between flocculation and fragmentation and the floc size distribution stay constant. This characteristic time must be reached in both flocculator explaining the similar residual turbidity found.

Patrick T Spicer, Sotiris E. Pratsinis, Judy Raper, Rose Amal, Graeme Bushell, Gabrie Meesters,(1997),"Effect of shear schedule on particle size, density, and structure during flocculation in stirred tanks",Powder Technology 97:26-34.

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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 Unable to render embedded object: File (500 NTU length 1 and 3.png) not found.
Figure 9: 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 500 NTU

Effect of the alum dose on flocculation:

For the next figure (figure 10) the residual turbidity has been plotted as a function of alum dose for water with a different influent turbidity in the short flocculator (2796 cm). We can observe a similar behavior between the plots. At the beginning, the residual turbidity is decreasing with an increase of alum dose until a limit is reached where the residual turbidity stays almost constant. These results show the optimal amount of alum that should be used; the least amount of alum in which the minimum effluent turbidity is reached will be the most cost effective alum dosage.

We can also observe on this figure that at a given alum dose, the mean residual turbidity is different for each influent turbidity. From this graph, highest residual turbidity is found in the alum doses: 500 NTU, 25 NTU, and 5 NTU respectively. The lowest residual turbidity is found for an influent water of 100 NTU. These observations can be explained by two phenomena ocurring at the same time. More time is required to produce large flocs at the lower turibidities of 5 and 25 NTU. The residence time in the 2796 cm long flocculator may result in higher residual turbidity because the collision potential is less and a large portion of colloids have not yet come into contact forming flocs. For higher influent turbidity, 500 NTU, a steady state is reached in the flocculator but as we are starting with a really high turbidity, the residual turbidity is high too. (The residual turbidity is not really high. Explain that in terms of pC* (-log removal of Cout/Cin) that you are removing as much turbidity or more compared to other turbidities. This may require some calculations) Logically, the optimum result should be found for a medium influent turbidity and it is found for 100 NTU. (We cannot control the influent turbidity unless we want to purposely make our water dirtier. Perhaps use an explanation of collision potential and sweep flocculation to explain why the 100 NTU was so effective. Can you characterize its pC* and see if this was better?)

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Figure 10:Plot of the residual turbidity as a function of alum dose at different influent turbidity with a 2796 cm long flocculator

The following steps of this analysis will consist in doing further experiments with wider alum range and more influent turbidity. The size distribution of flocs will be analized to see what would be the influence of different flocculator lengths and different alum doses on a floc blanket.