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

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

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 ? 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 ?: 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 ? 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 ?: 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 ? and ?, show mean sedimentation as a function of alum dose for three different lengths of flocculator. In figure ? , experiments were conducted with an influent water of 100 NTU and for figure ?, 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 ?: 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 ?: 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 ? 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 ?: 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 ? 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 ?: 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.

Mean 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.