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.
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.