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

Procedure

In Experiment 2 we investigated the effect of alum dosage on flocculator performance. The procedure for Experiment 1 contains information on the general setup used in this experiment. We used the same setup as in Experiment 1, but instead of varying flow rate, we varied the alum dosage. The plant flow rate was held constant at 5 mL/s (what is this shear rate?), and we used influent turbidities of 100 NTU and 500 NTU and three different flocculator lengths (28 m, 56 m, and 84 m).

To investigate alum dosage at a given turbidity and length, we planned to run two experiments. The first experiment would vary the alum dosage over a large range in larger intervals while holding the plant flow rate (shear), influent turbidity, and residence time constant. Then we planned to run a second experiment varying the alum dose across the best performing interval from the first experiment with small increments in order to determine the optimal dose.

For a flocculator length of 28 m, we began by testing water with an influent turbidity of 100 NTU. The alum dosage was varied from 20-60 mg/L in increments of 5 mg/L for each run. The alum dose was varied in process controller using the increment function shown in Figure 1:

FIGURE 1: Process Controller setpoints used to vary the alum dose.

The data was analyzed using the same methods as in Experiment 1.

However, since the optimal alum dosage for 100 NTU water has been well characterized by previous experiments and our the interval our data identified (see Results and Discussion) generally agreed with the known value of 45 mg/L, we did not continue with the second test at 100 NTU to narrow the optimal dose range and instead tested 500 NTU water. For the 500 NTU water, we maintained the same flow rate of 5 mL/s and flocculator length of 28 m. However, this time we varied the alum dose from 40 mg/L to 120 mg/L in increments of 10 mg/L. We then analyzed the data using the same methods as before.

We plan to continue our investigation of alum dose by performing a second experiment with 500 NTU water to find optimal dose based upon the best interval from our previous experiment. We will then repeat the process of varying alum dose over large intervals, and then narrowing to an optimal dose for 5, 25, 50, 100, and 500 NTU -water-water at extended flocculator lengths of 56 m and 84 m, or double and triple the length of the current flocculator.

The following Process Controller Files were used for the 100 NTU and 500 NTU tests.
100 NTU Test File
500 NTU Test File

Results and Discussion

The goal of this experiment is to study the effects of different alum doses and flocculator lengths in order to discover the most effective way to improve the design of the actual AguaClara flocculator. However, before beginning any analysis, we must first be aware of the characteristics of a good flocculator. What defines a good flocculator? A good flocculator must: first, have large enough resulting flocs at the end of the flocculator so that they will weigh down, sink, and settle out in the sedimentation tank; second, have this floc size under control to an optimal size-too large flocs will not facilitate floc blanket formation as they will settle out before the floc particles could ever become fluidized; and third, have 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 before reaching the sedimentation tank. In addition, the over use of alum will be expensive; cost efficiency is an important factor to be considered in building the AguaClara flocculator. To find an ideal alum dosage for a particular influent turbidity, in our experiments, we have varied the alum doses at a certain range and are going to see which dosage works best for each situation. The same goes to the different flocculator lengths; overly long flocculators will cause flocs to settle out before reaching the sedimentation tank and in addition will not be cost efficient. To find an ideal length, we have set up three apparatuses with varying tube lengths: 2796 cm, 5600 cm, and 8800 cm. For each alum dosage range, we will change the length of the flocculator in order to find the ideal combination of flocculator length and alum dose.

There are two important graphs that give the best overview of the experiment: the gamma PDF graph and the residual turbidity graph. The gamma PDF graph shows the probability of reaching a certain floc size for each run of experiment with varying alum dosage. The y-axis represents the probability and the x-axis represents the floc size. The residual turbidity graph is the gamma PDF graph focused at a certain range of sedimentation velocity. We chose this range based on our knowledge on capture velocity. Capture velocity is the minimum speed flocs must be in for them to settle in the sedimentation tank; any particle with a smaller velocity than this will not sink. Given that the capture velocity is 0.12 mm/sec, the residual turbidity graph focuses on the flocs that fail to reach this velocity. The turbidity from these particles shows the resulting turbidity produced after flocculation.

On November 11, 2009, an experiment was conducted with the turbidity set around 25 NTU, flocculator length 2796 cm, flow rate of 5 mL/s, and alum dosage ranged from 10 to 50 mg/L.

In the gamma PDF graph (Figure 1A), the alum dosage 10 mg/L showed a larger distribution of floc sizes with more probability to produce smaller flocs. In addition, looking at the residual turbidity graph (Figure 1B), the resultant turbidity this dosage gave was significantly higher than the rest. The residual turbidity graph also showed the alum doses 15 mg/L and 20 mg/L to be producing a slightly higher resultant turbidity. From the values of mean turbidity at settling state, starting from the alum dosage 35 mg/L, the mean turbidity seemed to be settling down to a constant value around 2.5 NTU.

1A 1B
FIGURE 1A: The graph plots normalized turbidity vs. sedimentation velocity for each Alum dose ranging 10 mg/L~50 mg/L; FIGURE 1B: The graph plots the residual turbidity vs. sedimentation velocity for each Alum dose ranging 10 mg/L~50 mg/L

On October 20, 2009, an experiment was ran with the set up of influent turbidity around 100 NTU, flocculator length of 2796 cm, flow rate of 5 mL/s, and an alum dosage ranging from 20 to 55 mg/L. The data obtained from this experiment was processed through Mathcad for a simplified overview of the results in graphic form.

Looking at the gamma PDF graph (Figure 2A), the alum dosage of 20 mg/L gave a widely distributed probability of reaching different floc sizes with a comparably low probability to reach its highest sedimentation velocity. In addition, the residual turbidity graph (Figure 2B)showed a high turbidity for this alum dose in its lower velocity range. Thus the alum dose 20 mg/L seems to be inefficient for this particular influent turbidity and flow rate. The mean turbidity resulting from alum dose 55 mg/L seems to be out of normal range; its NTU value is significantly lower than the values given from the previous, lower alum dosages. Hence, the result from this dosage is doubtful. Overall, after the alum dose of 35 mg/L (except for 55 mg/L), the mean turbidity seemed to be settling down to a constant value around 1.4 NTU.

2A 2B
FIGURE 2A: The graph plots normalized turbidity vs. sedimentation velocity for each Alum dose ranging 20 mg/L~55 mg/L; FIGURE 2B: The graph plots the residual turbidity vs. sedimentation velocity for each Alum dose ranging 20 mg/L~55 mg/L

On Oct 27, 2009, another experiment was conducted with the turbidity set around 500 NTU, flocculator length 2796 cm, flow rate of 5 mL/s, and alum dosage ranging from 10 to 90 mg/L. The data was run through Mathcad to get a simplified graphic overview of the result.

In both graphs (Figure 3A, Figure 3B), the alum dosage 10 mg/L gave a comparably different result from the rest; it produced a significantly higher settling turbidity and a large amount of smaller flocs. The overall mean turbidity for this dosage was almost twice the following dosages. After the alum dose 50 mg/L, the mean turbidity seemed to be settling down to a constant value around 2.5 NTU.

3A 3B
FIGURE 3A: The graph plots normalized turbidity vs. sedimentation velocity for each Alum dose ranging 10 mg/L~90 mg/L; FIGURE 3B: The graph plots the residual turbidity vs. sedimentation velocity for each Alum dose ranging 10 mg/L~90 mg/L

On Nov 19, 2009, another experiment was conducted with the turbidity set around 500 NTU, flocculator length 8388 cm, flow rate of 5 mL/s, and alum dosage ranging from 10 to 90 mg/L. The data was run through Mathcad to get a simplified graphic overview of the result.
As shown in figure 4.A, there is no big differences between the floc size distribution except for the lowest alum dose. As the previous experiment (500 NTU, length 2796 cm), the distribution is narrow wich means that we are producing flocs of almost the same size. Besides, the mean floc size seems to be the same than previously.

4A 4B
FIGURE 4A: The graph plots normalized turbidity vs. sedimentation velocity for each Alum dose ranging 10 mg/L~90 mg/L; FIGURE 4B: The graph plots the residual turbidity vs. sedimentation velocity for each Alum dose ranging 10 mg/L~90 mg/L

Conclusion

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 effluent turbidity. In the next figures (fugures 8 and 9), we are comparing 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 for different influent turbidity, 5NTU (figure 8) and 500 NTU (figure 9) .
As shown in figure 8, at 5 NTU the length has a real effect on flocculation. Low turbidity water is more difficult to flocculate than water with high turbidity because the collision potential required to make large flocs has to be higher. There are few colloidal particles in the water so the average volume occupied by a floc or a particle is bigger so that the average time between collisions is greater leading to a higher collision potential. (Mettre la formule) With a longer flocculator, the residence time is longer and the colloidal particles have more time to collide and form big flocs with a higher terminal velocity than the one at the end of the shorter flocculator. If the terminal velocity of the flocs is bigger, it means that more flocs are going to settle and it will reduce the residual velocity. As shown on figure 8, the residual turbidity is smaller for the longer flocculator.
With an effluent water of 500 NTU, the difference between the two lengths of flocculator can't really be seen in figure 9. The mean residual turbidity is even a little bit higher with the longer flocculator at high alum dose. This could be explained by the fact that we break up flocs. A high turbidity water is easier and faster to flocculate so we are making really big flocs even with the smaller flocculator and as the flocs grow bigger and bigger, they are easier to break. The hypothesis is that a sort of equilibrium is established in the flocculator. Flocs are growing but at one point they are too big and fluid shear stresses break the flocs into smaller fragments. But then these smaller fragments can flocculate again. After some time, a steady state is reached between flocculation and fragmentation and the floc size distribution stay the same. That's why the same residual turbidity is observed for the two lengths of flocculator: the steady state is reached in the two flocculators.

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

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:
The next figure (figure 10) we are plotting the residual turbidity 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 when we increase alum dose and then we reach a limit where the residual turbidity is almost constant when we increase alum dose. These results mean that we could optimize the cost by using the minimum alum dose that gives the minimum effluent turbidity.
We can also observe on this figure that at a given alum dose, the mean residual turbidity is different for each influent turbidity. The highest residual turbidity is found for 500 NTU then 25 NTU and 5 NTU. The lowest residual turbidity is found for an influent water of 100 NTU. This could be explained by the fact that two phenomena are happening at the same time. For the low turbidity, 5 NTU and 25 NTU, as I explained earlier, it is longer to produce large flocs and the residence time in the 2796 cm long flocculator might be too short so the residual turbidity is relatively high. For the high influent turbidity, 500 NTU, we reach a steady state in the flocculator but as we are starting with a really high turbidity, the residual turbidity is high too. Logically, the optimum result should be found for a medium influent turbidity and it is found for 100 NTU.

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