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.

Procedure

In this experiment 2 (? Give a more descriptive name than experiment 2) we investigated the effect of alum dosage on flocculator performance. The procedure for Experiment 1 (Your link name does not conform to wiki standards. It needs to include your team designation. Please fix.) 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 (a shear rate of 57.563 s-1 calculated using equation 1.9 (maybe we should try showing the equation here too) of [Ian's Thesis |Tube Floc Fall 09 Effect of Alum dose and Flocculator length on Tube Flocculator Performance^Ian Tse MS Thesis.doc]), and we used influent turbidities of 5, 25, 50, 100 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 first planned to vary the alum dose over a large range, incrementing the dose in set intervals while holding the plant flow rate (shear), influent turbidity, and residence time constant. We decided to first run an experiment at each value of influent turbidity at a length of 28 m and then repeat the experiment with the same alum dosage values at a flocculator length of 84 m. If we found a significant difference between the 28 and 84 m lengths for the same alum dosage and influent turbidity, we would then plan to test a third time at 54 m in order to understand the development of any trends we observed. Table 1 shows the alum ranges we tested for each flocculator length and influent turbidity:

TABLE 1: Alum dose ranges tested and the increment used (mg/L).

The alum dose was varied in process controller using the increment function shown in Figure 1:
Unable to render embedded object: File (Alum Dose Calculation.JPG) not found.
FIGURE 1: Process Controller setpoints used to vary the alum dose.

The data was analyzed using the same methods as in Experiment 1.
We plan to continue narrowing down the optimal alum dose ranges for each influent turbidity and length, and hope to learn whether or not increasing flocculator length will in fact improve performance as expected and how long this trend continues.

The following Process Controller Files were used in our experiments:
[5 NTU One Length Test File |Tube Floc Fall 09 Effect of Alum dose and Flocculator length on Tube Flocculator Performance^5NTU 2796cm 5mls Alum 10-50mgL by 5mgL.pcm]
[5 NTU Three Length Test File |Tube Floc Fall 09 Effect of Alum dose and Flocculator length on Tube Flocculator Performance^5NTU 8388cm 5mls Alum 10-50mgL by 5 mgL.pcm]
[25 NTU One Length Test File |Tube Floc Fall 09 Effect of Alum dose and Flocculator length on Tube Flocculator Performance^25NTU 2796cm 5mls Alum 10-50mgL by 5mgL.pcm]
[25 NTU Three Length Test File |Tube Floc Fall 09 Effect of Alum dose and Flocculator length on Tube Flocculator Performance^25NTU 8388cm 5mls Alum 10-50mgL by 5mgL.pcm]
[50 NTU One Length Test File |Tube Floc Fall 09 Effect of Alum dose and Flocculator length on Tube Flocculator Performance^50NTU 2796cm 5mls Alum 10-63mgL by 7mgL.pcm]
[50 NTU Three Length Test File |Tube Floc Fall 09 Effect of Alum dose and Flocculator length on Tube Flocculator Performance^50NTU 8388cm 5mls Alum 10-66mgL by 7 mgL.pcm]
[100 NTU One Length Test File |Tube Floc Fall 09 Effect of Alum dose and Flocculator length on Tube Flocculator Performance^100NTU 2796cm 5mls-1 alum 20-60mgL-1 by 5mgL-1.pcm]
[100 NTU Three Length Test File |Tube Floc Fall 09 Effect of Alum dose and Flocculator length on Tube Flocculator Performance^100NTU 8388cm 5mls Alum 20-60mgL by 5 mgL.pcm]
[500 NTU One Length Test File |Tube Floc Fall 09 Effect of Alum dose and Flocculator length on Tube Flocculator Performance^500NTU 2796cm 5mls Alum 10-90mgL by 10mgL.pcm]
[500 NTU Three Length Test File |Tube Floc Fall 09 Effect of Alum dose and Flocculator length on Tube Flocculator Performance^500NTU 8388cm 5mls Alum 10-90mgL by 10 mgL.pcm]

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.

with

With A longer flocculator the residence time is longer, and
enables longer collision times the colloidal particles have more time to collide to form big 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. since they weigh more, they will be able to settle faster, reducing the residual velocity. (The explanation above is not sufficient. Residual turbidity could be the result of flocs falling out of water, flocs sweeping up colloidal particles as they collide, and break up of flocs.)

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. From this trend, we can conclude hypothesize that the flocs may have been broken up due to too much collision because the shear rate the floc experienced exceeded floc strength in some part of the floc structure).

High turbidity water is easier and faster to flocculated more quickly (because?) +; even with a short flocculator large flocs are easily formed. However, because the bonding among the colloidal particles that have formed the flocs is weak, flocs are easily breakable +(The chemical bonding is not necessarily weak since we have an amorphous precipitation occuring. However, the shear rate of break-up may be significant enought to break up the floc into smaller pieces at certain points in the floc structure). Hence, we hypothesize that an optimal length of a flocculator must be set; longer flocculators do not always give better results. (Please comment on this. When flocs break up and reform are they stronger or weaker than before? Cite a literature source here.) Flocs grow only to a certain point where they become too big and vulnerable to fluid shear stresses that break the flocs (Good sentence, but should have been put somewhere before). Though the freed particles can flocculate again, the maximum floc size will remain the same, and thus, the particles will just form the same floc they have produced before the break up. Thus, the same residual turbidity is observed for the two lengths of flocculator: the steady state is reached in the two flocculators. (Again, not entirely sure of your claim here. Confirm this in literature.)

Unable to render embedded object: File (5NTU length 1 and 3.png) not found.
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 NTUEffect 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 the 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 the fact that two phenomena are happening ocurring at the same time. For the low turbidity, 5 NTU and 25 NTU, as mentioned earlier, more time is needed required to produce large flocs at the lower turibidities of 5 and 25 NTU. The residence time in the 2796 cm long flocculator may be too short 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. is relatively high. For the 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?)
Unable to render embedded object: File (residual turbidity_length1.png) not found.
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.