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.

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.

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.

Looking at the gamma PDF graph, 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 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.

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

TABLE 1: Shows the Lowest turbidity reached by specific alum dosage ranging 30 mg/L~55 mg/L during an influent turbidity of approximately 100 NTU.
(The table of data doesn't mean anything yet. First, graph data rather than creating tables of numbers. Second, the residual turbidity must have been observed at some time. That time corresponds to a settling velocity. So report the turbidity at a particular capture velocity! It appears that you didn't do actual data analysis, but that you somehow read numbers (perhaps from a graph) and then copied them down. That leads to subjective analysis. Use data analysis in MathCAD and create a graph and then show that in the wiki. This data is hardly any real justification for the alum dose of 45 mg/L. Why not use 40 mg/L?)

For the second experiment, the alum dose range was set to be from 40 mg/L to 110 mg/L varying with an increment of 5 mg/L (always leave a space between a number and the unit) and the influent turbidity set around 500 NTU. Repeating the process implemented for the first experiment, this data was run through Mathcad for an overview. However, unlike the previous experiment, looking at the gamma PDF graph (Figure 2) from Mathcad, the mean particle sizes did not vary significantly. With increasing alum doses, the floc size should have also differed. Yet comparing the size distribution for the lowest alum dose (40 mg/L) and the highest (110 mg/L), there was no comparable difference. In addition, reviewing the datalog, the approximate lowest NTU range also seemed to be similar (Table 2). From this analysis, we decided that either the accuracy of the result presented by this experiment was arguable or the alum dose of 40 mg/L was enough for effective flocculation. Since the dose of 40 mg/L seemed to have a similar effect as the higher alum doses, we decided to repeat this experiment with an expanded alum dose range to test even lower doses. An expansion of the range of alum dosages, from 10 mg/L to 100mg/L, was made to find the least alum dosage that will give effective flocculation.

FIGURE 3: The graph plots normalized turbidity vs. sedimentation velocity for each Alum dose ranging 40 mg/L~110 mg/L

TABLE 2: _Shows the Lowest turbidity reached by specific alum dosage ranging 30 mg/L~55 mg/L during an influent turbidity of approximately 500 NTU.

(I am again doubtful of the data shown in this table. It it is calculated data, then how did the numbers end up with a squiggle in front? And how did it turn into a range? Standard analysis would report a mean and a standard deviation. It appears that all of the alum dosages tried were too high. What was the capture velocity used for this analysis?)