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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 different 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 settlesink. Given that the capture velocity is 0.12 mm/sec, the residual turbidity graph focuses on the flocs that have not reached fail to reach this velocity. The turbidity from these particles shows the resulting turbidity produced after flocculation.

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. Looking at the gamma PDF graph (Figure 1) for this experiment, the alum dosage of 20 mg/L was immediately eliminated from consideration for an alum dosage because its probability to reach a large floc size was comparably smaller than the others. In addition, the alum dosage of 25 mg/L was eliminated because of its wide variation of floc sizes and relatively low probability to reach its largest size. Now narrowed down to an alum dosage range of 30~55mg/L, the datalog for the experiment was used to find the lowest turbidity range during the settling state. As shown in table 1, after the alum dosage of 40, there was not a considerable change in the lowest NTU range. So the alum doses lower than 45 mg/L were eliminated, and the dose range was again narrowed down to 45~55 mg/Lgave 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.

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

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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.

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

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