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, and we used influent turbidities of 100 NTU and 500 NTU and three different flocculator lengths (2296 cm, 5592 cm, and 8388 cm).

In order 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 to determine the optimal dose.

For a flocculator length of 2796 cm, we began by testing water with an influent turbidity of 100 NTU with a plant flow rate of 5 mL/s. 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.

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

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

Results and Discussion

To find the ideal alum dosage for a certain influent turbidity, there are several qualities that must be considered. First, the resulting floc size should be large enough so that by the time it reaches the sedimentation tank, it will weigh enough to sink and settle out. Second, the resulting NTU must be small indicating an efficient flocculation/sedimentation process and clean water, and third, the alum dosage should preferably be low to save cost.

For the first experiment, the alum dose ranged from 20 to 60 mg/L varying with an increment of 5 mg/L and the influent turbidity was around 100 NTU. The data given from this experiment was processed through Mathcad for a simplified overview of the result in graphic form.

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/L.

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.

For the second experiment, the alum dose range was set to be from 40mg/L to 110mg/L varying with an increment of 5mg/L 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.