PSS Summer 2009 Experiments Varying Alum Concentration

Overview of Methods

In these experiments the alum dosage supplied to the flocculation system was varied in order to study how floc and floc blanket formation affect the effluent turbidity produced by the tube settler.  The experimental set-up is identical to the one used in Spring 2009, and from our results we hope to analyze velocity gradient thresholds and possibly investigate how changing influent water chemistry affects the setter's efficiency.

Results and Discussion

Using the Spring 2009 team's process controller methods, we subjected an ideal geometry to non-ideal conditions. Though the Spring 2009 team had success with a 9.5 mm diameter tube, due to a change in influent water chemistry over the summer, (ineffective air bubble traps in the flocculator,) or the addition of a flow accumulator to the method, we experienced failure with this geometry. We achieved an acceptable effluent turbidity (less than 1 NTU) with a 15.1 mm diameter tube that had a length of 30.5 mm. With the ideal results, we then subjected this tube settler to varying alum dosage to investigate the dependency of the performance of the tube settler on this parameter. At each alum dosage, the tube settler was tested at a variety of capture velocities and at two different floc blanket levels.

Experiment 1: Alum Dose = 45 mg/L

Experiment 2: Alum Dose = 35 mg/L

Experiment 3: Alum Dose = 65 mg/L

Experiment 4: Alum Dose = 15 mg/L

Experiment 5: Alum Dose = 105 mg/L

Process Controller Files

Conclusions

Figure 1: Capture Velocity vs. Average Effluent Turbidity shown for each alum dose at low floc blanket level.


Figure 2: Capture Velocity vs. Average Effluent Turbidity shown for each alum dosage at high floc blanket level.

The above table shows the average effluent turbidities for each alum dosage, floc blanket state and capture velocity.

Overall, this system performed well and most of the effluent turbidities were below 1 NTU. The ideal alum dose of 45 mg/L and the slight underdose and overdose of 35 mg/L and 65 mg/L, respectively, performed best. Because the "overdose" and "underdose" did not fail, as expected, it was necessary to test more extreme doses. We tested 15 mg/L and 105 mg/L to observe more severe conditions. The extreme overdose of 105 mg/L demonstrated failure, as expected. The 15 mg/L extreme underdose, however, did not experience failure.

The major cause of failure for an underdose is an incomplete floc blanket as a result of smaller flocs that are formed, but the increased residence time in the flocculator creates larger flocs, which form a floc blanket more quickly and more effectively. Thus, although we expected that the extreme underdose of 15 mg/L would fail, the effluent turbidity fell within the acceptable range.

In contrast, an alum overdose forms a less dense, more "fluffy" floc blanket, which is not as effective in trapping flocs and filtering out particles. The extreme overdose of 105 mg/L shows failure as a result of this insufficient floc blanket.

Because the effluent turbidity using the alum underdose of 15 mg/L was acceptable once the floc blanket had formed, it seems that the dosage is unimportant once the floc blanket is completely formed. It appears that as long as the floc blanket is fully formed, which should occur with a higher dose so that it forms quickly enough, the alum dose can be lowered while still experiencing the same results.

The water chemistry in our system also contributes greatly to the unexpected results. The water in the lab is much more alkaline than the water in Honduras. As a result, the pH of the water in Honduras is more sensitive to changes in alum dose. There is an ideal range of pH values where flocculation occurs most effectively, and this range is harder to acheive in Honduras. Thus, the water in the lab allows the system to be more robust and able to acheive accepatable effluent turbidity even with a large range of alum dosages. This observation means that even though the system appears to work successfully regardless of the alum dose, the same will most likely not be true in Honduras. We must modify our findings for the plant in Honduras because the same results will not be achieved with a different water chemistry.