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The above table shows the average effluent turbidities for each alum dosage, floc blanket state and capture velocity.
Overall, the this system performed well and most of the average effluent turbidities were below 1 NTU. The overdose of alum did cause the effluent turbidity to be slightly higher than the ideal dose, however it was still within the range of ideal effluent turbidity. It was expected that the 35 mg/L alum dose would perform poorly. However, this dosage produced better results than the ideal alum dose, so further experiments are being performed to collect data with a lower alum stock concentration. (Include your results from 25 mg/L. Relate your graphs and results to your conclusions. What are these results indicating? Is alum dose affecting performance? Is it fair to indicate that these doses are over and under dosed? If not, do we need higher and lower doses? What does this mean for how we are dosing alum? Are the water chemistry conditions we have in lab applicable to Honduras? What would be different in Honduras about dosing that we do not see here.)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.