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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" slight underdose and overdose 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, despite averaging at less than 1 NTU, resulted in failure since the average effluent turbidity frequently spiked above 1 NTU. The 15 mg/L extreme underdose, however, did not experience failure.
OriginiallyOriginally, an alum dose of 45 mg/L was thought to be an ideal dose. However, this alum dose did not perform as well as was expected. In comparison to the other average effluent turbidities, 45 mg/L should either perform slightly better than an alum dose of 35 mg/L or somewhere between 35 mg/L and 65 mg/L. As shown in the above graphs, 45 mg/L performs worse than all of the other alum doses, including what was supposed to be extreme under and over doses (15 mg/L and 105 mg/L). This should not have happened, and experiments at this alum dose may need to should be re-run.
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.
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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 achieve in Honduras. Thus, the water in the lab allows the system to be more robust and able to acheive accepatable achieve acceptable 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.