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Overall, this system performed well and most of the effluent turbidities were below 1 NTU. The thesis on evaluation on parameters affecting steady-state performance of a floc blanket found that the ideal dosage was 45 mg/L for 100 NTU influent water. The the alum doses of 35 mg/L and 65 mg/L performed best. Because the slight underdose and overdose peformed well (average effluent turbidities were under 1 NTU), what was thought to be extreme under and over doses were also tested. We tested 15 mg/L and 105 mg/L to observe how the floc blanket formed under severe non-ideal conditions. The dose of 105 mg/L, despite averaging at less than 1 NTU, resulted in failure since the average effluent turbidity frequently spiked above 1 NTU. The 15 mg/L dose, however, had average effluent turbidity of less than 1 NTU, meaning that cannot be considered a failure.
Originally, an alum dose of 45 mg/L was thought to be an ideal dose, meaning that it was supposed to produce the lowest effluent turbidity. 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; possible reasons for these results include air bubbles in the tube settler
and the fact that the 45 mg/L experiments were run with an old apparatus with a flow accumulator. Experiments at this alum dose should be re-run.
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In contrast, an alum overdose forms a less dense, more "fluffy" floc blanket, which is not as effective because the flocs are weaker and breaking up in the floc blanket causing causes spikes in effluent turbidity. The extreme overdose of 105 mg/L shows higher turbidity as a result of these weak flocks in the floc blanket.
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The water chemistry in our system also contributes greatly to the unexpected results.(were the results unexpected except for These unexpected results include the poor performance of the system with the alum dose at 45 mg/L ?)and good performances with what was thought to be an exterme under and over dose. 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 achieve in Honduras. Thus, the water in the lab allows the system to be more robust and able to 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 ranges of alum doses that we tested, the same will most likely not be true in Honduras. A future study could include changing the alkalinity of the water to make the water pH more sensitive to changes in alum dose to confirm the applicability to Honduras.