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The very clear spike in effluent turbidity observed at the a flow rate of approximately 18 mL/min represents the point at which the floc particles began to roll up the tube settler, which was confirmed visually in the experimental apparatus. The velocity represents the critical velocity. At a certain velocity, the turbidity stabilizes, and stops increasing. This is because, at a certain point, the number of flocs rolling up in the settler cannot increase anymore and thus the turbidity cannot increase anymore. Given the current data that has been collected, we are not sure if this curve accurately represents how the turbidity should change during the ramp state function.
These experimental results can be compared with the expected results of our theoretical floc rollup calculations. Theoretical calculations for a 9.5 mm diameter plate settler tube predict that floc rollup should start to occur at a flow rate of 15.693 mL/min. Comparing this theoretical value with the observed floc rollup flow rate of approximately 18 mL/min above, we see that the experimental observations support the theoretical calculations quite well, within experimental error.
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Unlike the results for the 9.5mm tube, there is no obvious sharp increase in effluent turbidity. Although the turbidity of the effluent water does increase as the flow rate is increased, this change in turbidity is minimal compared to the 9.5 mm tube. The effluent tubidity is slightly higher than our standard of 1 NTU, but this is not a significant enough difference to assume that floc rollup has occured. Furthermore, theoretical calculations for a 15.3 mm diameter plate settler tube predict that floc rollup should start to occur at a flow rate of 65.557 mL/min. Analyzing the results plotted in Figure 2 above with this in mind, we see no visual confirmation of this predicted floc rollup flow rate in the form of a sharp peak in turbidity in the data around the predicted flow rate. Therefore, we have concluded that there is no clear evidence that floc rollup has occured. Due to the fact that the predicted values from our theoretical model calculations matched the observed results for the 9.5 mm tube quite well, we propose running more experiments with the 15.3 mm tube to verify whether the above discrepancy was due to experimental error or a problem with the theoretical model in predicting floc rollup in larger diameter tubes.
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