Experiment 5: Alum dose = 105 mg/L
This is a significant overdose. The graph below shows the effluent turbidity plotted against time for capture velocities of 0.058, 0.116, 0.174, and 0.231 mm/sec, for the low floc blanket level only.
Figure 1: Effluent Turbidity vs. Time for Floc Blanket on low, alum dose = 105 mg/L
Conclusions
This experiment was meant to demonstrate poor performance with an alum overdose. While the effluent turbidity averages less than 1 NTU, it is inconsistent and frequently spikes above 1 NTU for all capture velocities, meaning this is a demonstration of failure.
The failure of the overdose can be attributed to a less effective floc blanket. This occurs since the flocs that form with an overdose are larger and "fluffier" than they would be with normal dosing, forming a less dense floc blanket that is less effective in trapping and filtering out flocs. As a result, the system fails to consistently yield an effluent turbidity of less than 1 NTU.