Experiment 5: Alum dose = 105 mg/L
This is an 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 extremely high dose was tested to show what a failure looks like (the turbidity is greater than 1 NTU). While the effluent turbidity averages less than 1 NTU, there are many spikes above this acceptable value for all capture velocities, meaning this is a good demonstration of failure.
This overdose failed because the floc blanket was not effective because 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 cannot effectively trap flocs and filter. As a result, the effluent turbidity is higher and the system fails to clean the water fully enough.