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In order to investigate alum dosage at a given turbidity and length, we planned to run two experiments. The first experiment would vary the alum dosage over a large range in larger intervals while holding the plant flow rate (shear), influent turbidity, and residence time constant. Then we planned to run a second experiment varying the alum dose across the best performing interval from the first experiment with small increments to determine the optimal dose.
For a flocculator length of 2796 cm, we began by testing water with an influent turbidity of 100 NTU with a plant flow rate of 5 mL/s. The alum dosage was varied from 20-60 mg/L in increments of 5 mg/L for each run. During each run, the influent raw water combined with the correct alum dosage was allowed to run through the plant until two residence times had passed, insuring a steady-state effluent floc distribution. Then the pumps gradually ramped down, and a valve sealed off the settling column from the rest of the flocculator.
(see the following Process Controller File)
100NTU Experiment
500NTU Experiment

Results and Discussion

To find the ideal alum dosage for a certain influent turbidity, there are several qualities that must be considered. First, the resulting floc size should be large enough so that by the time it reaches the sedimentation tank, it will weigh enough to sink and settle out. Second, the resulting NTU must be small indicating an efficient flocculation/sedimentation process and clean water, and third, the alum dosage should preferably be low to save cost.

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