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Based on the results of this experiment, and alum dose of 35 mg/L is not an under dose. The system performed above expectations, so we are going to test the system with an alum dose at 25 mg/L to see how the tube settler performs with an actual under dose.
(Explain if possible the discrepancy between 15 and 20 m/day capture velocity. Why for the high is 15 m/day performing worse than 20 m/day? If you can't explain it, then it is an abberation and perhaps has to be re-run.)