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The Process Controller was used under the "Alum Not Increment Alum" setting, which starts the Alum Dose at 0 mg/L, and increments at 5 mg/L until it reaches a maximum of 40 mg/L. Each of the alum doses ran for a 30-minute period, or for about 3 times the residence time of the tank. After the completion of the test, data processing was performed to select the data from the last 10 minutes of each individual alum increment. The first 20 minutes of data were rejected because the residence time in the flocculator was 10 minutes, and the residence time in the tube settler was also about 10 minutes. Therefore, in order to get readings from the final turbidimeter that were representative of the alum dose that we were testing, we discarded the first 20 minutes of data at each alum dose. The outgoing turbidity was then analyzed for each increment, and the alum dose achieving the lowest turbidity was selected for the second part of the experiment.
However, one rule of thumb that may be used under lower temperatures and incoming tubidity levels can be to rely on the PAC (Polyaluminum Chloride) dosage use by the Cornell Water Filtration Plant Plant Operators. According to the active ingredients for the alum and PAC along with the equivalent ratios given for both coagulants, 1 ppm of PAC is approxmately equal to 0.8 mg/L of alum. This leads to an optimum dosage of approximately 4.0 mg/L of alum for the Pilot Plant Flocculator. Note that this may only apply for low temperatures and incoming tubidities.