Flocculant Dosing Instructions

Because the pilot plant takes water directly from a stream, environmental conditions change all the time and affect the incoming turbidity to the plant as well as the chemical composition of the particles causing turbidity. It is therefore necessary to determine the best alum dose for each day of testing to ensure the formation of good flocs. This requirement was implemented in the following way:

Process Controller can be used under the "Alum Not Increment" 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 run for a 30-minute period, or for about 3 times the residence time of the tank. After the completion of the test, data processing should be performed to select the data from the last 10 minutes of each individual alum increment. The first 20 minutes of data should be rejected because the residence time in the flocculator is 10 minutes, and the residence time in the tube settler is also about 10 minutes. Therefore, in order to get readings from the final turbidimeter that are representative of the alum dose that is being tested, discard the first 20 minutes of data at each alum dose. The outgoing turbidity should then be analyzed for each increment, and the alum dose achieving the lowest turbidity should be selected for subsequent experiments that day.

Since the effectiveness of alum as a flocculant varies with temperature (it is less effective in colder water), and the effectiveness of polyaluminum chloride (PAC) is not significantly effected by temperature (this information was obtained through discussion with plant operators regarding their dosing experience), trying to work out an equivalency ratio between the Cornell Water Filtration Plant dose (PAC) and the pilot plant dose (alum) is not recommended. If equivalency is desired, the pilot plant should be converted to PAC as a flocculant so that a direct concentration equivalency can be used. This is relatively easy, and is the methodology currently (Spring 2009) in use.