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For the majority of the testing done this summer, alum dose was set by the aforementioned log relationship equation (Y = A + B*log(NTU)). After A was adjusted from 15 to 10, this approach was effective for the low turbidities that the flocculator experienced this summer. At other times of year when the raw water turbidity is higher, the effectiveness of this relationship could be tested in a higher turbidity range. Through use of this equation, observations of the floc tank, and conversations with the operators at the water treatment plant, it has become apparent that there is still a lot of research that needs to be done regarding alum dose.
Observing the floc tank was helpful in being able to identify different kinds of floc and what different alum doses looked like in the tank. The water treatment plant has now switched to a different coagulant (PAC) but if they had to go back to alum they said they would use past experience and alum doses as well as jar tests to set their doses. This suggests that for each water treatment plant an equation, formula or at least a rule of thumb could be developed from past water treatment for future dosing. Due to variations from plant to plant in influent water, it is improbable that this formula would be useful at other water treatment plants. The run increment alum dose test helped to shed light on alum dosing as it allows the alum dose to be changed while at a relatively constant raw water turbidity. The data from this type of test should show either an optimum dose or a small range of optimal doses for specific settled water turbidity.
Methodology
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:
The Process Controller was 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 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.
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.