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In these experiments the alum dosage supplied to the flocculation system was varied in order to study how properties of flocs and the floc blanket affect the effluent turbidity produced by the tube settler. The experimental set-up is identical to the one used in Spring 2009, and from our results we hoped to analyze velocity gradient thresholds and possibly investigate how changing influent water chemistry affects the setter's efficiency.
The influent water in our system has a turbidity of 100 NTU. Varying alum doses were then added to this influent. The alum dosage significantly affects flocculation, and thus plays a direct role in the success of the system.
Alum Dosing Theory
Results and Discussion
Using the Spring 2009 team's process controller methods, we subjected an ideal geometry to non-ideal conditions. Specifically we altered the alum dose to see how different alum doses affected the effluent turbidity. Though the Spring 2009 team had success with a 9.5 mm diameter tube, due to what we think was ineffective air bubble traps in the flocculator, or the addition of a flow accumulator to the method, we experienced failure with this geometry. In our system, failure is defined as an effluent turbidity of above 1 NTU. We achieved an acceptable effluent turbidity (less than 1 NTU) with a 15.1 mm diameter tube that had a length of 30.5 mm. With the good experimental results, we then subjected this tube settler to varying alum dosage to investigate the affect of dosing on tube settler performance. At each alum dosage, the tube settler was tested at a variety of capture velocities and at two different floc blanket levels: the lower level is when the height of the floc blanket falls below the bottom of the tube settler, the high floc blanket level is when the floc blanket height is above that of the bottom of the tube settler.
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Because the effluent turbidity using the alum underdose of 15 mg/L was acceptable once the floc blanket had formed, it seems that the dosage is unimportant can be reduced once the floc blanket is completely formed, within a certain range of alum doses. It appears that as long as the floc blanket is fully formed, which should occur with a higher dose so that it forms quickly enough, the alum dose can be lowered while still experiencing the same results. However, the alum dose does need to be within a certain range for an effective floc blanket to form.
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