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Our experiments were all performed on the same Flocculator Residual Turbidity Analyzer (FReTA) setup that was developed previously (Spring 2009). FReTA consisted of five parts: an alum stock bucket at a stock concentration of 2.5 g/L, a kaolin clay stock bucketat stock bucket at a stock concentration of 10g/L, a raw water reservoir, a coiled tube (0.953 cm) serving as the flocculator, and the residual turbidty analyzer with a settling column. The raw water turbidity was controlled using a feedback loop mechanism; clay from the stock bucket was metered in automatically if the turbidity became too low. Peristaltic pumps were used to provide the flow rate and meter in the alum solution. All flow rates and chemical dosages were calculated, monitored and controlled using the Process Controller sotware ( software developed by Weber-Shirk (2008). For detailed information on FReTA setup and the Process Controller figuration, please see Ian Tse's MS Thesis. All general information on the setup can be found in Chapter 1 Sec. 1.3-1.4 of the thesis. Characteristics of the tap water used can also be found there. Details of the Process Controller states and setpoints used can be found in Appendix A.
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During each run, the influent raw water combined with the correct alum dosage was allowed to run through the plant until two residence times had passed, insuring ensuring a steady-state effluent floc distribution. Then the pumps gradually ramped down, and a valve sealed off the settling column from the rest of the flocculator. The turbidity was monitored every second for half an hour as the flocs settled out, and the data recorded in an EXCEL spreadsheet. We then analyzed the data using Mathcad files developed by the previous (Spring 2009) team to develop settling velocity probability density function for the flocs. Details of the data analysis procedures can be found in Appendix B of Ian Tse's Thesis. After the analysis, the results could be used to find the flow rate (shear) with the best performance (lowest residual turbidity, largest mean floc size) for the set turbidity and alum dosage.
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Results and Discussion
The floc terminal sedimentation rate velocity and the residual turbidity of flocculated suspension are important properties in a flocculator. (How? Why?) The Spring 2009 team evaluated quantitatively the effect of shear velocity on these parameters. To do so, they used the flocculation residual turbidity meter (FRETA) developed by the AguaClara team and a data processor to analyze these parameters automatically.
Our goal for these first experiments was to familiarize ourselves with the apparatus and the data processor (MathCAD file) made by the previous team (Spring 2009) and to try to replicate one of their last experiments to make sure that the apparatus and the MathCAD file were working properly.
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