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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 2.5 g/L, a kaolin clay stock bucketat 10g/L, a raw water reservoir, a coiled tube (insert diameter) 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 (Weber-Shirk 2008). For detailed information on FReTA setup and the Process Controller figuration, please see Ian Tse's MS Thesis. Characteristics of the tap water used can also be found there.
In order to verify that the equipment was working properly and to familiarize our team with FReTA, we first performed a similar experiment to one that had been done the previous semester. We modified an earlier Process Controller file(see process controller file attached)that been set to run at flocculator length of 2796 cm with an influent turbidity of 50 NTU and a constant alum dosage of 38 mg/L while varying the plant flow rate from 3-19 mL/s increasing by 1 mL/s each trial to run instead at an influent turbidity of 100 NTU with an alum dosage of 45 mg/L; we maintained the same flow rate variation.
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 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. After the analysis, the results could be used to find the flow rate (shear) with the best performance for the set turbidity and alum dosage. For details on how the data was collected by process controller, and how Mathcad was used to analyze the data, see Ian Tse's MS thesis.
! :Proecess Controller^image.gif! .Process Controller
Results and Discussion
The sedimentation rate velocity and the residual turbidity of flocculated suspension are important properties in a flocculator. 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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The previous team conclusion on this kind of experiment was that increased fluid shear not only decreased the average size and sedimentation velocity of flocs, but it resulted in higher residual turbidities as well. The results of Fall 2009 are consistent with the previous team results.
The goal of this experiment was not to analyze into depth the results but to replicate on of the experiment from Spring 2009 and understand the utilization of the data processor. The next set of experiments will analyze the evolution of the effluent turbidity with the alum dose and the length of the flocculator.