Procedure
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 (link to Ian 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.