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The Flocculator Residual Turbidity Analyzer (FReTA) allows us to gather data and investigate a number of different factors affecting flocculator performance, including shear (G), residence time (θ), alum dose, and influent turbidity. The shear rate in the flocculator can be controlled in Process Controller ProCoDA Software by either holding constant or varying the plant flow rate as desired. A given flow rate will define a particular shear rate in the flocculator. The shear rates in the tube flocculator can be calculated from flow rates and other characteristics of the setup using the following equations:
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We can also study the effect of increasing the residence time in the flocculator and holding shear constant by increasing the length of the flocculator while holding the flow rate constant; this will increase the amount of time water spends in the flocculator without changing the shear rate. Currently, setup can easily be modified to handle three different flocculator lengths, 27.96 m, 55.92 m, and 83.88 m. Process Controller ProCoDA Software can also be used to vary alum dosage, and set the desired influent turbidity for the raw water. This allows us complete control over what enters the flocculator, how long it spends in the flocculator, and how quickly it moves through the flocculator.
When running an experiment on FReTA, we allow 1.5-2 flocculator residence times to pass before collecting data. This ensures that the alum has the necessary time to react with the clay particles in order to produce a steady state distribution of flocs at the end of the flocculator.
After this loading time, Process Controller ProCoDA Software begins the actual data collection. The pumps ramp down gradually, and a ball valve is used to seal off the settling column (see Apparatus Setup) from the rest of the flocculator over a period of 6 seconds. The reason for this gradual shut down is to prevent turbulence that could disrupt flocs in the settling column. Process Controller ProCoDA Software then records the residual turbidity every second for half an hour (1800 s) at which point the valves open to begin backwashing for a new run.
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