Data Acquisition
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 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: (taken from Ian Tse's M.S. thesis )
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 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 for 1.5-2 residence times before collecting data. This means that a plug of water traveling through the flocculator would have time to move through the entire setup twice before we actually begin collecting data. This ensures that the alum and raw water have had enough time to mix properly and form a steady state mixture of flocs at the end of the flocculator, and that the results will not be affected by the water present in the tubing before a run begins.
After this loading time, Process Controller begins the actual data collection. After sealing off the settling column (see Apparatus Setup) from the rest of the flocculator, Process Controller 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.
The settling velocities corresponding to the time range we are studying are calculated by dividing the distance between the valve and the effluent turbiditmeter in the settling column (16 cm) by the time since settling began. Thus, the residual turbidity after 10 s would correspond all particles with a settling velocity of greater than 1.6 cm/s. We are not interested in recorded data after half an hour because this corresponds to extremely low settling velocities (>0.0889 mm/s) which means that these particles would not have time to settle out and would remain in the effluent.