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Figure 2.
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Process flow diagram of experimental setup.
Procedure
Process Controller, a software based on LabView and written by Dr. Monroe Weber-Shirk, is used to automate the experiment and record the data. The automation routine, written by James Leung, controls the power to the stirrers and the speed of the pumps, calculated from the plant flow and residence time parameters. All other parameters, such as alum dose and pump tubing sizes, are fixed and preset in the routine. Process Controller also records turbidity data at 5 s intervals to a tab-delimited text file.
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The graph below shows the effluent turbidities for various plant flows, plotted against dimensionless time, which is real time divided by the residence time of the plant at that plant flow. This is done for ease of comparison between different experimental flow rates. The effluent turbidities plotted are the average effluent turbidity values obtained through several experimental trials.
Figure 3. Average effluent turbidity
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vs. number of residence times elapsed
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Most of the plots show a large spike in effluent turbidity near the beginning of each experiment. This is due to the experimental procedure and can be ignored. Between each experiment, the water in the turbidimeter was not cleaned out, temporarily resulting in an artificially raised turbidity at the beginning of each experiment. Thus, these spikes can be neglected in our analysis of the results.
The graphs show that as plant flow rate decreases, the effluent turbidity also decreases, until we reach 50 mL/min. At 25 mL/min, we found that the effluent turbidity is unstable but not lower than that of 50 mL/min.
We plotted the actual amount of mixing as a result of shear and the maximum allowable shear in the figure below, as calculated by the MathCAD file versus flow rate. This calculation included losses in both the 180-degree bends as well as the vertical sections. This graph shows an increase in the degree of mixing and shear with increasing flow rate. The large G-theta calculated by the MathCAD file shown on the graph also shows that G-theta of the Demo Plant depends on both the loss in the 180-degree bend sections as well as in the vertical columns. This is due to the fact that the Demo Plant lamellas are very thin, compared to those in the actual plant, so that frictional interactions with the baffle walls are significant and cannot be neglected.
Figure 4. Degree of mixing & peak shear vs. plant flow.
To test if the lamellas in the sedimentation tank experienced very non-uniform flow rates, we added red dye to the last channel of the flocculator, and observed its progress through the sedimentation tank. We saw that the red dye proceeded mostly through the last 3 lamellas of each side of the sedimentation tank, and that it moved the fastest through the last ones. Also, upon further observation, we saw that at 100 mL/min, medium and small sized flocs were being carried up these channels of the sedimentation tank to the effluent turbidity meter.
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