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Figure 3 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 vs. number of residence times elapsed.

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Figure 4 below shows the total degree of mixing and the degree of mixing contributed by the 180° bends only, against plant flow. It also shows the maximum shear attained at the 180° bends and the average shear throughout the plant, againt plant flow. All values are calculated using a slightly modified version of the flocculator design program.

Figure 4. Degree of mixing & peak shear vs. plant flow.

The graph shows an increase in the degree of mixing and shear with increasing flow rate. It is also important to note that the degree of mixing in the Demo Plant flocculator depends on both the loss in the 180° bend sections as well as in the vertical sections. This is due to the fact that the flocculator channels are very narrow compared to those in the actual plant.  ThusThus,  the the frictional interactions with the channel the channel 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.

Simply by observing the Demo Plant as experiments were being run, we noted we noted that the largest flocs were created at a flow rate of 50 mL/min. These results also show us that the degree of mixing provided by the flocculator is sufficient at 50 mL/min to create very large flocs. However, at 25 mL/min, the flocs were very small, showing that the lower the lower shear had reduced the degree of mixing so much that the increased residence time could not compensate for it.

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