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Preliminary experiments were conducted to determine the appropriate time interval for the flow and increment states. These experiments consisted of running the system for an exorbitant length of time to identify the timeframe necessary to reach constant effluent turbidity. Ultimately, we decided to form the floc blanket for three hours; run each flow rate for six hours; stop effluent flow for ten seconds between flow rates; and allow about twenty minutes for the floc blanket to grow from its low to high depths. The floc blanket was reformed each time a new diameter was tested.
Results
Figure Table 1. Results
Let us begin by discussing the failure. As stated in the methods section above, this experiment primarily tested tube settlers of three diameters. That is, three diameters that did not exhibit utter failure under nearly every condition. In fact, tubes of 6.35mm 9.5 mm inner diameter were tested as well. These narrow tubes exhibited failure readily, as indicated in the table above chart. The Certainly, the highlighted values are far beyond exceed the disered desired effluent turbidity range, most above the initial turbidity of 100 NTU. Upon observation it was clear
Observation of the failing tubes revealed that the tubes would clog with flocs and then, eventually, the water flow would push the clog through to the effluent. This process of clogging did, however, lead led to a few interesting observations about the settling of flocs in tube settlers.
To begin, we observed a "rolling" movement of the floc as it travels up the tube. This movement can begin to be described by a quantitative drag analysis which is in the process of being completed. It is hypotheiszed We predict that the flocs in the smaller diameter tubes experience enough drag force tooverpower to overpower the settling force due to gravity.
The wider tube settlers, as expected, provided slightly more sensitive data. That is, the effluent turbidity was measurably impacted by changes in flow rate, floc blanket height, and tube diameter. The table above and the graph below illustrates illustrate the average effluent turbidity for the three diameters described in the methods section above.These results are displayed graphically below.
Graph 1. Effluent Turbidity vs. Critical velocity
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However, as the graph clearly illustrates, there are no well-defined trends between diameters and floc blanket height at the capture velocities where effluent turbidities are under ~.5 NTU. Perhaps this is because the effluent turbidity is exceptionally sensitive to disturbances when it is at such a low level. Overall the average effluent turbiditity values for the three largest tube diamemters did not exceed 1.2 NTU. This small range of values indicates that it may be possible to build a robust an efficient sedimentation tank with a small plate settler spacing.
Conclusions and Future
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Work
There appears to be is a trend of increasedaverage increasing average effluent turbidity with increasing critcal velocity for each tube diameter beyond a particular critcal velocity. Based on these results, the optimal capture velocity would be about is 9-13 m/day for a large range of the plate settler spacing. It is also important to mention spacings tested in this experiment. Also notable, is that the range of average effluent turbidities is relatively small over the range of critical velocities tested.
Further analysis on the effect of drag on settling floc will contribute to the understanding of the lower limit tube diameter constraints. The next set of experiments will determine whether maintaining geometric similitude by holding L/D constitute, constant will result in consistently low effluent turbidities.