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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
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Figure 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 above chart. The highlighted values are far beyond the disered effluent turbidity range, most above the initial turbidity of 100 NTU. Upon observation it was clear 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 to a few interesting observations about the settling of flocs in tube settlers.
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These results are displayed graphically below. _Graph 1. Effluent Turbidity vs. Critical velocity
!PSSAvgEffTurb vs Vc.jpg |width=700px height=1200px!
Graph 1. Effluent Turbidity vs. Critical velocity
As the results show, the second lowest capture velocity for each diameter tube resulted in the optimum settling efficiency at the low floc blanket heights. The results also show a general trend of increasing effluent turbidity once the capture velocity exceeds 11.0 m/day. it appears that the range of acceptable critical velocites is around 9 to 13 m/day. This indicates that the capture velocity designed to in the current plants is appropriate.
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 sedimentation tank with a small plate settler spacing.
Conclusions and Future work
There appears to be a trend of increasedaverage effluent turbidity for each tube diameter beyond a particular critcal velocity. Based on these results the optimal capture velocity would be 11 about 9-13 m/day for a large range of plate settler spacing. It is also important to mention that the range of average effluent turbidities is relatively small over the range of critical velocities tested.
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