PSS tube length experiment

Methods

In this experiment, we seek to determine what direct impact the diameter of the tube settler has on sedimentation efficiency. From the experiment detailed on the previous page, we were able to identify a range of capture velocities that produce low turbidity effluent regardless of tube diameter. For this experiment, we chose three capture velocities within this range and adjusted the length of the tube settlers to satisfy this velocity. Upflow velocity through the tubes was held constant. Recall that

is the capture velocity equation.

As the capture velocity equation above indicates, holding the capture velocity constant and varying the length to satisfy this velocity, allows isolation of the diameter of the tube as the parameter that should most directly impact settling efficiency.

Results

To this point (5/7/09), we have tested four different diameter tube settlers with a capture velocity of 8 m/day. Unfortunately, the data for the 9.5 mm inner diameter tube settlers are not convincing. As is evident from Figure 1 below, these data do not follow the trend of the other diameter tubes and the effluent turbidity is quite low. We will rerun the 9.5 mm tubes and be sure that water is flowing at all times. However, the other data gathered so far are quite interesting.

The graph below shows the average effluent turbidity over a six hour period for the four tube settler diameters.

Figure 1. Average effluent turbidity for various tube settler diameters with capture velocity of 8 m/day.

As Figure 1 above reveals, there is a correlation between tube settler diameter and effluent turbidity. In fact, the average effluent turbidity jumps 0.8 NTU when the inner diameter of the tube settler changes from 17 mm to 15 mm! This may indicate that there is a critical diameter, below which the sedimentation efficacy drops significantly.

Another preliminary observation of note is the consistency of the tube settler performance. For example, Figure 2 below shows the effluent turbidity of the 23 mm inner diameter tube settlers over several hours.

Figure 2. Effluent turbidity over time for 23 mm inner diameter tube settlers with capture velocity of 8 m/day.

This graph shows consistently low effluent turbidity over six hours for the 23 mm tubes. Conversely, the 15 mm and 17 mm diameter tubes showed a steady increase in effluent turbidity over time. Figure 3 below shows the effluent turbidity of the 17 mm inner diameter tube settlers over several hours. Although only the 17 mm effluent turbidities are shown below, the 15 mm tubes showed a similarly sloped linear increase.

Figure 3. Effluent turbidity over time for 17 mm inner diameter tube settlers with capture velocity of 8 m/day.

Future work

There is not information to draw any conclusions at this point, however, it is clear that there is a difference in performance in the tubes with the largest diameter and the smaller tubes. These experiments should be re-run at longer time intervals. The additional experiments will be added as a post-mortem addendum.