Replication of Spring 2009 experiment : Variation of flow rate

Results and Discussion

The floc terminal sedimentation velocity and the residual turbidity of flocculated suspension are important properties in a flocculator. Residual turbidity is the turbidity resulting from the flocs that failed to reach the capture velocity, which is approximately 0.12 mm/s for the plate settlers in AguaClara plants. Residual turbidity is a measurement of the anticipated performance of the plant if the effluent from the flocculator passed into the tube settlers without any other sedimentation process. The Spring 2009 team quantitatively evaluated the effect of shear velocity on these parameters. To do so, they used the flocculation residual turbidity meter (FReTA) developed by the AguaClara team and a data processor to analyze these parameters automatically ( see data acquisition)

Our goal for these first experiments was to familiarize ourselves with the apparatus and the data processor (MathCAD file) made by the previous team (Spring 2009) and to try to replicate one of their last experiments to make sure that the apparatus and the MathCAD file were working properly.

Table 1: Parameters for the Fall 09 experiment

*Figure 1 *: _ Evolution of the velocity gradient with flow rate_

The experiment was conducted with the parameters shown in table 1. The parameters were based on one of Ian's experiment (data from 5/13/2009) conducted with the same inputs, except for the flow rates which vary from 4 to 19 mL/s and the turbidity which was set at 50 NTU(MathCAD file).

The figures 2,3,4,5 show the plots, given by the data processor, with the data from our experiment (9/24/09) and the data from the Spring 2009 experiment (5/13/09). Comparing graphs at each state, we observed similar results when compared with the previous years experiments. Our experimental values, seen on the graphs below, are not exactly the same as Ian's Spring 2009 data because the experiments were not performed under the same conditions, but they closely resembled one another.
Figure 2, A and B, shows the normalized effluent turbidity vs. time during the settling state for Spring 2009 team's experiment and Fall 2009 team's experiment, respectively. In both cases, the turbidity of the water decreases rapidly. Moreover, we can observe that for low flow rates, the turbidity decreases more rapidly and the residual turbidity is lower in both experiments. Figure 3, A and B, represents the normalized turbidity as a function of Vs. As Vs is the inverse of time, we observe the opposite trend in the data. Figure 4, A and B, shows the cumulative distribution function and figure 5, A and B, show the resulting probability density function (PDF) of settling velocities obtained from the fitted gamma distribution for Fall 2009 experiment and Spring 2009 experiment. We can observe on the PDF curves that in both cases the mean velocities are decreasing with the flow rate (trace 1 being the lowest flow rate and trace 8 the highest).

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Figure 2:Plot of the normalized effluent turbidity (NTU) vs. time(s). Experiment of 5/13/2009(A). Experiment of 9/24/2009 (B)

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Figure 3:Plot of the normalized effluent turbidity (NTU) vs. Vs (m/day). Experiment of 5/13/2009(A). Experiment of 9/24/2009 (B)

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Figure 4:Plot of the normalized effluent turbidity (NTU) vs. Vs (m/day) fitted to a gamma distribution. Experiment of 5/13/2009(A). Experiment of 9/24/2009 (B)

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Figure 5:Plot of the probability distribution of the particle population vs. Vs (m/day). Experiment of 5/13/2009(A). Experiment of 9/24/2009 (B)

The previous team conclusion on this kind of experiment was that increased fluid shear not only decreased the average size and sedimentation velocity of flocs, but it resulted in higher residual turbidities as well. The results of Fall 2009 are consistent with the previous team results.
The goal of this experiment was not to analyze the results in depth but to replicate the experiment from Spring 2009 and understand how to use and analyze data with the data processor. The next set of experiments will analyze the evolution of the effluent turbidity with the alum dose and the length of the flocculator.