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The figures 4,5,6,7,8,9,10 and 11 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 can observe the same behavior on the result. The values are not the same on the graph because we did not run the experiment exactly in the same conditions but to the experiment conducted closely replicated one of the Spring 2009 experiments. Date smoothing and normalization are the first transformation performed on the raw turbidity data. The smoothing allows excluding outliers, really large flocs that cause important turbidity fluctuation passing in front of the light sensor of FRETA. The normalization allows comparing data sets with different influent turbidity. The settling velocity (Vs) was calculated by dividing the distance between the ball valve and the zone illuminated by the infrared LED of FRETA by the time elapsed. The plot of normalized turbidity vs. Vs can be interpreted as a cumulative distribution function (CFD) of turbidity with respect to Vs. A CFD describes the probability that a variable is less or equal to some value. To make the analysis more robust, the experimental data was fit to a gamma distribution. Then a derivative of the CFD of the gamma distribution gives a probability distribution of the particle population with respect to Vs.
Figure 4 and 5 show the normalized effluent turbidity vs. time during the settling state for respectively Fall 2009 team's experiment and Spring 2009 team's experiment. In both case, the turbidity of the water decreases rapidly. Beside, we can observe that for low flow rate, the turbidity decreases more rapidly and the residual turbidity is lower in both experiments. Figure 6 and 7 represent the normalized turbidity as a function of Vs. As Vs is the inverse of time, we observe the opposite trend in the data. Figure 8 and 9 show the cumulative distribution function and figure 10 and 11 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). For more details on the data processor see Ian Tse's M.S. thesis (put a link).