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It is interesting to observe the evolution of the coefficient of variation of sedimentation velocities as alum dose is increased. The coefficient of variation is the ratio of the standard deviation of the distribution of velocities divided by the mean of the sedimentation velocities:
Equation
Figure ? shows the evolution of the coefficient of variation as a function alum dose for an influent water of 100 NTU. It can be observed that the coefficient of variation decreases when we increase alum dose at a low alum dose. A decreasing coefficient of variation means that the distribution of particle size is getting narrower. However, after a certain alum dose, increasing the alum concentration makes the coefficient of variation increase. The spread of the sedimentation velocities is slightly getting larger with increasing alum dose.
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