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Experiments were conducted to find the optimum alum dosages for different lengths of the flocculator and different influent turbidities. From these experiments so far, it's evident that as alum dosages increase, the residual turbidity decreases. However, at a certain point, a limit is reached where residual turbidity remains constant as alum dosage increases. Also these experiments have shown that with increased flocculator length, the behavior of the floc depends on whether the influent turbidity is high or low. At low influent turbidity, increasing the flocculator length improved the residual turbidity. At high influent turbidities in general, an equilbrium is being reached wherein the flocculator and particle size does not very. Thus, the same residual turbidities are found for all lengths of the flocculator at higher influent turbiditiesThe optimum alum dose seems to decrease as we increase flocculator length. Alum dose and flocculator length also affect the mean sedimentation velocities as well as the coefficient of variation of the velocities distribution. These effects vary with influent turbidities. At 100 NTU, mean sedimentation velocities increase and the coefficient of variation decrease up to a certain alum dose and then velocities decrease and the coefficients of variation increase. For 500 NTU, Mean sedimentation velocities increase and coefficients of variation decrease until they both reach a threshold traducing the presence of equilibrium in the flocculator.
Introduction
The turbidity of water is caused by colloidal particles in suspension (and the presence of natural organic matter and other organic and inorganic contaminants). Colloidal particles are too small to settle and due to their negatively charged surfaces, electrostatically repel each other. Flocculation transforms colloidal particles into larger flocs that can settle out in the sedimentation tank. The probability (collision potential) that particles collide in a flocculator depends on energy dissipation rate and residence time in the flocculator. As flocs collide, they grow in size making it easier to remove them in subsequent processes.
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