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Influence of flocculator length and alum dosage on flocculation
Overview
Experimental Set-up
Abstract
Flocculation in water treatment plants is an essential step since it helps settle out colloidal particles with the use of a coagulant agent to facilitate particle collision and growth. For the AguaClara Tube Floc Team, the team's goal is to conduct experiments to further understand and optimize hydraulic flocculation. Specifically, the team will narrow down key design parameters such as the optimum length of the flocculator and optimum alum doses for each length and varying influent conditions tested. The experiments completed thus far include the replication of the Spring 2009 experiments and the study of the effects of alum dosage and length on flocculation.
Results from varying flow rate were consistent with the results from the previous team, which further confirmed that as energy dissipation rate increases the average floc size and the sedimentation velocity both decrease, while yielding higher residual turbidities. The success of replicating an experiment and obtaining similar data from Spring 2009 allowed the team to start conducting its own experiments to find the optimum alum dosages for different lengths of the flocculator. 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 turbidities.
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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Conventional design characterizes a flocculator with a laminar velocity gradient, G, and residence time, θ (Tambo and Watanabe, 1979). Because these are known parameters in literature the team has adopted them into its research, which can be characterized using a laminar tube flocculator. Currently the Tube Floc team is studying the effects of length and alum dose on varying influent turbidities and will determine the optimal values for these two conditions.
Experiments and Results from Fall 2009
*2009 Research and Experiments
Experiments and Results from Spring 2010
*2010 Research and Experiments
References
Tambo, N. and Watanabe, Y. (1979) "Physical characteristics of flocs. I. The floc density function and aluminum floc", Water Research, 13(5), 409-19