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The goal of any flocculation process is to transform suspended colloidal particles into flocs that can be removed by sedimentation. The design of sedimentation tanks is dictated by the settling velocity of the flocs. Floc capture requires that the fluid residence time in a sedimentation tank or in plate or tube settlers be greater than the time required for the flocs to settle out. (Not necessarily. Not all flocs have to settle out and the fluid residence time may or may not affect how and when the floc particles settle out.) Therefore, one design goal of removal via sedimentation requires the formation of flocs that have settling velocities greater than the average upflow velocity of the sedimentation tank. Therefore, an important design goal for AguaClara flocculators is to create flocs with sufficiently high mean sedimentation velocities larger than the design upflow velocity in the sedimentation tanks. Unfortunately, guidelines for proper design and operation of hydraulic flocculators are incomplete. The appropriate level of mixing (quantified by the energy dissipation rate required at different points along the flocculator that will produce the best flocs (what do you mean by "best"?) is not well understood) required to produce flocs with a narrow distribution of high sedimentation velocities in a hydraulic flocculator is yet unknown. It is expected that high energy dissipation rates will initially enhance the collision frequency of small particles creating larger floc aggregates; however, however high energy dissipation rates are also likely to cause break up of large flocs.
In orthokinetic flocculation, differential velocities cause flocs to collide. A percentage of these collisions result in adhesion and the further growth of flocs. It has been shown that the frequency of collisions is related to the magnitude of the energy dissipation rate present during orthokinetic flocculation. As flocs grow larger, they become more susceptible to breakup. Thus, a continuum of energy dissipation rates affects floc growth and breakup in orthokinetic flocculation. Eventually the particle size distribution can reach a pseudo-steady state during which breakup balances aggregation (Spicer & Pratsinis, 1996).
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