Sedimentation Tank Hydraulics Wiki

An improved sedimentation tank design in AguaClara facilities could significantly improve effluent performance, reduce clean water waste (from frequent draining of current sedimentation tanks), and reduce particle loading to filters (for plants considering filtration). We believe that the formation of floc blankets in full-scale sedimentation tanks is integral to achieving these objectives. A floc blanket is a fluidized and highly concentrated bed of particles. Floc blankets facilitate particle removal through mechanisms of flocculation and differential sedimentation. In lab tests, floc blankets were shown to significantly improve particle removal. Furthermore, the sedimentation tank would be drained and cleaned less often because a floc weir constantly removes flocs from the top of the floc blanket, and flocs would no longer settle to the bottom of the tank. Our underlying goal is to understand how we can form floc blankets in a scaled model and to identify the geometric parameters of the sedimentation tank that enable floc blanket formation so that these benefits can also be implemented at our full-scale facilities.

Conceptually, during floc blanket operation, the average upflow velocity in the sedimentation tank must match the terminal settling velocity of the concentrated bed of particles. Furthermore, as currently designed, the jets from the inlet manifold have a horizontal velocity component, which generates a circular flow within the sedimentation tank, as well as dead zones (areas that are quiescent with respect to fluid movement). Dead zones can also result from geometry. We believe that our current design of the sedimentation tank has potential dead zones and we will be focusing on designs that could possibly reduce dead zone.

Dead zones and non-uniform upflow conditions could hinder the formation of a floc blanket by respectively preventing floc re-suspension in some areas of the sedimentation tank, and excessive turbulence in other parts of the floc blanket, disturbing the floc-water interface and diminishing the ability of the blanket to catch particles. There are two important aspects of the sedimentation tank design (and possibly more) that can facilitate floc blanket formation: downward-pointing jets with an appropriate velocity to re-suspend large floc particles, and steep sloping sides that transport particles that have settled out to the re-suspending jet. Vertical tubes extending downwards from the inlet manifold, which we call diffusers, direct jets to re-suspend flocs on the tank floor and also eliminate the horizontal component of the jet velocity. We expect this to generate a more uniform upflow velocity distribution in the sedimentation tank.

We plan to construct a small-scale sedimentation tank incorporating these new design ideas to form floc blankets quickly and reliably in the lab before scaling up our design to the full size plant. If we create uniform upward flow and form a floc blanket in our model, we expect scaling up to be feasible because our model sed tank is designed to have the same upflow velocity as in the actual sedimentation tank, and floc size generated by our flocculator matches those created in full scale plants.

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