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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.
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AbstractThe Sedimentation Tank Hydraulics Team performs laboratory research to determine the geometry of the sedimentation tank and the design of the inlet manifold so that a floc blanket forms efficiently. This lowers the turbidity of effluent and is also an indication of uniform flow and the elimination of dead zones. Current modifications to be tested are 1) the insertion of 60˚ incline planes along the perimeter of the tank, 2)manifold diffusers that extend 5 in from the tank bottom that direct downward-flowing jets of flocculated water. It is hypothesized that the diffusers will eliminate non-uniform form with respect to the length of the tank and resuspend flocs that have settled out and were directed to the jets via the incline planes. In order to test these modifications, a tube flocculator and sedimentation tank have been designed so that the upflow velocity in the tank equals the settling velocity, which is dependent on floc size and therefore controlled by parameters of the tube flocculator, as confirmed by previous experiments. Introduction and ObjectivesThe current design of sedimentation tanks in Agua Clara plants consists of plate settlers placed above the inlet manifold that catch flocs formed in the flocculator while allowing water to flow past the settlers into exit launders. The flocs then settle out at the bottom of the sedimentation tank and are removed regularly by a sludge drain. Because of non-uniform flow, 1) flocs accumulate unevenly in the tank which causes difficulty in removal, 2) dead zones are created in the tank that lower the efficiency of the system. We wish to determine the geometry of sedimentation tanks so that uniform flow is achieved and dead zones are eliminated, which would improve the efficiency of the current tank design and decrease the labor needed to maintain a clean tank. Given a particular upflow velocity, under these hydraulic conditions we may also experiment with floc blanket formation, which we believe will provide a second filtration stage for upward flowing flocs in the sedimentation tank and lower the turbidity of the effluent. Flocs may then be drawn off the blanket with a weir, minimizing the effort needed to clean the tank. Experimental Methods and ResultsSpring 2011 Sedimentation Team Research Rationale
Sedimentation Team Research Rationale2
In our Research Rationales, we explained in detail our design procedure and the reasons for such design. It also contains list of potential experiments we will be running this semester. Sedimentation Team Research Report 1
Detailed report describing the experiments the results from the experiments we ran this semester. Sedimentation Team Final Research Report
Detailed Final Report, summarizes Spring 2011 semester's sedimentation team work. Summer 2011 Sedimentation Team Final Research Report
Zipped folder containing Final Research Report in Lyx format, together with attached images. Final Presentation Sedimentation Tank Hydraulics |
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