Sedimentation Tank Hydraulics
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Introduction
A floc blanket is a dense, fluidized bed of particles that forms in the sedimentation tank below the plate settlers. A floc blanket works like a mesh to trap small flocs, thereby reducing effluent turbidity. As flocs are continually resuspended in the blanket, sludge build up on the tank bottom is reduced. The sedimentation needs to be drained and cleaned less frequently, reducing the amount of clean water wasted.
The latest working design of the AguaClara sedimentation tank consists of an inlet manifold with vertical diffusers to channel water into the tank as a line source. A semi-circular half-pipe beneath the diffusers serve as a jet reverser to resuspend flocs. Plate settlers placed above the inlet manifold that catch flocs while allowing water to flow past the settlers into exit launders. The flocs that settle out at the bottom of the sedimentation tank are removed regularly by a sludge drain. Implementing a floc blanket and a floc hopper in the sedimentation tank provide an additional method of filtration and will reduce sludge build up, instead allowing flocs to be wasted at a constant rate from the floc hopper.
Current and Future Research
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One of our main goals of the semester was switching our coagulant to PACl instead of alum and see how the change affects performance. The PACl seemed to be much stickier than alum and we had problems with it sticking to the glass and insides of the flocculator. We also conducted experiments switching energy dissipation rates in the jet reverser. We were expecting to find a point where the energy dissipation rate is too high and performance decreased. We didn’t find a point where the floc breakup caused a decrease in performance. During these tests we actually found that momentum can play a large role in floc blanket failure because if the momentum of the floc sliding down the incline gets too large it can overpower the pet reverser and cause it to fill up. We also looked at sludge consolidation depending on the depth of the floc hopper. We found that the depth can make a large difference in consolidation. |
More Information
Sedimentation Team Research Rationale
Sedimentation Team Research Rationale2
Sedimentation Team Research Report 1
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IntroductionWhile our team is specifically looking at the floc blanket, some other aspects of the sedimentation tank may also be considered for an overall optimal design such as inlet manifold and the launder. You will be able to find past research on inlet manifold on the side bar. |
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Past Research Teams
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The Fall 2010 Inlet Manifold Team has focused on the theory that pressure recovery may be responsible for uneven distribution of the influent. Pressure recovery is the effect caused by the energy of forward motion in the pipe. This velocity is converted to static pressure as the velocity slows toward the end of the pipe, as dictated by Bernoulli's equation. In addition, the forward motion created a horizontal component to the discharge, as discussed in our results. From our Fall 2010 Team Page you can find the theory behind our work as well as our experimental set-up in the lab. Also on our team page you will find the results of our team's experiments. Our scale models demonstrate both a pressure recovery effect and horizontal flow components; we will continue to explore these behaviors and model corrective designs as the semester progresses. The Spring 2010 team used an ADV, or Acoustic Doppler Velocimeter, to measure the velocity at the ports of both a 10' and 20' manifold model. Their data proved inconsistent with expected theoretical values and pointed towards technical difficulties with the mechanics of the ADV. The AguaClara flocculator design is based on Computational Fluid Dynamics (CFD) models. Velocity measurements, using particle image velocimetry (PIV show that the minor loss coefficient predicted by the CFD models over estimates head loss. Incorporation of these results into the CFD models will eventually make flocculator design more efficient and cost-effective for future Honduran communities. The course CEE 4540, Small-Scale Water Supplies, was challenged to develop creative solutions to the uneven flow distribution of the inlet manifold. Projects included MathCAD files and power point presentations. |
