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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Inlet Manifold Research Team
Introduction
The sediment tank is a critical component of the Agua Clara design. Since Agua Clara does not yet utilize filtration, the sediment tank is the location where virtually all of the suspended material removal occurs. Moreover, the important goal of minimizing construction costs means the tank should be as small as practical. Because of its relatively small surface area and the lack of a redundant or back-up removal mechanism, optimization of the sediment tank is a critical design goal.
The specific design challenge for the Inlet Manifold Research Team is to design an optimal inlet manifold for the sedimentation tank. Past Aqua Clara designs have evolved to inlet systems that currently feature PVC pipes with multiple holes (exit ports) creating vertical (downward) flow along the bottom of the pipe. The intent of the design is to create a smooth, quiescent influent condition that creates slow, upward flow through the sediment tank, ideally with the flocculated inflow initially traversing through a settled floc "blanket" that aids in removal. Some performance issues of past plants (lack of settling floc in one area of a tank, for example) have suggested that there may be non-uniform flow conditions in the sediment tank, leading to non-optimal performance.
The goal of an optimal influent design is to investigate and correct any discovered unbalanced flow conditions, thereby creating the following optimal inlet conditions:
• Sufficient inlet velocity to prevent sedimentation prior to entry into the tank
• No excessive velocities anywhere in the pipe, outlets, or in the tank that might cause floc breakup.
• An even distribution of the flocculated influent along the length of the sedimentation tank without horizontal circulation or other non-uniform flow patterns that will create non-uniform rising flow and/or non-optimal sediment tank performance. Optimal flow is vertically upwards until reaching the lamella plates.
While our team is specifically looking at the inlet manifold, some other aspects of the sedimentation tank may also be considered for an overall optimal design. For example, the behavior of the effluent launders and overall geometry may create interactions with the flow from the inlet manifold and may be considered. However, the length and spacing of the lamella separation plates (a primary sedimentation tank technology in Agua Clara tanks) are the subject of another group's research and assessment and are not the focus of the Inlet Manifold Team.
Current Research Teams
The Fall 2010 Inlet Manifold Team has focused on the theory that pressure recovery (also called static regain) 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.
The theory of our work is found HERE
The experimental set-up is found HERE
Results of our team's experiments are found HERE. 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.
Future Research Goals
Future research goals will be developed as the semester advances.
Past Research Teams
Spring 2010 Inlet Manifold Team
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