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THEORY AND INTRODUCTION
Tube settlers are cylindrical devices used to filter andare designed to provide a low capture velocity to facilitate the removal of small flocs and hence lower the turbidity of dirty flocculated suspension water, and as can be seen from the final stages of the - they flocculated suspensions. They are integral to the overall effectiveness and performance of - the system we are analyzing AguaClara water treatment plants, as they are involved in the final particulate settling mechanism and thus the final effluent turbidity. The experimental apparatus contains a controlled clay stock that is added to raw water and mixed to a turbidity of 100 NTU. This raw water is sent through rapid mixing with a fixed dosage of alum, which then enters a flocculator (where the floc is actually created). From this stage, the water enters a settling column where it forms a floc blanket. The tube settlers draw water from this column and simultaneously filter it .
Tube settlers use the concept of capture velocity. Particles enter the settlers at a specific velocity, which for the purposes of our early experiments will be kept constant. The capture velocity is important because particles traveling faster than this quantity fall to the bottom of the tube settler as they enter and cascade back down to the floc blanket from which they were taken. This allows for clean, less turbid water to pass through the tube and out to be chlorinated. As the turbidity of the effluent water decreases, chlorination becomes more and more effective because pathogens in the water have more exposure to the cleaning chemical. Ultimately, this results in safer drinking water.
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plate settler length
plate settler diameter
plate settler angle
capture velocity
upward velocity
turbidity of source water
coagulant dosage
natural organic matter
Dependent
energy dissipation rate velocity gradient (turbidity, velocity, turbulence)
residence time (in plate settler and flocculation tubeflocculator and floc blanket)
EXPERIMENTAL DIRECTION
This summer , the plate settler team will first focus on the geometry of the tube settlers in order to gain insight into how parameters like the tubes' diameters and lengths affect the effluent turbidity, our main output and target of optimization. After the optimal geometry of the settlers has been determined under ideal conditions at two floc blanket levels--high and low--we need to test how robust this geometry actually is. To do so, we will create non-ideal circumstances by varying the influent turbidity and possibly the alum dosing in order to analyze how the system responds. Further, we hope to be able to characterize the system in terms of the fluid's residence time in the tube settlers in order to make correlations to the effluent turbidity.
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The tube residence time is not known to be an important parameter. I suggest keeping the focus on the capture velocity and on the effects of the velocity gradient at the tube walls. |
In past experimentation things like energy dissipation and velocity gradients within the tube settlers have been of much importance. Consequently, we are also interested in developing a physical model of our system that will allow us to explain such phenomena as floc roll-up, wherein velocity gradient thresholds are exceeded and floc enters the effluent water.
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work on generating a controlled ideal experiment as a basis for understanding the robustness of the plate settling system. We plan to test how the settlers function under a wide variance of conditions, since changes in alum dosage and influent turbidity inevitably affect the sedimentation process in many ways.
Velocity gradients within the tube settlers have proven to be very important. Particles that are supposed to settle out somehow make it into the effluent, and we would like to figure out why this is happening. Since it is likely that the density of our floc particles strongly depends on variable external parameters (like alum dosage and influent turbidity), our direction is highly geared towards a fluid dynamics anlysis
Velocity gradients are likely very important and it is possible that the length of the entrance region where the parabolic velocity profile of laminar flow is setup is also important. The Reynolds number influences the length of the entrance region where the velocity gradient at the tube walls will be higher.
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