h1. Plate Settler Spacing
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h4. Overview
This research is focused on a more thorough understanding ofand optimizing the lamellar sedimentation process so that AguaClara plants may remove flocculated particles from their effluent streams as efficiently as possible. Currently the plants use lamella, which are a network of stacked, sloped plates with narrow channels between them. These are used to provide more surface area for particles to settle out, thereby significantly decreasing the sedimentation tank plan area. As water flows through these channels gravity pulls out coagulated dirt particles, resulting in aimproved cleanfinished effluentwater. In the lab the Plate Settler Spacing Team (PSS) uses tube settlers to simulate the effects of lamella, where different tube diameters represent different spacing between the plates. The properties of these two technologies are analogous and our results from bench-scale experiments are applicable to plate settlers. The team is focusing on a failure mechanism called floc roll-up, where high velocity gradients present in small diameter tubes overcome the floc particles' settling velocity near the lab are directly applicable to plate settlers. wall causing flocs that would otherwise be captured to roll up into the effluent. Velocity Gradient theory (detailed in the [PSS Fall 2010 Velocity Gradients Experiments|PSS Fall 2010 Velocity Gradients Experiment]) dictates that performance deterioration due to floc roll-up will be more significant for tube settlers than for plate settlers, given that the tube diameter equals the plate spacing. This is due to the geometric differences between tubes and plates.
Since we are unable to control the turbidity level of the influent water entering the AguaClara plants, there is a significant interest in developing a robust system with high performance over a wide range of influentfield conditions. Nephelometric Turbidity Units, or (NTU,) are a measure of how concentrateddirty a dirty solution is based upon how much that solution scatters light. InOn the lablaboratory wescale, aimthe toteam producehas cleanproduced waterfinished with less than 1 NTU turbidity---and though this surpasses the WHO's, meeting the World Health Organization standard of 5 NTU, and demarcation,have itproduced doesresults notthat meet the EPA's 0.2- standard of 0.3 NTU. requirement,The whichPSS isteam's difficult to achieve without a filtration step. We striveobjective is to optimize the lamella design in order to achieve effluent water with 1 NTU or less turbidity, even under water chemistry fluctuations and variations in alum dosage. Some of the fundamental parameters which control the design of our experiments are plate spacing, capture velocity, and the formation of velocity gradients between the plates.
h4. Current Team Research Focus: Velocity Gradients
Past research has illuminated the importance of flow regime characteristics on the performance of tube settlers. Specifically, when velocity gradients established in the tube become too large, flocs at the bottom wall of the tube experience an upward force greater than the gravity pulling them down the plate. This causes flocs to roll up the side of the wall and exit with the finished water. The result of this phenomenon may vary from marginal increases in effluent turbidity to dramatic failure with turbidities closer to the system influent, depending on the magnitude of the velocity gradient in the tube. Current research focuses on using a bench scale system to validate the theoretical velocity gradient model developed last spring.
A schematic of the system used for measuring the performances of different tube sizes is given in Figure 1. The sequence of events for a typical experiment is as follows:
* The concentrated clay (10g/L) is diluted into the turbid water source until it reaches 100 NTU.
* The system switches to a floc blanket formation state, adding alum before mixing and flocculation. Prior experimental data indicated that an alum dose of 45 mg/L was optimal for 100 NTU influent.
* After the floc blanket forms, the system enters a loading state where tube settler effluent is sent to a reservoir (installed to prevent settling in turbidimeters that happens at around 50 mL/min for suspended clay particles). The reservoir delivers finished water to the turbidimeters at greater than 50 mL/min during a withdrawal state. As a consequence data collection for tube settlers is cyclical. The clarified effluent zone above the floc blanket is sampled so the tube settler effluent can be assessed relative to the tube settler influent. !SchematicExpFall2010.png|align=center!
Figure 1 - Schematic of the experimental design for testing different tube sizes with 10m/day capture velocity.
h2. Experimental Methods & Results
h3. Fall 2010
h5. [PSS Fall 2010 Velocity Gradients Experiments|PSS Fall 2010 Velocity Gradients Experiment]
This semester's research was designed the failure prediction of the velocity gradient model developed over the previous semesters for various sets of tubes diameters at upflow velocities of 1 mm/s, 2 mm/s, and 5 mm/s.
To isolate the effects of the velocity gradients from the effects of capture velocity, we chose set a constant capture velocity of 10 m/day for all tubes and tubes were designed without a constant length to diameter (L/D) ratio to meet these requirements. The L/D ratio is found by taking the length of a plate and dividing by the spacing between plates or in the case of tube settlers, the diameter. The design approach taken by the team was deemed acceptable because the research aims to propose improved plate settler design parameters (capture velocity and velocity gradient) over conventional parameters like L/D.
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h5. [PSS Dynamics Model|PSS Dynamics Model page]
h3. Spring 2010
h5. [Exploring the Effects of Velocity Gradients on Settler Performance|PSS Spring 2010 Velocity Gradients Experiment]
This experiment attempts to differentiate the effects of velocity gradients from capture velocity for different tube diameter and lengths.
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h5. [Exploring the Coupled Effects of Capture Velocity and Velocity Gradient on Settler Performance|PSS Spring 2010 Coupling Analysis Experiment]
This experiment attempts to hold geometric similarity in tubes of different diameters in order to explore changes in residual turbidity caused by the capture velocity and velocity gradient.
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[Subteam Semester Goals and Future Challenges|Plate Settler Spacing Goals]
[Weekly Subteam Progress|Plate Settler Spacing Meeting Minutes].
h3. [Previous Semester Research|PSS Summer 2008 to Fall 2009]
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h2. Additional Information
[Appendix - Fall 2010 - Useful Equations, definitions and some values for calculating the Pi-ratio|^WikiAppendix-Definitions_Equations_somevalues.pdf]
[Spring 2010 Team Presentations|PSS Presentations]
[Annotated Bibliography of Relevant Literature|PSS Bibliograhpy]
[Processor Controller Information|PSS Process Controller and Data Analysis]
[PSS Quiz for New Members |PSS Quiz]
[Fall 2008 Photo Gallery|Photo Gallery]
[PSS Apparatus Design] | 