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Our Clear Well Backwash system is completely gravity-driven. Our entire filtration system will consist of two granular filter beds, one clear well, an outlet for effluent water for distribution, an outlet for dirt particles removed from water, and a system of valves to control the flow of water between the above mentioned different components shown in Figure 1. (Your visual describes the system but could be improved.) In the AguaClara water treatment plant, the filtration system will be the final treatment process after the sedimentation tanks. During regular filtration operations, effluent from the sedimentation tank will flow through the filter which is set at a lower elevation than the sedimentation tank. The filter media is a rapid sand filter composed of a bed of anthracite coal, sand, and gravel entrapping colloid sized particles through a variety of mechanisms in the pore space. The effectiveness of the filter will be determinate in the final clarity of the water that is sent to the distribution system.
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We had mixed results with regards to Weber's equation for filter bed expansion. At low levels of filter bed expansion, the Weber equation accurately predicted the fluidization velocity required to achieve the targeted bed expansion. As the target bed expansion increased, so did the degree of error. At 9% expansion, the degree of error was at 14%. At 38% expansion, the degree of error was at 37%.
Figure 2: Agalteca Plant with Filter Designed from the Conservative Approach (Velocity of Backwash is 10 times the Velocity of Filtration)
Sources of Error
Human error:
Despite our best attempt at being consistent (by measuring and marking heights on the test tube, while also holding a ruler on the test tube wall), there will always be human error in observing the bed expansion visually.
Fix:The next expansion experiment should use a camera so there is record of the heights at each flow rate, and also tape a ruler to the filtration bed wall, rather than holding the ruler or drawing it on.
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