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Our Clear Well Backwash system is completely gravity-driven backwash system. 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 abovementioned different components. Please see figure 1 below. In the AguaClara water treatment plant, the entire filtration system will be the final treatment process after the sedimentation tanks. During regular filtration operations, influent water comes from the sedimentation tank and goes through the filter which is set at a lower elevation than the sedimentation tank. The filter will incorporate a rapid sand filter, which is a bed of anthracite coal, sand, and gravel that catches the dirt particles in the water running down through it. After going through granular filtration, the effluent water is sent to the distribution system.
In order to recharge charge the clear well, effluent water from the filter bed is diverted to the clear well by closing the valve leading to the distribution system and opening the valve leading to the clear well. Once the water flows through the filter, it is pushed up by pressure difference into the clear well which is at a higher elevation than the filter. The water level in the filter will eventually rise until the head difference is enough to recharge the clear well. Consequently, one of the test of feasibility is to determine the elevation difference between the sedimentation tank, clear well, and the granular filter. An outrageously large difference would make this system unfeasible. When the clear well is filled to the proper elevation, the valve leading to the clear well will be closed off. We would now have a supply of backwash water that we can confirm for quality and quantity necessary to thoroughly backwash our filter beds.
When the filter becomes clogged with dirt and needs to be cleaned, the plant operator will shut off the flow entering the filter and let the remaining water drain out. Next, the clear well valve is opened and the backwash water from the clear well will backwash the filter bed. This water elevates the sand particles in the filter, loosening the dirt particles that were caught in the sand. The water carries away the dirt particles into the backwash pipe, but not the sand particles because those are heavier. The sand bed will expand 30% for optimal cleaning. Once finished, the operator will close the backwash valve and begin filtration again or recharge the clear well.
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Our attempt to validate our clear well design consisted of three stages: 1) review of existing filtration/backwash technology and research, 2) development of a MATHCAD file that can generate backwash and filtration design parameters for both an actual AguaClara plant and the a bench-scale or pilot plant model of the plant for testing and 3) experiments of bench-scale or pilot plant model to confirm design success.
During the first stage, we conducted a literature and online review of existing filtration technology and research. We must determine determined the flow rate needed to sufficiently expand and clean the sand filter bed. This will help us determine how high the clear well needs to be above the filter, how large the flow pipes should be, and how much water should be in the clear well. Click here for the synopsis of our review of existing filtration technology and research.
Using what we learned from our research, we developed a MATHCAD file, which generate design parameters for our filtration/backwash system when provided with plant flow rate and properties of the granular filter. Besides being the basics for the final design of the filtration and backwash system for an actual AguaClara plant, this program would also generate scaled down parameters that we can scale down for bench-scale or pilot plant-scale testing. Click here for the description of the MATHCAD file and description.
The accuracy of our MATHCAD generated design parameters depend on the accuracy of the empirical fluidization velocity equations that we used. We developed a bench-scale model of our filtration system and conducted an experiment measuring the expansion of a filter bed as backwash velocity is varied. We then compared the empirically calculated fluidization velocities with the actual fluidization velocities required. Click here for the Fluidization Velocity Experiment.
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Our experiments demonstrate that Okun and Schulz equation for headloss through a fluidized bed is a valid tool to use in our design. Once fluidization is achieved, headloss through an expanded bed is constant. For a 5 cm filter bed, headloss was around .7 cm regardless of the level of bed expansion. This is a positive discovery because a constant value for expanded bed headloss means that we can estimate calculate all the headloss in our backwash system and design the height difference between the clear well and the filter bed appropriately.
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>> Repeating the Weber Fluidization Velocity experiment with a larger scale bench model to see if error decreases.
>> Repeating the Weber Fluidization Velocity experiment with multiple layer filter media.
>> Preliminary plant size design of the clear well backwash based filtration system for an actual AguaClara plant.