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In this experiment, we simulated the backwashing of a granular filter with a sand test vial as the bench scale model. Our bench scale model consists of a 5 cm deep sand filter with a diameter of 3 2.5 cm. The sand has D60 of 0.5mm and porosity of 0.4. The diameter of the flow control orifice is 0.2 cm. Please see figure 1 below. We essentially introduce introduced a backwash flowrate of water of known velocity from the bottom and measure measured the bed expansion. The attentuator between the pump is there to eliminate the pulsing action of the pump. We measured the bed expansion as we increased the flow rate from 20 mL/min to 380 mL/min, which is the maximum flow rate possible with our pump configuration.
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We plotted the actual fluidization velocity vs the calculated fluidization velocity as the target bed expansion is increased. As expected, higher bed expansion required high fluidization velocity. At lower target bed expansion around 10-20%, the Weber empirical equation predicts the velocity required with around 10% percent. The error increases difference between calculated and actual increased as the target bed expansion increasesvelocity increased.
Figure 2: Calculated Fluidization Velocity vs Actual Fluidization Velocity
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The above sources of error will be very difficult to control for the actual filtration design. Consequently, we surmise that we need to apply a safety factor of around 10-30% when applying the empirical fluidization velocity equation. The follow up experiments for multi-media experimentation with larger bench scale model will further specify the safety factor required and we expect the larger scale model to reduce the overall error.