...
In this experiment, a bench-scale granular filter was backwashed. Our bench scale model consists of a 5 cm deep sand filter with a diameter of 2.5 cm (see Figure 1, left image). The sand (classified as D60) has a diameter 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 introduced a backwash flowrate of water of known velocity from the bottom and measured the bed expansion. An attentuator, or small tank filled with water, was installed between the pump was installed and the filter to eliminate the pulsing action of the pump (see Figure 1, right image). The bed expansion flow rate was increased from 20 mL/min to 380 mL/min (Can you describe in units that will scale to full-scale such as average velocity?), which is the maximum flow rate possible with our pump configuration.
...
Results and Discussion
We plotted the actual experimentally found fluidization velocity vs the calculated fluidization velocity as the as target bed expansion is was increased (see Figure 2). As expected, higher bed expansion required high fluidization velocity. The However, the difference between calculated and actual velocity increased as the velocity increased. ( What sort of relationship is exhibited in Figure 2? Also, please reference all figures before you insert them in your document.) Experiment 1 flow rate increased. therefore, the experimental and the calculated data had a roughly direct relationship with calculated data having a steeper slope.
Experiment results data.
Figure 2: Calculated Fluidization Velocity vs Actual Fluidization Velocity
...
>> Human error: Despite our best attempt at being consistent, there will always be human error in observing the bed expansion visually.
>> Wall Friction: We can attribute the increase in error as flow rate increased due to the increase in wall friction on the test vial. We can minimize this by increasing the size of our bench scale experiments.
>> Sand Properties Parameters: We might have used an incorrect D60 and porosity for the filter bed in our equations. (If this is the case, then why not try to fit the empirical equation to different sand diameters or porosities to see if this corrects for the majority of the error?)
>> Preferential flow: Despite our best attempt to keep the test tube as level as possible, we might have introduced preferential flow in our experiment causing an unbalanced backwash flow. (This may be true, but has it been confirmed with a dye study? If this is true what is the next step in keeping the column level?)
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.