Page History

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Figure 1: Clear Well Basic Concept

Method

Our attempt to validate our clear well design consisted of three stages: 1) review 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 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.
We During the first stage, we conducted a literature and online review of existing filtration technology and research. We 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.
Research of Existing Work.

Important parameters for the final design of the filtration and backwash system for 2) Develop a MATHCAD file that generates backwash and filtration design parameters
We needed this for both an actual AguaClara plant and for a bench-scale or pilot plant -scale testing are in the following MATHCAD filemodel of the plant for testing.
MATHCAD File and description.

3) Experiments of bench-scale model to confirm design success
Bench scale modeling tests the effectiveness of a filtration design by shrinking the design parameters of the system (filter bed depth, filter bed surface area, and etc) to a smaller scale that is easier to test. For example, we would simulated a filter bed of 50 cm of sand with 5 cm of sand with the porosity and specific gravity of the sand being constant. This would also enable us to test the validity of the empirical equations that are behind our design.

The accuracy of our MATHCAD generated design parameters is based upon Our Mathcad design created two designs; one which was a conservative approach and another based upon empirical equations. The accuracy empirical fluidization velocity equations . We needed to be tested so 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.
Fluidization Velocity Experiment.

Results and Discussion

MathCad Results: Empirical vs. Conservative Approach

Experiment Results

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%.We believe the following to be sources of error:

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.

Wall Friction:

We can attribute the increase in error as flow rate increased due to the increase in wall friction on the test vial.
Fix:We can minimize the wall and tube friction 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. We show on the Fluidization Velocity Experiment page how small changes in these values could easily account for the error.
Fix: For the next experiment those parameters should be tested for the sand or material before conducting experiments.

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.
Fix:In the future, this hypothesis can be tested using dye. In addition, precuations can be taken, such as two levels could be clamped on the sides of the filter walls to ensure it is level or also use two clamps, rather than one.

Expansion Headloss:

The accuracy of our model also required the headloss occurring through the expanded bed to be more or less constant, which we did not have time to test (but is part of Recommended Future Research below).
Fix: Testing it would involve putting a pressure sensor into the system, connected on either end of the filter.

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