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At some point, the filter will become so clogged that the water level of the filter will begin to rise. Once the water level rises to a certain point, or the flow through the filter slows significantly, the filter has to be cleaned (how often this happens is usually plant and weather dependent). The plant operator will shut off the flow entering the filter and allow the remaining water to drain out. Next, the clear well valve is opened and the backwash water from the clear well will backwash the filter bed. This water fluidizes the sand particles in the filter, loosening the dirt particles caught in the sand and carries them into the backwash or sludge pipe. The backwash pipe will be at such an elevation so that the sand (which is larger and heavier than dirt particles) will remain in the sand filter. The clear well is designed so that as the last drop of water is flowing through the filter at the correct elevation to keep the sand particles elevated the target 30% for optimal cleaning. Once finished, the operator will close the backwash valve and begin filtration again or recharge the clear well. 
Figure 1: Clear Well Basic Concept
Method
1) Review of existing filtration/backwash technology and research
We conducted a literature and online review. 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.
2) Develop a MATHCAD file that generates backwash and filtration design parameters
We needed this for both an actual AguaClara plant and a bench-scale or pilot plant model 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.
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Below are rough proportional sketches of the plan view and side view representations of the clear well and two filters that we propose to build from the conservative design. These are pictured in relation to the flocculator and sedimentation tank at Agalteca.
Figure 2: Plan View of Agalteca Plant with Filter Design
Figure 3: Side View of Agalteca Plant with Filter Design
1) Our design based on simple hydraulics will work. However, it is a very large filter (see exact dimensions in Figure 2) and will not be sustainable economically. The material cost for construction will be too high.
2) The design based on the empirical Weber equation is smaller and less expensive. However, the validity of the empirical equations is not yet certain, in spite of our Fluidization Velocity Experiment. Therefore more testing needs to be done in pilot scale models.
3) If the empirical equations are valid, then we can change parts of the design, by changing the sand parameters. For example, lower the dimensions of the clear well by lowering the backwash velocity by decreasing the d60 and specific weight of the media. (see Fluidization Velocity Experiment for more specifics)
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 30% expansion (our target expansion), the degree of error was 110%.
Figure 4: Error Between Measured and Estimated Fluidization Velocities
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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, precautions 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 HeadlossHead loss:
The accuracy of our model also required the headloss head loss 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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- Complete putting fluid functions in the Mathcad code.
- Repeating the Weber Fluidization Velocity experiment with a larger scale bench model to see if error decreases.
- Determine the correct sand parameters to use to maximize filtration
- Testing the headloss head loss in the system through the expanded bed to ensure it is constant
- Create pilot scale model to determine any remaining error in the design before creating a plant scale