Clear Well Backwash System Research

Introduction and Objectives

The purpose of this research subteam is to develop a sustainable backwash system for granular filtration, which will be incorporated into the AguaClara water treatment process. The backwash system must meet the following requirements:

• When incorporated with the current AguaClara water treatment process, the granular filtration with the clear well backwash system must consistently meet the target effluent goal of 1 NTU or lower.
• Operate without electricity.
• Construction material must be relatively cheap and readily available in Honduras.
• Operation costs must be minimal.
• The filtration and backwash system design must be simple in order to facilitate operation and maintenance as much as possible.
• The filtration and backwash system must be open so that the operator can easily observe the workings of the system.

Concept of Operation

Basic Operation

Our Clear Well Backwash system is completely gravity-driven. 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 above mentioned different components shown in Figure 1. In the AguaClara water treatment plant, the filtration system will be the final treatment process after the sedimentation tanks. During regular filtration operations, effluent from the sedimentation tank will flow through the filter which is set at a lower elevation than the sedimentation tank. The filter media is a rapid sand filter composed of a bed of anthracite coal, sand, and gravel entrapping colloid sized particles through a variety of mechanisms in the pore space. The effectiveness of the filter will be determinate in the final clarity of the water that is sent to the distribution system.

Clear Well Operation

For filtered water to accumulate in 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 leading to the accumulation of water in the clear well after filtration. The water level in the filter will eventually rise until the elevation of water in the clear well is sufficient for backwashing. Consequently, one of the tests of feasibility is to determine the elevation differences between the sedimentation tank, clear well, and the granular filter for backwashing to be effective. A large difference, such as the clear well having to be more than 2 meters taller than the top of the filter, would make this system unfeasible mostly due to economic constraints. When the clear well is filled to the proper elevation, the valve leading to the clear well will be closed off. 

Backwash Operation

When the filter becomes clogged with dirt and needs to be cleaned, the plant operator will shut off the flow entering the filter and allow 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 fluidizes sand particles in the filter, loosening the dirt particles caught in the sand carries away the dirt particles into the backwash pipe. The backwash pipe will be at such an elevation so that the larger and heavier sand particles will remain in the sand filter. The sand bed will expand around 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.

Our Mathcad design created two designs; one which was a conservative approach (most commonly used), based on the simple hydraulics that the necessary velocity of the backwash is 10 times the velocity of filtration. The second design was based upon empirical equations, called the Weber Equation. The accuracy of the empirical fluidization velocity equations 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. Simple Hydraulics (Conservative) Approach

Figure 2: Agalteca Plant with Filter Designed from the Conservative Approach

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, inspite 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.

Figure 3: Small Change in D60 can fix the error at 30% Expansion by over 100%

4)An additional advantage to Clear Wells is that the distribution tank does not have to be below the filtration tank, and in fact, it could be the clear well as well.

Figure 4: Distribution Tank can be the Clear Well Tank

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%.

Figure 5: Error Between Calculated and Actual Expansions

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.

Recommended Future Research

Future Research should be devoted to the following objectives:

• 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.
• Testing the headloss 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