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The purpose of this research subteam is was 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

• 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.

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• Operate without electricity.

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• Construction material must be relatively

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• cost-effective and readily available in Honduras

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• Operation costs must be minimal.

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• The filtration and backwash system design must be simple in order to facilitate operation and maintenance as much as possible.

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• The filtration and backwash system must be

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• easily accessible to the operator so they can

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• observe the workings of the system (we assumed this to mean not pressurized).

The clear well design meets these objectives because it is a gravity run system that has water flow from the sedimentation tank through the gravity sand filter and then up into the clear well all by head loss differences. Once the clear well is filled, the water produced is sent to a storage tank to be distributed to the population.

A clear well is simply a large shallow tank that is utilized to clean the filter. When the filter becomes dirty, the flow to the filter is stopped and the clear well water flows back into the filter, elevating the sand particles and washing the dirt out of the filter. Back wash is designed to take about 10 minutes. Once this is completed, flow through the filter is continued and the clear well is filled back up.

Concept of Operation

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Basic Operation

Our Clear Well Backwash system is completely gravity-driven backwash system. Our entire filtration system will consist consists 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 . Please see figure 1 belowshown in Figure 1. In the AguaClara water treatment plant, the entire filtration system will be the final treatment process after the sedimentation tanks. During regular filtration operations, influent water comes effluent from the sedimentation tank and goes will flow through the filter which is set at a lower elevation than the sedimentation tank. The filter will incorporate media is a rapid sand filter, which is supported by a bed of anthracite coal, sandlayer of gravel, and gravel that catches the dirt the sand entraps colloid sized particles in the water running down through it. After going through granular filtration, the effluent water is sand pore space. The effectiveness of the filter is determined by the clarity of the water sent to the distribution system.
In order to recharge the clear well, effluent

Clear Well Operation

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. Once the water flows through the filter, it is pushed up by pressure difference into the clear well which is at a higher elevation than the filter. The water level in the filter will eventually rise until the head difference is enough to recharge elevation of water in the clear well . Consequently, one of the test of feasibility is to determine the elevation difference is sufficient for backwashing. The elevation differences between the sedimentation tank, clear well, and the granular filter . An outrageously large difference would make this system unfeasibleare key design elements for backwash to be effective. When the clear well is filled to the proper elevation, the valve leading to the clear well will be closed off. 

Backwash Operation

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 We would now have a supply of backwash water that we can confirm for quality and quantity necessary to thoroughly backwash our filter beds.
When the filter becomes clogged with dirt and needs to be cleaned, the plant operator will shut off the flow entering the filter and let 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 elevates fluidizes the sand particles in the filter, loosening the dirt particles that were caught in the sand . The water carries away the dirt particles into the backwash pipe, but not the sand particles because those are heavier. The sand bed will expand 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. Image Modified
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 the 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 must determine 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. Click here for the synopsis of our review of existing filtration technology and research.

Using what we learned from our research, we developed a MATHCAD file, which generate design parameters for our filtration/backwash system when provided with plant flow rate and properties of the granular filter. Besides being the basics for the final design of the filtration and backwash system for an actual AguaClara plant, this program would also generate parameters that we can scale down for bench-scale or pilot plant-scale testing. Click here for the description of the MATHCAD file and description.

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 The accuracy of our MATHCAD generated design parameters depend on the accuracy of the empirical fluidization velocity equations that we used. 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. Click here for the
Fluidization Velocity Experiment.

The accuracy of our model also required for the headloss occurring through an expanded bed to be more or less constant. According to our research, headloss should be constant once fluidization of the bed is achieved regardless of the degree of expansion. We simulated a variety of different bed expansion and observed headloss to be constant. Click here for the Expanded Bed Headloss Experiment.

Results and Discussion

Our experiments demonstrate that Okun and Schulz equation for headloss through a fluidized bed is a valid tool to use in our design. Once fluidization is achieved, headloss through an expanded bed is constant. For a 5 cm filter bed, headloss was around .7 cm regardless of the level of bed expansion. This is a positive discovery because a constant value for expanded bed headloss means that we can estimate all the headloss in our backwash system and design the height difference between the clear well and the filter bed appropriately.

Results and Discussion

We created the mathcad code to create design parameters for a plant the size of Agalteca with a flow rate of 6.3 L/s. We compared our two approaches, empirical and conservative (simple hydraulics).

Table 1. MathCad Results for a plant flow of 6.3L/s: Empirical vs. Conservative Approach

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.
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Figure 2: Plan View of Agalteca Plant with Filter Design
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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 38% 30% expansion (our target expansion), the degree of error was at 37%110%.
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Figure 4: Error Between Measured and Estimated Fluidization Velocities

Sources of Error

Wall Friction:

A very small error may be due to the increase of wall friction as the flow rate increased.
Fix: We can minimize the wall and tube friction We believe the following to be sources of error.
>> 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 (the diameter at which 60% of the particles are equal or smaller) 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.

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>> 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, 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.

Plan of Action for Remainder of Spring 2010 Semester

Expansion Head loss:

The accuracy of our model also required the 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.

Recommended Future Research

We recommend to do little future research in this area due to the great costs from the large scale of the proposed clear well. Future research into making the sedimentation tank be the backwash source might be fruitful, because this would make the filtration and backwash velocities the same because they are from the same source. However, if there is more research it should be focused on the following:

Future Research should The rest of the semester will be devoted to the following objectives:>>

• 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.

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• Determine the correct sand parameters to use to maximize filtration
• Testing the 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