Clear Well Backwash System Research

Introduction and Objectives

The purpose of this research subteam 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 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 cost-effective 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 easily accessible to the operator so they can 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

Basic Operation

Our Clear Well Backwash system is completely gravity-driven. Our entire filtration system 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 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, supported by a layer of gravel, and the sand entraps colloid sized particles in the sand pore space. The effectiveness of the filter is determined by the clarity of the water sent to the distribution system.

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. The water level in the filter will eventually rise until the elevation of water in the clear well is sufficient for backwashing. The elevation differences between the sedimentation tank, clear well, and the granular filter are 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 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.

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

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.

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

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 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.

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.

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 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.
• 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