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 NTU effluent goal of 1 NTU or lower.
>>Do not require electricity.
>>Economically sustainable-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 for the Clear Well Backwash System

Our Clear Well Backwash system is gravity-driven backwash system. Our entire filtration system will consist of two granular filter beds, one clear well, outlet for effluent water for distribution, outlet for dirt particles removed from water, and a system of valves to control the flow of water between the different components. Please see figure 1 below. 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 from the sedimentation tank and goes through the filter which is set at a lower elevation than the sedimentation tank. The filter will incorporate a rapid sand filter, which is a bed of anthracite coal, sand, and gravel that catches the dirt particles in the water running down through it. After going through granular filtration, the effluent water is sent to the distribution system.
In order to recharge 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. 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 the clear well. Consequently, one of the test of feasibility is to determine the elevation difference between the sedimentation tank, clear well, and the granular filter. An outrageously large difference would make this system unfeasible. When the clear well is filled to the proper elevation, the valve leading to the clear well will be closed off. 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 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 elevates 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 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

Our attempt to validate our clear well design consisted of three stages: 1) 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.

During the first stage, we conducted a literature and online review of existing filtration technology and research. We must determine 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.

The accuracy of our MATHCAD generated design parameters depend on the accuracy of the empirical fluidization velocity equations that we used. We developed a bench-scale model 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.

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.
>> 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 and porosity for the filter bed in our equations.
>> 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.

Plan of Action for Remainder of Spring 2010 Semester

The rest of the semester will 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 media.
>> Preliminary plant size design of the clear well backwash based filtration system for an actual AguaClara plant.