Clear Well Filtration

Introduction and Objectives

This is the first year that Aguaclara has investigated filtration and, if a successful design is created, filtration will greatly improve our water treatment plants. Our sub-team started out looking at stacked filtration and clear wells, and has decided to focus on clear well technology to determine its feasibility as a gravity fed system, and then maybe apply the technology to a stacked filtration system.

Concept of Operation for a Clear Well Filter

In the water treatment plant, the filter will be the final step after the flocculation and sedimentation tanks and it will be set at a lower elevation than the sedimentation tank. The filter will incorporate a rapid sand filter, which is a bed of sand that catches the dirt particles in the water running down through it. 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. When the clear well is filled to the proper elevation, the water will bypass the clear well and be sent to the distribution tank.

Periodically the filter becomes clogged with dirt and needs to be cleaned. At that time, the plant operator, will shut off the flow entering the filter and open a back wash drain and let the water drain out. Next, the clear well is opened and the water reenters the filter from the direction it left the filter. 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 30-50% for optimal cleaning. To achieve this, there is an optimal height of water in the clear well and an optimal height of the clear well above the filter that we have to achieve. Once finished, the operator will close the back wash pipe and begin filtration again.
Our immediate goal is to 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.

Figure 1: Clear Well Basic Concept

Initial Research:
In order to help us develop our clear well design, we conducted extensive literature and online research on granular filtration and backwash. We found Surface Water Treatment for Communities in Developing Countries by Christopher R. Schulz and Daniel A. Okun and Physicochemcial Processes for Water Quality Control by Walter J. Weber, Jr. to be useful sources of information in our research. These two documents are the only sources of information but there are solely mentioned because they essentially contain the same information as other objects of our online and literature review.
From Christopher R. Schulz and Daniel A. Okun, we learned the following useful information with regards to filtration:
• Filtration is the separation of suspended impurities from water by passage through porous media.
• Slow sand filtration consists of slowly filtering water through a layer of ungraded fine sand. Periodically, the top layer is clogged by impurities and is skimmed off the top.
• Rapid sand filtration rapidly conducts filtration in depth as compared to the slow sand filter which uses only the top layer to capture suspended particles. A lighter anthracite coal layer with larger pore spaces than sand is used on top of a sand layer to capture larger particles while allowing the smaller particles passage to be captured by the lower sand layer.(Schulz et. al 146)
• Backwashing is the act of removing the captured impurities in the filter bed by introducing enough water, usually from the effluent end, to fluidize and expand the bed and wash away the now released impurities. Backwashing is an art. There is neither a set time nor a set amount of backwash water required. If a filter is heavily clogged, significant length of backwash and greater backwash water for greater bed expansion are required. If the influent water is relatively low in NTU, less clogging may occur and a shorter backwash and less water may be required. Consequently, rapid filtration and required backwash operations necessitate a well trained crew of operators. This finding leads us to define our objective as not only providing a clear well design that works but a set of instructions in operating that design.(Schulz et. al 159)
• Headloss via expanded media can be calculated as below:

INPUT MATHTYPE Equation for 8.1 .(Schulz et. al 161)
From Walter J. Weber, Jr., we learned the following useful information with regards to filtration.
• INPUT MATHTYPE Equation 4-48 Pressure drop across Fluidized bed,
As the rate of backwash flow is increased, there is a linear increase in pressure drop to the point where some movement or reorientation of the granules takes places. At a slighter higher rate, the bed becomes fluidized and the pressure drop remains constant.
• INPUT MATHTYPE Equation 4-49 Minimal Fluidization Velocity (Weber 172)
• INPUT MATHTYPE Equation 4-50 to 4-53. Fluidization Velocity to achieve different degrees of expansion.(Weber 173)

Rest of Semester Plan

As stated above, the rest of the semester will be devoted to using the clear well technology applied to stacked filtration or other filter designs.

Assumptions/Design Elements

These are design elements we are trying to incorporate into any of our possible designs:
>Leave the system open for operator access
>Make system as simple as possible and understandable to operator as possible
>Make system of inexpensive materials and simple construction
>Make cleaning easy and understandable