Clear Well
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Backwash System Research
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.
These are the design qualities that we are trying to incorporate into 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
Concept of Operation for a Clear Well Filter
The purpose of the Clear Well Backwash System Research 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 bed, 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 It is 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 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 differences would make this system unfeasible. When the clear well is filled to the proper elevation, the water will bypass valve leading to the clear well and be sent to the distribution tank.Periodically 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. At that time, the plant operator , will shut off the flow entering the filter and open a back wash drain and let the remaining water drain out. Next, the clear well valave is opened and the backwash water reenters the filter from the direction it left 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 30-50% 30% 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 backwash valve and begin filtration again or recharge the clear well.
Figure 1: Clear Well Basic Concept
Method
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
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Our attempt to validate our clear well design consisted of three stages. During the first stage, we conducted a literature and online review of existing filtration technology and research. We then developed a MATHCAD file to generate filtration design parameters given state inputs. We then conducted bench scale experimentation of design parameters produced from the MATHCAD file.
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