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Our Clear Well Backwash system is completely gravity-driven. 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 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 composed of a bed of anthracite coal, sand, and gravel entrapping , supported by a layer of gravel, and the sand entraps colloid sized particles through a variety of mechanisms in the sand pore space. The effectiveness of the filter will be determinate in the final is determined by the clarity of the water that is sent to the distribution system.

Clear Well Operation

For filtered water to accumulate in the clear well, effluent 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 leading to the accumulation of water in the clear well after filtration. The water level in the filter will eventually rise until the elevation of water in the clear well is sufficient for backwashing. Consequently, one of the tests of feasibility is to determine the The elevation differences between the sedimentation tank, clear well, and the granular filter are key design elements for backwashing backwash to be effective. A large difference, such as the clear well having to be more than 2 meters taller than the top of the filter, would make this system unfeasible mostly due to economic constraints. When the clear well is filled to the proper elevation, the valve leading to the clear well will be closed off. 

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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 away the dirt particles 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 sand than dirt particles) will remain in the sand filter. The sand bed will expand around 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

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