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At some point, the filter will begin to be 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 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 sand particles in the filter, loosening the dirt particles caught in the sand carries away the dirt particles into the backwash pipe. The backwash pipe will be at such an elevation so that the larger and heavier sand particles will remain in the sand filter. The sand bed will expand around 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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