When the sand in a rapid sand filter becomes dirty, what is the process used to clean it?

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Backwashing is the process used to clean the dirty sand in a rapid sand filter. When the bed becomes clogged, filtration is stopped and the flow is reversed so water moves upward through the sand from the underdrain. This upward flow agitates and lifts the bed, breaking up the dirty cake and carrying the accumulated solids out of the filter with the wash water to the waste line. Air scour is often introduced to help loosen fine material. After the bed is cleaned and the effluent from the backwash runs clear, normal service flow is restored and filtration resumes.