In gravity filters, the raw water is introduced into the top of the basin and flows down through the media bed and is collected by the underdrain system. The driving force is the distance from the water level over the filter, to the first air break. If the discharge from the underdrain is below water level in the clearwell, that level is the determining factor.

The rapid sand, or gravity filter is the most common type used in water treatment to remove non-settleable floc remaining after chemical coagulation and sedimentation.

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