Granular filter media (effective sizes from 0.5 to 1.5 mm) generally remove particles larger than 7 to 10 percent of the smallest grains. Most water treatment applications try to remove color and all solids greater than 10 to 20 microns. Smaller particles are removed with the help of coagulating chemicals.In gravity filters, the conditioned raw water is introduced at the top of a basin, flows down through the media bed, and is collected by the underdrain system (Figure 1). The driving force is the distance from the water level over the filter to the first air break, which is usually
