Sand filters capture all types of suspended solids down to between 5 microns and 10 microns or less. They also remove oils efficiently. Sand filters differ mainly in their backwashing method. Traveling bridge sand filters backwash media cells using a scouring mechanism mounted on a bridge. When all cells are clean, the bridge returns to its rest area until the next backwash cycle.
Operators easily can vary backwash intervals to match solids loading.
Low life-cycle costs, simplicity and reliability make this type of media filter one of the most widely used. Air diffusers above the sand bed create high-velocity movement to keep solids in suspension, so they do not plate out rapidly on the media surface. During the filter run, periodic pulses of air from beneath the underdrain disrupt the surface layer, resuspending deposited solids and burying a major portion of the solids in the bed. Backwash water control separates backwash solids by gravity, eliminating return of those solids to the treatment plant. Clustered filters employ cluster arrangements of gravity filters to achieve a range of particle-removal efficiencies with a variety of single or dual media.