Current technologies include sand filters, clustered filters, automatic valveless gravity filters, multimedia and mixed-media filters, walnut shell media filters, activated carbon filters and precoat filters. Sand filters are the oldest and most common types of media filters. They trap particles as water flows via gravity through a bed of sand into an underdrain. 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.