Traditional systems use sand to trap the particles between the grains of sand.
The small sand granules are at the top of the filter bed and the larger sand granules are at the bottom. Therefore, all of the filtration takes place at the top. When the pressure loss becomes excessive, the filter bed is backwashed to purge the particulate matter from the bed. The depth filter has three different layers of material . Very coarse material of light density is used at the top.