The usual filter medium is sand but other materials - anthracite, garnet, manganese dioxide, are used for special applications. Water enters the top of the vessel and flows downwards through the bed of media and leaves through a collector system at the bottom. Contaminants are removed through straining and or or adsorption depending on the chosen media. shows a schematic of a typical granular media pressure filter. Water is passed through the filter until either the head loss across it rises to a preset value, or contaminant breakthrough is detected. It is then taken out of service and cleaned by backwashing with clean water.The backwash water enters the bottom of the filter vessel and flows upwards through the media bed, which starts to fluidize that are the media grains begin to separate from each other and to float freely.
The fluidization of the media releases the retained contaminants, a process which may be assisted by bubbling air through the bed (air scouring) which causes the sand grains to rub against each other dislodging the retained contaminants from the surface.