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The media used in deep-bed filters must be inert, resistant to fracture yet easily prepared in batches of graded particle size. Many different granular and crushed materials have been used to form the deep beds employed in the large gravity and
pressure filters common to the water purification and sewage treatment industries. In addition to sand, which is the classic and most common material, others used include garnet, ilmenite, alumina, magnetite, anthracite and quartz; coke and pumice have also been used but, because of their porosity, they are troublesome to clean and consequently give rise to the danger of uncontrolled breeding of bacteria.

The suitability of a granular material for use in a deep-bed filter depends both on the application and on the type of filter. Conventionally, there are two main types that operate with gravity flow downwards through a 0.6–1.0 m deep bed; these are identified respectively as ‘ slow ’ and ‘ rapid ’ sand filters, but only the rapid variant is truly a deep-bed filter. These utilize a velocity of 5–15 m/h and function by depth filtration within the bed. They are cleaned frequently by cessation of pro cess flow, followed by a reverse upward flow of wash water at such a rate that the bed expands and releases the trapped dirt particles; this cleaning flow may be augmented by some form of agitation, such as injecting compressed air below the bed or hydraulic jets impinging on the surface. This cleaning process has an important secondary effect, which is to reclassify the granules of the bed based on the combined influence of their size and their density, so that the washed bed is graded from finest at the top to coarsest at the bottom.

A size classification of one material does give the finest particles at the top, and therefore is more easily clogged than if there was a decrease in the size of flow channels downwards through the bed. The proper reclassification is best achieved by using a multi-layer filter. Two or more materials of different densities and sizes make up the bed, so that the hydraulic classification of cleaning places the finer, denser particles below the coarser, less dense particles (with filtration flow downwards).

The most modern version of the rapid sand filter is that which uses a moving bed of sand, whereby both filtration and cleaning proceed continuously and simultaneously. Recent evidence suggests that such filters can be as effective as membrane filtration plants in the removal of such pathogens as Cryptosporidia and Giardia from water intended for drinking.

Anthracite is usually one of the materials in a multi-layer bed. The anthracite particles are lighter and larger than sand particles, such that a mixed bed of the two provides good filtration in depth. The larger spaces between the anthracite particles enable high flow rates to be achieved, with low pressure drop losses, while the large surface area of the anthracite is efficient in removing algae, bacteria and turbidity.