Filter Media

The filter media is the part of the filter which actually removes the particles from the water being treated.  Filter media is most commonly sand, though other types of media can be used, usually in combination with sand.  The gravel at the bottom of the filter is not part of the filter media, merely providing a support between the underdrains and the media and allowing an even flow of water during filtering and backwashing. 

The sand used in rapid sand filters is coarser (larger) than the sand used in slow sand filters.  This larger sand has larger pores which do not fill as quickly with particles out of the water.  Coarse sand also costs less and is more readily available than the finer sand used in slow sand filtration.



Dual and Multi-Media Filters

In many cases, multiple types of media are layered within the filter.  Typically, the layers (starting at the bottom of the filter and advancing upward) are sand and anthracite coal, or garnet, sand, and anthracite coal.  The picture below shows a cross-section through a dual media filter. 

Layers of filter media.
Photo Credit: Christie Shinault

The media in a dual or multi-media filter are arranged so that the water moves through media with progressively smaller pores.  The largest particles are strained out by the anthracite.  Then the sand and garnet trap the rest of the particulate matter though a combination of adhesion and straining.  Since the particles in the water are filtered out at various depths in a dual or multi-media filter, the filter does not clog as quickly as if all of the particles were all caught by the top layer. 

Depth filter.
The largest particles are removed by the coal, the medium particles by the sand,
and the smallest particles by the garnet. 

The media in a dual or multi-media filter must have varying density as well as varying pore size so that they will sort back into the correct layering arrangement after backwashing.  Anthracite coal is a very light (low density) coal which will settle slowly, ending up as the top layer of the filter.  Garnet is a very dense sand which will settle quickly to the bottom of the filter.