Triple media filters

Dual or triple media filters are used to provide a coarse to fine filtration facility. Where an anthracite-sand media bed is employed, the anthracite, which is half the density of sand, acts as a roughing filter prior to the finer sand bed. The larger and irregular voids within the anthracite bed are particularly suitable for the removal of floes, and filtration occurs deep within the bed. Where fine crystalline precipitates of calcium carbonate or calcium phosphate occur, smaller grain sizes may be employed without risk of high head loss or low filter rates. 

FIGURE 7.1 (a) Dual-media filter (b) triple-media filter. 

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