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Multi-media filters represent a significant improvement over single-media filters. This is due primarily to improved filter bed action based on the innovative use and selection of filter media. Multi-media filtration permits delivery of high quality filtered water at much faster flow rates, as compared to a conventional sand filter.

In a conventional sand filter, lighter and finer sand particles are found at the top of the filter bed, and coarser, heavier sand particles remain at the bottom after backwashing. Filtration takes place in the top few inches of the filter bed.

The multi-media filter is radically different. The multi-media filter bed, in comparison to the sand filter bed, is upside down. Coarse, but lighter, particles backwash to the top, whereas finer, but heavier, particles remain at the bottom of the bed. The innovation lies in the selection of suitable media. This configuration has many advantages. The entire bed acts as a filter, rather than only the top few inches. Turbidity is trapped throughout the bed, enabling the filter to hold far more solids filtered from the water before backwashing is necessary.

Multi-Media Filter Below

Filter MediaTypically, the filter bed is made up of three layers of filter media. The total bed depth is about 26 to 40 inches. In a three layer filter the top layer is made up of large, lighter weight particles of anthracite coal and is from 15 to 18 inches in depth (particle size 1.0 to 1.5 millimeters, density 1.35 to 1.75). The middle layer contains from 8 to 15 inches of heavier and smaller particles of calcined aluminum silicate or sand (particle size 0.5 to 0.6 millimeters, density 2.65). The bottom layer contains from 3 to 6 inches of heavier garnet (particle size 0.2 to 0.3 millimeters, density 4.0 to 4.2). This semiprecious red silicate mineral is 50 to 60% heavier than sand.

A multi-media filter is backwashed in the same manner as a sand filter, using reverse or upward flow of water through the filter bed. The various layers of media retain their stratification because each material has a different density.

In a four-media filter a fourth or top layer contains from 3 to 6 inches of lighter and larger plastic pillows (particle size 2.0 to 4.0 millimeters, density 1.1 to 1.2). Their density is slightly above the density of water which is 1.0


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