17 DEC 2003
Table 5-2. Typical Properties of Filter Media Material
5-1.2.4 Media Shape. Grain shape is important because it affects the backwash flow
requirements for the media, the fixed bed porosity, and the head loss during filtration.
The measure of grain shape for granular media filtration is sphericity. It is defined as the
ratio of the surface area of an equal volume sphere (diameter of deq) to the surface of
the grain. The sphericity of filter media can be determined by measuring the pressure
drop through a sample and calculating the sphericity using the Carmen-Kozeny or Egun
equations for flow through porous media. This requires determining the equivalent
spherical diameter and the porosity of the sample first, so that the only unknown is
sphericity. Materials that are more angular, such as anthracite, have lower sphericity.
Typical values are presented in Table 5-2.
5-1.2.5 Fixed Bed Porosity. The fixed bed porosity of a granular media filter is de-
fined as the ratio of the void volume of the bed to the total bed volume and is expressed
as a decimal fraction. Fixed bed porosity is affected by the sphericity of the media;
those with lower sphericity will have a higher fixed bed porosity. Porosity is determined
by placing a media sample of known mass and density in a fixed diameter, transparent
cylinder. The depth of the sample in the cylinder times the cylinder area establishes the
total bed volume. The media volume is calculated by dividing the mass of the sample by
the density of the media. By subtracting the media volume from the total bed volume,
the void volume is determined. The porosity is then calculated as the ratio of the void
volume to the total bed volume of the sample.
5-1.2.6 Typical Media Properties and Design Standards. Some typically measured
values of density, sphericity, and porosity of different types of filter media are shown in
Table 5-2. Differences in the densities of the various materials are what permit their use
in dual media applications. Larger sizes of the lower density media--anthracite and
granular activated carbon--are used as a cap material. These lower density media also
have higher values of porosity, which will allow floc penetration. The larger media size
and greater porosity will typically result in better deep bed filtration.
5-22.214.171.124 Silica sand is the most common filtration media. Sand filters have historically
been used alone or in combination with other media. Silica sand is both economical and
fine-grained, which results in a satisfactory quality of effluent. But, single media sand
filters generally have short filter runs because the particles become trapped in the fine
grains at the top of the medium, quickly increasing head loss to an unacceptable level.
To overcome this, sands of varying sizes may be used in an unstratified bed. Alterna-
tively, coarser materials have been used in combination with fine-grained sand, where
the lighter, coarser materials will be found at the influent side of the bed. The most