reading the particle size from the sieve analysis curve corresponding to the 10% pass-

ing value and is typically noted as the *d*10 size. In general, with relatively uniformly sized

particles, the larger media size is, the greater the porosity or larger the flow passages

through the media will be.

5-1.2.2.3 The uniformity coefficient (*U.C.*) is a measure of the size range of the media

and is defined as the ratio of the opening size for which 60% of the grains by weight are

smaller compared to the opening size for which 10% of the grains by weight are smaller.

The uniformity coefficient can be denoted as follows:

The lower the uniformity coefficient value is, the closer the size range of the particles.

The uniformity coefficient is particularly important in the design and operation of dual

media filters as it influences the backwash rate required.

Typical ranges of values for the effective size and uniformity coefficient of

different types of media are presented in Table 5-1.

Silica sand

1.21.8

0.40.8

Anthracite coal

1.31.8

0.82.0

Garnet

1.51.8

0.20.6

Limonite

1.51.8

0.20.6

Source: Metcalf & Eddy (1991)

5-1.2.3 ** Media Density**. Media density is the mass per unit grain volume. The density

of the filter media affects the backwash flow requirements; for materials with the same

diameter, those with higher density will require higher backwash rates to achieve fluidi-

zation.

The specific gravity of a material is defined as the ratio of the mass of the

substance to the mass of an equal volume of water at a specified temperature (g/cm3).

Specific gravity is used to calculate the density of a material. The specific gravity of filter

media should be determined in accordance with ASTM C128-01. The test uses a dis-

placement technique at a temperature of 23 degrees C (73 degrees F). (There are three

alternative test methods for specific gravity--bulk specific gravity, bulk specific gravity

[saturated surface dry] and apparent specific gravity.) The bulk specific gravity (satu-

rated surface dry) would most closely represent conditions that exist with granular me-

dia filtration; however, results are difficult to reproduce. Apparent specific gravity is more

reproducible than the bulk specific gravity (saturated surface dry) for filter media and its

use is generally accepted for the backwash fluidization calculations. Test results for

specific gravity should be reported as the apparent specific gravity. Typical values are

presented in Table 5-2.

5-4