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8.3 KEY FILTRATION SYSTEM COMPONENTS 159
If the source water is relatively cold (i.e., the average annual temperature is below 15C/ 59F), and at the same time is of high organic content, a layer of granular activated carbon (GAC) of the same depth is used instead of deeper layer of anthracite, because the bio- filtration removal efficiency will be hindered by the low temperature. While during bio- filtration a portion of the soluble organics in the source water is metabolized by the microorganisms that grow on a thin biofilm formed on the granular filter media, the GAC media removes a portion of the source water organics mainly by adsorption.
Trimedia filters have 0.45e0.6 m (1.5e2.0 ft) of anthracite as the top layer, 0.2e0.4 m (0.7e1.3 ft) of sand as a middle layer, and 0.10e0.15 m (0.33e0.50 ft) of garnet or limonite as the bottom layer. These filters are used if the source water contains a large amount of fine silt or the source water intake experiences algal blooms dominated of microalgae (0.5e20 mm). Filter media density varies as shown on Table 8.2.
The effective size of the media is the size of the opening of the sieve for which 10% of the grains by weight are smaller in diameter. Uniformity coefficient is the ratio between the open- ing sizes of a sieve for which 60% of the grains by weight are smaller, divided by the effective size of the media:
UC 1⁄4 d60=d10 (8.1)
Uniformity coefficient is an important parameter because it indicates how similar the me- dia particles are in size. In general, for the same size media, increasing uniformity coefficient of the media allows to increase the length of the filter cycles.
The d60 value of the filter media can also be used to determine the filter backwash rate at 200C/680F (Qasim et al., 2000) using the following formulas:
Ub 1⁄4 d60 for sand (8.2) Ub 1⁄4 0:47  d60 for anthracite (8.3)
For temperatures different from 20C, the backwash time can be calculated applying an adjustment coefficient for water viscosity:
Ubt 1⁄4 Ub  K0:333
where K, is absolute viscosity of water at a given temperature (kg/m s).
(8.4)
TABLE 8.2
Media Type
Pumice Anthracite Silica sand Garnet
Typical Filter Media Characteristics
Typical Effective Grain Size (mm)
0.8e2.0 0.8e2.0 0.4e0.8 0.2e0.6
Specific Density tons/m3 (lb/ft3)
1.2 (75)
1.4e1.7 (87e104) 2.60e2.65 (162e165) 3.50e4.30 (218e268)
Uniformity Coefficient
1.3e1.8 1.3e1.8 1.2e1.6 1.5e1.8