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HIGH-RATE GRANULAR MEDIA FILTRATION 8.7

pores become rapidly plugged, and carbon life is quickly reduced. If turbidity, taste, and
odor are all significant problems, GAC beds should be preceded by conventional granu-
lar media filtration. If carbon adsorption is desired to remove organics, the depth of GAC
that can be provided in a converted gravity filter is likely to be too shallow to provide ad-
equate contact time; however, this may offer some additional carbon removal after con-
ventional sedimentation.
Uniform Media. Uniformly graded deep-bed filters use relatively coarse media, rang-
ing from 0.5 mm to as much as 6.0 mm. The UC is typically 1.2 to 1.3, but values as
high as 1.5 may be found. Greater media depth is substituted for the lack of fine media
in the bed. Such a substitution requires more vigilant operation of pretreatment systems
to avoid breakthrough. Depths of 4 to 6 ft (l.2 to 1.8 m) are common, and in some cases
media depths reach 8 ft (2.4 m). Filters of this type are not expanded during backwash,
and stratification of grain sizes does not occur. These filters are generally designed to use
air or air/water backwash.
There are many possible combinations of filter media size d and depth L. J. M. Mont-
gomery (1985) presents a methodology for determining the optimum relationship between
these two variables. The relationship between L and ES de (10% finer) of many high-rate
filters is shown in Figure 8.1. In this figure, the average ES for a dual- or mixed-media

Approximate average media grain size (d), mm
O.l 0.3 0.5 0.8 1.0 1.3 1.5 1.8 2.0
1.5 x 10 `3
f=
O. 50(
1.3 x 103 4.0
0.475--
t~
& •
1.0 x 103
3.0
E
Conventional
E
filters
(150 m/d max.)
8x102
.m_
o o c
E
o o~,,~
E
2.0
O O
iZ
L~
5x102
rl "f Sand filter for WTP
1.0
3x102
° o o o li• Reverse graded filter for WTP
Direct filtration filter
Hardinge Tertiary filter for WWTP
filters
OL
0 0.3 0.5 0.8 1.0 1.3 1.5
Effective size of media grain (de), mm*
*Weighted average to be used for dual or multimedia bed
FIGURE 8.1 Relationship between depth and size of media. (Courtesy of MWH Americas Inc.)