Page 186 -

HIGH-RATE GRANULAR MEDIA FILTRATION 8,5

all but the finest 10% (by weight) of the representative sample. It is also referred to as
the "10% finer" size. The UC is the ratio of the "60% finer" size to the ES. Common
practice in Europe is to express media sizes as the upper and lower limits of a range.
These limits may be expressed either as linear dimensions or as passing and retaining
sieve sizes (that is, 1.0 to 2.0 mm or -10 + 18 mesh).
The filtration process also affects the selection of the filter bed because of the special
requirements of each type of process. The direct and in-line filtration processes must have
filter beds with a large floc holding capacity. A reverse-graded filter bed, such as a dual-
media or coarse deep bed, satisfies this requirement. In two-stage filtration, the filter bed
of the first stage acts as a roughing filter and carries out the flocculation process. Data
obtained from pilot filter tests and actual installations using the two-stage filtration pro-
cess indicate that the first-stage filter bed may be designed in the same fashion as an or-
dinary filter (Kawamura, 2000).
Rapid sand filtration, with filtration rates ranging from 2 to 3 gpm/ft 2 (5 to 7.5 m/h),
usually uses medium-sized sand (0.5-mm ES). High-rate filters of 5 to 10 gpm/ft 2 (12.5 to
25 m/h) always consist of a reverse-graded filter bed or a deep, large-sized monomedia.
Filter beds may be classified as graded fine-to-coarse, ungraded, graded coarse-to-fine,
or uniformly graded, depending on the distribution of grain sizes within the bed during
filtration. Transition from the ungraded media of a slow sand filter to the fine-to-coarse
high-rate granular media filter resulted from dissatisfaction with the low loading rates and
laborious cleaning procedure characteristic of slow sand filters. Filters with uniformly
graded or coarse-to-fine beds are now operated at higher filtration rates and for longer
run times than are feasible with conventional rapid sand filters.

Ungraded Media. The slow sand filter is a primary example of an ungraded bed. Be-
cause slow sand filters are not backwashed, no hydraulic grading of the media occurs.
Distribution of the various grain sizes in the bed is essentially random. Typical slow sand
filter beds contain 2 to 4 ft (0.6 to 1.2 m) of sand with an ES of 0.15 to 0.35 mm and a
UC not exceeding 3.0. Refer to Chapter 9 for further information on slow sand filtration
design.

Fine-to-Coarse Media. Fluidization and expansion of rapid sand filter beds during back-
washing result in accumulating fine sand grains at the top of the bed and coarse grains at
the bottom. Consequently, filtration occurs predominantly in the top few inches, and head
loss increases relatively rapidly during operation. This sand medium typically has an ES
of 0.35 to 0.60 mm (generally 0.5 ram) and a UC of 1.3 to 1.8. Grains passing a no. 50
sieve (0.3 ram) or captured on a no. 16 sieve (1.18 mm) are normally limited by specifi-
cations to very small portions of the medium. Bed depths are typically 24, 30, or 36 in.
(0.6, 0.75, or 0.9 m), respectively.
Single-medium anthracite beds have been used in the same basic configuration as rapid
rate beds. Because anthracite is more angular than sand, the porosity of an anthracite bed
is higher than that of a sand bed containing media with the same ES. The porosity of a
sand bed is generally 40% to 45% whereas a typical anthracite bed has a porosity of 50%
to 55%. Consequently, anthracite does not perform in exactly the same manner as sand
of equivalent size. Because of the lower specific gravity, anthracite beds are also easier
to fluidize and expand than sand beds.

Coarse-to-Fine Media. In a coarse-to-fine bed, both small and large grains contribute
to the filtering process. The presence of fine media in a filter is desirable because of the
relatively large surface area per unit volume that fine media provide for particle adhesion.
Fine media are instrumental in achieving the best-quality filtered water. Coarse media,