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8.6 CHAPTER EIGHT

when placed before fine media in the filtering sequence, decrease the rate of head loss
buildup and increase available storage capacity in the bed.
Dual-media beds normally contain silica sand and crushed anthracite coal and are a
very common filter media design. Triple-media beds contain an additional layer of gar-
net or ilmenite sand. Specific gravities of materials used in filtration are roughly as fol-
lows:
• Silica sand, 2.55 to 2.65
• Anthracite coal, 1.5 to 1.75
• Garnet, 4.0 to 4.3
• Ilmenite, 4.5
A typical dual-media bed contains 6 to 12 in. (0.15 to 0.3 m) of silica sand (ES 0.45
to 0.55 ram) overlaid by 18 to 30 in. (0.46 to 0.76 m) of anthracite (ES 0.8 to 1.2 mm).
A typical mixed-media filter bed contains 3 to 4 in. (5 to 10 cm) of garnet (ES 0.15 to
0.35 ram), 6 to 9 in. (0.15 to 0.3 m) of silica sand (ES 0.35 to 0.5 ram), and 18 to 24 in.
(0.5 to 0.6 m) of anthracite (ES 0.8 to 1.2 ram).
The degree to which media layers are intermixed in the bed depends on the sizes and
shapes of the media used, the nature of the backwashing procedure, and the specific grav-
ities of the different media. Disagreement exists over whether distinct layers or intermixed
layers are most desirable. If layers mixed completely, the purpose of using more than one
medium would be defeated. If no mixing occurs, individual fine-to-coarse layers would
result, and the possibility of rapid clogging at interfaces would be raised.
Proponents contend that in a properly designed mixed-media filter, a gradual decline
in pore sizes from top to bottom of the bed is established after backwashing. The origi-
nal argument can be traced to Conley and Pitman (1960), Conley (1961), and Camp (t961,
1964) in the early 1960s. Brosman and Malina (1972) concluded that a slightly mixed bed
was superior to a distinctly layered bed in terms of head loss development, filter run time,
and filtered water turbidity. Cleasby and Sejkora (1975), however, disagree that superior
performance can be attributed to interfacial intermixing in and of itself; rather, it is a re-
sult of differences in the media sizes required to construct mixed and separated beds. They
found that to provide a relatively sharp interface in a dual-media bed, fairly coarse sand
was required. The resulting bed would not provide the same filtered water quality as a
bed using finer sand that mixed more readily with the coal.
The anthracite coal and silica sand used in dual-media filters inevitably result in some
intermixing of layers. In a triple-media bed, intermixing of silica sand and garnet sand
normally occurs more readily than mixing of silica sand and coal. Cleasby and Woods
(1975) suggest that, as a rule of thumb, the ratio of the average particle size of coarse sil-
ica grains to the size of coarse garnet grains should not exceed 1.5, to ensure that some
garnet remains at the bottom of the bed. They also suggest that a ratio of coarse coal grain
size to a fine silica sand grain size of about 3 results in a reasonable degree of mixing in
dual- or mixed-media beds. Brosman and Malina (1972) found that anthracite sand filter
media with a size ratio at the interface of less than 3:1 exhibits little mixing and that the
zone of mixing increases linearly as the size ratio increases above 3:1.
In a number of U.S. installations, taste and odor removal and filtration have been com-
bined in a single unit using GAC (Hager, 1969; Hansen, 1972; Blanck and Sulick, 1975;
McCreary and Snoeyink, 1977). GAC is sometimes added to existing rapid sand units
from which some sand has been removed. GAC depths of 12 to 48 in. (0.3 to 1.2 m) over
silica sand layers of 6 to 18 in. (0.14 to 0.5 m) have been reported. Typically, GAC with
an ES of 0.5 to 0.65 mm has been used. This technique is usually applicable only where
taste and odor, and not turbidity, are of primary concern. If turbidity levels are high, GAC