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HIGH-RATE GRANULAR MEDIA FILTRATION 8.23

• Total area of orifices (surface area of bed): 0.0015 to 0.005:1
• Cross-sectional area of lateral (total area of orifices served): 2 to 4:1
• Cross-sectional area of manifold (total area of laterals served): 1.5 to 3:1
Orifices are normally spaced at 3 to 12 in. (8 to 30 cm) and laterals at roughly the
same spacings as the orifices. Approximately 18 in. (45 cm) of support gravel is required
to cover a lateral network. Three to five graded layers are usually involved, with sizes
varying from 11/~ to 1/8 in. (38 to 3 mm). The bottom layer should extend 4 in. (10 cm)
above the highest washwater outlet.
A commonly used block underdrain consists of vitrified clay blocks with lA-
Blocks.
in.-diameter (6-mm) dispersion orifices located across the top of each block. Support gravel
is required with this type of underdrain system. The size and arrangement of these blocks
and typical support gravel layers are shown in Figure 8.12. In mixed-media applications,
the third gravel layer is replaced by garnet of similar size. This type of underdrain sys-
tem is suitable only for water washing. However, auxiliary air scour may be provided by
adding an air piping grid at the sand-gravel interface, as shown in Figure 8.13.
Another type of block underdrain is designed for concurrent air/water wash. Blocks
are constructed of polyethylene and consist of a primary feeder lateral (lower) and a sec-
ondary compensating lateral (upper), shown in Figure 8.14. Small control orifices open
from the feeder lateral directly into the compensating lateral. Washwater and air are ad-
mitted to and flow through the feeder lateral and rise to discharge from the control ori-
rices into the compensating lateral. The triangular shape of the primary lateral distributes
incoming washwater and air uniformly along its length. Support gravel is typically used
with this type of underdrain, graded in an hourglass configuration. As a replacement for

FIGURE 8.12 Typical gravity filter. (Source: F. B. Leopold Co., Inc.)