Page 453 - Water and wastewater engineering

11-26 WATER AND WASTEWATER ENGINEERING

TABLE 11-4
Suggested media depth as a function of effective size
Media depth( )
D
Effectivesize( ),
E
or D/E Applications
1,000 Ordinary monosand and dual-media beds
1,250 Typical tri-media beds (coal, sand, and garnet)
1,250–1,500 Coarse, deep, monomedium beds (E 1.2 to 1.4 mm)
1,500–2,000 Very coarse, monomedium beds (E 1.5 to 2.0 mm)
Data source: Kawamura, 2000.

Kawamura (2000) suggests that the depth of the filter bed follow the relationship shown in
Table 11-4 .
Filter Support and Underdrains
The underdrain system serves to support the filter medium, collect filtered water, and distribute
backwash water and air scour if it is used.
Many filters use underdrain systems with openings larger than the filter medium to be sup-
ported. To prevent the medium from leaking into the underdrain system, several layers of graded
gravel (called filter support ) are placed between the underdrain and the medium. GLUMRB’s
specifications when graded gravel is required are given in Table 11-5 .
There are five major categories of underdrain: manifold pipe system, blocks, screens, false
bottom with nozzles, and porous bottom. A few of these are illustrated in Figure 11-10 .
Pipe lateral underdrains were once popular because of their relatively low cost. Problems
with relatively high headloss and poor wash water distribution has resulted in a decline in their
use. Pipe lateral underdrains require support gravel.
One common type of block is made with 6 mm diameter orifices on the top of the block. Sup-
port gravel is required for this type of underdrain to keep the media from being washed out of the
filter. Air scour may not be used with this system. A variation of this system uses polyethylene
blocks. Air scour can be incorporated in this block.

TABLE 11-5
GLUMRB specifications for graded gravel
From the top layer down:
Torpedo sand: 8 cm depth, E 0.8 mm, U 1.7
Minimum of four gravel layers from the following:
Size range, mm Depth, mm
3–6 50–75
5–12 50–75
12–20 75–125
20–40 75–125
40–65 125–200

Source: Adapted from GLUMRB, 2003.