Page 214 -

HIGH-RATE GRANULAR MEDIA FILTRATION 8.33

FIGURE 8.18 Typical gravity filter piping. (Courtesy of CDM.)

washing. These same troughs also normally distribute influent flow uniformly across the
filter media's top surface. This limits horizontal travel required and equalizes static head
on the underdrain system. In contrast, European designs often feature narrow beds with
overflow walls on one or both sides, but not suspended over the media. In these designs,
tilting side weirs, horizontal water jets, and a procedure allowing influent water to enter
the filter on the side opposite the overflow wall are sometimes used to aid the movement
of scoured solids to waste. This method is termed cross-wash. Spacing troughs in U.S.
practice are usually at 5- to 7-ft (1.5- to 2-m) centers to limit horizontal travel distances
to 2.5 to 3.5 ft (0.8 to 1 m).
If troughs are placed too close to the surface of the unexpanded bed, media may be lost
during backwash. The design elevation of the weir edge of the trough may be determined
by adding the depth required for maximum bed expansion (usually 50%) and the overall
depth of the trough, plus a small margin of safety of 6 to 12 in. (0.15 to 0.3 m). If air scour
is practiced, additional care must be taken. In a conventional trough, simultaneous use of
washwater and air must be stopped when the level of water rises to the bottom of the trough,
or media will be lost. In a limited number of plants, special baffled plates on either side
of the trough minimize the loss of media during concurrent water/air wash.
French (1981) has given the following criterion for the location of the top of the troughs
based on trough spacing:

H = 0.34S