Filtration - Public Water Systems - page 407

depths up to 6 feet (1.82 m) are used in an attempt to minimize bed lifting. In the
United States, parallel plates or a metal grid is placed at the top of the fine media.
The spacing of the plates or the size of the openings in the grid is such that the media
grains arch across the open space to restrain the bed against expansion. These restrain-
ing bar systems have about 75 percent open area in the best designs developed to date.
Figure 12-15 illustrates an upflow filter with a restraining grid system. Even with the
use of a restraining grid or a deep bed, there may be problems with excessive pressures
or sudden variations in pressure that break the sand bridge or cause the bed to expand
and lose its filter action.
The frequency of breakthrough is rare, but the fact that it can occur at all—say,
with poor operation—has been sufficient to raise questions concerning public health
implications and to limit the use of upflow filters for potable water applications. In
areas that are free of health considerations, upflow filters have found wide application
Fig. 12-15. Upflow filter with restraining grid