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HIGH-RATE GRANULAR MEDIA FILTRATION 8.13

t FfLTER BOX 1
_ Z

WATER LEVEL
, FILTERING HEAD

[ z ,L ~, .,./
PLENUM/ [ DIRECTION OF FLOW DURING BACKWASHING L DIRECTION OF FLOW DURING FILTERING
FIGURE 8.6 Self-backwashing filter. (Source: Monk, 1987.)

Washing Methods. The AWWA Subcommittee on Backwashing of Granular Filters has
acknowledged four basic backwash methods:
• Upflow water wash without auxiliary scour
• Upflow water wash with air scour
• Upflow water wash with surface wash
• Continuous backwash
The application normally dictates the method selected. Filter bed expansion during up-
flow water washing results in media stratification. Air washing results in filter media mix-
ing. If stratification is desired, air scour must be avoided or must precede fluidization and
expansion with water. There are a number of conflicting opinions on the best method of
backwash.
Upflow Water Wash without Auxiliary Scour. Upflow water wash alone may be suf-
ficient in some filters receiving low solids loadings. In the absence of auxiliary scour,
washing in an expanded bed occurs as a result of the drag forces on the suspended grains.
Grain collisions do not contribute significantly to washing (Camp, Graber, and Conklin,
1971; Cleasby, Stangl, and Rice, 1975; Cleasby et al., 1977).
Maximum shear on the grains theoretically occurs (for typical filter sand) at a bed ex-
pansion of 80% to 100% (Cleasby et al., 1977). The increase in shear with increasing bed
porosity is relatively slight beyond the point at which expansion begins. Optimal expan-
sion may be less than 20% (Johnson and Cleasby, 1966). Normally, when water wash is
applied exclusively, an expansion of 20% to 50% is used. Water wash at a sufficient rate
to substantially expand (10% or greater) a granular bed is generally referred to as high-
rate water wash. Water wash incapable of fully fluidizing a bed (i.e., less than 10% ex-
pansion) is generally referred to as a low-rate water wash.
Experience in the United States with high-rate water wash used alone is extensive. It
is generally successful for applications that filter iron precipitates from groundwater or
remove color from otherwise high-quality surface water. The relatively weak cleaning ac-
tion of water wash without auxiliary scour of some type, however, generally renders it