UFC 3-280-04
17 DEC 2003
washing process. Auxiliary cleaning is recommended, particularly when filtering waste-
waters. Surface wash and surface air scour are used to loosen and remove deposits
from upper levels of the medium. Air scour may also be used to reduce wash water re-
quirements and clean the deeper portion of the filter bed.
The USEPA has promulgated rules under the Safe Drinking Water Act
Amendments of 1996 regarding the disposal of backwash water. Most HTRW applica-
tions are not regulated by the Safe Drinking Water Act but, if the Act does apply to a
particular application, then the designer should be familiar with the Filter Backwash Re-
cycling Rule (40 CFR Parts 9, 141 and 142). Summarily, the Ten State Standards (criti-
cal reference) may include backwash criteria applicable to potable water.
Factors governing backwash system design include size distribution, depth,
shape, and specific gravity of media, density of bed, influent solids characteristics, pre-
treatment, any supplemental cleaning by surface wash or air scour, and disposal of
backwash waste.
5-1.3.4.2 Process Alternatives. Water backwash uses the shearing action of the water
to dislodge the accumulated material on the media. The dislodged material is flushed
through the bed and wasted through the wash water gutters. Traditionally, the media
have been fluidized or expanded to assist in the shearing and removal of solids. Some
manufacturers have found that water wash alone is insufficient to adequately clean the
filter bed, especially when filtering wastewater, and provide for supplemental surface
wash.
Surface wash can be used to provide additional shearing force. The surface
wash system produces high velocity water jets 50 to 80 mm (2 to 3 in.) above the unex-
panded media. The jets are introduced by orifices located on a fixed piping grid or on a
rotating arm. Surface wash water rates are generally from 40 to 120 (L/min.)/m2 (1 to 3
gpm/ft2) at 3.5 105 to 7 105 Pa (50 to 100 psi). The cycle is started 1 to 3 minutes
before water backwash, is operated for a period (5 to 10 minutes) simultaneous with
water backwash, and is then shut off. The orifices will become submerged during the
water backwash. The surface wash should be shut off at least 1 minute before the end
of the backwash cycle. This is particularly important with dual media and multi-media
beds, where the horizontal currents must be dissipated before the media settles and re-
stratifies.
The diameter of a sweep washer should be selected so that approximately 80
mm (3 in.) of clearance is available at the nearest wall. If the filter is constructed in a
rectangular shape, it may be advisable to use multiple surface washers to cover the
area adequately. The washers should be located such that they remain parallel to the
media surface. Sufficient clearance must exist beneath the wash troughs to allow for
rotation as well as 5080 mm (23 in.) between the washer arm centerline and the me-
dia surface. Auxiliary agitation may also be achieved by air scour. Air is introduced at
the bottom of the filter medium prior to water backwash at approximately 1 to 1.5
(m3/min.)/m2 (3 to 5 cfm/ft2) for 3 to 10 minutes. Water backwash is then initiated, and
air scour may continue until the water is about 250 mm (10 in.) from the wash water
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