UFC 3-280-04
17 DEC 2003
either horizontal or vertical axes. Backwash is executed in substantially the same man-
ner as for gravity filters.
Figure 5-2. Pressure Filter
5-1.3.2.2 Single medium stratified filter beds are not typically used for wastewater treat-
ment, except in continuous backwash filters. However, for HTRW and industrial applica-
tions, single media stratified beds may be considered. For typical gravity and pressure
filtration systems, single medium filter beds will become stratified with finer grains on top
after backwash. This results in unfavorable head loss buildup resulting from surface
straining of the solids within the finest medium layer. Instead, if a single medium is to be
used, the bed should be unstratified. Two types of unstratified single medium beds have
been used. One type uses a single, uniform, coarse medium (approximately 13 mm in
diameter) in a deep filter (approximately 2 m or 6.5 ft). Effluent quality may suffer
somewhat with use of this type of bed, as the coarse media may not entrap fine parti-
cles. Additionally, prohibitively high backwash rates may be required to fully fluidize the
bed. For example, the minimum backwash velocity needed to fluidize 2-mm-diameter
sand grains is approximately 1800 (L/min.)/m2 (45 gpm/ft2) as opposed to a more typical
required backwash velocity of 600 (L/min.)/m2 (15 gpm/ft2). The second type of unstrati-
fied single media bed uses a single medium of varying sizes to a depth of approximately
1 m (3 ft) with a combined airwater backwash. Use of this type of unstratified bed re-
sults in uniform average pore size throughout the filter bed. Therefore, in-depth filtration
is more likely, which results in longer filter runs. Use of airwater backwash eliminates
the need for fluidization and consequent stratification of the media. This type of filter has
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