UFC 3-280-04
17 DEC 2003
to ensure that the design proposed by the chosen manufacturer is adequate for the in-
tended application.
5-1.3.1 Description of Gravity Filter. Gravity filters are granular filters that are open
to the atmosphere. Suspended solids are removed as the influent passes through the
porous, granular media. The removal occurs within the interstices of the filter medium
by interception, impaction, and straining. The hydrostatic pressure over the bed pro-
vides the driving force to overcome head loss through the unit. Maximum head loss
typically is less than 2.5 to 3 m (8 to 10 ft), and depends on the hydraulic profile of the
treatment system. Backwash is initiated at this limiting head loss. The direction of water
flow may be downflow, upflow, or biflow. The downflow designs are most common. If an
upflow configuration is used, a retaining grid must be placed above the media bed to
minimize loss of media in the effluent. This limits filtration rates in upflow filters. Biflow
configurations, where influent is introduced to the bottom and top of the bed with the ef-
fluent withdrawn from a strainer placed within the bed, have been used principally in
Europe. Backwash is always upflow, regardless of the operating flow direction. Back-
wash may consist of water wash in conjunction with surface wash or air scour. Flow rate
may be controlled using either constant-rate filtration or declining-rate filtration. These
controls will be discussed in Paragraph 6-3. A typical gravity filter is shown in Figure 5-
1.
Figure 5-1. Gravity Filter
5-1.3.2
Description of Pressure Filtration System
5-1.3.2.1 Pressure filtration systems operate in essentially the same manner as gravity
filtration systems, except that pressurized conditions, achieved by pumping, supply the
required driving force. Pressure filters may be operated with terminal head losses up to
10 m (30 ft). A typical pressure filter is shown in Figure 5-2. Again, downflow, upflow,
and biflow configurations are available. In addition to control by constant-rate filtration
and variable-declining-rate filtration, pressure filtration can also be operated at constant-
pressure. Pressure filtration units are usually constructed of cylindrical steel shells with
5-15
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