UFC 3-280-04
17 DEC 2003
5-1.2.12.4 Common materials used for backwash gutters are concrete, steel, aluminum,
and fiberglass. Materials of construction should be chosen based upon compatibility
with the water to be filtered (TM 5-814-3).
5-1.2.13 Backwash
5-1.2.13.1 Process Description. A necessary component for long-term operating suc-
cess of granular media filters is adequate bed cleaning. Traditionally, this has been ac-
complished using an upflow water wash with full-bed fluidization. Backwash water is in-
troduced into the bottom of the filter bed through the underdrain system. The filter media
gradually assumes a fluidized state as backwash flow is increased. Recently, surface
washing or air scour have been used to supplement water backwash. Surface wash
systems consist of orifices located 50 to 80 mm above the fixed-bed surface that inject
water over the bed prior to and during water backwash. Air scour supplies air to the full
filter area from orifices located under the filter medium. Air scour may be used either
prior to the water backwash or simultaneously with the water backwash. Proprietary
systems have been developed in which the media are cleaned continuously. This is ac-
complished in the deep bed continuous backwash filter by removing media from the fil-
tration zone for cleaning and returning the media once cleaned.
5-1.2.13.2 Disposal Options. Backwash water is usually disposed of by re-filtering,
settling in an upstream clarification unit, or dewatering to concentrate the solids. Gener-
ally, it is advisable to provide either treatment (e.g., clarification or dewatering, or both)
or storage prior to re-filtering the backwash stream. Storage is more typically used, ex-
cept for continuous backwash systems. The water can be stored and delivered at a
uniform rate to the influent flow. A storage tank is usually necessary to avoid sending a
high solids or high volume "slug" through the filter at once. To dewater, the waste
stream is typically collected, conditioned, and settled. If dewatering is used, the wet
stream from the dewatering unit is often returned to the process stream ahead of the
filtration unit. If a separate treatment train is not desirable, or the process train configu-
ration lends itself to simple re-treatment, the waste stream may be returned to upstream
settling and clarification units for solids separation. The designer should always consider
the hydraulic effects on upstream unit processes when the backwash waste is returned
directly to the treatment train. Alternatively, the backwash water may be disposed of off-
site. The disposal of both the backwash water, and eventually the media, is a significant
design consideration for hazardous and toxic waste applications. The designer is re-
ferred to the Resource Conservation and Recovery Act (RCRA) and the Clean Water
Act regulations and other applicable Federal, state and local regulations to determine
the required treatment or permitting prior to release. The designer should try to minimize
all waste streams that are subject to regulation and treatment as a hazardous waste.
5-1.3
Gravity and Pressure Filtration. The following is a brief description of how
gravity and pressure filtration systems work and general design parameters. Package
systems have been designed by their respective manufacturers and generally come as
a unit that is inserted into the treatment process. The range of options available to a
system designer specifying package systems is limited, although each manufacturer
may configure its system in a slightly different fashion. It is the designer's responsibility
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