UFC 3-280-04
17 DEC 2003
5-1.4.4 Media Configuration. The frequency of backwash and filter configuration for
both the traveling bridge and deep bed filters allow for silica sand to be used effectively
as a single medium. Silica sand can be used as a single medium in traveling bridge fil-
ters because surface filtration is the primary method of solids removal within the filter.
The frequency of backwash makes this practicable. Surface filtration allows the traveling
bridge filter media bed to be relatively shallow. Typical bed depths are approximately
300 mm (1218 in.).
Continuous backwash filtration systems successfully use silica sand as a sin-
gle medium. Traditional common problems associated with use of a single medium are
avoided because the bed is continuously moving. The absence of a backwash cycle re-
sults in no stratification of the bed. Also, surface straining and resultant solids matting
and head loss buildup are avoided. However, one drawback of the continuous back-
wash system is generally higher suspended solids in the effluent as compared to gravity
or pressurized granular media filters (typically 10 mg/L TSS for continuous backwash
filters vs. 5 mg/L for traveling bridge filters and removal in the 46 micron range for
deep bed granular filters). Typically, continuous backwash filters maintain a bed of ap-
proximately 1 m (40 in.), with sand approximately 1.2 mm in diameter. Deeper and
shallower beds are available from certain manufacturers for appropriate applications.
5-1.4.5 Underdrain. Traveling bridge filters generally use porous plate underdrains
with no gravel layer. The porous plate is advantageous for this application because no
air scour is needed to supplement water backwash owing to backwash frequency and
no gravel is required, helping to minimize total bed depth. (Aqua-Aerobic Systems, Inc.;
Infilco Degremont, Inc.)
No support system is required for downflow and upflow continuous backwash
systems because the media moves through the filter shell. This eliminates the need for
both media support and backwash distribution, the purposes of the underdrain.
5-1.4.6 Design Considerations. In addition to the filter tank, media, media support,
distribution and collection devices, and the necessary controls, the traveling bridge filter
also has the traveling backwash hood and rail upon which it moves. Traveling bridge
filters are proprietary systems. They are constructed of concrete or steel. The bridge
design and construction can vary substantially based on the hood itself and the trans-
port system. The filter bed is divided horizontally into several cells. Each cell operates
as a gravity filter. Backwashing occurs under the hood. Backwash can be automatic or
based on a triggering mechanism. Package filtration systems are commercially avail-
able.
All continuous backwash filters are proprietary. The manufacturers offer sys-
tems that operate at a range of capacities. Generally, the filters use a single medium
(sand), which is housed in a cylindrical shell. These shells may be stand-alone units or
multiple units may be housed in a concrete tanks if the influent flow warrants. The sys-
tems can be manufactured from a variety of materials, ranging from mild-steel with vari-
ous coatings to fiberglass-reinforced plastics to stainless steel. Several different sizes of
upflow and downflow continuous backwash systems are commercially available. The
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