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been most commonly used in potable water treatment. Generally, it is advisable to use
dual media filters.
5-18.104.22.168 Tables 5-3 and 5-4 present typical media designs for filters. Additional
information is presented in TM 5-814-3 and TM 5-813-3.
5-1.3.3 Design Considerations. Typical filtration rates for granular filters are 40 to
100 (L/min.)/m2) (1 to 2.5 gpm/ft2) for rapid filters and 100 to 600 (L/min.)/.m2 (3 to 15
gpm/ft2) for high rate filters. Higher filtration rates are generally preferred to decrease
the capital cost of the filter (less filter area required) and the higher filtration rates result
in greater penetration of solids into the bed. The trade-off is potentially poorer effluent
quality. When designing a filter for a specific net production (m3/hr or gpm), downtime
for backwash and time associated with treatment of the backwash water, if applicable,
must be considered. Head losses of approximately 3 m (10 ft) permit a reasonably long
run in gravity filters. Lower head losses (2 m [6.5 ft]) may be acceptable for dual media
configurations. The loss of head through the filter is determined by summing the incre-
mental losses through the underdrain (and supporting gravel, if applicable), media,
static height, and valves and piping.
Gravity filters may be of concrete or steel shell construction. Concrete gravity
filter boxes are usually arranged in rows along one or two sides of a common pipe gal-
lery, minimizing piping required for influent, effluent, wash water supply, and wash water
drainage. Concrete units are usually rectangular and steel units are round. Generally,
the steel units are made for smaller influent flows than the concrete units and may be
more practical for HTRW applications.
Pressure filters shells can be either steel or fiberglass and must withstand
high operating pressures. They must be manufactured in strict accordance with the
American Society of Mechanical Engineers (ASME) standards for pressure vessels
(ASME Boiler and Pressure Vessel Codes, Section VIII, Divisions 1,2 and 3). Pressure
filter units are sized to use commercially available shells. The shells can be mounted
either vertically or horizontally. The vessel will house the media; media support struc-
tures; distribution and collection devices for influent, effluent, backwash water and
waste; supplemental cleaning devices; and necessary controls. Media support struc-
tures are typically pipe laterals with nozzles or orifices or a plenum with porous plate-
type structure using a framework similar to a well screen. Allowable head losses ap-
proach 10 me (30 ft) (WEF, 1992). With pressure filtration, only single pumping typically
is required. If pretreatment is not needed, water may be pumped from wells, for exam-
ple, through the filters and to further waste treatment or storage facilities.
5-22.214.171.124 Process Description. Granular media filters are cleaned by reversal of the
flow through the bed based on a triggering measurement, such as effluent quality or
head loss or after a predetermined time. During backwashing, the media are usually
fluidized to allow the captured particulate to be released into the water and collected in
wash water troughs. Air injection, surface wash, or jets of water may supplement the