TM 5-813-3/AFM 88-10, Vol 3
waters, but some have been installed for the treatment
have been developed and may be employed at the dis-
of potable water.
cretion of the designer. In general, each filter must
c. Pressure filters. Pressure filters are similar in
have five operating valves: influent, wash water,
construction and operating characteristics to rapid
drain, surface wash, and filter-to-waste. It is empha-
sand filters. However, in a pressure filter the media,
sized that the filter-to-waste piping must not be direct-
gravel bed, and underdrains are enclosed in a steel
ly connected to a plant drain or sewer. An effluent
shell. There are a variety of new pressure filters in use
sampling tap must be provided for each filter. Valves
today. The most common of these are the conventional
can be manually, electrically, hydraulically, or pneu-
downflow filter, the high-rate downflow filter and the
matically operated. Butterfly type valves are recom-
up flow filter. An advantage of any pressure filter is
mended for filter service. Design velocities commonly
that any pressure in waterlines leading to the filter is
employed for major filter conduits are as follows:
not lost, as in the case of gravity filters, but can be
used for distribution of the filter effluent. Between 3
and 10 feet of pressure head are lost through the filter.
The primary disadvantage of a pressure filter is that,
due to the filter being enclosed in a steel shell, access
to the filter bed for normal observation and mainte-
nance is restricted. Also, the steel shells require care-
The effluent conduit must be trapped to present back-
ful periodic maintenance to prevent both internal and
flow of air and provide a seal for the rate controllers.
external corrosion. The use of pressure filters is not ad-
The filter pipe gallery should have ample room and
vantageous in most systems. However, if the pressure
good drainage, ventilation, and lighting. Dehumidifi-
requirements and conditions in a particular system are
cation equipment for the gallery should receive careful
such that repumping of filtered water can be elim-
inated, cost savings will be realized,
consideration. Filters should be covered by a super-
structure except under favorable climatic conditions.
(1) Conventional downflow filters. Conventional
Drainage from the operating floor into the filter
downflow pressure filters consist of a bed of granular
should be prevented by a curb. Access to the entire bed
media or multi-media and are good in removing sus-
should be provided by a walkway at operating floor
pended solids comprised of floe. The advantages over
level around the filter. Filters may be manually or
gravity filters include lower installation cost and
automatically controlled from local or remote loca-
adaptability y to different piping systems. Hydraulic
loadings range from 1 to 4 gpm/sq. ft.
tions. Facilities permitting local, manual control are
recommended irrespective of other control features.
(2) High-rate downflow filters. High-rate down-
Used backwash water should be discharged to a wash
flow filters have filtration rates of 10-20 gpm/sq. ft.
water recovery basin or to a waste disposal facility.
The higher downflow velocities require coarser media
Regulatory agencies generally view filter wash water
which allow suspended solids to penetrate deeper into
as a pollutant and forbid its direct discharge to the
the medium. As a result, more solids can be stored in
natural drainage.
the filter bed before backwashing is required, Many
(10) Essential instrumentation. Minimum essen-
units exhibit a 1-4 lbs/sq. ft. solids-loading capacity,
The higher filtration rates also allow smaller or fewer
tial instrumentation for each filter will be provided as
follows: rate=of-flow indicator; loss-of-head indicator;
filters to be used over conventional filters. However,
effluent turbidity indicator; wash water rate-of-flow
the high solids-loading capacity of this filter requires
indicating and totalizing meter. If a wash water stor-
higher backwashing flow rates and hence larger back-
age tank is provided, it must be equipped with a water-
washing water storage tanks.
level indicator. While not absolutely required, a tur-
(3) Upflow filters. Upflow multi-media filters
bidity indicator on the main filter influent is desirable.
allow filtration of high solids-loaded liquids in concen-
b. Diatomite filters. Filtration is accomplished by a
tration up to 1,000 mg/L. The advantage of upflow
layer of diatomaceous earth supported by a filter ele-
multi-media filters is that the coarser material at the
ment termed a septum, This layer of diatomaceous
inlet collects the heavier particles, while the finer ma-
earth is about l/8-inch thick at the beginning of filtra-
terial collects the smaller particles, thus efficiency of
tion and must be maintained during filtration by a
the filter is increased.
constant feed of diatomaceous earth (body feed) to the
(4) Upflow continuous backwash sand filters. Up-
influent water. At the conclusion of a filter run, the
flow continuous backwash sand filters continuously
layer of diatomaceous earth will have increased in
clean the filter medial by recycling the sand internally
thickness to about 1/2 inch. Filtration rates generally
through an air lift pipe and sand washer. The regen-
vary from 0.5 to 2.0 gpm per square foot. The princi-
erated sand is then redistributed to the top of the sand
pal use of diatomite filters has been for swimming pool
bed. Once the sand migrates down to the bottom of the
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