TM 5-813-3/AFM 88-10, Vol 3
on a 50-mesh (U.S. Series) sieve. Approximately 100
mally, individual filters sizes do not exceed about 2100
percent by weight, of the sand should pass the
square feet corresponding to a capacity of about 6 mgd
16-mesh sieve and 90 to 100 percent be retained on a
at a flow rate of 2.0 gpm per square foot. A unit of this
50-mesh sieve. Filter sand should be clean silica sand
size would require a maximum backwash water rate of
having a specific gravity of not less than 2.5. The
about 60 mgd, which is excessive. Consequently, it
hydrochloric acid volubility of the sand should be less
should be divided into two parts of equal size arranged
than 5 percent.
for separate backwashing. Total filter depth should be
(b) Anthracite. Anthracite is an alternative me-
at least 9 feet.
dium consisting of hard anthracite coal particles. The
(7) Filter backwash. Backwash facilities should be
effective size commonly ranges from about 0.45 mm to
capable of expanding the filter media 50 percent. This
0.6 mm with a uniformity coefficient not to exceed
will require wash rates in the range of 10 to 20 gpm
1.7. The hardness should not be less than 2.7 on the
per square foot. Backwash water can be supplied by a
Moh scale and the specific gravity not below 1.4. Also,
backwash pump or from elevated storage provided spe-
cifically for this purpose. Filter down-time during
the anthracite should be visibly free of clay, shale, and
wash periods commonly average 10 to 20 minutes in-
dirt.
cluding a 5- to 15-minute wash period. For a 15-minute
backwash of a single unit, at maximum rate, the wash
or three layers of media of different size and specific
water volume will be 300 gallons per square foot of fil-
gravity. A common arrangement, the dual media
filter, is 20 inches of anthracite overlaying a sand
tration area in that unit. In addition to backwashing,
layer of approximately 8 to 12 inches. The anthracite
auxiliary scour is commonly provided. This aids in
cleaning the filter and is commonly accomplished by
layer has size range of about 0.8 to 2.0 mm; the sand
rotary or fixed surface-wash equipment located near
layer, about 0.4 to 1.0 mm. Tri-media filters employ an
18-inch anthracite layer, an 8-inch sand layer, and an
the top of the bed. It is operated for a time period
equal to that of the backwash, Water pressures of
underlying 4-inch layer of garnet or ilmenite having a
40-100 psi are required for surface-wash operation at
size range of 0.2 to 0.4 mm. Garnet has a specific grav-
it y of about 4, and ilmenite about 4.5.
a rate of 0.5 gpm per square foot. Air scour may also
(3) Filter gravel and underdrains, The filter media
be employed but is not generally used. If an independ-
is commonly supported by a 10- to 18-inch layer of
ent washwater storage tank is used, it must refill be-
coarse sand and graded gravel. The gravel depth may
tween washes. Tank capacity should be at least 1.5
range from 6 inches to 24 inches, depending on the fil-
times the volume required for a single wash.
(8) Wash water troughs. Wash water troughs
ter underdrain system chosen. The gravel should con-
sist of hard, rounded stones having a specific gravity
equalize the flow of wash water and provide a conduit
of at least 2.5 and an acid volubility of less than 5 per-
for removal of used water. Two or more troughs are
cent. A 3- to 4-inch transition layer of coarse (torpedo)
usually provided. The elevation of the trough bottoms
sand, having a size range of about 1.2 to 2.4 mm, is
should be above that of the expended bed. The clear
placed on top of the filter gravel. Gravel size usually
horizontal distance between troughs should not exceed
ranges from about 0.1 inch to about 2.5 inches. Filter
5 to 6 feet, and the top of the troughs not more than 30
underdrains may be constructed of perforated pipe
inches above the top of the bed.
grids or various proprietary underdrain systems. A
(9) Filter piping and equipment. Essential filter
variety of the latter are available. Design details for
control valves, etc., are shown schematically in figure
2-4. Each filter should be equipped with a rate-of-flow
pipe underdrains are given in numerous texts and
controller plus associated equipment for automatic fil-
handbooks. Manufacturers will furnish design and in-
stallation criteria for proprietary systems.
ter water-level control. The latter senses the water
level in the main influent conduit and transmits a sig-
(4) Sand, anthracite, gravel specifications. De-
nal to the flow controllers. The controllers, in response
tailed specifications for filter sand, anthracite and
gravel are contained in AWWA B100.
to this signal, adjust filtration rates to match the in-
(5) Number of filters. Not less than two filters
flow from the sedimentation basins. Thus, within prac-
tical limits, total filter outflow always equals total in-
should be installed regardless of plant size. For large
flow and the filter water level remains virtually con-
plants, rough guidance as to the number of filters to be
stant. A device that will sense maximum permissible
provided may be obtained from:
clearwell level should also be provided. This should be
arranged so that at maximum allowable clearwell
--N = number of filter units
water level, a shut-off signal will be transmitted to all
Q = design capacity in mgd
Thus, a 9 mgd plant would require eight filters.
filter controllers and also to an audible alarm. Other
-
(6) Size of filter units. The maximum filter size is
designs, not involving rate controllers, such as "in-
related to wash water flow rate and distribution. Nor-
fluent flow splitting" and "variable declining rate"
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