TM 5-814-9
per unit area (in acres) which is
area is then divided into two or more
applied to the surface of the filter. The
filter cells. This design minimizes the
application rate shall be from 490,000
size of the wastewater distribution
and 980,000 gallons per acre per day
network on the filter surface and
(524 and 1048 liters per square meter
lowers the effect of taking one filter
per day), which is the equivalent to
cell off-line for servicing. Because the
flooding the filter to a depth from 6 to
distribution lines must be easily
12 inches (15.2 to 30.4 centimeters)
removable for servicing the filter
every 8 hours. The daily dosing rate of
surface, it is useful to limit their length
the filter currently is chosen according
and size. Small filters also minimize
to soil conditions. Soil type (St) for the
pump capacity and velocity of flow
installation (chap 3) should he used in
through the distribution systems. Cell
evaluating the design loading rate.
sizes of approximately 1/4 acre (1012
m2) are desirable; however, they may
CVWFs serving installations with
mainly sandy soil (St=1) and little or
be larger.
no suspended clay in the training areas
(e)
Example Sizing. Assume that a CVWF
may be designed at higher loading
requires an average weekly wash
rates. However, since some clay-type
water flow of 1.6 million gallons per
soils (St=5) occur at most installations,
day for 5 days/week; soiling conditions
design should he adjusted toward the
indicates a loading rate of 650,000
lower end of the recommended loading
gallons/acre/ day.
rate.
Then, total filter surface area = 1,600,000 gpd x 5 days/
(b)
650,000 gal/day/acre x 7 days = 1.76 acres
volume is applied to the filters in three
intervals to maintain wetness of the
Further assume that two filters will be used, each divided into
filter and also allow the filter to
four cells:
"breathe" between doses, thus
Cell size
= 1.76 acres/2 filters x 4 cells/filter
maintaining the aerobic biological
= 0.22 acres/cell
treatment which results from
intermittent dosing. Normally, a filter
Dosing frequency: one of the eight cells is dosed every
will have more than one cell and each
hour.
cell must be dosed every 8 hours, but
the actual dosing interval may he
Volume per filter dose=8,000,000gal/21 doses= 380,950-
alternated from cell to cell. A dosing
gal dose.
sequence using equal dosing per day
per filter to he delivered by an
Volume of dose per filter cell = 380,950 gal/8cells =
automatic dosing device or pumps is
47,620gal.
recommended. Using the available
filters on an alternating basis, with rest
Dosing rate = 150 gpm/1000 square feet x 0.22 acre
periods between doses, effectively
(9,583 square feet) = 1438 gpm.
reduces the hydraulic dosing rate. This
procedure is achieved by using
Pumping time to dose each cell = 47,620 gal/I 438 gpm
electronic timers to control the
= 33 min.
application of wastewater to the filters.
Distribution system. The designer may
(4)
(c)
Dosing rate. This value is the flow
choose one of several distribution systems to
rate to the filter during each of the
apply waste water evenly over the surface of
three dosing intervals. Dosing flow to
the sand. Whichever system is chosen, the de-
the filter should be 95 to 190 gpm per
signer must ensure that it will be easy to
1000 square feet of filter surface area
remove and replace for cleaning. One design
(6 to 8 inches [15.2 to 20.3
uses perforated plastic pipes placed on the
centimeters]) applied for 20 to 40
surface of the sand in a grid pattern (fig 6-l3).
minutes to give the desired daily
The drilled orifices in the pipes should be at
loading rate.
least 0.16 inch (4 millimeters) in diameter or
larger to prevent the openings from clogging.
(d)
Filter surface area. The total filter
The pipes must be sized to distribute the
surface area is calculated by dividing
water as evenly as possible. Another
the average weekly flow by the daily
distribution system uses wooden troughs
loading rate x 7 as the filter can be
placed on the surface of the filter (fig 6-14)
designed to operate 7 days per week
and positioned so as to apply water to the
unattended. The total filter surface
6-17
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