TM 5-814-9
(8) Intermittent sand filters. The installation has
depth of 12 feet (3.7 meters). The water surface area can then
indicated that the facility need not be fully
be calculated as follows:
operational for 7 days after a major washing
operation that takes about 2 days (54 hours).
The designer chooses two filters, which will
allow one filter to operate while the other one
Using the method described for sizing the equalization basin
is allowed to rest. A loading rate of 8 inches
and assuming a square basin gives the average basin size at the
(20.3 centimeters) per filter dose will be used.
midpoint of water depth is 301 feet by 301 feet (92 meters by
The filter dose frequency is every 8 hours. The
92 meters). With the side slopes of I to 3.5, the dimensions of
designer calculates the area required for the
the basin at the top of the water level would be 336 feet by 336
sand filters:
feet (102.4 meters by 102.4 meters). With the 3 feet (0.9
meters) of freeboard added, the overall top dimension of the
basin will be 357 feet by 357 feet (109 meters by 109 meters).
The bottom of the basin would be 252 feet by 252 feet (77
meters by 77 meters).
Makeup water. An economic analysis has
(11)
Thus, each filter should be 0.47 acres or 20473 sf(1904 SM).
revealed that, if wells are drilled as the
primary source of makeup water for the
(9) Dosing base. The designer has performed an
CVWF, the return on investment will be 37
economic study which showed that a dosing
years. Since the design life of the facility is
basin would be cost-effective when compared
only 20 to 25 years; the installation chooses
with installing larger pumps. Water can flow
to use the existing potable water supply as a
by gravity to the sand filter. Small pumps can
source of makeup water since the supply is
take water from the equalization basin to the
dependable. The existing 10-in (25.4-cm)
dosing basin. With a dosing basin, the de--
water line running adjacent to Range Road
signer is able to use smaller pumps for
can be tapped. Makeup water will be added
charging the dosing basin over a long period of
to the water supply basin at night when
time. The basin is sized to hold the volume of
demand on the potable water supply is
water to charge one filter. The volume is
lowest.
calculated as follows:
Emergency discharge. The closest sanitary
(12)
sewer connection is approximately 0.75 mi
(1.2 km). The designer decides to provide a
sewer from the water supply basin to the
existing sanitary sewer line to handle
emergency overflows. This will prevent
Water supply basin. Design criteria call for
(10)
overflows from otherwise being discharged
the size of the water supply basin to be the
to the environment.
larger of Vmax (724522 cf or 20287 CM); or
Checking calculations. Figure A-l 1 shows
Vave for a S day wash period.
(13)
the expected water volume in the
equalization basin at any given time during
the washing activities. This graph confirms
that at no time during a major wash will the
Since Vmax is larger than five times Vave, the designer uses a
equalization basin volume of 5.5 mg be
safety factor of 1.25 times Vmax to size the water supply basin or
exceeded during the peak water usage
905653 cf (25358 CM). Because of the large water volume
period. The projected volume for the water
required, the designer chooses an effective depth of 10 feet (3.0
supply basin is also plotted (fig A-12). This
meters) with 2.0 feet (0.6 meters) of dead storage for a total
graph shows that 5.5 Mg is sufficient to
ensure that an adequate supply of wash
water is available at all times.
A-18
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