MIL-HDBK-1164
each filter should be determined by actual experience at the plant. The rise rate is
related to the backwash rate, as illustrated in the following calculation:
15 gallons
1 cubic foot
2 feet
=
x
minute x square foot
7.48 gallons
minute
10 liters
1 cubic meter 0.01 meter
=
x
second x square meter 1,000 liters
second
0.01 meter 60seconds 1,000 mm 600 mm
=
=
x
second
minute
meter
minute
a.
Media Expansion. Filter media should be expanded at least
20 to 25 percent for good cleaning action, although a greater expansion may be
optimum in some cases. Higher expansions risk washing out some filter media along
with the accumulated solids. The degree of expansion is affected by many variables
associated with the filter media and the water. Filter media variables include size and
gradation as well as shape and density. Water variables include viscosity and density
which, in turn, vary with water temperature. Figure 7 relates media size and specific
gravity to backwash rate and gives approximate temperature correction factors. Media
cleaning is also affected by interparticle abrasion, although the bulk of the cleaning
action is due to the force of the rising backwash water. Expansion can be measured by
attaching cups to a pole at suitable intervals, then dipping the pole into the
backwashing filter bed; the highest cup that contains sand indicates the height of bed
expansion. A waterproof flashlight attached to a pole works well to show the top of the
sand, but only after the backwash water becomes relatively clear.
b.
Hook Gage. A multiple hook gage (Figure 8) is a series of
vertical, sharp, pointed rods held in a frame that may be hung on the side of the filter.
The tips of the sharp, pointed rods are set accurately at 2- or 3-inch (50- to 75-mm)
spacings. The hook can be used to check the rate of filtration or backwashing,
although its primary use is for measuring backwash flow rate. Hang the frame on the
side of the filter and accurately record the time required for the water level to fall or rise
between the points. The volume of water in the filter box between the gage points can
easily be calculated. From the recorded time, the flow rate can accurately be
determined.
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