Geoscience Reference
In-Depth Information
■
EXAMPLE 23.88
Problem:
A total of 66,000 gal of water will be required for backwashing a filter at a rate of 8000
gpm for a 9-min period. What depth of water is required if the backwash tank has a diameter of 50
ft?
Solution:
Use the volume equation for cylindrical tanks:
Volume (gal) = 0.785 × (Diameter)
2
× Depth (ft) × 7.48 (gal) l /f (ft)
3
66,000 gal = 0.785 × (50 ft)
2
×
x
ft × 7.48 gal l /f ft
3
x
= 66,000/(0.785 × 50 × 50 × 7.48) = 4.5 ft
23.6.8 b
aCKWash
p
umping
r
ate
(
gpm
)
The desired backwash pumping rate (gpm) for a filter depends on the desired backwash rate in
gpm/ft
2
and the ft
2
area of the filter. The backwash pumping rate (gpm) can be determined by using
Equation 23.75:
Backwash pumping rate (gpm) = Desired backwash rate (gpm/ft
2
) × Filter area (ft
2
)
(23.75)
■
EXAMPLE 23.89
Problem:
A filter is 25 ft long by 20 ft wide. If the desired backwash rate is 22 gpm/ft
2
, what back-
wash pumping rate (gpm) will be required?
Solution:
The desired backwash flow through each square foot of filter area is 20 gpm. The total
gpm flow through the filter is therefore 20 gpm times the entire square foot area of the filter:
Backwash pumping rate = Desired backwash rate (gpm/ft
2
) × Filter area (ft
2
)
= 20 gpm/ft
2
× (25 ft × 20 ft) = 10,000 gpm
■
EXAMPLE 23.90
Problem:
The desired backwash pumping rate for a filter is 12 gpm/ft
2
. If the filter is 20 ft long by
20 ft wide, what backwash pumping rate (gpm) will be required?
Solution:
Backwash pumping rate = Desired backwash rate (gpm/ft
2
) × Filter area (ft
2
)
= 12 gpm/ft
2
× (20 ft × 20 ft) = 4800 gpm
23.6.9 p
erCent
p
roduCt
W
ater
u
sed
For
b
aCKWashing
Along with measuring filtration rate and filter run time, another aspect of filter operation that is
monitored for filter performance is the percent of product water used for backwashing. The equation
for percent of product water used for backwashing calculations used is shown below:
Backwash water(gal)
Wate
Backwash water (%)
=
×100
(23.76)
rrfiltered(gal)
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