Geoscience Reference
In-Depth Information
Equation 23.68 can be modified as shown in Equation 23.69 to calculate the unit filter run vol-
ume given filtration rate and filter run data:
Unit filter run volume = Filtration rate (gpm/ft
2
) × Filter run time (min)
(23.69)
■
EXAMPLE 23.78
Problem:
The average filtration rate for a filter was determined to be 2 gpm/ft
2
. If the filter run time
was 4250 minutes, what was the unit filter run volume in gal/ft
2
?
Solution:
Unit filter run volume = Filtration rate (gpm/ft
2
) × Filter run time (min)
= 2 gpm/ft
2
× 4250 = 8500 gal/ft
2
The problem indicates that, at an average filtration rate of 2 gal entering each square foot of filter
each minute, the total gallons entering during the total filter run is 4250 times that amount.
■
EXAMPLE 23.79
Problem:
The average filtration rate during a particular filter run was determined to be 3.2 gpm/ft
2
.
If the filter run time was 61.0 hr, what was the UFRV in gal/ft
2
for the filter run?
Solution:
Unit filter run volume = Filtration rate (gpm/ft
2
) × Filter run (hr) × 60 min/hr
= 3.2 gpm/ft
2
× 61.0 hr × 60 min/hr = 11,712 gal/ft
2
23.6.4 b
aCKWash
r
ate
In filter backwashing, one of the most important operational parameters to be determined is the
amount of water in gallons required for each backwash. This amount depends on the design of the
filter and the quality of the water being filtered. The actual washing typically lasts 5 to 10 min and
usually amounts to 1 to 5% of the flow produced.
■
EXAMPLE 23.80
Problem:
A filter has the following dimensions:
Length = 30 ft
Width = 20 ft
Depth of filter media = 24 in.
Assuming that a backwash rate of 15 gal/ft
2
/min is recommended and 10 min of backwash is
required, calculate the amount of water in gallons required for each backwash.
Solution:
Given the above data, find the amount of water in gallons required:
1. Area of filter = 30 ft × 20 ft = 600 ft
2
2. Gallons of water used per square foot of filter = 15 gal/ft
2
/min × 10 min = 150 gal/ft
2
3. Gallons required for backwash = 150 gal/ft
2
× 600 ft
2
= 90,000 gal
Typically, backwash rates will range from 10 to 25 gpm/ft
2
. The backwash rate is determined by
using Equation 23.70:
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