Science of Water : Concepts and Applications

Water Treatment Calculations 375
Example 10.89
Problem:
A fi lter is 25 ft long and 20 ft wide. If the desired backwash rate is 22 gpm/ft 2 , what backwash pumping
rate, gpm, will be required?
Solution:
The desired backwash fl ow through each square foot of fi lter area is 20 gpm. The total gpm fl ow
through the fi lter is therefore 20 gpm times the entire square foot area of the fi lter:
2
Backwash pumpingrate, gpm (desired backwash rate, gpmft )(filterarea,
sqft)
2
(20 gpmft )(25 ft)(20 ft)
10,000 gpm
Example 10.90
Problem:
The desired backwash pumping rate for a fi lter is 12 gpm/ft2 . If the fi lter is 20 ft long and 20 ft wide,
what backwash pumping rate, gpm, will be required?
Solution:
2
2
Backwash pumpingrate, gpm (desired backwash rate, ft )(filterarea,
ft )
2
(12 gpm/ft )(20 ft)(20 ft)
4800 gpm
PERCENT PRODUCT WATER USED FOR BACKWASHING
Along with measuring fi ltration rate and fi lter run time, another aspect of fi lter operation that is
monitored for fi lter performance is the percent of product water used for backwashing. The equation
for percent of product water used for backwashing calculations is shown below.
Example 10.91
backwash water, gal
Backwash water, % 100 (10.77)
water filtered, gal
Problem:
A total of 18,100,000 gal of water were fi ltered during a fi lter run. If 74,000 gal of this product water
were used for backwashing, what percent of the product water was used for backwashing?
Solution:
backwash water, gal
Backwash water, % 100
water filtered, gal
74, 000 gal
100
18,100,000 gal
04 .%