Water Treatment Processes - Mathematics and Statistics for the Environment - page 641

Flow rate (gpm)
Filterarea(f

Backwash rate = (23.70)

2

tt)

■ EXAMPLE 23.81
Problem: A filter 30 ft by 10 ft has a backwash rate of 3120 gpm. What is the backwash rate in
gpm/ft 2 ?

Solution:

Flow rate (gpm)
Filterarea(f

3120 gpm
(30ft10ft)

2

Backwash rate = = = 10.4 gpm/ft

2

tt) ×

■ EXAMPLE 23.82
Problem: A filter 20 ft by 20 ft has a backwash rate of 4.85 MGD. What is the filter backwash rate
in gpm/ft 2 ?

Solution:

Flow rate = 4,850,000 gpd/(1440 min/day) = 3368 gpm

Flow rate (gpm)
Filterarea(f

3368 gpm
(20ft20ft)

2

Backwash rate = = = 8.42 gpm/ft

2

tt) ×

23.6.5 b aCKWash r ise r ate
Backwash rate is occasionally measured as the upward velocity of the water during backwashing,
expressed as in./min rise. To convert from gpm/ft 2 backwash rate to an in./min rise rate, use either
Equation 23.71 or Equation 23.72:

2
3

Backwash rate (gpm/ ft ) 2in./ft)

7.48 gal/ft

× (

Backwash rate (in./min) = (23.71)

Backwash rate (in./min) = Backwash rate (gpm/ft 2 ) × 1.6 (23.72)

■ EXAMPLE 23.83
Problem: A filter has a backwash rate of 16 gpm/ft 2 . What is this backwash rate expressed as an in./
min rise rate?

Solution:

2 2

Backwash rate (gpm/ft) × ( 12 i in./ft) 16 gpm/ft × (

.48 gal/ft

2in./ft)

Backwash rate = = = 25 . in./min

3 3

7.48 gal/ft 7

■ EXAMPLE 23.84
Problem: A filter 22 ft long by 12 ft wide has a backwash rate of 3260 gpm. What is this backwash
rate expressed as an in./min rise?

Solution: First calculate the backwash rate as gpm/ft 2 :

Flow rate (gpm)
Filterarea(f

3260 gpm
(22ft12ft)

2

Backwash rate = = = 12.3 gpm/ft

2

tt) ×