Water Treatment Processes - Mathematics and Statistics for the Environment - page 638

■ EXAMPLE 23.71
Problem: The influent valve to a filter is closed for 5 min. During this time, the water level in the
filter drops 0.8 ft (10 in.). If the filter is 45 ft long and 15 ft wide, what is the gpm flow rate through
the filter? Water drop equals 0.16 ft/min.

Solution: First calculate cfm flow rate using the Q = A × V equation:

Q = Length (ft) × Width (ft) × Drop velocity (ft/min)

= 45 ft × 15 ft × 0.16 ft/min = 108 cfm

Then convert cfm flow rate to gpm flow rate:

108 cfm × 7.48 ga l /f t 3 = 808 gpm

23.6.2 F filtration r ate
One measure of filter production is filtration rate (generally ranging from 2 to 10 gpm/ft 2 ). Along
with filter run time, it provides valuable information for operation of filters. It is the gallons of water
filtered per minute through each square foot of filter area. Filtration rate is determined as follows:

Flow rate (gpm)

2 =

Filtrationrate(gpm/ft) (23.67)

2

Fi lter surface area (ft )

■ EXAMPLE 23.72
Problem: A filter 18 ft by 22 ft receives a flow of 1750 gpm. What is the filtration rate in gpm/ft 2 ?

Solution:

Flow rate (gpm)
Filter surfa

1750 gpm
18 ft

ft 2

Filtrationrate = = = 4.4 gpm/

2

ce area (ft ) × 22 ft

■ EXAMPLE 23.73
Problem: A filter 28 ft long by 18 ft wide treats a flow of 3.5 MGD. What is the filtration rate in
gpm/ft 2 ?

Solution:

Flow rate = 3,500,000 gpd/(1440 min/day) = 2431 gpm

Flow rate (gpm)
Filter surfa

2431 gpm
28 ft

ft 2

Filtrationrate = = = 4.8 gpm/

2

ce area (ft ) × 18 ft

■ EXAMPLE 23.74
Problem: A filter 45 ft long by 20 ft wide produces a total of 18 MG during a 76-hr filter run. What
is the average filtration rate in gpm/ft 2 for this filter run?

Solution: First calculate the gpm flow rate through the filter:

Flow rate = 18,000,000 gal/(76 hr × 60 min/hr) = 3947 gpm