Science of Water : Concepts and Applications

Water Treatment Calculations 353
Then use the Q = AV equation to calculate velocity:
QAV 84 cfm (18 ft)(12 ft)( x fpm)
84
x
(18)(12)
0.4 fpm
Example 10.51
Problem:
A rectangular sedimentation basin 50 ft long and 20 ft wide has a water depth of 9 ft. If the fl ow to the
basin is 1,880,000 gpd, what is the mean fl ow velocity in ft/min?
Solution:
Because velocity is desired in ft/min, the fl ow rate in the Q = AV equation must be expressed
in cfm:
1,880,000 gpd
(1440 min/day)(7.48 gal/ft )
Then use the Q = AV equation to calculate velocity:
3
QAV 175 cfm (20 ft)(9 ft)( x fpm)
175 cfm
x
(20)(9)
097 . fpm
175 cfm
WEIR LOADING RATE (WEIR OVERFLOW RATE)
Weir loading rate (weir overfl ow rate) is the amount of water leaving the settling tank per linear foot
of weir. The result of this calculation can be compared with design. Normally, weir overfl ow rates
of 10,000–20,000 gal/day/ft are used in the design of a settling tank. Typically, weir loading rate
is a measure of the gpm fl ow over each foot (ft) of weir. Weir loading rate is determined using the
following equation:
Example 10.52
Problem:
flow, gpm
Weir loading rate, gpm/ft (10.53)
weir length, ft
A rectangular sedimentation basin has a total of 115 ft of weir. What is the weir loading rate in gpm/ft
when the fl ow is 1,110,000 gpd?