Advanced Waterworks Mathematics, 2019a

Now you can substitute the values into the Weir Overflow Rate formula and
solve for the weir length.
Weir Overflow Rate (MGD/ft) =
4.3 MGD
? ft
0.07488 MGD/ft
Rearrange the terms and solve for the weir length in feet.
Weir Length (ft) =
4.3 MGD
0.07488 MGD/ft
57.425 ft = 57.43 ft
It doesn’t matter whether you calculate the weir length using MGD or gpm. In
either case, the weir length is 57.43 ft. However, it is important that all of the
units in the problem are the same – MGD to MGD/ft or gpm to gpm/ft.
Example:
A treatment plant processes 12.5 MGD. The weir overflow rate through a
circular clarifier is 33.8 gpm/ft. What is the diameter of the clarifier?
Since the plant flow rate is provided in MGD and the WOR is provided in gpm per
foot, you have the option of converting gpm to MGD or MGD to gpm in order to
solve. Here is the conversion for gpm to MGD.
33.8 gal 1 MG 24 hour 60 min
min 1,000,000 gal 1 day 1 hour =
ft
0.048672 MGD
ft
Weir Overflow Rate (MGD/ft) =
12.5 MGD
? ft
0.048672 MGD/ft
Weir Length (ft) =
12.5 MGD
0.048672 MGD/ft
256.8212 ft
It is a circular weir. Therefore, the length calculated is actually the circumference
of a circle. To calculate the diameter, use the circumference formula and solve
for D.
Circumference = D = 256.8212 ft
D =
256.8212 ft
3.14
= 81.79 ft = 81.8 ft
120 | A dvanced Waterworks Mathematics