Wastewater Calculations - Mathematics and Statistics for the Environment - page 673

24.3.2 s urFaCe l loading r ate (s urFaCe s ettling r ate /
s urFaCe o verFloW r ate ) C alCulations
Surface loading rate is the number of gallons of wastewater passing over 1 ft 2 of tank per day. This
can be used to compare actual conditions with design. Plant designs generally use a surface loading
rate of 300 to 1200 gpd/ft 2 :

Flow (gpd)

2 =

Surface loadingrate(gpd/ft ) (24.15)

2

Ta nk surface area (ft )

■ EXAMPLE 24.13
Problem: The circular settling tank has a diameter of 120 ft. If the flow to the unit is 4.5 MGD, what
is the surface loading rate in gpd/ft 2 ?

Solution:

4.5 MGD × 1,000,000 gal/ MGD

398 gpd/ft 2 Surface loadingrate = =

0.785 × 120 ft × 120 ft

■ EXAMPLE 24.14
Problem: A circular clarifier has a diameter of 50 ft. If the primary effluent flow is 2,150,000 gpd,
what is the surface overflow rate in gpd/ft 2 ?

Solution:

Key Point: Remember that area = 0.785 × 50 ft × 50 ft.

Flow (gpd)
Tank surfac

2,150,000 gpd
0.785

Surface overflow rate = =

t = 1096 gpd/ft

2

eearea (ft ) × 50 ft × 50 f

■ EXAMPLE 24.15
Problem: A sedimentation basin 90 ft by 20 ft receives a flow of 1.5 MGD. What is the surface
overflow rate in gpd/ft 2 ?

Flow (gpd)
Tank surfac

1,500,000 gpd
90 ft

gpd/ft 2

Surface overflow rate = = = 833

2

eearea (ft ) × 20 ft

24.3.3 W eir o verFloW r ate (W eir l loading r ate ) C alCulations
A weir is a device used to measure wastewater flow. Weir overflow rate (weir loading rate) is the
amount of water leaving the settling tank per linear foot of water. The result of this calculation can
be compared with design. Normally, weir overflow rates of 10,000 to 20,000 gpd/ft are used in the
design of a settling tank:

Flow (gpd)
Weir

Weir overflow rate (gpd/ft) = (24.16)

length (ft)

Key Point: To calculate weir circumference, use total feet of weir = 3.14 × weir diameter (ft).