Wastewater Calculations - Mathematics and Statistics for the Environment - page 674

■ EXAMPLE 24.16
Problem: The circular settling tank is 80 ft in diameter and has a weir along its circumference. The
effluent flow rate is 2.75 MGD. What is the weir overflow rate in gallons per day per foot?

Solution:

2.75 MGD × 1,000, 000 gal

Weir overflow rate (gpd/ft) = = 10,947 gpd/ft

3.14 × 80 ft

Note: 10,947 gpd/ft is above the recommended minimum of 10,000.

■ EXAMPLE 24.17
Problem: A rectangular clarifier has a total of 70 ft of weir. What is the weir overflow rate in gpd/
ft when the flow is 1,055,000 gpd?

Solution:

Flow (gpd)
Weir

1,055,000 gpd
70 ft

Weir overflow rate (gpd/ft) = = = 15,071 gpd

length (ft)

24.3.4 p rimary s edimentation b asin C alCulations
■ EXAMPLE 24.18
Problem: Two rectangular settling tanks are each 8 m wide, 26 m long, and 2.5 m deep. Each is
used alternatively to treat 1800 m 3 in a 12-hr period. Compute the surface overflow rate, detention
time, horizontal velocity, and outlet weir overflow rate using an H-shaped weir with three times
width.

Solution: First determine design flow Q :

3

1800 m
12 hr

24 hr
1day

3

Q = × = 3600 m/day

Compute the surface overflow rate, v o :

3

Q
A

3600 m/day
8m

32

v

o == ×

= 17.3 m(mday) ⋅

26 m

Compute the detention time, t :

2.5 m21040 m 3 Tank volume ( V =× × 8 m26m × =

3

V
Q

1040 m
3600 m/day

t == = 0.289 days = 6.9 hr

3

Compute the horizontal velocity, v h :

3

3600 m/day
8m

v h = = 180 m/day = 0.125 m/m in = 0.410 ft/min

× 2.5 m