Water Treatment Processes - Mathematics and Statistics for the Environment - page 635

The overflow rate is a standard design parameter that can be determined from discrete particle set-
tling analysis. The overflow rate or surface loading rate is calculated by dividing the average daily
flow by the total area of the sedimentation basin:

Q
A

Q
lw

u == (23.65)

where
u = Overflow rate (m 3 /m 2 ·d, gpd/ft 2 ).
Q = Average daily flow (m 3 /d, gpd).
A = Total surface area of basin (m 2 , ft 2 ).
l = Length of basin (m, ft).
w = Width of basin (m, ft).

Note: All particles having a settling velocity greater than the overflow rate will settle and be
removed.

Rapid particle density changes due to temperature, solids concentration, or salinity can induce
density currents which can cause severe short-circuiting in horizontal tanks (Hudson, 1989).

■ EXAMPLE 23.66
Problem: A water treatment plant has two clarifiers treating 2.0 MGD of water. Each clarifier is 14
ft wide, 80 ft long, and 17 ft deep. Determine: (a) detention time, (b) overflow rate, (c) horizontal
velocity, and (d) weir overflow rate, assuming the weir length is 2.5 times the basin width.

Solution:

(a) Compute detention time ( t ) for each clarifier:

2 MGD

2

1 ,000,000 gal

day

1ft
7.48 gal

1day
24 hr

3 3

Q = = × × = 5570 ft /hr = 92.8 ft /min

(b) Compute overflow rate u :

Q 1,000,000 gpd

14 ft

u = = = 893 gpd/ft

LengthWidth × × 80 ft

(c) Compute horizontal velocity V :

3

Q 92.8 ft /min

14 ft

V = = = 0.39 ft/min

WidthDepth × × 17 ft

(d) Compute weir overflow rate u w :

Q 1,000,000 gpd

14 ft

u

w =

= = 28 571 , gpd/ft

25 . × Width 25 . ×