Page 393 - Water and wastewater engineering

10-10 WATER AND WASTEWATER ENGINEERING

w

v
h v s 0

l
FIGURE 10-6
Dimensional definition of settling zone for Camp’s deriva-
tion of overflow rate for a rectangular horizontal-flow sedi-
mentation tank.

that it reaches the bottom of the tank during the time ( t o ) the water spends in the tank (the deten-
tion time ). That is, the settling velocity must equal the depth of the tank divided by t o
h
v s (10-14)
t o

where the detention time is defined as
V
t (10-15)
o
Q

3
where V volume of tank, m

3
Q flow rate, m /s
Using the definition of detention time from Equation 10-15 and substituting into Equation 10-14 :
h hQ
v s (10-16)
(V / Q) V

Because tank volume is described by the product of the height, length, and width
hQ Q
v s (10-17)
l w h l w

The product ( l w ) is the surface area ( A s ). Therefore,

Q
v s (10-18)
A s
which is the overflow rate ( v o ). From this, Camp proposed that the overflow rate is the critical
design parameter that determines the removal efficiency of settling tanks. From a theoretical