Civil Engineering Reference
In-Depth Information
s
particle density, lb
m
/ft
3
(kg/m
3
)
l
liquid density, lb
m
/ft
3
(kg/m
3
)
C
D
drag coefficient, dimensionless
V
p
particle volume, ft
3
(m
3
)
A
p
projected particle area in the direction of flow, ft
2
(m
2
)
Equation 11-1 identifies several properties that affect sedimentation: particle den-
sity, liquid density, and the size and shape of the particles. The settling velocity of a
particle varies inversely with liquid density and liquid kinematic viscosity. Kinematic
viscosity is related to water temperature, as shown in Table 11-1.
Water temperature has an important effect on sedimentation basin design. The set-
tling velocity of a particle is directly related to the ratio of the kinematic viscosity at
50
F (10
C) to the kinematic viscosity at any other temperature. For example, increas-
ing the water temperature from 50
F (10
C) to 86
F (30
C) would increase the settling
velocity of a particle by 1.307 / 0.801, or 1.63, times (see Table 11-1). Similarly, re-
ducing the temperature from 50
F (10
C) to 32
F(0
C) would reduce the settling
velocity to 1.307 / 1.787, or 0.73, times its initial value. This illustrates the fact that,
other conditions being equal, with cold waters, sedimentation basin overflow rates
should be lower than with warmer waters.
Settling velocity also is a function of the specific gravity and the size of the par-
ticles. Table 11-2 presents some relative settling velocities that dramatically illustrate
the effect of particle size.
The behavior of nonflocculent particles is often used to describe ideal settling. An
ideal settling basin is divided into four zones, as illustrated in Figure 11-1: inlet, outlet,
settling, and sludge zones.
Ideal settling theory results in the following equation for surface loading (i.e., sur-
face overflow rate):
Q
V
(11-2)
0
A
where:
V
0
settling velocity of particle that settles the depth of the basin in detention
time
t
0
, ft/sec (m/s)
TABLE 11-1. Relationship of Kinematic Viscosity to Water Temperature
Temperature
Kinematic Viscosity,
F
C
10
5
ft
2
/ sec
10
6
m
2
/s
32
0
1.923
1.787
41
5
1.634
1.519
50
10
1.406
1.307
59
15
1.227
1.140
68
20
1.08
1.004
86
30
0.862
0.801
Source:
Adapted from Reference 1.
