Civil Engineering Reference
In-Depth Information
TABLE 11-2. Velocities at Which Particles of Sand and Silt Subside in Still Water
Hydraulic
Subsiding
Rate
(mm/s)
Diameter of
Particle
(mm)
Comparable Overflow Rate
gpm/ft
2
Classification
m/h
10.0
Gravel
1,000
1,480
3,600
1.0
100
150
360
0.6
63
90
230
0.4
Coarse sand
42
60
150
0.2
21
30
75
0.1
8
12
30
0.06
3.8
5.6
14
0.04
Fine sand
2.1
3.1
7.6
0.02
0.62
0.91
2.2
0.01
0.154
0.23
0.55
0.004
Silt
0.0247
0.036
0.087
Source:
Reference 2. (Reprinted from
Water Treatment Plant Design,
3d ed., by permission. Copyright
1998,
American Water Works Association.)
Note:
Temperature, 50
C); specific gravity of sand and silt particles, 2.65; values for 10-mm to 0.1-mm
particles from Hazen's experiments; values for 0.02-mm to 0.004-mm particles from Wiley's formula; inter-
mediate values interpolated from connecting curve.
F (10
Q
rate of flow through the basin, ft
3
/ sec (m
3
/ s), or gpd (m
3
/ d) with appropriate
conversion factors
A
surface area of the basin, ft
2
(m
2
)
Also:
C
t
(11-3)
0
Q
where:
t
0
basin detention time, sec
C
volume of settling zone, ft
3
(m
3
)
All particles with a settling velocity greater than
V
0
are removed. Particles with
settling velocities (
V
i
) less than
V
0
are removed only if they enter the basin within a
vertical striking distance
h
i
V
i
t
0
from the sludge zone.
Assuming a uniform distribution of particles in the inlet zone, these particles are
removed in the ratio of
V
i
/
V
0
. If the term
ƒ
i
is the fraction of particles with a settling
velocity of
V
0
or less, the removal efficiency
E
for the tank is
ƒ
i
V
i
E
(1
ƒ
)
d
ƒ
(11-4)
i
V
0
0
If
V
0
is taken to be the terminal settling velocity of the critical particles, Equation
11-2 equates terminal velocity to overflow rate. However, the overflow,
Q
/
A
, should
