Page 420 - Water and Wastewater Engineering Design Principles and Practice

SEDIMENTATION 10-37

f. Check the approach velocity.

3
Q 43 200 m /d
,
v approach 0 0116 m/s
.
)
(
)
(
A 2 ( tanks 6 m 3 6 m)(86 400, s/d)
m
.
This is slightly high but acceptable.
g. Check the Reynolds and Froude numbers.
The angle of the settler tubes is 60 . The area of the settler module calculated in (b)
2
above is 144 m . The velocity in the settler is
3
.
Q 0 5 m /s
v fc A sin 2 ( tanks 144)( m )( sin 60)
2
0 0020 m/s
.
This is a little low but acceptable. Assuming a 50 mm (0.05 m) hydraulic diameter of the
tube, the hydraulic radius is
⎛ d ⎞ ⎛ 1 ⎞ d 005 m
2
.
R h A x ⎜ ⎟ 0 0125 m
.
P w ⎝ 4 ⎠ ⎝ d ⎠ 4 4
2
6
From Appendix A at a temperature of 10 C, the viscosity is 1.307 10 m /s. Using
Equation 10-32, the Reynolds number is
.
.
(0 0020 m/s )(0 0125 m )
R 19.112 or 19
2
1 307 10 6 m /s
.
This is less than 50 and is acceptable. Using equation 10-33 with v fc for the velocity, the
Froude number is
(0 0020 m ) 2
.
Fr 3 26 10 5
.
.
(9 81 m/s 2 )(0 0125 m )
.
5
This is greater than 10 and is acceptable.
h. Design the launders.
Provide launders over the length of the settler. L Launder 24 m
Place them at 1 m intervals on center so that there are three in the tank.
i. Check the weir loading rate.

3
43 200 m /d
,
3

WL 150 m /dm
2 ( tanks 3 launders/tank 24 m/launder)(2 sides)
)(
)(
3
This is well below the limit of 300 m /d · m and is acceptable.