Page 414 - Water and wastewater engineering

SEDIMENTATION 10-31

If the sludge zone is not counted, the depth of the water is less than the design recom-
mendation of 3 m.
f. Check the length-to-depth ratios.

.
55 5 m
L/D 27 75. or 28 1:
2 m
The L:D ratio is acceptable.
g. Check the velocity and then check the Reynolds and Froude numbers.
3
Q 0 5 . m /s
v f
h
A ( 6 tanks)( 2 m depth)( 4 m width)
.
0 0104 m/s
This is within the acceptable range of 0.005 0.018 m/s.

R h A x (2 mdeep )(4 m wide ) 10 m.
P w 2 m 4m 2m
2
6
From Appendix A at a temperature of 10 C, the viscosity is 1.307 10 m /s and the
Reynolds number is
.
.
(0 0104 m/s )(1 0 m )
R 7957, .15 or 8 000
,
1
2
1 307 10 6 m /s
.
This is less than 20,000 and is acceptable.
(0 0104 m ) 2
.
Fr 2 11. 10 5
.
.
(981 m/s )(1 0 m )
5
This is greater than 10 and is acceptable.
h. Design the launders.
Provide launders for 1/3 of the tank length
55 5 . m
L Launder 18 5. m
3
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 64 .86 or 65 m /d
m
m
( 6 tanks)( 3 launders/tank)( 18..5 m/launder)( 2 sides)
3
This is well below the limit of 250 m /d · m and is acceptable.