Page 419 - Water and wastewater engineering

10-36 WATER AND WASTEWATER ENGINEERING

Example 10-5. Design the settling tank(s) for the city of Stillwater’s water treatment plant
3
expansion using high-rate settlers. The maximum day design flow is 0.5 m /s. Assume a well set-
tling alum floc, a water temperature of 10 C, that the angle of the settler tubes is 60 , and that the
tubes have a hydraulic diameter of 50 mm.
Solution:
a. Find the surface area of the settler.
First change the flow rate to compatible units:

3 3
(0 5 m /s )(86 400 s/d 43 200 m /d
.
,
,
)
3 2
Using an overflow rate of 150 m /d
m , the surface area is
3
,
43 200 m /d
A s 288 m 2
3
150 m /dm 2

b. Select the number of tanks.
Two tanks is the minimum number. For this flow rate make trial calculations using two tanks.
2
288 m 2
144 m /tank
2 tanks
c. Select a trial width for calculation.
The maximum width for the chain-and-flight sludge collector is 24 m in 0.3 m incre-
ments. Assume a width of 6 m. This is the maximum width per train.
d. Check length-to-width ratio.

2
144 m /tank
L settler 24 m
6 m/tank
Setting the settler at 75% of the length of the basin, the tank length is

24 m
L 32 m
.
075
e. Select a trial side water depth (SWD).
Assume a depth of settler of 0.6 m. Provide allowance above the settler for launder of
1.0 m (0.6 m clearance and 0.4 m depth of trough). Provide 2 m for sludge zone.

.
SWD 06 10 2 3 6 m
.
.
The depth of the water plus the sludge zone is greater than the minimum design recom-
mendation of 3 m. With the addition of freeboard, the tank depth is 4.2 m.