Page 413 - Water and wastewater engineering

10-30 WATER AND WASTEWATER ENGINEERING

Example 10-4. Design the settling tank(s) for the city of Stillwater’s water treatment plant ex-
pansion using the design overflow rate found in Example 10-3 . The maximum day design flow is
3
0.5 m /s. Assume a water temperature of 10 C.

Solution:
a. Find the surface area.
First change the flow rate to compatible units:
3
3
.
)
,
,
(0 5 m /s )(86 400 s/d 43 200 m /d
Using the overflow rate from Example 10-3 , the surface area is
3
43 200 m /d 2
,
A s 1 329 233 or 1,330 m
,
.
,
3

32 5 . m /dm 2
b. Select the number of tanks.
Two tanks is the minimum number. For this flow rate make trial calculations using six
tanks.
,
1 330 m 2 2
221 66 or 222 m /tank
.
6 tanks
c. Select a trial width for calculation.
The maximum width for the chain-and-flight sludge collector is 6 m increments. Assume
a width of 4 m.
d. Check length-to-width ratio.
2
222 m /tank
L 55 5 . m
4 m/tank
55 5 . m
L/W 13 8. or 13 81
3
.
:
4 m
This is larger than the ratio of 6:1 and is acceptable.
e. Select a trial depth.
Because the column depth used to calculate the overflow rate was 2 m, this is a starting
point for setting the design depth. An allowance for the sludge depth of 1 m is added to
this depth. In addition the tank should be provided with 0.6 m of freeboard. The total
depth of the tank is then

2 m 1 m 0 6 m 3 6 m
.
.
Side water depth (SWD) 3.0 m.