Civil Engineering Reference
In-Depth Information
does provide a starting point and the results should be checked against normally ac-
cepted design practice.
For
G
1,000 sec
1
,
t
opt
53.7 sec
•
For
G
700 sec
1
,
t
opt
76.7 sec
•
According to
Water Treatment Plant Design
,
45
for mixing times greater than 40
sec, a
G
-value of 700 sec
1
should be used. Higher
G
-value (700-1000 sec
1
) should
be used if detention time is less than 30 seconds. In this example, a detention time of
60 sec and a
G
-value of 700 sec
1
would be appropriate.
The volume of the rapid mix chamber is calculated as:
V
3.0 mgd
1.547 cu ft / sec / mgd
60 sec
3
280 cu ft (7.9 m )
Design for at least two units and make the dimensions of each unit compatible with
the raw-water influent channel or pipe.
Using two units:
280
Volume
140 cu ft
2
Use a square chamber with 5-foot side dimensions:
140
Depth
5.6 ft
5
5
Design the rapid mix chamber for 6-foot depth for 150 cu ft volume. Use a turbine
mixer with incline blades. The actual mixer dimensions and recommendations should
be obtained directly from the manufacturer.
Compute motor horsepower by rearranging Equation 10-15 as follows:
P
G
V
2
2
700
2.9
10
5
150
2130 ft-lb / sec
2130
Water hp
3.9 hp
550
Using an overall efficiency for motor and drive of 80 percent:
3.9
Motor hp
4.8 hp
(Use 5 hp motor)
0.8
Jet Injection
Figure 10-9 illustrates a recommended design for in-line jet mixer presented by Chao
and Stone.
45
Since specific design parameters closely associate with each manufac-
