Page 252 - Water and Wastewater Engineering Design Principles and Practice

COAGULATION AND FLOCCULATION 6-29

Example 6-5. Using Table 6-4 select an in-line blender for an alum coagulant. The jar test
3
data resemble that shown in Figure 6-11 . The design flow rate is 383 m /h, and the design water
temperature is 17 C.

Solution:
a. Based on the jar test results, it appears that adsorption/destabilization is the predominant
1
mechanism of coagulation. G values in the range of 3,000 to 5,000 s and detention
times on the order of 0.5 s are recommended for adsorption/destabilization reactions.
b. As a first trial, select model AZ-6, with a reaction chamber diameter of 36 cm (dimen-
sion C in Table 6-4 ) and a length of 60 cm (dimension D in Table 6-4 ) and calculate the
volume of the reaction chamber.
(36 cm ) 2 3
V (60 cm ) 61 072 cm,
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c. Check the detention time using Equation 6-13

3 6 3 3
(61 072 cm )(10 m /cm ) 4
,

.
t 1 5910 hor 0 57 s
.
3
383 m /h
This is sufficiently close to the 0.5 s guideline to be acceptable.
d. Estimate the value of G assuming that the water power is 80% of the motor power. From
Table 6-4 find the motor power is 1,500 W
P (08 )(1 500 W ) 1 200 W
,
,
.
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Using Appendix A, find the viscosity of the water is 1.081 10 Pa · s at 17 C.
From Equation 6-12
⎛ 1 200 W ⎞ 0 5 .
,
G ⎜ ( ⎝ 1 081 10 3 Pa
s)( 61 072 cm )(10 m /cm 3 ⎟ , 1
4263 s
) ⎠

3
6
3
.
(
,
This meets the velocity gradient criteria.
Comment. If either the detention time or the velocity gradient criteria had not been met, an-

other trial model would have been selected and checked. In an actual design, more than one
manufacturer’s models may have to be examined to find a satisfactory match.
In-Line Static Mixing. As shown in Figure 6-15 , this mixer consists of a pipe with in-line heli-
cal vanes that rotate and split the flow to increase turbulence. The vanes are segmented so that
the number of vanes may be adjusted to fit local conditions. These segments are called elements.
The element size is specified in terms of the diameter of the pipe they are in, that is, the length of
element divided by the pipe diameter ( L / D ). This is called the aspect ratio. Generally, the aspect
ratio varies from 1.0 to 1.5. The mixers come in sizes as small as 0.5 cm for research application