Page 253 - Water and Wastewater Engineering Design Principles and Practice

6-30 WATER AND WASTEWATER ENGINEERING

FIGURE 6-15
Static mixer.
1 Element

to as large as 300 cm for industrial and water treatment use. Generally, they are made in standard
pipe diameters.
These mixers have two advantages: (1) there are no moving parts and (2) no external energy
source is required. They have the disadvantage that the degree of mixing and mixing time is a
function of flow rate.
Although it applies to all mixers, a measure of the uniformity of the blend of the chemical
and the water has been found to be particularly useful in selecting static mixers. The measure
used is the coefficient of variation with time (COV) of the blended mixture. It is defined as
⎛ ⎞
COV ⎝ C ⎠ (100 %) (6-14)

where COV coefficient of variation with time
standard deviation of concentration, mg/L
C average concentration over time, mg/L
The standard deviation is defined as

) 2
C
(C t
(6-15)
n 1

where C t concentration of sample, mg/L
n number of samples
The following design criteria that may be used in selection are: (1) COV of 1 to 10 percent
with an average of 5 percent, (2) Gt in the range 350–1700, (3) a mixing time of 1 to 3 s, and (4) a
maximum headloss of 0.6 to 0.9 m. The design should specify that the mixing elements be remov-
able so they may be cleared and/or cleaned (Bayer et al., 2003; Kawamura, 2000; MWH, 2005).
The selection process is highly dependent on the approach suggested by the manufacturer for
their mixer. The following method is representative:
• Select the number of mixing elements to achieve the desired COV. Bayer et al. (2003) sug-
gest that three elements will yield a COV of about 10 percent and that six elements will
yield a COV of about 1 percent for mixers designed for turbulent flow (Reynolds number
greater than 5,000). Turbulent flow may be assumed for water entering water treatment
plants from pumped sources.