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6.6 CHAPTER SIX

DESIGN OF CHEMICAL MIXING

Chemical mixing can be accomplished by several different types of equipment designed
to mix the applied chemicals with the source water as quickly as possible.

Mixing Intensity
The intensity of agitation required for optimum rapid mixing and flocculation is measured
by the G value. The G value concept, developed by Camp and Stein in 1943, is widely
used in designing rapid mixing and flocculation processes and is defined by the equation
(pf/2
a=--~

where G = root-mean-square velocity gradient, or rate of change of velocity, (ft/s)fft
P = power input, ft • lb/s
/.~ = dynamic viscosity, lb • sift 2
V = volume, ft 3
Equations are also available to calculate G for various types of mixing arrangements,
and manufacturers of mixing and flocculation equipment provide information on G val-
ues for their equipment. Another parameter used in designing mixing systems is Gt, which
is the dimensionless product of G and detention time t in seconds.

Temperature Effects on Mixing

Rapid mix and flocculation systems design is temperature-dependent because water vis-
cosity varies with temperature, as shown in Table 6.1. Guidelines for adjusting detention
times in both rapid mix and flocculation basins are shown in Table 6.2.

Types of Rapid Mixing Systems
Coagulant chemicals can be mixed by several methods, including
• Mechanical devices in a dedicated basin
• In-line blenders

TABLE 6.1 Water Viscosity and Water Temperature
Temperature, °C Temperature, °F /~, cP p~, lb • s/ft 2
0 32 1.792 3.75 x 10 -5
5 41 1.520 3.17 × 10 -5
10 50 1.310 2.74 X 10 -5
15 59 1.145 2.39 x 10 -5
20 68 1.009 2.10 X 10 -5
25 77 0.895 1.87 x 10 -5
30 86 0.800 1.67 x 10 -5