Page 247 - Water and wastewater engineering

6-24 WATER AND WASTEWATER ENGINEERING

Mechanical mixing causes unequal shearing forces on the floc, and some of the floc are
broken up. After some period of mixing, a steady state distribution of floc sizes is achieved and
formation and breakup become nearly equal.

Differential Settling
Because the floc particles are of different size, they settle at different rates. Differences in the
settling velocities cause the particles to collide and flocculate.

Chemical Sequence
The addition of multiple chemicals to improve flocculation is common practice. The order of
addition is important to achieve optimum results at minimum cost. Typically, the addition of
a polymer after the addition of hydrolyzing metal salts is most effective. Ideally, the polymer
addition should be made 5 to 10 minutes after the addition of the hydrolyzing metal salt. This
allows for the formation of pinpoint floc that is then “bridged” by polymer. In conventional water
treatment plant design this is rarely possible because of space limitations.

6-6 MIXING THEORY
The crux of efficient coagulation is the efficiency of mixing the coagulant with the raw water.
Efficient flocculation requires mixing to bring the particles into contact with one another.
The following discussion includes the theoretical considerations in mixing coagulants, floc-
culation, and the practical aspects of selecting a mixing device. Many aspects of this discussion
also apply to pH adjustment, softening (Chapter 7), and disinfection (Chapter 13).

Velocity Gradient
In the 1940s Kolmogorov (1941) and Camp and Stein (1943) independently developed a method of
quantifying the energy dissipation in a vessel. Camp and Stein further proposed that the root-mean-
square (RMS) of the velocity gradient ( G ) of the fluid, that is dv / dy in Figure 6-12 , be used to esti-
mate energy dissipation. They further proposed that the rate of flocculation is directly proportional
to G. Subsequent research demonstrated that the proportionality also applied to coagulation with
both metal ion coagulants and polymers (Harris, et al., 1966; Birkner and Morgan, 1968).

dv
dy

FIGURE 6-12
Velocity gradient.