136 ENVIRONMENTAL ENGINEERING
Raw,
water
Chemicals- Rapid mixing
- Flocculation
- Settling
- Filtration
- Chlorination
- Clear well storage
- Pumping to distribution system
iished
water
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Figure 7-1. Diagram of a typical water treatment facility.prevented from coming together to form large particles that could more readily be
settled out. The removal of these particles by settling requires first that their charges
be neutralized and second that the particles be encouraged to collide with each other.
The charge neutralization is called coagulation, and the building of larger flocs from
smaller particles is called flocculation.
A fairly simple but not altogether satisfactory explanation of coagulation is avail-
able in the double-layer model. Figure 7-2 is a representation of the static electric
field surrounding the particle. The solid particle is negatively charged, and attracts
positively charged ions - counterions - from the surrounding fluid. Some of these
negative ions are so strongly attracted that they are virtually attached to the particle
and travel with it, thereby forming a slippage plane. Around this inner layer is an outer
layer of ions consisting mostly of positive ions, but they are less strongly attracted, are
loosely attached, and can slip off. The charge on the particle as it moves through the
fluid is the negative charge, diminished in part by the positive ions in the inner layer.
The latter is called the zeta potential.
If the net negative charge is considered a repulsive charge, since the neighboring
particles are also so charged, the charge may be pictured as in Fig. 7-3A. In addition
to this repulsive charge, however, all particles carry an attractive electrostatic charge,
van der Waals force, that is a function of the molecular structure of the particle. This
attractive charge is also shown in Fig. 7-3A. The combination of these forces results
in a net repulsive charge, an energy barrier, or “energy hill,” that prevents the par-
ticles from coming together. The objective of coagulation is to reduce this energy
barrier to 0 so that the particles no longer repel each other. Adding trivalent cations
to the water is one way to reduce the energy barrier. These ions are electrostatically
attracted to the negatively charged particle and, because they are more positively
charged, they displace the monovalent cations. The net negative charge, and thus the
net repulsive force, is thereby reduced, as shown in Fig. 7-3B. Under this condition,