Page 237 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

192 Fundamentals of Water Treatment Unit Processes: Physical, Chemical, and Biological

9.1.2.2 Coagulation Effectiveness are about 1 mm, algae are 1–50 mm, protozoa 5–100 mm. The
Coagulation effectiveness has been measured traditionally by coagulation process is likely to be effective for most of these
removal of turbidity. Color removal is another traditional particles (Singley et al., 1971, p. 99).
objective in drinking water treatment, but for esthetic reasons.
Since about 1980, however, color has became a health issue as 9.2.2 PARTICLE CHARACTERISTICS
well, since it is caused mostly by NOM.
Table 9.1 lists some of particles that may be coagulated and
gives sizes, ‘‘zeta potentials,’’ and representative counts. As
9.2 PARTICLES IN AMBIENT WATERS seen, except for chrysotile, the particles have negative zeta
Coagulation theory is based on the mineral colloid ‘‘model.’’ potentials; the chrysotile is listed only to indicate that a
The main characteristics of the mineral colloid model are (1) a positive charge, albeit the exception, may occur. The particles
negative charge with a diffuse double layer, and (2) a particle listed include viruses, bacteria, algae, protozoan cysts, plant
size 10 mm. In practice, however, a variety of kinds of debris, nematode eggs, and other kinds of particles having
particles, both mineral and biological, occur in ambient waters biological origin and reflect the character of the water.
and must be removed. There are perhaps thousands of differ-
9.2.2.1 Colloids
ent species of microscopic organisms found commonly in
water supplies. The general groups include: viruses, bacteria, A colloid is a discrete particle that remains in suspension, for
cysts, algae, spores, rotifers, fecal debris from rodents, etc. example, does not settle in water. Clay is a typical colloid, as
Other particles may include organic debris such as parts of seen in Table 9.1, and remains in suspension due to its small
animal fecal matter, and organic molecules such as humic and size, that is, 10 mm, and negative charge, the latter causing
FAs, that is, NOM, which are essentially macromolecules. mutual repulsion between particles. Such a suspension is
called a ‘‘sol’’ and is termed, ‘‘stable.’’ An objective of coagu-
lation is to ‘‘destabilize’’ the suspension, that is, reduce the
9.2.1 PARTICLE VARIETY
magnitude of the colloid negative charge, which is done in
Figure 9.1 indicates the different kinds of particles found in traditional water treatment practice by trivalent cations, for
ambient surface waters (i.e., in rivers and lakes). Size ranges example, Al 3þ or Fe .
3þ
are shown and compared with several common references, for
example, visible to the unaided eye, laboratory microscopic, 9.2.2.2 Microscopic Particles
and the electron microscope. As seen, colloids are those In addition to mineral colloids, ambient water has a variety of
particles nominally 10 mm, viruses are 10–800 Å, bacteria particle categories, mostly biological, that include: viruses,

Size (m)
10 –10 10 –9 10 –8 10 –7 10 –6 10 –5 10 –4 10 –3 10 –2 10 –1 10 –0

Colloidal range Coarse turbidity

Range visible to the eye
Ultramicroscope
Electron microscope
Laboratory microscope

Stereo microscope

Nominal coagulation removal range

Molecules Protozoa
Bacteria
12 μ quartz particle will
Viruses settle at 20°C 1.0 m in 2h
as calculated by Stoke's law
Algae

1 Å 1 μ 1 mm 1 cm 1 m
Size (in units shown)
FIGURE 9.1 Sizes of particles in coagulation removal range. (Adapted from Riddick, T.M., J. Am. Water Works Assoc., 53(8), 1007,
August 1961; Amirtharajah, A. and O’Melia, C.R., Coagulation processes: Destabilization, mixing, and flocculation, in Pontius, F.W., Ed.,
Water Quality and Treatment, 4th edn., McGraw-Hill, New York, 1990, Chapter 6.)