Page 236 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

9 Coagulation

The term coagulation refers to the process of inducing con- 9.1.1.4 Rapid-Mix
tacts between a chemical and colloidal particle to effect a Mixing (Chapter 10) is the means to cause contacts between
reaction. The reaction product is called here, a microfloc. the coagulant chemicals and the particles to be removed in
Engineering the process has two phases: (1) choosing the the raw water. The mixing unit in coagulation is termed
proper chemicals, the proper dosages, and the proper pH to ‘‘rapid-mix.’’ Several design forms are found in practice,
achieve a microfloc product, and (2) causing contacts between for example, ‘‘back-mix’’ reactor, ‘‘flash-mix,’’ baffles,
the coagulant chemicals and the colloidal particles. The latter elbows, etc.
is the topic of Chapter 10. Inducing the growth of microflocs
to form flocs is the topic of Chapter 11. The effectiveness of 9.1.1.5 Flocculation
the coagulation process determines the effectiveness of the The collision between microflocs to form larger aggregated
subsequent processes, for example, flocculation–flotation; particles is called ‘‘flocculation’’ (Chapter 11) and the result-
flocculation–sedimentation; flocculation–settling–filtration, ing particles are called ‘‘flocs.’’
or filtration.
9.1.1.6 Themes of Coagulation Theory
9.1 COAGULATION IN-A-NUTSHELL The three themes that have emerged in coagulation theory are
as follows: (1) a sequence of reactions between aluminum and
Coagulation is chemical in character. Simply stated, coagula-
water, (2) adsorption destabilization with positively charged
tion is the reaction between a chemical and particles to form a
polynuclear aluminum species, and (3) ‘‘sweep-floc’’ based
‘‘microfloc.’’ Actually, a body of knowledge is involved; key
on positively charged aluminum hydroxide.
elements of the topic are summarized in this section.
9.1.1 DEFINING COAGULATION 9.1.2 COAGULATION PRACTICE
An array of terms are involved in the coagulation process and The many aspects of coagulation practice include
are explained here. The glossary includes a more extended
. Selection of type of coagulant (generally alum or a
list.
ferric salt)
9.1.1.1 Particles to Be Removed . Selection of a coagulant aid, that is, a polymer
Ambient waters contain a wide variety of particles in the (which may or may not be used)
. Determination of dosage of each chemical
general categories: mineral (e.g., clays), biological (viruses,
. Design of rapid-mix
bacteria, algae, protozoan cysts, etc.), and organic matter (e.g.,
. Evaluation of coagulation effectiveness
natural organic matter (NOM)). These particles range in size
from nanometers to perhaps 200–300 mm.
The preceding enumeration has related, traditionally, to par-
9.1.1.2 Coagulation ticle removal, with ancillary focus on removing color, a sur-
Trivalent metal ions, that is, Al 3þ and Fe , are the traditional rogate for NOM. In the mid-1970s NOM was identified as a
3þ
coagulants used in water treatment practice. They react with precursor to halogenated organics, the latter being considered
water to form ‘‘hydrolysis products’’ and=or precipitate, which carcinogenic. NOM became a major focus of coagulation
react in turn with negatively charged particles to form what is practice along with particles.
called here, ‘‘microflocs.’’ This process is termed, ‘‘coagulation.’’
9.1.2.1 Dosage
9.1.1.3 Microflocs Coagulation is the most critical phase of the depth filtration
Small discrete particles that leave the rapid-mix are called process (Chapter 12) and coagulant dosage is the determining
here, ‘‘microflocs.’’ In the case of coagulation with Al , the factor (along with pH). Generally, dosage is determined by a
3þ
objective is that the reaction products of Al 3þ and water jar test. The term, ‘‘enhanced-coagulation’’ refers to the add-
combine with the microscopic particles as found in the ambi- ition of an alum or ferric dosage sufficient to remove NOM as
ent water being treated. well as particles.

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