Page 336 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

11 Flocculation

Flocculation is the unit process in which larger particles are 11.1.1.2 Chemical Floc
formed from smaller particles due to collisions between them. Floc formed from a chemical substance as the primary par-
The collisions occur by the ‘‘transport’’ mechanisms of vel- ticles is chemical floc. The most common ones are those
ocity gradients (induced by laminar flow or turbulence), formed from alum, ferric ion, lime, etc.
Brownian motion, or a sequence of the two.
In water treatment, the starting point is the rapid mix where 11.1.1.3 Primary Particles
‘‘microflocs’’ form from coagulation chemicals and which may The smaller particles that comprise floc are called primary
incorporate ‘‘primary’’ particles, e.g., mineral turbidity, micro- particles; generally the size is d 1 mm. In the case of
organisms, and other ‘‘microscopic particulates.’’ The next chemical flocs, the primary particles are microflocs (Ham
step is flocculation; the objective is to cause collisions such and Christman, 1969, p. 483); see also Chapter 9. In the
that the microflocs grow in size to become ‘‘floc’’ particles. If case of bioflocculation, the primary particles are individual
the treatment train is conventional filtration, the flocculation microbes. Also, primary particles may include viruses, bac-
objective is to produce large settleable flocs. In flotation, on the teria, cysts, other microscopic organisms, and turbidity.
other hand, the objective is to form a floc of small size, e.g.,
50 mm, that rises readily when small gas bubbles attach.
The common flocculation technology is the paddle wheel. 11.1.2 FLOCCULATION
Other technologies include impeller basins, baffles, and pro- The term flocculation is the process of causing collisions
prietary innovations. These various technologies provide the between
‘‘transport’’ mechanisms that induce floc collisions.
. Primary particles
. Primary particles and floc
11.1 DEFINITIONS
. Floc particles and other floc
The flocculation terms used here were based on usage in the
literature. In a few cases, the literature is ambivalent and The purpose of flocculation is to cause growth in particle size
therefore some liberties were taken in gleaning what was such that the floc will settle or be amenable to flotation.
believed the most coherent definition that fitted with past Effective flocculation requires that the colliding particles
usage. The glossary provides a more inclusive listing. adhere to one another.

11.1.2.1 Orthokinetic Flocculation
11.1.1 FLOC
Collisions induced by turbulence is called orthokinetic floccu-
Floc is an agglomeration of ‘‘primary particles’’ combined lation (Langelier and Ludwig, 1949, p.165; Argaman and
with ‘‘microflocs’’ (see Sections 9.1, 9.1.1, and Glossary of Kaufman, 1968, p. 5). The collision process is most effective
Chapter 10); the latter are usually of diameter, d 1 mm. with eddy sizes that are approximately the same size as the floc
The flocs grow in size by collisions with one another, i.e., particles (Casson and Lawler, 1990, p. 68). Large eddies are
by ‘‘flocculation.’’ Size may vary from ‘‘pinpoint,’’ i.e., not effective except to maintain the particles in suspension.
5 d 10 mm, to perhaps 1–5 mm depending on the turbu-
lence intensity. A general description is that a floc is an 11.1.2.2 Perikinetic Flocculation
aggregate with a complex random structure with low average Collisions induced by Brownian motion is called perikinetic
density (Meakin, 1989, p. 250). flocculation. The terms ‘‘perikinetic flocculation’’ and ‘‘orthoki-
netic flocculation’’ were used by Langelier and Ludwig (1949,
11.1.1.1 Biological Floc p. 165), as adopted from the field of soil chemistry. Later
Biological floc is an agglomeration of the diverse microorgan- Argaman and Kaufman (1968, p. 5) used the terms and credited
isms as found in a biological reactor. As it occurs in an their first use to Smoluchowski in his 1916 and 1918 papers.
activated sludge reactor, the biological floc is termed
‘‘mixed liquor suspended solids’’ (MLSS). The character of 11.1.2.3 Flocculent
the floc is important in settling, i.e., as in ‘‘final settling.’’ A polymer added prior to flocculation to add a ‘‘toughness’’ to
Filamentous bacteria, for example, result in floc that does not the floc is called a flocculent. Note that here we say that a
settle well. chemical addedbeforerapid mixisa‘‘coagulant’’and achemical

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