Page 370 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

Flocculation 325

Flocculent: (1) A chemical, e.g., a polymer, that contributes of fully turbulent motion in a turbulent flow field
to the aggregation and growth of floc particles; the (Casson and Lawler, 1990, p. 55) (see also
mechanism would be ‘‘bridging,’’ i.e., two or more Chapter 10).
floc particles are attached to a molecular strand of Kolmogorov’s universal equilibrium theory: Relates to the
polymer. (2) A coagulant chemical, e.g., alum or net rate of change in the energy contained in the
ferric ion is sometimes called a ‘‘flocculent’’ (this small-scale motions to the energy dissipation rate
definition is mentioned only because of its use in of these motions (Casson and Lawler, 1990, p. 55).
the literature and is not defined unequivocally; Monodisperse: A suspension having but one particle size.
rather, its definition is implied). Orthokinetic flocculation: (1) Refers to flocculation induced
Fractal: (1) A structure of formed aggregates (Gmachowski, by a velocity gradient (Argaman and Kaufman,
1995, p. 1815). (2) See Chapter 9. (3) The concepts 1968, p. 5; Ives, 1978, p. 39). (2) See Chapter 9.
of fractal geometry provide a mathematical frame- Perikinetic flocculation: (1) Refers to flocculation induced
work for description of the structure of irregular flocs by a Brownian motion (Argaman and Kaufman,
(Spicer and Pratsinis, 1996a, p.1052). 1968, p. 5; Ives, 1978, p. 39). (2) See Chapter 9.
Primary particles: (1) Particles at the commencement of
Fractal dimension: (1) An exponent in the relation, N ¼
z(d=d o ) D F that characterizes the aggregate mass flocculation in which all particles are considered to
of fractal (dimensionless). (2) A solid three- be covered with coagulant and have diameters nearly
dimensional body has a mass which depends on the equal (Tambo and Watanabe, 1979, p. 430). (2)
third power of some characteristic length (such as Destabilized particles ready for aggregation (Arga-
the diameter of a sphere), so that a log–log plot of man, 1971, p. 775). (3) The particles to be removed
mass against size should give a straight line with a from the raw water, e.g., turbidity, microorganisms
slope of 3. When such plots are made for aggregates, (viruses, bacteria, cysts, oocysts, algae, and other
lower slopes are found, with non-integer values. The microscopic particulates).
slope of the line is known as the fractal dimension, Root mean square velocity gradient: The root mean square
D F . In three-dimensional space, D F may take values (rms) of the velocity fluctuation, u ,is defined,
0
between 1 and 3, the lower value representing a
1=2
linear aggregate and the upper one an aggregate of 02
u u
0
uniform density or porosity. Generally, intermediate
values are found, and the lower the fractal dimen-
sion, the more ‘‘open’’ or ‘‘stringy’’ the aggregate In a turbulent flow field, the velocity at any point
structure (Gregory, 1989, p. 215). fluctuates randomly; this fluctuation is dealt with
statistically as the rms.
G: A term defined by Camp and Stein (1943) as G ¼ dv=dy ¼
0.5
[P=(mV)] . Scale of turbulence: Distance across which the velocity of an
Heterodisperse: A suspension having a distribution of par- eddy changes (François, 1987b, p. 1023). Energy
ticle sizes. content of an eddy depends upon the scale of turbu-
Hydrogen bond: Considered having a minor role in intra- lence; the large eddies contain most of the energy of
particle bonding because of their short range; ener- the system and to not dissipate energy.
gies are 3–10 kcal=mol (Hannah et al., 1967, p. 846). Sludge blanket clarifier: See solids contact unit.
Considered, however, to be a major force in attach- Solids contact unit: (1) A tank that has a center well used for
ment of flexible polymer chains to floc particles to coagulation, with outer portion maintained as a
permit bridging and agglomeration. ‘‘sludge blanket.’’ The microflocs resulting from
Isotropic turbulence: Isotropic turbulence is characterized coagulation are forced up through the blanket, per-
by equal strain rates, i.e., velocity gradients, with haps 0.7 m (2 ft) depth and fluidized, of larger
respect to direction. According to Tennekes and floc particles; during their flow through the blanket,
Lumley (1972, p. 262), small eddies exhibit ‘‘local these primary particles make contact during with
isotropy,’’ where any sense of direction is lost with these previously formed floc particles. The solids
turbulence being increasingly scrambled at small blanket is maintained at a desired depth by waste
scales. The range of wave numbers exhibiting local flow of sludge from the bottom. The effluent flow
isotropy is called the equilibrium range (see also leaves the clarifier by peripheral weirs. For those
anisotropic turbulence). floc particles that are suspended in the flow leaving
Kolmogorov’s microscale: (1) The Kolmogorov’s micro- the solids blanket, they may fall back to the
scale of turbulence is a particular eddy size, calcu- sludge blanket. (2) Proprietary devices that combine
lated by the fluid viscosity and the energy dissipation rapid mixing, flocculation, and sedimentation in
one unit. These units provide separate coagulation
(François, 1987, p. 1023), i.e., l(Kolmogorov) ¼
3
(n =e) 1=4 . (2) Based on the universal equilibrium and flocculation zones and are designed to cause
theory, the Kolmogorov’s microscales of length, contact between newly formed floc and settled
time, and velocity are defined as the smallest scales solids.