Page 276 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

Coagulation 231

Dielectric coefficient: The field between a pair of oppositely Electron volt: Work done to an electron in moving it
charged parallel plates and the induced charges on through an electric field under a potential differ-
the surfaces of a dielectric adjacent to the plates. ence of 1 V. 1.0 eV ¼ 1.60217733 10 19 C; also,
An insulator, or dielectric, is a substance within 1.0 eV ¼ 1.6022 10 19 J. [C ¼ 1.0 A 1.0 s]
which there are a few or no charged particles free Electrophoresis: The movement of a charged particle in an
to move under the influence of an electric field. electric field. A charged particle moves toward a
Representative values of the coefficient, D, are plate of opposite charge at a velocity proportional
D(vacuum) ¼ 1, D(glass) ¼ 5–10, D(rubber) ¼ to the potential gradient in V=cm.
2.5–35, D(water) ¼ 81, D(air) ¼ 1.00059 (Sears, Electrophoretic mobility (EM): The electrophoretic mobil-
1947, p. 177). See also permittivity. ity, EM, is the particle velocity per unit of field
Diffuse layer: Ion cloud of counterions having charge oppos- strength with SI units, m=s=V=m (Pilipovich et al.,
ite the surface charge of the colloid; also called the 1958, p. 1469; Gregory, 1975, p. 65, 67).
‘‘Gouy’’ layer. Emulsion: Suspension of one liquid in another, for example,
Disinfection by-product (DBP): Products formed due to globules of fat in water (Saville, 1917, p. 80).
the reaction between a disinfectant and organic Enhanced coagulation: The addition of alum (or ferric ion)
carbon. in excess of that required for removal of colloidal
Dispersed phase: Two disperse phases important in water particles to the extent that a substantial amount of
systems include a gas in a liquid and particles in NOM is also removed. The term has been around
water (Stumm and O’Melia, 1968, p. 532; Myers, since about 1984 in general usage, being introduced
1991, p. 191). by Kavanaugh (1978) as ‘‘modified’’ coagulation.
Dispersion: A colloidal dispersion is the solid and liquid Enhanced coagulation end point: Dosage of coagulant
phases together. A system of fine particles is called and=or pH value which, when achieved, no longer
a colloidal dispersion, or a sol (Sennett and Olivier, produces significant TOC reduction (Pizzi and
1965, p. 33). Rodgers, 2000, p. 4).
Dispersion forces: The same as London-van der Waals Equilibrium: As pertaining to a chemical reaction, the prod-
forces and sometimes called dispersion forces ucts and reactants have concentrations that do not
because the electron oscillations involved are also change with time.
responsible for the dispersion of light; the term is Equilibrium constant: The ratio of product concentrations in
distinguished from dispersions of particles (Gregory, mol=L to reactant concentrations in mol=L, the stoi-
2006, p. 67). chiometric coefficients are exponents. The equilib-
Dissolved organic carbon (DOC): Organic carbon passing rium constant is designated, K a . The negative log of
through a 0.45 mm filter (Randtke, 1988, p. 43). K a is designated, pK a , that is, log K a ¼ pK a .As an
DLVO theory: A theory of colloid stability synthesized by example of the use of pK a , for carbonate equilibria,
four investigators in colloid science, Derjaguin– HCO 3 is predominant in the range, 4.35 < pH <
Landau–Verwey–Overbeek that integrates electrical 10.33, which are the respective pK a that separate
repulsion between particles with van der Waals HCO 3 from H 2 CO 3 and CO 3 , respectively.
2

attraction. Equilibrium diagram: A diagram that delineates equilib-
Double layer: A negatively charged colloid in water has rium concentration relationships between a pair of
both a ‘‘fixed layer’’ of positive ions bound to the species in a chemical reaction.
particle (as described by Helmholz and Stern) and an Faraday: Amount of electric charge required to liberate 1 g
‘‘ion cloud’’ that emanates from the fixed layer (as equivalent of material at an electrolytic cell
described by Gouy and Chapman). These two 1 Faraday ¼ 96,514 C
parts, the fixed layer and the ion cloud, were termed Ferric iron salt: The ferric salts used in water treatment
the ‘‘double layer.’’ The diffuse layer is characterized include FeCl 3 and Fe 2 (SO 4 ) 3 ; usually, they are
by a gradual change in potential, due to the hydrated with 6 and 4.5 waters, respectively.
thermal energy of the ions (Sennett and Olivier, Floc: An aggregation of microflocs induced usually by
1965, p. 36). hydraulic turbulence causing a growth in size such
Electric field: A potential gradient. Also, a charged particle at that settling occurs readily. Size may range 0.1–5mm.
a point, A, causes an electric field (Sears, 1947, Flocculation: Flocculation is the physical process of bringing
p. 17). the coagulated particles into contact to promote floc
Electrokinetic: When two phases exist, that is, a solid and a formation. The term, flocculation comes from the
liquid, each surface is likely to have a charge. When Latin word, flocculus—a small tuft of wool or a
one phase moves past the second phase, four effects loosely fibrous structure (Black, 1967, p. 277).
are possible, depending on the way the motion is Flocculent: A chemical added to produce growth in size of
induced: (1) electrophoresis, (2) electroosmosis, (3) destabilized particles or precipitate. Usually floccu-
streaming potential, (4) sedimentation potential lents are commercial polymers introduced into the
(Hunter, 1981, p. 2). flow prior to mechanical flocculation.