Page 208 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

8 Flotation

The flotation process involves (1) generation of air bubbles, fine particles are required such as colloidal metals, calcium
(2) contact between the air bubbles and the particles to sulfate, and metal hydroxides; finely divided coal; scale and oil
be removed, (3) flotation of particles by the buoyant in steel mill wastewater; solids and fatty acids in laundry wastes;
force created, and (4) removal by skimming. In modern prac- grease from meat product wastewaters; free or emulsified oil
tice, flotation utilizes dissolved air as a source of bubbles from refineries; pulp and paper wastewaters; and recovery of fats
and is called ‘‘dissolved air flotation,’’ with the common and oils in soap manufacturing (Vrablik, 1960).
acronym ‘‘DAF.’’
Examples of particles to be floated include algae; chemical
8.2 DAF SYSTEM DESCRIPTION
precipitates; coagulant flocs such as alum or ferric, perhaps
strengthened with a polymer; and biological flocs. The object- The DAF ‘‘process’’ comprises a system, i.e., a coordinated
ive may be either to separate solids and water or to ‘‘thicken’’ collection of components that results in achieving its object-
the solids, e.g., to raise the solids concentration from say 1% ives. The objectives depend, of course, on the application, but
to perhaps 4% (such as in the case of activated sludge). the common thread of all is separation of water and solids by
flotation.
8.1 DEVELOPMENT OF FLOTATION
The flotation process was developed about 1875 for ore separ- 8.2.1 SYNOPSIS OF DAF PROCESS
ations in the mining industry, and in the 1950s it was adopted for
Figure 8.1 shows details of a DAF system, e.g., rapid mix,
treatment of industrial wastewaters. During the decades since,
flocculators, air saturator, and the flotation basin. The descrip-
applications have been extended to include thickening of acti-
tions that follow refer to Figure 8.1.
vated sludge, removal of algae from oxidation pond effluents,
and in drinking-water clarification in lieu of gravity settling.
8.2.1.1 Coagulation
Coagulant chemicals, e.g., alum or ferric ion, are added in the
8.1.1 BEGINNING DESIGN PRACTICE
rapid mix, where particles to be removed are charge neutral-
Some of the same empirical design parameters as are current ized, resulting in ‘‘microflocs.’’
were seen in the 1950s literature. For example, Eckenfelder
et al. (1958, p. 257) proposed guidelines for the air-to-solids 8.2.1.2 Flocculation
(A=S). Then, Vrablik (1960) gave guidelines for overflow rate The ‘‘micro-flocs’’ formed grow in size in the flocculation
and saturator pressure (Section 8.4). In other words, those basins to form ‘‘flocs,’’ preferably to a size of 10–50 mm but
active in that time evidently had insight into critical process not larger than 50 mm. The size of the flocs is controlled by
parameters. the coagulant dose, flocculation turbulence intensity, i.e., G
1
Design theory evolved further during the 1990s based in s , and detention time, q 10 min. Recommended G
on research by Edzwald, Fukushi, and Haarhoff and their values are G 70 s 1 for alum or ferric coagulation, and
coworkers (Edzwald, 1995; Edzwald and Walsh, 1992; G 30 s 1 for polyaluminum chloride. For wastewater,
Fukushi et al., 1998; Haarhoff and Rykaart, 1995; Haarhoff suggested limits were 60 G 80 s 1 (Ødegaard, 1995).
and van Vuuren, 1995) directed mostly toward water
treatment. Principles delineated were based on saturator pres- 8.2.1.3 Contact Zone
sure, bubble–particle contact probability, and rise-velocities After flocculation, the raw-water flow enters the ‘‘contact
of bubble–particle agglomerates. zone’’ of the flotation basin where the floc particles contact
precipitated gas bubbles in the recycle flow from the saturator.
The recycle flow is emitted just below the contact zone in jets
8.1.2 WATER AND WASTEWATER APPLICATIONS
from a bank of nozzles attached to the manifold.
Kalinske (1958, p. 228) listed 11 applications of flotation
in industrial wastewater treatment. Only one application was 8.2.1.4 Saturator
mentioned in municipal wastewater treatment, i.e., thickening The saturator is a tank where ‘‘gas transfer’’ occurs, i.e., the
of activated sludge, which was not established as a technology. water gains dissolved oxygen and nitrogen (and minor gases
Some of the specific applications of DAF for industrial such as argon and carbon dioxide) from air under pressure.
wastewater treatment have included: canneries in which organic The air flows upward from beneath a ‘‘packing’’ material and
suspended solids are removed; chemicals in which recoveries of the gas transfer occurs in the ‘‘recycle’’ water flowing down.

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