Page 284 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

10 Mixing

The mixing unit process is found throughout treatment plants. material in a solution is some arbitrary fraction of an ultimate
Examples include rapid mix (also called ‘‘initial mixing’’), value, e.g., 0.99.
flocculation, disinfection; activated sludge, the anaerobic pro-
cess, and gas dissolution. Usually, the mixing is not visible 10.1.1.2.2 Dispersion
since the equipment may be located under floors, in pipes, in The dictionary defines dispersion as ‘‘scatter,’’ which results in
tanks, or behind walls. mixing. In flow through a porous medium, the mechanism is
For most unit processes, mixing is a critical supporting step usually random velocity variation along any flow path. The
causing (1) contacts between reactants, (2) creation of inter- same mechanism applies to turbulent diffusion (e.g., in the case
facial area, or (3) reduction of the ‘‘film’’ thickness (to maxi- of open channel flow or pipe flow). Examples include
mize the diffusion gradient across an interface). In design, the the spread of a solute in porous media flow (both longitudinally
objective is to ensure, to the extent feasible, that mixing is not and laterally), the mixing of two rivers (lateral dispersion),
a rate-limiting factor. the mixing of pipeline discharge into a stream, mixing by a
Design questions include the selection of type of mixing submerged jet (lateral and longitudinal), and peeling-off of
unit, its size, and power input. For a given mixing application, eddies from main circulation currents in a mixing basin.
a certain type of mixing technology may be more appropriate
than another. The intent of this chapter is to cover mixing 10.1.1.2.3 Agitation
principles and practice. Agitation involves the induced motion of a material (McCabe
et al., 1993, p. 235), i.e., a disturbance of the status quo.
The chemical engineering literature seems to use this term
10.1 DEFINITIONS AND APPLICATIONS preferentially.
The world of mixing includes an array of technologies, any
one of which may be employed for one of the numerous 10.1.2 APPLICATION CATEGORIES
applications. The principles involved, i.e., advection, turbu-
lence, diffusion (Box 10.1), are applicable also to the variety Processing pairs applicable to water treatment include
of mixing situations that occur in the ambient environment, (1) liquid–solid, (2) liquid–gas, (3) two immiscible liquids,
e.g., re-aeration in streams, chemical and biological reactions (4) two miscible liquids, and (5) fluid motion. Descriptions of
in streams and lakes, and dispersion in streams. each are given with reference to an impeller–tank. Jet mixers,
static mixers, baffled basins, etc., are other technologies that
could be applicable.
10.1.1 DEFINITIONS
10.1.2.1 Liquid–Solid
The term ‘‘mixing’’ seems self-explanatory. Several ancillary
terms are used, however, which are near-synonyms. A ‘‘liquid–solid’’ pair includes particles that dissolve in water.
For example, a solid chemical in pellet or powder form must
be suspended and ‘‘agitated’’ to achieve mass transfer from
10.1.1.1 Mixing
the solid phase to the dissolved phase.
Mixing is defined as inducing the random distribution of two
or more initially separate phases through one another
10.1.2.2 Liquid–Gas
(McCabe et al., 1993, p. 235). Some commonplace examples
A gas flow emerging from a tube below an impeller is ‘‘dis-
are: in the kitchen, e.g., tossing a mixed salad with dressing,
persed’’ throughout the liquid volume first by shear to create
blending chocolate chips or nuts into cookie dough; in the bar,
bubbles and then by pumping and shear along the advected
e.g., mixing drinks; or in the garage, e.g., mixing paint.
stream to spread them; the diffusion of gas from the bubbles
to the water is the final step.
10.1.1.2 Near-Synonyms
The following terms are found in the literature without fine 10.1.2.3 Immiscible Liquids
distinctions. Nevertheless, each has its own connotation. High molecular weight polymers are often dissolved in an oil
base. Prior to metering into a water flow, the emulsion must
10.1.1.2.1 Blending be dispersed by means of a high-shear zone, i.e., an impeller,
In blending, one substance may lose its identity within within a small flow of water, which is then metered into the
another. A criterion may be when the concentration of a main flow.

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