Page 285 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

240 Fundamentals of Water Treatment Unit Processes: Physical, Chemical, and Biological

10.1.3 MIXING AS RATE LIMITING
BOX 10.1 MIXING IN-A-NUTSHELL
Mixing may control the reaction rate for anaerobic reactors,
Mixing can be described in terms of a simple task such aerobic reactors, coagulation in water treatment, oxygen dis-
as stirring sugar into a cup of coffee or tea. The sugar solution, gas stripping, etc. For diffusion-limited processes,
granules, poured into a cup, come to rest on the bottom the rate of the creation of surface area governs the total area
and dissolve. If the solution is quiescent, the dissolved available as a mass transfer interface. At the same time, with
sugar molecules diffuse, in accordance with Fick’s law, fresh area being created, a steep concentration gradient
forming a concentration gradient from the bottom of the (small d) is maintained between the concentration in the
cup to the top. Over hours of time the gradient bulk of solution, C o , and the interface concentration, C i .In
approaches uniformity. other words, mixing is important in paving the way for other
As everyone knows, however, stirring causes the processes to function effectively. In such cases, mixing deter-
sugar granules to be ‘‘dispersed’’ rapidly throughout mines the rate of mass transfer to some asymptote limit, where
the cup. Two things occur in stirring, e.g., by a spoon: higher mixing energy will have no further effect.
(1) the dissolved sugar circulates throughout the cup,
i.e., is ‘‘advected’’ by the ‘‘large-eddies’’ from the spoon,
and (2) a ‘‘wake eddy’’ is induced and the sugar granules 10.2 HISTORY OF MIXING
and molecules ‘‘blend’’ with the water molecules.
Mixing has been an empirical practice since the early days of
In summary, three transport mechanisms are opera-
water treatment, i.e., about 1900 with theory getting its start in
tive, i.e., advection, turbulence, and diffusion. They
the 1940s with the work of Camp and Stein (1943). This
apply to virtually all mixing situations, e.g., gas trans-
section covers the evolution of theory and practice.
fer, chemical dissolution, polymer blending, coagula-
tion, disinfection, and oxidation reactions.
10.2.1 DRINKING WATER TREATMENT
Mixing in drinking water treatment is important in co-
10.1.2.4 Miscible Liquids agulation (i.e., initial mixing or rapid mix), in flocculation
The term ‘‘blending’’ is sometimes given to one miscible (Chapter 11), and in disinfection. Also, mixing is required to
liquid being dispersed, i.e., ‘‘blended’’ into another. Metering dissolve solid chemicals (e.g., lime), to disperse polymer
liquid alum into a water flow is an example. emulsions, to dissolve solid polymers, to blend a liquid
chemical (e. g., fluoride), to dissolve a gas (e.g., CO 2 for pH
10.1.2.5 Fluid Motion reduction), etc.
When the required fluid motion is achieved, the other parts of
the process will also be satisfied (Oldshue, 1983, p. 5). For an 10.2.1.1 Initial Mixing
impeller–basin system, the characteristics of fluid motion Willcomb (1932, p. 1427) stated that none of the plants built
include the pumping capacity of the impeller, the flow pat- during the period 1900–1911 under his direction had mixing.
terns within the basin, and the shear zones and their extent and Rather, the gravity plants had their alum introduced just
intensities. before the raw-water meter, and the pumping plants had
their alum dose added to the pump suction. By the time of
10.1.2.6 Pumping and Shear his article in 1932, rapid mix as a unit process was becoming
established. Alternative mixing methods mentioned included
The impeller energy is divided into pumping and shear.
The proportion of each depends upon the impeller–tank sys- aeration, the current of a flume, the turbulence associated with
tem. Whether advection flow is preferred at the expense a valve, and the hydraulic jump.
of shear or vice versa depends upon the application (Oldshue, The importance of mixing was recognized by Hansen
1983, p. 8). (1936) who noted that mixing of coagulants with raw water
was essential, followed by flocculation (he also cited
the importance of Langelier’s work in Sacramento in 1919,
10.1.2.7 Examples
Langelier, 1921). Despite the trend toward recognizing the
Applications of mixing in water treatment include rapid mix
importance of initial mixing, Babbitt and Doland (1939,
after the addition of chemicals; disinfection after the addition
p. 529) did not wholly endorse such a position with the
of chlorine (or other oxidant); the dispersion of mixed liquor
following view:
return activated sludge into the reactor flow; the dispersion of
an immiscible liquid, e.g., a polymer in oil in a water concen-
It is probable that rapid mixing devices for the somewhat
trate prior to metering into raw water; the dispersion of a
violent initial mixing of the chemicals with the raw water
polymer for sludge thickening; the dispersion of a gas through will not come into general use, because of their cost, both
water as small bubbles. In the anaerobic process, advection for construction and for operation, is not justified by the slight
mixing predominates, while for the activated sludge process increase in efficiency of operation. Where low lift pumps are
the shear rate is higher. available, they may serve the purpose without added expense.