Page 104 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

Unit Process Principles 59

TABLE 4.1 (continued)
Unit Processes of Water Treatment
Unit Process Principle Categories Technologies=Forms=Examples
Biological treatment—anaerobic Substrate is metabolized under anaerobic conditions Anaerobic digesters Large tanks—u 30 days
Anaerobic reactors Sealed tank
Disinfection Interference with organism survival and reproduction Contact basin
Membrane filtration Straining small particles Microfiltration Tubular—cross flow
Straining viruses Ultrafiltration Ceramic—cross flow
Straining organic molecules Nanofiltration Spiral wound
Straining ions Hyperfiltration Hollow fiber
Electrodialysis Electrodialysis

4.2.2 TRANSPORT
Transport mechanisms are at two levels: macro and molecu- 1 2 3 4 5 6
lar. The macro level includes advection of fluid mass, turbu-
lent diffusion in open channels and pipes, and dispersion in
1 2 3 4 5 6
porous media. Molecular diffusion is the other mechanism. As
a rule, these mechanisms occur in two or more combinations.
1 2 3 4 5 6
4.2.2.1 Macro Transport: Sedimentation
The settling of a particle through a fluid under the influence of
gravity is a transport mechanism. The sink is the surface, that FIGURE 4.1 Illustration of dispersion in flow through porous
is, the bottom of a tank or an inclined plate, where the particle media (numbers indicate hypothetical sequence of ‘‘steps’’ of tagged
settles. In flotation, the water surface is the sink. molecules).
4.2.2.2 Macro Transport: Advection
lengths. Each movement of the advective flow is a ‘‘step.’’
At the macro level, advection (sometimes called convection) Along the top stream path the particular random sequence of
is the transport of a fluid mass under a pressure gradient. [In six pore velocities shown, and the six ‘‘steps’’, will result in a
open channel flow, gravity provides the pressure gradient.] In tagged molecule moving ahead of the others. Along another
other words, advection transports the bulk flow of a fluid. flow path, say the bottom, the random sequence of pore vel-
ocities results in a tagged molecule lagging the others. The
4.2.2.3 Macro Transport: Turbulent Diffusion average of all pore velocities for a given cross section is the
Turbulence is a transport mechanism found in both ambient average advective pore velocity for the flow. The actual vel-
and engineered environments. It is superimposed on advective ocities vary continuously with pore size and also have micro
transport. Turbulence causes a random transport of molecules components that vary from the straight lines shown.
and particles; the transport rate is proportional to the intensity The effect of dispersion would be seen if a pseudo vertical
of turbulence. The turbulence intensity is dependent upon the line of tagged molecules, say 1000 in number, is placed on the
rate of energy dissipation and is important in rapid left side of the porous media shown in Figure 4.1. As an initially
mix=coagulation, flocculation, activated sludge, and other flu- vertical line of tagged molecules is translated to the right, it
idized bed reactors. In some unit processes, such as sedimen- becomes abell-shaped (Gaussian) curve. The bell-shaped curve
tation, turbulence is unwanted. spreads as the translation progresses. This is the effect of
dispersion. In addition, lateral dispersion occurs, and is super-
4.2.2.4 Macro Transport: Porous Media Dispersion posed on the translation. The effect may be observed if tagged
In flow through porous media, the advective flow follows molecules are followed from some arbitrary point source on the
‘‘streamlines.’’ At the micro scale, the flow follows a tortuous left side of Figure 4.1 and would be seen as a lateral ‘‘spread.’’
path due to the irregular pore sizes, which causes commen-
surate flow velocities that vary randomly about the mean 4.2.2.5 Molecular Transport: Diffusion
advective velocity. This is porous media dispersion. In any gas or liquid, the molecules comprising the medium,
To illustrate the idea of dispersion in porous media, consider and any contaminants within, are buffeted about randomly by
the depiction of porous media in Figure 4.1 with irregular grain successive incessant collisions with other molecules, called
sizes, random packing of media, and random pore sizes. As ‘‘Brownian motion.’’ This random motion causes a net flux of
with turbulent diffusion and molecular diffusion, the velocities molecules from a higher to a lower concentration, and is
are random and are illustrated by the varying random arrow diffusion.