Civil Engineering Reference
In-Depth Information
Kinetics
Over the past century, several models have been proposed to explain gas transfer
theory. Of these models, the two-film model proposed by Lewis and Whitman in 1924
is the simplest and is referred to the most frequently.
4
Two other models, the penetra-
tion model by Higbie and the surface renewal model by Danckwertz, have also been
used to explain gas transfer theory.
5,6
The rate of mass transfer of a volatile substance from water to air is generally
proportional to the difference between the concentration of the contaminant in solution
at the system temperature, as defined by Henry's law. The relationship is expressed as
follows:
M
Ka
(
C
*
C
)
L
where
M
rate of mass transfer, lb / hr / ft
3
(kg/hr/m
3
)
K
L
a
overall mass transfer coefficient, hr
1
C
*
equilibrium concentration of the gas in the liquid, lb / ft
3
(kg/m
3
)
C
bulk liquid-phase concentration, lb / ft
3
(kg/m
3
)
The driving force for the mass transfer is the difference between the equilibrium and
bulk liquid-phase concentrations for the gas. As mentioned earlier, the equilibrium
conditions are defined by Henry's law for most drinking water applications. The overall
mass transfer coefficient,
K
L
a
, is a function of the gas, the process used for gas transfer,
and physical parameters, such as temperature and dissolved solids. For most drinking
water applications,
K
L
a
is controlled by the liquid-phase resistance. Therefore, gas
transfer processes should be designed to maximize the liquid film mass transfer rate.
UNIT PROCESSES
The design and operation of aeration processes are well established. Aeration—or air
stripping, as it is sometimes called—can provide treatment at low costs and with few
operation and maintenance (O&M) requirements; hence, it is particularly attractive to
small communities. The most common application of aeration in water treatment is
the removal of gases—including hydrogen sulfide, methane, volatile organic com-
pounds, carbon dioxide, and radon—from groundwater. It is also used for the oxidation
of iron and manganese, enabling their subsequent removal by filtration. Aeration pro-
vides taste-and-odor removal and is gaining recognition as an effective corrosion con-
trol strategy.
In all aeration or air-stripping processes, an air-water interface is created across
which mass transfer of a compound can occur. There are three general categories of
aeration:
Diffused aeration, which involves the injection of air into water
•
Spray aeration, which involves the injection of water into air
•
