Page 616 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

18 Gas Transfer

The term ‘‘gas transfer,’’ as used here, is a special case of the gas may be transported to the vicinity of the gas–water
‘‘mass-transfer’’ limited to gases. A further limitation, for the interface by advection and turbulence with diffusion being the
purposes of this chapter, is that the focus is on the transfer of final transport step to the interface. The water surface becomes
gases to and from water. The gases may be any, for example, ‘‘saturated’’ with respect to the gas concentration at the inter-
oxygen, carbon dioxide, chlorine, chlorine dioxide, ozone, face; the gas may be transported from the interface, that is, in
etc., and various volatile organic compounds (VOCs). the aqueous phase, by molecular diffusion, turbulence, or
advection (or some combination). The process may be con-
18.1 DESCRIPTION trolled by the rate of interfacial area created, which, in turn, is
controlled by fluid transport processes, for example, the rate
The principles of mass transfer are founded on the three of pumping in the case of a turbine aerator. The same prin-
themes common to most reactor problems: (1) a materials ciples apply to natural processes, for example, reaeration in
balance, (2) equilibrium, and (3) kinetics. The approach of streams with oxygen, deoxygenation of streams of supersat-
this chapter is to apply such fundamentals as the basis for urated oxygen concentration, carbon dioxide transport, etc. In
operational equations, which differ by the characteristics of each case, the system is striving for equilibrium, which is
the particular application, for example, surface aerators, dif- never attained in any ‘‘open’’ system.
fused aeration, packed towers, stream reaeration, etc. The
differences are due essentially to the means in which inter-
facial surface area is created, for example, surface renewal or 18.1.2 APPLICATIONS
bubbles, and its rate of creation, for example, rate of pumping
Gas transfer to the aqueous phase occurs during aeration of
or airflow, and to the turbulence scale and intensity.
water in activated sludge reactors, in trickling filters, in rotat-
ing biological contactors, in chlorination, in ozone uptake, etc.
18.1.1 GAS TRANSFER IN-A-NUTSHELL Gas transfer from the aqueous phase to the gas phase occurs in
aeration to remove odors, such as hydrogen sulfide or radon
Gas transfer involves (1) getting gases into solution, for
(prevalent in some groundwaters); and in air stripping to
example, oxygen, carbon dioxide, ozone, chlorine, chlorine
remove VOCs, ammonia, etc. In nature, gas transfer occurs
dioxide, etc., and (2) getting gases out of solution, for example,
in stream reaeration; photosynthesis, for example, due to
ammonia, nitrogen, and a variety of VOCs. For each purpose,
supersaturation with oxygen; anaerobic reactions in benthic
specialized industrial equipment has been developed, for
muds with methane and carbon dioxide as products. Gas
example, surface aerators, diffusers for bubble aeration, pack-
precipitation occurs in the latter situations. Table 18.1 gives
ing for packed-towers, chlorinators, etc.
some examples of each category of gas transfer and indicates
18.1.1.1 Comparison with Other Mass-Transfer the technology involved; the ‘‘notes’’ describe the context for
Processes each application.
Every natural and engineered reactor embodies mass-transfer
processes, for example, advection, dispersion, turbulence, and
18.1.3 HISTORY
molecular diffusion. In flotation, bubbles must be brought into
contact with particles (by differential settling and=or turbu- The use of dissolved gases in water treatment goes back to
lence); in filtration, particles must contact granular media (by 1902 when ozone disinfection was applied at Paderborn,
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advection, sedimentation, diffusion); in adsorption columns Germany (Q ¼ 1.00 m =min) and then in 1906 at the Bon
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molecules must reach the particle–water interface, and then Voyage WTP at Nice, France (Q ¼ 13 m =min) (Hill and
diffuse to the interior; in activated sludge, oxygen molecules, Rice, 1982, p. 11). The latter plant was considered the ‘‘birth-
bacteria aggregates, and organic substrates are brought into place’’ use of ozone in drinking water treatment. Chlorine was
contact by turbulence and then by diffusion to the point of adopted for municipal drinking water starting in 1909 in
reaction; etc. While the purposes are different in each case, the Poughkeepsie, New York, in the form of hypochlorites (chlor-
mass-transfer processes involve common principles. ide of lime) and as chlorine gas in 1913 in Philadelphia
(Baker, 1948, p. 340, 342, respectively). Removal of dis-
18.1.1.2 Process Description solved gases in drinking water treatment goes back to the
As stated, gas transfer involves transport either (1) from the 1920s for removal of taste and odors (Hale, 1932). Methods
gas phase to the dissolved aqueous phase or (2) from the for the latter included the use of spray nozzles, perforated
dissolved aqueous phase to the gas phase. In the first case, trays, diffused air, or the construction of cascades.

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