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AERATION AND AIR STRIPPING 5.9

rate of approach to it are not independent of each other. Under similar conditions, the fur-
ther the air-water system is from equilibrium, the more rapid the interchange of gas in the
direction of attaining equilibrium.

Significance of Films
Films at the air-water interface appear to have an important bearing on the rate of gas
transfer. Both liquid and gas films can retard the rate of exchange of volatile material, but
the liquid film is a more important factor in the transfer of gases of low solubility, such
as oxygen and carbon dioxide.
Film resistance is influenced by many factors, but the most important are turbulence
and temperature. High temperature and turbulence promote gas transfer by reducing film
thickness. Increased temperature also increases the rate of molecular diffusion.

Rate of Transfer

The rate of transfer of a volatile substance from water to air is generally proportional to
the difference between the existing concentration and the equilibrium concentration of the
substance in solution. The relationship is expressed as follows:
M = KLa (Ci* - Di)
where M = mass of substance transferred per unit of time per unit of volume (lb/h)/ft 3
[(kg/h)/m 31
KL = overall liquid mass-transfer coefficient, ft/h (m/h)
a = effective area for mass transfer, ft2/ft 3 (m2/m 3)
C/* = liquid-phase concentration in equilibrium with gas-phase concentration,
lb/ft 3 (kg/m 3)
D i = bulkhead liquid-phase concentration, lb/ft 3 (kg/m 3)
The driving force for mass transfer is the difference between actual conditions in the
air stripping unit and conditions associated with equilibrium between the gas and liquid
phases. Equilibrium concentration of a solute in air is directly proportional to the con-
centration of the solute in water at a given temperature.
Henry's law states that the amount of gas that dissolves in a given quantity of liquid,
at constant temperature and total pressure, is directly proportional to the partial pressure
of the gas above the solution. Thus Henry's law constant can be considered a partition
coefficient describing the relative tendency for a compound to separate, or partition, be-
tween the gas and the liquid of equilibrium; it indicates a contaminant's volatility and its
affinity for the aeration process. Substances with high Henry's law constants are easily
removed by air stripping, and those with low constants are difficult to remove. Table 5.1
lists the Henry's law constants for several compounds. Vinyl chloride has an extremely
high constant relative to the other VOCs.
The mass-transfer coefficient KL is a function of the compound being stripped from
water, the geometry and physical characteristics of the air stripping system, and the tem-
perature and flow rate (contact time) for the liquid. It also incorporates the diffusion re-
sistance to mass transfer in both liquid and gas phases and is related to local gas- and
liquid-phase mass-transfer coefficients kg and kz, respectively.
For most stripping applications in water treatment, the bulk of resistance to mass trans-
fer resides in the liquid phase. As a result, air stripping process design should be based
on maximizing the liquid mass-transfer coefficient.