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5.8 CHAPTER FIVE

PRINCIPLES OF AIR STRIPPING AND AERATION

Transfer by aeration of a volatile material to or from water depends on a number of fac-
tors, including
• Characteristics of the volatile material
• Water and surrounding air temperatures
• Gas-transfer resistance
• Partial pressure of the gases in the aerator atmosphere
• Turbulence in gaseous and liquid phases
• Area-to-volume ratio
• Time of exposure

Equilibrium Conditions
The term equilibrium applied to gases dissolved in water signifies a steady-state concen-
tration of dissolved substances. Aeration promotes the establishment of equilibrium be-
tween dissolved, volatile constituents in the water and the constituents in the air to which
the water is exposed. For example, when water is exposed to air, oxygen and nitrogen
dissolve in the water until a state of equilibrium is reached. The function of aeration is to
speed up this natural process. True equilibrium may not be attained by aeration unless the
air-water exposure period is relatively long. From a practical standpoint, however, it is
generally not necessary to achieve absolute equilibrium.

Saturation Value
The concentration of a gas dissolved in a liquid at equilibrium is known as its saturation
value. This value is an important characteristic of a dissolved gas. Saturation value is prin-
cipally dependent on water temperature, partial pressure of the gas in the atmosphere in
contact with the water, and presence of dissolved solids. The higher the partial pressure,
the greater the dissolved gas concentration. This relationship is known as Henry's law.
At a fixed partial pressure, the higher the temperature, the lower the solubility or satura-
tion value of a gas. Gas solubility is also reduced by dissolved solids.
Saturation value has considerable practical and theoretical significance. It is the dif-
ference between the saturation value of a gas and its actual concentration in the water that
provides the driving force for the interchange of gas between air and water. Water defi-
cient in oxygen will absorb it when brought into contact with air, and the air-water equi-
librium will be reached from the direction of oxygen deficiency. Prolonged aeration
produces oxygen saturation.
On the other hand, if water contains more oxygen or, as is more commonly encoun-
tered, more carbon dioxide than the saturation amount, air stripping brings about release
of the gas. In this instance, equilibrium is approached from the direction of supersatura-
tion. The final result of prolonged air stripping, however, is the same--saturation.

Rate of Achievement
Equilibrium conditions are important in the aeration process, but of even greater signifi-
cance to the design engineer is the rate of achievement of equilibrium. Equilibrium and