Air Stripping
Description
Air stripping is a technology in which volatile
organic
compounds (VOCs)
are transferred from extracted water to air. Typically, air stripping
takes
place in a packed tower (known as an air stripper) or an aeration
tank.
The "air stripper" includes a spray nozzle at the top of the tower.
It sprays groundwater that has been pumped to the surface over the
packing in
the column. As the water descends, air is forced up through the column,
stripping off the volatile compounds. Packing or baffles within the
tower
increase the surface area of the contaminated water that is exposed to
air,
thus maximizing the amount of volatilization.
A sump
at the
bottom of the tower collects decontaminated water. Auxiliary equipment
may
include an air heater to improve removal efficiency and air emission
"scrubbers."
Traditional air strippers vary in height, and the height is
correlated to
the chemical concentration of the contaminated water. A recent
innovation in
air strippers is the low-profile air stripper. These units have a
number of
trays that are set almost horizontally. Water is cascaded over the
trays to
maximize air-water contact while minimizing vertical space. Because
they are
not so visible, they are increasingly being used for groundwater
treatment.
Limitations and Concerns
Air strippers transfer contaminants from one medium
to
another. There is no destruction of the contaminant. Consequently, the
risks of
emitting pollutants into the air must be carefully evaluated. Often,
the air
stream (or off-gas)
is treated before it is emitted to the atmosphere.
Algae, fungi, bacteria, and fine particles may foul the equipment,
requiring
pretreatment or periodic column cleaning.
Air stripping is effective only for water contaminated with VOC or
semi-volatile concentrations with a Henry's Law constant greater than
0.01. (Henry's
Law
is a measure of the extent to which a chemical separates between water
and air.
The higher the Henry's Law constant, the more likely substances will
volatize
rather than remaining in water.) Compounds with low volatility at
ambient
temperature may require preheating of the groundwater.
The visual impacts associated with air strippers should be
considered, as
they are often viewed as a nuisance when located in residential areas.
Applicability
Air stripping is used to separate VOCs from contaminated groundwater.
Technology Development Status
Air strippers are commercial and widely used.
Web Links
http://clu-in.org/download/citizens/airstripping.pdf
http://www.frtr.gov/matrix2/section4/4-46.html
Other Resources and Demonstrations
See the descriptions of Granular Activated Charcoal.