v°/EPA
United States
Environmental Protection
Agency
Office of Emergency and
Remedial Response
Washington, DC 20460
Office of
Research and Development
Cincinnati, OH 45268
Superfurid
EPA/540/2-91/022
October 1991
Engineering Bulletin
Air Stripping of Aqueous
Solutions
Purpose
Section 121(b) of the Comprehensive Environmental Re-
sponse, Compensation, and Liability Act (CERCLA) mandates
the Environmental Protection Agency (EPA) to select remedies
that "utilize permanent solutions and alternative treatment
technologies or resource recovery technologies to the maxi-
mum extent practicable" and to prefer remedial actions in
which treatment "permanently and significantly reduces the
volume, toxicity, or mobility of hazardous substances, pollut-
ants, and contaminants as a principal element." The Engineer-
ing Bulletins are a series of documents that summarize the latest
information available on selected treatment and site remedia-
tion technologies and related issues. They provide summaries
of and references for the latest: information to help remedial
project managers, on-scene coordinators, contractors, and other
site cleanup managers understand the type of data and site
characteristics needed to evaluate a technology for potential
applicability to their Superfund or other hazardous waste site.
Those documents that describe individual treatment technolo-
gies focus on remedial investigation scoping needs. Addenda
will be issued periodically to update the original bulletins
Abstract
Air stripping is a means to transfer contaminants from
aqueous solutions to air. Contaminants are not destroyed by
air stripping but are physically separated from the aqueous
solutions. Contaminant vapors are transferred into the air
stream and, if necessary, can be treated by incineration, ad-
sorption, or oxidation. Most frequently, contaminants are
collected in carbon adsorption systems and then treated or
destroyed in this concentrated form. The concentrated con-
taminants may be recovered, incinerated for waste heat recov-
ery, or destroyed by other treatment technologies. Generally,
air stripping is used as one in a series of unit operations arid can
reduce the overall cost for managing a particular site. Air
stripping is applicable to volatile and semivolatile organic com-
pounds. It is not applicable for treating metals and inorganic
compounds.
During 1988, air stripping was one of the selected rem-
edies at 30 Superfund sites [1]*. In 1989, it was a component
of the selected remedy at 38 Superfund sites [2]. An estimated
1,000 air-stripping units are presently in operation at sites
throughout the United States [3]. Packed-tower systems typi-
cally provide the best removal efficiencies, but other equipment
configurations exist, including diffused-air basins, surface aera-
tors, and cross-flow towers [4, p. 2] [5, p. 10-48]. In packed-
tower systems, there is no clear technology leader by virtue of
the type of equipment used or mode of operation. The final
determination of the lowest cost alternative will be more site
specific than process equipment dominated.
This bulletin provides information on the technology ap-
plicability, the technology limitations, a description of the
technology, the types of residuals produced, site requirements,
the latest performance data, the status of the technology, and
sources of further information.
Technology Applicability
Air stripping has been demonstrated in treating water
contaminated with volatile organic compounds (VOCs) and
semivolatile compounds. Removal efficiencies of greater than
98 percent for VOCs and greater than or equal to 80 percent
for semivolatile compounds have been achieved. The technol-
ogy is not effective in treating low-volatility compounds, metals,
or inorganics [6, p. 5-3]. Air stripping has commonly been used
with pump-and-treat methods for treating contaminated
groundwater.
This technology has been used primarily for the treatment of
VOCs in dilute aqueous waste streams. Effluent liquid quality is
highly dependent on the influent contaminant concentration.
Air stripping at specific design and operating conditions will yield
a fixed, compound-specific percentage removal. Therefore, high
influent contaminant concentrations may result in effluent con-
centrations above discharge standards. Enhancements, such as
high temperature or rotary air stripping, will allow less-volatile
organics, such as ketones, to be treated [6, p. 5-3].
Table 1 shows the effectiveness of air stripping on gen-
eral contaminant groups present in aqueous solution. Ex-
amples of constituents within contaminant groups are pro-
vided in Reference 7, "Technology Screening Guide for
Treatment of CERCLA Soils and Sludges." This table is based
on the current available information or professional judgment
* [reference number, page number]
-------
Table 1
Effectiveness of Air Stripping on General Contaminant
Groups from Water
Contaminant Croups
o
O
o
o
1
o
o
QC
Halogenated volatiles
Halogenated semivolatiles *
Nonhalogenated volatiles
Nonhalogenated semivolatiles
PCBs
Pesticides
Dioxins/Furans
Organic cyanides
Organic corrosives
Volatile metals
Nonvolatile metals
Asbestos
Radioactive materials
Inorganic corrosives
Inorganic cyanides
Oxidizers
Reducers
Effectiveness
m
V
•
j
'3
a
u
LJ
J
J
J
J
J
J
J
J
Demonstrated Effectiveness: Successful treatability test at some scale
completed
Potential Effectiveness: Expert opinion that technology will work
No Expected Effectiveness: Expert opinion that technology will not
work
Only some compounds in this category are candidates for air strip-
ping.
where no information was available. The proven effectiveness
of the technology for a particular site or contaminant does
not ensure that it will be effective at all sites or that the
treatment efficiencies achieved will be acceptable at other
sites. For the ratings used for this table, demonstrated effec-
tiveness means that, at some scale, treatability testing dem-
onstrated the technology was effective for that particular
contaminant group. The ratings of potential effectiveness
and no expected effectiveness are both based upon expert
judgment. Where potential effectiveness is indicated, the
technology is believed capable of successfully treating the
contaminant group in a particular matrix. When the tech-
nology is not applicable or will probably not work for a
particular contaminant group, a no-expected-effectiveness
rating is given.
Limitations
Because air stripping of aqueous solutions is a means of
mass transfer of contaminants from the liquid to the air stream,
air pollution control devices are typically required to capture or
destroy contaminants in the offgas [8]. Even when offgas treat-
ment is required, air stripping usually provides significant id-
vantages over alternatives such as direct carbon adsorption
from water because the contaminants are more favorably sorbed
onto activated carbon from air than from water. Moreover,
contaminant destruction via catalytic oxidation or incineration
may be feasible when applied to the offgas air stream.
Aqueous solutions with high turbidity or elevated levels
of iron, manganese, or carbonate may reduce removal effi-
ciencies due to scaling and the resultant channeling effects.
Influent aqueous media with pHs greater than 11 or less than
5 may corrode system components and auxiliary equipment.
The air stripper may also be subject to biological fouling. The
aqueous solution being air stripped may need pretreatment to
neutralize the liquid, control biological fouling, or prevent
scaling [6][9].
Contaminated water with VOC or semivolatile concentra-
tions greater than 0.01 percent generally cannot be treated by
air stripping. Even at lower influent concentrations, air strip-
ping may not be able to achieve cleanup levels required at
certain sites. For example, a 99 percent removal of
trichloroethene (TCE) from groundwater containing 100 parts
per million (ppm) would result in an effluent concentration of
1 ppm, well above drinking water standards. Without heating,
only volatile organic contaminants with a dimensionless Henry's
Law constant greater than 102 are amenable to continuous-
flow air stripping in aqueous solutions [6][5]. In certain cases,
where a high removal efficiency is not required, compounds
with lower Henry's Law constants may be air stripped. Ashworth
et al. published the Henry's Law constants for 45 chemicals
[10, p. 25]. Nirmalakhandan and Speece published a method
for predicting Henry's Law constants when published constants
are unavailable [11]. Air strippers operated in a batch mode
may be effective for treating water containing either high
contaminant concentrations or contaminants with lower Henry's
Law constants. However, batch systems are normally limited
to relatively low average flow rates.
Several environmental impacts are associated with air strip-
ping. Air emissions of volatile organics are produced and must
be treated. The treated wastewater may need additional treat-
ment to remove metals and nonvolatiles. Deposits, such as
metal (e.g., iron) precipitates may occur, necessitating periodic
cleaning of air-stripping towers [6, p. 5-5]. In cases where
heavy metals are present and additional treatment will be re-
quired, it may be beneficial to precipitate those metals prior to
air stripping.
Technology Description
Air stripping is a mass transfer process used to treat ground-
water or surface water contaminated with volatile or semivola-
tile organic contaminants. At a given site, the system is de-
signed based on the type of contaminant present, the
contaminant concentration, the required effluent concentra-
tion, water temperature, and water flow rate. The major design
variables are gas pressure drop, air-to-water ratio, and type of
packing. Given those design variables, the gas and liquid
loading (i.e., flows per cross-sectional area), tower diameter
and packing height can be determined. Flexibility in the system
design should allow for changes in contaminant concentration,
air and water flow rates, and water temperature. Figure 1 is a
schematic of a typical process for the air stripping of contami-
nated water.
Engineering Bulletin: Air Stripping of Aqueous Solutions
-------
Figure 1
Schematic Diagram of Air-Stripping System [8, p. 20][13, p. 43]
OFFGAS TREATMENT
(5)
Stripper
Offgas
Stack
Mist Eliminator
Contaminated
Groundwater
or —
Surface Water
PRE-
TREATMENT
STORAGE
TANKS
(1)
Feed
Pump
Gas
Liquid
Packed Bed
Recycle (optional)
In an air-stripping process, the contaminated liquid is
pumped from a groundwater or surface water source. Water to
be processed is directed to a storage tank (1) along with any
recycle from the air-stripping unit.
Air stripping is typically performed at ambient temperature.
In some cases, the feed stream temperature is increased in a heat
exchanger (2). Heating the influent liquid increases air-stripping
efficiency and has been used to obtain a greater removal of semi-
volatile organics such as ketones. At temperatures close to 100°C,
steam stripping may be a more practical treatment technique [8,
p. 3].
The feed stream (combination of the influent and recycle)
is pumped to the air stripper (3). Three basic designs are used
for air strippers: surface aeration, diflused-air systems, and
specially designed liquid-gas contactors [4, p. 3]. The first two
of these have limited application to the treatment of contami-
nated water due to their lower contaminant removal efficiency.
In addition, air emissions from surface-aeration and diffused-air
systems are frequently more difficult to capture and control.
These two types of air strippers will not be discussed further.
The air stripper in Figure 1 is an example of a liquid-gas
contactor.
The most efficient type of liquid-gas contactor is the packed
tower [4, p. 3]. Within the packed tower, structures called
packing provide surface area on which the contaminated water
can form a thin film and come in contact with a countercurrent
flow of air. Air-to-water ratios may range from 10:1 to iOO:1 on
a volumetric basis [14, p. 8], Selecting packing material that
will maximize the wetted surface area will enhance air strip-
ping. Packed towers are usually cylindrical and are filled with
either random or structured packing. Random packing consists
of pieces of packing dumped onto a support structuie within
the tower. Metal, plastic, or ceramic pieces come in standard
sizes and a variety of shapes. Smaller packing sizes generally
increase the interfacial area for stripping and improve the mass-
Treated Liquid
transfer kinetics. However, smaller packing sizes result in an
increased pressure drop of the air stream and an increased
potential for precipitate fouling. Tripacks", saddles, and slotted
rings are the shapes most commonly used for commercial
applications. Structured packing consists of trays fitted to the
inner diameter of the tower and placed at designated points
along the height of the tower. These trays are made of metal
gauze, sheet metal, or plastic. The choice of which type of
packing to use depends on budget and design constraints. Ran-
dom packing is generally less expensive. However, structured
packing reportedly provides advantages such as lower pressure-
drop and better liquid distribution characteristics [4, p. 5].
The processed liquid from the air-stripper tower may con-
tain trace amounts of contaminants. If required, this effluent is
treated (4) with carbon adsorption or other appropriate
treatments.
The offgas can be treated (5) using carbon adsorption,
thermal incineration, or catalytic oxidation. Carbon adsorption
is used more frequently than the other control technologies
because of its ability to remove hydrocarbons cost-effectively
from dilute (< 1 percent) air streams [8, p. 5].
Process Residuals
The primary process residual streams created with air-
stripping systems are the offgas and liquid effluent. The offgas
is released to the atmosphere after treatment; activated carbon
is the treatment most frequently applied to the offgas stream.
Where activated carbon is used, it is recommended that the
relative humidity of the air stream be reduced. Once spent, the
carbon can be regenerated onsite or shipped to the original
supplier for reactivation. If spent carbon is replaced, it may
have to be handled as a hazardous waste. Catalytic oxidation
and thermal incineration also may be used for offgas treatment
[15, p. 10] [8, p. 5]. Sludges, such as iron precipitates, build up
Engineering Bulletin: Air Stripping of Aqueous Solutions
-------
within the tower and must be removed periodically [6, p. 5-5].
Spent carbon can also result if carbon filters are used to treat
effluent water from the air-stripper system. Effluent water
containing nonvolatile contaminants may need additional treat-
ment. Such liquids are treated onsite or stored and removed to
an appropriate facility. Biological, chemical, activated carbon,
or other appropriate treatment technologies may be used to
treat the effluent liquid. Once satisfactorily treated, the water is
sent to a sewage treatment facility, discharged to surface water,
or returned to the source, such as an underground aquifer
Site Requirements
Air strippers are most frequently permanent installations,
although mobile systems may be available for limited ;jse.
Permanent installations may be fabricated onsite or ma> be
shipped in modular form and constructed onsite. Packing is
installed after fabrication or construction of the tower. A concrete
pad will be required to support the air-stripper tower in either
case. Access roads or compacted soil will be needed to transport
the necessary materials.
Standard 440V, three-phase electrical service is needed.
Water should be available at the site to periodically clean scale
or deposits from packing materials. The quantity of water
needed is site specific. Typically, treated effluent can be used to
wash scale from packing.
Contaminated liquids are hazardous, and their handling
requires that a site safety plan be developed to provide for
personnel protection and special handling measures. Spent
activated carbon may be hazardous and require similar han-
dling. Storage may be needed to hold the treated liquid until it
has been tested to determine its acceptability for disposal or
release. Depending upon the site, a method to store liquid lhat
has been pretreated may be necessary. Storage capacity will
depend on liquid volume.
Onsite analytical equipment for conducting various analy-
ses, including gas chromatography capable of determining
site-specific organic compounds for performance assessment,
make the operation more efficient and provide better informa-
tion for process control.
Performance Data
System performance is measured by comparing contami-
nant concentrations in the untreated liquid with those in the
treated liquid. Performance data on air-stripping systems, rang-
ing from pilot-scale to full-scale operation, have been reported
by several sources, including equipment vendors. Data ob-
tained on air strippers at Superfund sites also are discussed
below. The data are presented as originally reported in the
referenced documents. The quality of this information has not
been determined. The key operating and design variables are
provided when they were available in the reference.
An air-stripping system, which employed liquid-phase GAC
to polish the effluent, was installed at the Sydney Mine site in
Valrico, Florida. The air-stripping tower was 4 feet in diameter,
Table 2
Performance Data for the Groundwater Treatment
System at the Sydney Mine Site, FL. [13, p. 42]
Contaminant
Volatile organic*
Benzene
Chlorobenzene
1,1-dichloroethane
Trans-1,2-dichloropropane
Ethylbenzene
Methylene chloride
Toluene
Trichlorofluoromethane
Meta-xylene
Ortho-xylene
Extractable organics
3-(l,1-dimethylethyl) phenol
Pesticides
2,4-D
2,4,5-TP
Inorganics
Iron (mg/L)
Concentration
Influent Effluent
11
1
39
1
5
503
10
71
3
2
32
4
1
11
NDa
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
<0.03
aND = Not detected at method detection limit of 1 (ig/L for volatile
organics and 10 ng/L for extractable organics and pesticides
42 feet tall, and contained a 24-foot bed of 3.5-inch diameter
polyethylene packing. The average design water flow was 150
gallons per minute (gpm) with a hydraulic loading rate of 12
gpm/ft2 and a volumetric air-to-water ratio of approximately
200:1. The air-stripping tower was oversized for use at future
treatment sites. Effluent water from the air stripper was pol-
ished in a carbon adsorption unit. Table 2 summarizes the
performance data for the complete system; it is unclear how
much removal was accomplished by the air stripper and how
much by the activated carbon. Influent concentrations of
total organics varied from approximately 25 parts per billion
(ppb) to 700 ppb [1 3, p. 41].
Air stripping was used at well 12A in the city of Tacoma,
Washington. Well 12A had a capacity of 3,500 gpm and was
contaminated with chlorinated hydrocarbons, including 1,1,2,2-
tetrachloroethane; trans-1,2-dichloroethene (DCE); TCE; and
perchloroethylene. The total VOC concentration was approxi-
mately 100 ppb. Five towers were installed and began operation
on July 15, 1983. Each tower was 12 feet in diameter and was
packed with 1-inch polypropylene saddles to a depth of 20
feet. The water flow rate was 700 gpm for each tower, and the
volumetric air-to-water ratio was 310:1. The towers consis-
tently removed 94 to 98 percent of the influent 1,1,2,2-
tetrachloroethane with an overall average of 95.5 percent re-
moval. For the other contaminants, removal efficiencies in excess
of 98 percent were achieved [16, p. 112].
Another remedial action site was Wurtsmith Air Force Base
in Oscoda, Michigan. The contamination at this site was the
result of a leaking underground storage tank near a mainte-
Engineering Bulletin: Air Stripping of Aqueous Solutions
-------
Table 3
Air-Stripper Performance Summary
At Wurtsmith AFB
[17, p. 121]
C/L
(vol)
10
10
10
18
18
18
25
25'
25
Water Flow
(L/min)
Single Tower
(% Removed)
1,135
1,700
2,270
1,135
1,700
2,270
1,135
1,700
2,270
95
94
86
98
97
90
98
98
98
Series Operation
(% Removed)
99.8
99.8
96.0
99.9
99.9
99.7
99.9
99.9
99.9
Influent TCE concentration: 50-8,000 ug/L Water temperature 283°K
nance facility. Two packed-tower air strippers were installed to
remove TCE. Each tower was 5 feet in diameter and 30 feet tall,
with 18 feet of 16mm pall ring packing. The performance
summary for the towers, presented in Table 3, is based on
evaluations conducted in May and August 1982 and January
1983. Excessive biological growth decreased performance and
required repeated removal and cleaning of the packing. Op-
eration of the towers in series, with a volumetric air-to-water
ratio of 25:1 and a water flow of 600 gpm (2,270 L/min),
removed 99.9 percent of the contaminant [1 7, p. 119]
A 2,500 gpm air stripper was used to treat contaminated
groundwater during the initial remedial action at the Verona
Well field site in Battle Creek, Michigan. This well field is the
major source of public potable water for the city of Battle Creek.
The air stripper was a 10-foot diameter tower packed to a
height of 40 feet with 3.5 inch pall rings. The air stripper was
operated at 2,000 gpm with a 20:1 volumetric air-to-water
ratio. Initial problems with iron oxide precipitating on the
packed rings were solved by recirculating sodium hypochlorite
through the stripper about four times per year [8, p. 8-9]. The
total VOC concentration of 131 ppb was reduced by approxi-
mately 82.9 percent [15, p. 56]. The air stripper offgas was
treated via vapor phase granular activated carbon beds. The
offgas was heated prior to entering the carbon beds to reduce
its humidity to 40 percent.
An air stripper is currently operating at the Hyde Park
Superfund site in New York. Treatek, Inc., which operates the
unit, reports the system is treating about 80,000 gallons per
day (gpd) of landfill leachate. The contaminants are in the
range of 4,000 ppm total organic carbon (TOC). The air
stripper is reportedly able to remove about 90 percent of the
TOCs [18]. A report describing the performance of the air
stripper is expected to be published during 1991.
The primary VOCs at the Des Moines Superfund site were
TCE; 1,2-DCE; and vinyl chloride. The TCE initial concentration
was approximately 2,800 ppb and gradually declined to the
800 to 1,000 ppb range after 5 months. Initial groundwater
concentrations of 1,2-DCE were unreported while the concen-
tration of vinyl chloride ranged from 38 ppb down to 1 ppb.
The water flow rate to the air stripper ranged from 500 to 1,850
gpm and averaged approximately 1,300 gpm. No other design
data were provided. TCE removal efficiencies were generally
above 96 percent, while the removal efficiencies for 1,2-DCE
were in the 85 to 96 percent range. No detectable levels of vinyl
chloride were observed in the effluent water [12, p. B-1 ].
VOCs were detected in the Eau Claire municipal well field in
Eau Claire, Wisconsin, as part of an EPA groundwater supply
survey in 1981. An air stripper was placed on-line in 1987 to
protect public health and welfare until completion of the reme-
dial investigation/feasibility study (RI/FS) and final remedy selec-
tion. Data reported on the Eau Claire site were for the period
beginning August 31, 1987 and ending February 15,1989. Dur-
ing this period, the average removal efficiency was greater than
Table 4
Air-Stripper Performance at
Eau Claire Municipal Well Field [12, p. C-1]
Contaminant
1,1-Dichloroethene
1,1-Dichloroethane
1,1,1 -Trichloroethane
Trichloroethene
88 percent for the four chlorinated organic compounds studied.
The average removal efficiencies are shown in Table 4. The air
stripper had a 12-foot diameter and was 60 feet tall, with a
packed bed of 26 feet. Water feed rates were approximately 5 to
6 million gallons per day (mgd). No other design parameters
were reported [12, p. C-1 ].
In March 1990, an EPA study reviewed the performance
data from a number of Superfund sites, including the Brewster
Well Field, Hicksville MEK Spill, Rockaway Township, Western
Processing, and Gilson Road Sites [15].
Reported removal efficiencies at the Brewster Well Field site
in New York were 98.50 percent, 93.33 percent, and 95.59
percent for tetrachloroethene (PCE); TCE; and 1,2-DCE; respec-
tively. Initial concentrations of the three contaminants were
200 ppb (PCE), 30 ppb (TCE) and 38 ppb (1,2-DCE) [15, p. 55].
The 300 gpm air stripper had a tower diameter of 4.75 feet,
packing height of 17.75 feet, air-to-water ratio of 50:1, and
used 1-inch saddles for packing material [15, p. 24].
A removal efficiency of 98.41 percent was reported for methyl
ethyl ketone (MEK) at the Hicksville MEK spill site in New York.
The reported influent MEK concentration was 15 ppm. The air
stripper had a 100 gpm flowrate, an air-to-water ratio of 120:1, a
tower diameter of 3.6 feet, a packing height of 15 feet, and used
2-inch jaeger Tripack packing material. Water entering the air
stripper was heated to approximately 180° to195°F by heat ex-
changers [15, p. 38].
Influent
Concentration
(Ppb)
0.17-2.78
0.38-1.81
? 4.32-14.99
2.53-11.18
Removal
Efficiency
(%)
88
93
99
98
Engineering Bulletin: Air Stripping of Aqueous Solutions
-------
Table 5
Air Stripper Performance at Rockaway
Township, NJ [15, p. 53]
Contaminant
Trichloroethylene
Methyl-tert-butyl ether
1,1 -Dichloroethylene
cis-1,2-Dichloroethylene
Chloroform
1,1,1 -Trichloroethane
1,1-Dichloroethane
Total VOC
Influent
Concentration
(ppb)
28.3
?r 3.2
4.0
me 6.4
1.3
20.0
2.0
65.2
Removal
Efficiency
99.99
99.99
99.99
99.99
99.99
99.99
99.99
99.99
Table 7
Air-Stripper Performance at the
Gilson Road Site, NH [15, p. 65]
Contaminant
Isopropyl alcohol
Acetone
Toluene
Dichloromethane
1,1,1-Trichloroethane
Trichloroethylene
Chloroform
Total VOC
Influent
Concentration
(ppb)
532
473
14,884
236
1,340
1,017
469
18,951
Average Removal
Efficiency
(%)
95.30
91.93
99.87
93.79
99.45
99.71
99.06
99.41
The Rockaway Township air stripper had a flowrate of
1,400 gpm, tower diameter of 9 feet, packing height of 25
feet, air-to-water ratio of 200:1, and used 3-inch Tellerettes
packing material. The performance data are shown in Table 5
[15, p. 18].
The Western Processing site had two air-stripping towers
treating different wells in parallel. The first tower had a 100
gpm (initial) and 200 gpm (maximum) flowrate, a tower diam-
eter of 40 feet, a packing height of 40.5 feet, an air-to-water
ratio of 160:1 (initial) and 100:1 (maximum), and used 2-inch
Jaeger Tripack packing material. The second tower had a 45
Table 6
gpm (initial) and 60 gpm (maximum) flowrate, a tower diam-
eter of 2 feet, packing height of 22.5 feet, air-to-water ratio of
83.1:1 (initial) and 62.3:1 (maximum), and used 2-inch Jaeger
Tripack packing material [15, p. 31 ]. The performance data are
presented in Table 6.
The Gilson Road Site used a single column high-tempera-
ture air stripper (HTAS) which had a 300 gpm flowrate (heated
influent), tower diameter of 4 feet, packing height of 16 feet, air-
to-water ratio of 51.4:1, and used 16 Koch-type trays at 1-foot
intervals [15, p. 42-45]. The performance data are provided in
Table 7. Due to the relatively high influent concentration and
the high (average) removal efficiency, this system required supple-
mental control of the volatiles in the offgas.
Air-Mnpper performance at
Western Processing, WA [15, p. 61]
Contaminant
... ._
Benzene
Carbon tetrachloride
Chloroform
1,2-Dichloroethane
1 ,1 -Dichloroethylene
1 ,1 ,1 -Trichloroethane
Trichloroethylene
Vinyl chloride
Dichloromethane
Tetrachloroethylene
Toluene
1 ,2-Dichlorobenzene
Hexachlorobutadiene
Hexachloroethane
Isobutanol
Methyl ethyl ketone
Influent
Concentration
(ppb)
73
5
781
22
89
1 ,440
8,220
159
8,170
378
551
11
250
250
10
1,480
Removal
Efficiency
(%)
- - .
93.15
99.36
77.27
94.38
99.65
99.94
99 37
.7 ^ . J /
99.63
98.68
99.09
54.55
96.00
96.00
0.00
70.27
Another EPA study, completed in August 1987, analyzed
performance data from 1 77 air-stripping systems in the United
States. The study presented data on systems design, contami-
nant types, and loading rates, and reported removal efficiencies
for 52 sites. Table 8 summarizes data from 46 of those sites,
illustrating experiences with a wide range of contaminants [1 9].
Reported efficiencies should be interpreted with caution. Low
efficiencies reported in some instances may not reflect the true
potential of air stripping, but may instead reflect designs in-
tended to achieve only modest removals from low-level con-
taminant sources. It is also important to recognize that, be-
cause different system designs were used for these sites, the
results are not directly comparable from site to site.
Technology Status
Air stripping is a well-developed technology with wide
application. During 1988, air stripping of aqueous solutions
was a part of the selected remedy at 30 Superfund sites [1 ]. In
1989, air stripping was a part of the selected remedy at 38
Superfund Sites [2].
The factors determining the cost of an air stripper can be
categorized as those affecting design, emission controls, and
operation and maintenance (O&M). Design considerations such
as the size and number of towers, the materials of construction,
and the desired capacity influence the capital costs. Equipment
cost components associated with a typical packed-tower air strip-
Engineering Bulletin: Air Stripping of Aqueous Solutions
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Table 8
Summary of Reported Air-Stripper Removal Efficiencies from 46 Sites [19]
Influent
No. of Concentration
Data Points (^g/L)
Contaminant Average
Aniline 1 226
Benzene 3 3,730
Bromodichloromethane 1 36
Bromoform 1 8
Chloroform 1 530
Chlorobenzene 0 95
Dibrgmochloromethane 1 34
Dichloroethylene 7 409
Diisopropyl ether 2 35
Ethylbenzene 1 6,370
Ethylene dichloride 7 1 73
Methylene chloride 1 1 5
Methyl ethyl ketone 1 1 00
2-Methylphenol 1 160
Methyl tertiary butylether 2 90
Perchloroethylene 1 7 355
Phenol 1 198
1,1,2,2-Tetrachloroethane 1 300
Trichloroethane 8 81
Trichloroethylene 34 7,660
1,2,3-Trichloropropane 1 29,000
Toluene 2 6,710
Xylene 4 14,823
Volatile organic compounds 3 44,000
Total Volatile Organics 46 11,120
Range
NAb
200-10,000
NA
NA
1500
NA
NA
2-3,000
20-50
1 00-1 ,400
5-1,000
9-20
NA
NA
50-130
3-4,700
NA
NA
5-300
1 -200,000
NA
30-23,000
1 7-53,000
57-130,000
Reported
Removal Efficiency0
(%)
Average
58
99.6
81
44
48
NDC
60
98.6
97.0
99.8
99.3
100
99
70
97.0
96.5
74
95
95.4
98.3
99
98
98.4
98.8
12-205,000 97.5
Range
NA
99-1 00
NA
NA
NA
ND
NA
96-1 00
95-99
NA
79-1 00
NA
NA
NA
95-99
86-100
NA
NA
70-100
76-100
NA
96-100
96-100
98-99.5
58.1-100
aNote that the averages and ranges presented n this column represent more data points than are presented in the second column of this table because the
removal efficiencies were not available for all ar strippers.
bNA = Not Applicable. Data available for only one stripper.
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According to Hydro Croup, Inc., the cost of air stripping
may range from $0.04 to $0.17 per 1,000 gallons [21, p. 7].
The Des Moines Superfund site unit cost for groundwater treat-
ment is estimated to be about $0.45/1,000 gallons based on a
1,250 gpm treatment rate and an average O&M cost of
$200,000/year for 10 years at 10 percent interest. The Eau
Claire site had a unit cost of roughly $0.14/1,000 gallons
assuming a 5-year operation period and an average treat-
ment rate of 7 million gpd [12, p. C-6].
Recent developments in this technology include high-
temperature air stripping (HTAS) and rotary air stripping. A
full-scale HTAS system was demonstrated at McClellan AFB to
treat groundwater contaminated with fuel and solvents from
spills and storage tank leaks. The combined recycle and makeup
was heated to 65°C, and a removal efficiency of greater than
99 percent was achieved [8, p. 9]. The rotary design, marketed
under the name HIGEE, was demonstrated at a U.S. Coast
Guard air station in East Bay Township, Michigan. At a gas-to-
liquid ratio of 30:1 and a rotor speed of 435 rpm, removal
efficiencies for all contaminants, except 1, 2-DCE, exceeded 99
percent. The removal efficiency for 1,2-DCE was not reported
[4, p. 19].
Raising influent liquid temperature increases mass-transfer
rates and the Henry's Law Constants. This results in improved
removal efficiencies for VOCs and the capability to remove
contaminants that are less volatile. Table 9 illustrates the
influence that changes in liquid temperature have on contami-
nant removal efficiencies. Note that steam stripping may be
the preferred treatment technology at a feed temperature
approaching 100°C, because the higher temperatures associ-
ated with steam stripping allow organics to be removed more
efficiently than in HTAS systems. However, steam stripping
uses more fuel and therefore will have higher operating costs.
Additionally, the capital costs for steam stripping may be higher
than for HTAS if higher-grade construction materials are needed
at the elevated temperatures used in steam stripping [8, p. 3j.
Table 9
Influence of Feed Temperature on Removal of Water
Soluble Compounds from Groundwater [8, p. 15]
Compound Percent Removed at Selected Temperature
2 - Propanol
Acetone
Tetrahydrofuran
72°C
10
35
50
35°C
23
80
92
73°C
70
95
>99
Rotary air strippers use centrifugal force rather than gravity
to drive aqueous solutions through the specially designed pack-
ing. This packing, consisting of thin sheets of metal wound
together tightly, was developed for rotary air strippers because of
the strain of high centrifugal forces. The use of centrifugal force
reportedly results in high removal efficiencies due to formation of
a very thin liquid film on wetted surfaces. The rotary motion also
causes a high degree of turbulence in the gas phase. The
turbulence results in improved liquid distribution over conven-
tional gravity-driven air strippers. The biggest advantage of
rotary strippers is the high capacity for a relatively small device.
Disadvantages include the potential for mechanical failures and
additional energy requirements for the drive motor. Water
carryover into the air effluent stream may cause problems with
certain emission control devices used to treat the contaminated
air. Cost and performance data on rotary air strippers are very
limited [4, p. 16].
EPA Contact
Technology-specific questions regarding air stripping of
liquids may be directed to:
Dr. James Heidman
U.S. Environmental Protection Agency
Risk Reduction Engineering Laboratory
26 West Martin Luther King Drive
Cincinnati, Ohio 45268
FTS 684-7632
(513) 569-7632
Acknowledgments
This bulletin was prepared for the U.S. Environmental Pro-
tection Agency, Office of Research and Development (ORD),
Risk Reduction Engineering Laboratory (RREL), Cincinnati, Ohio,
by Science Applications International Corporation (SAIC) under
contract No. 68-C8-0062. Mr. Eugene Harris served as the EPA
Technical Project Monitor. Mr. Gary Baker was SAIC's Work
Assignment Manager. This bulletin was authored by Mr. Jim
Rawe of SAIC. The Author is especially grateful to Mr. Ron
Turner, Mr. Ken Dostal and Dr. James Heidman of EPA, RREL,
who have contributed significantly by serving as technical con-
sultants during the development of this document.
The following other Agency and contractor personnel have
contributed their time and comments by participating in the
expert review meeting and/or peer reviewing the document:
Mr. Ben Blaney
Dr. John Crittenden
Mr. Clyde Dial
Dr. James Gossett
Mr. George Wahl
Ms. Tish Zimmerman
EPA-RREL
Michigan Technological University
SAIC
Cornell University
SAIC
EPA-OERR
8
Engineering Bulletin: Air Stripping of Aqueous Solutions
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REFERENCES
1. ROD Annual Report, FY 1988. EPA/540/8-89/006, U.S.
Environmental Protection Agency, 1989.
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Environmental Protection Agency, 1990.
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Presented at the First US/USSR Conference on
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Compounds From Groundwater: A Survey of the Technolo-
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Contract DE-AC05-84OR21400, 1989.
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16. Byers, W.D., and C.M. Morton. Removing VOC from
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Strips Trichloroethylene from Groundwater. Environ-
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18. Personal communication with vendor.
19. Air Stripping of Contaminated Water Sources - Air
Emissions and Controls. EPA/450/3-87/01 7, U.S.
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49th Annual Conference of the Indiana Water Pollution
Control Association, August 19-21, 1985.
Engineering Bulletin: Air Stripping of Aqueous Solutions
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