PB88-239082
Air Strippers and Their Emissions
Control at Superfund Sites
Research Triangle Inst.
Research Trianqle Park, *IC
Prepared for
Environmental Protection Agency, Cincinnati, OH
Aug 88
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EPA/600/D-88/153
Aufiust 1988
AIR STRIPPERS AND THEIR EMISSIONS CONTROL
AT SUPERFUNO SITES
by
Benjanln L. Blaney
Hazardous Uaste Engineering Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati, OH 45263
and
Marvin Branscome
Research Triangle Institute
Research Triangle Park, NC 27709
EPA Project Officer
Benjamin L. Blaney
EPA Contract 68-02-3992
HAZARDOUS WASTE ENGINEERING RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
RESEARCH TRIANGLE PARK, NC 27711
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TlCMNlCAl M»OKT £
EPA/600/D-88/153
*CCtU-O*
AIR STRIPPERS AND THEIR EMISSIONS CONTROL
AT SUPERFUND SITES
k «I»O«1 **~t. - --
August 1988
!»O* COOI
B. L. Blaney and M. Branscome
it
*«o AOOMU
U.C. Environmental Protection Agency
Cincinnati, Ohio 45268 and
Research Triangle Institute
Research Triangle Park, North Carolina 27709
'I VO*>f 0*1*6
D *DO*IU
o»
Hazardous Waste Engineering Research Laboratory
U.S. Environmental Protection Agency
Office of Research and Development
Cincinnati. Ohio 45268
Air stripping, a traditional means of making slightly contaminated ground-
water potable. 1$ being applied increasingly to more severe ground*ater pollution
at remedial action sites. Concentrations of volatile and semi volatile compounds
at such sites may reach hundreds of parts per million. As a result, several
changes have resulted in air stripping technology. New air stripping technologies
are being employed to achieve very high (>99 percent) removal of volatile com-
pounds and to increase the removal of semivolatlies. New stripper designs are
being investigated for compactness and mobility. In addition, emissions controls
are being added because air pollution Impacts are larger. This paper discusses
these trends and provides examples from groundwater cleanup at remedial action
sites in the United States.
•t*
M **o Docwwctrr
Ctotif
:t«rr
None
Relo
Public
None
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NOTICE
This document has been reviewed in accordance with U.S. Environmental
Protection Agency policy and approved for publication. Mention of trade
names or commercial products does not constitute endorsement or recommenda-
tion for use.
11
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ABSTRACT
Air stripping, a traditional means of making slightly contaminated ground-
water potable, is being applied increasingly to more severe groundwater pollu-
tion problems at remedial action sites. Concentrations of volatile and semi-
volatile compounds at such sites may reach hundreds of parts per million. As a
result, several changes have resulted in air stripping technology. New air
stripping systems are being employed to achieve very high (>99 percent) removal
of volatile compounds and to increase the remo/al of semivolatlles. New stripper
designs are being investigated for compactness and mobility. In addition,
emissions controls are being added because air pollution impacts are larger.
This paper discusses these trends and provides examples from groundwater cleanup
at superfund sites in the United States.
Ill
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INTRODUCTION
Air stripping has been a frequently used option for removal of volatile
organic compounds (VOC) from groundwater to make it potable. In the past* the
levels of groundwater contamination to which air stripping was applied were in
the low to mid parts per billion (ppb). In such cases, air stripping readily
reduced VOC concentrations to below detection levels (typically 10 ppb). Air
emission controls were not generally required because the mass of VOC released
to the air was low.
More recently, environmental agencies have been faced with levels of ground-
water contamination at remedial action sites which are ten to hundreds of times
higher, sometimes reaching hundreds of parts p*»r million (ppm). The need to
efficiently remove higher concentrations in an environmentally safe manner has
resulted in two major changes in the types of air stripper systems that are
being used at some remedial action sites. First, the design of the strippers
has been changed to achieve very high VOC removal efficiences and to achieve
semivolatile removals of 80 percent and above. This is often accomplished by
raising the operating temperatures of the strippers to 60°C (HO°F) or higher.
Second, air pollution control devices have been added to strippers because
their organic emissions may result in a significant health hazard to site
neighbors. For example, a system processing 100 gprr. cf water with 10 ppm of
benzene would release 2 Mg/year of that compound to the atmosphere when operating
at 99 percent efficiency. The health effects of such emissions may be significant,
depending on local meteorology, compound toxicity and population distributions.
This paper describes these changes In air stripper technology that are
resulting from their Increased use at remedial action sites.
Air Stripping
Air stripping Is a dynamic physical separation process which relies on the
contact between clean air and contaminated media (typically water or soil) to Induce
transfer of the contaminant to the air. By continually replenishing the system
with uncontaminated air the contaminants are stripped away from the polluted media.
Two niajor parameters Influence the efficiency of air stripping. The first
Is the rate at which a contaminant will transfer from the liquid to air.
The larger the ratio of the air to the liquid concentrations of a compound at
equilibrium, the higher the rate of transfer from liquid to air during stripping.
Volatile organic compounds (VOC) are only sl1ght1> water soluble and have high air-
to-water equilibrium coefflcents. Therefore, they are readily removed from water
by air stripping [1],
Compounds which are more water soluble, such as acetone, are not as amenable
to air stripping because their equilibrium coefficients are low. However, since
the air-to-water equilibrium coefficient will increase with temperature, heating
of the Influent water is one means of Increasing the efficiency of air stripping
for these compounds.
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The second parameter which has a major influence on air stripping efficien-
cies is the air to water contact area. The design of the air stripper will
influence this parameter. Spray aeration, diffused aeration and multiple tray
aeration all promote VOC transfer to air. However, they are not as efficient as
packed tower aeration, the most common air stripper design.
Figure 1 shows a typical packed tower aerator which has a countercurrent
flow of clean air and contaminated water. The unit generates contaminated air
and a water if:".ue-i: from which essentially all VOC has been removed. Obviously,
air stripping ..., ,. ist transfer an environmental problem from one media to
another if there 1s no means of capturing the stripped organics.
Applications of Air Stripping at Remedial Action Sites
Table I summarizes the operating characteristics of several ambient and
high temperature air strippers at remedial action sites in the United States.
For each site, data are provided on full-scale unit designs or on actual operating
parameters of pilot-scale or full-scale units. Flow rates and concentrations
of operating systems represent typical values during extended operation. Influent
concentrations are approximate, due to daily variations. When the site data
provided only a range of influent concentrations, upper limits are used.
Removal efficiencies are either based on stated values in the literature or
based on the Influent and effluent concentrations In Table I. The table shows
that air strippers have been designed to handle a wide range of influent concen-
trations and flow rates.
Frequently, Influent concentrations will drop by one or two orders of
magnitude during the course of operation at a remedial action site as contaminated
groundwater is diluted. As this occurs, feed rates can be increased and/or the
air to water ratio reduced to optimize stripper operation. At some sites,
several air strippers may Initially be ir. series to achieve high cleanup
efficiencies and then operated in parallel as contamination levels decrease [12],
The data in this table show that air stripping can be used to treat a
wide range of volumes and degrees of contamination of groundwater at remedial
action sites. Large volumes (e.g. Tacoma, UA) may require a number of units in
parallel. Large concentrations and stringent effluent limitations may be most
economically handled by several units in series. High temperature air strippers
(HTAS) are also being used to Increase VOC removal efficiencies, as well as to
remove water soluble orjanlcs from groundwater.
INNOVATIONS IN AIR STRIPPING
High Temperature Air Stripping
Since groundwater at certain remedial action sites contains relatively
high (>1 ppm) organics, some of which are highly misclble In water (e.g. methyl
ethyl ketone, acetone). Improvements in air stripping have been undertaken
to Increase removal efficiencies. This Is often accomplished by heating the
influent stream, a technique called high temperature air stripping (HTAS).
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The higher efficiencies obtained using HTAS are due to the increase with
temperature of the air-to-water equilibrium constants of organic compounds.
Only limited experimental data are available on the variation of equilibrium
constants with temperature. However, temperature increases from 0°C (32°F) to
30°C (86°F) result in equilibrium constants increasing by a factor of 4 to 6
for a number of volatile compounds [15, 16]. Therefore, one would expect that
increasing the influent stream temperature to 60°C (140°F) could improve removal
efficiencies by at least a factor of ten.
Lamare. et al., have performed pilot-scale studies investigating the in-
creased rate of removal of semivolatile compounds from water as a function of
stripper feed temperature. This work was done as part of the development of a
groundwater cleanup system at the Gil son Road site, Nashua, NM, which has a
number of water soluble contaminants, as well as chlorinated and non-chlorinated
volatile organics. Table II shows the removal efficiencies obtained at various
temperatures with an air to water ratio of 500 over a temperature range of 12°C
(54°F) to 75°C (170°F). Removal efficiencies are markedly increased by Increasing
feed temperature [13],
The full-scale treatment system at Gllson Road will utilize a 300 gpm
HTAS. Iron and manganese will be removed upstream to prevent fouling of the
stripper packing. The feed stream will be heated with an economizer and a
trim heat exchanger before entering the stripper [17],
Johnson, et. al, found similar Improvements in removal efficiencies for
methyl ethyl ketone, another water soluble solvent which Is frequently found
at rem3dial action sites. Johnson's earlier tests had shown that a mobile
air stripper capable of removing over 98 percent of more volatile compounds
such as trichloroethene (TCE) could remove only 25 percent methyl ethyl ketone
(HER). Over 95 percent HER removal was achieved In pilot-scale tests at 60°C
and 150 air:water ratio and at 70°C with a 75 air: water ratio. Johnson proposed
a series of four heated air strippers, each capable of achieving 99S removal.
In order to obtain an effluent below 50 ppb from a feed stream containing
1,000 ppm HER [12].
Recently, a high temperature air stripper was Installed at McClellan A1r
Force Base (AFB), Sacramento, CA as part of a groundwater treatment system.
Initial testing of the stripper Indicates that It r«anoves all VOC to below
detection limits, while appreciable amounts of MEK and acetone are also
removed. This system is discussed in more detail at the end of this paper.
It should be noted that as the design feed temperature of an air stripper
approaches 100 °C (212 °F), steam stripping may be a preferrable treatment
technique. This will be especially true if a condenser can be used as an
emission control device. Steam stripping (which utilizes steam Instead of air
as the stripping medium) rer.oves organics more efficiently than HTAS because
It operates at higher temperatures. Steam stripping has been demonstrated for
decontamination of groundwater containing ketones, alcohols and chlorinated
solvents at concentrations up to 5,600^ ppm [18]. However, steam stripping
has higher capital and operating costs than HTAS due to the additional fuel
use and, in some cases, the need for higher grade materials of construction
[19. 20].
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Centrifugal Force Air Stripping [6, 7]
Another Innovation in air stripping technology is the use of centrifugal
force distillation devices for the separation of volatiles from water. Such
devices are more compact, making them readily mobile and easily sheltered in
cold climates. They also are reported to have fewer fouling problems and require
lower air-to-water ratios than conventional air strippers.
Figure 2 1s the schematic diagram of a Higee* system which is being
tested at the U.S. Coast Guard Base, Traverse City, MI. Filtered groundwater
enters the center of a rotating bed of packing with high surface area-to-
volume raLio. The bed's rotation produces centrifugal forces of 100 to
1,000 G*;. Air enters the device countercurrently and mass transfer takes
place in the packing, which has a pore size of 150 to 200 urn. The decontam-
inated w<»ter is removed by gravity drain, while the VOC-laden air exits through
the va'x>r outlet. The unit can process 100 to 300 gallons per minute of water
anr* te-.ts have shown that particles up to 100 urn 1n diameter have passed
through the packing. During normal testing, a 50 um filter Is used upstream
of the unit.
The Hlgee* 1s designed to handle a wide range of groundwater concentrations.
Concentrations at the Coast Guard site were originally predicted to be on the
order to 5 to 10 ppm. However, by the time the unit was brought into operation,
tarlier remediation action, »rh1ch promoted in-situ biodegradation, had p.-ocessed
enough water so that subsurface dilution resulted in much lower concentrations
of benzene, toluene and xylene (typically 100-500 ppb). The unit has obtained
greater than 99% removal for these compounds, even In performance tests in
which water was spiked to concentrations of 8 ppm benzene and 17 ppm toluene.
The system operators. The Traverse Group7 Inc., Indicate that the unit should
be cost competitive with air stripping columns and less expensive than carbon
adsorption at Influent concentrations of 10 ppm or higher. The company Is
currently developing designs for more compact, truck-mounted units.
COHTROL OF AIR STRIPPER EMISSIONS
Regulatory Requirements
The groundwater concentrations at remedial action sites may be over a hun-
dred times higher than what has traditionally been encountered when air stripping
was used for drinking water cleanup. The air emissions from such systems are
proportionately greater and the resulting health Impacts may be significant.
Federal policy requires any treatment technologies Installed as part of a
remedial action at a Superfund site to meet all "applicable, relevant and
appropriate" environmental regulations, whether Federal or State. Some States,
such as California and Ohio, have standards set based on the hazard to the
surrounding community as determined by the levels of human exposure to the
emitted organics. Others do not require an operating permit unless the emissions
exceed some level set for VOC sources in general.
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Michigan has VOC emission regulations which place stringent requirements on
air strippers. The State requires all sources of VOC emissions to utilize best
available control technology (BACT) to reduce emissions. For groundwater
cleanup operations, the permittee must also analyze other treatment alternatives
that cause less air pollution and demonstrate that they are not more cost
effective than an air stripper with or without controls. Once air stripping is
decided on and an air emission control is chosen, an impact analysis must be
done to show that the controlled emissions result in an "acceptable environmental
impact." Presently, health risks to the maximally exposed fence line individual
are evaluated to determine that impact. The measure of acceptability is dependent
upon the type of health effect of each of the emitted compounds. For most
compounds, the 8-hour time weighted average must be less than 1 percent of Its
threshold limit value (TLV). For carcinogenic compounds, the lifetime cancer
risk to the individual must be under 10~6 b-ised on annual average pollutant
concentrations. Best available lexicological data are used for other compounds to
determine maximum annual exposure limits [21],
Another important part of Michigan's air permitting policy is that emitters
must use worst case emission rates when determining exposure limits. Since
groundwater cleanup often results in decreased emissions with time, the control
technology must be designed to handle initial emissions rates. The State does
allow permitters to reapply for revised permits at a later date, a provision
which would allow for reduced operating costs, if granted [21].
Control Options
The^e are basically three control options* available to the cleanup of gases
from air stripping:
1. Carbon adsorption
2. Thermal incineration, and
3. Catalytic incineration
Condensers will not be considered here because they are generally not
effective for gas streams containing less than 10,000 ppm organics [22], There
are advantage and disadvantages to each, as will now be discussed. To provide
a perspective on the size and concentrations of the off-gas streams to which
these are, or might be, applied. Table III provides such information for the
air strippers previously characterized in Table I.
Carbon Adsorption
Carbon adsorption is the most frequently used control technology for air
stripping, having been shown to be cost-effective for removing hydrocarbons
from dilute (<1 percent) air streams from a number of industrial processes.
Package adsorber systems are available for turn-key Installation from a number
of manufacturers which simplifies their use, especially with mobile remedial
action treatment units.
The capital cost of a system is principally dependent upon the amount
and type of carbon chosen. Carbon requirement is giver, by [23]:
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Carbon requirement (1b carbon/lOOU scf) = (CM/k) x Z79
where we define:
C = VOC concentration (n.jle fraction)
M » VOC molecular weight (Ib/lb mole)
k = Carbon capacity (Ib VOC/1CO Ib carbon).
While the VOC concentration and molecular weight may be estimated for a given
air stripper off-gas, the operating carbon capacity is influenced by a number
of factors. These include the properties of the carbon, the humidity and
composition of the feed stream and the desired percent of contaminant removed.
For industrial waste streams, which typically only have a few compounds
in the air stream, the carbon requirement can be estimated by vendors based
on adsorption isotherms. However, remedial action site strippers are usually
producing off-gas streams containing a large number of organics whose concen-
trations vary with time. Therefore, it is important to monitor the adsorber
exhaust stream periodically to determine when breakthrough is occuring so that
the carbon can be replaced or regenerated.
Several other factors must be considered when designing a carbon adsorber.
Gas entering the unit must be free of particles or liquid aerosols, which will
block air flow through the adsorber. This can be a problem for units on air
strippers which have high air:water ratios. Demisters must be added in such
situations.
Also, carbon adsorber efficiency drops dramatically if -jas stream relative
humidity exceeds 50? because adsorbed water decreases the bed's adsorption
capacity. When humidity levels exceed this threshold, the off-gas stream is
heated. For ambient temperature air strippers (13 to 25°C), a temperature
increase of 17°C (30°F) will decrease the relative humidity from 100 to 40
percent [24], The reduction in humidity will increase the adsorptive capacity
of the carbon for many compounds by a factor of three to four, which greatly
exceeds the decrease in absorptive capacity of the carbon resulting from the
rise in temperature.
The off-gas from HTAS units may be particularly difficult to treat by
carbon adsorption because of decreased adsorptive capacity from both high
temperature and a high loading of water vap^r.
The Verona Wall Field site in Battle Creek, MI uses carbon adsorption
for ambient temperature air stripper-emissions control. Details are provided
at the end of this paper.
Thermal Incineration
Gas incinerators generally operate at 760 to 1200°C (1400 to 2200°F). The
lower ranges are adequate for volatile organics of high heat of combustion,
while higher temperatures (typically over 1800°F) are required to destroy hydro-
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carbons of low heat content, such as halogenated compounds. Because the incin-
erator design and fuel utage are specific to the gas stream to be combusted,
generalities about size cannot be made based solely on gas flow rate and VOC
concentration.
Figure 3 shows a typical thermal incinerator. The unit is designed to
promote good mixing of air, *aste gas, and auxiliary fuel (if needed) prior to
combustion. It is also important to have good turbulence in the combustion
zone, along with at least a 0.5 to 1.0 second residence time. Energy can be
recovered from the hot exhaust gases and used to preheat con-.bustion air or the
influent water stream to an air stripper.
Combustion air with a heating value of less than 1.9 NJ/scm (50 Btu/scf)
usually requires auxiliary fuel to maintain desired combustion temperatures.
This will typically be the case for air stripper exhaust gases which have much
lower heat content. For example, stripper gas with 100 ppm benzene has a heat
content of only 20 KJ/scm (0.53 Ctu/scf). If the waste gas contains water
droplets, additional fuel is required for water vaporization. Therefore,
demisters should be used before incinerators.
Packaged, single unit thermal incinerators are available for gas rates
ranging from about 0.M sen/sec (3CO scfm) to 24 scm/sec (50,000 scfm). These
units can achieve greater than 9T. percent destruction efficiency for most VOC [25],
If emissions are predominately halogenated oryanics, special considerations
must be given to incinerator design. Such compounds require high (>1800°F)
combustion temperatures to achieve high destruction efficiencies. Also, hydrogen
chloride is the principal conbuvlion product for such compounds. Acid gas
scrubbers are required on large hazardous waste incinerators but will not
generally be reeded on air stripper control incinerators because HC1 emissions
are low.
The Gilson Road site uses an oil-fired boiler as a thermal incinerator to
treat air emissions from a groundwater/leachate air stripping process. This
air strean contains primarily tetrahydrofuran, methyl ethyl ketone, butyl
alcohol, toluene, and smaller amounts of other onjanics. The desired destruction
efficiency is 99.99 percent [20]. The unit is currently being tested.
The thermal incinerator at ftcClellan Air Force Base is discussed later
in this paper.
Catalytic Incineration
A catalytic incinerator, or catalytic oxidizer, operates at lower tempera-
tures than a thermal incinerator. Combustion temperatures are typically 320 to
6'jO°C (600 to 1200°F). The catalyst serves to promote oxidation reactions that
require high temperatures for thermal oxidation. This reduces the fuel require-
ments an«j associated costs.
While catalytic incinerators require less fuel, they have two drawbacks
that may limit their applicability to controlling air stripper off-gases.
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First, catalysts are specific to certain classes of compounds and may be
poisoned by others. For example, catalysts will not usually work efficiently
on both halogenated and nonhalogenated hydrocarbons. Second, catalytic incin-
erators are less forgiving than thermal incinerators to variations in feed
stream composition.
An Engelhard TOR VEX® Catalytic Reactor is in use at the Avenue A Area
Site, U.S. Coast Guard Base, Traverse City, Michigan. The unit is being used
to destroy emissions from the stripping of mainly nonchlorinated organics
(e.g., benzene, xylene, and toluene) from groundwater. It has a 2,000 cfm
capacity and operates at 260 to 320°C (500 to 600°F). It cost approximately
$72,000, consumes $80/day of natural gas, and requires approximately 1 person-
day every two weeks for operation and maintenance. Detailed air emissions
measurements have not been performed on the unit, but its efficiency is
purported to be over 90 percent by tne vendor [6, 7],
CASE STUDIES
In order to provide more detailed examples of the application of air
strippers and their emission controls at remedial action sites, two case
studies are briefly presented. Besides demonstrating some of the general
points made earlier In this paper, details about system design and operating
problems are provided.
Verona Uell Field [9-11]
The use of air stripping at th? Verona Well Field site in Battle Creek,
Michigan demonstrates (1) how a system can be designed to accommodate high
Initial concentrations in the groundwater and (2) the use of carbon adsorption
for air pollution control.
The Verona Well Field is the major source of public potable water of the
City of Battle Creek. In August 1981, it was determined that a number of
private and city wells in the field were contaminated. An Initial Remedial
Measure (IRH) was approved which Included the use of a 2,500 gpm air stripper
operating at 5,000 cfm to provide hydraulic blocking to encroachment of con-
taminated groundwater Into the field. Table IV shows the design concentrations
for major compounds in the influent and the actual concentrations during 29
months of operation. During this IRN phase, the load on the stripper was 2,000
gpm.
Recently, a Source Control Action (SCA) at one of the major pollution sources
In the well field was Initiated which will Involve decontamination of the
groundwater at a rate of 400 gpm. This stream will be combined with the 2,000 gpm
IRM strean for a total feed of about 2,400 gpn to the stripper. Table 4 shows
the anticipated total VOC concentrations of this groundwater stream over the first
150 days of extraction from the SCA site. Even allowing for dilution of this 400 gpm
stream at the inlet to the stripper, these concentrations are Initially orders
of magnitude above the stripper design concentrations. To ceal with this, the
EPA required that liquid-phase carbon beds be used to treat the 400 gpm flow
for the first month of operation. The Implementation of this source control
action started in early 1987.
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The Verona Well Field air stripper has a carbon adsorption unit to treat
the off-gas air. The system consists of two parallel beds, each approximately
4 ft deep and 10 ft In diameter. Each bed contains 9,500 pounds of carbon.
The stripper offgas Is heated prior to entering the carbon to reduce its
humidity to 40 percent.
The capital cost of the air stripper and carbon adsorption beds. Including
design and installation, was $550,000.
Table IV shows the air permit limits set by the State for emissions from
the air stripper during the IRM phase. The off-gas from the units are being
monitored 3 or 4 times per year to detect breakthrough. The carbon has been
replenished about once per year during the IRM phase. The last two columns of
Table IV present emissions from stripper and the carbon beds collected on
August 2, 1985, about 11 months after system startup. Based on the breakthrough
of dichlorlnated hydrocarbons shown here, the decision was made to change the
carbon. The cost of replenishing both beds with regenerated carbon is approx-
imately $18,000.
The air stripper has had few operational problems during the IRM phase.
Iron oxide was Initially plating out on the packed rings, but this problem was
alleviated by recirculating sodium hypochlorite through the stripper about four
times per year.
McClcllan Air Force Base [14]
Me del Ian Air Force Base in Sacramento, California has groundwater contam-
inated with fuel and solvents from spills and storage tank leaks. There are
volatile and semlvolatile organic* at ppm concentration in the groundwater.
A treatment system composed of air stripping and liquid-phase carbon adsorption
has been Installed to remove these compounds to below detection limits. A
biological treatment unit will be added between the two processes, shortly
(Figure 4).
The groundwater Is pumped through a series of heat exchangers to preheat
the feed stream to the air stripper to 60 to 65°C (140 to 150°F). The stripping
column is 8 feet In diameter and has 23 feet of packing, a maximum air flow
of 4,000 cfm, and a feed rate of 1,000 gpm with provisions for recycling of
500 to 1,000 gpm. The extent of recycle .s determined by the rate of groundwater
flow, the temperature of tfie stripper feed and the proper a1r:water ratio. The
bottoms from the air stripper pass through the primary heat exchanger to preheat
the feed. A portion of the bottoms Is recycled and the balance Is further
treated by carbon adsorption prior to discharge.
The liquid phase carbon adsorption process consists of three granular
activated carbon trains In parallel, each with two contact vessels. Each vessel
is 10 feet in diameter and 10 feet high and contains approximately 60,000
pounds of carbon.
The stripper offgas passes through a demlster to reduce the water aerosol
load on the incinerator and then through a heat exchanger, where the vapors
are heated by the incinerator's offgas. Hydrocarbons In the stripper offgas are
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10
destroyed by thermal combustion in a vapor phase Incinerator that burns natural
gas and is designed to achieve 99.99 percent destruction. The incinerator operates
at 980°C (1800°F) and has an estimated thermal capacity of 5.5 million BTU/hr.
Heat is recovered from the hot exhaust gases in a series of heat exchangers* which
cool the offgas to about 230°C (450°F). These gases are then scrubbed with a
solution of 20 percent caustic to remove HC1 prior to discharge to the atmosphere.
The system includes three heat exchangers. Two utilize waste heat from the
Incinerator to heat the groundwater feed to the stripper and to preheat the
stripper offgas prior to incineration. A third transfers heat from the stripper
bottoms to the groundwater feed.
An interesting feature of the vapor system is the collection and treatment
of vapors from other sources. Gas vents from the caps placed over the con-
taminated soil and from the groundwater storage tank are vented through the
air stripper to the Incinerator to destroy the VOC in these vapors.
The groundwater treatment system was evaluated in a 30 day shakedown test
conducted early in 1987 and the results are listed In Table VI. During these
tests, a 620 gpm recycle was used combine with a flow of 180 gpm of contaminated
water. The stripper air flow during these tests was 2,500 cfm, the feed rate
was 800 gpm, and the resulting air-to-water ratio w*'. 20:1. The results shown
in Table VI demonstrate that the system as a whole obtained its design objectives,
with the exception of acetone which exceeded 1 ppm. Limited sampling of the
stripper effluent Indicated that VOC concentrations were reduced to below
detection limits and acetone was reduced by about 30 percent. A biological
treatment unit 1s being added after the stripper to further reduce ketone
concentrations.
The total Installed capital cost of the treatment train (including the
air stripper. Incinerator, and carbon adsorber) was approximately $3.4 million.
This includes the system's design, construction, and installation. The only
major operating problem experienced to date has been fouling of the system,
especially the heat exchangers, from biological activity. Initially the system
was back flushed to reduce the fouling. The groundwater is now being chlorinated
In order to minimize this source of fouling over the longer term.
CONCLUSIONS
As a result of advances in design, air stripping can be used to remove
water soluble, as well as volatile, organic .compounds from aqueous streams.
At sufficiently high temperatures, removal efficiencies of over 90 percent can
be obtained.
Uncontrolled stripping of groundwater at remedial action sites may result
In significant air emissions impacts. Carbon adsorption and Incineration are
the most frequently used emission control options. Thermal Incineration
appears to be the control technique of choice at sites where high temperature
air stripping is used because the waste heat from the incinerator can be used
to heat the stripper feed.
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ACKNOWLEDGMENTS
We Mould Hke to thank the Individuals who provided data for use in this
article, particularly those whose private communications are listed in the
references. The authors assume full responsibility for any misinterpretation
of that data.
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12
REFERENCES
1. Kavanaugh. H. C. and Trussell, R. R., Design of Aeration Towers to Strip
Volatile Contaminants from Drinking Water. Journal AWWA 80 (12). 684 (1980).
2. Gross, R. L. and TerMath, S. G.. Packed Tower Aeration Strips Trichloro-
ethylene from Groundwater, Environmental Progress 4 (I:), 119 (1985).
3. Stall ings, R. L. and Rogers, T. N., Packed Tower Aeration Study to Remove
Volatile Organics from Groundwater at Wurtsmith AFB, HI, U.S. Air Force
Report No. E5L-TR-84-60 (1985).
4. ftlntrye, G. T. et al.. Design and Performance of a Groundwater Treatment
System for Toxic Organlcs Removal, Journal UPCF 58 (1), 41 (1986).
5. Byers, U. D. and Morton, C. D., Removing VOC from Groundwater; Pilot,
Scale-Up and Operating Experience, Environmental Progress 4 (2), 112 (1985).
6. Armstrong, J. H. and Dietrich, C., The Traverse Group, private communications,
7. U.S. Environmental Protection Agency, Trip Report: Prcsurvey of the Hygee
Air Stripper. U.S. Coast Guard, Traverse City, HI.USEPA Contract No.
68-03-3253, draft (1986).
8. NUS Corporation, Treatabllity Study Report, Tyson's Dump Site, Draft (1986).
9. U.S. Environmental Protection Agency, Operable Unit Feasibility Study,
Verona Wall Field-Thomas Solvent Co., Raymond Rd. Facility, Battle
Creek, MI, EPA WA38.5 H51.0 (1985).
10. J. Tanaka, Site Management Section, USEPA, Region V, private communications.
11. P. McKay, Groundwater Quality Division, MI DNR, private communications.
12. Johnson. T. et al.. Raising Stripper Temperature Raises MEK Removal,
Pollution Engineering 17_ (9), 34 (1985).
13. Lamarre, B. L. et al.. Design, Operation and Results of a Pilot Plant for
Removal of Contaminants from Groundwater, Proceedings of the Third National
Symposium on Aquifer Restoration and Grrundwater Monitoring, D, M. Hi el son,
Ed., national Water Well Assoc., 113 (1933).
14. Mackenzie, D., McClellan AFB. private communications.
15. Byers, W. D. and Morton. C. M., Removing VOC from Groundwater, Pilot, Scale-
up, and Operating Experience, Environmental Progress 4_ (2), 112 (1985).
16. Gossett, J. M., Packed Tower Air Stripping of Trlchloroethylene from Dilute
Aqueous Solution, U.S. Air Force Report ftp. ESL TR 81-38 (J983).
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13
17. U.S. Environmental Protection Agency, Superfund Record of Decision (EPA
Supe
,~IW
Region 1), Sylvester Site, Nashua, NH, EPA/RQD/R01-S3/007 (1983).
18. Nakles. D. V. and Bratina, J. JE., Technology for Remediation of Groundwater
Contamination, Proceedings of the National Conference on Hazardous Waste and
Hazardous Materials, 133 (1986).
19. Boegel, J., Air Stripping and Steam Stripping, Standard Handbook for
Hazardous Waste Treatment, H. Freeman, ed., McGraw Hill (to be published).
20. HcArdle, J. L. et al., A Handbook on Treatment of Hazardous Waste Leachate,
EPA Contract No. 68-03-3248 (1986).
21. Edwards, G., Air Quality Division, MI ONR, private communications.
22. U.S. Environmental Protection Agency, Control Techniques for Volatile
Organic Compound Emissions from Stationary Sources, 3rd ed., 450/3-85-008.
(1985).
23. Blackburn, J. U., Organic Emission Control Device, Environmental Progress 1
(3), 182 (1982).
24. Stenzel, H. H. ar.d Gupta, U.S., Treatment of Contaminated Groundwaters with
Granular Activated Carbon and A1r Stripping, JAPCA 35 (12), 1304 (1985).
25. U.S. Environmental Protection Agency, Organic Chemical Manufacturing,
Volume 4: Combustion Control Devices, EPA-450/3-80-026 (1980).
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14
TABLE I. Operational Data for Some Air
Strippers at Remedial Action Sites
Flow Water
Site (gpm) Influent
Concentration
(ppb)
Ambient Teirperature Infl
Wurt smith AFB
- TFE Plume [2] 900
(2 parallel units)
- Benzene Plume
PT [3] 85
Sydney Mines
Design [4] 150
Tacoma, UA Design [5]
(5 parallel
units) O.500
Traverse City
USCG Base [6.7] 90
Tysons Dump PT [8] 5
Verona Well
Field [9-11] 2.000
HTAS Operation
Hydro Group Design [12]
(60°C. 3 units in
series) <100
Roy Ueston PT [13]
(60°-70°C) NA
KcClellan AFB [14]
(620 gpm recycle.
180 gpm makeup
65°C) 800
uent
500
<8,820
2.225
<1,000
1.000
<47.000
<41
IxlO6 MEK
NA
<4,400C
Effluent Percent AirrWater
Concentration Removal Ratio0
a Objective3
(ppb)
<1.5 >99 30
NA >90 65
.
NA NA 200
ru >89 310
<10 99 52
<500 >98 250
<7 >90 20
<500 MEK >99 200
NA <76% propanol 50-500
995 THF
<0.5 >99 20
Air Flow
(cfm)
3.900
730
4.000
145.000
600
170
5,000
2,700
NA
2,500
Abbreviations: Not available (NA) and pilot test (PT).
a T"»:l volatile organlcs, unless otherwise indicated.
b Volumetric.
c Sum of highest concentrations of VOC detected in groundwater stream, diluted
by 180/800 to account for recycle. -
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15
TABLE II. Influence of Feed Temperature on Removal of
Water Soluble Compounds from Groundwater
Percent Removed at Selected Temperatures
Compound 12°C 35°C 73°C
Compound
1.1, -OCA
1,2-OCA
1,1.1-TCA
1.2-DCE
1,1-DCE
TCE
PCE
2-Propanol 10 23
Acetone 35 80
Tetrahydrofuran 50 92
Source: Reference 12.
TABLE IV. Verona Well Field Air Stripper VOC
and Air Emissions During IRM Phase
Influent
Concentration (ppn) Permi
Design Actual*1
38 5.5 250
8 <1.0 350
150 12 1.000
230 9.7
11 <1.0
52 <1.1 420
120 11 810
70
95
>99
Influent
Air Emissions
t Stripper
(8/2/85)
243
4.6
1.014
487
ND
92
785
(ug/m3)
Carbon
(8/2/85)
234
ND
50
424
ND
ND
ND
Abbreviations: Initial Response Measure (IRM), Not Available (NA).
dichloroethane (OCA), dlchloroethylene (DCE), trichloro-
ethane (TCA), trichloroethylene (TCE), perchloroethylene (PCE),
methylene chloride (HeCl) and vinyl chloride (VC).
a Average of monthly measurements from September 1984 to January 1987.
Source: References 10 and 11.
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16
TABLE III. Air Stripper Emissions Stream Characteristics
and Control Techniques at Remedial Action Sites
Site
Uncontrolled Stripper Off-Gas
Air Flow
(cfm)
Emission
Rate
(g/sec)
Approximate Control
VOC Device
Concentration
(ppm benzene)a
Ambient Temperature Feed
WurtsmHh AFC
- TCE Plume 3,900
(2 parallel units)
- Benzene Plume PT 730
Sydney Mines Design 4,000
Tacoma, UA Design
(5 units In
parallel) 145,000
Traverse City
USCG Base 600
Tysons Dumo PT 170
Verona Well
Field 5,COO
Heated Feed
Hydro Group PT 2,700
(60°, 3 units in series)
Roy Weston PT NA
(60°-70°C)
McClellan AFB 2,500
(620 gpm recycle,
180 gpm makup. 65°C)
0.027
<0.041
<0.022
<0.19
0.004
0.014
0.068
NA
23 None
<2.4xl02 None/PT
<22 None
<5.2
26
3.3X102
54
<9.0xl03
flA
None
Catalytic
Incinerator
None/PTb
Carbon
Adsorber
None/PT
None/PTc
<1.8xl03d Thermal
Incinerator
Abbreviations: Not available (NA), total volatile organics compounds (VOC),
and pilot tests (PT).
a Calculated in terms of benzene equivalents.
b Carbon adsorber is planned for full-scale stripper.
c Thermal Incinerator Is being used at site for which these pilot tests were done.
d Upper limit to mass flow and concentration based on sum of maximum concentrations
of individual VOC detected in groundwater.
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17
TABLE V. Modelled VOC Concentrations in
Extracted Groundwater at Source
Control Action, Verona Well Field
Time
(days)
1
8
15
22
29
149
Average Total VCC
Concentration (uy/1)
125.000
11.000
6.500
4.600
4.400
2.800
Source: Reference 9.
TABLE VI. Influent and Effluent Concentrations of Major Compounds3
Processed in Groundwater Treatment System at McClellan AFB
During 30 Day Startup Tests
Coumpound
Concentrations (ppb)
Influent Effluent
1.1-OCE
1,2-OCE
VC
TCE
1.1.1-TCA
MEK
MIBK
Acetone
750 -
<0.5 -
41 •
300 -
210 -
4.900 -
1.200 -
5.100 -
6.5CO
6.100
2.400
1.300
1.150
25,000
3.700
35,000
<0.5
<0.5
<0.5
<0.5
<0.5
45 - 800
45 - 130
100 - 6,300
Abbreviations: Dkhlorjethylene (DCE), vinyl chloride (VC), trichloro-
ethylene (TCE). trichloroethane (TCA), methyl ethyl
ketone (MEK) and methyl Isobutyl ketone (MIBK).
a Compounds in groundwater with maximum concentrations exceeding 1 ppm,
Source: Reference 14.
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Groundwater
Air Intake — — -
Air Blower
Offgaito
Catalytic
Incinerator
— - - - - Air
Water
Mlgee
Air Stripper
oo
) ^ Motor and Orlv*
Rotating Packing
Effluent
Holding
Tank
Pump
Figure 2. Hlgee® air stripper system at U.S. Coast Guard Station, Traverse City, Ml.
-------
Wntt QM
Auxiliary
Fuel Burntf
(dltcrata)
Air
Mixing
Section
Combustion
Section
Stack
Optional
Heat
Recovery
FlguraS. Thermal Incinerator.
-------
Orovftdwatff <
Natural Oa»
1
1
Tfiarmal
Indnarator •"" Irwlnarator
Q"iif *
990'C 1
1
*
Stora* faad _
Tank ' * Hart
fidiangart
< ' 1
1
Macyda *4\
I
Air StHppar ~ J
1
*
Cteanatf QM
w e«*c _
^" ~ T Strlppar •—«••— QM
OM,«
Air
Strlppar
Bettomi _ wwwviai Carbon
58 83 C (propotad)
Figure 4. Groumtwator treatment system. McClellan Air Fore* Ba%». Sacramento, CA.
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