&EPA
United States Office of Air Quality
Environmental Protection Planning and Standards
Agency Research Triangle Park, NC 27711
EPA-451/R-93-005
April 1993
Air/Superfund
AIR/SUPERFUND
NATIONAL TECHNICAL GUIDANCE
STUDY SERIES
ESTIMATION OF AIR IMPACTS
FOR THERMAL DESORPTION UNITS
USED AT SUPERFUND SITES
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EPA 451/R-93-005
ESTIMATION OF AIR IMPACTS
FOR THERMAL DESORPTION
USED AT SUPERFUND SITES
Report ASF - 35
U.S. Environmental Protection Agency
Region 5, Library (PL-12J)
77 West Jackson Boulevard, 12th Floor
Chicago, IL 60604-3590
Prepared for:
Office of Air Quality and Standards
U.S. Environmental Protection Agency
Research Triangle Park, North Carolina 27711
April 1993
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DISCLAIMER
This report has been reviewed by the
Office of Air Quality Planning and Standards,
U.S. Environmental Protection Agency, and has
been approved for publication as received from
the contractor. The contents reflect the
views and policies of the Agency, but any
mention of trade names or commercial products
does not constitute endorsement or
recommendation for use.
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TABLE OF CONTENTS
Page
INTRODUCTION 1
PROCESS DESCRIPTION 1
ESTIMATION OF VOC AIR EMISSIONS 5
Average Long-Term Uncontrolled VOC Emission Rate 6
Short-Term Uncontrolled VOC Emission Rate ". . 6
ESTIMATION OF PARTICULATE MATTER AIR EMISSIONS 8
Uncontrolled PM Emission Rate 10
Uncontrolled Emission Rate for Metals . . . . 10
ESTIMATION OF AMBIENT AIR CONCENTRATIONS 11
ESTIMATION OF HEALTH EFFECTS 18
Cancer Effects Due to Long-Term Exposure 18
Non-Cancer Effects Due to Long-Term Exposure 30
Short-Term Exposure 32
EXAMPLE 32
CONCLUSIONS 36
REFERENCES 36
APPENDIX A PHYSICAL PROPERTY DATA FOR SELECTED ORGANIC
COMPOUNDS
APPENDDC B PHYSICAL PROPERTY DATA FOR SELECTED SEMI-VOLATILE
ORGANIC COMPOUNDS
11
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LIST OF TABLES
1 Default Values for VOC Emissions Model for Thermal Desorption Units 9
2 Default Values for PM Emissions Model for Thermal Desorption Units 12
3 Partition Factors for Metals in Soils 12
4 Example Scenarios for Rotary Dryers and Asphalt Aggregate Dryers 14
5 Example Scenarios for Thermal Screws 14
6 Long-Term and Short-Term Health-Based Action Levels for Organic
Compounds in Ambient Air ^ . . . 19
7 Long-Term and Short-Term Health-Based Action Levels for Selected
Semi-Volatile Organic Compounds in Air 27
8 Long-Term and Short-Term Health-Based Action Levels for Metals
in Ambient Air 29
9 Estimated Emission Rates and Ambient Air Concentrations 35
10 Action Level Concentrations 35
LIST OF FIGURES
1 Generalized Process Diagram for Thermal Screw-Based Thermal Desorption .... 3
2 Generalized Process Diagram for Rotary Dryer-Based Thermal Desorption 4
3 One-Hour Average Downwind Dispersion Factor Versus Distance
for Rotary Dryers and Asphalt Aggregate Dryers with Thermal
Oxidizer Control Unit : ... 15
4 One-Hour Average Downwind Dispersion Factor Versus Distance
for Rotary Dryers and Asphalt Aggregate Dryers with Off-Gas
Cooling Control Unit 16
5 One-Hour Average Downwind Dispersion Factor Versus Distance
. for Thermal Screws with Condenser Control Unit 17
111
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INTRODUCTION
The U.S. Environmental Protection Agency's Office of Air Quality Planning and
Standards and the Regional Air Offices have been given the responsibility to evaluate air
impacts from Superfund sites. An important part of this program is the assessment of air
impacts from various alternatives for cleaning up Superfund sites. Since these assessments
are frequently required for planning purposes prior to actual cleanup, they depend on
estimated emissions and ambient concentrations rather than on field measurements.
This report provides screening procedures for estimating the ambient air
concentrations associated with thermal desorption. These procedures are analogous to
procedures for air strippers, soil vapor extraction systems, and excavation that have
previously been published1-2-3. Thermal desorption is a treatment process where heat is used
to physically remove organic compounds from soils and sludges. Procedures are given to
evaluate the effect of treatment rate and contaminant concentration on the emission rates and
on the ambient air concentrations at selected distances from the treatment area.
Health-based ambient air action levels are also provided for comparison to the
estimated ambient concentrations. Many of the health levels have not been verified by EPA
or are based on extrapolations of oral exposures or occupational exposures. Their use could
either under or over estimate the potential health effects. The use of action levels that are
neither EPA-verified nor EPA-approved should be considered on a case-by-case basis. The
statements and conclusions presented in this report are those of the authors and do not reflect
U.S. EPA policy.
PROCESS DESCRIPTION
The following discussion and emissions models are adapted from a recent compilation
of emission models4. A summary of air emissions from thermal desorption units is presented
in Eklund, et al5. Additional general information about thermal desorption is contained in a
guidance document that is currently in press6.
1
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In the thermal desoiption process, volatile and semi-volatile contaminants are removed
from soils, sediments, slurries, and filter cakes. This process typically operates at
temperatures of 200°-1000°F but is often referred to as low temperature thermal desorption
to differentiate it from incineration. At these lower temperatures, the process promotes
physical separation of the components rather than combustion. At operating temperatures
near or above 1000°F, some pyrolysis and oxidation may occur in addition to the
vaporization of water and organic compounds. Volatile organic compound (VOC) removal is
enhanced if the soil contains 10-15 percent moisture prior to treatment.since the vaporized
water will force out some VOCs.
The contaminated soil is first removed from the ground and then transferred to
treatment units, making- thermal desorption an ex situ process. After it is excavated, the
waste material is screened to remove objects greater than 1.5" in diameter before being
introduced to the desorber. In general, three desorber designs are used: a directly fired
rotary dryer, an asphalt aggregate dryer, or an internally heated screw auger (i.e., thermal
\
screw). Generalized process diagrams for rotary dryers and thermal screws are given in
Figures 1 and 2, respectively. The treatment systems include mobile process units designed
specifically for treating soil and asphalt kilns adapted to treat soils. Direct or indirect heat
exchange vaporizes the volatile compounds producing an off-gas that is treated before being
vented to the atmosphere.
Collection and control equipment such as afterburners, fabric filters, activated carbon,
or condensers are used to prevent the release of the contaminants to the atmosphere. Most
rotary dyers have thermal oxidizers (afterburners) to control emissions of organic
compounds, while thermal screws typically have condensers for off-gas treatment. Most
systems have a baghouse (fabric filter) or a cyclone to remove paniculate matter.
While thermal desorbers are designed to remove volatile and semi-volatile organic
compounds, some substances with higher boiling points such as polychlorinated biphenyls
(PCBs) and dioxins may also be removed. The higher boiling point compounds usually are
associated with particulate matter (PM) and removed in the PM control device.
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Classifier
Contaminated
Soil
Oversize Objects
Particulate
Control
Oust
Condenser
Clean Off-gas
Oil/Water
Separator
Wate
1 '
Carbon
Adsorber
i *
Liquid ,
Gas/Liquid was Aftorburnar tl
^Organic SeParator ^ "^ /
Liquid /
r
» Spent Carbon
Treated Water
Figure 1. Generalized Process Diagram for Thermal Screw-Based Thermal Desorption
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Classifier
Contaminated
Soil
Oversize Objects
Paniculate
Control
Dust
Clean Off-gas
Afterburner
Figure 2. Generalized Process Diagram for Rotary Dryer-Based Thermal Desorption
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The specific point sources of air emissions from thermal desorption units vary widely
with each process design. The stack of an afterburner vents combustion products, as does a
fuel-fired heating system if the combustion gases are not fed into the desorber. The fuel-
fired heating system typically operates with propane, natural gas or fuel oil. If control
devices are present and working properly, the stack will vent small concentrations of the
original VOC contaminants, as well as products of any chemical reactions that might occur
from the control devices, such as a baghouse or a scrubber. Relative to incineration, the off-
gas volume from the thermal desorption unit may be smaller, there is less likelihood of
creating dioxins and other oxidation products, and metals are less likely to partition to the
gas-phase. As with incineration, air emission control devices are always part of the system
design; the estimates of uncontrolled emissions obtained from this manual can be used to
help estimate the required removal efficiency of an emission control system or the size and
cost of a given control system.
Fugitive emissions from area sources may contribute-significantly to the total air
emissions from a remediation site. Probably the largest source is excavation of the
contaminated soil. Other sources of fugitive emissions may include the classifier, feed
conveyor, and the feed hopper. Fugitive emissions from the components of the thermal
desorption system and control devices are possible as well. Emissions may also emanate
from the waste streams such as exhaust gases from the heating system, treated soil, dust
collected with the particulate control system, untreated oil from the oil/water separator, spent
carbon from a liquid- or vapor-phase carbon adsorber, treated water, and scrubber sludge.
ESTIMATION OF VOC AIR EMISSIONS
There are several alternative approaches for estimating the emissions from thermal
desorption processes. The best method is to directly measure the emissions during mil-scale
or pilot-scale operations. The next best method is to estimate the emissions using predictive
equations with site-specific inputs. If site-specific information is not available, a
conservative estimate can be made using default values for the input parameters. Equations
are given below for estimating an average long-term emission rate (Equation 1) and a short-
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term emission rate (Equation 2). Equation 2 is the recommended equation for estimating
YOG emissions; Equation 1 can be used as a gross check of total emissions.
Average Long-Term Uncontrolled VOC Emission Rate (Worst Case)
A simple check of the total emissions potential for the site should be made by
dividing the total mass of a given contaminant to be removed by the expected duration of the
clean-up:
(SV)(Q(P)(D
where ER^ = Average worst case emission rate (g/sec);
Sv = Volume of contaminated soil to be treated (m3);
C Average contaminant concentration G*g/g);
|3 = Bulk density of soil (g/cm3);
1 = Constant (g/106 /tg * 10* cnWrn3); and
IK = Duration of remediation (sec).
The volume of contaminated soil and the total mass of each contaminant of concern present
typically are determined during the remedial investigation (RI) of the site, while the fraction
of contaminated soil that must be treated typically is determined during the feasibility study
(FS) for the site. Final clean-up criteria also should be considered when calculating the
volume of soil to be treated. The duration of the clean-up will usually be limited by the
operational rate of the treatment process. For Equation 1, a typical default value for bulk
density of uncompacted soil is 1.5 g/cm3.
Short-Term Uncontrolled VOC Emission Rate
The primary factors affecting the emission rate of a given compound from a thermal
desorption unit are the concentration of the contaminant in the soil, the mass rate of soil
being treated, and the volatility of the contaminant. Uncontrolled VOC emissions from a
thermal desorption unit can be estimated by using the following mass balance approach:
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1-2)
1,000^3,600^1°°;
where ER - Emission rate for contaminant of interest (g/sec);
C = Concentration of the contaminant in the soil 0*g/g);
1000 » Conversion factor Gtg/g / g/kg);
FT = Total feed rate of waste into process unit (kg/hr);
3600 = Conversion factor (sec/hr); and
V = Amount of contaminant volatilized (%).
Equation 2 does not take into account emissions from excavation or materials handling, but
these emissions should be considered when determining the total air impacts for a thermal
desorption unit. Fugitive emissions from the desorber system must be calculated on an ad
hoc basis. It is assumed in Equation 2 that no control device is present, so any combustion
gases or PICs from fume incinerators would require further modeling.
Site-specific field data must be collected (e.g., during the RI/FS) to provide the input
data necessary to generate reasonably accurate estimates of air emissions. The minimum
field data required are knowledge of the specific contaminants present in the soil to be treated
and the average contaminant concentration.
The preferred source of input data for Equation 2 is field measurements for the
thermal desorption system of interest. Some of this information may be obtained from the
thermal desorption vendor or from design specification documents. The removal efficiency
(i.e., percent volatilized) of the thermal desorption unit for various compounds will vary.
The operating temperature and residence time of the process unit will obviously affect the
amount of volatilization. In addition, the moisture content of the waste material and the
concentration range will also influence the fraction of a given organic contaminant that is
volatilized. Field test data should be obtained to estimate this parameter. Once the thermal
desorption unit is in operation, stack sampling of emissions from the system can be
performed to confirm the emission estimates.
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If the feed rate or the amount of volatilization is not available from other sources,
then the defaults values listed in Table 1 can be used. As shown in Table 1, the percentage
of a given compound that is volatilized is somewhat dependent on its vapor pressure.
Organic compounds can be divided into VOCs and semi-volatile organic compounds
(SVOCs) based on their vapor pressure at 25°C:
VOCs = Vapor pressure .>. 1 mm Hg
SVOCs = Vapor pressure < 1 mm Hg.
Vapor pressure data for organic compounds can be found in Appendices A and B.
The VOC control devices generally will reduce the emissions by one to two orders of
magnitude. Equation 2 can be modified to account for the effectiveness of any control
device by adding the following term:
(1 - CE/100)
where CE - Control efficiency (%).
ESTIMATION OF PARTICULATE MATTER AIR EMISSIONS
The equations presented in this section were developed by Eklund, et al.4 and
modified slightly by IT Corporation to estimate the uncontrolled PM emission from thermal
desorbers8. In actual practice, however, control devices for paniculate matter are almost
always used. Most of the available thermal desorption systems utilize baghouses or cyclones
for PM control. Baghouses are especially efficient in removing paniculate matter from off-
gas streams.
The procedures for estimating emissions outlined below may be useful for estimating
the required removal efficiency of a proposed control device or for estimating the size and
cost of a control device capable of achieving a given removal efficiency. Information on the
effectiveness of various control devices is available from various sources5-*'9'10.
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Table 1
Default Values for VOC Emissions Model for Thermal Desorption Units
Parameter
Feed rate
% Volatilized
Symbol
FT
V
Units
kg/hr
%
Default Value
27,200
Desorber Temperature
of200-600°F
VOCs/BTEX 99
SVOCs/PNAs 90
THC 95
PCBs 50
Desorber Temperature
of600-1000°F
VOCs/BTEX 99.99
SVOCs/PNAs 99
THC 99.9
PCBs 99
Expected Range
2700 - 90,800
Reference
5
7
7
Other Parameters of Possible Interest*
Mass of soil to be treated M
Residence Time RT
kg
min
4.5x10* - 2.3xl07
3-70
5
5
These parameters may be used to find the treatment rate, FT, using:
FT = M * 60 / RT
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Uncontrolled PM Emission Rate
The paniculate matter emissions from thermal desorption units can be estimated from
a simple mass-balance formula which gives a conservative estimate:
ER - (0.18XQ) (Eq. 3)
where ER = PM Emission rate (g/sec);
0.18 = PM loading in stack emissions (g/dscm); and
Q = Exit gas flow rate (dscm).
The maximum allowable PM emissions under RCRA regulations is 0.18 g/dscm. This limit
is not always achieved in actual operation, however3. Typical levels of controlled emissions
can be estimated by replacing the PM loading value of 0.18 g/dscm in Equation 3 with a PM
loading concentration term, CPM. Default values for this term are given later in this section
(see Table 2).
Uncontrolled Emission Rate for Metals
If the dust is contaminated, the PM emissions from Equation 3 may be translated to
emissions rates of the contaminant using Equation 4. In general, the paniculate matter at a
site will contain a higher fraction of metal species than the bulk soil at the site; in other
words, the paniculate matter is enriched with the metals. 10
- 0.278 F
where ER = Emission rate (g/sec);
0.278 = Conversion factor (g/sec / kg/hr);
F,,, = Feed rate of metal (kg/hr); and
?F = Partitioning factor of metal (%).
The feed rate of elements in the waste stream can be calculated from the total feed
rate and the concentration of metal species in the soil using Equation 5.
(Eq. 5)
10
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where FT = Total feed rate (kg/hr);
C. = Concentration of metal in the bulk soil 0*g/g); and
10* = Conversion factor (g//tg)-
The minimum field data required to estimate emissions from thermal desorption systems are
the specific contaminants present in the material and their average and maximum
concentrations in the soil. Values for the flow rate of material to the thermal desorption
unit, exhaust gas flow rate, and the efficiency of any control devices may be obtained from
design specification documents or from field measurements. If this information is not
available from other sources, default values for use with Equations 3 and 5 are presented in
Table 2. Table 3 contains default values for metal partitioning factors for soils for use in
Equation 4.
These equations are meant to predict the behavior of thermal desorption systems used
at "typical" Superfund sites. If a particular site has soil with a high silt content, or other
unusual condition, the accuracy of these models may be affected. It is prudent to always
monitor actual field emissions to verify the model predictions. The PM control devices
generally will reduce the emissions by one to two orders of magnitude. Equations 3 and 4
can be modified to account for the effectiveness of any control device by adding the
following term:
(1 - CE/100)
where CE = Control efficiency (%).
ESTIMATION OF AMBIENT AIR CONCENTRATIONS
Estimates of short-term, worst-case ambient concentrations should be obtained by
using site specific release parameters in the EPA's TSCREEN model11. Estimates of long-
term concentrations should be obtained by using EPA's Industrial Source Complex (ISCLT)
model. Here, for simplicity, the annual average estimates are derived by multiplying the
short-term estimate obtained from the TSCREEN model, by a conversion factor to account
for variations of wind direction over time. This approach results in a higher estimate of the
annual average concentration than if the ISCLT model, with site specific data, is used.
11
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Table 2.
Default Values for PM Emissions Model for Thermal Desorption Units
..:, .';.:: Parameter .
Feed Rate
Exit Gas Flow Rate*
PM Loading in Stack Emissions
Symbol
FT
Q
Cm
Units
kg/hr
mVsec
g/dscm
g/dscm
Default Value
27,200
8.8
0.46b
0.08C
Expected Range
2,700 - 90,800
1.8 - 16.2
0.01-0.17
Reference
5
-
5
5
Assumes dry standard m3/sec (20°C, 1 atm)
bAsphalt plant with controls
Notary dryer with controls
Table 3.
Partition Factors for Metals in Soils
Parameter
Metal Partitioning Factor
Symbol
PF
Units
%
Default Value
100
20
10
Metal
Mercury
Lead
Beryllium
Chromium
Copper
Iron
Zinc
Reference
7
12
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Typical remediation scenarios for rotary dryers and asphalt aggregate dryers are given
in Table 4, and typical scenarios for thermal screws are given in Table 5. The scenarios
were based on information obtained from a review of the existing literature and conversations
with manufacturers of thermal desorption units. The worst-case, short-term downwind
dispersion of emitted gases from each of these scenarios for an emission rate of 1 gram per
second, is illustrated in Figures 3, 4, and 5. Of the variables listed in Tables 4 and 5, the
stack height and the exhaust gas velocity and temperature are used to estimate the downwind
dispersion.
Figures 3 and 4 illustrate the downwind dispersion for rotary dryer and asphalt
aggregate dryer thermal desorption units. The results are presented for two exhuast gas
temperatures. The higher temperature shown in Figure 3 is typical for thermal oxidation as
an off-gas treatment process and the lower temperature shown in Figure 4 represents off gas
that has been cooled by a heat exchanger, a quench chamber, or a scrubber. Generally,
baghouses require gas temperatures of less than 150°F. Figure 5 illustrates the downwind
dispersion of emitted gases for thermal screws. The exit gas from thermal screw thermal
desorption units is generally around 21 °C because condensation is the most common off-gas
treatment process for these units.
The curves in all figures were calculated according to the following assumptions: 1)
the combined emission rate is 1 gram per second; 2) a flat terrain without any structures
near the desorption unit; 3) the emission plume is of low, positive buoyancy; 4) the stack is
the only downwash structure; and 5) the receptors are at ground level. The third-order
inflection points in the curves are an artifact of the model. Ideally, the curves should follow
a smooth decay from peak values.
Figures 3, 4, and 5 can be used to estimate the maximum hourly ambient air
concentration for an emission rate of 1 gram per second at selected distances downwind from
a thermal desorption unit. The dispersion factor, in micrograms/m3 per g/sec, obtained from
13
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Table 4.
Example Scenarios for Rotary Dryers and Asphalt Aggregate Dryers
.: . Parameter :'- ';" -
Feed rate (soils)
Gas Volume*
Stack Height
Stack Diameter
Exit Gas Velocity*
Exit Gas Temperature1'
Units
kg/hr
mVmin
cfm
m
m
m/sec
°C
System
Small
7,300
110
4,000
9.1
0.4
15
: Medium
27,200
530
18,700
7.6
1.3
6.7
Large
59,000
7,400
26,000
6.1
1.3
9.3
Gas volume and exit velocity assume dry standard conditions at 7% Oj (20 °C, 1 atm).
"Exhaust gas temperature is highly dependent on the types of control devices used. For
thermal oxidation with no off-gas cooling assume 815 °O (1500 °F). For any configuration
with off-gas cooling by a heat exchanger, quench chamber, or scrubber, assume 120 °C (250
Table 5.
Example Scenarios for Thermal Screws
Parameter
Feed Rate (Soils)
Gas Volume*
Stack Height
Stack Diameter
Exhaust Gas Velocity
Exhaust Gas Temperature11
Units
kg/hr
nrVmin
scfm
m
m
m/sec
°C
System
Small
3200
3.7
130
6.7
0.2
2.0
21
Large
8200 '
24.8 i
875 I
4.6
0.2
13.2
21 1
*Gas volume and exit velocity assume dry standard conditions at 7% Oj (20 °C, 1 atm).
bAssumes off-gas treatment is condensation, which is typical for thermal screws.
14
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Figure 3, 4, or 5 can be substituted into Equation 6 to estimate the maximum hourly ambient
concentration and into Equation 7 to estimate the annual average ambient air concentration
for a given downwind distance. Since TSCREEN provides maximum short-term estimates,
the factor of 0.08 in Equation 7 is used to convert the short-term estimate to an annual
average estimate. This assumption has been recently revised by the U.S. EPA; it is still
under review by EPA, however, and may be subject to further change.
C. = (ER)(F) (Eq. 6)
C. - (ER)(F)(0.08) (Eq. 7)
where Cm = Maximum hourly ambient air concentration Og/m3);
C, = Annual average ambient air concentration (pig/m3);
ER = Emission rate (g/sec); and
F = Dispersion factor from Figure 3, 4, or 5 Gtg/m3 / g/sec).
ESTIMATION OF HEALTH EFFECTS
Cancer Effects Due to Long-Term Exposure
Potential cancer effects resulting from long-term exposure to substances emitted to the
air can be evaluated using inhalation unit risk factors. Inhalation unit risk factors are a
measure of the cancer risk for each /xg/m3 of concentration in the ambient air. They are
available on EPA's Integrated Risk Information System (IRIS), the U.S. EPA's preferred
source of toxicity information. User Support can be contacted at (513) 569-7254. The next
best source of inhalation unit risk factors is EPA's Health Effects Assessment Summary
Tables (HEAST) which are updated annually12. Inhalation unit risk factors listed in IRIS as
of January 1993 or in HEAST (FY 1992) are given in Table 6 for 168 volatile and semi-
volatile organic compounds. This is an updated version of the same table contained in
References 2 and 3. Similar information is given in Tables 7 and 8 for selected semi-volatile
organic compounds and metais, respectively. Emission data for thermal desorpuon amis
reviewed to develop the list of SVOCs given in Table 7.
18
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Equation 8 can be used to estimate the cancer risk at a specified distance downwind of
the thermal desorption unit. Cancer risk is a measure of the increased probability of
developing cancer in a lifetime as a result of the exposure in question. Equation 8 assumes
continuous exposure (24 hours/day, 365 days/year for 70 years) to the estimated annual
average concentration in air.
R = (CJ(IUR) (Eq. 8)
where R = Cancer risk from long-term exposure to a specific compound in air
(dimensionless);
C, = Annual average ambient concentration, from Equation 7 (/ig/m3); and
IUR = Inhalation unit risk factor, from Table 6, 7, or 8 (/ig/rn3)"1.
If the source operates for less than 70 years, multiply C. by x/70, where x is the
expected operating time of the source in years before using Equation 8. If more than one
contaminant is present, the cancer risks for each contaminant can be summed to derive the
total cancer risk at a specified distance downwind of the source.
Non-Cancer Effects Due to Long-Term Exposure
Non-cancer effects can be evaluated by using chronic inhalation reference
concentrations (RfCs). An inhalation RfC is an estimate (with uncertainty spanning perhaps
an order of magnitude) of continuous exposure of the human population to contaminants in
the air that is likely to be without appreciable risk of deleterious effects during a lifetime.
During the past three years, the EPA has become increasingly active in the development of
chemical specific RfCs.
If inhalation RfCs were not available from either IRIS or HEAST, then chronic oral
reference dose (RfD) data (in mg/kg/day) were multiplied by 70 kg (average body weight of
an adult), then divided by 20 mVday (average adult inhalation rate), and finally multiplied by
1000 pg/mg to derive a value in pg/m3. -This methodology was selected as the best available
30
-------
approach for this screening document. The EPA, however, does not condone derivation of
RfCs from data for other routes of exposure on a chemical specific basis.
The EPA considers the minimum basis for the derivation of an RfC to be a properly
conducted inhalation study that examines portal of entry effects. Portal of entry effects are
particularly important with respect to inhalation. There are many known cases where
respiratory effects due to inhalation exposure are much more severe than effects resulting
from equivalent oral doses. Therefore, ambient air action levels based on extrapolated oral
data should be used cautiously since there may be significant differences between the oral
and inhalation exposure pathways.
For compounds lacking RfC or RfD values, action levels were based on occupational
exposure levels recommended by the Occupational Safety and Health Administration
(OSHA)13 and the American Conference of Governmental Industrial Hygienists (ACGIH)14.
The action levels were estimated by using the lower of the OSHA Permissible Exposure
Limit-Time Weighted Average (PEL-TWA) level (or ceiling value) or the ACGIH Threshold
Limit Value-Time Weighted Average (TLV-TWA) level (or ceiling value). The lower value
was divided by 1000 to compensate for differences between occupational and residential
exposures. It should be noted that occupational exposure levels are not intended for
evaluation or control of community air pollution or in estimating the toxic potential of
continuous, uninterrupted exposures. Therefore, ambient air action levels based on
occupational exposure limits are not precise distinctions between safe and dangerous ambient
air concentrations, nor are they necessarily indices of toxicity.
Long-term ambient air action level concentrations for non-carcinogens based on RfCs,
extrapolated RfDs and occupational exposure levels are also listed in Tables 6, 7, and 8.
The action levels are in units of /xg/m3 to facilitate comparison to the ambient air
concentrations estimated from Equation 7.
31
-------
-------
Short-Term Exposure
The short-term (one hour) action levels, in /xg/m3, are presented in the last column of
Tables 6, 7, and 8. The listed values were obtained by dividing the lowest of (1) the OSHA
PEL-TWA or (2) the ACGIH TLV-TWA (or ceiling limits if 8-hour averages are not
available) by 100. Division by 100 accounts for variations in human sensitivity (occupational
levels are designed to protect healthy adult workers) and for uncertainties in using
occupational exposure levels to derive ambient air action levels.
The occupational exposure levels on which the short-term action levels are based are
subject to change. To check the values in Tables 6, 7, and 8 (or to derive values for
compounds not listed in the tables), determine the current OSHA PEL-TWA values by con-
sulting 29 CFR Section 1910 and the most recent edition of the ACGIH publication entitled
Threshold Limit Values and Biological Exposure Indices.
The short-term action levels listed in Tables 6, 7, and 8 can be compared directly
with the estimated maximum hourly ambient air concentrations obtained by using Equation 6
and Figures 3, 4, and 5. Use of the short term action levels should consider that no EPA
accepted method exists to determine the short-term concentrations of airborne chemicals
acceptable for community exposure.
EXAMPLE
The following steps illustrate the use of the procedures presented in this document.
The goal is to estimate the maximum hourly and annual average ambient air concentrations at
the nearest receptor to a thermal desorption process and compare these values to the action
level concentrations listed in Tables 6, 7, and 8.
Step 1 First, collect all necessary information. For this example, assume a
site that has approximately 10,000 m3 of soil contaminated with
benzene, toluene, and lead at concentrations in the soil at 1.0 /*g/g,
24.0 jig/g, and 100 /tg/g, respectively. A rotary dryer will be used and
the treatment rate is estimated to be 7.5 tons/hr (6800 kg/hr). The
32
-------
amount of benzene and toluene volatilized in the thermal desorption
process is 99.48% and 99.98%, respectively. The exhaust gas flowrate
is assumed to be 110 mVmin (1.83 mVsec). The thermal desorption
process is expected to be in continual operation for 90 days (7.776 x
106 seconds). The bulk density of the soil at the site averages about
1.5 g/cm3. The nearest off-site downwind receptor is 400 meters away.
Step 2 Estimate the total emissions potential for the site. Using Equation 1,
the average long-term emission rate of benzene would be:
ER = . . L93 x KT'g/sec
(7.776 x 106)
The average long-term emission rate for toluene is 0.046 g/sec, and for
lead is 0.19 g/sec. All these rates assume that 100% of the
contaminant is lost to the atmosphere. This is obviously an overly
conservative assumption for lead, since it will not partition completely
to the gas-phase. The estimate for VOCs is based on no controls
present, so it too may be overly conservative.
Step 3 Estimate the VOC emission rate of each compound from the thermal
desorption operation. The appropriate data are used with Equations 2
and 3. For benzene, the emission rate would be:
1/1000 » 6800/3600 * 99.48/100 = 1.88x10° g/sec
The emission rate for toluene is 4.51x10"* g/sec.
Step 4 Estimate the PM and metal emissions. The particulate matter emissions
are the emissions from the thermal desorption unit. From Equation 3,
the emissions are:
ER = (0.18)(1.83) = 0.33 g/sec
The feed rate of lead can be determined using Equation 5:
PI,., = (6800)(100)(10^) = 0.680 kg/hr
The elemental feed rate can then be used along with a partition factor
of 20% from Table 3 to calculate the emission rate of lead using
Equation 4:
= (0.278)(0.680)(20/100) = 3.78xl(J-2 g/sec
33
-------
Step 5 Compare the estimated emission rates from Steps 3 and 4 to those from
Step 2. The comparison is:
; i Compound
Benzene
Toluene
Lead
Equation 1
Emission Rate
(g/sec)
1.93xlO-3
4.6xlO-2
1.9X10-1
Equation 2
Emission Rate
(g/sec)
l.SSxlO-3
4.51xlO-2
3.78xlO-2
The VOC emission rates estimated using Equation 2 are essentially the
same as the total emissions potential for the site. This is expected since
the default value used for the percent of the contaminant that is
volatilized is nearly 100%.
»
The estimated emissions of lead are below the total amount of lead
present, which is as expected since far less than 100% of the lead
present would be expected to become airborne. The estimated VOC
and PM emission rates are used below to assess ambient air
concentrations.
Step 6 Estimate the downwind ambient air concentrations. From Figure 3, the
maximum hourly ambient air concentration at a distance of 400 meters
for a small unit is approximately 20 ug/m3 per g/sec emission rate.
This corresponds to an annual average dispersion factor of 1.6 ug/m3
per g/s (20 x 0.08 = 1.6). Using Equation 6, the hourly average
ambient air concentration for benzene would be:
Cm = (1.88xlO-3)(20) = 0.038 ug/m3
Using Equation 7, the annual average air ambient concentration for
benzene would be:
C. = (0.038)(0.08) - 0.0030 ug/m3
The ambient air concentrations estimated from Equations 6 and 7 are
presented in Table 9.
Step 7 Compare the downwind concentrations to the action level ambient air
concentrations. The short-term and long-term action levels from Tables
6 and 8 for the compounds of interest are presented in Table 10. Of
the estimated maximum hourly ambient concentrations, only lead is at
all close'to the action level. The annual average ambient
concentrations show also that only lead is approaching the action level.
34
-------
Table 9.
Estimated Emission Rates and Ambient Air Concentrations
"' .':...-..' ' - '"'
Benzene
Toluene
Lead
Paniculate Matter
Soil Concentration
For Example
Problem Gtg/g)
1.0
24.0
100
-
Emission Rate
(g/s)
1.88 x 10-3
4.55 x 10-2
3.79 x 10-2
0.33
Ambient Concentrations
fctg/nr5)
Maximum
1-Hour
0.038
0.91
0.76
~
Annual
Average
0.0030
0.073
0.061
Table 10.
Action Level Concentrations
, '-. '-'
Benzene
Toluene
Lead
Action Levels /*g/mj
Short-Term
320
3,750
1.5
Long-Term
0.121
- 4002
0.153
'Based on 10*, 70-year risk.
2Based on reference dose concentrations (RfCs).
3Based on occupational exposure limits
35
-------
Step 8 Document the results of the air pathway analysis arid define a future
course of action. Based on these screening level results, a more
rigorous analysis of the air impacts is not necessary. The air impacts
should be re-examined, however, when the site-specific data become
available, if the inputs differ from the default values. Also, it would be
adviseable to perform an ambient air monitoring program during
remediation to document the actual worker and community exposures.
CONCLUSIONS
The procedures presented here are not intended to negate the need for rigorous
analyses that consider site specific meteorological conditions and the health effects of the
specific compounds involved. Although the procedures are based on what is typical and
reasonable for cleaning up Superfund sites, the underlying assumptions need to be kept in
mind. For example, emission models assume typical operating conditions, dispersion models
assume Gaussian distribution of the plume, and many of the health levels are not endorsed by
the Environmental Protection Agency. EPA's Regional Toxicologist should be contacted for
general lexicological information and technical guidance on evaluation of chemicals without
established toxicity values.
REFERENCES
1. Eklund, B., S. Smith, and M. Hunt. Estimation of Air Impacts For Air
Stripping of Contaminated Water. EPA-450/1-91-002. May 1991.
2. Eklund, B,, S. Smith, P. Thompson, and A. Malik. Estimation of Air
Impacts For Soil Vapor Extraction (SVE) Systems. EPA Contract No. 68-D1-
0031, WA13. December 2, 1991.
3. Eklund, B., S. Smith, and A. Hendler. Estimation of Air Impacts for the
Excavation of Contaminated Soil. EPA-450/1-92-004. March 1992.
4. Eklund, B. and C. Albert. Models for Estimating Air Emission Rates From
Superfund Remedial Actions. EPA-451/R-93-001. March 1993.
5. Eklund, B., P. Thompson, A. Inglis, and W. Dulaney. Air Emissions From
the Treatment of Soils Contaminated with Petroleum Fuels and Other
Substances. EPA-600/R-92-124. July 1992.
36
-------
6. Troxler, W.L., JJ. Cudahy, R. P. Zink, and S.I. Rosenthal. Thermal
Desorption Guidance Document for Treating Petroleum Contaminated Soils.
EPA Draft Report to James Yezzi, U.S. EPA, Edison, NJ. January 1992.
7. de Percin, Paul (EPA). Personal Communication from Paul de Percin to Bart
Eklund of Radian Corporation. August 1992.
8. n Corporation. Screening Procedures For Estimating the Air Impacts of
Incineration at Superfund Sites. EPA Contract No. 68-02-4466, WA 91-77.
September 1991.
9. Eklund, et al. Control of Air Toxics at Superfund Sites. EPA/625/R-92-012.
U.S. EPA, Center for Environmental Research Information, Cincinnati, OH.
November 1992.
10. Eklund, B., et al. Air/Superfund National Technical Guidance Study Series,
Volume ffl: Estimation of Air Emissions from Cleanup Activities at
Superfund Sites. Report No. EPA.-450/1-89-003. NTIS PB89 180061/AS.
January 1989.
11. U.S. EPA. A Workbook of Screening Techniques for Assessing Impacts of
Toxic Air Pollutants. EPA-450/4-88-009. September 1988.
12. Health Effects Assessment Summary Tables (HEASTl. U.S. Environmental
Protection Agency, Wash. D.C., 1990, OERR 9200.6-303(92-1), NTIS No.
PB91-92199, March 1992.
13. 29 CFR ch. XVII. Subpart Z. Section 1910.1000. July 1. 1990.
14. 1992-^993 Threshold Limit Values for Chemical Substances and Physical
Agents and Biological Indices. American Conference of Governmental
Industrial Hygienists, Cincinnati, Ohio, 1992.
37
-------
APPENDIX A
PHYSICAL PROPERTY DATA
FOR SELECTED ORGANIC COMPOUNDS
(For compounds in Table 5 of the report)
Source: Reference 4
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APPENDIX B
PHYSICAL PROPERTY DATA
FOR SELECTED SEMI-VOLATILE ORGANIC COMPOUNDS
(For compounds in Table 6 of the report)
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