l ('pin1 r
TP360
.E824
1988
United States Office of the Administrator
Environmental Protection Science Advisory Board Apr!,
Agency Washangton. D. C. 20460 Final
Report of the Environm*
Effects, Transport and
Committee
SABEETFC882
Evaluation of Scie
Issues Related
i
Municipal Waste Co
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UNITED STATES ENVIRONMENTAL PROTECTION AGENCY
WASHINGTON. D C 20460
April 26, 1988
The Honorable Lee M. Thomas TMr °"!"°1
THE ADMlPMaTW
Administrator
U.S. Environmental Protection Agency
401 M. Street, S.W.
Washington, D.C. 20460
Dear Mr. Thomas:
The Municipal Waste Combustion Subcommittee of the Science
Advisory Board's Environmental Effects, Transport and Fate
Committee has completed its report entitled "Evaluation of
Scientific Issues Related to Municipal Waste Combustion". The
evaluation was initiated at your request, along with two other
charges related to municipal waste combustion, all of which are
now complete. The Subcommittee began gathering information in
April of 1986 and has achieved consensus on a number of
conclusions and recommendations in the intervening time. These
findings are summarized below.
The Subcommittee recognizes that regardless of the
technologies a society employs to reduce or dispose of municipal
waste, there will always be a degree of residual risk to both the
public and the environment. Members of the Subcommittee do not
attempt to evaluate all of the issues that municipalities must
weigh as they consider incineration as a waste management option,
but instead strive to inform citizens and decision makers of
current risks and uncertainties accompanied by recommendations
for increasing knowledge to reduce such risks and uncertainties.
The report examines a series of generic scientific issues that
policy makers must address in an order that reflects the movement
of potential pollutants through and from a municipal waste
combustion facility. In particular, such issues as combustor
feedstocks; the design and operation of municipal incinerators;
the performance of incinerators with various degrees of pollution
control equipment; stack emissions; ash disposal; operator
training and certification; environmental transport and fate of
combustion residues and by-products; pathways to and potential
for exposures of humans and ecosystems; and potential public
health and environmental effects are addressed.
The Subcommittee concludes that, in general, the performance
side of the technology, including design and pollution control,
has greatly improved, and is likely to continue to improve. In
the Subcommittee's judgment, two critical needs at present are
expanded and more rigorous operator training requirements, and
U S. Environmental Protection Agency
Region 5 Library (PL-12J)
77 West Jackson Blvd., 12th Floor
Chicago, IL 60604^590
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data collection and analyses aimed at enabling scientists and
decision makers to better estimate health and environmental
exposures from this technology.
Since technological improvements have created highly
efficient stack emission control systems, fly ash with relatively
smaller particle size and increased concentrations of pollutants
such as heavy metals and trace organics has resulted. The
Subcommittee recommends that EPA develop a series of alternative
techniques for: 1) analyzing ash samples and the compounds
present in ash extracts; 2) assessing the toxic potential of ash;
and 3) managing ash disposal.
The Subcommittee also recommends that the potential for
health and environmental effects be addressed by developing a
more comprehensive data base through field studies. Little
information is presently available on the fate of chemicals from
MWC facilities, and information is needed to estimate deposition
of particulate and gaseous emissions, to model transport and
diffusions operations, and to understand environmental
transformation and dispersal of technology by-products that may
pose risk.
i
Finally, the Subcommittee recommends that EPA assist local
decision makers and the public by developing ways to collect and
analyze data that will allow more informed choices regarding the
management of municipal solid waste. Approaches should be
developed for assessing exposure and risk and these tools should
be transferred to 'the parties responsible for making the
decisions. Appropriate tools may include guidance for evaluating
waste management options, and means for comparing exposure and
risk between available options.
i
The Subcommittee appreciates the opportunity to conduct this
scientific review. We request that the Agency formally respond
to the scientific advice transmitted in the attached report.
Sincerely,
\\
Norton Nelson, Chairman
Executive Committee
Science Advisory Board
Rolf" Hartung, Clrslrman
Municipal Waste
Combustion Subcommittee
Enc, cc: A. James Barnes
J. Winston Porter
Vaun Newill
Donald E. Barnes
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SAB-EETFC-88-25
EVALUATION OF SCIENTIFIC ISSUES
RELATED TO MUNICIPAL WASTE
COMBUSTION
REPORT OF THE MUNICIPAL WASTE COMBUSTION
SUBCOMMITTEE
ENVIRONMENTAL EFFECTS, TRANSPORT AND FATE COMMITTEE
SCIENCE ADVISORY BOARD
U.S. ENVIRONMENTAL PROTECTION AGENCY
APRIL 1988
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NOTICE
This report -has been written as a part of the activities of
the Science Advisory Board, a public advisory group providing
extramural scientific information and advice to the Administrator
and other officials of the Environmental Protection Agency. The
Board is structured to provide a balanced expert assessment of
scientific matters related to problems facing the Agency. This
report has not been reviewed for approval by the Agency, and,
hence the contents of this report do not necessarily represent
the views and policies of the Environmental Protection Agency,
nor of other agencies in the Executive Branch of the Federal
government, nor does mention of trade names or commercial
products constitute endorsement of recommendation for use.
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U.S. ENVIRONMENTAL PROTECTION AGENCY
SCIENCE ADVISORY BOARD
ENVIRONMENTAL EFFECTS, TRANSPORT AND FATE COMMITTEE
MUNICIPAL WASTE COMBUSTION SUBCOMMITTEE
Chairman
Dr. Rolf Hartung, Professor of Environmental Toxicology, School
of Public Health, University of Michigan, Ann Arbor, Michigan
48109
Members
Dr. Martin Alexander, Professor, Department of Agronomy, Cornell
University, Ithaca, New York 14853
Dr. Stanley Auerbach, Environmental Sciences Division, Oak Ridge
National Laboratory, Oak Ridge, Tennessee 37831
Mr. Allen Cywin, P.E., 1126 Arcturus Lane, Alexandria, [Virginia
22308
Dr. Walter Dabberdt, National Center for Atmospheric Research,
P.O. Box 3000, Boulder, Colorado 80307-3000 '
Dr. Robert Huggett, Professor of Marine Science, (Virginia
Institute of Marine Science, School of Marine Sciences,1 College
of William and Mary, Gloucester Point, Virginia 23062 i
Mr. Alfred Joensen, Associate Professor, Department of Mechanical
Engineering, Iowa State University, Ames, Iowa 50011
i
*Dr. Renate Kimbrough, Centers for Disease Control, Center for
Environmental Health, 1600 Clifton Road, Atlanta, Georgia! 30333
Mr. Raymond Klicius, Environment Canada, 351 St. Joseph's
Boulevard, Hull Quebec, Canada K1AOE7
Dr. William Lowrance, Senior Fellow and Director, Life sciences
and Public Policy Program, The Rockefeller University, 1230 York
Avenue, New York, New York 10021 '
1
Dr. John Neuhold, Professor of Fisheries and Wildlife, Cc-llege
of Natural Sciences, Utah State University, Logan, Utah 84322
Dr. Adel Sarofim, Department of Chemical Engineering,
Massachusetts Institute of Technology, Cambridge, Massachusetts
02139
Mr. Charles 0. Velzy, Charles R. Velzy Associates, 355 Main
Street, Armonk, New York 10504
*Dr. Kimbrough served on the Subcommittee until May 11, 1987
ii
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Staff Director
*Dr. Donald Barnes, U.S. EPA, Science Advisory Board (A101), 401 M
Street, S.W., 1145 WT, Washington, D.C. 20460
Executive Secretary
Ms. Janis C. Kurtz, U.S. EPA, Science Advisory Board (A101-F),
401 M Street, S.W., Room 508, Washington, D.C. 20460
Staff Secretary
Mrs. Lutithia Barbee, U.S. EPA, Science Advisory Board (A101-F),
401 M Street, S.W., Room 508, Washington, D.C. 20460
*Dr. Terry F. Yosie served as Director until February 28, 1988
iii
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Table of Contents
Page Number
I. Executive Summary 1
A. Municipal Waste Combustion: Process and 3
Technology
B. Operator Training and Certification 5
C. Air Pollution Control Technologies 6
D. Ash Characterization and Disposal 8
E. Environmental Transport and Fate 9
F. Assessment of Risk to Public Health and 11
the Environment
II. Introduction 12
A. Charge and Scope of the Review 12
B. Major Assumptions and Limitations of the 14
Review
III. The National Municipal Waste Management Problem 16
IV. The Process and Technology of Incinerating 20
Municipal Waste
A. Feedstock 20
B. The Incineration Process 21
C. Description of Combustion Systems 22
1. Mass Burning of Unprocessed Municipal 23
Waste
2. Modular/Starved Air Burning of 25
Unprocessed Municipal Waste
3. Dedicated Stoker Boilers Burning 25
Coarsely Processed Refuse
4. Cofiring of Coal and Municipal Solid 29
Waste Burning Processed Refuse
D. Combustion System Design and Operating 29
1. Stages of Combustion Operation, Old and 29
New Plant Designs
2. Emissions from the Combustion Chamber 31
a. Acid Gases 31
b. Fly Ash and Residues 32
c. Trace Metals 33
d. Organic Compounds 33
E. Operator Training and Certification 35
F. Conclusions and Recommendations 36
1. Conclusions 36
2. Recommendations 37
V. Performance of Air Pollution Control Technology 40
A. Potential Air Pollutants of Concern 40
B. Description of Air Pollution Control Systems 40
1. Electrostatic Precipitators 40
2. Fabric Filters 44
3. Scrubbers 44
iv
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V. Performance of Air Pollution Control Technology
(Continued)
C. Historical Perspective of Air Pollution 46
Control for MSW Incinerators
D. Air Pollution Control Experience 50
1. Particulates 50
-2-.- Metals 51
3. Acid Gases 53
4. Trace Organics 54
5. Conventional Combustion Gases 56
6. Ash Disposal 57
7. Ongoing Research and Development 57
E. Conclusions and Recommendations 58
1. Conclusions . 58
2. Recommendations 59
VI. Environmental Transport and Fate 60
A. Dispersal and Persistence in Environmental 60
Media
1. The Atmosphere 61
2. The Terrestrial Environment 67
3. The Aquatic Environment 70
B. Conclusions and Recommendations 73
1. Conclusions 73
2. Recommendations 73
C. Transport and Fate of MWC Ash 74
1. Considerations 74
2. Conclusions and Recommendations 76
VII. Potential Exposure and Effects- 78
A. Environmental Loadings 79
B. Exposures 80
1. Human Exposures 80
2. Ecosystem Exposures 81
3. Approaches for Estimating Exposures 82
C. Effects 82
1. Human Health Effects 82
2. Environmental Exposures 83
D. Conclusions and Recommendations 84
1. Conclusions 84
2. Recommendations 85
"• VIII. Concluding Perspectives 87
LITERATURE CITED 91
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APPENDICES
Page Number
A: Assessing EPA's Risk Assessment Methodology A-l
for Municipal Incinerator Emissions:
Key Findings and Conclusions
B: Dioxin Toxic Equivalency Methodology B-l
Subcommittee Report: Executive Summary
C: Review of the Municipal Waste Combustion C-l
Research Plan
D: Description of Refuse Derived Fuel (RDF) D-l
Categories
E: Glossary of Terms and Units E-l
vi
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List of Tables
Table Page Number
Table 1: Existing MWC Facilities by Design Type 19
Table 2: Current and Predicted Composition of 20
Discarded Residential and Commercial
Solid Waste
Table 3: Concentration of Stack Emission Components 52
for MWC Equipped with Scrubber/
Fabric Filters
Table 4: Scrubber/Fabric Filter Performance 55
Table 5: Distance in km where Dry Deposition 65
Depletes the Mass of a Plume
by 50 percent
Table 6: Comparative Advantages of Selected Waste 89
Disposal Options
List of Figures
Figure 1: Mass Burning Incinerator 24
Figure 2: MSW Grate 26
Figure 3: Starved Air Combustors 27
Figure 4: RDF-Fired Combustor 28
Figure 5: Electrostatic Precipitation Process • 42
Figure 6: Arrangement of Electrostatic Precipitators 43
Figure 7: Fabric Filter 45
Figure 8: Dry Scrubber 47
Figure 9: Wet-Dry Scrubber 48
Figure 10: Transport of MWC Emissions from an 62
Incinerator Facility Through the
Ecosystem
vii
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I. EXECUTIVE SUMMARY
The present problem of safely disposing of municipal solid
waste (MSW) stems from practices of modern industrial society
that emphasize storage of wastes over disposal methods that
destroy wastes or minimize them at the source. Regardless of
which technologies a society uses to reduce or otherwise dispose
of municipal solid wastes, there will always be a degree of
residual risk to the public and the environment. Incineration
offers particular environmental advantages and disadvantages as a
waste disposal option, and may be applied as a technology with
varying degrees of safety and effectiveness. Safe and effective
application requires well designed plants, state-of-the-art
pollution control equipment and appropriately skilled operating
personnel.
Because different technological options exist for municipal
waste disposal, EPA has an important task — prior to
establishing a comprehensive strategy for regulating municipal
waste combustion — to generate and evaluate data, and develop
methodologies, for assessing the relative risks resulting from
incineration and other municipal waste disposal processes. EPA
needs to perform this task to enable citizens, scientists and
public officials to compare risks across various environmental
media for each waste disposal and management option. Such
comparative analysis can provide the technical basis for
choosing among technological options.
Communities must evaluate available technological
alternatives for waste disposal and choose the technology (or
combination of technologies) that presents acceptable levels of
public health and environmental risk. Given the local
differences in waste composition, available landfill capacity,
urban and rural locations, population density, cost, and other
factors, no single disposal technology is likely to be uniformly
efficient or safe in all regions of the country. Thus, the
overall goal, for individual communities and for society in
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general, is to choose the particular technological option(s) that
is both cost-effective and presents the least risk to the
population and the environment.
The Municipal Waste Combustion Subcommittee of the
Environmental Effects, Transport and Fate Committee of the
Science Advisory Board has evaluated a series of technical issues
related to the performance of waste combustion technologies. It
has examined, in particular, such issues as combustor feedstocks;
the design and operation of municipal incinerators; the
performance of incinerators with various degrees of pollution
control equipment; stack emissions; ash disposal; operator
training and certification; environmental transport and fate of
combustion residues and by-products; pathways to and potential
for exposures of humans and ecosystems; and potential public
health and environmental effects.
Evaluating the human health and environmental impacts of
municipal waste combustion is a difficult task. This is true for
a number of reasons including: 1) difficulty in identifying
and/or obtaining a representative or "average" sample of
municipal waste; 2) variability in the conditions of combustion;
3) limited information on the identity of emitted compounds; 4)
lack of validation of transport and fate models; 5) the relative
lack of data on the environmental loadings contributed by
incinerators compared to other combustion sources (including coal
and oil fired power plants, automobiles, and wood stoves and
fireplaces); and 6) large uncertainties in estimating human
health and environmental effects from municipal incineration in
comparison to other combustion sources.
In evaluating the issues identified above, the Subcommittee
has reached the following major conclusions and recommendations:
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A. Municipal Waste Combustion; Process and Technology
o Municipal solid waste (MSW) is heterogeneous in
composition. MSW composition is heavily dependent on location,
time of year and patterns of consumption. Even simple
constituents such as moisture content may fluctuate widely^
Because of the inherent variability of the MSW feedstock, it is
difficult to predict the composition of stack emissions that may
result from combustion. Wide variations in feedstock composition
can affect combustion conditions in the incinerator furnaces, and
can cause cycles of poor combustion. Poor combustion conditions
have a direct impact on emissions. It is important to design
incinerators with state-of-the-art features that will provide
operators with the ability to accommodate wide variations in
feedstock composition to reduce the potential for poor combustion
and increased emissions.
o Organic materials containing only carbon and hydrogen are
completely combusted or burned in an oxygen-containing atmosphere
theoretically producing water vapor and carbon dioxide as the
products of combustion. Municipal solid waste, however, is not
composed entirely of organic materials or carbon and' hydrogen,
and, therefore, many other products of combustion are released to
the environment. In addition, combustion is not always complete,
resulting in release of products of incomplete combustion (PIC).
Proper or complete combustion depends not only on sufficiently
elevated temperatures but also on the residence time needed for
the materials to burn fully, and the need for turbulent
conditions in the furnace in order to achieve proper mixing of
air and the gases evolved from the burning fuel, which all vary
with the composition of the waste.
o Increased competition for the growing market for
incinerators is leading to improvements in engineering design and
especially to an increased understanding and sophistication of
the technology of combustion. Recognition of environmental
problems has also been a key factor motivating the development of
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improved combustors and emission control equipment. Earlier
designs of mass burners did not incorporate the flexibility for
controlling the location and amount of introduced combustion air,
or the sophistication of instrumentation for control of feedback
combustion air that newer plant designs provide. Thus, older
plants generally do not achieve the efficiency of combustion
attainable in modern plants.
o The Subcommittee concludes that EPA should investigate
the hypothesis that polychlorinated dibenzodioxins (PCDD) and
polychlorinated dibenzofurans (PCDF) can result from free radical
reactions that take place in fuel-rich zones of incinerator
flames. These reactions may yield polycyclic aromatic
hydrocarbons (PAH), oxygenated compounds such as phenol and
perhaps, in the presence of chlorine, some PCDD and PCDF.
These compounds may also be present in MWC feed stock since
they are by-product contaminants in a number of chemicals, most
notably chlorinated phenols and polychlorinated biphenyls (PCB).
These compounds may persist beyond combustion only if
temperatures are sufficiently cooled by excesses in local air
flow. Condensation reactions involving the chlorinated phenols,
phenol ethers, and biphenyls may also produce PCDD and PCDF.
o The design and operation of an incinerator combustion
chamber has major impact on the concentration of the pollutants
entering the air pollution control devices. In well-designed and
operated incinerators, the emissions of organic compounds
currently measured can be reduced to levels close to the limits
of detection with existing analytical methods.
Recommendations
o EPA and private vendors should fund research to gain a
better understanding of 1)" municipal solid waste composition; 2)
the affects of furnace design and operating conditions on the
combustion process; 3) the relation between inorganic and organic
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emissions and; 4) PCDD and PCDF formation. To obtain this
information, research on full-, pilot-, and laboratory-scale units
is needed.
o Well-planned field testing that evaluates different
operating conditions will generate a realistic correlation of
emissions with operations, and will provide data for establishing
emission indices. Pilot-plant and laboratory-scale testing can
be used to critically investigate hypotheses derived from field
studies. Small-scale equipment facilitates such testing because
of the greater ease of independently varying design parameters,
operating conditions, and feed composition.
o Research on the relationship between the composition of
MSW and emissions should be carried out over a wide enough
temperature range to be useful in testing the various hypotheses
for formation of PCDD and PCDF. Specific research is required
to understand post-combustion formation of PCDD/PCDF by
condensation reactions that occur as the flue gas cools in the
heat recovery process.
o Continuous monitors to detect upsets in operating
conditions should be developed. Carbon monoxide and hydrocarbons
are currently being explored as potential indicators of emissions
of PCDD and PCDF. Alternatives such as polycyclic aromatics,
detectable by their fluorescence or ultraviolet irradiation, may
be more appropriate surrogates.
B. Operator Training and Certification
o The combustion of municipal solid wastes at resource
recovery facilities is exempt from the Subtitle C requirements of
the Resource Conservation and Recovery Act, provided that the
owners or operators take precautions to ensure that hazardous
wastes are not burned. Because of this exemption, no national
policy related to operator training and/or certification is
required for municipal solid waste combustion facilities.
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However, proper operation of municipal incinerators requires a
thorough understanding of the complexities of the combustion
process. An understanding of the composition and variability of
the feedstock, fundamentals of the combustion process,
requirements and consequences of adequate emission controls,
procedures for handling upset conditions, and elements of safe
operator practice are required for efficient and effective
municipal waste disposal. At present, there are no recommended
criteria for selecting MWC staff nor is there an existing pool of
trained, experienced personnel to operate municipal waste
combustion facilities.
o New facilities planned and/or under construction are much
more complex than existing facilities. The need for proper
operation of new plants is made even more critical by the rapidly
increasing complexity of regulatory requirements and the need for
increases in capacity and efficiency of pollutant control
devices to ensure environmentally safe plant operations.
Recommendations
o EPA, the states and private vendors should support and
promote efforts to ensure that adequate training programs are
developed to provide a reservoir of technically competent
personnel to staff municipal waste combustors. Training
programs should be readily available, developed with appropriate
expertise, and tailored to the specific technology being
utilized, and the programs should lead to certification when
sufficient expertise is demonstrated.
C. Air Pollution Control Technologies
o There appear to be trade-offs between the influence of
combustor design and operation and the technology of emissions
control. For example, higher incinerator temperatures more
thoroughly destroy organic compounds; but at those higher
temperatures certain metals volatilize more readily creating the
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potential for emissions with greater metal concentrations.
Increased nitrogen oxide production can also result at higher
temperatures.
o Until about 1985, stack sampling of only very limited
scope was conducted at several scrubber/fabric filter
installations in Europe, principally for emissions of
particulates, acid gases (hydrochloric acid and sulfur dioxide)
and certain metals. These studies generated a narrow data base
of somewhat limited use, since trace organic compounds were often
not studied or, at best, only PCDD/PCDF were. Moreover, the
operating conditions of the incinerators and the identities of
the pollution control devices were often not well documented, and
the studies did not examine a range of different operating
conditions.
In 1985, Environment Canada completed an extensive testing
program providing the first thorough data base for evaluating the
performance of these control systems for a wide range of
pollutants of concern. Testing of more limited scope conducted
in Denmark paralleled these efforts. The results are encouraging
and indicate that, at appropriate temperatures, scrubber/fabric
filter technology can significantly reduce not only particulates
and acid gases, but also a range of trace organics (e.g., PCDD,
PCDF, chlorophenols, chlorobenzenes, PCB, and polycyclic aromatic
hydrocarbons), and a host of metals (including cadmium,
chromium, lead and mercury). Equipment design and operating
conditions necessary to achieve high removal of these compounds
were identified in these studies on a pilot scale.
o The scrubber/fabric filter is currently an effective
technology and the data base is growing rapidly to substantiate
its capability to reduce stack emission to low levels (in some
cases approaching the analytical detection limits for compounds
such as PCDD, PCDF, and certain metals). This conclusion does
not represent a Subcommittee endorsement that the scrubber/fabric
filter technology is the only one to use. Other technologies may
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offer equal or even better performance with less associated
capital cost. Furthermore, the capability for continued
performance at such low emission levels under a variety of
operating conditions remains to be demonstrated for full-scale
municipal solid waste incinerators.
Recommendations
o EPA and the private sector should examine the long-term
performance of air pollution control systems under a variety of
operating conditions.
o EPA's determination of "Best Available Control
Technology" should be sufficiently flexible to allow adoption of
improvements in control technologies.
D. Ash Characterization and Disposal
o The concentration of various metals and organic compounds
in ash is highly dependent on whether it is bottom grate ash,
boiler hopper ash, or ash from emission control devices. Most
compounds of concern appear to become progressively more
concentrated in the ash sampled or removed from the flue gas
stream further downstream in the process. Highly efficient stack
emissions control systems result in fly ash with relatively
higher concentrations of pollutants, e.g. heavy metals and trace
organics, since those substances tend to concentrate on the
smaller particles that are more efficiently removed by these
systems.
o EPA has considered requiring compliance with RCRA
Subtitle C if ash residues from municipal waste combustion
contain waste constituents defined as "hazardous". Alternatives
under consideration include regulating municipal incinerator ash
as non-haza-rdous waste. Leachate tests on incinerator ash
conducted by EPA and other organizations have identified lead and
cadmium levels above the Extraction Procedure (EP) toxicity
8
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limits. The EP test, originally developed by EPA for
characterizing the toxicity of hazardous waste liquids, has not
been validated as a test for municipal incinerator ash.
Recommendations
o State-of-the-art analytical chemical techniques should be
employed on ash samples, and as many of the compounds in the
extracts as feasible should be identified in order to provide a
broad-scale data base.
o EPA should re-examine the appropriateness of using the EP
test or its successor, the Toxicity Characteristic Leaching
Procedure (TCLP), to assess the toxicity of municipal incinerator
ash.
o EPA should evaluate a number of alternative techniques for
managing ash disposal from municipal incinerators. These may
involve solidification or vitrification of the waste material, or
grouting of disposal trenches, sometimes in combination with
liners. The Subcommittee recognizes that these techniques may
need to be modified to meet the particular chemical
characteristics of incinerator bottom ash and fly ash, although
the experience of disposing of fly ash from coal-fired power
plants may have relevance.
E. Environmenta1 Transport and Fate
o The atmospheric transport and fate of emissions from
municipal solid waste incinerators involve a broad spectrum of
physical and chemical processes. The processes that need to be
addressed include stack emission phenomenon, including plume rise
and downwash; plume chemistry, involving changes of physical
state and chemical reactions; atmospheric transport and
diffusion; gravitational settling; dry deposition; and wet
deposition due to in-cloud and below-cloud processes. A
scientific basis exists to support model simulations of the
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atmospheric transport of pollutants provided the emissions are
properly characterized along with the atmospheric and topographic
characteristics of each site. However, considerable uncertainty
surrounds the ability to properly simulate both wet and dry
deposition processes.
o Chemicals which are emitted to the atmosphere, or are
deposited on soil or in water, undergo a variety of
transformations. Such transformations can result in the
destruction of the parent compounds and the simultaneous
formation of one or more products. Some of the products may be
toxic. The transformation may be photochemical, may proceed in
the dark, or may be mediated by biological processes. For
assessing potential effects, the identity, quantity and rate of
destruction of the parent chemical in various environmental media
and the identity, concentration, and persistence of the products
are of great importance. Little information is presently
available on the fate of chemicals from MWC operations because of
the paucity of information on the parent compounds released and
the absence of a research program to identify and quantify
products formed from the parent substances. In some instances
where the parent compounds have been identified, scientists can
make reasonable predictions of fate based on published studies.
Recommendations
o EPA and the private sector should develop a more
comprehensive data base through field studies at several
representative MSW facilities. The data base should provide
information that can be used to estimate deposition (wet and dry)
of particulate and gaseous emissions, and also to evaluate
mathematical and fluid models of transport, diffusion and
deposition in urban and suburban environments. The data base
should include measurements of MSW emissions (stack and
fugitive), plume rise, dispersion, wet and dry deposition.
10
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F. Assessment of Risk to Public Health and the Environment
o The Subcommittee concluded in a-separate, previous report
that the proposed EPA methodology for assessing risks from
municipal incinerators through multiple environmental pathways
represents a considerable improvement over other multi-media risk
assessment methodologies previously developed by EPA and reviewed
by the Science Advisory Board (See Appendix A) . The current
methodology is more comprehensive and, in general, provides a
conceptual framework that should be expanded to other environmental
problems. The Subcommittee identified areas in this methodology
that need further consideration or improvement, including: the
inappropriate use of the Hampton incinerator facility and
associated data to represent typical mass burn technology; the
failure to use data from current best available control
technology facilities for model validation; separate treatment of
particulate and gaseous emissions and their fate, i.e. downwash;
the need to use best available kinetics in predicting soil
degradation; exposure resulting from the disposal of ash; over
emphasizing the maximally exposed individual (MEI) concept; and
the treatment of plant (and herbivore) exposure.
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II. INTRODUCTION
A. Charge and Scope of the Review
At the request of the Administrator of the U.S.
Environmental Protection Agency (EPA), the Science Advisory Board
(SAB) Executive Committee agreed on April 23, 1986, to review a
number of scientific issues related to the incineration of
municipal wastes. The Executive committee assigned the
responsibility for conducting the review to its Environmental
Effects, Transport and Fate Committee which, in turn, established
a Municipal Waste Combustion Subcommittee.
The Subcommittee's review encompasses current municipal
waste incineration technologies, the combustion process, and
emissions to the atmosphere, including associated air pollution
control equipment. In addition, it covers such issues as ash
disposal, transport and fate of process residues, and assessment
of potential effects on human and ecological receptors. The
Subcommittee recommended research to reduce scientific
uncertainties associated with incineration technologies.
The Municipal Waste Combustion Subcommittee reviewed several
separate documents prepared by EPA on aspects of the municipal
waste combustion problem. On November 10-11, 1986, the
Subcommittee reviewed a methodology jointly prepared by the
Office of Air Quality Planning and Standards (OAQPS) and the
Environmental Criteria and Assessment Office (ECAO) entitled:
Methodology for the Assessment of Health Risks Associated with
Multiple Pathway Exposure to Municipal Waste Combustor Emissions.
EPA intends that the methodology serve as a principal technical
basis for its decision on whether to regulate municipal
combustors. EPA was required by a court settlement to publish a
decision on this issue in the Federal Register by July 2, 1987.
Because the Subcommittee desired to advise the Administrator in a
timely fashion, the review of this methodology was issued as a
separate report on April 9, 1987 (reprinted in Appendix A).
12
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During the course of the Subcommittee's review, the
Assistant Administrator for Air and Radiation, J. Craig- Potter,
requested that the Science Advisory Board review a methodology
prepared by EPA's Risk Assessment Forum entitled, "Interim
Procedures for Estimating Risk Associated with Exposures to
Mixtures of Chlorinated Dibenzo-p-dioxins and Dibenzo-p-furans
(CDD and CDF)". The purpose of this methodology is to provide
EPA with a tool for risk assessment, specifically to address the
toxicity of various congeners of CDD and CDF in relation to
2,3,7,8- TCDD. The SAB Executive Committee, recognizing the
relationship between this procedure and the issues undergoing
review by the Municipal Waste Combustion Subcommittee,
established a Dioxin Toxic Equivalency Methodology Subcommittee
to conduct a review of the former, and provided for overlapping
membership between the two Subcommittees. This enabled a joint
consideration of information pertinent to assessing risk from
exposure to CDD and CDF and municipal waste combustion. The
executive summary of the Dioxin Toxic Equivalency Methodology
Subcommittee's report is presented in Appendix B.
The Municipal Waste Combustion Subcommittee separately
reviewed a research strategy prepared by EPA's Office of
Research and Development (ORD). The ORD strategy document
entitled Draft Municipal Waste combustion Research Plan
(February 1987), is a chapter in EPA's Comprehensive Report on
Municipal Waste Incineration which was prepared for and submitted
to Congress. The Subcommittee had access to various draft
versions of EPA's comprehensive report, but reviewed only the
research plan. A summary of the Subcommittee's review of the
research plan is presented in Appendix C.
During its review, the Subcommittee held eight public
meetings and solicited scientific testimony from a number of
individuals-, groups and organizations. State and local
regulatory officials, private industrial firms, and environmental
groups presented their views on a series of issues. These issues
13
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included the evolution of municipal waste combustion
technologies, projected scenarios of the growth in demand for
municipal incinerators, problems encountered in permitting
municipal waste combustors and the potential of public health and
environmental risks resulting from the use of this technology.
The Subcommittee made site visits to operating incinerators; in
Hampton, Virginia on May 29-30, 1986, and in Baltimore, Maryland
on July 28-29, 1986.
B. Mai or Assumptions and Limitations of the Review
Because of the complexity of the scientific issues under
review, the data limitations for many of these issues, and the
time constraints for providing advice to EPA, the Subcommittee
adopted a number of assumptions and recognized several
limitations in defining its charge. They included the following:
o The Subcommittee considered but did not evaluate
information on alternatives to municipal waste combustion, such
as landfilling, recycling, and waste minimization. Nor did it
assess potential risks from these alternatives in a rigorous
manner. in principle, the subcommittee believes that MWC is one
of several acceptable waste management technigues. However, it
recognizes that some degree of risk or hazard is associated with
the application of any waste management technology.
o The Subcommittee recommends that the potential effects of
municipal waste combustion be compared with those associated with
other common combustion processes. For example, emissions from
coal-fired or oil-fired power generators, internal combustion
engines, and wood-burning stoves and fireplaces should be
compared to emissions contributed by waste combustors to better
define no antecedents refers to what respective contributions to
health and environmental risks. The Subcommittee did not compare
potential emission characteristics from the various combustion
sources in common use.
14
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o The Subcommittee report presents an evaluation of a
series of generic scientific issues that policy makers at the
national level must address. This report is not designed to
evaluate all the issues, such as optimal incinerator location,
that municipalities typically encounter as they evaluate waste
combustion as a part of their local or regional waste disposal
strategy. The issues that the Subcommittee reviewed for the
purpose of advising national policy makers may not be of equal
relevance or importance for making local decisions or site-
specific assessments.
This report analyzes issues in an order that reflects the
movement of potential pollutants from a specific incinerator
source. This analysis follows the process from combustion,
through emissions resulting from the combustion process (either
directly through the stack, or fugitive emissions), through
environmental transport and fate of emissions through various
media (e.g., air, land and water) and finally through potential
human health and environmental effects.
o The Subcommittee did not initiate or conduct any economic
analysis of alternatives for municipal waste disposal or
management.
15
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III. THE NATIONAL MUNICIPAL WASTE MANAGEMENT PROBLEM
All societies produce municipal solid waste as a by-product
of their industrial activities and consumption patterns. In
general, the larger and more complex the society, the greater and
more complex its municipal wastes. One of the key technologies
available for managing the growing amounts of garbage or trash is
incineration. This is not a new technology; incinerator plants
have been in use in both Europe and the United States for
decades.
EPA estimates that the United States generated approximately
126-159 million tons of municipal solid waste (MSW) in 1980.
Only about 6 million tons, or approximately 4 percent of such
wastes, were incinerated in approximately 100 municipal waste
combustors (MWCs) [1]. In comparison, Sweden currently
incinerates approximately 50 percent of its municipal waste and
Japan combusts approximately 70 percent.
Over 90 percent, or about 137 million tons of MSW are buried
»
in the United States each year in about 10,000 municipal, and
privately operated sanitary landfills. Currently, EPA estimates
that 12.7 million tons/year of industrial solid waste is recycled
and recovered as raw material for manufacturing. There is some
potential for waste reduction due to waste minimization efforts.
In the past decade a number of intersecting events have com-
bined to alter the nation's awareness, and the public policy
framework, regarding municipal waste management. These include:
growing amounts of municipal waste to be collected and disposed;
limitations — such as the need for greater efforts by government
to provide technology transfer and consumer awareness and, in
some areas, economic disincentives — in the current potential
for recycling waste and reducing the volume of waste generated;
shrinking landfill capacity in many areas of the country;
escalating costs for transportation and storage of municipal
wastes; and stricter controls on landfills increasing operating
16
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costs and owner's legal liability. Public health and
environmental concerns over waste/management alternatives, such
as landfilling, incineration, and ocean dumping, have also been
heightened, as evidenced by difficulties in gaining public
acceptance to new landfill or incinerator sites and concerns over
the potential for groundwater contamination. Through the
Hazardous and Solid Waste Act Amendments of 1984, Congress
declared a national policy preference for more permanent methods
of disposal, such as incineration, over the storage of wastes,
such as landfilling. In general, these factors are stimulating a
wider reliance upon incineration technologies and are encouraging
expansion of this industry.
The present municipal waste problem stems in large part from
the fact that, to date, the nation has chosen disposal methods
that favor storage of wastes over methods that favor
destruction. In the future, waste minimization and recycling can
reduce the overall volume of waste, but ultimately there is a
requirement for some form of disposal. Municipal waste combustion
currently represents a technological alternative that can reduce
the volume of waste by over 90 percent. In addition, it may
provide a source of energy recovery under certain conditions.
EPA projects significant growth in the use of municipal
waste combustion in the United States between 1985 and the year
2000. By that time the Agency estimates that as many as 311
additional MWCs may be in service with a design for total
capacity of about 252,000 tons of MSW per day. This compares
with 1985 design capacity of approximately 45,000 tons
incinerated per day in more than 100 combustors [1]. Table I
identifies currently operating incinerators by design type. Data
on facilities now being planned or built suggest to EPA that MWCs
with a design capacity of more than 1,000 tons per day will
constitute more than 50 percent of the new facilities built by
1990.
17
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During the course of its review, the Subcommittee became
aware of the many changes under way in combustion technology as
well as the improvements in emissions control technology. The
Subcommittee is also aware that other related waste disposal
technologies are under development or show promise for wider use,
given favorable economics and ease of practice. Two of these
developing approaches—preprocessing and resource recycling—have
been considered but not fully evaluated by the Subcommittee.
These approaches singly, or in combination, have the capability
to further minimize the generation of potentially hazardous
residues. Minimizing solid wastes will reduce the amount of land
needed for disposal of waste, and will increase the potential for
returning materials to the economic cycle, potentially reducing
pressure on natural resources.
No matter which methods society uses to reduce and dispose
of municipal wastes, it will encounter some degree of public
health or environmental risk. In this respect, waste disposal,
including combustion, is no different than most other
technologies which serve our needs. It is important for EPA to
develop the means for and to undertake comparative risk
assessments across media for each waste management option. Such
comparative analyses would provide a basis for selecting among
the different options and would help to identify the option
presenting the least adverse risk. State and local decision
makers could also utilize this technique for site-specific
assessments. Furthermore, comparative analysis would facilitate
risk management, taking economic, societal, and other factors
into account.
It is necessary for individual municipalities to evaluate
all available technological alternatives for waste disposal, and
to choose the technology(ies) that presents acceptable levels of
risk to the local population and environment. It should be
recognized that all technological alternatives (including the
maintenance of the status quo) impose (voluntarily or
involuntarily) some form of risk. The overall societal objective
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is to select the most cost-effective technology that imposes the
least adverse risk to the population and environment.
TABLE 1
EXISTING MWC FACILITIES BY DESIGN TYPE3
DESIGN TYPE
MASS Burn0
With Heat Recovery
Without Heat Recovery
Total
MODULAR INCINERATOR0
With Heat Recovery
Without Heat Recovery
RDF PROCESS
Total
d
With Heat Recovery
Without Heat Recovery
Total
INSTALLED
DESIGN CAPACITY
(TONS/DAY)
20,900
9,800
30,700
3,300
500
3,800
10,700
0
10,700
NUMBER OF
FACILITIES
25
16
41
33
16
49
9
0
GRAND TOTAL
45,200
99
aSource: Radian Corp., [1]
"Mass burn - The burning of unprocessed MSW, typically in
refractory or waterwall furnaces
GModular incinerator - Factory preassembled mass burn units
usually employing controlled air combustion
technology to incinerate considerably lower
volumes of waste than those employed by mass
burn or RDF units
dRDF - Refuse derived fuel processes subject MSW to varying
degrees of processing to improve fuel quality
for better combustion efficiency and to achieve
some material recycling or recovery (see Appendix D
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IV. THE PROCESS AND TECHNOLOGY OF INCINERATING MUNICIPAL WASTE
A. Feedstock
Municipal solid waste is extremely heterogeneous in nature,
and its composition is, in part, a function"~bf consumption pat-
terns that differ with geographic locations and vary
significantly with time of the year. There is a substantial
data base describing MSW by major constituents—paper, plastics,
glass, wood, cardboard and ferrous and nonferrous metals (see
Table 2) . The data contain not only proximate and ultimate
analysis, but also chemical analysis of ash. This information
can be useful in making the standard combustion calculations,
including combustion air requirements, inorganic stack gas
emissions (such as acid gases and volatile metals), and bottom
ash characterization.
TABLE 2
CURRENT AND PREDICTED COMPOSITION OF DISCARDED
RESIDENTIAL AND 1COMMERCIAL SOLID WASTE (WEIGHT PERCENT)a
Component
Paper and Paperboard
Yard Wastes
Food Wastes
Glass
Metals
Plastics
Wood
Textiles
Rubber and Leather
Miscellaneous
1980
33.6
18.2
9.2
11.3
10.3
6.0
3.9
2.3
3.3
1.9
Year
1990
38.3
17.0
7.7
8.8
9.4
8.3
3.7
2.2
2.5
2.1
2000
41.0
15.3
6.8
7.6
9.0
9.8
3.8
2.2
2.4
2. 1
TOTAL
100.0
100.0
100.0
aSource: Radian Corp., [1]
20
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Due to the inherent variability of MSW, and shortcomings of
current computer models of incinerator combustion, it is
difficult to accurately predict the composition of stack
emissions. Accordingly, it is important to consider the impact
of variation in MSW composition when designing furnace and
emission/control systems in order to minimize solid and gaseous
emissions.
The American Society for Testing Materials (ASTM) has
classified municipal solid waste used as a fuel as Refuse Derived
Fuel (RDF-1, RDF-2, RDF-3, RDF-4, or RDF-5) based on the degree
of MSW processing required. Appendix IV provides a further
description of these categories.
B. The Incineration Process
Organic materials that are completely combusted or burned in
an oxygen atmosphere will theoretically produce water vapor and
carbon dioxide as gaseous products of combustion. This assumes
that the organic materials contain only carbon and hydrogen.
Municipal solid waste, which is usually composed of 50-75
percent organic materials, is a fuel that contains many
constituents other than organic materials, such as free moisture
and inorganic materials including minerals and trace metals.
Thus, the products of combustion, whether complete or incomplete,
will leave the incinerator in various forms. These forms include
stack emissions as flue gas and suspended particulates, bottom
ash falling off the grate at the end of the burning fuel bed, or
fly ash removed by pollution control devices.
Complete combustion depends on temperature, turbulence and
residence time. The temperature required for proper combustion
varies with the raw material. The turbulence required in the
furnace to achieve the proper mixing of combustion air and
product gases evolved from burning materials also varies and
influences efficient combustion. Similarly, the amount of time
21
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needed for materials to fully combust depends on the elemental
and physical characteristics of the feedstock and has an
influence on combustion efficiency.
Thermodynamic properties of chemical constituents in J1SW
indicate that, under excess air conditions and the temperatures
typical of incinerators, emissions of organic compounds should be
so low as to be considered zero. However, field sampling data
show significant emissions of trace organic compounds.
Organic compounds, which include hydrocarbons, can be formed
during MSW combustion. Some of these hydrocarbons may raise
toxicological concerns or may be precursors to potentially toxic
compounds. The heterogeneous characteristics of the fuel can
prevent complete and uniform mixing of volatile gases and thereby
prevent complete combustion. Fuel-rich pockets develop in the
furnace leading to hydrocarbon formation. Chemical kinetic
considerations indicate that these hydrocarbons should be
destroyed rapidly in the presence of oxygen at elevated
temperatures.
The objective of the combustion control process is to
provide for effective mixing of the fuel with oxygen at a
temperature sufficiently high and for a time sufficiently long to
promote the destruction of all organic species. Thus, organic
emissions can be eliminated or reduced to minimal amounts by the
proper implementation of combustion control, which includes
efficient furnace design, sufficient instrumentation for
combustion air control and proper unit operation.
C. Descriptions of Combustion Systems
Increased competition in the growing incinerator market is a
prime motivation for continued improvements in the engineering
design of incinerators, especially in the combustion process.
Both competitive pressures and concerns over environmental
22
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performance are forces leading to improvements in the design of
municipal waste combustors and emissions control equipment.
Major municipal waste combustion systems can be grouped into
several categories:
1. Mass Burning of Unprocessed Municipal Waste (RDF-1)
Mass burning usually implies an incinerator that employs a
waterwall furnace enclosure positioned over the combustor grate.
The flue gases that are products of combustion leave the furnace
then flow through a convective (back-pass) heat recovery boiler
(see Figure 1) . Older systems may also consist of refractory
furnace walls combined with a convective (back-pass) heat
recovery boiler.
Early mass burning units introduced the waste into the
furnace and onto the grate by gravity through a feed chute.
Newer units utilize hydraulic rams to meter the fuel onto the
grate. Grate designs use some form of fuel bed agitation through
reciprocating, oscillatory or rotary motion or some combination
of these movements. This bed agitation allows for more uniform
burning and maximum burnup. Grate area is designed to maximize
the heat release rate.
In such units combustion air is introduced as undergrate
(primary) air and as overfire (secondary) air. Overfire air is
introduced via nozzles positioned in the front, rear, and
sidewalls of the furnace over sections of the grate. Excess air
levels in such units usually range from approximately 80 percent
for waterwall plants to 150 percent or more for refractory wall
units. Flue gases exiting the furnace usually pass through a
convection heat transfer boiler. The non-combustible matter in
the fuel, along with unburned carbon, fall off the end of the
grate as bottom (hopper) ash, or will be carried up as fly ash in
the flue gases passing through the burning fuel bed. Bottom ash
is the residue remaining after nearly complete combustion of the
organic matter achieved in current design and operation. Bottom
23
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Figure 1
MASS BURNING INCINERATION
WATERWALL—l
MEMBRANE
FRONT
ARCH
FEED
CHUTE
COMBUSTION
CHAMBER
PRIMARY
AIR SYSTEM
RAM
FEEDER
PRIMARY
AIR SYSTEM
24
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ash usually drops off into a water-filled hopper (for quenching)
and is usually transported to a landfill for disposal (see Figure
2).
2. Modular/Starved Air Burning of Unprocessed Municipal Waste
fRDF-li
In small-scale facilities, starved air or controlled air
combustors use two-stage or double combustion chambers (see
Figure 3) . Usually, sub-stoichometric air is supplied to the
primary refractory lined chamber to control exit temperatures of
gases and to reduce particulate entrainment by the flue gas from
the burning bed. A variation in the system design of the two-
stage combustor is known as a controlled-air incinerator. In
this design, excess combustion air is supplied to both primary
and secondary chambers. To minimize fly ash carry-over, the
excess air in the primary chamber is relatively low. In either a
starved air or an excess air unit, a heat recovery boiler is
located downstream, followed by appropriate equipment for
particulate removal.
3. Dedicated Stoker Boilers Burning Coarsely Processed Refuse
(RDF-2)
RDF-2 may be combusted in "conventional" stoker fired
boilers which consist of a waterwall furnace and a convective
back-pass heat recovery boiler (See Figure 4). Fuel is injected
into the furnace by air swept spouts (or essentially pneumatic
injection) . Traveling grates drop the bed ash into hoppers as
they move towards the front wall of the boiler. Optimum amounts
of excess air range from 70-90 percent. Several levels of
overfire air nozzles are normally positioned above the grate in
the front and back waterwalls. These nozzles induce turbulence,
providing the necessary mixing of partially combusted flue gas as
it exits the grate bed. New units are also being designed with
arches located in several of the waterwalls to promote further
25
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Figure 2
Refuse
Drying
Grate
Burning Grate
Birnout Grate
AshPit
MSW Grate
26
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Figure 3
WASTE HEAT
RECOVERY
CONTROLLED AIR
SECONDARY
SR >100%
CONTROLLED AIR TO
PRIMARY CHAMBER
SR<100%
HYDRAULIC RAM
CHARGING
Starved Air Combustors
27
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Figure 4
Refuse Fired Bciter
Stack-
Trailer Storage Area
Electrostatic
Precipitate
Ash Hopper
RDF-Fired Combustors
28
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turbulent mixing. Some volatile matter in the RDF-2 undergoes
drying and ignition while in suspension.
4. Co-firina of Coal and Municipal Solid Waste Burning
Processed Refuse (RDF-3. RDF-4, and RDF-5)
The practice of co-firing MSW with coal involves the use of
processed fuel such as RDF-3, RDF-4, or RDF-5. Co-firing may be
accomplished either in a spreader stoker or in a utility steam
generator co-fired with pulverized coal.
Fluff RDF (RDF-3) is pneumatically injected into the furnace
of stoker units at firing rates of up to 50 percent BTU of total
heat input from fuel, or up to 20 percent BTU heat input in
pulverized coal units.
D. Combustion System Design and Operation
1. Stages of Combustor Operation. Old and New Plant Designs
As previously stated, completeness of combustion depends on
oxygen supply, time, temperature, and turbulence. Sufficient
temperature and residence time are required for the fuel to
undergo complete oxidation. Proper amounts of excess air aid in
developing necessary furnace flue gas temperatures and
turbulence.
Grate combustion of municipal solid waste takes place in
three often overlapping stages. Multi-sectioned grates are used
to accomplish these steps in both American and European designs
for mass burn systems. These three stages are illustrated in
Figure 1 and are described below:
o Drying-Volatilization: As the waste is heated/ moisture
and volatile matter is released, leaving a carbonaceous residue.
The combustible content of the volatiles burns partially within
29
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the refuse bed and partially in suspension over the grate (see
"refuse drying, Figure 1).
o Fixed Carbon Combustion: The carbon residue produced by
the devolatilization of the waste burns on the grate, leaving an
inorganic residue (see "burning grate", Figure 1).
o Final Ash Burnout: Additional time on the burnout grate
is required to reduce the remnants of carbon embedded in the ash
to an acceptable ( <5 percent ) level (see "burnout grate, Figure
1).
In older units, undergrate combustion air was supplied by
use of a single damper-controlled compartment (wind-box) for each
section of the grate system. In the past, the incinerator
operator often depended on visual inspection to achieve a "good-
looking" fire, with the hope of maintaining "good11 combustion.
Modern designs use sectionalized undergrate air compartments to
supply varying amounts of primary combustion air to different
areas of the grate. Automatic controls on the combustion system
integrate signals from CO, 02, or C02, waste feed and/or steam
production, as well as combustion air control to produce
optimized burnout of the products of combustion.
Turbulent mixing of the flue gas leaving the grate in mass
burners or dedicated boilers is essential and can be obtained by
appropriate location of the overfire air nozzles. Proper
combustion control yields the correct ratio of
undergrate/overfire air. This ratio provides good flue gas
mixing at a constant excess air setting. Too little combustion
air will result in the generation of products of incomplete
combustion including soot, while too much overfire air will
result in a quenching of the combustion process causing formation
of the products of incomplete combustion. Sometimes the emission
of a white smoke will result from the condensation of the
quenched, unburned hydrocarbons.
30
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Earlier designs for mass burners did not incorporate
flexibility in introducing combustion air, or the sophistication
of combustion air control, used in newer plant designs. Thus,
older plants generally did not achieve the efficiency of
combustion attainable in modern plants.
Furnace designs feature arches over the grate in the front
and rear walls to allow for faster drying and distillation of the
fuel volatile matter, in fuel and also improve the mixing of
stratified flue gases to permit more complete combustion and ash
burnup (See Figure 1). Plugging or jamming of the drag conveyer
for bottom ash has been the source of some boiler load upsets
resulting in the release of products of incomplete combustion to
the environment.
2. Emissions from the Combustion Chamber
Data on the identity and concentration of different
pollutants emitted from the combustion chamber provide useful
information in designing air pollution control devices, but such
data are relatively scarce. The following information pertains
primarily to mass-burn incinerators and is provided to illustrate
the relationship among emissions, feed composition, and combustor
design and operation.
a. Acid Gases
HC1 and S02 are produced as a result of chlorine and sulfur
containing materials in the feedstock. Approximately 60 percent
of the chlorine in the waste ends up as HC1; the remainder occurs
primarily in the solid residue as inorganic chloride and may be
combined with trace amounts of gaseous organic compounds. The
sulfur in the feedstock oxidizes to S02. Part of the S02 will
react further, such as with alkali in the waste, to form
sulfates. The ash residues may retain from 10 to 90 percent of
the sulfur depending upon the alkali and sulfur content of the
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waste. The remainder of the sulfur will be emitted with the
combustion products, primarily as S02-
The oxidation of nitrogen, present either in the air or in
organic compounds in the feed, produces nitrogen oxides (NOX).
The organically-bound nitrogen, found in high concentrations in
proteins and some plastics, can be converted to NOX with
efficiencies of up to 50 percent. The NOX produced from the
oxidation of atmospheric nitrogen in combustion air increases
markedly with increases in combustion temperature and strongly
depends upon combustion conditions.
b. Fly ash and Residues
Unlike organic compounds, elemental or non-combustible
materials are not destroyed during the incineration process. The
composition of feedstock or incoming wastes, therefore, provides
a measure of the total inorganic residue. Most of the inorganic
residue and the products of incomplete combustion of organic
compounds leave the furnace as either fly ash or bottom ash.
Bottom ash drops off the end of the grate and is conveyed to
hoppers, while fly ash is elutriated with the flue gas to be
collected by air pollution control devices or emitted as
particulate out of the stack. The distribution of elements
between bottom ash and fly ash carried over to the air pollution
control device(s) depends upon the design and operation of the
incinerator and the composition of the feedstock. The amount of
ash carried out with the flue gases leaving a burning refuse bed
increases with increasing underfire air and with bed agitation.
For this reason, starved air incinerators with low underfire air
flow tend to have less particulate emissions than conventional
mass-burn units. The amount of fly ash carried from the
combustion chamber will be influenced by the particle size of the
inorganic content of the MSW.
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The distribution of elements between the different
components of refuse has a strong influence on the their
environmental fate. For example, TiO2 used as a pigment in paper
products has a particle size of about 0.2 urn and will be carried
off by the flue gases passing through the refuse bed, whereas
Ti02 in glass will accumulate in the bottom ash. Up to 20
percent of the total inorganic content of the waste will usually
be entrained in the flue gas causing the burning refuse bed to
form fly ash particles in the 1 to 20 urn size range. The
remainder will end up in the bottom ash.
c. Trace Metals
Volatile elements and their compounds, usually present in
trace amounts in the feed, will vaporize from the refuse and
condense in the cooler portions of a furnace. They will condense
either as ultra fine aerosol (less than 1 um size) or on the
surface of the fly ash, preferentially on the finer ash
particles. A large fraction of the mercury, arsenic and selenium
in the feed will be volatilized. Elements such as sodium, lead,
zinc and cadmium will be distributed between the volatiles and
the residues in amounts that depend on the chemical composition
of the substances that contain the elements. For example, sodium
in glass will be retained in the ash residue but that in common
salt will be volatilized.
d. Organic Compounds
Relating characteristics of organic emissions to the
composition of MSW and to the operation of a unit during upset
conditions is very difficult. Some episodic releases of organic
emissions can be related to operating upsets resulting from
changes in feed composition. High moisture content of fuel
delays ignition and yields lower furnace temperatures which, in
turn, retards complete combustion. High concentrations of
certain plastics, solvents or other highly volatile materials
will result in surges in the emission of combustible gases from
33
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the grate that may overwhelm the local air supply. The
capability of modulating the air supply and changing the
distribution of air can -control the effects of such sudden surges
in heat release. Mixing the MSW in the receiving or collection
pit may also help control surges of heat release in the furnace.
As combustion takes place, polycyclic aromatic hydrocarbons
(PAH) are formed during the fuel rich combustion of gas, oil, and
coal, as a consequence of free radical chemical reactions in the
high temperature flame. Quenching of partially combusted fuel due
to interaction with cooled surfaces is another PAH formation
mechanism that occurs with internal combustion engines, diesel
engines and oil-fired home heating furnaces. In such
circumstances a high fraction of the polycyclic compounds are
oxygenated. Upset conditions leading to local air deficiency may
also result in the emission of organic compounds such as PAH.
One hypothesis deserving further analysis is that similar
free radical reactions take place in fuel rich zones of incinera-
tor flames yielding PAH, oxygenated compounds such as phenols,
dioxins and furans and, in the presence of chlorine, some PCDD
and PCDF. This hypothesis is supported by the observation of
PCDF in the combustion products of pine wood only when it had
absorbed HC1. The argument for the high temperature synthesis of
PCDD and PCDF is also supported by the demonstrated increase in
the concentration of the pollutants across a heat recovery
boiler.
The above free radical mechanism should be further
investigated to determine if it is the dominant source of PCDD
and PCDF in incinerators. These compounds may also be present as
contaminants in a number of chemicals, therefore they may be
present in MWC feedstock. The presence of chlorinated phenols,
and polychlorinated biphenyls (PCB) may result from the use of
these chemicals (uses that have been discontinued in some cases)
as fungicides and bactericides (phenol derivatives), and as heat
exchanger and capacitor fluids (PCB) contaminated with low levels
34
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of PCDF. These compounds may persist beyond combustion only if
process gases are cooled to temperatures below those required for
their decomposition and reaction by large excesses in local air
flow.
PCDD and PCDF may also be produced by condensation reactions
involving the chlorinated phenols and biphenyls. The observed
formation of PCDD when fly ash from MSW incinerators is heated to
250°-300°C suggests such catalyzed condensation reactions of
chlorinated phenols. PCS can be a precursor to PCDF; pyrolysis
tests with PCS in laboratory reactors at elevated temperatures
have yielded PCDF.
E. Operator Training and Certification
i
i
The proceeding sections underline the importance of
controlling combustion air flow rates, air distribution, and
furnace, operating temperatures for minimizing emissions. To
minimize the potential for hazardous emissions, facilities must
be operated properly. The proper operation of MWCs requires a
thorough understanding of the complexities of incineration,
includilng knowledge of the composition and variability of the
feedstock, the fundamentals of the combustion process, and
requirements and consequences of adequate emission controls. In
addition, operators must be trained in procedures for managing
upset conditions in order to prevent or mitigate the release of
hazardous compounds.
I
1
The combustion of municipal solid wastes at resource
recovery facilities is exempt from Subtitle C requirements of the
Resource Conservation and Recovery Act (RCRA), providing that the
owners or operators assure the permitting authorities that the
burning of hazardous wastes will be prevented. Due to this
exemption, no national policy on operator training and/or
certification for MWC facility operators has developed.
35
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Since many new resource recovery facilities are, or will
soon be, under construction, there is an urgent need for approp-
riately trained, and technically qualified operators. Incinera-
tors in the planning and/or construction stages are generally
larger and much more complex than existing facilities. The
performance of these newer plants is becoming more critical in
view of the increasing complexity of regulatory requirements,
the requirement for increased efficiency of pollutant control
technologies in newly permitted facilities and the heightened
public concern for environmentally safe disposal of residue from
the combustion process.
There is no existing pool of trained plant operating
personnel that private industry or municipalities can draw upon
to staff MSW plants. Some states have promulgated regulations
requiring plants to be operated by certified personnel, but these
states do not have formal training programs leading to
certification. At present, training courses for plant operating
personnel are available only to a limited extent, and most of
these are one week general training programs. Such programs do
not provide the necessary understanding of the concepts and
details of combustion system design and operation, and emissions
control. Vendors must deal with the normal problems of plant
startup while providing extensive on-the-job training for
personnel who are basically unfamiliar with the facilities.
F. Conclusions and Recommendations
1. Conclusions
o The design and operation of combustion chambers has a
major influence on the type and concentration of the pollutants
entering air pollution control devices. In well-designed, well-
operated incinerators with state-of-the-art systems for air
pollution control, the emissions of organic compounds of concern
36
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can be reduced to levels close to the limits detectable by
currently available sampling and analytical methods.
o When high concentrations of organic compounds are
emitted, it is usually a consequence of poor mixing of
combustible volatiles generated in the burning refuse bed with
air, or from quenching of the partially combusted products by
excess quantities of air or contact with cold surfaces.
Inadequacies of design and/or operation of overfire air jets or
underfire air compartments may result in the improper
distribution of air causing inefficient mixing and quenching
before volatiles are combusted.
o Extensive field 'testing has been conducted to establish
general emission concentrations. There are fewer data on
systematic variation o£ operating and design parameters to
provide insight into the mechanisms governing organic emissions.
[2, 3, 4]
I
o The wide variety of polycyclic aromatic compounds and the
large number of congeners of PCDD and PCDF observed in the
emissions from incinerators appear to be consistent with the
pyrosynthesis of these compounds in the high temperature flame
zone.
o Feedstock composition has an important impact on the
emissions of inorganic compounds. Chlorine, sulfur, and volatile
I
trace metals will be transferred with relatively high efficiency
to the gaseous and fine) particulate matter carried out of the
combustion chamber. In addition, particulate matter will be
carried from the combustor in amounts that will depend upon the
fineness of the mineral constituents in the refuse, bed
agitation, and the underfire air flow rate.
o Municipal waste combustion is a complex process that
depends on many factors that begin with initial feedstock
variability and end with emissions control. Technologies under
37
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development will add to this complexity. Operators are seldom
trained to operate existing or new incinerators, nor are they
required to be certified for incinerator operation. The lack of
trained operators may slow the application of MWC technology and
may compromise efficient and safe plant operation.
2. Recommendations
There is a need for better understanding of the relationship
of inorganic and organic emissions, and PCDD and PCDF in
particular, to MSW composition, furnace design, and operating
conditions. This requires research on full-, pilot-, and
laboratory-scale units. Well-planned field testing under
different operating conditions will generate a more realistic
correlation of emissions to operations, in addition to providing
data for establishing emission indices. Pilot-plant and
laboratory-scale testing can be used to critically test
hypotheses on the routes and mechanisms of pollutant formation,
because of the ability to independently vary operating and
design parameters and feed composition in small-scale equipment.
The following specific tasks need to be undertaken:
o The relationship between underfire and overfire air
distribution and emissions needs to be understood in order to
establish guidelines for adjustments in air flow rates that are
responsive to changes in MSW composition and feed rate. A
complementary study is needed on the emission of combustible
volatiles from burning refuse beds since the overfire air
distribution should be matched to the evolution of combustible
volatiles. The effect of transient operation is of particular
interest.
o The kinetics of pyrosynthesis and condensation reactions
as they relate to the formation of PCDD and PCDF should be
further investigated. An understanding of the factors governing
the distribution of congeners and isomers of these compounds
38
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would be useful for assessing health effects and as an aid in
diagnosing the genesis of emissions. For example, the
relationship of the composition of MSW to emissions should be
studied over a wide enough temperature range to be useful in
testing the various hypotheses for formation of PCDD and PCDF.
o EPA and the private sector should work cooperatively to
develop continuous monitors to detect upsets in operating
conditions. Carbon monoxide and total hydrocarbons are currently
being explored as potential indicators of emissions of PCDD and
PCDF. Alternatives, such as polycyclic aromatics, may be
appropriate surrogates.
o Private industry and Federal, State! and local governments
should initiate efforts to plan and implement an operator train-
ing program leading to certification. This plan should provide
the operator with a basic understanding of the combustion pro-
cess, management of plant equipment, and 'impact of operational
parameters on environmental emissions. EPA and state authorities
should provide guidelines to facilitate operator training and to
maximize assurance that hazardous materials will not be burned in
MWC. Certification should be valid nationally and transferable
from state to state. Implementation of this recommendation will
provide a reservoir of appropriately trained personnel to staff
the increasing number of MWCs.
39
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V. PERFORMANCE OF AIR POLLUTION CONTROL TECHNOLOGIES
A. Potential Air Pollutants of Concern
As outlined in the previous chapter, the combustion process
results in the generation of flue gases and particulates which
contain various pollutants. These can be grouped into several
categories:
Particulates
Heavy Metals
Acid gases
Trace Organics
To prevent or reduce emission of these compounds into the
atmosphere, various air pollution control systems can be
installed between the incinerator/boiler and the stack. These
are discussed below.
For some pollutants, there appear to be trade-offs between
combustor design, unit operation and emission controls. For
example, higher incinerator temperatures can destroy trace
organic compounds, but also cause an increase in NOX production.
In addition, metals like mercury volatilize more readily and are
carried from the combustor to the control devices in greater
amounts at higher incinerator temperatures.
B. Description of Air Pollution Control Systems
The following are the main types of air pollution control
systems or devices that can be installed on municipal waste
combustors:
1. Electrostatic Precipitators (ESP)
Electrostatic precipitators have demonstrated capability to
remove particulate matter but do not remove gaseous pollutants.
40
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They can be used alone, linked in a series of 2 - 5 fields or
linked with other pollution control devices such as scrubbers.
The precipitation process follows these basic steps: (a)
development of a current of negative ions from a high voltage
corona discharge to charge dust particles in the gas stream; (b)
the presence of an electric field in the gas space between the
high voltage discharge electrode wires and the collection plate
that propels the negatively charged particulate matter toward the
positive collection plate; and (c) removal of the collected
particulate matter into hoppers by use of a rapping mechanism.
Figure 5 illustrates the basic principles of electrostatic
precipitation.
Electrostatic precipitation occurs within an enclosed ciham-
ber. A high voltage transformer and a rectifier modify; the
electrical power input. Suspended within the chamber arei the
grounded collection electrodes (metal plates) connected to the
grounded steel framework of the supporting structure. Suspended
between the collection plates are the high voltage discharge
(wire) electrodes (corona electrodes) insulated from ground and
negatively charged with voltages ranging from 20 kv to 100 kVDC.
The last step of this process involves dust removal fro^ the
collection electrodes. In dry ESPs, this is accomplished by
periodic striking of the collection plates and discharge
electrode with a rapping device. Hoppers collect the fly ash and
it is conveyed to storage or disposal points.
i
In North America, electrostatic precipitators have ti^adi-
tionally been used alone for particulate control. In Europe,
several installations use a scrubber in combination with an
electrostatic precipitator. The physical arrangement of a
typical electrostatic precipitator having two independent
electrical fields is illustrated in Figure 6.
41
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Figure 5
Earthed Collector
Electrode at
Positive Polarity
Gas Flow — i
Charged
Particle
Uncharged Particles
&
Particles Attracted
to Collector Electrode.
Forming a Dust Layer
Ctean Gas Exit
Discharge Electrode at
Negative Polarity
Electrostatic Precipitation Process
42
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Figure 6
insdator Compartment
High Voltage System
Upper Support Flame
Casing
Transformer/Rectifier
Reactor
I
Electrical Equipm
Platform
Collecting Surfaces
Cdecting Surface Rapp
Hopper
Field __
Arrangement of Electrostatic
Precipitators
43
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2. Fabric Filters
Operation of this technology involves impaction of dust
particles on a fabric filter to form a dust cake on the cloth
surface, with filtration of the gas as it passes through the cake
and cloth. The fabric typically used is a woven or felted
material. The dust cake formed on the filter plays a key role
in the overall efficiency of particulate collection.
Periodically the dust cake is removed from the filter surface via
a cleaning cycle that may consist of shaking the bag, reverse air
cleaning or blow-back by compressed air via pulse jets. The cake
remaining after cleaning forms a base for collection of particles
as the bag is put back on line.
The type of cleaning cycle used is a factor in distinguish-
ing the different designs of fabric filter type dust collectors.
Figure 7 illustrates a small pulse-jet cleaning fabric filter.
Fabric filters have not been used alone on MSW incinerators, but
are used in combination with lime injection scrubbers, described
below.
3. Scrubbers
Three widely used types of scrubbers exist. These include
wet, dry, and wet-dry scrubbers. A wet scrubber can be designed
with several different configurations, but they have in common
an underlying principle of intimate contact of a gas stream with
a liquid that may also contain some absorbent and/or reagent for
removal of acid gases. Although some wet scrubbers have been
installed in the past, typically on older incinerators, these are
not likely to be used in the future due to several disadvantages,
including the generation of a liquid waste effluent and a wet
plume from the stack.
44
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Figure 7
BfowPipa
Reduced Row
Bag Retaner
Dirty Air Inlet and
Diffuser
— Housing
To Clear Air Outlet
and Exhauster
Filter Tube
Hopper
Rotary Air Lock
Fabric Filter
45
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Dry scrubbers are typically cylindrical vessels where
powdered dry sorbent is injected into the gas stream by
compressed air. Intimate mixing of the sorbent and gas occurs,
then dry gases flow into a highly efficiency device for
particulate removal, such as a fabric filter or an electrostatic
precipitator. For temperature control, a dry scrubber is often
preceded by a heat exchange system which may also involve a water
spray system to cool the gases. Figure 8 depicts a dry scrubber
system with water sprays and fabric filter.
A wet-dry scrubber is also called a spray dryer or semi-dry
scrubber, or even a dry scrubber. In a wet-dry scrubber a
liquid sorbent stream is sprayed into a gas stream and the amount
of liquid is carefully controlled so that all the liquid
evaporates into the gas stream, yielding a dry fly ash product.
A high efficiency particulate removal device, such as a fabric
filter or an electrostatic precipitator, is required to remove
the particulates from the gas stream prior to discharge up the
stack. Figure 9 illustrates a wet-dry scrubbing system with a
fabric filter.
C. Historical Perspective of Air Pollution control for MSW
Incinerators
The air pollution control systems used to reduce stack
emissions from municipal solid waste incinerators are undergoing
continued design improvement. In post-1980 North America, two-
field electrostatic precipitators were succeeded by three, four
and, more recently, five fields for enhanced removal of
particulate matter from flue gas.
Beginning in the late 1970s, several air pollution control
systems, consisting of a combination of a dry scrubber or a wet
dry scrubber followed by either a fabric filter or an
electrostatic precipitator, were installed in Europe and Japan.
It appears that facilities adopting this technology initially
46
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Figure S
WCMCIUTO*
WfTOH»
tcmuftt*
Wet-Dry Scrubber
47
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Figure 9
Dry Scrubber
48
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sought improved acid gas control. As concerns over trace organic
compounds and toxic metals emerged during the past few years,
questions arose regarding the capability of this equipment for
removing these substances, as well as acid gases.
Up to 1985, limited sampling occurred at several
scrubber/fabric filter installations in Europe, principally for
emissions of particulates, acid gases (hydrochloric acid and
sulfur dioxide) and certain metals. These studies generated a
narrow data base of somewhat limited use, since the analysis of
trace organic compounds was often omitted or, at best, confined
to PCDD/PCDF or even TCDD/TCDF. In addition, the operating
conditions of the incinerators and pollution control devices were
often not well documented, and the studies did not examine a
range of different operating conditions.
In 1985, Environment Canada completed extensive testing on
a pilot-scale unit with pollution control equipment. This
testing resulted in the first thorough data base for evaluating
the performance of these control systems for a wide range of
pollutants [4]. Testing of a more limited scope in Denmark [5]
paralleled these efforts. The results of these tests indicate
that, at appropriate temperatures, the scrubber/fabric filter
technology can significantly reduce not only particulates and
acid gases, but also a range of trace organics (PCDD, PCDF,
chlorophenols, chlorobenzenes, PCS, polycyclic aromatic
hydrocarbons), and a host of metals (including mercury, chromium,
cadmium, and lead) in the stack emissions. Equipment design and
operating conditions necessary to achieve high removal of these
compounds were identified on a pilot scale in these studies.
The first full scale scrubber/fabric filter installation on
a waste-to-energy incinerator in North America was tested and the
stack data for PCDD/PCDF [6] show comparable concentrations to
the emissions found in the pilot-scale studies discussed above.
Several municipal solid waste incinerator facilities are now
49
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operating in North American with this type of pollution control
equipment.
The scrubber/electrostatic precipitator combination has been
installed at several incinerators in Europe. However, data to
evaluate this combination and compare its performance to that of
the scrubber/fabric filter are currently limited to inorganic
compounds. There is a reluctance on the part of regulatory
agencies to permit such facilities because of the limited test
data on scrubber/electrostatic precipitator installations.
Equipment manufacturers and system suppliers also have some hesi-
tation to guarantee that such facilities will meet very low
emission levels required for some new plants. EPA may test a new
scrubber/electrostatic precipitator installation at an incinera-
tor in Massachusetts in 1988.
The scrubber/fabric filter technology reduces stack
emissions to low levels approaching the detection limits for
certain compounds, such as PCDD, PCDF and some metals. The data
base to substantiate this capability is growing rapidly.
Nevertheless, the reader should not construe that the
Subcommittee endorses the scrubber/fabric filter as the only
technology to use. Other technologies may offer equal or even
better performance in the future. The potential development of
other improved systems should not be hindered by undue insistence
on the use of a scrubber/fabric filter.
D. Air Pollution Control Experience
1. Particulates
Tests on incinerators equipped with the conventional two-
field electrostatic precipitator have shown a wide range of
particulate emissions, varying from 50 to 300 mg/Nm3. The three-
and four-field electrostatic precipitators achieve emissions of
20 to 75 mg/Nm3. An emission level below 20 mg/Nm3 is
technically possible. However, there is a high capital cost
50
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associated with constructing a precipitator with a sufficient
number of fields and adequate treatment area to consistently
achieve such performance. Furthermore, the reliability of
continued performance at such low emission levels remains to be
demonstrated.
Scrubber/fabric filter control systems are capable of
operating within a range of 2 to 20 mg/Nra3. The material
selected for the filter bags can have an important effect on
filtering efficiency and the emission levels achieved. In
general, test results to date for the scrubber/fabric filter
indicate lower particulate emissions than those for electrostatic
precipitators on municipal solid waste incinerators. However,
there is considerable controversy that electrostatic precipita-
tors can be as effective. The longer-term reliability and cost
effectiveness of the various control processes also need to be
considered.
2. Metals
Data exist on emission levels for approximately 30 different
elements. Among those present in stack emissions from municipal
waste incinerators are the following: lead, chromium, cadmium,
arsenic, zinc, antimony, mercury, molybdenum, calcium, vanadium,
aluminum, magnesium, barium, potassium, strontium, sodium,
manganese, cobalt, copper, silver, iron, titanium, boron,
phosphorus, tin, and others. (See Table 3)
A number of sampling studies for metal emissions were re-
viewed by M. Clarke [7]. Since the condensation point for metals
such as lead, cadmium, chromium, and zinc is above 300°C, ultra
fine aerosol particles will form for which removal efficiency
depends largely on the efficiency of the particulate control
system used. Efficient removal, defined as exceeding 99 percent,
has been observed for most metals with the scrubber/fabric filter
system. Conversely, relatively high metal emissions are
associated with lower efficiency precipitators. Many existing
51
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TABLE 3
CONCENTRATION OF STACK EMISSION COMPONENTS FOR
MWC EQUIPPED WITH SCRUBBER/FABRIC FILTERS3
COMPOUND
CONCENTRATION IN (UNITS)
O TRACE ORGANIC COMPOUNDS
PCDD
PCDF
CB
CP
PAH
PCB
ng/Nm3 @ 12% C02b
1-5
1-5
100 - 1000
200 - 1000
10 - 200
1-10
o METALS
Zinc
Cadmium
Lead
Chromium
Nickel
Arsenic
Antimony
Mercury
ug/Nm3 @ 12% C02
5-10
0.5
1-6
0.2 - 1
1-2
0.02 - 0.1
0.2 - 0.6
10 - 40
o PARTICULATES
All particulates
mg/Nm3 @ 12% C02
2-10
O ACID GASES
HC1
SO,
ppm
10 - 30
10 - 40
a Source: Environment Canada [3]
b To convert to mass flow rates, use approximately 5000
nm3 flue gas @ 12% C02 per ton of refuse as fired.
52
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facilities have inefficient particulate control equipment,
resulting in higher levels of metal emissions.
Since mercury is a relatively volatile metal, several
studies indicate that both sufficient cooling of the flue gas
(below 140°C, based on tests conducted to date) and a highly
efficient particulate removal system are required to achieve high
mercury removal. The scrubber/fabric filter system can achieve
efficient mercury removal, provided that the flue gas is
adequately cooled.
3. Acid Gases
Municipal solid waste incineration typically generates
levels of 300-1000 ppm HCl, 50-200 ppm S02, 1-10 ppm HF, and 75
to 320 ppm NOX. Lime injection into a scrubber/fabric filter
system has resulted in removal efficiencies of 90-99 percent for
HCl and 70-90 percent S02, provided that the flue gas temperature
and the stoichometric ratio are suitable. This has reduced HCl
to levels below 20 ppm and SO2 to levels below 40 ppm. This
technology has also been extensively used in other applications
for acid gas removal [1, 2, 8].
The scrubber/electrostatic precipitator combination provides
about 90 percent HCl removal, but typically less S02 removal
(about 50 percent). Since precipitators and baghouses alone have
no effect on HCl and S02 removal, lime injection into the furnace
has been tested with some success (about 50-70 percent
efficiency). Some sampling to determine HF removal has been
reported. In general, HF removal of approximately 50 percent has
been reported where HCl removal exceeded 90 percent.
The Commerce Waste-to-Energy facility in Los Angeles
recently achieved significant NOX reduction through the use of
Selective Non-Catalytic Removal technology (SNCR). Start-up
operation testing has shown NOX reduction up to 50 percent. Most
53
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MSW plants under permit review in California propose to use SNCR
for NOX control [9].
4. Trace Organics
Organic compounds for which emission data exist include
PCDD, PCDF, chlorobenzenes (CB), chlorophenols (CP), PAH and PCS.
Available test data identify a number of other organic compounds,
including aldehydes, chlorinated alkanes, and phthalic acid
esters. Since public perception has focused on PCDD and PCDF
emissions, there are more data for these compounds, especially
for the tetra homologues, and especially the 2,3,7,8 substituted
isomers. The other compounds have been analyzed at only a few
facilities.
Data clearly show that chlorinated dioxins and furans exit
the boilers and, depending on the emission control devices
employed, some fraction enters the atmosphere either as gases or
sorbed onto particulates. In addition, the solids remaining
behind in fly ash or bottom ash contain most of the same com-
pounds, which become another potential source of environmental
release of these substances.
Worldwide, there are data pertaining to PCDD/PCDF in stack
emissions for about 35 incinerators. It is important to
recognize that this data base was developed using somewhat
inconsistent sampling and analytical techniques. Reported
emission concentrations for PCDD fall into three ranges:
low emissions, in the range of 20 to 130 ng/Nm3,
- typical emissions, from 130 to 1000 ng/NM3, and
- high emissions, over 1000 ng/Nm3.
Average PCDD emissions from older plants may be expected to
range from 500 to 1000 ng/Nm3. Concentrations of the 2,3,7,8
54
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isomers represent only small fractions of these levels. The "low
emission" levels tend to be achieved by newer, well operated mass
fired facilities such as waterwall plants and modular design
incinerators. In most testing programs, adequate operating data
were not collected to correlate emissions with incinerator
operations. Researchers in the field theorize that combustion
conditions can play a role in minimizing PCDD emissions [10].
Several studies are underway in Canada and the United States to
define this role more exactly [11, 12].
Recently, Environment Canada has evaluated a scrubber/fabric
filter system control for PCDD emissions, and has reported PCDD
removal efficiencies exceeding 99 percent. This has resulted in
PCDD concentrations at the stack that approach the analytical
detection limit of the sampling and analytical equipment
currently available. Emissions of PCDF exhibit a similar range
of values, and the scrubber/high efficiency particulate removal
combination can reduce PCDF to very low or non-detectable levels.
Some limited data on emissions of CB, CP, PCB, and PAH are
available. Most sampling programs for PCDD/PCDF have
unfortunately neglected to analyze for these compounds. Maximum
levels from two Canadian studies follow in Table 4.
TABLE 4
SCRUBBER/FABRIC FILTER PERFORMANCE
COMPOUND
EMITTED
INLET
ng/m3
OUTLET
ng/m3
CB 17,000 3,000
CP 30,000 8,000
PCB 700 Non-detectable
PAH 30,000 130
55
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The scrubber/fabric filter technology generally achieved
removal rates of 80-99 percent for these compounds in the
Canadian studies. Very few studies report on other products in
the flue gas. Some data from tests on older plants have been
reported for aldehydes and certain volatile hydrocarbons [8],
Unfortunately, no data are available from newer plants.
5. Conventional Combustion Gases
The conventional combustion gas measurements include CO,
total hydrocarbons (THC) , C02, and H20. Both CO and THC have
been considered as potential surrogates or indices of combustion
efficiency for dioxin/furan production; however studies published
before 1985 report no strong correlations. Nonetheless, a few
authors have attempted to correlate CO and dioxin/furan data
obtained from several different facilities [10]. From such
comparisons, low CO levels (below 100 ppm) are associated with
low dioxin/furan emissions. Higher CO levels, (ranging from 100
ppm to more than 1000 ppm), indicate high dioxin/furan emissions,
but correlations are not consistent. During poor or upset
combustion conditions, CO levels of 1000 ppm have been observed
and THC levels have risen from a typical 1-5 ppm to 100 ppm and
above.
A few studies have attempted to determine CO and dioxin
emission data under varying operating conditions on the same
incinerator, successfully demonstrate a direct correlation [4,
11] . Since one of the measures of optimized combustion that is
available to incinerator operators is minimal CO production, one
could hypothesize from the above noted correlations that
dioxin/furan emissions could also be minimized. However,
presence of high CO has been used more as an indicator of furnace
upset, alerting the unit operator to take corrective action.
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6. Ash Disposal
MSW incineration facilities generate several residues for
disposal. These include incinerator bottom ash,
boiler/economizer ash, electrostatic precipitator ash, scrubber
ash, and fabric filter ash. The formation of the latter three
types of ash depends upon the type of air pollution control
equipment utilized.
Environment Canada studies report that the concentration of
various organic and metallic compounds in ash greatly depends
upon their source: bottom ash, boiler ash, or fabric filter ash.
In general, most compounds present in ash appear to become
progressively more concentrated in ash found further downstream
in the combustion/pollution control process. For example, PCDD
concentrations in fabric filter ash were reported as 200 - 700
ng/g, whereas 30 - 150 ng/g were detected in boiler/economizer
ash, and PCDD were non-detectable in bottom ash. PAH have shown
a mixed trend, with highest values in the bottom ash, lower
values in the scrubber ash, and increased values in fabric
filter ash. Most metals show a progressive increase in
concentration (i.e. more in fabric filter ash than in scrubber
ash) ; however, some metals such as chromium and nickel show the
reverse trend. Highly efficient air emission control systems
result in fly ashes with relatively higher concentrations of
heavy metals and trace organics, since air pollutant removal is
more efficient.
7. Ongoing Research and Development
Recent research results, based upon the more modern plant
design and operation in the United States, Canada, Germany and
Japan have contributed measurably to the existing knowledge base
relative to emission control capabilities. Several ongoing
studies in Canada, Germany,'and the United States, will generate,
during the next year, data that will provide additional
information on the role of incinerator design in limiting
57
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dioxin/furan emissions. A reduction of these emissions in the
furnace will result in lower concentrations to be treated in
subsequent air pollution control devices. This will result in
lower concentrations of pollutants in the fly ash removed and,
thus, can potentially reduce the concern with ash disposal.
E. Conclusions and Recommendations
1. Conclusions
o Recent studies of municipal waste incinerator particulate
emissions indicate that state-of-the-art control devices reduce
these emissions to levels of 20 mg/Nm3 and less. It has yet to
be demonstrated whether such levels of control can be maintained
over long periods of time under all normal operating conditions.
o Acid gas control for HC1, SO2, and HF can achieve 90-99
percent removal of HCL, and lower removal of S02 and HF where
lime injection is used in conjunction with a wet scrubber, a dry
scrubber or a wet-dry scrubber.
o Removal of heavy metals (over 99 percent) including
mercury (over 95 percent), can be achieved provided that the flue
gas temperature is maintained below 140°C and a highly efficient
particulate control device is used (fabric filter or a properly
designed electrostatic ptecipitator).
o Scrubber/high efficiency particulate removal technology
offers the possibility of reducing PCDD/PCDF emissions to very
low levels, well below 10 ng/Nm3. This is 1 to 3 orders of
magnitude below emissions data reported for incinerators lacking
this type of control technology. In addition, this control
technology is capable of removing a significant portion of CB,
CP, PCB, and PAH.
Table 3 provides a summary of emission results from an
Environment Canada study, using a pilot-scale scrubber/fabric
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filter control system. These results provide further
significant documentation of the low emission levels achievable
with state-of-the-art air pollution control equipment.
o Currently, the only New Source Performance Standards
[NSPS] for MSW incinerators relate to particulate control. There
are no Federal standards directly controlling organic or metal
emissions from MSW incinerators. Several states have established
their own regulations and permit procedures as municipalities and
private industry have proposed facilities. This inconsistency of
emission requirements among various levels of government has
contributed to public uncertainty regarding the use of this
technology and has caused further complexities in the permitting
process.
2. Recommendations
Adequate data may exist to begin to develop technology based
emission standards for municipal incinerators. However, EPA and
private industry should continue research to better define trace
emissions, and the relationship between combustion, control
technology and emission of these hazardous substances.
Conducting this research will provide an improved data base for
risk assessments that can lead to more scientifically informed
decisions for adequate protection of public health and the
environment.
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VI. ENVIRONMENTAL TRANSPORT AND FATE
A. Dispersal and Persistence In Environmental Media
Pollutants emitted to the atmosphere, entering soils or
waters, or deposited in landfills are subject to a variety of
dispersal processes and fate mechanisms, including
transformation. Transformations can result in the destruction of
the parent compounds and the simultaneous formation of one or
more chemical products. The parent compounds or the products may
have long or short periods of persistence. The transformations
may be photochemical as can occur via atmospheric processes in
the airborne plume, on the surfaces of soil and vegetation, and
I
near the tdp of water columns. They may be chemical reactions
that can proceed in the dark. They may be mediated by physical
i
and biological processes at or below soil surfaces and in surface
waters. From the viewpoint of assessing potential effects, the
degree of persistence or rate of destruction of the parent
chemical in the various environmental media and the identity,
quantity, and persistence of the products are of great
importance.
i
Assessments of fate — transformation being one of many fate
processes — clearly rely on knowledge of the identities and
quantities of the parent compounds. Little information is avail-
able on the fate of chemicals from MWC operations because of the
paucity of information on the parent compounds released in either
stack emissions or ash. Furthermore, there is currently no
substantive! program designed to identify and quantify products
formed from the parent substances. In some instances, in which
the parent compounds have been identified, scientists can make
reasonable predictions of fate based on published studies.
Assessments of the environmental transport and fate of
chemicals also depend upon the availability of validated
mathematical models which can make efficient use of the available
data. The availability of such models is, in turn, dependent on
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an understanding of the transport and fate processes occurring in
different media. Unfortunately, current levels of knowledge
regarding the transport and fate of chemicals vary greatly for
different environmental media. In general, knowledge of
atmospheric transport and fate processes is much more advanced
than that for other media. This factor is reflected in the pages
to follow where discussions relating to atmospheric transport and
fate are more extensive than those for soil and water.
Figure 10 schematically depicts the major transport pathways
that may disperse MWC emissions through the ecosystem. However,
current understanding of complex terrestrial food webs,
biotransformation and bioaccumulation processes, and the
influence of these environmental processes on the quantitative
transport and fate of chemicals, is rudimentary at best.
1. The Atmosphere
The atmospheric transport and fate of emissions from
municipal solid waste incinerators are governed by a broad
spectrum of physical and chemical processes. These include
emission dynamics, such as plume rise and downwash; plume
chemistry, involving changes of state and chemical reactions;
atmospheric transport and diffusion; gravitational settling; dry
deposition; and wet deposition, due to in-cloud and below-cloud
processes. Model simulations are scientifically feasible
provided the emissions are properly characterized along with the
atmospheric and topographic structure of each site. The use of
so-called generic conditions can result in model results that are
not representative of realistic potential impacts. Model
simulations are impeded further by the scarcity of information on
combustion products and the atmospheric transformations of those
products from any emission sources.
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Figure 10
MUNICIPAL
WASTE
INCINERATOR
\f_
BOTTOM
ASH
DISPOSAL
SITE
STACK
EMISSIONS
Transport of MWC Emissions
From an Incinerator Facility
Through the Ecosystem
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a. Stack Emissions
Understanding emission products requires characterization of
the plume constituents and the factors pertaining to their
release, including temperature, velocity, and physical character-
istics of the materials exiting the stack. Emissions need to be
characterized according to chemical state, rate of release, and,
for particulates, size distribution, density, and mass. The
description of the release must take into account factors such as
stack height and diameter, efflux velocity, and sensible and
latent heat content in order to calculate plume rise and the
effective release height. The latter also requires characteriza-
tion of the vertical (and sometimes the horizontal) structure of
the ambient atmosphere--winds, temperature, and humidity.
Several algorithms are available which provide representative
estimates of the effective plume rise [13].
Downwash can occur where the stack height is low with res-
pect to the incinerator or adjacent buildings. This phenomenon
occurs due to aerodynamically generated, horizontal-axis vortices
•
or eddies in the lee of the stack/structure that transport the
stack plume to the ground, thereby effectively creating a ground-
level volumetric source. Case-specific analyses are required to
assure the absence or prevention of adverse downwash effects in
the vicinity of a given incinerator. In general, however, it is
possible to estimate minimum stack-height requirements using the
so-called "2.5 times rule" that suggests that stacks discharge
their emissions at a height at least 2.5 times the height of the
tallest nearby structure. "Nearby" is interpreted to include
structures whose horizontal separation from the stack is less
than five times the height or width of these structures
(whichever is greater).
EPA's Administrator has promulgated regulations (40 CFR Part
51) that define the use of good engineering practice (GEP) to
limit the stack heights that can be used to avoid downwash. EPA
has developed guidelines for determining GEP stack height [14],
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and to provide guidance for the use of fluid modeling (i.e. wind
tunnel simulations) to determine GEP stack heights [15].
b. Atmospheric Dispersion and Transformation
The transport and diffusion of gaseous and particulate MWC
emissions are governed by the magnitude and variability of the
wind, the thermal stratification of the lower troposphere, and
the aerodynamic characteristics of the ground surface (including
manmade structures). These factors interact, and their
characterization is frequently difficult (but tractable),
especially in urban areas and complex terrain.
Well established and representative modeling techniques are
available from the Agency, especially for determining long-term
impacts rather than short-term case studies [1(5]. It is impor-
tant, however, that the atmospheric measurements used as inputs
to these models be representative of conditions both at the
source and downwind. For example, wind and temperature profiles
recorded from observing instrumented weather balloons located at
airports are frequently not representative of the urban
environment. Similarly, stability estimates based on airport
surface weather observations may not be representative in urban
areas, or when extrapolated to areas that are bounded by large
water bodies. A useful overview of dispersion parameterization
methods is provided by Hanna, Briggs and Hosker, 1982, [17] as
well as many other sources.
I
\
c. Deposition \
Removal of particulate emissions and gaseous constituents
by atmospheric deposition is also an important fate process. The
removal and deposition at the earth-air interface occurs by dry
deposition and precipitation scavenging. Although there has been
extensive theoretical and observational research on these fate
mechanisms, there is still considerable uncertainty surrounding
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both physical processes and modeling. Hosker and Lindberg, [18]
have prepared a critical review and summary of these issues.
o Dry Deposition
Dry deposition of large particles occurs by gravitational
settling. Several mechanisms remove small particles and gases at
the surface, including impaction, electrostatic attraction,
adsorption, and chemical processes. The fall velocity of
particles is determined by the balance between gravitational
forces and aerodynamic drag forces, and depends on particle size,
density, and shape. Inert materials typically deposit more
slowly than reactive materials or charged particles, and
vegetated surfaces effect greater deposition than bare surfaces.
Dry deposition estimates for MSW emissions are, therefore,
subject to considerable uncertainty. Hanna, Briggs and Hosker,
have estimated the distance from sources of several heights at
which 50 percent of the plume is depleted through dry deposition
for a wind speed of 1 m s'1 and a deposition velocity of 1 cm s'1
[17]. These data are summarized in Table 5, and they clearly
show that dry deposition can be an effective removal process for
certain combinations of source height and stability.
TABLE 5
DISTANCE IN km WHERE DRY DEPOSITION DEPLETES
THE MASS OF A PLUME BY 50 PERCENT FOR A WIND OF 1 m s'1
AND A DEPOSITION VELOCITY OF 1 cm s"1
Meteorological
Stability Class
A-B
C
D
E
F
Source Height (m)
0 10 50
>10 km
1.3 IS 43
0.4 3.5 8.6
0.15 2.2 8.3
0.10 2.0 10.0
100
60
19
17
28
Source: Hanna, Briggs and Hosker, 1982 [17]
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o Wet Deposition
Wet deposition of plume material occurs both as the result
of in-cloud scavenging by cloud droplets (i.e. rainout) and
below-cloud scavenging by rain or snow, called washout.
Scavenging is a function of pollutant reactivity, solubility,
size distribution (for particulates), rainfall intensity, and
cloud residence time. Therefore, it is important that both
clouds and pollutants be carefully characterized in order to
provide accurate estimates of wet deposition.
Washout and rainout are typically combined and expressed in
terms of either a scavenging coefficient or washout ratio. The
scavenging coefficient is more appropriate to single episode
events and is used to express the decrease of concentration with;
time. The washout ratio averages conditions over multiple!
precipitation events. It is defined as the effluent
concentration in precipitation normalized by the effluent
concentration in air, and tends to decrease with time of
precipitation in a given storm and increases with the overall
precipitation rate. Worst-case wet deposition rates and
distributions could be obtained using data from specific eventsi
while long-term wet deposition amounts may be better estimated!
using the washout-ratio concept.
o Deposition on Soil
i
The Agency has presented a number of possible approaches to,
evaluate exposures to emissions from municipal waste combustion.!
However, literature available to the Subcommittee contained only
a few examples that sought to predict environmental
concentrations of chlorinated dioxins around municipal waste
incinerators. These approaches, although simplistic and limited,
may be helpful in indicating whether more sophisticated analyses
should be undertaken. Beychok made an effort to calculate
exposure of soil to PCDD based on air concentrations. He
calculated a value of 7.5 x 10"10g/g of soil [20]. The
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Subcommittee also conducted its own evaluation of possible soil
deposition which might be associates with the emissions of the
Hampton, Virginia incinerator facility. The Subcommittee
concluded that simplistic environmental transport models, which
did not include any allowance for the possible breakdown of
emitted compounds, did indeed provide similar estimates to those
cited above. Dispersion modeling of PCDD by Higgins, predicted
maximum ground-level atmospheric concentrations on the order of
10~12 to 10~13 g/m3 [21]. Average ground-level concentrations
were predicted to be approximately 5 orders of magnitude lower
than stack concentrations. However, until confirmed by
measurements, the significance of these simplistic modeling
exercises will remain very controversial.
2. The Terrestrial Environment
The Subcommittee has examined the terrestrial fate of PCDD
and PCDF as examples of the terrestrial fate of MWC emissions
because emissions of PCDD/PCDF have generally been studied more
intensively than the emissions of other compounds, such as metals
and acid gases. Unfortunately, knowledge of the fate and
transport of PCDF/PCDD emissions is largely based upon the
application of mathematical models. In this case, knowledge of
the physical transport models is superior to our understanding of
the fate of these chemicals during transport and subsequent
deposition.
Despite a limited understanding of the terrestrial fate of
dioxins and furans that have the potential to be deposited on
ground and vegetation surfaces as a result of municipal waste
combustion, there is some evidence that incinerators are the
source of dioxins found in nearby surface soils [22]. Given this
evidence, major questions arise concerning the fate and mobility
of dioxins in the terrestrial environment due to the lack of
definitive information concerning the physical nature of the
stack emissions, atmospheric transformation, photodegradation,
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volatilization, mobility in soil, and translocation in and
retention by plants.
Whereas particulate dioxin emissions are characterized by
condensation onto fly ash, virtually all research on the
environmental fate of dioxins has focused on pure dioxin, dioxins
in solvents and herbicides, dioxins in aqueous solution, and
dioxins from the 1976 ICMESA accident in Seveso, Italy.
Furthermore, most research on the fate of dioxins has been
concerned with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) which
is a solid at standard atmospheric temperature and pressure, has
low solubility in water but high solubility in organic solvents
and has a very low vapor pressure. Approximately half of all
incinerator emissions are gaseous and dioxins may be emitted
along with these gases, however, most relevant research has been
conducted with TCDD, a solid. Therefore, the fate of gas-phase
dioxin emissions is largely unknown. Except as noted, the
following discussion refers to the environmental fate of 2,3,7,8
- TCDD.
In the atmosphere, 2,3,7,8-TCDD may be subject to photolysis
and oxidation by the hydroxyl radical. A photolysis half-life on
the order of 5 to 24 days is estimated for typical sunlight
conditions. No quantitative estimates of oxidation rate are
known to be available, presumably due in part to the lack of
information on atmospheric abundance and distribution of the
hydroxyl radical.
Photodegradation of dioxins appears to be the principal loss
mechanism, although the findings are in some cases contradictory
and poorly understood. Research indicates that TCDD is unstable
when dissolved in solvents and exposed to ultraviolet light [23,
24], while thin films of pure TCDD applied to glass plates are
reported to be stable in sunlight [24]. Later studies indicate
that TCDD photodegrades when contained in a herbicide solution;
the loss rate is greater when the solution is applied to plant
leaves and less when applied to soil surfaces (presumably due to
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shading) [25]. Subsequently, Plimmer confirmed that pure TCDD
can decompose by UV photolysis when applied to glass in very thin
layers, while soil appears to protect against UV degradation
[26].
Studies of fate in soils indicate that TCDD can be
persistent under certain conditions with an apparent half-life of
approximately one year [24, 27]. TCDD is nearly immobile in the
soils tested by Helling [28] and Matsumura and Benezet [29]. The
latter also found microbial degradation of TCDD to be very slow
as did Isensee and Jones [30]. On the other hand, Young et al.
report biodegradation losses with apparent half-lives of 250
days. They also report that plant uptake and transport are very
slow [31].
Perhaps highly relevant environmental studies regarding soil
transport and fate are those of DiDomenico et. al., [32, 33], who
made measurements following the ICMESA chemical plant accident
and release of 2,3,7,8-TCDD. Surface soil samples (7 cm deep)
were analyzed at 44 locations 1, 5 and 17 months after the
accident. Overall, concentrations decreased significantly
between the' first and second surveys with an equivalent mean
half-life of 10 or 14 months. Concentrations changed very little
between the second and third surveys, and the apparent half-life
was estimated to be greater than 10 years. These changes in
apparent half-lives with time show that half-life, or first-
order kinetics are inappropriate for describing the persistence
of TCDD. Concentration profiles were also measured at about 32
sites to depths of 136 cm, up to 17 months after the accident.
Significant soil penetration was observed, although
concentrations decreased rapidly away from the soil surface. As
a generalization, the concentration of TCDD more than 8 cm from
the surface was ten fold lower than that in the upper 8 cm. Some
of the soil profiles showed maximum concentrations in the 0.5 to
1.0 cm layer, rather than in the uppermost layer, suggesting
degradation at the surface as well as migration away from the
surface. Loss mechanisms at the surface were not identified, nor
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were the specific transport phenomena, although it was suggested
that "migration in the soil may have occurred along with soil
colloids and particles to which TCDD may have been bound."
3. The Aquatic Environment
Even less information is available on the transport and fate
of MWC emissions and residues in aquatic systems. Contaminants
associated with particulates emitted from MWCs are subject to
deposition on surfaces downwind from the facility. This fallout
is subsequently subject to dissolution or suspension in rain or
meltwater from precipitation events. Eroding soil and flowing
water enter nearby water bodies, and suspended solids may settle
out and become incorporated with the sediment in such water
bodies. The dissolved portion may also infiltrate into the
ground, recharging groundwater and may be re-evaporated into the
atmosphere.
Again, PCDD and PCDF emissions, specifically TCDD, have been
studied in more detail than other identified contaminants and,
therefore, they are used as examples. The transport of dioxin -
contaminated soil into lakes and streams by erosion is evidenced
by the detection of 2,3,7,8-TCDD in water samples from a Florida
pond adjacent to a highly contaminated land area [34].
Additionally, several laboratory studies have shown that lakes or
rivers can become contaminated with minute quantities (ppt) of
2,3,7,8-TCDD and possibly other dioxins through leaching from
contaminated sediments. In a study reported by Isensee and Jones
2,3,7,8-TCDD was adsorbed to soils, which were then placed in
aquariums filled with water and various aquatic organisms [30].
Concentrations of the dioxin in the water ranged from 0.05 to
1330 ppt. These values resulted from initial soil concentrations
of 2,3,7,8-TCDD ranging from 0.001 to 7.45 ppm. The
investigators concluded that dioxin adsorbed to soil could lead
to significant concentrations of 2,3,7,8-TCDD in water if the
dioxin-laden soil was washed into a pond or other small body of
water.
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Other investigations have given similar results. Using
radiolabeled 2,3,7,8-TCDD, Ward and Matsumura showed that, after
dissolution from contaminated lake bottom sediment, dioxin
concentrations in water ranged from 0.3 to 9 percent of the
original dioxin concentration found in the sediment [35].
Results of another test indicated that a total of about 0.3
percent of the applied dioxin concentration passed through sand
with leaching water [29]. In some cases, the observed
concentration of TCDD in the water was greater than its water
solubility (0.2 ppb). The dioxin present in the aqueous phase
probably results from presence of 2,3,7,8-TCDD metabolites, and
binding or adsorption of TCDD onto organic matter or sediment
particles suspended in the water. In another study, application
of 0.1 ppm TCDD to a silt loam soil led to TCDD concentrations in
the water ranging from 2.4 to 4.2 ppt over a period of 32 days
[32].
The findings of such investigations are consistent with
recent reports that TCDD migrates to nearby water bodies from
industrial chlorophenol wastes buried or stored in various
*
landfills. At Niagara Falls, New York, for example, 1.5 ppb TCDD
has been detected at an onsite lagoon at the Hyde Park dump where
3300 tons of 2,4,5-TCP wastes are buried. Sediment from a creek
adjacent to the Hyde Park fill is contaminated with ppb levels of
the dioxin. There is growing evidence that TCDD has migrated
from process waste containers in the landfill of a former 2,4,5-T
production site in Jacksonville, Arkansas. The dioxins have been
found in a large pool of surface water on the site (at 500 ppb),
downstream of the facility in the local sewage treatment plant,
in bayou bottom sediments, and in the flesh of mussels and fish.
TCDD is also apparently being leached into surface and
groundwaters from an 880-acre dump site of the Hooker Chemical
Company at Montague, Michigan. Dioxins were found on the site at
a level approaching 800 ppt [37].
A recent study, [38], considers the fate of 1,3,6,8-
tetrachlorodibenzo-p-dioxin in aquatic systems. This congener,
71
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though less toxic and persistent than 2,3,7,8-TCDD, has been
reported as a component of fly ash from municipal incinerators.
A major portion of the total TCDD isomers entering the
environment from both herbicide and combustion sources is
1,3,6,8-TCDD. Movement and accumulation of 1,3,6,8-TCDD were
monitored in air, water, sediment and vegetation over a 426-day
period. Sediments were shown to be the major reservoir for the
compound, accounting for 5 to 14 percent of the added compound
after 426 days. Five to eight percent of the applied compound
could be accounted for in the water. The experimenters concluded
that volatilization losses may represent a significant route of
release following slow release from sediment and decayed plant
material. TCDD isomers degrade only under conditions of high
microbial activity, such as in the presence of decaying plant
material, and in situations where the compound is bioavailable.
Direct and indirect photolysis are major paths of chemical loss
in shallow water; however, little of the compound is degraded in
waters shielded from sunlight. I
i
The above information on the fate and dispersal of TCDD in
waters is relevant to assessments of MWC emissions and residues,
but these compounds are not the only ones likely to be present in
such emissions and residues, as previously pointed out. Metals
and acidic gas components should also be assessed to adequately
characterize the transport and fate of MWC by-products in aquatic
environments. A considerable data base gives insight into the
participation of metals in environmental processes. MWC
emissions, however, have not been well characterized, and likely
constituents have not been identified or verified. Witihout an
understanding of the metal contaminants that are likely to result
from MWC emissions, environmental transport and fate
determinations cannot be scientifically supported. Similar
problems with different complexity surround acid gas emissions
and associated transport and fate determinations.
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B. Conclusions and Recommendations
1. Conclusions
Atmospheric concentrations and surface depositions of MSW
emissions result from a complex relationship among such factors
as emission rates, plume rise, downwash, dispersion, chemical
reactions, and wet and dry removal processes. Casual attempts to
simulate or estimate ambient and surface pollutant levels can
lead to large uncertainties in the results. Notable gaps in
current knowledge include identification and quantification of
the organic or other chemicals contained in stack emissions, and
aquatic and terrestrial transport and fate processes. There are
other cases where current information is inadequate to permit
reliable predictions of concentrations and compounds that result
from MSW emission.
2. Recommendations
o A comprehensive data base should be developed through
t
atmospheric field studies at several representative MSW
facilities. The data base should be used to estimate deposition
(wet and dry) of particulate and gaseous emissions, and the
organic compounds generated should be identified and quantified.
Such field studies and the resultant data base should also be
used to evaluate mathematical and fluid models of transport,
diffusion and deposition in urban and suburban environments. The
data base should include measurements of MSW emissions (stack and
fugitive), plume rise, dispersion, and wet and dry deposition,
and should incorporate the use of inert gaseous tracers and
soluble particle tracers; both long-term and case-study (i.e.
intensive) measurement programs should be conducted.
o Fluid modeling studies should be conducted for urban MSW
incinerators and those likely to be affected by complex terrain.
EPA should implement fluid modeling methods for GEP stack height
determination in the design and siting of MSW facilities.
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Fugitive particulate emissions also need to be considered.
Estimation of their impact on areas downwind is complicated by
the need to model the processes by which the particles become
airborne. Major difficulties include description of the source
term and specification of the accompanying local wind field.
o There have been significant advances in the design and
performance of municipal waste incineration facilities. An
environmental monitoring program should be instituted that will
assess the occurrence of combustion products from diverse sources
before a state-of-the-art municipal waste incinerator begins
operation, and then assesses the incremental contribution of
combustion products from the municipal waste incinerator after it
begins to operate.
C. Transport and Fate of MWC Ash
I. Considerations
The solid waste or ash generated by municipal waste
incinerators potentially contains any and/or all of the same
chemical substances found in stack emissions. The masses of most
individual components are likely to be greater in the solids
since the chemicals preferentially partition to the solid
particles. Unfortunately, in few data sets are the solids well
characterized chemically.
The ashes remaining after combustion, and those collected by
pollution control devices, can pose a threat to humans and the
environment if not properly handled and disposed. For example,
ash from particulate control devices at one incinerator visited
by the Subcommittee was poorly contained and was observed to be
partially dispersed into the ambient air through large, unsecured
openings in the exterior walls of the incinerator building.
Fugitive dust (ash) was also observed to be blowing off the top
of uncovered dump trucks.
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It is common practice to dispose of incinerator-generated
solids in municipal landfills. Once landfilled, the solids and
associated pollutants are mixed with other wastes, some of which
may be liquid. These wastes vary considerably in their chemical
and solvent characteristics. They are composed of mixtures of
organic solvents, decomposing organic materials, and high or low
pH liquids. Just how the pollutants will react and interact as
they associate with added MWC ash under such conditions is
unknown. Some of the organic pollutants may leach from the ash
particles, especially if they are exposed to organic fluids.
Some of the trace metals may be dissolved under normal conditions
in ground water.
A Canadian study has been recently completed which focuses
on the leachability of metals and trace organic compounds for
different ashes from a dry scrubber/fabric filter system, and a
wet/dry, scrubber/fabric filter system [3]. Batch leaching tests
with distilled water on a laboratory scale indicate that there
was no organic contaminant mobility from any of the ashes, except
for chlorophenols. However, long-term leaching of organic
compounds was not determined.
An extensive investigation of the disposal of bottom and fly
ash in a separate ash landfill subjected to acid precipitation
showed no significant mobility of metals [39]. Although the ash
contained significant quantities of metals, most were not
mobilized at the expected pH of the ash/leachate system. In
contrast, some metals, such as copper, lead, zinc, and boron,
were leached to varying degrees when subjected to waters of
varying pH in a Canadian Study [3]. Significant quantities of
cadmium, lead, zinc, and copper may be leached in the short-term,
suggesting that further investigation and special handling of
these ashes are needed for safe disposal.
Landfilling of fly and bottom ash, without some
stabilization, may or may not pose hazards that surpass those
presented by burying wastes that have not been incinerated. The
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mass of municipal waste may be reduced considerably by
incineration, but some of the constituents in the ash, especially
heavy metals, may become correspondingly more concentrated.
Also, the chemical form of many elements may change during the
incineration process, affecting the subsequent transport and fate
of the resulting compounds. Such changes may result in either
increases or decreases in the leachability of the substances.
EPA has developed two leaching tests with potential
applicability to assess transport and fate of pollutants in ash.
These tests are the Extraction Procedure (EP) test, and the
Toxicity Characteristic Leaching Procedure (TCLP). Such tests
were designed for purposes other than assessing the pollution
potential of MWC ash, and, therefore, the salient features of
these tests should be evaluated to determine their applicability
for assessing MWC ash.
2. Conclusions and Recommendations
a. Conclusion
Insufficient data exist on the identities and quantities of
chemicals in ash residues, preventing a rigorous scientific
evaluation of the transport and fate of contaminants discharged
from municipal waste incinerators.
b. Recommendation
State-of-the-art analytical chemical techniques should be
employed to characterize ash samples, and as many of the
extracted compounds from selected installations should be
identified as feasible, to provide a useful data base. The goals
of this effort should include determination of the speciation
and mobility of trace metals and trace organics released from
municipal solid waste combustion facilities.
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c. Conclusion
The practice of disposing of bottom ash and fly ash from
municipal waste combustors by landfilling will be of increasing
concern as more MWC employ state-of-the-art pollution control
technologies. It is time to "consider the disposal of ash as a
discrete problem, independent of the combustor itself. A number
of approaches already being applied to the disposal of hazardous
residuals could be utilized. Generally these involve
solidification or vitrification of the waste material. Grouting
of disposal trenches, sometimes in combination with liners, is
another technique that may be applicable. The Subcommittee
recognizes that these techniques may need to be modified to meet
the particular chemical characteristics of bottom ash and fly
ash, although much work has been done on the utilization of coal-
fired power plant fly ash.
d. Recommendation
The present handling and disposal practices of ash,
especially fly ash, should be examined closely in light of data
regarding the potential for movement of heavy metals, "contained
in MWC ash, into the environment. This examination should
include identification and quantification of the inorganic and
organic chemicals that may leach from both fly and bottom ash.
Determination of the transport and fate of identified chemicals
should follow, and should include determinations of
bioavailability.
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VII. POTENTIAL EXPOSURE AND EFFECTS
The ensuing discussion of potential exposures and effects
associated with municipal waste combustion should not be
construed as a risk assessment of this technology. Rather, it is
a discussion of the data needed to further improve EPA's ability
to conduct a risk assessment of municipal waste combustion
emissions and discharges and to enhance the public's
understanding of this technology. The Subcommittee's critique of
EPA's risk assessment methodology (See Appendix I) indicates that
the procedure that has been developed is a significant
preliminary step that aids decision makers in evaluating the
risks of municipal waste combustion.
It is clear that a certain fraction of the components of
stack gases and particulates, fly and bottom ash, and scrub
waters will reach the ambient environment. Their presence in the
ambient environment may result in human and environmental
exposures. The preceding chapters have identified research needs
pertaining to the quality and quantity of emissions and
discharges, and their potential transformations and delivery to
sites where ecosystems and humans could be exposed. In this
section, the Subcommittee examines the need for data that will
improve the characterization and prediction of potential
exposures and effects to both humans and ecosystems.
On the basis of current scientific information, the
Subcommittee cannot state that no risk is posed from municipal
waste combustion. From both a scientific and a policy
perspective, the two most critical unsolved questions are as
follows:
o What is the relative contribution of pollutants emitted
by municipal waste combustors relative to other combustion
sources?
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o What is the probability that human or environmental
receptors will be exposed to harmful amounts of incinerator
emissions or discharges?
At best, obtaining answers to these questions can help
resolve the issue of the risk associated with~-this technology.
At a minimum, such answers can reduce the current uncertainties
faced by scientists and decision makers as they seek to develop
environmentally "safe" alternatives for municipal solid waste
disposal.
A. Environmental Loadings
The by-products of municipal waste incineration (stack
emissions and ash discharges, for example), contain constituents
that are virtually all already present in the environment. They
originate from a variety of combustion sources including
vehicles, smelters, home wood burners, and fossil fuel power
plants. The emissions generated by municipal waste incineration
will add to emissions from other sources to yield the total
environmental load. The relative proportion that MWC discharges
will contribute to total ambient levels- is open to question, and
will vary from site to site.
The Subcommittee concludes that, with state-of-the-art
combustor designs, controlled operating conditions, and effective
emission control devices, the emissions from MWC alone are not
likely to significantly increase total environmental loadings on
a national basis over the next generation. This conclusion rests
on the observation that the reported environmental levels of
chemicals known to be emitted from MWCs do not appear to be
significantly greater in Sweden or Japan, countries that practice
a much greater degree of incineration than that projected for the
United States. In addition, this conclusion assumes that, over
time, existing facilities, both controlled and uncontrolled, will
be replaced or retrofitted with advanced design features,
controlled operating conditions, and emission control equipment.
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This conclusion can, of course, vary with the total loadings
generated from this and other sources in specific municipalities.
It should not be expected that loadings from an incinerator in
Philadelphia or Boston will be of the same proportion (relative
to other combustion sources) as loadings from an incinerator in
the rural southwest. The policy issues faced by decision makers
in these two regions of the country may also be different. In
the former case, a chief issue will be the incremental risk that
is experienced by urban populations relative to other sources; in
the later instance, a major issue will be the impacts of a new
source, a risk in the absence of other major combustion sources.
Members of this Subcommittee cannot answer the question of
whether such risks under either scenario are acceptable. What
scientists and engineers can do is inform citizens and decision
makers of current risks and uncertainties, and develop
recommendations for further reducing them.
B. EXPOSURES
1. Human Exposures
The ambient environment at any particular MWC site may
present a hazardous exposure to either humans or ecosystems.
Assessing exposure to MWC emissions is particularly difficult.
The compounds present are generated from a variety of sources,
and isolating the contribution of pollutants from MWC is a
complex undertaking. The magnitude of exposure to MWC emissions
depends upon 1) the density of respective populations, and 2) the
extent to which the environment already receives discharges from
other sources. Individual life styles also influence the body
burdens of these chemicals. For instance, cigarette smokers may
have higher levels of cadmium in their kidneys than individuals
of the same age who do not smoke. Cadmium may also be a
component contributed by MWC ash residues. The level of human
exposure to municipal waste combustion discharges will be highly
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site-specific, and its significance will be dependent on
contributions relative to other combustion sources.
Based on present and proposed incinerator sitings, it is
likely that municipal incinerator facilities will be sited in or
near areas with dense human populations. Siting areas are likely
to be industrial and urban, as opposed to agricultural and rural.
A significant amount of background information on the
toxicity to and tolerance of humans to many of the components
identified in MWC emissions has been collected. Much of this
information exists in the general toxicological literature. The
data also appear in secondary sources, where the information has
been summarized and evaluated, such as in criteria documents and
health advisory documents prepared by the U.S. EPA, in criteria
documents prepared by the National Institute of Occupational
Safety and Health, and in various publications of the National
Research Council. When combined with existing data on
precombustion exposure levels, adequate criteria can be developed
to both protect human health and facilitate the permitting
process for plant construction and operation.
2. Ecosystem Exposures
When compared to humans, much less is known about the
exposures of plants and animals to MWC discharges and potential
toxicants. Moreover, even less is known about how well (or
poorly) ecosystems respond to, tolerate or recover from exposure
to these substances.
EPA supports research on the tolerance of aquatic
vertebrates and invertebrates to such substances dissolved in
water. These efforts, along with work in structure/activity
relationships among chemicals, toxic equivalency research,
investigations into the tolerance relationships among species,
and research into the community and ecosystem level responses to
toxicants, serve to provide a basis for the kind of information
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necessary for ecological risk assessment in general. However,
little of this effort is directed specifically to municipal waste
combustion emissions.
The ecosystem components most likely to encounter risk from
municipal waste incineration emissions, in the short-term, will
be aquatic and terrestrial life existing near the facilities.
Ecosystems may experience longer-term or chronic risk depending
upon the persistence and accumulation of emitted substances.
3. Approaches for Estimating Exposure
EPA's approach, to date, for estimating exposures of humans
and ecosystems has been to develop models that make a number of
assumptions, in the absence of data, as to how pollutants reach
human and environmental receptors through direct and indirect
pathways. The Subcommittee believes that modeling represents a
first but only a preliminary step towards answering the two major
questions of this chapter. Modeling in the absence of even
limited data or validation is simply too uncertain a tool from
which to present statements to the public on the presence or
absence of risk from a technology. What is needed is for EPA,
the private sector and the public to take the next step, that is
to develop a strategy for: 1) measuring the emissions and
discharges from major combustion sources and the proportion of
such emissions and discharges attributable to municipal
incinerators, 2) measuring selected human and environmental
receptors in urban and rural areas to discern impacts, and 3)
comparing the source-receptor relationships that emerge.
C. Effects
1. Human Health Effects
The impact on health from ingestion, inhalation and dermal
absorption of individual chemicals or chemical mixtures emitted
by municipal waste combustors depends on the dose humans receive
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and the duration of the dose. The Agency has maintained an
active program to develop health advisories, criteria documents,
and risk assessments that characterize the toxicology data base
and evaluate the dose-response relationships in humans for many
of the chemicals identified as by-products of MWC. Understanding
the impacts from MWC was not the primary motivation for such
efforts. Nonetheless, pertinent information and analyses have
resulted. Additional relevant information is available in the
toxicological literature. Very little information is available
on the effects of specific isomers or on effects of mixtures.
Interactions between and/or among compounds may enhance or
eliminate their toxicity or bioavailability.
Although a data base exists for many compounds, the effects
caused by a significant number of substances are relatively
unknown. There may also be chemical constituents in MWC
emissions or residues that have not been identified.
2. Environmental Effects
The response of individual organisms to toxic substances is.
a function of concentration, toxicity and duration of exposure.
Combinations of these functions may produce lethality or more
subtle responses, such as behavioral changes or reproductive
inhibition. These sublethal effects can take on a multitude of
forms with varying effects. They are often difficult to detect
under field or laboratory conditions.
Much of the bioassay research on environmental pollutants
has been conducted using fish and aquatic invertebrates. Less
research has been performed with plant, mammalian and avian
species and with microbes. To a large extent, this research has
yielded data on the levels of a toxicant to which species produce
an acute response, while research on chronic and behavioral
responses is now under way.
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Beyond studies of individual organisms, the responses of
populations, communities and ecosystems are important to assess.
Because these environmental units are not easily subjected to
experimentation, simulation models are often utilized. However,
models are often limited in their capability to predict
responses.
Unlike the human health effects data base, very little
information is available on the environmental effects that result
from compounds contributed by MWC technology. While EPA and
other research organizations support work on the toxicity of
atmospheric toxicants to terrestrial plants and animals, little
of this effort is directed to specific evaluation of MWC
emissions or residues.
D. Conclusions and Recommendations
1. Conclusions
o The Subcommittee concludes, based on currently available
information, that emissions from state-of-the-art, well-
controlled and operated municipal waste incinerators are not
likely to significantly increase total pollutant loadings to the
environment on a national basis. However, background levels will
vary with the sites selected for the incinerator plants, and it
is important to distinguish background levels from new emissions
before adequate exposure and effects assessments can be
developed.
o There are very limited data for evaluating both exposure
and effects of MWC emissions or residues. It is clearly not
feasible to conduct toxicity tests on representatives of all
species, but effects on animals, terrestrial plants and microbes
have not been well characterized when compared to fish and
aquatic invertebrates.
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o More data are available for assessing environmental
effects on individuals and populations than on more complex
communities and ecosystems. Very little is known about how
ecosystems respond to toxicants from any particular source,
including municipal waste incinerators.
o Exposure data for some selected compounds that are
potentially emitted by MWC are available for evaluating human
health effects; however, data on specific isomers and other
compounds identified as by-products of MWC have not been
collected. In addition, mixtures of compounds and associated
interactions under environmental conditions have not been
investigated or evaluated.
o Throughout its report, the Subcommittee has separated the
evaluation of municipal waste combustion into various components
including: feedstock, the incineration process, combustion
system design, performance of pollution control technologies,
operator capabilities, environmental transport and fate
processes, and potential exposures and effects. Each of these
can also be thought of as a critical component for assessing the
risk of this technology.
2. Recommendations
o EPA, private industry and other interested organizations
should initiate efforts to characterize emissions into the
ambient environment or conduct baseline surveys through site-
specific sampling and field monitoring techniques. EPA should
consider whether to require such data collection as part of the
permitting process. By accumulating and analyzing such
background data, the foundation for an accurate comparison of
post-combustion environmental effects can begin to emerge.
o The Subcommittee recommends that a higher priority be
placed on evaluations of environmental exposure and effects.
Individual or species level toxicity testing should be conducted
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with compounds potentially emitted by MWCs. This testing should
occur in species that are known to be of primary importance in
community interactions, including terrestrial plants, animals and
microbes for which data are scarce.
o The exposure and effects data base for human risk
assessment should be expanded to include more of the compounds
identified in MWC emissions and residues, as well as their
transformation products. The use of toxic eguivalency and
structure/activity relationships should be expanded and refined
through collection of such data. In addition, toxicity
evaluations of mixtures and products that are predicted from
interactions should improve the risk assessment process.
To improve the utility of such investigations for decision
makers, data should be generated and evaluated from a range of
incinerators (controlled and uncontrolled, as well as both new
and old designs) in a variety of locations. From such efforts,
scientists can obtain data that can be used to test the "ground
truth" and the sensitivity of previous modeling efforts. In
combination, modeling and measurements will provide decision
makers with more powerful tools to assess the relative
contribution of pollutants from MWCs relative to other sources,
and the probability of exposures reaching human and environmental
receptors.
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VIII. CONCLUDING PERSPECTIVES
The Municipal Waste Combustion Subcommittee recognized a
central fact throughout every phase of its deliberations on the
incineration of municipal wastes - the existence of some degree
of hazard, or risk, associated with the application of any
technological alternative (landfilling, preprocessing and
recycling, etc.) for managing the disposal of solid waste.
Limitations in current scientific understanding make it difficult
to make precise statements on the relative risk from the various
alternatives on a large scale and over time. Since every
recognized alternative practice has some associated risk, it is
important to compare the risks posed by all waste management
alternatives. Risk-based comparisons of the various options can
provide a valuable perspective to aid local decision makers in
choosing the most appropriate option for their community.
The previous chapters have pointed out some of the
deficiencies in the data base for conducting a formal MWC risk
assessment. Nevertheless, the Subcommittee finds that a
significant amount of research has been carried out on the
biological and human health effects of dioxins and, that
considerable data are becoming available on levels of emissions
and on the impact that control technologies have on emissions at
incinerators currently in use. In addition, government agencies,
industry and other researchers are currently characterizing
potential human and environmental exposures from air emissions
and from ash, and have begun to establish research plans.
Unfortunately, the other waste management options are also
plagued by significant data gaps, preventing scientifically
rigorous analyses that could lead to comparative risk
assessments. On the basis of risk assessment alone, therefore,
the Subcommittee believes that no single waste management
alternative is universally applicable to the range of site-
specific solid waste problems that municipalities encounter. At
the same time, the Subcommittee concludes that well-designed,
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well-operated and well-controlled incinerators represent one
alternative that should be available for use by localities.
As stated at the beginning of this report, the Subcommittee
did not evaluate the other waste management technologies in
depth, but it is possible to illustrate certain commonalties and
differences among these alternatives. In Table 6, the
Subcommittee compares the relative advantages of landfilling
(without prior treatment), municipal waste combustion and
recycling. For the purposes of this illustration the reader
should assume that incinerator ash is landfilled, that the
portion of waste that can not be recycled is landfilled, and that
recycling procedures include processing after collection. The
relative importance of each issue can differ significantly,
depending in part upon site-specific conditions. The relative
advantage for each factor, as judged by the Subcommittee, is
denoted as (a) for most advantageous, (b) moderately advantageous
and (c) as least advantageous.
While Table 6 considers only a limited number of issues, it
illustrates that when different management options are compared,
they may exhibit advantages or disadvantages, depending upon
which issues are highlighted. Comparisons among these options
for a single issue are difficult. For instance, in the case of
groundwater contamination, incineration may produce a major
impact as metals leach out of ash. The same metals are present
in raw, landfilled waste, but metals become more concentrated in
ash. The form of metals also affects their leachability. In the
case of recycling, the potential problems are intimately related
to the exact recycling process used, and can range from
negligible to very significant.
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TABLE 6
COMPARATIVE ADVANTAGES OF SELECTED WASTE DISPOSAL OPTIONS
ISSUES
INCINERATION LANDFILL RECYCLING
Air Emissions
Methane Generation
Land Volume Required
Transportation
Energy Recovery
Leachable Metals
Leachable Organics
Reduction of Infectious
Agents
Reduction of Rodents
Groundwater Contamination
Surface Water Contamination
Capital Cost
a - most advantageous
b = moderately advantageous
c = least advantageous
? » uncertain
+ a methane generation can
c b a
a c+ a
a c b
depends on distance
a c b
b-c b-c a
a c a
a c ?-c
a c . b
b b a-c
a a a-c
c a b-c
be harnessed to advantage
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o Conclusion
All waste management options entail some form of risk. Each
option has environmental advantages and disadvantages. Although
the assessment of municipal waste incineration per se generates
useful information, the value of that information is very limited
when one seeks to develop guidance for the selection of the
preferred waste management options in specific communities. In
order for decision makers and the public to make informed
decisions on the most appropriate waste management options for
their community, it is important to have comparative assessments
of the options that are under consideration for specific sites.
o Recommendation
EPA can assist local decision makers and the public by
developing ways to collect and analyze data that will allow more
informed choices regarding the management of municipal solid
waste. Such support can be provided by developing approaches for
assessing exposure and by generating models for assessing risk.
In addition, means should be provided or improved to facilitate
the transfer of such tools and information to the parties
responsible for making the decisions. The Agency should develop
guidance for evaluating individual waste management options, as
well as comparative exposure and risk assessments between
available options.
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LITERATURE CITED
[1] Radian Corporation. Appendix A: Characterization of the
Municipal Waste Combustion Industry. Draft of October
31, 1986.
[2] Environment Canada. The National Incinerator Testing and
Evaluation Program: Two-Stage Combustion. Report EPS
3/UP/l. September 1985.
[3] Environment Canada. The National Incinerator Testing and
Evaluation Program: Air Pollution Control Technology.
Report EPS 3/UP/2. September 1986.
[4] Midwest Research Institute. Results of the Combustion and
Emissions Research Project at the Vicon Incinerator Facility in
Pittsfield, Massachusetts. Project No. 8649-L(12) for New York
State Energy and Development Authority. May 1987.
[5] Denmark National Environmental Protection Agency. Formation
and Dispersion of Dioxins, Particularly in Connection with
Combustion of Refuse. December 1984.
[6] Hahn, Jeffrey L. Testimony before the California Air
Resources Board at a Public Hearing to consider the adoption
of a regulatory amendment identifying Chlorinated Dioxins
and Dibenzofurans as toxic air contaminants. July 25, 1986.
[7] Clarke, Marjorie J. "Emission Control Technologies for
Resource Recovery", presented at the Symposium on Environmental
Pollution in the Urban Area, Brooklyn Polytechnic University.
March 15, 1986.
[8] Howes, J.E.; Kohler, D.F.; DeRoos, F.L.; Riggin, R.M.; and
Barbour, R.L. Characterization of Stack Emissions from Municipal
Refuse-to-Energy Systems. Prepared by Battelle Columbus Labora-
tories for U.S. EPA under Contract No. 68-02-3458. Draft of
May 16, 1985.
[9] Hanson, James C. U.S. EPA Region 9, 215 Fremont St., San
Francisco, Ca 94105. Personal communication to the Subcommittee,
April 30-May 1, 1987.
[10] Hasselriis, Floyd. Technical Guidance Relative to Municipal
Waste Incineration. Prepared for the Task Force on Municipal
Waste Incineration, New York State Department of Environmental
Protection. August 1985.
[11] Environment Canada. The National Incinerator Testing and
Evaluation Program. Mass Burning Technology Assessment.
September 1987, Statistics published separately IN: NITEP
Update. November/December/January, 1987.
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[12] U.S. EPA and Environment Canada. Joint Project on Refuse-
Derived Fuel Combustion Technology. Status Report IN: NITEP
Update ; November/December/January 1987.
[13] Briggs, G.A. Plume Rise Predictions IN Lectures on Air
Pollution and Environment Impact Analysis. Workshop Proceedings.
American Meteorological Society, Boston, Massachusetts. 1975.
[14] U.S. EPA. Guidelines for Determination of Good Engineering
Practice Stack Height (Technical Support Document for Stack
Height Regulations). Report No. EPA-450/4-80-023, Office of Air
Quality Planning and Standards, Research Triangle Park, N.C.
27711. 1981.
[15] U.S. EPA. Guideline for Use of Fluid Modeling to Determine
Good Engineering Practice Stack Height, EPA Report No.
EPA450/4-81-003, Office of Air Quality Planning and Standards,
Research Triangle Park, N.C. 27711 (NTIS No. PB 82-145327).
1981.
[16] Turner, D.B. Atmospheric dispersion modeling: A critical
review, J. Air Pollution Control Association 29:518-519, 1979.
[17] Hanna, S.R. ; G.A. Briggs, and R.P. Hosker, Jr. Handbook on
Atmospheric Diffusion. Pub. No. DOE/TIC-11223 (De 82002045) U.S.
Department of Energy, Oak Ridge, Tennessee. 1982.
[18] Hosker, R.P. and S.E. Lindberg. Review: Atmospheric
Deposition and plant assimilation of gases and particles.
Atm Env. 16(5): 899-910. 1982.
[19] McMahon, T.A. and P.J. Dennison. Empirical Atmospheric
Deposition Parameters - A Survey Atmos . Environ. 13 :
571-585. 1979.
[20] Beychok, Milton R. A data base of dioxin and furan
emissions from municipal refuse incinerators. Draft Manuscript
submitted to Atmospheric Environment. 1986.
[21] Higgins, Gregory M. An Evaluation of Trace Organic Emissions
from Refuse Thermal Processing Facilities. Systech Corp. Xenia,
Ohio. 1982.
[22] Amendola, Gary A. Soil Screening Survey at Four Midwestern
Sites. U.S. EPA-905/4-85-005, #194. June 1985.
[23] Isensee, A.R. and G.E. Jones. Absorption and translocation
of root and foliage applied 2,4-dichlorophenol, 2,7-dichloro-
dibenzo-p-dioxin, and 2,3,7,8-tetrachlorodibenzo-p-dioxins,
Aar. Food Chem. . 19: 1210-1214. 1971.
[24] Crosby, D.G.; A.S. Wong, J.R. Plimmer and E.A. Woolson.
Photodecomposition of chlorinated dibenzo-p-dioxins. Science
173: 748. 1971.
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[25] Crosby, D.G. and A.S. Wong. Environmental Degradation of
2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Science
195: 1337. 1977.
[26] Plimmer, J.R. Photolysis of TCDD and trifluralin on
silica and soil, Bull. Environm. Contain. Toxicol. .
20: 87-92. 1978.
[27] Kearney, P.C., E.A. Woolson, A.R. Isensee and C.S. Helling.
Tetrachlorodibenzodioxin in the environment: Sources, Fate and
Decontamination, Environ. Health Perspectives, 5: 273. 1973.
[28] Helling, C.S. Pesticide mobility in soils II. Applications
of soil thin-layer chromatography, Soil Science Soc. Amer. Proc.
35: 737. 1970.
[29] Matsumura, F. and H. Benezet. Studies on the bioaccumula-
tion and microbial degradation of 2,3,7,8-tetrachlordibenzo-p-
dioxin, Environ. Health Perspectives. 5: 523. 1973.
[30] Isensee, A.R. and G.E. Jones. Distribution of 2,3,7,8-
tetrachlorodibenzo-p-dioxin (TCDD) in an aquatic ecosystem,
Environ. Sci. Technol.. 9(7) 667. 1975.
[31] Young, A.L., et.al. Fate of 2,3,7,8,-Tetrachlorodibenzo-p-
dioxin (TCDD) in the Environment: Summary and Decontamination
Recommendations. USAFA-TR-76-18.
[32] DiDomenico, A., V. Silano, G. Viviano and G. Zapponi,
Accidental release of 2,3,7,8,-tetrachloro-dibenzo-p-dioxin
(TCDD) at Seveso, Italy, II. TCDD Distribution in-the soil
surface layer. Ecotoxic. Enviro. Safety. 4: 298-320.
1980a.
[33] DiDomenico, A., V. Silano, G. Vivano, and G. Zapponi.
Accidental release of 2,3,7,8-tetrachlorodibenzo-p-dioxin
(TCDD) at Seveso, Italy, IV: Verticle Distribution of TCDD
in soil, Ecotoxic. Environ. Safety. 4: 327-338. 1980b.
[34] Bartelson, F.D., D.D. Harrison, and J.B. Morgan. Field
Studies of Wildlife Exposed to TCDD Contaminated Soils. Air
Force Armament Lab. Eglin Air Force Base, FL. 1975.
[35] Ward, C. and F. Matsumura. Fate of 2,4,5-T Contaminant
2,3,7,8-tetrachloro-p-dioxin (TCDD) in Aquatic Environments.
NTIS PB-264187. 1977.
[36] Yockim, R.S., A.R. Isensee, and G.T. Jones. Distribution
and Toxicity of TCDD and 2,4,5-T in an Aquatic Model Ecosystem.
Chemosohere. 7(3): 215-220. 1978.
[37] U.S. EPA. Dioxins, Volume 1. Sources, Exposure, Transport
and Control. EPA-600/2-80-156. June, 1980.
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[38] Corbet, R.L., G.R. Barrie Webster, and D.C.G. Muir. Fate
of 1,3,6,8-Tetrachlorodibenzo-p-dioxin in an outdoor aquatic
system. Environ. Tox._ Chem. 7: 167-180. 1988.
[39] Seever, W.R., Lanier, W.S., and Heap, M.P. Municipal Waste
Combustion Study: Combustion Control of Organic Emissions.
Energy and Environmental Research Corporation, Irvine, CA.
Draft Report to EPA, January, 1987.
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Appendix A
ASSESSING EPA'S RISK ASSESSMENT METHODOLOGY FOR MUNICIPAL
INCINERATOR EMISSIONS: KEY FINDINGS AND CONCLUSIONS
A-l
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ASSESSING EPA'S RISK ASSESSMENT METHODOLOGY
FOR MUNICIPAL INCINERATOR EMISSIONS:
Key Findings and Conclusions
Introduction
On November 10, 1986, the Municipal Waste Combustion
Subcommittee of the Environmental Effects, Transport and Fate
Committee of EPA's Science Advisory Board reviewed a draft
document entitled "Methodology for the Assessment of Health Risks
Associated with Multiple Pathway Exposure to Municipal Waste
Combustor Emissions" prepared by the Office of Air Quality
Planning and Standards (OAQPS) and the Environmental Criteria and
Assessment Office (ECAO). This document will be referred to
hereafter as the "methodology".
The purpose of the risk assessment and exposure methodology
developed in the document under review is to examine the
potential health and environmental effects exposed populations
are likely to experience as a result of municipal waste
combustion (MWC) technologies. This asessment allows comparison
of variations in the efficiency of combustor design and operation,
and is also intended to predict the effects resulting from
multiple exposures to emissions from more than one source.
OAQPS and ECAO requested the Subcommittee to evaluate the
scientific validity of the methodology for assessing health
risks associated with multiple pathway exposures to municipal
waste combustor emissions. Specifically, the Subcommittee was
asked to determine whether the methodology provides a reasonable
scientific approach to evaluating effects on public health given
the available data, the validity of exposure assessments, and the
appropriateness of transport and dispersion models. The
Subcommittee's key findings are reported in the following pages;
detailed comments and meeting transcripts have been provided to
appropriate Agency authors.
General Coamanta and Methodology Overview
Overall, the Subcommittee considers the proposed methodology
to be conceptually thorough, although it identifies a number of
areas where specific technical improvements are needed. Since
the methodology will be used as a technical support document for
regulatory decision making, a thorough technical effort is
necessary. The approach also makes reasonably effective use of
existing scientific data and exhibits the degree of accuracy and
understanding needed for using models. The Subcommittee
consensus is that the methodology is a credible effort towards
developing a tool for assessing multiple media exposures from
this source category.
The Subcommittee commends the authors on both the tone and
the detail used in documenting the assumptions that support the
methodology. The uncertainties and possible consequences of
A-9
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using the methodology are clearly presented in a number of •
instances, such as limitations created by focusing on stack
pollutants rather than total pollutant loadings (e.g., ash
residues, aqueous residues, and stack emissions). Another con-
cern is the uncertainty in identifying specific pollutants in
emissions from a municipal waste combustor, since characterizing
emissions improves the ability to predict the physical and
chemical properties and effects of emitted substances. The
authors are clearly aware that the methodology they have deve-
loped is but a step in a development process to expand current
risk assessment methodologies to include other pathways, in
addition to atmospheric, exposures beyond-inhalation and nort-human
effects.
The Subcommittee has several recommendations for placing the
scientific issued raised by the use of this technology into
better perspective. These recommendations include:
° The methodology should attempt to predict the risk
posed from both combustion as a whole and from specific
activities, such as automobile use, industrial practices (e.g.,
coal combustion for energy production), and both hazardous
chemical and municipal incineration.
0 While individual scenarios are modeled in this
methodology, calculating dose from the source and dispersal
through various pathways does not lead the reader to understand
the entire risk perspective that incineration technologies
present.
° In applying the models, the methodology utilizes
two separate sites as examples: 1) Hampton, Virginia, and 2) a
proposed, or hypothetical, state-of-the-art facility to be
located in Florida. Although both sites are individually
discussed and evaluated as to the risks they presumably pose,
they are not compared. Since risk assessment is a comparative
tool, the Subcommittee recommends that the chosen sites be
evaluated in comparison to one another, and for reasons to be
discussed later, recommends that facilities in addition to
Hampton b« used for this comparison.
° The subcommittee believes that the most appropriate
data for monitoring MWCs may be derived from combining actual
field measurements with predictions from mathematical models. For
the field measurements, this presupposes that measurements have
been made in appropriate locations, at appropriate times, and
with appropriate methods. It also presupposes, for the
mathematical models, that they have been validated at least to
the extent that their limitations are understood and that the
range of divergence between model predications and reality can be
quantified. In this context it is important to consider both
statistical variability and its propagation through the model, as
well as conceptual biases which inherently result from making the
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simplifying assumptions required for the construction of models.
The Subcommittee recognizes that elements of this recommendation
are best carried out through a longer-term research program.
The document should definitely state that, even when models
^are validated, actual data are preferable to results predicted by
models. Also, the methodology should caution that the existence
of a useful model should not substitute for or discourage the
collection of site specific data. In addition, the methodology
should encourage the use of field data and model application in
concert.
The methodology appropriately states that much of the
information needed to further support the methodology does not
exist, and that some assumptions about non-existent data must be
made to make initial predictions. However, the specific choices
in such assumption raise several questions for the Subcommitte,
which are addressed in the sections to follow.
The Subcommittee recommends that uncertainties be identified
as to whether they are the result of limitations in the
understanding of the MWC process itself, or a result of the
predictive capability of the model.
Technology and Emissions
The document cover attempts to represent a broad perspective
of exposure patterns. However, the Subcommittee is concerned
that the drawing depicts a worst-case exposure scenario without
illustrating the problem-solving aspects of the technology. This
concern centers around the negative impression that may result
from the depiction of a particulate emissions plume. It was also
noted that the illustration represents a rural setting, and does
not depict the urban environment, where most incinerators may be
built.
The methodology reviews the state-of-the-art for existing
and projected municipal waste combustors, and provides useful
background information. However, various sections on existing
and projected facility sites are inconsistent with regard to
future locations. In addition, projections for California may be
misrepresented. The Subcommittee believes that it is important to
distinguish between the number of facilities and the number of
incinerator furnaces, since most facilities consist of several
incinerators that can be operated independently.
Using a combination of dry scrubber and fabric filter
technology for pollution control is reported to reduce mercury
emissions by 50 percent. Data actually demonstrate that at 140
degrees Celsius (C) or below, 95-97 percent collection is
achieved, while at 209 degrees C, no collection is achieved. The
average may be 50 percent, but averaging this type of data does
not accurately represent the performance of the control system.
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The methodology discusses many factors that may influence
emissions. The apparent and ultimate conclusion is that the
efficiency of the air pollution control system determines the
emission level of particulate matter (PM) and associated
pollutants from the stack. This conclusion should be clearly
stated.
The Subcommittee disagrees with the use of the Hampton
facility to represent existing incinerators and their emissions.
Use of this inappropriate example will yield a gross overestima-
tion of emissions from new incinerators. The Hampton data set may
be extensive, but the technology used at the facility is hardly
representative of typical mass burn technology. The design and
operating practices used at Hampton should be explained, along
with the fact that this design is not in common use. This
facility provides a worst case scenario that is not representa-
tive of most recent installations. The results of modeling will
be very different when best available control technology (BACT)
is used. The Subcommittee recommends that EPA develop more
scenarios, including one for BACT, that can be used to evaluate a
more complete range of source and emission characteristics for
existing and proposed MWC facilities.
The methodology cites three reasons to explain the presence
of polychlorinated dibenzo-dioxins and furans (PCDDs and PCDFs,
respectively) in MWC flue gases. A fourth reason should be added,
since these organic compounds may be formed in the boiler during
cooling, in the presence of fly ash (post-combustion formation).
It should also be stated that little is known about reactions
that occur between gaseous species within emission plumes.
The methodology recognizes that the available emissions data
are limited in both quantity and quality. Few specific chemicals
have been identified, although much of the total mass has been
characterized as silicates and forms of carbon. There is reason
to suspect that some of the chemical components of MWC emissions
that remain to be identified may be toxic. However, these
chemical components, such as polyaromatic hydrocarbons (PAHs),
may be contributed by sources other than municipal incinerators,
and background levels are not adequately established. Major data
gaps exist with regard to chemical identity, toxic potential, and
total environmental burden of MWC emissions, making the
assessment of risk posed by the technology itself, and in
comparison to other alternatives, difficult to predict.
Exposure Models
0 Industrial Source Complex (ISC) Model
The introduction to the ISC model would be improved by a
discussion of the likely uncertainties of the estimates for
models of gaseous dispersion, particle dispersion, and wet and
dry deposition of gases and particles. This discussion should
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address uncertainties that arise both as a result of limitations
in the understanding of the processes and those due to random
variation in deposition and dispersal processes.
Although some of the assumptions made in parameterizing wet
deposition may be rather crude (e.g., assumptions regarding the
spatial distribution of precipitation), they are not likely to
present a problem when annualized computations are made.
However, the parameterization of dry deposition, particularly for
emission of chemicals for which loss mechanisms are not under-
stood, is not clear. The methodology seems to imply that gaseous
components are not considered. This point needs to be clarified.
The use of data concerning the size distribution of particles
obtained from the Braintree MWC may not be representative, and
the data on emission rates seem to be conservative.
The methodology for atmospheric dispersion and deposition
of emissions should separately consider particulate and gaseous
emissions and their fate. The contribution from chemicals in
different physical and chemical states should be evaluated with
respect to to direct and indirect routes of exposure. Variability
in the size and solubility of particles should be considered. The
biological availability of emitted materials is also affected by
the degree of sorbtion to particles that occurs. The discussion
should specify the assumptions made about emission characteristics.
•
The effects of buildings on lateral and vertical dispersion
of emissions has been considered in the methodology. However,
careful consideration of downwash is also necessary. The
proximity of other structures in urban areas and the potential
for downwash are not treated in the methodology. Since one of the
strengths of the ISC model is the ability to consider multiple
sources, the document should also address the issue of the
proximity of other incinerator facilities.
The methodology does not consider the exposure of people who
do not reside at ground level. This factor could be significant
for urban residents, and is compounded by the likely concentra-
tion of incinerators in urban settings.
° Human Exposure Model (HEM)
The HEM is used to estimate the carcinogenic risk posed to
populations by inhalation of predicted ambient air concentrations
of MWC emissions. The model assumes equivalency of indoor and
outdoor concentrations, an assumption that the Subcommittee finds
suspect for two reasons: 1) the finite length of typical
infiltration rates (> 1 hour, typically), and 2) the significance
of indoor sources of~certain chemicals.
The HEM estimates do not consider the short or long-term
mobility of the population. It also assumes a 70-year lifetime
for MWCs. In other parts of the methodology, a more realistic
30-year estimate is utilized. The assumption of continuous
operation of MWC facility is also an unrealistic assumption.
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Specific aspects of the locality and siting of the MWC
facility need to be considered because of their significant
effect on concentration and dispersal of pollutants.
The document should refer to the discussion of quantitative
risk assessment modeling found in EPA revised guidelines for
cancer in order to provide the reader with a better understanding
of the range of assumptions and models used in cancer risJc
assessment.
0 Terrestrial Food Chain (TFC) Model
This model is used to predict the deposition of MWC
emissions on soil and vegetation. Its pathways assess the
exposure to humans, animals, soil biota and vegetation, and
associated effects on the food chain. The TFC model has separate
components for examining the potential for human exposure from
ingesting contaminated soil and from consuming vegetation and
animal tissues containing the contaminants. The potential for
children to be exposed as a result of ingesting soil is also
estimated. However, pathways of human exposure via consumption of
herbivorous animals are not clearly explained. The assumption
that herbivores are exposed only by ingesting soil or by
consuming plants that have assimilated emitted materials
deposited on soil neglects consideration of the component
presenting the highest exposure potential. Herbivores are likely
to receive the highest exposure from ingesting leaves of plants
upon which particulate emissions have been deposited.
The Subcommittee questions the appropriateness of using
sludge or pesticide amendment practices as surrogates for
predicting fallout from MWC emissions. The burden of toxic
compounds and metals that is created by applying sludges to soils
should be compared to that presented by the assumption that rates
of dioxin or furan emissions will equal or exceed 2.7 kg/ha over
50 km linear dimension as a result of MWC.
This model uses a hypothetical Florida MWC as an example for
making predictions, but the input factors, such as rates of
emissions, soil characteristics, and design and operation, are
not documented. It is not clear whether the Florida MWC
represents a best or worst case illustration. More exposition is
needed with respect to both input and output parameters. These
improvements would greatly enhance the reader's understanding of
the methodology.
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0 Exposure Pathways
The assumptions required for determining the maximally ex-
posed individual (MEI) need to be considered more carefully to
prevent the overconservatism which may result from combining the
basic MEI concept with those resulting from the multi-exposure
models. The MEI concept estimates the effect on only one
hypothetical human subject; population effects and effects over
generations are not determined. The MEI concept also does not
consider acute exposure or exposures to other biota. These
oversimplifications result in conservative estimates of human
exposure. A new concept should also be developed which includes
the cumulative probability of MEI exposure.
Another flaw in the methodology is the assumption of flat
terrain. Urban or hilly settings may, in actuality, result in
greater levels of human exposure.
The methodology does give appropriate consideration to soil
type. Soils differ greatly, making the selection of a specific
standard soil density and penetration depth tenuous. Compounds
from MWC emissions will be deposited at different concentrations
and will be found at varying depths in the soil, depending on
soil type. Assumptions that toxicants will be concentrated in
the upper centimeter of soil may bo incorrect for some locations
because of differences in soil density, moisture and composition.
Some toxicants will be concentrated near the soil surface, while
others may move down from the surface and be dispersed.
Degradation of chemicals in soil is often assumed to be a
first-order reaction, even when data for specific chemicals
indicate that the degradation rate is not first order. The best
available kinetics should be used, since first order kinetics may
often be inappropriate.
In the methodology, trace metal contaminants are assumed to
persist indefinitely unless loss constants are available. A
reasonable loss constant, which can be derived from soil pH
values, should be used instead of making a blanket assumption
that contaminants will persist.
Assuming that no degradation and no retardation takes place
for chemicals in the plow depth layer is of concern when there is
a lack of data to support this assumption. The fate of chemicals
is known to be altered in plow depth layers composed of organic
clays as a result of biologic activity.
° Surface/Ground Water Models
Tier one of the surface/ground water methodology assumes
that all material deposited during a single year is incorporated
into the water in the same year. This model does not take into
account the potential for build-up over periods of more than one
year, or the potential for this large amount of material to be
released by a single storm event at some future time. In drier
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climates (i.e., the Intermountain West and the Southwest deserts)
major storms or "gully washers" can occur as seldom as once in 10
years, rendering doubtful the assumption thai all toxicants
adhering to particulates are flushed out in a one year period.
Furthermore, in wet climates the opposite may be true, as some
toxicants may not build up appreciably.
0 Other Exposures Not Considered
As the authors point out, no consideration is given to
exposures from landfilling ash. Similarly, consideration is not
given to the potential for change in emission characteristics
that may result from incinerator upsets. These data gaps are
significant, but consistent with the inadequate knowledge
regarding MWCs. The Subsommittee recommends that the methodology
address these issues.
Estimation of Risk to Humans
The equation used to calculate the adjusted reference intake
(RIA) is logical for application, since the use of the acceptable
daily intake (ADI) is well established. Also, the use of excess
concentration over background in the equation is an established '
measure of the potential for human health effects. However, the
definition of total background intake (TBI) of pollutants from all
existing sources needs some clarification.
Examples presented in the methodology use national averages
to define the TBI, although these values may not be
representative of the,particular sites where risk is to be
evaluated. The approach taken for risk assessment is based on
the location with the minimum RIA, although people at this
location may not be those with the maximum exposure to the
pollutant. The Subcommittee believes that the values selected may
not be valid for the particular sites being evaluated.
Defining the TBI as the sum of contributions from individual
sources assumes that no interactions, such as synergism or
antagonism, occur when sources are combined and individuals are
exposed by multiple routes. There are many instances where this
concept is not supported by the available data.
There la inconsistency in the methodology's treatment of
exposure to background concentrations of different chemical
substances. For some chemicals, such as cadmium, contributions
from MWC emissions are added to contributions from all background
sources to give total exposure. For other substances, such as
benzo(a)pyrene, exposures to background concentrations are
ignored and assessment is conducted in terms of additional risk
posed by MWC contributions alone. The methodology should assess
exposure to chemical substances in a consistent manner.
The prediction of inhalation exposure, which assumes that
individuals are exposed to emissions only in gaseous form,
neglects the potential for particulate absorption and particle
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deposition. Pathways other than inhalation, such as dry
depositon of particulate emissions and related dermal absorption,
need —to be considered.
The methodology postulates that some noncarcinogenic effects
that exhibit thresholds occur only after nearly an entire life-
time of exposure. This assumption does not reflect the actual
situation. For example, fibrotic lung diseases occur after less
than a full life span of exposure, and their onset is very
gradual. For many chemicals, the reported latency periods tend
to be measured in terms of weeks or months, rather than years.
Relative effectiveness (RE) is used in the methodology to
standardize effects of exposure by one route to the effects of
exposure by another. There may not be scientific justification
for this conversion factor. However, the concept is useful as
long as users realize that the effect of an exposure does not
relate solely to absorption efficiency, but is also related to
differences in the sensitivity of absorption sites to damage, and
to differences in toxicokinetics between exposure routes. The
methodology should acknowledge the assumptions required for using
this approach.
Consumption of fish by the general population is discussed,
but the discussion does not take into account the fact that fish
may coma from a variety of sources with varying degrees of
contamination. A similar situation exists for drinking water.
Drinking water obtained from any one tap may consist of water
from a local source, may contain water that originates outside of
the localized delivery area, or may be a mixture of both.
Alternatively, drinking water may be obtained from individual
wells drawing on ground water from a large source or deep aquifer.
Local contamination is not always represented in the localized
supply of drinking water.
With regard to water consumption, the amount of fluid
intake documented is low. It is not clear whether this amount
represents total fluid intake or the intake of water alone. It is
usually assumed that fluid intake for adults averages 2 liters
per day. It is questionable, therefore, that females between the
ages of 14 and 16 would only take in 586 ml water per day, as
reported in the document.
Ecological Effects
The treatment of plant uptake as a linear function is
erroneous unless no other information is available. Many
toxicants, especially metal salts, are actively transported
across membranes or cell walls and, therefore, cannot be described
by a linear function.
The Subcommittee disagrees with the assumption that plants
are exposed to contaminants mainly through uptake from soil.
Greater exposure is likely to occur from foliar deposition.
Estimates of deposition can be obtained from acid deposition
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studies and also from studies of the nuclear energy industry,
e.g., deposition of radioiodine (I131).
The Subcommittee also questions the method used to average
bioconcentration data for aquatic species. Even when means are
calculated separately for bivalves and fin fishes, misleading
interpretations can result. The bioconcentration data should be
correlated with human dietary factors. For example, humans con-
sume more oysters than mussels, and oysters may accumulate
significantly more contaminants than mussels. Averaging biocon-
centration factors together for oysters and mussels may create a
significant source of error in calculating exposure to
bioaccumulated chemicals.
The document summary mentions measurement of adverse effects
on natural ecosystem vitality. The definition of ecosystem
vitality is unclear, as are the endpoints to be used in measure-
ment. Uptake from water is modeled, but few other environmental
endpoints are considered. One important component not treated is
the highest trophic level, predators. Predators play an
important role in community regulation. There is also a need to
consider the potential for concentration of materials in
sediment, since sediments may serve as a source of contamination
for overlying waters, and materials concentrated in sediment may
be biologically available to benthic organisms and organisms
dwelling in the water column. Assessments of exposure cannot be
derived from water quality concentrations for benthic dwellers,
since they are exposed in a totally different way.
In closing, the Subcommittee agrees that the methodology
represents an appropriate step towards modeling and predicting
exposure from. MWC emissions. Some conceptual assumptions can be
strengthened by closer examination of the complexities associated
with pollutant emission to and interaction with the environment,
while others must await collection of actual field data to fill
in knowledge voids and elucidate environmental interactions.
Finally, the methodology, over time, must be validated with actual
data to evaluate and demonstrate its utility, and to guide its
further development and refinement.
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Appendix 3
DIOXIN TOXIC EQUIVALENCY METHODOLOGY SUBCOMMITTEE REPORT:
EXECUTIVE SUMMARY
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A. Major Subcommittee Conclusions
EPA has proposed inter in procedures for estimating health risks for
CDDs and CDFs based on the premises that: (a) toxicity equivalence factors
can be assigned'to untested (or incompletely-tested) compounds on the basis of
structure/activity relationships; and (b) the toxicity of mixtures of these
compounds can be approximated for policy purposes by the sums of their TEF
times concentrations. Empirically, the present proposal falls generally
between the positions adopted by certain European countries, which rank
2,3,7,8 TCDD far above any other congener in toxicity, and that initially
proposed by the state of California, which equates all the dioxin congeners.
All have.used similar scientific assumptions in developing policy.
The Subcommittee agrees that the congeners of CDDs and CDFs constitute
a class of chemical substances that share similar structural relationships
and qualitatively similar toxic effects and, therefore, can reasonably be
considered together. From the limited toxicologic data available it seems
reasonable, too, to consider those tetra-to hexa-chlorinated compounds with
chlorine substitutions at the lateral 2,3,7,8 positions as a closely related
subclass in terms of biological activity and environmental fate.
The Subcommittee also concurs that the problems in assessing the health
risks of dibenzo-£-dioxins and dibenzofurans are two-fold. They include:
limited information frcm human or experimental studies about the hazards from
exposure to these compounds (few of the 75 CDDs and 135 CDFs have been tested
at all) and even more limited information about their possible interactions
in mixtures. Indications of interactions, mostly additive, are found in
certain experimental model systems (e.g. binary combinations). Not addressed
in the draft document, however, is the possibility of chemical and toxicologic
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interactions with other types of ccrpcunds in ccrpiex environmental mixtures,
especially solvents that might affect uptake and retention by the body.
EPA should address the latter subject in the TEF document, perhaps with
nore specific reference to its recently published Risk Assessment Guidelines
and to three National Academy of Sciences' reviews on toxicological interacticr
the last of which is currently being prepared for EPA and the National
Institute of Environmental Health Sciences. The Subcommittee also questions
the basis for including or excluding other chemicals with effects similar
to CDDs and CDFs, such as chlorinated biphenylenes.
Based upon its review of the draft document/ the Subcommittee concludes
that the method proposed by EPA is a reasonable interim approach to assessing
the health risks associated with exposure to mixtures of CDOs and CDFs for
risk management purposes. It is necessary, however, as lessons are learned
• * •
fron toxicologic research and from application, the approach should be-
re-evaluated systematically by EPA. Moreover, attempts should be made to
validate the method by selected experimental testing of hypotheses. For
example, more data are needed on in vivo potencies of additional PCDDs and
PCDFs to compare with in vitro test results. The assumption of additivity
can be evaluated by comparing observed activities with predicted activities
in selected tests.
The Subcommittee recommends that EPA place more emphasis on the interim
nature of the method in the document. The Subcommittee anticipates that,
over time, the method will be modified and eventually superseded as pore
precise data became available. Meanwhile, the general method proposed
appears to have utility for this and for other classes of closely related
compounds where toxicologic data are incomplete. Application of structure
activity relationships is an old and established practice of demonstrated
usefulness in pharmacology and toxicology.
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However, EPA should not abandon its exploration of other approaches to
estimating risks for substances in mixtures. For example, where variability
in the composition of environmental samples is noc wide, a 'reference standard
<•
approach might be used (similar to those used in toxicology for selecting a
reference cigarette or a representative blend of gasolines). As another
example, the incorporation of a small amount of radiolabeled test compound
into a representative and defined mixture might be one useful way of determininc
in vivo whether the uptake and metabolism of one congener is-greatly modified
by the presence of other substances in a mixture.
Some additional technical comments that the Subcoitnittee wishes to draw
to the Agency's attention include: 1) perceptions by many Subcommittee • •
•
members of an over-reliance upon the postulated mechanisms of the Ah
receptor/AHH enzyme induction upon which to gauge relative and absolute
toxicity; 2) the need to discuss the work of Matsumura, Rozman, Greenlee,
Poellinger and others on additional toxicologically significant effects of
the dioxins other than those associated with receptor binding or with
cytochrone P-450 induction; 3) observations of a disassociation between AHH
induction and cytotoxicity in studies on the gonado toxicity of TCDD; and
4) examination of the extent to which the longer biological half-life of
higher* chlorinated dioxin isomers, as compared to 2,3,7,8-TCDD, counter-
balances their lesser in vivo potency.
B. Major Subcommittee Recommendations
The Subcommittee has several recommendations for improving the report.
First, the draft report narrative is relatively brief and may not be
readily understood by those not familiar with dioxins. For example, four
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possible approaches are introduced and one (TEF) selected, but the document
does not clarify what the other three aproaches are and the- reasons for
their rejection.' The first approach, long-term animal testing, might be
appropriate for municipal incinerator fly ash, where analytic data suggest
there is a characteristic pattern of composition. The second approach
(short-term assays) is not clearly described (not even whether they are
in vivo or in vitro). The third approach, additivity of -the toxicity of
components, is at first rejected in the narrative but then forms the
basis for handling the equivalents to 2,3,7,S-TCDD in mixtures.
Because the draft document presents a procedure, it is essential
that the decision steps be clearly articulated, the assumptions made'
* . ' 9
m '
explicit, and the mechanics of calculating be illustrated in a stepwise
fashion. To approach the subject from the viewpoint of studying the
whole class of pollutants and to avoid bias by selecting data, the Subcom-
mittee recommends that the tabular data be enlarged to include all compounds
with zero to eight substituted chlorines. Biological activity has been
reported for di- and tri-CDDs, and carcinogenicity studies exist for DD
and 2,7 DCDD, as examples. Moreover, the activity of brcminated and
other substituted compounds should also be indicated and a specific
effort encouraged to collect data on non-chlorine substituted compounds.
In contrast with the document's first priority on carcinogenic and
then on teratologic effects in animals, the Subcommittee recommends that
the TEF methodology assign first priority to human data when it exists. In
evaluating experimental data, EPA should continue to follow the current
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toxicologic practice of evaluating all endpoints, and selecting the ones
most reliable, 'sensitive, and inportant for risk assessment. Thus, columns
should be added to the tables in the document for other inportant toxic
'
endpoints including immunotoxicity, thyme atrophy, body weight, and enzyme
induction in vivo. The limited data points from which TEFs are currently
derived (e.g. carcinogenicity of 2,3,7,8-TCDD, 2,3,7,8-Hx CD'Ds and repro-
ductive effects of those compounds plus 2,3,7,8-TCDF) should be critically
re-examined and the range of experimental data and estimated potencies from
all studies tabulated. The Subcommittee also recommends that EPA consider
assigning higher relative TEFs to GDFs in general, and 2,3,4,7,8-PeCDF in
particular.
• '*'•' *
The Subcommittee strongly believes that EPA should assign greater
•
priority to obtaining and using data on toxicokinetics, including metabolism.
The rates of uptake and distribution of compounds alone and in mixtures
are important measures of bioavailability and dosimetry. The kinetics of
metabolism and excretion, along with those of receptor kinetics and"
affinities, should be especially useful for interspecies comparisons and
for estimating risks for this particular class of compounds.
The Subcommittee wishes to emphasize that the method proposed may lack
scientific validity. The associated errors have not.been quantified. It
is important, therefore, that the Agency make every effort to validate
the method. The Subcommittee recommends periodic review and analysis as
better data are obtained, and that EPA make systematic efforts to obtain
critically important data, including that from in vivo tests on compounds
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with representative positional substitutions. Efforts should continue to
develop methods for assaying the biologic activity of important mixtures
(e.g. fly ash) in in vitro systems, using other cells in addition to
hepatocytes and'other endpoints in addition to AHH activity. Until the
uncertainties are reduced, the interim TEF method should be largely
reserved for specific situations where the components of the mixture are
known, where the composition of the mixture is not expected to vary much
with time, and where the extrapolations are consistent with existing
animal data.
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Appendix C
REVIEW OF THE MUNICIPAL WASTE COMBUSTION RESEARCH PLAN
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UNITED STATES ENVIRONMENTAL PROTECTION AGENCY
WASHINGTON. D.C. 20460
OFFICE OF
THE ADMINISTRATOR
April 11, 1988
The Honorable Lee M. Thomas
Administrator
U.S. Environmental Protection Agency
401 M. Street, S.W.
Washington, D.C. 20460
Dear Mr. Thomas:
The Municipal Waste Combustion Subcommittee of the Science
Advisory Board's Environmental Effects, Transport and Fate
Committee has completed its review of the Office of Research and
Development's (ORD's) "Municipal Waste Combustion Research Plan".
The review was initiated at your request, along with two other
charges related to municipal waste combustion, and was reviewed
concurrently with other issues on March 10, 1987.
The Subcommittee concludes that the research plan is well
defined and reflects considerable thought, however, the proposed
level of. funding for the research appears grossly inadequate in
view of the large number of scientific uncertainties associated
with this technology, and EPA's responsibility to develop
scientifically credible regulatory decisions. Important areas,
such as ecological effects, are entirely left out or are
addressed in a cursory fashion, which is understandable since
allocated funds are inadequate for the areas that are addressed.
Prioritization of research emphasizes avenues with short-term
goals which may be necessary to meet the needs for technical
guidance in permitting the many MWCs that are being planned or
are already in operation.
The Subcommittee believes that emissions should be
characterized as a first priority through analytical chemistry
projects, methods development, and field testing. Risk
assessment, health effects prediction and emission control cannot
be adequately conducted without a thorough knowledge of the
quality and quantity of the emissions, both gaseous and residual.
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Page two
Following such characterization, environmental transport, fate
and bioavailability should be determined, since they are key to
assessing both risk and exposure to humans and the environment.
Monitoring is also considered by the Subcommittee to be an
important research priority, and research directed towards
monitoring goals will insure the development of tools to ensure
compliance with guidelines or standards that may be set. In
addition, monitoring is important for the validation of
predictive models which have been developed for air transport of
stack emissions.
The Subcommittee agrees that major areas of promising
research have been proposed and developed to investigate
important areas of uncertainty with respect to municipal waste
combustion technology. However, budgetary constraints shed
doubt, in the Subcommittee's opinion, on EPA's ability to reach
the objectives defined in the program. Considerations of
priority might be revisited to allow identification of research
areas with high priority and attainable objectives.
The Subcommittee appreciates the opportunity to conduct this
scientific review. We request that the Agency formally respond
to the scientific advice transmitted in the attached report.
Sincerely,
Norton Nelson, Chairman
Executive Committee
Science Advisory Board
.f Hartung, Cftwrrman
Municipal Waste
Combustion Subcommittee
Enclosure
cc: A. James Barnes
Vaun Newill
Alfred Lindsey
Larry Fradkin
Terry Yosie
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Appendix D
DESCRIPTION OF REFUSE DERIVED FUEL (RDF) CATEGORIES
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CLASS
FORM
Appendix D
ASTM CLASSIFICATION OF RDFs
DESCRIPTION
RDF-1
RDF-2
RDF-3
RDF-4
RDF-5
RDF-6
RDF-7
Raw Municipal solid waste with minimal processing
to remove oversize bulky waste.
Coarse MSW processed to coarse particle size with or
without ferrous metal separation such that
95% by weight passes through a 6-inch-square
mesh screen.
Fluff Shredded fuel derived from MSW processed for
the removal of metal, glass, and other
entrained inorganics; particle size of this
material is such that 95% by weight passes
through a 2-inch-square mesh screen.
Powder Combustible waste fraction processed into
powdered form such that 95% by weight passes
through a 10-mesh screen.
Densified Combustible waste fraction densified
(compressed) into pellets/ slugs, cubettes,
briquettes, or similar forms.
Liquid Combustible waste fraction processed into a
liquid fuel.
Gas
Combustible waste fraction processed into a
gaseous fuel.
Source: Hickman, H.L., "Thermal Systems for Conversion of
Municipal Solid Waste: Overview," Argonne National
Laboratory/CNSV-Tm-120, Volume 1, May 1983.
A measured RDF particle size distributions indicated that 95
percent by weight of the RDF is smaller than 2 inches, and that
over 99 percent by weight of the RDF is smaller than 2.5 inches,
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Appendix E
GLOSSARY OF TERMS AND UNITS
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GLOSSARY OF TERMS AND UNITS
Acid gases
As
Back-pass convective
heat recovery boiler,
convective back-pass
BACT
Bed agitation
Bottom ash
BTU
Burning refuse bed
CB
Cd
CO
Congeners
CP
Compounds, such as hydrochloric acid
(HC1), sulfur dioxide (£02),
nitrogen oxides (NO..) and
hydrofluoric acid (HF) that are in
the gaseous state.
- Arsenic
the heat recovery boiler at the
furnace outlet generating steam by
convective heat transfer.
Best Available control technology
- Agitation of the burning fuel bed
by mechanical movement of the
furnace grate
- Residual ash resulting from the
burning of garbage, as discharged
from the bottom of the incinerator
- British Thermal Units
- MSW bed burning on the furnace grate
Chlorobenzenes
f
Cadmium
- Carbon Monoxide
- A group of closely related chemical
compounds such as the 75 chlorinated
dibenzodioxins or chlorinated
dibenzofurans
Chlorophenols
DC
Dioxins
Downwash
Dry Deposition
Direct Current
See PCDD below
Downward air movement in lee of
buildings and structures due to
aerodynamic forces
Turbulent exchange of gases and small
particles from the atmosphere to the
ground surface
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EPA
ESP
Efflux velocity
Electrostatic
precipitator (ESP)
Elutriate
Excursion
Fabric filter, or
fabric filter baghouse
Feed pit
Fly ash
Front, rear arch
geometry
Gasification of MSW
Gravitational settling
HC1
HF
ng
Environmental Protection Agency
Electrostatic precipitator
Flue gas velocity leaving the
furnace grate passing up through
the combustion chamber
An air pollution control device
designed to remove particulate
matter from a gas stream using
electrostatic forces.
Particulate ash carried up from
a furnace fuel bed by the gas
velocity through the grate
Deliberation from normal
operating conditions resulting
in incinerator upset conditions
- An air pollution control device
used to remove particulate matter
from a gas using filtration
principles
- Receptor pit used for MSW
storage from which the fuel is
introduced into the MWC
- General term for all ash carried
up from the grate and out from
the incinerator/boiler by the
flue gas
furnace wall design configuration
Heating of the MSW at the entry
point of the furnace grate - which
drives off MSW Moisture and Volatile
hydrocarbon constituents of fuel
- Settling of particulate matter by
force of gravity
- Hydrochloric acid
- Hydrofluoric acid
- Mercury
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Homologue groups
Hydrometeors
Isomer
KV
mg/Nm3
MSW
MWC
Mass-burn units
Modular incinerator
Na
ng/g
ng/Nm-
NOX
PAH
Pb
PCB
PCDD
- Group of chemicals which vary in
structure but have the same
composition, such as degree of
chlorination
Solid and liquid water particles
and droplets
2 particular members of a
homologue group
- Kilovolts
- Milligrams per normal cubic meter
- Municipal Solid Waste
- Municipal Waste Combustion
(or Combustors)
Incinerators that burn unprocessed
MSW, typically in refractory or
waterwall furnaces
Factory preassembled mass burn units
usually employing controlled air
combustion technology to incinerate
considerably lower volumes of waste
than those employed by mass burn or
RDF units
^ Sodium
- Nanograms per gram
Nanograms per normal cubic meter at
normal temperature and pressure
conditions
Oxides of Nitrogen, such as NO2,
or nitrogen dioxide
- Polycyclic aromatic hydrocarbons
- Lead
- Polychlorinated biphenyls
- Total of all Comers and/or all
homologue groups of polychlor-
inated dibenzo dioxins
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PCDF
Plume rise
Pneumatic injection
ppb
ppm
Proximate Analysis
Pyrolysis of MSW
RCRA
RDF
RDF processes
Rotary combustors
Scavenging coefficient
Scrubber/baghouse
Se
- Total of all isomers and/or all
homologue groups of polychlor-
inated dibenzo furans
A term used to describe the rise of
a steady stream of flue gas due to
buoyant effects after it leaves a
stack
- Air injection of MSW or processed
refuse into the furnace
Parts per billion
Parts per million
- The gravimetric composition of
moisture, Ash, volatile matter and
fixed carbon in a MSW fuel
- The heating of MSW in the absence of
oxygen, which drives off moisture and
volatile matter in municipal waste
Resource Conservation and Recovery
Act
- Refuse-derived fuel (unprocessed
or processed municipal solid waste)
Refuse derived fuel processes that
subject MSW to varying degrees of
processing to improve fuel quality
for better combustion efficiency and
to achieve some material recycling or
recovery.
- MSW combustion occurring in a
rotating drum (or kiln)
the coefficient describing the expo-
nential decrease with time of
atmospheric contaminants due to
capture by rain and cloud droplets;
usually applied to single precipita-
tion events
- An air pollution control system
consisting of a scrubbing device
(lime injector) followed by a
fabric filter dust collection
- Selenium
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so.
- Sulfur dioxide
Spreader stoker
Stack
Starved air incinerators -
Sub-stoichometric air
TCDD
TCDF
Tetrahomologues
THC
TiO
TPD
Ultimate analysis
Underfire air
Washout ratio (Wr)
Waterwall incinerator
Wet deposition
Coal or RDF injection over a forward
moving traveling grate
Chimney through which gases and
particulate residues are emitted
MSW combustion occurring in primary
combustion chambers supplied with
sub-stoichometric air
Combustion air supplied that is
less than that theoretically
required to burn the fuel completely
Any tetra isomers or the tetra
homologue. group of dioxins
Any tetra isomers or the tetra
homologue groups of furans
As applied to PCDO and PCDF - those
isomers and homologues which are
chlorinated at 4 positions
Total hydrocarbons
Titanium oxide
- Tons per day
- A gravimetric fuel analysis giving
mass composition of fuel elements
necessary to do combustion
calculations
- Combustion air introduced under
the grates of an incinerator or
furnace
- The effluent concentration in
precipitation normalized by the
effluent concentration in air
used to describe average conditions
over many precipitation events
- The furnace of a MWC that is *xned
with tubes recovering heat for
steam generation
- Removal of atmospheric contaminants
as a result of capture by cloud
droplets as well as precipitation
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Windboxes - multiple
air compartments
Zn
2,3,7,8 TCDD
The use of multiple compartments
under a furnace grate to allow for
better undergrate combustion air
distribution
- Zinc
2,3,7,8 tetrachlorodibenzo-p-
dioxin
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DATE DUE
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