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Copyright © 2000 National Academy of Sciences
All right reserved.
Incineration is widely used to reduce the volume of municipal solid waste, to reduce the potential infectious properties and volume of medical waste, and to reduce the potential toxicity and volume of hazardous chemical and biological waste. In the United States, more than 100 facilities incinerate municipal solid waste, and more than 1,600 facilities incinerate medical waste. Also, almost 200 incinerators and industrial kiln facilities, and many industrial boilers and furnaces combust hazardous and nonhazardous waste.
Whether incineration is an appropriate means of managing waste has been the subject of much debate in this country. A major aspect of the debate is the potential risk to human health that might result from the emission of pollutants generated by the incineration process; some of those pollutants have been found to cause various adverse health effects. Although such effects have generally been observed at much higher ambient concentrations than those usually produced by emissions from an incineration facility, questions persist about the possible effects of smaller amounts of pollutants from incineration facilities, especially when combined with the mix of pollutants emitted from other sources. The possiblesocial, economic, and psychologic effects associated with living or working near an incineration facility also have been topics of concern.
This report was prepared by the National Research Council's Committee on Health Effects of Waste Incineration. The committee was formed to assess relationships between waste incineration and human health and to consider specific issues related to the incineration of hazardous waste, municipal solid waste, and medical waste. The committee was asked to consider various design, siting, and operating conditions at waste-incineration facilities with respect to releases of potentially harmful pollutants to the environment. It was also asked to consider appropriate health-based approaches for demonstrating that an incineration facility meets and maintains established levels of health protection. Issues related to communication of information on waste incineration were also within the study charge. The committee was asked to consider types of information that should be provided to government officials, industry managers, and the general public to help them in future efforts to understand and weigh the risks associated with waste incineration and its alternatives. Finally, the committee was asked to consider factors that might affect public perceptions of waste incineration.
The committee was not charged to assess risks posed by any particular waste-incineration facility or to compare the risks of incineration with risks posed by various waste-management alternatives, such as landfilling. The committee focused its attention on wastes that have reached an incineration facility-it was not asked to address the collection or storage of wastes at, or their transportation to, any incineration facility; nor was it asked to consider treatment of residual ash away from a facility.
WASTE-INCINERATION PROCESSES AND EMISSIONS
The principal gaseous products of waste incineration, like other combustion processes, are carbon dioxide and water vapor. And, like many combustion processes, incineration also produces byproducts such as soot particles and other contaminants released in exhaust gases, and leaves a residue (bottom ash) of incombustible and partially combusted waste that must be emptied from incinerator chambers and properly disposed. The composition of the gas and ash byproducts is determined, at least in part, by the composition of the wastes fed into an incineration facility. This feedstream composition can be altered by other waste-management activities, such as reducing the amount of waste generated, reusing materials, and recycling waste materials for use as feedstocks for various manufacturing processes.
The exhaust gases from waste incineration facilities may contain many potentially harmful substances, including particulate matter; oxides of nitrogen; oxides of sulfur; carbon monoxide; dioxins and furans; metals, such as lead and mercury; acid gases; volatile chlorinated organic compounds; and polycyclic aromatic compounds. Some pollutant emissions are formed, in part, by incomplete combustion that may in turn lead to the formation of pollutants such as dioxins and furans. The formation of products of incomplete combustion is governed by the duration of the combustion process, the extent of gas mixing in the combustion chamber, and the temperature of combustion. Good combustion efficiency depends upon maintaining the appropriate temperature, residence time, and turbulence in the incineration process. Optimal conditions in a combustion chamber must be maintained so that the gases rising from the chamber mix thoroughly and continuously with injected air; maintaining the optimal temperature range involves burning of fuel in an auxiliary burner during startup, shutdown, and process upsets. The combustion chamber is designed to provide adequate turbulence and residence time of the combustion gases.
Operation of the incinerator also affects the emission of heavy metals, chlorine, sulfur, and nitrogen that may be present in the waste fed into the incinerator. Such chemicals are not destroyed during combustion, but are distributed among the bottom ash, fly ash, and released gases in proportions that depend on the characteristics of the metal and the combustion conditions. Mercury and its compounds, for example, are volatile, so most of the mercury in the waste feed is vaporized in the combustion chamber. In the cases of lead and cadmium, the distributions between the bottom ash and fly ash depend on operating conditions. At higher combustion-chamber temperatures, more of the metals can appear in the fly ash or gaseous emissions. Therefore, combustion conditions need to maximize the destruction of products of incomplete combustion and to minimize the vaporization and entrainment of heavy metals, especially when adequate control of emissions is lacking. Formation of oxides of nitrogen is promoted by high temperatures and the presence of nitrogen-containing wastes.
In addition, air-pollution control devices can greatly influence emissions from waste-incineration facilities. For example, airborne particles can be controlled with electrostatic precipitators, fabric filters, or wet scrubbers. Hydrochloric acid, sulfur dioxide, dioxins, and heavy metals can be controlled with wet scrubbers, spray-dryer absorbers, or dry-sorbent injection and fabric filters. Oxides of nitrogen can be controlled, in part, by combustion-process modification and ammonia or urea injection through selective catalytic or noncatalytic reduction. Concentrations of dioxins and mercury can be reduced substantially by passing the cooled flue gas through a carbon sorbent bed or by injecting activated carbon into the flue gas.
With current technology, waste incinerators can be designed and operated to produce nearly complete combustion of the combustible portion of waste and to emit low amounts of the pollutants of concern under normal operating conditions. In addition, using well-trained employees can help ensure that an incinerator is operated to its maximal combustion efficiency and that the emission-control devices are operated optimally for pollutant capture or neutralization. However, for all types of incinerators, there is a need to be alert to off-normal (upset) conditions that might result in short-term emissions greater than those usually represented by typical operating conditions or by annual national averages. Such upset conditions usually occur during incinerator startup or shutdown or when the composition of the waste being burned changes sharply. Upset conditions can also be caused by malfunctioning equipment, operator error, poor management of the incineration process, or inadequate maintenance.
Typically, emissions data have been collected from incineration facilities during only a small fraction of the total number of incinerator operating hours and generally do not include data during startup, shutdown, and upset conditions. Furthermore, such data are typically based on a few stack samples for each pollutant. The adequacy of such emissions data to characterize fully the contribution of incineration to ambient pollutant concentrations for health-effects assessments is uncertain. More emissions information is needed, especially for dioxins and furans, heavy metals, and particulate matter.
Government agencies should continue to improve-or in some cases should begin-the process of collecting, and making readily available to the public, substantially more information on the following:
The effects of design and operating conditions on emissions and ash. Such information should show how specific emissions and ash characteristics are affected by modifying the operating conditions of an incinerator to maximize its combustion efficiency. It should also indicate the types and combinations of operating conditions that optimize the effectiveness of emission-control devices. New combustor designs; continuous emission monitors; emissions-control technologies; operating practices; and techniques for source reduction, fuel cleaning, and fuel preparation, including records of demonstrated environmental performance and effects on emissions and ash. Emission and process conditions during startup, shutdown, and upset conditions. Emissions testing has usually been performed under relatively steady-state conditions. However, the greatest emissions are expected to occur during startup, shutdown, and malfunctions. Such emissions need to be better characterized with respect to possible health effects. Therefore, data are needed on the level of emissions, the frequency of accidents and other off-normal performance, and the reasons for such occurrences.
ENVIRONMENTAL PATHWAYS OF HUMAN EXPOSURE
After pollutants from an incineration facility disperse into the air, some people close to the facility may be exposed directly through inhalation or indirectly through consumption of food or water contaminated by deposition of the pollutants from air to soil, vegetation, and water. For metals and other pollutants that are very persistent in the environment, the potential effects may extend well beyond the area close to the incinerator. Persistent pollutants can be carried long distances from their emission sources, go through various chemical and physical transformations, and pass numerous times through soil, water, or food.
Dioxins, furans, and mercury are examples of persistent pollutants for which incinerators have contributed a substantial portion of the total national emissions. Whereas one incinerator might contribute only a small fraction of the total environmental concentrations of these chemicals, the sum of the emissions of all the incineration facilities in a region can be considerable. Many older incinerators have been closed down and replaced by modern low-emitting units, so the relative contribution of incineration to the current concentrations of chemicals in the environment is uncertain.
Results of environmental monitoring studies around incineration facilities have indicated that the specific facilities studied were not likely to be major contributors to local ambient concentrations of the substances of concern, although there are exceptions. However, methodological limitations of those studies do not permit general conclusions to be drawn about the overall contributions of waste incineration to environmental concentrations of those contaminants.
Although emissions from incineration facilities can be smaller than emissions from other types of sources, it is important to assess incinerator emissions in the context of the total ambient concentration of pollutants in an area. In areas where the ambient concentrations are already close to or above environmental guidelines or standards, even relatively small increments can be important.
Computational models for the environmental transport and fate of contaminants through air, soil, water, and food can provide useful information for assessing major exposure pathways for humans, but, in general, they are not accurate enough to provide estimates of overall environmental contributions from an individual facility within a factor of 10. The models suggest that fish consumption is the major pathway of human exposure to mercury, and that meats, dairy products, and fish are potentially the major exposure pathways for dioxins and furans. For assessment of persistent pollutants, there is usually a poor correlation between total ambient concentrations and local emissions from an incinerator.
Environmental assessment and management strategies for emissions from individual incineration facilities should include a regional-scale framework for assessing dispersion, persistence, and potential long-term impacts on human health. Better material balance information-including measurements of source emissions to air and deposition rates to soil, water, and vegetation-are needed to determine the contribution of waste-incineration facilities to environmental concentrations of persistent chemicals. The variation of these emissions over time needs to be taken into account: for the short term to determine if any important emission increases occur at an incineration facility, and for the long term to measure changes due to the replacement of less-efficient incinerators with modern, lower-emitting units. To facilitate evaluation of the overall contributions of incinerators to pollutants in the environment, estimates of dispersion of incinerator emissions into the environment should be gathered. The additional information would allow conversion of emissions estimates into environmental concentration estimates. Government agencies should link emissions and facility-specific data from all incineration facilities to characterize better the contributions of incinerators to environmental concentrations. Existing databases should be linked to provide easy access to specific operating conditions of an incinerator, height and diameter of the emission stack, flow rate and temperature of the gases leaving the stack, local meteorological conditions, air-dispersion coefficients as a function of distance from a facility, and precise geographic location of the emission point. Data should be standardized for uniform reporting.
Few epidemiologic studies have attempted to assess whether adverse health effects have actually occurred near individual incinerators, and most of them have been unable to detect any effects. The studies of which the committee is aware that did report finding health effects had shortcomings and failed to provide convincing evidence. That result is not surprising given the small populations typically available for study and the fact that such effects, if any, might occur only infrequently or take many years to appear. Also, factors such as emissions from other pollution sources and variations in human activity patterns often decrease the likelihood of determining a relationship between small contributions of pollutants from incinerators and observed health effects. Lack of evidence of such relationships might mean that adverse health effects did not occur, but it could also mean that such relationships might not be detectable using available methods and data sources.
Pollutants emitted by incinerators that appear to have the potential to cause the largest health effects are particulate matter, lead, mercury, and dioxins and furans. However, there is wide variation in the contributions that incinerators can make to environmental concentrations of those contaminants. Although emissions from newer, well-run facilities are expected to contribute little to environmental concentrations and to health risks, the same might not be true for some older or poorly run facilities.
Studies of workers at municipal solid-waste incinerators show that workers are at much higher risk for adverse health effects than individual residents in the surrounding area. In the past, incinerator workers have been exposed to high concentrations of dioxins and toxic metals, particularly lead, cadmium, and mercury.
Excerpted from WASTE INCINERATION & PUBLIC HEALTH Copyright © 2000 by National Academy of Sciences. Excerpted by permission.
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|1||Scope of the Committee's Effort||12|
|2||Waste Incineration Overview||17|
|3||Incineration Processes and Environmental Releases||34|
|4||Environmental Transport and Exposure Pathways of Substances Emitted from Incineration Facilities||71|
|5||Understanding Health Effects of Incineration||112|
|6||Regulation Related to Waste Incineration||182|
|7||Social Issues and Community Interactions||217|
|8||Uncertainty and Variability||246|
|App. A||Biographical Information on the Committee on Health Effects of Water Incineration||295|
|App. B||Off-Normal Operations of Six Facilities||301|
|List of Abbreviations||311|