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.

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.

HEALTH EFFECTS

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.

REGULATION OF WASTE-INCINERATION FACILITIES

Waste-incineration facilities are required to comply with a combination of federal, state, and local regulations that vary from place to place and time to time. EPA has proposed or has promulgated separate regulations for incineration of medical, hazardous, and municipal solid wastes to reduce emissions to values achieved by the best-controlled 12% of incinerators. This standard is known as “maximum achievable control technology,” or MACT. Facilities that meet the MACT requirements are generally expected to have substantially lower emissions. The intended reduction in emissions would lower exposures and possible risks to populations surrounding incinerators, especially for particulate matter, lead, mercury, and other metals. However, the effects of such regulations are less apparent when emissions of the most-important pollutants from all incineration sources are considered on a regional scale. For example, the collective contribution of dioxins from multiple incineration sources might remain problematic despite MACT regulations. Because the collective effects of incinerators on metropolitan or regional scales are largely unknown, it is uncertain whether implementation of MACT standards for incinerators will substantially reduce the actual risks posed by persistent environmental pollutants at those scales.

Based on estimates of incinerator emissions, environmental transport and fate, potential total exposure, and relative toxicity of the individual substances inferred from studies not involving incineration, the committee concludes

• Compliance with MACT regulations is expected to reduce substantially local population exposures, especially for particulate matter, lead, mercury and other metals, acidic gases, and acidic aerosols.
• Substantial concerns about regional dioxin and furan exposures and moderate concerns about regional exposures to metals are not expected to be relieved by MACT regulations, because the regulations may not adequately reduce risks attributable to cumulative emissions on a regional basis.
• Substantial concerns about workers' exposures to particulate matter, lead, mercury, and dioxins and furans are not expected to be relieved by MACT compliance, because those regulations were not designed to affect workers' exposures.

SOCIAL ISSUES AND COMMUNITY INTERACTIONS

In addition to possible physical-health effects, a waste-incineration facility may have other effects on individuals, groups, or the entire population in the surrounding area. The effects might be economic (such as job creation or decrease in property values), psychological (such as stress or stigma), or social (such as community factionalization or unity). However, there is little rigorous information on those impacts of waste-incineration facilities.

Citizen concerns need to be heard and understood. Conflicts can increase the time and expense of conducting waste incinerators and other facilities that might be potentially beneficial to society. Opposition to the facilities also can indicate that important concerns are not being addressed adequately.

Much public opposition to waste incineration might be due to a lack of understanding of the relative health risks posed by incineration in comparison with other waste-management methods. But health is not the only issue, and the differences between expert and public perceptions are not due merely to differences in information and understanding; they can also be due to differences in social values. People's perceptions are often extraordinarily resistant to change, in part because they reflect underlying values. Efforts that ignore or try to change these perceptions radically are likely to fail. Risk communication should accept as legitimate the perceptions and concerns of various members of the public and involve them in consultative, participatory processes. Not only do members of the public have a right and responsibility to be involved in the assessment and management of hazards in their communities, but such involvement might result in improved assessments and management strategies.

Developing effective participatory programs is very difficult, but some general principles are beginning to emerge. The process of public involvement should be open, inclusive, and substantive, and members of the public in an affected area should be involved early and often. Major concerns are likely to include issues of safety, compensation, and local oversight and control. Satisfying the public's need for information on incinerator safety requires continual assessment and demonstration of regulatory compliance with existing standards.

UNCERTAINTY AND VARIABILITY

Incineration facilities vary with regard to types of waste incinerated, operating practices, allowable magnitudes of emission, emission-control technologies, types of substances emitted, environmental conditions, proximity to other sources of contaminants, and frequency of process upsets. The people who might be exposed to the contaminants are likely to differ in their susceptibilities and activity patterns. Some uncertainties are specific to waste incineration, and some are inherent in any activity that releases contaminants into the environment.

Some of the uncertainties and variability can be reduced or better accounted for; others will remain intractable. The most-effective decisions concerning the siting, design, operation, and regulation of incineration facilities are the ones that take uncertainty and variability fully into account.