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<Municipal Solid Waste Management>

Sound Practices

1.5.4 Managing environmental impacts of air emissions and reshuffle ash

The major concerns about the environmental risks of MSW incinerators are the potential emission of contaminants into the air through exhaust stacks and into water through ash leachate. Proper planning to minimize environmental damage, as well as public education and involvement that directly address these issues, are essential to successful incineration programs.

Air emissions

The combustion of any substance will generate byproduct emissions that could be released to the air. The following air emissions are usually the main concerns associated with incineration facilities: metals, especially mercury, lead, and cadmium; organics such as dioxins and furans; acid gases such as sulfur dioxide and hydrogen chloride; particulate matter such as dust and grit; nitrogen oxides (which are ozone precursors); and other substances, such as carbon monoxide.

People can be exposed to emissions directly by inhaling contaminated air, or by ingesting or having skin contact with contaminated soil and dust. A number of studies have suggested that ingestion and skin contact pose more significant risks than inhalation of emissions from MSW incineration facilities. Exposure can also occur indirectly by eating foods that have been contaminated with these substances. In addition to human health, plants and animals could also be adversely affected by emissions from incineration facilities. The ultimate effects will depend on the concentrations of the contaminants in emissions, the type of environmental controls employed, the height of the emission stack, the location of the facility, and prevailing weather and geographical conditions.

One related concern is contamination of the area close to the incinerator, underneath its emissions plume. Whatever pollutants do escape are more likely to reach the ground closer to the incinerator. It is therefore particularly important to site an incinerator in as isolated an area as possible. In general, industrial areas make more sense than other areas because the contamination levels found there may already have induced the taking of precautions.

Air pollution control equipment
In order to meet current environmental standards, modern pollution control equipment is designed to remove, and can effectively remove, the vast majority of the emissions of concern. Research from a wide variety of facilities in the US and elsewhere has found that, when properly operated, the best air pollution control equipment can potentially remove up to 99% of dioxins and furans, more than 99% of heavy metals, more than 99% of particulate matter, more than 99% of hydrogen chloride, more than 90% of sulfur dioxide, and up to 65% of nitrogen oxides.


Treatment Facility of Exhaust Gases
Exhaust gases from an incinerator can be forced through a wet scrubber to remove toxic gases.
(credit: Osaka City Public Cleaning Bureau)

The major air emission control technologies available for incinerators are fabric filters, electrostatic precipitators, and scrubbers. Fabric filters, also known as baghouses, are extremely effective in controlling emissions of metals and organic compounds that attach to fine particulates. Fabric filters consist of several cylindrical bags that filter emissions. Electrostatic precipitators also can be used to control particulate emissions; they electrically charge particulate emissions and then draw the particles to oppositely charged collection plates. The collection plates are shaken periodically to remove the particles, which constitute fly ash.

Scrubbers are used primarily to control acid gases, but they also remove some heavy metals. Wet scrubbers use a moving alkaline liquid solution to neutralize acids. Dry scrubbers use either a fine alkaline spray or powder to neutralize acids. The generally accepted state-of-the-art MSW air pollution control system for maximum pollution control is dry scrubbing followed by a baghouse.

After-burn systems, where potentially harmful emissions are minimized, typically serve as the primary air pollution control method in modular units. Some modular units employ additional air pollution control equipment that controls emissions as effectively as mass-burn facilities, and many modular units are planning to retrofit existing controls to meet higher standards. Others, however, will close due to the expense associated with upgrading units to meet more stringent air emission standards. Interest in modular incinerators is thus decreasing.

Good combustion practices
Proper control of air emissions, however, requires more than the presence of these control technologies. MSW incineration facilities must be well operated and well maintained to ensure that emissions are as low as possible. Good combustion practices can control emissions by ensuring that the temperature in the combustion chamber and the time the MSW remains in the combustion chamber are kept at optimal levels. Major variations in these or other incineration operations could lead to a limited but significant belch of contaminated air emissions. Newer incinerators are equipped with computer control systems to help maintain a high degree of consistency in plant operations. Air pollution control equipment must also be carefully maintained to prevent releases of contaminants.

These demanding technical considerations are among the obstacles to incineration in most developing countries. The difficulty, and the necessity, of maintaining emissions control systems in essentially perfect order over a long period of time is daunting even to industrialized countries. Small mistakes in the operation of such facilities can easily lead to significant emissions of toxic substances.

An important point is that these emission controls are not perfect. But designing more expensive controls to remove even more of the pollutants is unlikely to be cost-effective, compared to the environmental benefits that can be achieved by investments in other MSW technologies, such as leachate collection systems at landfills. Moreover, it should be recognized that incinerators are not the only source of these pollutants, so that controlling all emissions at the incinerator would not eliminate them from the environment.

Residual incinerator ash

MSW incineration generates ash, representing about 10% by volume and 25-35% by weight of the waste incinerated. Ash can be divided into two categories: bottom ash and fly ash. Bottom ash is completely or partially combusted material that passes through or is discharged from the combustion grate. Fly ash is the term for particulate matter captured from flue gas by the air pollution control system; it could include scrubber residue, baghouse dust, and what is shaken from precipitators. The total ash generated by mass-burn and modular incinerators is 75-85% bottom ash and 15-25% fly ash, by weight. Localities considering incinerators must therefore plan how and where ash will be managed in an environmentally sound manner; this planning is best done early in project development.

Bottom ash and fly ash are often managed together and referred to as “combined ash.” Incinerator ash is usually disposed of in an MSW landfill (ideally in a special section) or an ash-only landfill known as an ash monofill. Ash monofills are specially designed to reduce the ability of heavy metals to migrate from the ash into the environment. Monofills are often co-located with MSW incinerators or existing landfills to reduce transportation distances and siting difficulties.

Environmental impacts
Incinerator ash can contain concentrations of heavy metals such as lead, cadmium, mercury, arsenic, copper, and zinc, which originate from plastics, colored printing inks, batteries, certain rubber products, and hazardous waste from households and small industrial generators. Organic compounds such as dioxins and furans have also been detected in incinerator ash.

The principal environmental concern of the public regarding incinerator ash is that when ash is disposed of in a landfill, the metals and organic compounds can leach (i.e., dissolve and move from the ash through liquids in the landfill) and migrate into ground water or nearby surface water. In addition to possibly contaminating water supplies, incinerator ash could also affect human health through direct inhalation or ingestion of airborne or settled ash.

The actual magnitude of these risks has been intensely debated by researchers, industry, and the public. Incinerator ash sometimes fails hazardous waste tests for toxicity, but some studies have shown that these tests over-predict the actual environmental risks posed by the ash. In addition, field tests performed on leachate from actual ashfills indicate that the metals content is below US hazardous waste classification levels, and usually satisfies US drinking water standards. Despite the controversy, local MSW public education programs should explain these risks and the management techniques used to minimize them.

Potential solutions
Because incinerator ash is usually disposed of in an MSW landfill, the environmental controls typically installed for environmentally sound sanitary landfills (e.g., liners and leachate collection/treatment) become all the more important. Ash monofills can be specially designed to reduce the ability of heavy metals to migrate from the ash into the environment.

Ash can be stabilized and solidified by encasing in concrete prior to disposal, thereby significantly reducing the potential for the contaminant to migrate. Some individuals also advocate managing fly ash and bottom ash separately, with additional stabilization of the fly ash through vitrification or pyrolysis, because fly ash can contain higher concentrations of metals. In addition to landfilling, incinerator ash has been used in the production of road bedding, concrete, brick, cinder block, and curbing. These uses have drawn some criticism as awareness has grown of the presence and the leachability of toxic constituents of these materials.

There is some evidence to suggest that recovering batteries and composting yard waste reduces the environmental impact of MSW incinerators, but this is not proven. According to some studies, diverting yard waste from incinerators to composting facilities could reduce the facility’s NOx emissions, which react in the atmosphere to form ozone. Most heavy metals (e.g., mercury, cadmium, and lead) come from items commonly found in MSW such as household batteries, thermostats, fluorescent lamps, plastics, and solder-bearing items (e.g., consumer electronics, light bulb sockets, and plated metals). Removing these items from the waste stream, at the household, commercial, and industrial sources, might therefore result in a significant reduction in the metals found in incinerator ash.

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