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274 Waste Management Practices: Municipal, Hazardous, and Industrial
will a particular metal leach from ash and enter the groundwater supply or some other environ-
mental receptor? Some have questioned the “leachability methods”, however, for being incomplete
in terms of assessing toxicity. The total concentrations of both metals and PCDDs must be consid-
ered in assessing ash toxicity, because humans and the environment can be exposed to ash through
many routes in addition to contaminated groundwater. For example, humans can inhale ash parti-
cles into the lungs after which toxins on the particles are directly absorbed into the tissue or blood-
stream. In addition, ash particles may be ingested, either directly or through contaminated food or
water. Because these exposure routes can be highly significant, a full assessment of the hazards
posed by ash must include data of its total chemical composition.
Reducing the hazard relating to metals and other toxins in ash requires several actions (Denison
and Rustin, 1990):

• Keeping toxic metals out of products that may enter the waste stream
• Keeping metal-containing materials out of incinerators
• Chemically or physically treating ash prior to disposal (e.g., mixing with Portland cement
and allowing to set)
• Disposing of all ash in secure facilities that do not contain other types of waste (this prac-
tice is termed “monofilling”)
• Compacting the ash prior to or during landfilling

3
3
Noncompacted MSW ash may have a density of 900 kg/m (1500 lb/yd ). If the ash is com-
3
3
pacted the density increases to as high as 1980 kg/m (3300 lb/yd ). At this density the ash is highly
impermeable; permeability may be as low as 1 x 10 9 cm/sec (Vesilind et al., 2002). As more and
more ash is being produced and landfill space is becoming more difficult to acquire, alternative uses
for ash are being sought. Some other uses include:
• Road base material
• Structural fill
• Gravel drainage ditches
• Capping strip mines
• Mixing with cement to make building (construction) blocks

In addition, ash from MSW combustion contains metals that can be reclaimed, especially steel
and aluminum.

9.7 MSW INCINERATION IN THE UNITED STATES: THE FUTURE
A number of factors have been operating over the last several years to expand the use of incinera-
tion in managing MSW. These include (Denison and Rustin, 1990):

• Diminishing landfill capacity, especially in heavily urbanized areas of the Northeast,
along with rising landfill costs and difficulties with siting.
• An aggressive marketing campaign conducted by incinerator vendors.
• A public perception that waste-to-energy is cleaner and more conserving of resources
than landfilling.
• A perception that the convenience that incineration provides is preferred, albeit more
expensive, to implement than are municipal-level recycling programs.

However, a number of arguments continue regarding the suitability of incineration. Factors
restricting the development of incineration include:

• Intense public opposition
• Unresolved risk issues relating to air emissions and ash residues