United States
Environmental Protection
Agency
Office of Health and
Environmental Assessment
Washington, DC 20460
Research and Development
EPA/600/S8-91/031 March 1992
EPA Project Summary
Methodology for Assessing
Environmental Releases of and
Exposure to Municipal Solid
Waste Combustor Residuals
Matthew Lorber
This document provides risk asses-
sors with guidance on evaluating the
exposure and risk that could arise from
the disposal of ash from municipal solid
waste (MSW) combustors. The risk
commonly associated with MSW com-
bustors has been from direct exposure
to combustor emissions. This docu-
ment now allows for a more complete
evaluation of exposure and risk from
release of contaminants of MSW com-
bustion. This document accomplishes
the following: (1) summarizes existing
information on MSW combustor design,
types, and location of MSW facilities
nationally; beneficial uses of ash; MSW
ash contaminant measures; and con-
taminant concentrations of ash, (2)
summarizes the complete MSW com-
bustion operation to identify points of
environmental release of ash until ulti-
mate disposal in a landfill, (3) provides
methodologies to quantify these re-
leases, and (4) directs the risk asses-
sor to other documents particularly
which detail fate and transport models,
cind exposure and risk algorithms. The
document closes with an example of
the methodologies applied to an or-
ganic contaminant, 2,3,7,8-tetrachloro-
dibenzo-p-dioxin, and an inorganic con-
taminant, cadmium, both common in
MSW combustor ash.
This Project Summary was developed
by EPA's Office of Health and Environ-
mental Assessment, Washington, DC,
to announce key findings of the re-
search project that is fully documented
in a separate report of the same title
(see Project Report ordering informa-
tion at back)
Introduction
Incineration of municipal solid waste
(MSW) is increasingly being used as a
means of reducing the volume of waste
that must ultimately be disposed in a land-
fill. However, the combustion residuals
that are generated by the combustion of
such wastes must still be disposed of in
some manner. In the past, risk assess-
ments of MSW incineration have been
concerned with human health and envi-
ronmental impacts resulting from stack
emissions. This document focuses on
another fundamental issue, that of expo-
sure to municipal waste combustion
(MWC) residuals.
The objective of this document is to
develop guidance for assessing human
exposure to MWC residuals or their chemi-
cal constituents that is consistent with
methods that are already in use within the
U.S. Environmental Protection Agency
(EPA). Existing guidance includes meth-
ods for selection and use of media-spe-
cific fate and transport models, and meth-
ods for performing multi-media pathway-
specific exposure assessments. To avoid
redundancy with existing documents, this
methodology will direct assessors to the
appropriate EPA source materials and fo-
cus on issues germane to the assess-
ment of risk posed by exposure to MWC
residuals.
Background Information
The materials flowing out of a MSW
incinerator are ashes, quench water, and
gases. The ashes can be divided into two
main categories: bottom ash and fly ash.
Bottom ash consists of slags and cinders
remaining in the combustion chamber af-
Printed on Recycled Paper
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ter burning. These are generally non-com
bustible materials and materials with boil-
ing points greater than the combustion
temperature. The bottom ash is usually
removed from the combustion chamber
by a conveyor and then passed through a
quench system to wet and cool the ash.
Fly ash includes those particulates and
fine particles that are collected from the
stack and pollution control devices. The
small particles that make up fly ash are
non-combustible materials and may be
carried by the combustion gases.
The different effluents that result from
MSW incineration may be managed in a
number of ways. Bottom and fly ash may
be mixed together or managed separately.
Quench water may be completely or par-
tially recycled within the facility, or dis-
charged as effluent. The bottom and fly
ash make up the bulk of municipal waste
combustion residuals, and their manage-
ment represent a principal concern.
A number of different incinerator types
are currently in operation in the United
States. Mass burn incinerators make up
almost 90 percent of MWC facilities in
operation in the United States. A mass
burn incinerator is so termed because it
incinerates unprocessed municipal waste.
The central component of a mass burn
incinerator is the furnace. Refuse derived
fuel (RDF) combustion uses MSW that
has been processed to some degree. RDF
incinerators are less commonly used than
mass burn incinerators in the United
States. RDF has been classified accord-
ing to the amount of processing it has
undergone.
Approximately 140 to 155 MWC facili-
ties were in operation in the United States
in 1988. These facilities have an esti-
mated total installed capacity of 78,700
tons per day, or about 16 percent of the
total municipal solid waste stream. How-
ever, these capacities are not actually
achieved, so that approximately 10% of
the total waste stream is actually treated
in this manner. The total number of incin-
erators in the United States has been
projected to increase to 227 by 1992.
Ash Characterization
High concentrations of heavy metals and
other inorganic constituents have been re-
ported to be present in ash. Higher con-
centrations of inorganic chemicals have
generally been found in fly ash than in
bottom ash. For example, fly ash usually
has greater mass values of cadmium and
lead than bottom ash. However, bottom
ash has generally been found to have
greater total mass values for silicon, alu-
minum, calcium, iron, copper, and zinc.
Polycyclic aromatic hydrocarbons
(PAHs), phthalates, chlorobenzenes, and
chlorophenols are the most prevalent types
of organic compounds found in municipal
waste combustor ashes. The concentra-
tions of organic constituents are generally
greater in fly ash than in bottom ash,
while the concentrations of organic con-
stituents in combined ash have intermedi-
ate values. The various data sets also
indicate an absence of volatile organic
compounds. Volatile compounds would
not be expected in materials that are com-
busted at temperatures that are much
higher than their boiling points, such as
those temperatures present in municipal
waste combustors.
PCDDs, PCDFs, and PCBs, and their
homologs, have been detected and quan-
tified in MWC ash. These compounds are
usually found in greater concentrations
among the smaller particle sizes, such as
found in fly ash. This differential partition-
ing may be explained by the fact that
smaller sized particles have larger surface
areas relative to weight and therefore have
a greater area for sorption per mass unit.
Laboratory leaching procedures, includ-
ing the Extraction Procedure (EP-Tox) and
the Toxicity Characteristic Leaching Pro-
cedure (TCLP), have been used to deter-
mine whether a waste, including ash,
should be handled under hazardous waste
regulations. A wide range of concentra-
tions for common inorganic contaminants
(lead, arsenic, zinc, etc.) in laboratory
leachates is noted for these contaminants,
and significant differences are also noted
in a study comparing concentrations of
inorganic contaminants in field and labo-
ratory leachates from the same ash source.
Few data exist on the concentrations of
most organic constituents in fly and bot-
tom ash extracts from laboratory leaching
tests. The concentrations of most organic
constituents analyzed are generally below
detection limits or at trace levels.
Estimating Environmental
Releases
Disposal of MWC residual in a landfill
is by far the most common fate for such
wastes. The sources of potential environ-
mental release of residuals during dis-
posal activities include:
• Vaporization and fugitive emissions
within the incinerator conveyor sys-
tem during quenching and movement
of ash to the storage/transport con-
tainers,
• Contaminated water releases from
spray and quench water,
• Fugitive emissions, ground contami-
nation, and runoff when the ash is
dropped into the transport/storage
containers or dumped into temporary
storage piles or pits,
• Fugitive dust emissions during truck
loading and travel,
• Fugitive emissions during unloading
and spreading operations at the dis-
posal site, and
• Fugitive emissions, runoff, and
leachate generation at the disposal
site.
The last four bullets above discuss a
key category of environmental release -
that of fugitive dust emissions. Estimating
such emissions can be accomplished us-
ing "AP-42" emission factors. These have
been developed by EPA's Office of Air
Quality Planning and Standards (EPA,
1985; EPA, 1988). An example of such
an emission factor equation is the one
given for emissions resulting from vehicu-
lar traffic on unpaved surfaces:
Eup = k[1.7(s/12)(Vs/48)(W/2.7)07
(nw/4)05(365-P)/365)]
where:
Eup = emission factor for unpaved
surfaces (kg/VKt - vehicle kilo
meter traveled)
k = particle size multiplier
(dimensionless)
s = silt content (%)
Vs = mean vehicle speed (km/hr)
W = mean vehicle weight (kg)
nw = mean number of wheels
P = number of days with at least
0.254 mm (0.01 inch)
precipitation per year.
This emission equation, like others,
gives total emissions in kg of ash and as
a function of a specific activity - in the
above example, the amount of vehicle ki-
lometers traveled over the roadway. Esti-
mating the contaminant released in a fugi-
tive emission requires several additional
steps. Site-specific data or assumptions
are needed to assign values to the s, Vs,
W, nw, and P parameters. The k param-
eter is a function of the dust particle size
for which estimations are desired, with k
assigned for 2.5 um (k = 0.095) to 30 fim
(k = 0.80). Estimating the vehicle kilome-
ters traveled requires assumptions on the
length of impacted roadway and the num-
ber of vehicles traveling over that road-
way per day. Not included in emission
factor equations is an additional "control
efficiency factor," which reduces estimated
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emissions based on control practices. For
dust emissions from roadways, common
controls include wetting or chemical dust
suppression. Finally, the contaminant con-
centration on the emitted dust needs to
be known or estimated.
Simple water balance models are used
to estimate the amount of water leaching
or running off disposal and storage sites.
Water release totals are estimated in terms
of centimeters (or inches) and are con-
verted to a volume based on the area of
disposal or storage. Also required is the
concentration of the contaminant in the
runoff or leachate water.
Tabular summaries of concentrations of
contaminants in ash and in leachate pro-
vide initial estimates for these key terms
in the absence of site-specific data. Also
provided is general guidance on fate and
transport models, including key citations,
which estimate concentrations individuals
are exposed to downwind (or downstream)
of the release.
Example Application
The document closes with a compre-
hensive example demonstrating all meth-
odologies on a hypothetical facility pro-
ducing 200,000 tons of combined fly and
bottom ash per year. The example uses
two contaminants, cadmium (Cd) and
2,3,7,8-tetrachlorodibenzo-p-dioxin
(TCDD), to demonstrate the methodology.
The concentration of cadmium and TCDD
in fugitive ash emissions is assumed to
be equal to the upper-bound reported con-
centrations of Cd, 100 u.g/g, and TCDD,
7.8 x 10"4 |ig/g, in combined bottom and
fly ash. The concentration in water re-
leases is assumed to be equal to the EP-
Tox limit for Cd, 100 u.g/L, and the upper-
bound reported concentration for TCDD,
2.3 x 10-* jig/L, in extract from combined
ash using the TCLP.
The example describes the many as-
sumptions required for assessing environ-
mental releases. The assumptions required
for estimating fugitive emissions from road-
ways in the hypothetical landfill were, for
example:
Particle size multiplier: Total emis-
sions will be estimated. Therefore, the k
value for the largest sized particles, those
with diameters <30 |im, will be used: k =
0.80.
Silt Content: MRI (1990) took two
samples on unpaved haul routes in two
landfills, and obtained silt contents of 6.7
and 20.1%. The mean of these two val-
ues, 13.4%, will be assumed.
Vehicle Speed: MRI (1990) assumed
a vehicle speed of 24 km/hr (15 mph) in
their assessment of dust resuspension.
This will be assumed in this example.
Vehicle Weight: A range of vehicle
weights for vehicles disposing materials in
industrial solid waste (ISW) landfills was
gliven as 14-40 x 103 kg in MRI (1990).
The midpoint of this range, 27 x 103 kg
will be assumed here.
Number of Wheels: A range for the
number of wheels was given as 6-14 in
MRI (1990). The midpoint of this range,
10, will be assumed here.
Precipitation: A value of 121 days,
one-third of the time and reasonable for
mid-continent conditions, will be applied
to the example scenario.
Length of Haul Route: MRI conducted
a survey of MWC ash disposal sites, in-
cluding ISW landfills (described in MRI,
1990). The median haul route length from
46 ISW landfills was 407 m (1320 ft). Like
the assessment of resuspended dust in
the vicinity of storage areas, it will be
assumed that 25% of this length, or 102
rn, is impacted by the ash disposal activi-
ties.
Vehicle Passes per Day: In this same
survey, MRI determined that the median
number of daily vehicle transactions, ex-
cluding ash haulers, was 26. However,
not all 26 vehicles will pass over the same
impacted roadway, since different portions
of the landfill could be active. Assuming
half of them pass over the impacted road-
way, or 13 trucks, this translates to 26
passes. This is added to the 20 ash haul-
ers per day, or 40 passes, for a total of 66
vehicle passes per day for this example.
The total fugitive emission of dust as a
result of vehicular resuspension was 506.4
kg/km travelled. Assuming a control effi-
ciency of 90%, this emission is reduced to
51 kg/km. MRI (1990) took further data
showing that the ratio of concentrations of
metals in ash being disposed of and in
roadside particulates ranged from 0.01 to
0.45. Finally, estimates of Cd and TCDD
emitted were 2.1 x 109 uxj/yr and 1.64 x
104 u,g/yr respectively.
References
Environmental Protection Agency (EPA),
1985. Compilation of Air Pollutant Emis-
sion Factors. Volume 1. Stationary
and Area Sources. Fourth Edition. U.S.
Environmental Protection Agency, Of-
fice of Air Quality, Planning and Stan-
dards. Research Triangle Park, NC.
Environmental Protection Agency (EPA),
1988. Compilation of Air Pollutant Emis-
sion Factors. Volume 1. Stationary
and Area Sources. Fourth Edition.
Supplement B. U.S. Environmental Pro-
tection Agency, Office of Air Quality,
Planning and Standards. Research Tri-
angle Park, NC.
Midwest Research Institute (MRI), 1990.
Special Management Standards for Mu-
nicipal Waste Combustion (MWC) Ash.
Prepared by Midwest Research Insti-
tute, Kansas City, MO, for U.S. EPA,
Municipal Solid Waste Program. EPA
Contract No. 68-01-7287. June 29,
1990.
•&U.S. GOVERNMENT PRINTING OFFICE: I992 - 648-080/40213
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The EPA author, Matthew Lorber (also the EPA Project Officer, see below) is
with the Office of Health and Environmental Assessment, Washington, DC
20460.
The complete report, entitled "Methodology for Assessing Environmental Releases
of and Exposure to Municipal Solid Waste Combustor Residuals," (Order No.
PB92-109 149/AS; Cost: $26.00; subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Office of Health and Environmental Assessment
U.S. Environmental Protection Agency
Washington, DC 20460
United States
Environmental Protection
Agency
Center for Environmental
Research Information
Cincinnati, OH 45268
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