United States
Environmental Protection
Agency
Office of Health
and Environmental Assessment
Washington. DC 20460
Research and Development
EPA/600/S6-91/004 Sep. 91
EPA Project Summary
Development of Risk
Assessment Methodology for
Municipal Sludge Incineration
This Is one of a series of reports that
present methodologies for assessing
the potential risks to humans or other
organisms from management practices
for the disposal or reuse of municipal
sewage sludge. The management prac-
tices addressed by this series Include
land application practices, distribution
and marketing programs, landfllllng, In-
cineration and ocean disposal. In par-
ticular, these reports deal with meth-
ods for evaluating potential health and
environmental risks from toxic chemi-
cals that may be present In sludge.
This document addresses risks from
chemicals associated with sludge In-
cineration practices.
These proposed risk assessment pro-
cedures are designed as tools to assist
In the development of regulations for
sludge management practices. The pro-
cedures are structured to allow calcu-
lation of technical criteria for sludge
disposal/reuse options based on the
potential for adverse health or environ-
mental Impacts. The criteria may ad-
dress management practices (such as
site design or process control specifi-
cations), limits on sludge disposal rates
or limits on toxic chemical concentra-
tions In the sludge.
This Project Summary was developed
by EPA's Environmental Criteria and
Assessment Office, Cincinnati, OH, to
announce key findings of the research
project that Is fully documented In a
separate report of the same title (see
Project Report ordering Information at
back).
Introduction
Sludge incineration involves the com-
bustion of sludge within a mechanically
controlled environment. Incineration is a
treatment method used before disposal or
reuse of the ash. Because incineration
drastically reduces the volume and mass
of sludge, it has been traditionally consid-
ered a disposal method similar to landfilling
and ocean dumping. Land applications (in-
cluding distribution and marketing of
sludge) are considered use options, since
in these cases beneficial properties of
sludge are utilized.
Currently, only three types of incinera-
tion technology are employed for dedi-
cated sludge combustion in the United
States:
1. Multiple-hearth furnace (MHF);
2. Fluidized-bed (FB) furnace; and
3. Infrared electric furnace.
Uniform feed of sludge is critical to the
satisfactory operation of incineration sys-
tems. Realization of uniform feed requires
good control of sludge thickening, blend-
ing, sludge age control and pumping be-
fore dewatering. If these tasks are man-
aged and maintained properly and the de-
watering equipment is operated correctly,
the output of the dewatering equipment
will be uniform. A variation of ±10% of the
selected feed rate over 8 hours is accept-
able. Uniform feed assures stable opera-
tion and prevents upsets that could lead
to excessive emissions.
Virtually all incinerators currently oper-
ating in the United States are equipped
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with wet scrubbers for emission control.
The wide variety of wet scrubbers in use
includes fixed and variable-throat Venturi
impingement plates and cyclonic scrub-
bers. Most sludge incinerators, particu-
larly those built in the last 10 years, are
equipped with variable-throat Venturi
scrubber units and an impingement
subcooling tray separator. Over 70% of
the incinerators installed since 1978 are
equipped with combination Venturi/im-
pingement tray scrubbers.
Identification of Key Pathways
The air emissions, ash residue (wet or
dry) and scrubber water represent the
major pathways by which pollutants enter
the environment and potentially affect hu-
man health. The air emissions pathway
begins with the paniculate and gaseous
emissions generated by the combustion
process. These emissions pass through a
wet scrubbing air pollution control system
that reduces the paniculate and gaseous
pollutant concentrations in the exhaust gas.
There are several regulatory programs that
exercise control over air pollutants emit-
ted from sludge incineration processes.
Municipal sludge incineration in well-
operated facilities produces an odorless
ash weighing between 30 and 60% of the
weight of the original sludge on a dry
basis. Municipal wastewater plants in the
United States generate about 72 million
dry tons of sludge each year. Approxi-
mately 2 million tons of this are inciner-
ated, resulting in approximately 700,000
tons of ash that must finally be disposed
of in an environmentally compatible man-
ner.
The scrubber water is usually treated
separately by flocculation and sedimenta-
tion to reduce its solids content. Residue
produced during scrubber water treatment
may be dewatered and disposed of with
the incinerator ash. Treated scrubber wa-
ter effluent after solids removal is almost
universally recycled back through the
wastewater treatment plant influent.
Overview of Method
The greatest concern associated with
air emissions from incinerators is the po-
tential public health risk associated with
the inhalation of airborne gases and par-
ticles. Certain emitted contaminants are
suspected human carcinogens, and oth-
ers can exert other acute or chronic ef-
fects. Thus, incinerator air emissions are
selected as the critical pathway in evalu-
ating the human health risk from the sludge
incineration process.
The resulting methodology was devel-
oped considering the following:
1. Identify the human health and envi-
ronmental impacts and exposures consid-
ered acceptable.
2. Define the human population exposed
to the ground level ambient concentra-
tions for representative facilities.
3. Define, for the air exposure route,
the maximum allowable daily and annual
ground level concentrations of the pollut-
ants of concern that will satisfy the health
and exposure criteria in Step 1.
4. Define the stack modeling character-
istics, such as stack height, exit diameter,
gas fbw, gas temperature, and meteorol-
ogy for selected model plants.
5. Select the appropriate air dispersion
model(s) for the models/plants and calcu-
late the emission rates associated with
the acceptable ambient concentrations.
6. Calculate allowable sludge concen-
trations and mass loadings for the pollut-
ants of concern.
Exposure and Assessment of
Health Effects
Two approaches for exposure health
assessment may be performed by this
methodology: an aggregate risk approach
and the most exposed individual (MEI)
approach; the human exposure model
(HEM) assesses aggregate risk.
The U.S. EPA's Office of Air Qu*>lu-
Planning and Standards' Pollutant As
ment Branch has developed a huma
posure model. The impact parameters
(exposure, hazard and risk) are the basis
of the HEM computations of the individual
and community health effects resulting
from the emissions of chemical species.
These concepts are defined as follows:
Population — The number of persons
in contact with the concentration.
Exposure — The population multiplied
by the concentration.
Carcinogenic Potency — This param-
eter is quantified by the unit risk factor,
the probability of developing cancer due
to continuous exposure to 1 ug/m3 of the
species over a 70-year lifetime.
Hazard — Concentration multiplied by
the unit risk factor.
Risk — Exposure multiplied by the unit
risk factor.
The human exposure model uses a
finely detailed national census data base
to compute the impact parameters of ex-
posure and dose. The resulting risk pat-
terns are dependent not only on concen-
tration but also on population patterns.
The degree of contaminant exposure to
individuals residing in an area where emis-
sions from a municipal sludge incinerator
exist depends upon the following: the '<••-
ration of exposure; the volume of a
haled; the particle size distribution ft
cinerator emissions; the annual average
contaminant concentrations; and the num-
ber of people exposed.
The most exposed individual (MEI) is
assumed to reside in the area of the maxi-
mum annual ground level concentration
and is exposed 24 hours a day. The
incinerator(s) will be assumed to be op-
erational for the life of the individual (70
years) and to be operating 100% of the
time. The 70-year exposure is a valid
estimate since sludge incinerators are ex-
pected to be operational indefinitely. A
reference air concentration (RAC) is cal-
culated as the maximum concentration that
the MEI will be permitted to be exposed to
for any particular contaminant.
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Norm Kowal is the EPA Project Officer (see below).
The complete report, entitled "Development of Risk Assessment Methodology for
Municipal Sludge Incineration," (Order No. PB91- 228114/AS; Cost: $23.00, subject
to change) will be available onty from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Environmental Criteria and Assessment Office
U.S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection
Agency
Center for Environmental
Research Information
Cincinnati, OH 45268
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