Office of Health and
Environmental Assessment
Washington DC 20460
EPA 6007B-89/OB2F
January 1990
&EFK
Summary of Potential
Risks from Hospital
Waste Incineration:
Pathogens in Air
Emissions and Residues
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ECAO-R-0238
JANUARY 1990
REVIEW
SUMMARY OF POTENTIAL RISKS FROM HOSPITAL
WASTE INCINERATION: PATHOGENS IN
AIR EMISSIONS AND RESIDUES
U.S. ENVIRONMENTAL PROTECTION AGENCY
OFFICE OF RESEARCH AND DEVELOPMENT
OFFICE OF HEALTH AND ENVIRONMENTAL ASSESSMENT
ENVIRONMENTAL CRITERIA AND ASSESSMENT OFFICE
RESEARCH TRIANGLE PARK, NC 27711
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DISCLAIMER
This document has been reviewed in accordance with U.S. Environmental Protection
Agency policy and approved for publication. Mention of trade names or commercial products
does not constitute endorsement or recommendation for use.
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CONTENTS
1. INTRODUCTION 1
2. SURVIVAL OF MICROORGANISMS 2
3. AIR EMISSIONS 10
4. RESIDUES 17
5. COMPARISON OF HOSPITAL WASTE WITH OTHER WASTE 22
6. SAMPLING METHODS 25
6.1 STACK EMISSIONS 25
6.2 ASH RESIDUES 28
7. SUMMARY AND CONCLUSIONS 29
8. REFERENCES 32
111
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TABLES
Number Page
1 Pathogens Which May be Found in Hospital Waste for Which
CDC Recommends Special Handling 3
2 Hospital Incinerator Stack Gas and Ambient Air Bacterial
Concentrations 11
3 Bacteria Concentration in Hospital Incinerator Stack Gas
and Outdoor Air 13
4 Number of Stack Gas and Outdoor Air Bacteria Collected,
Tested and Identified 13
5 Stack Gas and Outdoor Air Bacteria Identification 14
6 Bacterial Identification From the Hospital Incinerator Room 14
7 Recovery of B. Subtilis Spores From Incinerator Stack 16
8 Incinerator Characteristics 19
9 Percent Destruction of Microorganisms in Four Municipal Solid
Waste Incinerators 20
10 Salmonella Isolations From Solid Waste and Incinerator
Residue and Quench Water 21
11 Bacteria, Fungi, and Viruses Pathogenic to Man That May be
Present in Sewage and Sludge 26
IV
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FIGURES
Number
Bacterial concentrations (cfu/g) and their statistical
parameters in hospital wastes and in household refuse 23
Relative frequency of bacterial groups in hospital wastes
and household refuse 24
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1. INTRODUCTION
The purpose of this report, which was requested by EPA's Office of Air Quality
Planning and Standards, is to summarize what is known regarding the risk of infection
through exposure to the emissions or residues of incineration of biomedical wastes. These
wastes include refuse from hospitals, biomedical research laboratories and similar institutions.
It has been estimated that the total hospital waste generation in the United States is 3.8 million
tons per year (Office of Technology Assessment, 1988). Such wastes are incinerated in
incinerators at hospital sites or in larger units which incinerate biomedical waste with
municipal solid waste; however, it is not known what percentage of the total is incinerated.
The Council of State Governments (Moreland and Hinson, 1988) has estimated that
36 states specify incineration as recommended treatment of infectious wastes under existing or
proposed regulations. EPA's Office of Solid Waste (U.S. Environmental Protection Agency,
1986) in its "Guide for Infectious Waste Management" recommends incineration as treatment
of all types of infectious waste. Infectious waste was designated in the following categories;
isolation waste (from patients with communicable disease); cultures of infectious agents;
human blood and blood products; pathological waste from surgery, autopsy, and biopsy;
contaminated needles, scalpels, syringes and glass; and contaminated animals and bedding.
This report summarizes studies which have evaluated the microbiological aspects of the
air emissions and waste residues of hospital and municipal waste incinerators. Studies that
have compared the microbiological load in hospital wastes with that of municipal solid waste
are also included. Pathogens are also found in sewage sludge and significant volumes of
sewage sludge are also incinerated, .so some comparison can be considered with that source as
well. The testing methods used by various investigators for microorganisms in stack gasses
and ash residues are also summarized and compared.
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2. SURVIVAL OF MICROORGANISMS
Among the microorganisms that can potentially be found among hospital wastes are
viruses, bacteria, fungi (molds) protozoa and helminths. Examples of specific species of
these are listed in Table 1. The list of microorganisms was taken from the CDC Guideline
for Isolation Precautions in Hospitals (Garner and Simmons, 1983). It is important to keep in
mind that many of these species are found commonly in the population and in the
environment.
Some bacteria and molds are capable of growth and multiplication within containers
holding wastes, provided that favorable temperature, proper nutrients and adequate moisture
are present, and that adverse conditions of pH, dessication, or disinfectants are absent.
Organisms such as some viruses, protozoans, and molds, however, are especially susceptible
to dessication. Viruses cannot multiply outside the cells of their host and would therefore not
increase their number during storage and transport of virus-contaminated waste. The virus
population in waste awaiting incineration may decrease over time due to adverse conditions of
temperature and drying.
Survival of microorganisms and infectivity are not synonymous terms. Survival or
viability is defined as the ability of the organisms "to propagate indefinitely when placed in a
suitable environment" (Davis et al.s 1968). Survivability is necessary for infectivity, but
some microbes, such as certain bacteria and viruses, can lose their ability to infect their host,
yet can still be recovered as viable particles (Cox, 1987). Irreversible lipid-phase changes
and/or carbohydrate protein complex changes in the membranes of bacteria and viruses or in
the protein coat of viruses can alter the ability of such microbes to interact with the
membranes of their host cells. Such changes can prevent attachment and infection (Cox,
1987). Infectivity is of greater relevance to estimating risk than measurements of viability
are; infectivity is measured using cell cultures when animal or human viruses are collected,
but measurement of bacteria is more likely to measure viability only.
Virulence, or the ability of a specific pathogen to cause disease, is also of concern in
attempting to measure risk, but is more difficult to ascertain. Virulence depends on a
combination of host factors and microbial characteristics (Wistreich and Lechtman, 1976).
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TABLE 1. PATHOGENS WHICH MAY BE FOUND IN HOSPITAL WASTE
FOR WHICH CDC RECOMMENDS SPECIAL HANDLING
Viruses Protozoa:
Varicella Dientamaeba fragilis
Zoster (Herpes zoster) Crvptosporidium
Herpes simplex Giardia lamblia
Influenza
Rabies
Rubella Rickettsia
Vaccinia
Rubeola
Coxsackie virus Chlamvdia
Echovirus
Enterovirus
Rotavims Other
Hepatitis virus A, B, non A, non B
polio virus Sarcoptes scabiei
HTV (scabies mite)
EEEV
Norwalk agent
Adenovirus
Bacteria
Corvnebacterium diptheriae
Yersinia pestis
Yersinia enterocolitica
Neisseria gonorrheae
Neisseria meningitidis
Streptococcus group A & B
Haemophilus influenzae
Bordetella pertussis
Staphvllococcus aureus
Streptococcus pneumoniae
Mvcobacterium tuberculosis
Vibrio cholera
Vibrio parahaemoliticus
Bacillus anthracis
Salmonella sp.
Shigella sp.
Clostridium difficile
Clostridium welchii
E.coli (enteropathogenic, enterotoxic or enteroinvasive)
Treponema pallidum
Brucella sp.
Francisella tularensis
Source: Garner and Simmons (1983).
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Among the microbial characteristics that determine virulence are invasiveness, the ability
to produce toxins antagonistic to the host, and the ability to survive and reproduce in the
presence of the host's defense mechanisms. The presence or absence of bacterial capsules
influences the virulence of such bacteria as Bacillus anthraxis, Haemophilia influenzae,
Klebsiella pnewnoniae, Meningococci groups A and B, Pneumococci, and some strains of
Staphylococci, in that the presence of a capsule increases the organism's ability to resist the
host's phagocytotic defenses (Wistreich and Lechtman, 1976). The ability of some bacteria to
produce certain extracellular enzymes such as coagulase, streptokinase or hyaluronidase, also
aids their invasiveness. Some bacteria also produce toxins which affect the host in a variety
of ways, for instance by destroying specific components of the host cells or by inhibiting
certain cellular activities. The exotoxin of Clostridium botulinum affects nerve cells by
blocking the release of the neurotransmitter acetylcholine, while the exotoxin from Diphtheria
inhibits host protein synthesis, and is generally destructive to a variety of host tissues.
Clostridium perfringens produces a toxin which can destroy cell membranes. Other bacteria,
such as Staphylococcus aureus, produce endotoxins which are released upon lysis of the
bacterial cell, and which cause headache, nausea and vomiting in the host (Wistreich and
Lechtman, 1976).
Host resistance is a large factor in pathogenic invasion by microorganisms. It varies
from individual to individual, and therefore cannot be calculated from measuring microbial
concentrations. Also important in establishing virulence from the airborne route is particle
size of the aerosol, aerosol age, and aerosol matrix. Particle size determines where in the
respiratory system the pathogen deposits, which determines host susceptibility and the
exposure of organisms to host defense mechanisms. Aerosol age and aerosol matrix are
factors that determine how physical parameters such as relative humidity, temperature and
suspending medium influence the survival and infectivity of the microbe.
It is clear that estimates of risk of disease from biological aerosols that might be emitted
from hospital incinerators need to be concerned more with infectivity than with survivability
of organisms. However, the many complex factors that determine infectivity are not readily
delineated, so that quantitative risk estimates are not presently possible. What is possible is a
quantitative measurement of pathogenic particles, and in some cases, some estimate of
infectivity to cultured cells.
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A number of physical factors influence both the ability of various microorganisms to
survive, and to remain infective. Relative humidity (RH), temperature, presence of oxygen,
and open air factor are among the physical parameters which would be relevant to survival
under incinerator conditions.
Animal viruses (which include those that infect humans) are variable in their response to
changes in RH. Lipoproteins apparently denature most easily at high to mid-range RH, while
proteins do so more easily at low RH (Cox, 1987). Poliovirus is most stable in aerosols
whose RH is above 60%, below which the protein coat is inactivated through loss of water
molecules (Harper, 1963). A similar pattern is seen for the picornavirus meningovirus 37A,
which is unstable below RH of 70% (Akers and Hatch, 1968), and the picornavirus
encephalomyocarditis (EMC), which survives poorly at RH values below 60% (De Jong
et al., 1974). The presence of structural lipid in the coats of viruses may affect the ability of
such lipid-containing viruses to survive and remain infective in the airborne state (Cox,
1987). Like poliovirus, foot-and-mouth disease virus does not contain lipid in its coat, and is
stable at high RH (Harper, 1963; Benbough, 1971). Viruses such as Langat and Senliki
Forest virus, (Benbough, 1971; Cox, 1976), vesicular stomatitis virus, (Warren et al., 1969),
vaccinia, Venezuelan equine encephalomyelitis virus (Harper, 1961, 1963) and influenza virus
(Harper, 1961; Schaffer et al., 1976), which contain structural lipids, are most stable at low
RH. HTLV-III/LAV (now termed HIV-III) has an inactivation rate of one log of titer
reduction per nine hours in a dried state, with complete inactivation between 3 to 7 days. No
reverse transcriptase activity was detectable after 24 hours in the dried state. The
concentration of HIV-III used for this test is many orders of magnitude greater than those
derived from patient specimens. Concentration did have an effect on virus survival, with
greater concentrations being associated with longer survival times (Resnick et al., 1986).
Temperature effects on viruses tend to follow the same denaturation kinetics that hold
for other microorganisms (Cox, 1987). Resnick et al. (1986) examined the effects of
frequently encountered clinical and laboratory temperatures (room temperature: 23° to 27°C;
36° to 37°C; and 54° to 56°C) on the infectivity of HIV-III. Heating of the virus in 1 ml of
medium containing 50% human plasma, in a water bath at 54° to 56°C resulted in a
reduction of virus titer at a rate of about 1 log reduction in TDIC^ every 20 minutes. After
five hours, no infectious virus was detectable. No residual reverse transcriptase activity was
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detectable after 30 minutes exposure to this temperature (Resnick et al., 1986). Exposure of
HIV-in virus under similar conditions at room temperature allowed infectious virus to be
detected after 15 days, while complete inactivation was seen at 36° to 37°C between 11 and
15 days (Resnick et al., 1986). Gordon et al. (1988) report reduced stability and infectivity
of HTV on heating above 42°C, and report that this may be due in part to the abolishment of
thermo-dependent lipid domains in the virus.
As stated previously, temperature effects follow denaturation kinetics for the inactivation
of viruses, as well as other organisms. Irreversible changes in the protein- and lipoprotein-
carbohydrate structures of the coats of viruses, and of the membranes or protective capsules
of bacteria, as well as of their spores, or in the membranes of other microbes, account for the
loss of infectivity. Interference with the ability to use oxygen, or the inactivation of vital
enzyme systems is also involved. Inability to use oxygen prevents the use of repair
mechanisms, with resultant loss of viability, while denaturation of surface structures prevents
viral attachment, also causing loss of infectivity (Cox, 1987).
Some species of vegetative bacteria are inactivated by oxygen and their susceptibility to
oxygen increases with increasing oxygen concentration, degree of desiccation, and with time
of exposure. Among the bacteria that are oxygen sensitive are Escherichia coli B, Serratia
marcescens 8UK, (Cox and Heckly, 1973), Micrococcus candidus, Klebsiella pnewnoniae,
and Francisdla tularensis (Strange and Cox, 1976). Other microorganisms, such as spores of
spore-forming bacteria, phages and viruses survive equally in air or in pure nitrogen (Cox,
1987).
Another factor that affects the survival of microbial aerosols, and that might be a factor
in incinerator emissions once they leave the stack and mix with ambient air, is the Open Air
Factor or OAF. Druett and May (1968) compared survival of microorganisms in laboratory
and ambient air, and found that under similar conditions of photoactivity, temperature and
RH, outside air was much more toxic to the microorganisms than inside air. Considerable
research concerning the nature of OAF has led to the conclusion that the reactions between
ozone, olefins, and other hydrocarbons in air pollutants, produce the chemicals which act on
bacteria (Cox, 1987). Organisms sensitive to OAF include bacteria such as E. coli, S.
marcescens, F. tularensis, Brucella suis, Group C streptococcus, Micrococcus albus, and
Erwinia strains, and viruses such as vaccinia, Semliki Forest virus and some of the
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coliphages. Spores of Bacillus subtilis var. niger, and B. anthracis are not sensitive to OAF
in the dark, and Micrococcus radiodurans is also resistant (Cox, 1987).
Of those microorganisms that could contaminate waste, those with the highest potential
for survival are spore-forming bacteria. Formation of bacterial spores, unlike formation of
mold spores which constitute a reproductive structure, is a means for protecting bacteria
against heat, drying, lack of nutrients, damage from some chemicals, and sometimes even UV
radiation. Spore-forming bacteria are largely soil flora; while there are pathogenic species
among them, such as Clostridiwn sp., and some species of Bacillus, they are not generally
found among the flora that contaminate hospital wastes. Because of their high potential for
surviving adverse conditions, however, they make ideal test organisms for determining the
effectiveness of sterilization techniques, and they have been used to test the efficiency of
incinerators in killing potential contaminants. If incineration destroys the most heat-resistant
organisms, and none of those deliberately inoculated into wastes can be detected in stack
emissions, then an assumption can be made that less heat tolerant organisms such as viruses,
protozoa, fungi, and heat-sensitive bacteria, would be killed.
Conditions which destroy such test organisms, as well as more sensitive ones, have been
investigated extensively relative to sterilization. Among the techniques that are relevant to
hospital incineration are wet- and dry-heat treatment, and the actual testing of incinerators,
which will be described in Section 3.0 of this document.
Temperature (a measure of heat energy level) is the most important variable in both wet-
heat and dry-heat destruction of organisms, and its action is a function of time (Pflug and
Holcomb, 1977). Wet- and dry-heat destruction of microorganisms are used for sterilization
of objects or apparatus when damage to such reusable equipment is to be avoided.
Moist heat is used in steam sterilizers or autoclaves. It is used to sterilize reusable
equipment and instruments not damaged by the process, and is inexpensive, rapid, and
effective since all organisms are susceptible. Its effectiveness is attributed to the high latent
heat of water (540 cal/gram), which is transferred to the organisms, resulting in the thermal
denaturation of proteins and death. Fungi, most viruses, and various pathogenic bacteria are
sterilized within a few minutes at 50° to 70°C, and even the more heat-resistant spores of
Clostridia and other spore-forming pathogens are sterilized within a few minutes at 100°C
(Davis et al., 1968). In order to assure complete inactivation of all organisms, with a margin
7
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of safety, autoclaving at 15 psi pressure at a temperature of 121.5°C (250°F), for 15 minutes
is recommended (Davis et ah, 1968; Wistreich and Lechtman, 1976).
Many factors govern the efficiency of the sterilization process, however. Load size,
degree of compaction, distribution of materials in the load, and heat penetration are among
those that determine the length of time required for complete sterilization to take place. Glick
et ah (1961) conducted experiments on various loads that were autoclaved at 121 °C at 15 psi,
using thermocouples, and spores of Bacillus subtilis var. niger, to determine sterilization
requirements.
Among the loads tested were stacked animal cages containing bedding of various
thicknesses, garbage cans filled with moistened animal bedding, packed guinea pig carcasses
in a fiberboard container, and eggs packed in five gallon containers. Nested animal cages
(37.5 x 54.6 x 35.6 cm), with 2.5 cm bedding composed of wood shavings and chips,
required 2 hours to reach sterilization temperatures throughout. When bedding was 7.6 cm
deep, 4 hours were required to reach the same temperature. Viable spores of B. subtilis were
recovered at the top of the can, and 15 and 30 cm deep, in garbage cans containing 32 cm of
moistened animal bedding, after 4 hours autoclaving at 121 °C at 15 psi. Control tubes used
to test sterilization were placed in the abdomens of 20 guinea pig carcasses packed into a
fiberboard container. More than 8 hours (but less than 16 hours) were required before
sterilization was achieved. Discarded eggs in five gallon containers packed three-quarters full
did not reach sterilization temperatures after six hours.
These experiments show that even when recommended procedures for autoclaving are
followed, sterilization may not occur. If autoclave contents are large, bulky, unusually
compacted, or contain a large amount of moisture, the time required to achieve sterilization
may be long. These results impact the present discussion in two ways. One is in the
definition of what constitutes infectious waste. Waste which has been autoclaved for
15 minutes at 15 psi (standard autoclaving procedure) may be assumed sterile and therefore
non-infectious, when in fact it is not. Secondly, the factors that affect sterilization efficiency
in autoclaves may also hold true for incinerators. These factors affect whether or not
complete combustion occurs and, therefore, whether or not pathogenic organisms are killed
during their residence time in the incinerator.
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Dry heat sterilization, which usually takes place in ovens constructed for the purpose,
requires higher temperatures and longer exposure times to achieve sterilization. Temperatures
of 160 to 180°C, for periods of 2 hours or longer, depending on the size of the package to be
sterilized, are recommended (Davis et al., 1968; Wistreich and Lechtman, 1976).
Incineration of wastes implies that the destruction of the substrate material, as well as
the contaminating organisms, is desired. The differences between the processes are the
extremes of temperature and the lengths of time for applying heat. Wet/dry-heat destruction
of microorganisms rarely exceeds temperatures of 125°C, while exposure times may be as
long as 139 hours (Pflug and Holcomb, 1977). Incinerator temperatures may exceed 800 °C,
and exposure times may range from seconds to hours.
The relevance of discussing wet/dry-heat considerations is that the effect of such
parameters as relative humidity (RH) and open/closed systems on the destruction of
microorganisms are significant. Wet-heat destruction of microorganisms occurs at RH of
100 percent (the air above saturated with steam) and implies that some water is present in the
system. Dry heat destruction occurs at RH of less than 100 percent (RH can range from 0 to
99 percent), and no water is present. Chapman and Pflug (1973) and Anderson (1959) tested
the heat resistance of bacterial spores using wet-heat and dry-heat methods, as well as
enclosures, and found that spores enclosed with liquids had a longer survival time than those
dried on filter paper and subjected to dry-heat sterilization techniques. These procedures use
temperatures much lower than those found in incinerator conditions.
Microenvironments within wastes fed into incinerators may be subject to wet-heat
destruction due to water generated from hydrocarbon combustion or water associated with
containment in flasks or tubes. The latter would also constitute a closed system for wet-heat
destruction of the contaminating organisms. Effectiveness of the incineration process on
destruction of organisms would depend on the temperature to which they and their substrates
are subjected, together with the residence time at that temperature. Requirements for
effective destruction of wastes are discussed in Section 3.0 of this report.
Survival of viruses has not actually been tested in incinerators. Their susceptibility to
conditions of very high temperature, dry air, presence of oxidant gases, and OAF (should
they reach the ambient air) makes surviving the incineration process unlikely, though it cannot
be ruled out. Viruses differ in their ability to survive as aerosols depending on the nature of
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the virus, the air temperature, RH, nature of the suspension fluid, presence of atmospheric
gases (especially oxidants), and irradiation (Satter and Ijaz, 1987). Ability to survive is
inversely proportional to air temperature (Satter and Ijaz, 1987), and most viruses are
inactivated at temperatures above 60°C (Kabrick et al., 1979). Lipid content of the virus
affects its susceptibility to RH. Lipid-free viruses survive better at high RH; lipid-containing
viruses survive better at low RH; and some other viruses survive best at intermediate RH
ranges (Satter and Ijaz, 1987).
3. AIR EMISSIONS
The two studies most pertinent to hospital waste incinerators are those of Kelly et al.
(1983) and Allen et al. (1989). Only air emissions were tested in these two studies.
Kelly et al. (1983) took 15 samples from the stack of a hospital waste incinerator which
had been charged with hospital waste containing pathogenic material. Waste from a 400-bed
hospital was burned in a two-chamber incinerator at a rate of 500-800 pounds per hour. The
temperature at the exit from the secondary chamber was 650-1,065°C (1,200-1,960°F).
Ambient air and incinerator stack gas microbial samples were collected simultaneously using
Shipe impingers. After sampling, the impingement fluid was transferred to a sterile container
and filtered. The filters were then cultured in trypticase soy broth aerobically for 48 hours,
after which the colonies were counted. Representative samples of the bacterial colonies were
transferred to streak plates, incubated for 48 hrs, and stained. The bacterial concentrations
are shown in Table 2. A mean concentration of 231 bacterial colonies/cubic meter was found
in the stack gasses; the level in ambient air was 148 colonies/cubic meter. The authors
concluded that the presence of some bacteria in the stack samples could be due to the more
than 200% excess ambient air added to the primary and secondary chambers, but the ambient
air did not account for all the bacteria found in the stack gas.
The authors concluded that due to experimental uncertainty, there was no statistically
significant difference between bacteria in stack emissions and in ambient air (0.05 < p<0.1).
There was not a statistically significant difference (0.25 < p<0.5) between the ratios of
concentrations of gram-positive bacteria to the total bacterial concentration in the incinerator
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TABLE 2. HOSPITAL INCINERATOR STACK GAS
AND AMBIENT AIR BACTERIAL CONCENTRATIONS
Sample
number
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
MEAN
Incinerator
Sampler 1
17
33
60
90
58
0
0
14
536
0
194
15
97
63
0
79
Concentration*
Sampler 2
10
844
33
852
66
0
0
31
288
15
2,297
37
1,270
0
15
384
Average
Stack Gas
Concentration"
13
438
47
471
62
0
0
23
412
8
1,246
26
684
32
7
231
Ambient
Air
Concentration"
8
36
252
574
128
104
73
20
81
14
221
189
140
341
37
148
"Total colonies/m3 of air sampled.
Source: Kelly et al. (1983).
emissions and the ambient air. The levels of bacteria in the original waste were not measured
in this study.
Allen et al. (1989) tested bacterial emissions from a hospital incinerator, using a
composited waste consisting of wadded newspaper, bond paper, cardboard, and tap water;
spores of Bacillus subtilis were added for testing purposes. The composited waste was used
for testing because the normal hospital waste produced hydrochloric acid when burned (due to
its plastics content), which caused problems in sampling for bacteria. The incinerator had a
capacity of 100 Ibs/hr and was manually loaded. The burn cycle time was longer than
20 minutes. The two chambers were maintained at a temperature of 1,400°F; natural gas was
used as an auxiliary fuel. During routine operation a plume was visible from the stack, which
the authors concluded was an indication of incomplete combustion.
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Stack gas bacteria samples were collected by impinger at six different times for
20 minutes each. Simultaneous samples of outdoor air bacteria were also collected. The
impingement fluid was filtered and each filter was placed in a petri dish with trypticase soy
broth. Colonies were counted after 48 hours and then transferred to streak plates (5% sheep
blood agar) and incubated for 24 hours. The freshly grown colonies were then stained and
identified by comparison with known bacterial species. The bacterial concentration in the
hospital incinerator stack gas and in outdoor air is shown in Table 3. The total volume of
samples varied depending on flow rate and sampling time (non-isokinetic sampling). The
stack gas bacteria concentration given is the total of the bacteria collected in the impingers,
divided by the total volume of the stack gas sampled. The number of bacteria collected in
each stack gas and outdoor air sample, the number analyzed, and the number identified are
shown in Table 4. The average temperature of the stack gas during the sampling period is
also shown. The stack gas bacteria concentration had no relationship to stack gas temperature
over this range.
The bacteria species identity and frequency of occurrence are shown in Table 5. The
number of Bacillus and Staphylococcus species in the stack gas and the outdoor air were
considerably different. Bacterial analysis of the incinerator room air is shown in Table 6; the
four species of Staphylococcus found account for 91 out of 96 colonies found in the stack gas;
2 percent found were Bacillus. No Bacillus subtilis, which had been added to the waste, was
isolated from the stack gas; this may indicate that the bacteria found in the stack gas did not
originate from incompletely-burned waste. Much of the excess air enters the incinerator
afterburner and is retained for only a fraction of a second at high temperature, which would
kill only the most heat-sensitive organisms. The authors concluded that the source of bacteria
in the stack gas was combustion air, which was drawn from the room housing the incinerator,
a conclusion similar to Kelly et al. (1983). The authors recommended an additional study to
sample stack gas, outdoor air, and combustion air simultaneously.
Barbeito and Gremillion (1968) tested a refuse incinerator to determine minimal
operating temperatures required to prevent release of viable microorganisms into the
atmosphere. Their study was accomplished by disseminating an aerosol suspension of
Bacillus subtilis var. niger spores of known concentration into the incinerator. Dry spores
mixed with animal bedding were also used. The minimum requirement for destruction of wet
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TABLE 3. BACTERIA CONCENTRATION IN HOSPITAL
INCINERATOR STACK GAS AND OUTDOOR AIR
Average
Stack Gas
Temperature
CO
192
188
178
177
171
159
Stack Gas
Bacteria
Concentration
(colonies/m3)
202
2.1
1157
379
ND(4.0)"
ND(4.0)a
Outdoor Air
Bacteria
Concentration
(colonies/m3)
24.0
16.2
48.0
8.0
8.0
16.0
"Not detectable, number in parentheses is the lower detection limit
Source: Allen et al. (1989).
TABLE 4. NUMBER OF STACK GAS AND OUTDOOR AIR
BACTERIA COLLECTED, TESTED AND IDENT1MKD
(colony forming units)
Bacteria Collected
Stack Outdoor
1
226
94
5
0
0
2
6
1
3
1
2
Bacteria Tested
Stack Outdoor
1
87
17
5
0
0
2
6
1
3
1
2
Bacteria Identified
Stack Outdoor
1
74
16
5
0
0
2
6
1
3
1
2
Source: Allen et al. (1989).
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TABLE 5. STACK GAS AND OUTDOOR AIR BACTERIA IDENTIFICATION
Sample
Organism
Frequency
of
Occurrence
Stack
Outdoor
Bacillus circulans
Bacillus megaterium
Staphvlococcus simulans
Staphvlococcus warneri
Staphvlococcus epidermidis
Staphvlococcus saprophvticus
Staphvlococcus hominis
Staphvlococcus auricularis
Bacillus megaterium
Bacillus pumilus
Bacillus cereus
Bacillus coagulans
Staphvlococcus epidermidis
Staphvlococcus saprophvticus
1
1
2
1
61
25
2
3
2
2
1
1
5
3
Source: Allen et al. (1989).
TABLE 6. BACTERIAL IDENTIFICATION FROM THE HOSPITAL
INCINERATOR ROOM
Orsanism
Number
of
Colonies
Staphvlococcus epidermidis
Staphvlococcus saprophvticus
Staphvlococcus hominis
Staphvlococcus auricularis
Pseudomonas stutzeri
Pseudomonas maltophilia
Corvnebacterium species
Flavobacterium species
CDC Group VE-1
CDC Group VE-2
9
3
2
1
1
1
2
4
1
2
Source: Allen et al. (1989).
14
-------
spores was 302°C (575°F) for firebox air temperature, and 196°C (385°F) for the firebrick
lining. For dry spores, these temperatures were 371 and 196°C (700 and 385°F). The
incinerator was a gas expansion chamber type with a capacity of 4,000 Ibs/hr. Oil or gas was
used as fuel. The retention time for particles flowing through the incinerator was 41 seconds
at 302°C (575°F) and 26.5 seconds at 371 °C (700°F). During testing, the incinerator was
brought to operating temperatures before adding the bacterial spores, and the incinerator air
temperature was adjusted for separate tests.
The bacterial aerosols were sampled inside the stack near ground level and on top of the
stack; the concentration of spores recovered per cubic foot of air sampled was decreased by
one log as they passed through the stack. Barbeito and Gremillion (1968) considered exhaust
stack height and exhaust velocities to be significant; the stack provided additional retention
time and increased effectiveness for sterilization. Change in stack height or exhaust velocity
would affect retention time and required incinerator temperature. Complete destruction of
microbes was a function of incinerator air, firebrick temperature, and retention time. Thus,
incinerators should be brought to operating temperatures prior to loading, and not used
intermittently in order to prevent cooling between burns. High moisture content of refuse can
also cause the incinerator temperature to drop below that required to destroy microorganisms.
The authors also concluded that siting of incinerators in relation to prevailing winds and
nearby buildings was not often considered. Finally, they concluded that temperature and time
sufficient to kill spore-forming bacteria would destroy viruses.
Barbeito et al. (1968) tested two incinerators constructed of metal, as opposed to the
brick incinerator in the earlier study. The incinerators were air-tight, and fueled with oil.
Combustion was supported by the contaminated air stream in continuous operation. Aerosols
of liquid and dry suspensions of Bacillus subtilis var. niger spores and dry vegetative cells of
Serratia marcescens were disseminated into the two incinerators to determine the temperature
required to destroy the bacteria. At a combustion chamber temperature of 371 °C (700°F),
the retention time was approximately five seconds. Wet spores of B. subtilis
(1.74 x lO'/cubic foot) required from 329 to 371 °C (625 to 700°F) for complete removal.
The authors stated that the bacterial concentrations used were much higher than would be
normally contained in infectious waste. They pointed out that the temperature and retention
time established in their study would apply only to incinerators of the same design. Barbeito
15
-------
and Shapiro (1977) did a microbiological evaluation of emissions from a pathological
incinerator which burned waste from a primate research laboratory by adding spores of
Bacillus subtilis var. niger to solid or liquid waste. The incinerator burned propane or fuel
oil and had two chambers. They determined that temperatures of 760°C (1,400°F) in the
primary chamber and 871 °C (1,600°F) in the secondary chamber were necessary to prevent
release of viable microorganisms in stack emissions. The retention time in the secondary
chamber was 1.2 sec. The stack sampling was done using an air sampling unit (Gelman)
fitted with a sterile membrane filter having a 99.995% efficiency for particles of 0.3 /xm
diameter. The recovery of spores was reported per cubic foot of stack gas (Table 7). The
rate of liquid feed to the incinerator was 3.8 liters (1 gal) per minute; the rate of solid waste
feed ranged from 170 kg (375 lb)/hour to 236 kg (520 lb)/hour.
TABLE 7. RECOVERY OF B. SUBTILIS SPORES FROM INCINERATOR STACK
Challenge Concentration Spores Recovered
Refuse Test Spores/ft3" Total sporesb Per ft3 Total
Solid
Liquid0
1
2
3
4
3.0 x 107
3.0 x 107
3.0 x 107
7.2 x 102
6.0 x 109
6.0 x 109
6.0 x 109
1.4 x 105
0.333
negative
negative
11.5
67
negative
negative
2.3 x 103
Temperature - Primary chamber = 649°C (1,200°F); secondary chamber = 760°C
(1,400°F); top of stack = 399-566°C (750-1,050°F).
"Calculated concentration when mixed with incinerator air.
bMaximum possible number in air sampled (200 ft3).
'Concentration of spores in storage tank 1.8 x 105 spores/ml.
Source: Barbeito and Shapiro (1977).
16
-------
Among the factors listed by the authors which could prevent the necessary time-
temperature exposure to destroy microorganisms were: loading before operating temperature
is obtained, temperature gradients caused by intermittent use, and exceeding design capacity.
The authors recommended burning clean refuse until sufficient temperatures are reached,
before loading pathological waste, if the incinerator is not used continuously.
Glysson et al. (1974) used an Anderson sampler to determine the microbiological quality
of the air inside and outside of an incinerator facility. Stack emissions were not sampled.
The highest number of organisms were found in the air above the charging or tipping floor
where waste is dumped before being pushed into the incinerator itself. Staphylococci colonies
were present in relatively low numbers. This study supports the suggestion by Brenniman and
Allen (1988) that air drawn from inside the facility to provide combustion air for the
incinerator could be a source of microorganisms in the stack emissions.
Duckett et al. (1980) took 18 samples of airborne dusts at an incinerator site where
municipal solid waste was shredded and burned for heat recovery. No incinerator stack
emissions were sampled. They found that although the total level of airborne bacteria was
high (up to 9.0 x 106 organisms/cubic meter), average concentrations of fecal coliforms and
fecal streptococci were lower by more than two orders of magnitude. No Salmonella or
Shigella were found; Aspergillus fumigatus was identified in two samples; Staphylococcus
aureus was found in seven samples; and Klebsiella pneumoniae was identified in one sample.
The authors stated that this facility may represent a worst case in terms of concentrations of
microbiological aerosols due to the shredding and separating process for the solid waste.
However, they concluded that,.assuming that all fecal coliforms were enteropathogenic
Escherichia coli, a worker at the plant would accumulate less than the infective dose. The
risk for people living near the facility would be even less.
4. RESIDUES
There were no published studies found which discussed the presence of pathogens in the
ash residues of hospital waste incinerators. The following studies deal with pathogens in
municipal solid waste (MSW) and MSW incinerator residues. Peterson and Stutzenberger
17
-------
(1969) sampled solid waste and residues of four municipal incinerators for total bacteria, total
coliforms, fecal coliforms, and heat-resistant spore formers. Air emissions were not sampled.
Selected samples of food and fecal matter refuse from incinerator storage pits were taken and
would probably represent the higher range of microbiological load. Selected spot samples of
residue were also removed. All samples were removed with sterile tongs, placed in 200 ml
specimen cups and homogenized to slurries in cold phosphate buffer in sterile vessels; the
supernatant fluid was sampled and diluted for measurement. The engineering characteristics
of the incinerators in this study are shown in Table 8. The continuously-fed rotary kiln was
the most modern unit used.
The average numbers of viable bacteria from the refuse samples were relatively similar
among the 4 incinerators. The corresponding percent removal of microorganisms by
incineration is shown in Table 9. The values for counts per gram of samples, and spores per
gram of sample for the bacterial populations are geometric means of 24 refuse samples and
22 residue samples. Numbers of heat resistant bacteria are expressed as spores/gram of
sample; all others are expressed as counts per gram. None of the incinerators in this study
produced a sterile residue. The authors suggested that one way of judging the relative
efficiency of incinerator operation was by the relative number of heat-resistant versus heat-
sensitive organisms surviving. A more efficient incinerator would select for heat-resistant
spores, with few or no vegetative heat-resistant or heat-sensitive bacteria found.
The purpose of the study by Peterson and Klee (1971) was to optimize sample weight,
incubation temperature, culture medium, and plating media for the recovery of Salmonella
from municipal solid waste (MSW), mixtures of solid waste and sewage sludge, and ash
residue of MSW incinerators. Their methods are discussed in detail in Section 5. Table 10
contains the results of Salmonella isolations for four incinerators, which appear to be the same
incinerators studied in Peterson and Stutzenberger (1969). In the earlier Peterson and Klee
(1971) study, incinerator I had the lowest percent removal of total bacteria. The Peterson and
Stutzenberger (1969) study found Salmonella in residue and quench water as well as in the
solid waste. The authors pointed out that the recorded temperatures (1,200 - 2,000°F) of
these MSW incinerators should destroy all microorganisms if all parts of the waste
approached these temperatures. However, they observed unburned materials such as food
matter in the residue of some incinerators. They suggested that a large mass of organic
18
-------
TABLE «. INCINERATOR CHARACTERISTICS
Incinerator
Characteristics
Design capacity"
Number of furnaces
Feed mechanism
Grate
Operating temp
I
500
2
Continuous
Traveling
1,800-2, 000 F
II
500
4
Batch
Circular
1,800-2, 000 F
III
1,200
4
Continuous
Rotary-kiln
1,200-1, 700 F (Primary)
IV
200
4
Batch
Reciprocating
1, 800-2,000 F
Duration of burning
(hr)
Total burning rate
(tons/hr)
Quench water
recirculated
Estimated volume
reduction
980-1,090 C
1.75-2.0
22
980-1,090 C
1.5-1.75
20
No No quench water
80-85%
80-85%
650-925 C
1,700-2,200 F (Secondary)
925-1,200 C
0.5-1.5
50
Yes
80-85%
" Expressed as tons per 24 hr.
Source: Peterson and Stutzenberger (1969).
980-1,090 C
1.0
6.5
No quench water
80-85%
-------
TABLE 9. PERCENT DESTRUCTION OF MICROORGANISMS IN FOUR
MUNICIPAL SOLID WASTE INCINERATORS
Incinerator
I
n
in
IV
Bacterial
Population
Total cells
Heat resistant
Total coliforms
Fecal coliforms
Total cells
Heat resistant
Total coliforms
Fecal coliforms
Total cells
Heat resistant
Total colifonns
Fecal coliforms
Total cells
Heat resistant
Total coliforms
Fecal coliforms
Solid Waste
(W)
7.6 x 107
4.2 x 104
6.2 x 105
9.1 x 10"
4.1 x 108
6.8 x 10"
4.8 x 106
4.0 x 105
5.6 x 107
2.7 x 10"
5.4 x 105
1.2 x 105
3.8 x 108
1.7 x 104
1.2 x 105
2.3 x 10"
Residue
(R)
4.4 x 107
1.0 x 103
1.5 x 104
2.4 x 103
1.7 x 10*
2.0 x 104
7.3 x 102
9
1.2x 106
3.9 x 103
4.1 x 10'
5
7.1 x 103
4.4 x 103
5
<1
Percent Removed
(1-R/W) x 100
42
-138*
97
97
>99
70
>99
>99
97
85
>99
>99
>99
74
>99
>99
"Value may be due to selection of heat-resistant cells during incineration.
Source: Peterson and Stutzenberger (1969).
matter is a poor heat conductor. Such a mass can also become wet through water of
combustion, and parts of the mass may not even reach pasteurization temperature. Hospital
waste has a higher plastics content than MSW (U.S. Environmental Protection Agency,
1988), so the factors of heat conduction and water of combustion may not be as important
when hospital waste alone is burned.
20
-------
TABLE 10. SALMONELLA ISOLATIONS FROM SOLID WASTE AND INCINERATOR RESIDUE AND QUENCH WATER
Incinerator type and
design capacity
Continuous feed,
traveling grate 500
tons per 24 h
Batch feed, circular
grate, 500 tons per
24 h
Batch feed, recipro-
cating grate, 200
tons per 24 h
Continuous feed, rotary
kiln, 1,200 tons per
24 h
Type of
sample
Solid waste
Residue
Quench water
Solid waste
Residue
Quench water
Solid waste
Residue
Quench water
Solid waste
Residue
Quench water
Total
Total
samples
4
4
4
4
4
4
4
4
4
3
3
3
45
No.
positive
1
1
2
0
0
0
0
0
0
0
0
0
4
Salmonella serotype
Salmonella Group Q
Salmonella saint paul
Salmonella saint paul
none found
none found
none found
none found
none found
none found
none found
none found
none found
Source: Peterson and Klee (1971).
-------
5. COMPARISON OF HOSPITAL WASTE
WITH OTHER WASTE
Microbiological studies have been done comparing municipal waste and hospital waste.
In the report by Althaus et al. (1983), waste from two hospitals was selected based on direct
contact from patients, including swabs, dressings, syringes, and catheters. The average mass
of waste generated was l/2 kg (solid) per bed per day and 5.4 liters (liquid) per bed per day
with a specific weight of 1 kg/f. This waste was compared microbiologically with household
waste from three dumps. The hospital refuse was represented by 264 waste samples - 21
samples were from dumps. Qualitative and quantitative tests were performed. The authors
concluded that hospital wastes present no more risk of infection than household waste.
Kalnowski et al. (1983) examined hospital wastes from a surgical unit, an intensive care
unit, and a nursing station, and compared them to household refuse (2 to 3 days accumulation
of garbage) with respect to bacterial concentrations and species pattern. Waste samples were
macerated, liquified, mixed and suspended in 0.9% NaCl with a surfactant (Tween 80), and
aliquots of the eluate were cultured. Results were reported as colony forming units (cfu) per
gram (see Figure 1). The concentrations of the four types of bacteria are shown for
household waste and three sources of hospital waste. The relative frequency of bacterial
groups—aerobic bacteria, facultative anaerobic bacteria, gram-negative bacteria and type D-
Streptococci—indicated that hospital wastes were no more contaminated than household waste.
The relative frequency of bacterial groups in hospital waste and household refuse is shown in
Figure 2. The authors concluded from their results that hospital wastes were no more
contaminated with pathogens than household waste. A similar conclusion was made by Knoll
(1974), who cited hospital hygiene and disinfection procedures.
Thus, wastes originating in households can contain as much of a microbiological load as
hospital waste. One reason could be that people harboring disease organisms live at home for
a time prior to developing symptoms requiring hospitalization. Another would be that
infectious patients in hospitals are isolated, and their associated wastes dealt with more
stringently. For instance, disinfectants which are bacteriostatic and bacteriocidal are
frequently used in cleaning the environs of patients with infectious diseases. Household
wastes may also sit in warm, moist garbage cans for several days until they are picked up for
22
-------
Types of Waste
rv>
CO
OJ 11
0 10
t/>
I 9
^ 8
g" 7
1 6
O
|i 5
c 4
o
0 3
O
0) 2
O
_J 1
0
Private
Household
- B
v
_
—
—
-
—
—
—
f
ri
s
C0
"
K
I
I
E
1
•
i
Nursing
Unit
\
>
> CS E
}'t
-
1
,-
>
3
-
-
-
Intensive
Care Unit
K
H
B C
, j
i
-•
*o
/o
I
K Operating
f 1 B U"" -
1 r
f- ^O-' •<>-* i
T cs ^
TT
O
—
1
T rL
fl
A
—
—
K
U -4- W -»- -
10 10 9 9 13 14 14 13 12 14 12 13 10 10 10 10
Number of Samples
Figure 1. Bacterial concentrations (clu/g) and their statistical parameters in hospital
wastes and in household refuse.
B = Blood-Agar (bacterial group, aerobic bacteria)
CS = Clostridium Selective Agar [bacterial group: (facultative) anaerobic bacteria]
E = Endo Agar (bacterial group: gram negative bacteria)
K = Kanamycin Esculin Azide Agar (bacterial group: D streptococci)
The figure shows the minimal and maximal values ( o ), the arithmetic mean value (A),
the upper and lower quartile and the median.
Values A = 0 are plotted as A = Ig 0.
Source: Kalnowski etal. (1983)
-------
Types of Waste
ro
100
90
^ 80
c
8 70
Q) 60
Q.
50
40
30
20
10
0
Nursing
r, » Unit
Private E/B
Household
K/B
-
E/B
" ft
" P
- I
—
—
—
_
A
n
CS/B
-<•
r»
n £
-
•*^ toe* L
— ^O ^
y
^
< -
CS/B
T
- -i
t
i-l LoJ i<
\
i^
a
—
—
Operating
Intensive Unit
Care Unit E/B
CS/B
E/B
-r K/B
I
A
^A,
^ i^j «^- •_*_
--
A
-o-
"*
A
-
CS/B
•9- —
K/B rfi
lo-l 1<>J -o- KM —
999
12 11 11 11 11 11
Number of Samples
666
Figure 2. Relative frequency of bacterial groups in hospital wastes and household refuse.
Percentage of the concentrations (cfu/g) of gram-negative bacteria (E), P-streptococci (K)
and (facultative) anaerobic bacteria (CS) on the concentrations of aerobic bacteria (B).
The figure shows the minimal and maximal values ( o ), the arithmetic mean value (A), the
upper and lower quartile and the median.
Source. Kalnowski etal. (1983)
-------
transport to land fills. Organisms may multiply in such an environment. This reasoning
leads to a need to define infectious waste more closely.
Sewage sludge may contain pathogenic organisms (Table 11). Not all the pathogens
listed are likely to be routinely found in sewage sludge, but some of the same species may be
found in hospital waste such as (Salmonella and Shigella). Approximately 2 million dry
metric tons of sewage sludge and 6.6 million tons of MSW are incinerated annually in the
United States (U.S. Environmental Protection Agency, 1988). Approximately 3.8 million
tons of biomedical waste are produced each year (U.S. Environmental Protection Agency,
1988); however, it is not known what percentage is incinerated. Potentially, there could be
as much risk of infection from sewage sludge incineration and MSW incineration as from
biomedical waste incineration.
6. SAMPLING METHODS
6.1 STACK EMISSIONS
Armstrong (1970) developed a portable sampling device that concentrates
microorganisms into a small quantity of phosphate buffer from large volumes of incinerator
stack emissions. This device provides a method for qualitative, nonisokinetic sampling. The
sampler is adjustable to isokinetic conditions for quantitative results. A vacuum pump draws
1 cubic foot/min into a sterile 700 ml beaker, containing 300 ml phosphate buffer, through a
1/4 O.D. stainless steel probe which is inserted into the stack. A flowmeter is mounted
between the beaker and the pump. The microorganisms are impinged into the liquid; after
sampling, the liquid is cultured. If the stack velocity is known and remains constant, the
sample flow rate can be adjusted to yield quantitative results.
Allen et al. (1989) used a sterile Shipe impinger fitted with a liquid trap, a critical flow
orifice, and a vacuum pump. Flow rate was 6-9 liters/min at 25°C and 760 mm Hg.
Impingers were cooled with ice during sampling of incineration stack gases to prevent death
of collected organisms. The impinger apparatus did not allow for isokinetic sampling; that is,
the correlation between the stack flow and sampler flow was not constant. Each sample was
collected for a 20 minute sampling period.
25
-------
TABLE 11. BACTERIA, FUNGI, AND VIRUSES PATHOGENIC TO MAN THAT
MAY BE PRESENT IN SEWAGE AND SLUDGE
Group
Pathogen
Bacteria
Protozoa
Salmonella (1700 types)
Shigella (4 spp.)
Enteropathogenic Escherichia coli
Yersinia enterocolitica
Campvlobacter ieiuni
Vibrio cholerae
Leptospira
Entamoeba histolvtica
Giardia lamblia
Balantidium coli
Cryptosporidium
Funsi
Enteroviruses:
Aspergillus fumieatus
Candida albicans
Crvptococcus neoformans
Epidermophvton spp. and Tricophvton spp.
Trichosporon spp.
Phialophora spp.
Poliovirus
Echovirus
Coxsackievirus A
Coxsackievirus B
New enteroviruses
(Types 68-71)
Hepatitis Type A
(Enterovirus 72)
Norwalk virus
Calicivirus
Astrovirus
Reovirus
Rotavirus
Adenovirus
Pararotavirus
Snow Mountain Agent
Epidemic non-A non-B hepatitis
Source: U.S. Environmental Protection Agency (1988).
26
-------
Barbeito and Shapiro (1977) used a participate membrane filter (0.3 pirn) to sample
emissions from a solid and liquid pathological waste incinerator. The sampling time was
10 min at a stack flow rate of 9.4 £/sec (20 cubic feet/min). Spore (Bacillus subtilis)
recovery was reported per cubic foot of stack gas.
Rich and Cherry (1986) suggest that even if there are no visible emissions from an
incinerator, only specific testing can determine whether viable pathogens are being emitted.
They recommend testing with spores of Bacillus subtilis var. niger added to the waste by
adusting the number of spores added and sampling time such that a theoretical capture of
> 1 x 103 spores would occur if none were destroyed. They identified variation in waste
composition, waste feed rate, combustion temperature, air and fuel (if auxiliary fuel is used)
feed rates, and waste flow path, as factors which may result in incomplete destruction of
pathological waste. If bacterial spores are not added to the waste to test for destruction, then
those organisms existing in the waste must be identified so that they can be tested for in the
emissions.
Duckett et al. (1980) sampled airborne dusts at a facility which shredded and incinerated
municipal solid wastes. No stack emissions were tested. Samples were collected using
impactor preseparators and cascade impactors which formed an eight-stage particle collection
assembly operating at 1.25 mVhr (45 ftVhr). Dust-laden air entered the sterilized sampling
apparatus through a duct which accelerated the air from ambient velocity to the isokinetic
sampling velocity of the impactors. Accumulations from particle size samplers were collected
on sterile filter papers. After sampling the filters were placed in sterile bottles, and the
microorganisms dispersed in 0.1 % peptone containing 0.1% Triton xlOO. The bottles were
then agitated, and 1:10 and 1:100 dilutions prepared. Total aerobic microorganisms, total
coliforms, total fecal coliforms, and fecal streptococci were determined quantitatively.
Qualitative tests determined the presence of Aspergillus velocity of the impactors.
Accumulations from particle size samplers were collected on sterile filter papers. After
sampling the filters were placed in sterile bottles, and the microorganisms dispersed in 0.1 %
peptone containing 0.1% Triton xlOO. The bottles were then agitated, and 1:10 and 1:100
dilutions prepared. Total aerobic microorganisms, total coliforms, total fecal coliforms, and
fecal streptococci were determined quantitatively. Qualitative tests determined the presence of
27
-------
Aspergillusfianigatus, Staphylococcus aureus, Klebsiella pnewnoniae, Salmonella sp., and
Shigella sp. Procedures of the Public Health Association (1976) were used where applicable.
6.2 ASH RESIDUES
There were no studies which measured pathogens in residues of biomedical waste
incinerators. Peterson and Stutzenberger (1969) assessed samples of solid waste and its
residue after incineration. An average of 24 samples (mean weight, 43 g) and 22 residue
samples (mean weight, 76 g) were homogenized as slurries in cold 0.067 M phosphate buffer
(pH 7.2) in sterile stainless-steel vessels. Each homogenate was overlaid with sterile
cheesecloth to clarify the liquid. Serial dilutions (10-fold) were pipetted in phosphate buffer.
Total viable cell counts were made after plating 0.1 ml samples on blood-agar plates.
Total and fecal coliforms were estimated by the Most Probable Number Method.
Confirmation of the presence of coliforms was done by streaking on stained agar.
The number of heat-resistant spores in the homogenates were determined by heating
diluted samples to 80°C, and plating. Populations were calculated as cell numbers per gram
(wet weight) of raw refuse or quenched residue.
The authors noted that the appearance of residue was related to the microbiological
quality. Ash with relatively lesser amounts of unburned material such as residue with
unburned vegetables and animal waste had high microbial concentrations. They also observed
that residue containing such material was found when the incinerators were operating at loads
over designed capacity.
Peterson and Klee (1971) sampled municipal solid waste, quench water, and incinerator
residue for the presence of Salmonella. Random samples of MSW and residue (100 to 200 g)
were collected with sterile tongs and placed in sterile 200 ml plastic cups. The random
samples were separately pooled for both waste and residue into one 2,000-4,0000 g composite
sample. After mixing, two 30-g composite subsamples were cultured. Quench water was
collected in sterile, 250-ml bottles. Each of four incinerators was sampled three to four
times, and 15 solid waste, 15 residue, and 15 quench water samples were examined.
28
-------
The 30 g subsamples were inoculated into Selenite F enrichment medium or SBG
enrichment medium for 16-18 hr at 39.5°C. Quench water samples were filtered, and the
filtered material cultured in two types of media for 16-18 hr at 39.5°C.
Loops from incubation flasks were then streaked on agar plates, incubated at 37°C for
24-48 hours, and transferred to agar slants. After overnight incubation at 37°C, identification
was done based on CDC procedures.
Williams and Hickey (1982) examined the ash residue of an incinerator, which burned
municipal refuse and hospital waste, for organic content. Samples analyzed contained 0.08 to
0.12 percent volatile material; the authors concluded that there was "no indication that
hospital wastes are leaving the incinerator partly unbumed". No microbiological analysis was
done.
7. SUMMARY AND CONCLUSIONS
Incineration of biomedical wastes is the most frequently used method of refuse disposal
from hospitals, biomedical research laboratories, and similar institutions. Such wastes may be
incinerated at hospital sites or transported to municipal waste incineration sites. This report
summarizes information regarding the microbial contamination of waste, survival of microbes
in wastes and their destruction by incineration, and survival of organisms in incinerator
emissions and residue. Roles of residence time and temperature considerations in producing
destruction of microorganisms during incineration, as well as modes of operation and
efficiency of operation of incinerators are also discussed. Comparisons of microbial
contamination of biomedical wastes with contamination of other kinds of waste and with
sewage sludge are also made.
Among the microorganisms that can potentially be found in hospital wastes are viruses,
bacteria, fungi (molds) and protozoa. Viruses cannot multiply outside the cells of their host
and are very susceptible to inactivation through drying or exposure to elevated temperatures.
The ability of viruses to survive as aerosols is inversely proportional to air temperature, and
viruses do not survive short-term exposure to temperatures above 65 °C. Protozoa are also
very susceptible to drying and elevated temperatures. Bacteria and molds, except for spore-
29
-------
forming genera of bacteria, tend to be heat-sensitive and can be killed by exposure to
temperatures of 125 °C and below if the exposure time is of sufficient length.
Time/temperature parameters needed to kill bacteria vary with the species and variety. Even
spore-forming bacteria, which are the most heat-resistant group of microorganisms, can be
inactivated at 125°C with extended exposure times. Higher temperatures can kill spore-
forming bacteria in shorter periods of time, and time-temperature relationships for cell death
of such bacteria can be calculated. Spore-forming bacteria are used as test organisms for
determining the effectiveness of sterilization procedures, including incineration.
Whether or not bacteria survive the incineration process has been tested in a number of
studies. One study compared bacteria collected from stack emissions with bacteria collected
from the ambient air, and found no significant difference between them. While bacteria were
not measured in the waste material burned, the authors concluded that the very presence of
bacteria in emissions indicated that they had come from another source, namely the 200%
excess ambient air that had been added to the primary and secondary combustion chambers.
The assumption is that these bacteria did not spend sufficient time in either chamber
(secondary chamber temperature 650-1,065 °C) to be heat inactivated. A second study used a
composited sham waste that was inoculated with cultures of Bacillus subtilis, a spore-forming
nonpathogenic bacterium. With a burn cycle of 20-30 minutes, and a constantly maintained
burn temperature of 760°C, bacteria were isolated from both stack gas and ambient air. No
Bacillus subtilis were isolated, however, indicating that the inoculated bacteria had been
destroyed, and that those bacteria found may have originated from sources other than the
waste burned. The authors postulated that the source was the incinerator room air, and
analysis of this air accounted for 91 of 96 colonies found in the stack gas. Most of the excess
air entered the afterburner and was retained for only a fraction of a second at high tempera-
ture - a condition which would have killed only the most heat-sensitive organisms.
It is hypothetically possible to do a risk assessment for pathogens actually present in
hospital waste relative to the infection probability of people exposed to incinerator emissions
or ash residues. Similar risk assessments have been designed for pathogens in sewage sludge
disposed by composting and landspreading (U.S. Environmental Protection Agency, 1988),
and involve mathematical descriptions of pathogen survival, transport, and exposure. Such a
risk assessment would be extremely difficult, however, due to the number of variables
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involved and assumptions made. Also, each incinerator would require a separate risk
assessment based on design type, the range of operating conditions, waste load, location, and
other factors.
In order to do a quantitative risk assessment of pathogens potentially released from
medical waste incineration, an exposure assessment of affected populations (living near
hospital waste or MSW incinerators) is needed. Presently no such information is available.
There is a need for emissions measurement from stacks, not only under experimental
conditions that involve different kinds of incinerators, under varying operating conditions, but
also of incinerators in actual use under normal operating conditions. Ambient air sampling of
sites near hospital and municipal incinerators burning medical wastes, using indicator
organisms, as well as sampling for those organisms measured in pre-incineration wastes is
also required for exposure assessment, since stack emissions do not reflect what happens to
pathogens once they are exposed to ambient air conditions and would not reflect what a
potentially exposed human being would breathe.
Quantitative risk assessment also requires data on potential infectivity for a spectrum of
organisms, including a variety of species of bacteria, viruses, and molds and protozoa, which
could theoretically be emitted in an infective state from the incineration process. Data on
infectivity vs. survivability is lacking at this time.
Because of the lack of two essential components of the quantitative risk assessment
process, dose-response data and exposure assessment, it is not possible at this time to do a
quantitative risk characterization. However, other useful risk information could be obtained,
once emissions and ambient data are available. For instance, relative hazard ranking of kinds
of incinerators and recommendations for proper and effective operating conditions could then
be made. Quantitative statements of potential hazard to susceptible populations, such as high
risk groups (e.g., immunosuppressed patients in or out of hospital, patients in burn units, or
ICU) could be made. Optimum conditions for handling of infectious wastes could be
described.
The usefulness of such a risk assessment would be limited compared to a testing
protocol whereby spore-forming bacteria cultures are added to the waste, so that the
incinerator parameters could be adjusted for complete spore destruction. Given that such
spores are eliminated, it could be assumed that no other microorganisms in the waste would
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survive, as discussed in Section 2.0. Litsky et al. (1972) make the point that adequate
destruction of pathogens is only accomplished by sufficiently high temperatures, and air
pollution control devices should not be considered a substitute for effective removal by heat.
Also, given the microbiology of municipal and sewage sludge waste, equal concern should be
given to incineration of those wastes as to biomedical waste.
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UNITED STATES ENVIRONMENTAL PROTECTION AGENCY
; Environmental Criteria and Assessment Office (MD-52)
Research Triangle Park, North Carolina 27711
••< PaO'V~
DATE: April 16, 1990
SUBJECT: Summary of Potential Risks from Hospital Waste Incineration:
Pathogens in Air Emissions and Residues
FROM: Lester D. Grant, Ph.D., Director
Environmental Criteria and Assessment Office (MD-52)
TO: Gerald Emison, Director
Office of Air Quality Planning and Standards (MD-10)
THROUGH: William Farland, Ph.D., Director
Office of Health and Environmental Assessment (RD-689)
Attached is the final version of the Summary of Potential Risks from
Hospital Waste Incineration: Pathogens in Air Emissions and Residues, as prepared
by the Environmental Criteria and Assessment Office, Research Triangle Park, NC.
This document has been reviewed for scientific and technical accuracy by
recognized experts within and outside the agency. It was previously sent to
OAQPS for review and comment. No changes were suggested after this review, so
the document is now being forwarded for publication.
Incineration of biomedical wastes is the most frequently used method of
disposal of refuse from hospitals, biomedical research laboratories, and similar
institutions. This report summarizes information regarding the microbial
contamination of waste, survival of microbes in wastes and their destruction by
incineration, and survival of organisms in incinerator emissions and residues.
The roles of residence time and temperature considerations in producing
destruction of microorganisms during incineration, as well as the roles of modes
of operation and efficiency of operation of incinerators in such destruction of
organisms is also discussed. Comparisons of microbial contamination of medical
wastes with contamination of other kinds of municipal wastes and with sewage
sludge is also discussed.
Lack of exposure assessment of affected populations, which would include
measurement of emissions from different kinds of incinerators and stacks, seeding
actual incinerators with test organisms under varying operating conditions, and
ambient sampling in the vicinity of such operating incinerators, prevents the
development of a quantitative risk assessment at this time. Such quantitative
risk assessment would also require data on potential infectivity from a spectrum
of organisms, including a variety of species of bacteria, viruses, molds and
protozoa, which could in theory be emitted in an infective state from the
incineration process. Data on survivability versus infectivity are also lacking
at this time.
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