Metal Behavior During Medical Waste Incineration - American

Chapter 15

Metal Behavior During Medical Waste Incineration C. C. Lee and G. L. Huffman Risk Reduction Engineering Laboratory, U.S. Environmental Protection

Downloaded by CORNELL UNIV on July 25, 2016 | http://pubs.acs.org Publication Date: December 23, 1992 | doi: 10.1021/bk-1992-0515.ch015

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Toxic metals such as lead, cadmium, and mercury are contained in medical waste. Consequently, the incineration of medical waste may r e s u l t in the emissions of trace metals into the environment, i f incinerators are not properly designed and operated. EPA's Risk Reduction Engineering Laboratory i n i t i a t e d a study in 1988 to document what i s known about medical waste treatment, p a r t i c u l a r l y in the area of medical waste i n c i n e r a t i o n . This paper i s to summarize the findings from this study regarding the behavior of metals in incineration processes. Highlights of these findings are as follows: (1) Lead and cadmium are the two most-often-found metals in medical waste; (2) Metals can p a r t i t i o n into different phases (gas, l i q u i d or solid) but cannot be destroyed during i n c i n e r a t i o n ; (3) There are several potential pathways that metals follow to reach the environment. They exit incinerators with s i f t i n g s , bottom ash, f l y ash, scrubber waste, and flue gas; (4) Data on the capture e f f i c i e n c y of metals by a i r pollution control equipment used at medical waste incinerators is very l i m i t e d ; and (5) Wet scrubbers generally capture cadmium moderately well but normally perform poorly in removing chromium and lead. Fabric filter systems e f f i c i e n t l y capture a l l metals.

Medical waste contains toxic metals such as lead, cadmium, and mercury. These metals w i l l only change forms (chemical and physical states) but w i l l not be destroyed during i n c i n e r a t i o n . They can be emitted from incinerators on small p a r t i c l e s capable of penetrating deep into human lungs. Thus, the emission of trace amounts of heavy metals from medical waste incinerators i s one of the major concerns to those who are involved in medical waste management. A clear understanding of metals behavior in medical waste incinerators i s c r i t i c a l l y needed.

This chapter not subject to U.S. copyright Published 1993 American Chemical Society Khan; Clean Energy from Waste and Coal ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

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CLEAN ENERGY FROM WASTE AND COAL

EPA's Risk Reduction Engineering Laboratory i n i t i a t e d a study in 1988 to document what i s known about medical waste treatment, p a r t i c u l a r l y in the area of medical waste i n c i n e r a t i o n . Potential toxic metal emissions from medical waste incineration was one of main subjects studied. This paper i s to summarize the findings of that study.


Metal Sources University of C a l i f o r n i a at Davis researchers conducted a study to identify the sources of toxic metals in medical wastes (l). The research e f f o r t focused on lead and cadmium because they were the two most-often-found metals in medical waste. They concluded that p l a s t i c s in the waste contributed most to the presence of these two metals. Cadmium i s a component in common dyes and thermo- and p h o t o - s t a b i l i z e r s used in p l a s t i c s . Lead was found in many materials including p l a s t i c s , paper, inks, and e l e c t r i c a l cable i n s u l a t i o n . However, the primary source of lead appeared to be plastics. Like cadmium, lead i s used to make dyes and s t a b i l i z e r s which protect p l a s t i c s from thermal and photo-degradation. It i s i r o n i c to note that the dyes made from lead and cadmium are used to color p l a s t i c bags. Thus, part of the lead and cadmium emissions could be due simply to the "red bags" that infectious waste i s placed i n . Under the authority of the hazardous waste program required by the Resource Conservation and Recovery A c t , EPA has i d e n t i f i e d ten (10) metals of most concern from 40 CFR 261 Appendix VIII. Four of the ten metals are c l a s s i f i e d as carcinogenic and the other six metals are considered to be t o x i c . The EPA's Carcinogen Assessment Group has estimated the carcinogenic potency for humans exposed to low levels of carcinogens. An assigned "Unit Risk" indicates the r e l a t i v e health threat of the metals. Unit Risk (UR) i s the incremental r i s k of developing cancer to an individual exposed f o r a l i f e t i m e to ambient a i r containing one microgram of the compound per cubic meter of a i r . Inhalation i s the only exposure pathway considered in determining UR. Data on t o x i c i t y are used to define concentrations f o r the six toxic metals below which they are not considered dangerous. Ambient concentrations should not exceed t h i s concentration. The EPA has defined the maximum toxic concentration, or Reference A i r Concentration (RAC), for each metal. If ground level concentrations of any of these metals exceeds i t s RAC, adverse health effects are likely. The Unit Risk of the four carcinogenic metals and the RAC of the six toxic metals are l i s t e d in Tables I and II. Table I.

Unit Risk (UR) Values for Four Carcinogenic Metals Metals species

Unit r i s k

Arsenic (As) Beryllium (Be) Cadmium (Cd) Chromium (Cr )

0.0043 0.0025 0.0017 0.012

+6

UR: incremental l i f e t i m e cancer r i s k from exposure to 1 /xg/cubic meter

Khan; Clean Energy from Waste and Coal ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

15. LEE & HUFFMAN Table II.

Metal Behavior During Medical Waste Incineration

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Reference A i r Concentrations (RACs) for Six Toxic Metals 3

Metals species

RAC (/ig/m )

Antimony (Sb) Barium (Ba) Lead (Pb) Mercury (Hg) S i l v e r (Ag) Thallium (Tl)

0.025 50.00 0.09 1.70 5.00 500.00


Emission Pathways A majority of metal emissions is in the form of s o l i d particulate matter and a minority is in vapor form. It was generally concluded that particulate emissions from the incineration of medical wastes are determined by three major f a c t o r s : (1)

Suspension of noncombustible inorganic materials;

(2)

Incomplete combustion of combustible materials (these materials can be organic or inorganic matter); and

(3)

Condensation of vaporous materials (these materials are mostly inorganic matter).

The ash content of the waste feed materials i s a measure of the noncombustible portion of the waste feed and represents those materials which do not burn under any condition in an incinerator. Emissions of noncombustible materials result from the suspension or entrainment of ash by the combustion a i r added to the primary chamber of an incinerator. The more a i r added, the more l i k e l y that noncombustibles become entrained. Particulate emissions from incomplete combustion of combustible materials result from improper combustion control of the incinerator. Condensation of vaporous materials results from noncombustible substances that v o l a t i l i z e at primary combustion chamber temperatures with subsequent cooling in the flue gas. These materials usually condense on the surface of other fine p a r t i c l e s . The transformation of mineral matter during combustion of metalscontaining waste i s shown in Figure 1. The Figure i s s e l f explanatory. There are several potential pathways to the environment that metals may follow. Most metals remain in the bottom ash. A small fraction of the ash (on a weight basis) is entrained by the combustion gases and carried out of the primary chamber as f l y ash. V o l a t i l e metals may vaporize in the primary combustion chamber and leave the bottom ash. These metals recondense to form very small p a r t i c l e s as the combustion gases cool. Some of the entrained ash and condensed metals are captured



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HOMOGENEOUS CONDENSATION

Figure 1. Metal transformation during incineration. Reproduced from ref. 2.


15. LEE & HUFFMAN

Metal Behavior During Medical Waste Incineration

193

in the a i r pollution control equipment (APCE). The rest enters the atmosphere. Four key variables affecting the vaporization of metals are ( 2 ) : t

Chlorine concentration in the waste;

•

Temperature p r o f i l e s in the

•

Metal species concentration in the waste; and

•

Local oxygen concentration.

incinerator;


Current Control Practice T y p i c a l l y , two strategies are used to minimize metals emissions: (1) The primary chamber is operated at conditions which do not promote vaporization or entrainment of metals; and (2) Any metals which do escape can be captured in the APCE, i f present. The parameters usually used to control the escape of metals from the primary chamber are the primary chamber temperature and gas v e l o c i t y . The key APCE parameters used are s p e c i f i c to the device which i s utilized. (1)

Combustion control : Most operating medical waste incinerators are simple single-chamber units with an afterburner located in the stack. The a b i l i t y of batch incinerators to control metals emissions i s limited because only the temperature in the stack is usually monitored. Most new incinerators are starved-air u n i t s . The primary chamber is designed to operate at low temperatures and low gas flow rates. This minimizes the amount of materials entrained or vaporized. To ensure that metal emissions are minimized, operators must maintain the primary chamber at the temperatures and gas flow rates for which i t was designed. Usually the only parameter that system operators can d i r e c t l y control i s feed rate. High feed rates can lead to high temperatures and high gas v e l o c i t i e s . Thus, many operators c a r e f u l l y control the feed rate. The feed rate is reduced when primary temperatures increase.

(2)

APCE control : When metals reach the APCE, they are present in one of three forms. Non-volatile metals are on large entrained p a r t i c l e s . Metals which have vaporized and recondensed are usually present on f l y ash p a r t i c l e s with diameters less than 1 micron. Extremely v o l a t i l e metals are present as vapors. Table III summarizes the a b i l i t y of common APCE to control these different metal forms. The Table i s based on data and worst case p r e d i c t i o n s . Wet scrubbers are often used to minimize the temperature of the


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flue gases. Use of low temperatures ensure that a l l metal vapors have condensed. As indicated in Table III, vapors are much more d i f f i c u l t to capture than p a r t i c l e s (2). TABLE III.

Typical APCE Control E f f i c i e n c i e s


Control E f f i c i e n c y (%) APCE

Particulate

Fume

Vapor

Venturi scrubber 20" pressure drop

90

85

60

Venturi scrubber 60" pressure drop

98

97

90

Fabric f i l t e r

95

90

50

Spray d r i e r / fabric f i l t e r

99

95

90

Emission Data Figure 2 compares the concentration of arsenic (chosen merely f o r i l l u s t r a t i v e purposes) in flue gases before any APCE, and in emitted gases for a variety of incinerators. As shown, a wide variety of flue gas cleaning equipment i s used. The Figure indicates the effectiveness of the various types of APCE. Arsenic i s predicted to be r e l a t i v e l y v o l a t i l e , compared to other metals. S i g n i f i c a n t amounts of arsenic are therefore expected to vaporize in an incinerator. Figures 3 and 4 present s i m i l a r data f o r the two most common metals found in medical waste, lead and cadmium (2). Conclusion Some metals and metal species found in medical waste are v o l a t i l e and w i l l vaporize at the conditions found in medical waste incinerators. The vapors are carried away from the waste by the exhaust gas and they recondense as the gas c o o l s . The vapors condense both homogeneously to form new p a r t i c l e s and heterogeneously on the surfaces of existing f l y ash p a r t i c l e s . To control metal emissions, metals which are of a highly v o l a t i l e nature are of main concern in terms of i n s t a l l i n g the proper APCE. Because there are many APCE sizes and types, i t i s very important to f u l l y understand metal emissions c h a r a c t e r i s t i c s , combustion control and operating p o s s i b i l i t i e s , and expected APCE performance so that metal emissions can be minimized from medical waste i n c i n e r a t o r s .


15. LEE & HUFFMAN

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> Data not available « Metal below detection limit

1 gr/dscf = 2.29 g/Ncm

Figure 4. Cadmium emissions. Reproduced from ref. 2.


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CLEAN ENERGY F R O M WASTE A N D COAL

Literature Cited 1. Hickman, D. C., "Cadmium and Lead in Bio-Medical Waste Incinerators," Master of Science Thesis, University of C a l i f o r n i a , Davis, 1987. 2. Energy and Environmental Research Corporation, "State-of-the-Art Assessment of Medical Waste Thermal Treatment," A Draft Report to EPA's Risk Reduction Engineering Laboratory, A p r i l 1991.


RECEIVED September 15, 1992


Metal Behavior During Medical Waste Incineration - American

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