PROVOKING A FIRES ORM: $‘ Industry and government have accepted incineration as the treatment method of choice for many types of waste. Benefits of incineration include the destruction of toxic organics, volume reduction, low public health risk, potential energy recovery, and wide range of application. In a matter of seconds or minutes, an incinerator can destroy a waste that might otherwise survive for hundreds of years in a landfill. Because of these benefits, EPA has embraced incineration as the preferred treatment method for a wide range of waste streams. Waste generators appreciate the lower long-term liability of incineration compared with the more traditional and cheaper landfill and deep-well injection alternatives. Because of EPA’s recent research efforts in particular, more is known about incineration technology and its effects on the environment than virtually any other waste management alternative (1). Results of this research have shown that a properly designed incinerator is an effective treatment device that can be operated safely and with negligible apparent environmental impact or health risks (2).EPA’s research and regulatory framework also has significantly improved incineration practices over the past decade. These improvements include better performance and control, reduced emissions, and standardized sampling and analytical methods. Despite the benefits and the regulatory and industrial acceptance, incineration lacks public acceptance. The discovery and widespread publicizing of major environmental ca1809 Environ. Sci. Technol., Val. 25,
E. Malone Stevenon Science Applications International Corp. Idaho Falls, ID 83401
tastrophes resulting from improper past disposal practices have heightened public awareness. This has led to social activism that has resulted in major waste management legislation. The public, however, is still concerned about the ability of industry to safely operate waste management facilities and about the ability of the government to enforce compliance on these facilities. As a result, the siting of an incinerator has become a difficult endeavor at a time when increased capacity is urgently needed. Status Statistics. Estimated numbers of operating incinerators in the United States are given in Table 1. Although the largest category of waste incinerators by number encompasses medical waste incinerators, the largest category by capacity comprises municipal waste incinerators. At the other end of the spectrum in number and capacity are radioactive waste incinerators. Table 1 also compares the waste incineration capacity with the waste generation rate where the information is available. This comparison shows that only 1-356 of the hazardous waste generated is processed by incinerators, boilers, and industrial furnaces (BIFs).Although the municipal waste combustion capacity may be as high as 29% of that generated ( 3 ,it also has been re-
No. 11, 1991
ported that only 14% of all municipal waste currently is incinerated ( 4 ) . This suggests that only about half of the available capacity is being used. The incineration capacity for medical waste exceeds the generation rate by an order of magnitude. Primarily, this is because medical waste incinerators associated with hospitals and other institutions, which make up the bulk of medical waste incinerators, typically are operated on an intermittent basis rather than continuously. Although only 1-3% of the hazardous waste generated in the United States is incinerated, EPA and industry prefer this treatment method to other disposal options. These numbers suggest the potential for an increase in the number of hazardous waste incinerators in the United States. EPA’s land disposal restrictions, which mandate the incineration of many hazardous waste types before disposal, support this prospect (5).The primary need will be for sludge and solids burners because boilers and industrial furnaces provide adequate capacity for liquids (2).This suggests a demand for rotary kiln incinerators in the near future because of their ability to handle hazardous solids and sludges effectively. Needs for other types of iucinerators also are expected to grow in the near future. For example, the number of municipal incinerators is expected to increase by 70 units withi n the next five years because nearly half of the municipal landfills now in use probably will be full or closed in the same period. The number of medical waste incinera-
0013-936)(191/0925-1808$02.50/0 0 1991 American Chemical Society
7 I Waste incinemtor in Stanislaus County, CA
tors is expected to increase by 125 on-site hospital and 15 commercial facilities per year within about five years as more generators try to reduce their liability (6). Mobile and transportable incinerators, used for contaminated soil remediation, represent a significant potential market that is estimated as high as $300 billion over the next 30-40 years (7). Regulations. Some recent regulatory amendments have had a major impact on the waste incineration industry. The most important changes include the Clean Air Act (CAA) Amendments of 1990, the new Resource Conservation and Recovery Act (RCRA), Boiler and Industrial Furnace Regulations ( 1 1 ) , and the proposed RCRA incinerator regnlations (12). The CAA Amendments require EPA to establish new standards for municipal and medical waste incin-
erators. The original standards, set in 1971 and revised in 1986, address only particulate matter. The first of these measures, new standards for large municipal waste incinerators (greater than 250 tons per day), was announced in Jan. 1991. These new standards are intended to reduce the air emission of certain pollutants by 90% by 1994.Similar regulations for new and exi.sting medical waste incinerators are scheduled for proposal in Aug. 1992 and for promulgation in Sept. 1993.
The BIF regulations, announced in Dec. 1990 and effective Aug. 1991,close a 1o.ophole that allowed the unregulated burning of hazardous waste in boilers and furnaces. These facilities process about half of all hazardous waste burned in the United States. As a result of these regulations, boilers and industrial
furnaces now are regulated more stringently than are hazardous waste incinerators. The regulations include risk-based standards for toxic metals and required controls on particulate, organics, and hydrochloric acid (HC1). The new proposed RCRA hazardous waste incinerator regulations are similar to the BIF regulations. Under the existing regulations, toxic metals are assumed to be controlled by the particulate standard. The new proposal includes riskbased emission limits on the 10 toxic metals listed in Table 2. The new standards also establish a risk-based evaluation to ensure the control of products of incomplete combustion (PIC) and of HC1 and chlorine emissions. Although these regulations have not yet been promulgated, EPA is now imposing the standards in all new permits (23)under the
Environ. Sci. Technol., Vol. 25. NO.11, 1991 1809
IP.WLL
I
Estimated numbers of operatln US. incinerators, treatment capacities, and waste generate3
i azardous
175
Municipal4
ru0-300 6850 8 200
Medicalc
3adbactived 31Fs' Mobile' ~~
~
65
2-3
265
NA
NA
'MWn me m s : soma values hws been mwstiad 10 MMTlyaar using 300 operating days per
'LYkrs operating in 1989 :Numbers opermlng in 1991
'Numbers opsrahng in 1989 6d bailers and rating in 1990 I
omnibus authority of RCRA (14). The new regulations that require more stringent controls on a wider range of pollutants are driving the industry to use more advanced technologies. Perhaps the most universal and costly effect is the need for air pollution control equipment (APCE). Before the new standards imposed by the CAA, BIF, and new hazardous waste incinerator regulations, many facilities were able to meet the emission standards without APCE. Now, operators of many facilities will consider adding scrubbers or other types of air pollution control equipment to their systems. This is particularly true for municipal incinerators because of the HC1, SO,, and NO, standards. Issues Air emissions. It is probably safe to say that the emissions of waste incinerators have been studied more intensely in the past 15 years than they were in all the previous time. This study has resulted in standardized sampling and analytical methods, improved operating practices, monitoring, and controls, and an effective regulatory framework. The stack emissions of hazardous waste incinerators have received the most attention, probably because of the public's concern with this type of incineration. EPA-sponsored tests and regulatory trial burns have provided detailed characterization of incinerator stack emissions. The conclusion derived by EPA from the significant quantity of emissions data accumulated is that a well-operated incinerator, boiler, or industrial furnace is capable of achieving RCRA performance standards (2). Of the performance
I
tific community is that of dioxins. Table 3 presents a comparison of the dioxin and furan emissions from various incineration sources. Although the ranges are very wide, it is apparent that the emission levstandards, the particulate emissions els from medical and municipal inlimit has been the most difficult for cinerators are higher than those from hazardous waste burning defacilities to attain. The emission of products of in- vices. The very wide range in levels complete combustion has been the from municipal incinerators sugsubject of considerable debate be- gests that dioxin and furan emiscause it casts doubt on the effective- sions can be controlled a great deal ness of EPA's current regulatory ap- better than they are in some faciliproach. In response, the agency has ties. Generally, the emissions from proposed the addition of a new controlled municipal incinerators technology-based performance tend toward the lower end of the standard for hazardous waste incin- range, whereas the emissions from erators to control organic emissions medical incinerators are relatively and has included this standard in high (9). This difference is thought to be the result of the lack of APCE the new BIF regulations. EPA believes that the best way to and good combustion control on control organic emissions is to en- medical incinerators. The higher levels of dioxin emissure that the incinerator is operating at a high combustion efficiency. sion from municipal and medical Although the agency has not been incinerators, compared with those able to establish a direct link be- from other types of devices listed in tween carbon monoxide (GO) levels Table 3, are likely to result from opand PIC emissions, EPA's trial burn erating practices. The current regudata indicate that when CO levels latory program for hazardous waste are below 100 ppmv, PIC emissions incinerators is much more stringent always are at levels that pose ac- than that for other types of incineraceptable health risk. Therefore, EPA tors. These more stringent regulahas issued regulations for BIFs and tions have led to tighter controls on has proposed regulations for haz- operating conditions and to better ardous waste incinerators that re- training of operators. A dioxin-furan emission limit of quire the continuous monitoring of offgas GO. When the CO level ex- 30 ng/Nm3 recently was promulgated for new municipal incinerators, ceeds 100 ppmv (corrected to 7% OJ, waste feed to the incinerator and a similar standard is expected must be automatically stopped (12). for medical incinerators in the next Meanwhile, the agency has commit- two to three years. The limits for exted itself to studying the PIC issue isting incinerators were set at 60250 ng/Nm3 depending on the size in greater detail. EPA has also issued regulations of the combustor and the degree to requiring continuous GO monitor- which fuel is cofired (25).This level ing and limits as a control on the or- is significantly higher than the mganic emissions from municipal rent European dioxin-furan emiswaste incinerators. Similar mea- sion standard of 0.1 ng/Nm3 (note, sures are likely to be imposed on however, that the European standard is expressed as the numericalmedical waste incinerators. The organic emission that has re- ly much lower "TCDD equivaceived the greatest amount of atten- lents"). The U S . standard usually tion from the public and the scien- can be met by good combustion
1810 Envimn. Sci. Technol.. Vol. 25, NO.11. 1991
Dioxin and furan emissions from incinerators (nglNm') Source.
Medical (3) Wunicipal (7) iazardous (4)
3oiler (4 hazardous waste) he-cement kilns (4hazardous waste)
Polychlorinated dibenzopdlorin
Polychlorinated dibenzopluran
117450 1-10700 NDb-16 ND-1 ND
52-30,300 2-37,500
'Number of incinerators ~ a r n p ,,, ,~~ 'None detected Source: References 20 and 25.
practices using CO as an indicator. The European standard, on the other hand, has been met only through recent developments in APCE (16). Other emissions currently receiving much attention are those of toxic metals. Toxic metal-bearing wastes are commonly fed to incinerators of all types. In municipal waste incinerators, for example, batteries are the primary source of lead and cadmium. Although organic compounds are destroyed by incineration, metals are not. The effect of incineration on metals is usually the formation of metal oxides and metal chlorides that become part of the ash or scrubber liquid streams or are emitted with the stack gas. Residues. The management of solid residues remains a concern in the incineration industry. The management and disposal of municipal waste ash is the subject of considerable controversy. The combination of unclear disposal regulations and confusion over whether the ash is hazardous or not has restricted the use of incineration for municipal wastes. The most likely solution to the problem is for Congress to enact legislation governing the management and disposal of municipal incinerator ash. Meanwhile, EPA should provide technical guidance on municipal ash disposal (1 7). The challenges facing producers of hazardous waste incinerator ash are more technical. Land disposal restriction regulations (5) require that the organic contaminants be destroyed and the metal contaminants immobilized to defined standards before disposal. These standards are defined for each of EPA's hazardous waste codes. However, because many incinerators burn mixtures of EPA-defined waste streams, the resulting ash must meet the most stringent treatment
standards of all the waste codes. Testing to verify that the residues meet these standards is a challenging, time-consuming, and expensive task. Facility operators must provide a high level of assurance that the standards are being met while controlling costs of analysis, meeting costs and limits of residue storage, and reducing test turnaround time (181. In addition, many operators are having to turn to ash vitrification and other expensive ash treatment methods to meet metals immobilization standards. Risk. Much attention is focused on the emissions of various pollutants from an incinerator and the ability of the incinerator to comply with the regulatory treatment standard. What is most important, however, is the public health risk that these emissions pose. EPA conducted a risk assessment to examine the health and environmental effects of the 1982 RCRA incinerator regulations. The analysis used the results of emissions data from nine fullscale incinerator tests. Similar analyses have been done on several municipal waste incinerators. Table 4 presents a summary of the results of incinerator risk assessments. Although total risk is low, the hazardous waste incinerator data show that the increased cancer risk posed by metals may be as much as five orders of magnitude greater than the risk from organics. It is this apparent risk that prompted EPA to propose risk-based metal emissions limits for RCRA-regulated incinerators. The total risk posed by municipal waste incinerators may be higher than the risk from hazardous waste incinerators, although it is difficult to compare numerical expressions of risk (Table 4). The contribution of individual contaminants to total risk, however, can be compared. Al-
though toxic metals dominate the risk associated with hazardous waste incinerator emissions, the total risk from municipal combustors appears to be split evenly between metals and organics. Dioxins and furans are the organic compounds that contribute the greatest risk from municipal incinerators (191. On the other hand, the risk posed by dioxins and furans from hazardous waste incinerators is low. These risk assessments, however, were done before the promulgation of organic emissions control regulations for municipal incinerators in Feb. 1991.The additional risk that municipal incinerators pose to public health will decrease over the next three to five years as compliance with these regulations is achieved. Similarly, the enforcement of the proposed metal emission controls on hazardous waste incinerators and new metal emission controls on municipal and medical waste incinerators in coming years will affect the risk posed by these devices. The net effect of these regulatory changes should be a reduction in total measured risk by at least one order of magnitude. Risk assessments to date show that a well-operated, well-designed incinerator presents an acceptable risk to public health. This depends on perception of acceptable risk. EPA's definition of acceptable risk is an additional lifetime (70-year) individual cancer risk to the potential maximum exposed individual (ME11 of 1 in 100,000 or lo". (The potential ME1 is defined as a hypothetical individual at an off-site location where ambient pollutant concentrations created by a facility are highest; whether anyone actually spends any time at that location is immaterial.) The public's perception of acceptable risk usually approaches zero more closely than EPA's definition when it comes to incinerators. The question also arises of whether incinerator risk analyses truly represent the risk. Risk assessments have to rely to a great extent on conservative assumptions and professional judgment because of a deficiency of data and guidelines for methodology (2.201. A small fraction of the necessary chronic lowlevel exposure effects data is presently available. Considerable uncertainty also is involved in extrapolating the available data of the effects of high doses on animals to low doses on humans. The lack of
Environ. Sci. Technol., Val. 25, No. 11, 1991 1811
knowledge of the interactive effects of complex mixtures of pollutants compounds the uncertainty (2). A risk assessment guidelines document for hazardous waste incineration will be promulgated together with the new regulations. If the use of this guideline is codified, it will standardize the general methodology and eliminate some of the uncertainty. With standardized methodology, the result will be a useful index of risk even if it is not a quantitative measure. Public acceptance. Clearly, the biggest challenge currently facing the incineration industry is public acceptance. Delays in incinerator siting caused by public pressure could have serious consequence! including overburdened landfill! unsafe storage of hazardous and radioactive wastes, and delays in remediation of contaminated soil sites, which would result in further contamination. Much of the research on incinerators done by EPA and industry has been directed at developing means of improving the public acceptance of incineration technology. These attempts have included work on continuous emissions monitors to improve regulatory oversight and allay the public's often-mentioned fears of process upsets. Many attempts also have been made to educate the public about the actual physical risk of incinerator emissions. The hope was that the public would begin to see incineration in the same light as do EPA and industry. Unfortunately, it appears that nonphysical aspects, including psychological, social, economic, and political factors, are th major contributors to the oppositio of waste management facilities. These factors can be summarized as follows: Social and psychological: Incinerators are perceived as imposed rather than voluntary risk, and the images associated with incinerators are undesirable (21,22); Economic: The public believes that the presence of an incinerator will lower property values (23);and Political: A general distrust of government and industry (24). Because public attitudes probably are influenced most by nontechnical issues, they are not likely to change in the near future.
od of choice for a wide range of wastes. Incineration offers many benefits, including the following: highly efficient toxic organics destruction, enormous volume reduction, low public health risk, potential energy recovery, and a wide range of application. Because of government and private research conducted over the past 10-20 years, incineration has become perhaps the most well understood of all waste treatment options. The results of this research
Benefits and skepticism EPA and industry have accepte incineration as the treatment met1 1812 Envimn. Sci.Technol., Vol. 25, No. 11. 1991
have shown that emissions from well-designed and well-operated incinerators present very low risk to the public health. EPA's research has led to an effective regulatory framework. The regulatory program governing the burni n g of h a z a r d o u s w a s t e i n incinerators, boilers, and industrial furnaces is the most mature. New regulatory programs for municipal incinerators have been promulgated, and rules for medical waste are being developed under the Clean Air Act of 1990. The new regulations that require more stringent controls on a wider range of pollutants are moti-
vating industry to use more sophisticated technologies. This will lead to improved emission c;ontrols and reduced public health risk.
Because of the effectiveness of incineration as a waste treatment, the demand for incinerators i s escalating. Primary reasons are t h e
TABLE
Total excess lifetime cancer risk from incinerator emlssions to
the maximum exposed individual' Incinerators
Organics Metals
Municipal waste"
10-io-10-7 10"-10~
I 0-7-1 o4
I 0-9-1 o4
10--106
Total individual 'Reference 26.
Hazardout wasteb
=,,-
3-*in-t
pollutant conc
10-7-104 I
facility are hip-$
"Reference 27.
Medical, radioactive waste. The incinerators most frequently used for medical and radioactive wastes are of the hearth type or controlled-air incinerators (9, IO). These units are popular for medical waste destruction because of their low cost, modular design, and ability to meet particulate standards without APCE. Hearth incinerators also are preferred for radioactive waste because of their adaptability in meeting requirements for contamination and particulate control. Municipal waste. The furnaces used to burn municipal waste are designed for enhanced energy recovery. These include modular and field-erected furnaces. The modular mass burn-type furnace, operated in the Starved or excess air mode, is the most frequently used. Other furnaces in wide use include mass-bum waterwall, mass-burn refractory, mass-burn rotary waterwall, and fluidized-bed (circulating and bubbling) types. New technology. Many innovative thermal treatment technologies have been developed during the past decade. These technologies have been developed primarily for specialized applications such as soil remediation and radioactive waste treatment. The type of innovations currently sought are those that fix toxic metals and radionuclides, such as vitrification technologies, and those that make a system more portable or less costly. The development of innovative technologies has been promoted by EPA through the Superfund Innovative Technology Evaluation Program and by the Depaltment of Energy's Office of Technology Development. These efforts have resulted in technologies that use alternatives to basic combustion bum. ers such as electric radiant heating elements, oxygen-enriched burners, anc plasma-arc torches. Developing these techniques will lead to the reduction 01 offgas volume generated by waste treatment and to smaller, more transport. able systems. The oxygen-enriched burners and plasma-arc torches also in. crease the processing temperature to the point at which organics are completely destroyed and residual matter is melted or vitrified. The resulting glasslike material has been found to retain toxic metals and radionuclides effectivel) when tested for leachability. In contrast, incinerator ash residues usually r e quire additional treatment, such as cementation, to bind toxic metals beforf disposal.
regulatory requirement of incineration before disposal for many hazardous waste streams and the cost and reduced number of municipal landfills. Now that municipal incinerator regulations have been promulgated, only t h e issues of ash disposal and siting remain as public policy barriers t o the increased use of incineration to reduce our dependence on landfills. Because of these issues, however, only a fraction of the incinerable waste generated in this country i s being incinerated. Despite the benefits and despite regulatory and industrial acceptance, incineration has not gained public acceptance. The discovery and widespread publicizing of major environmental catastrophes resulting from improper past disposal practices have heightened public awareness and led to genera l skepticism toward incinerators. The public i s very concerned about the safety and health impacts of incinerators and the ability of the government to enforce compliance b y the facilities. Besides the physic a l aspects, the p u b l i c may be more influenced by psychological, social, economic, and political factors. As a result, the siting of an incinerator has become a very diffic u l t endeavor at a t i m e when increased capacity i s needed.
References (1) Hazardous Waste Incineration: A Resource Document American Society of Mechanical Engineers: New York, 1988.
(continued on next page)
4
rlYYnC I
State-of-the-art rotaw kiln incinerator and offgas svstern
E. Malone Steverson is a senior engineering specialist with Science Applications International. He specializes in operations technical support, federal and state permitting, and the development of incinerators and other thermal waste treatment systems. Steverson serves on the American Nuclear Society Standards Committee for low-level waste volume reduction. He received his M.S.in chemical engineering from the University of Idaho. Environ. Sci. Technoi., Voi. 25, No. 11, 1991 1813
Oppelt, E. T. J. Air Pollut. Control AsSOC. 1987, 37(5), 558-86. Incineration In(3) Oberacker, D. A. U.S. dustry Status; 1991 Incineration Conference Basics Course, Knoxville, TN, May 1991; EPA: Cincinnati, OH, 1991. (4) j . Air Waste Manage. Assoc. 1991, 42(3), 259. (5) Code ofFederal Regulations, Title 40. Part 268, 1986. (6) Durkee, K. R.; Eddinger, J. A. Presented at the 1991 Incineration Conference, Knoxville, TN, May 1991. (7) Cudahy, J. J.; Troxler, W. L. Presented at Haztech International 89, Cincinnati, Sept. 1989. (8) Vogel, G. Presented at the EPA 12th Annual Research Symposium, Incineration and Treatment of Hazardous Wastes, Cincinnati, OH, Aug. 1986. (9) Barton, R. G.; Lanier, S.; Seeker, W. Presented at the 1990 Incineration Conference, S a n Diego, CA, May 1990. (10) Mayberry, J. L. Preliminary Systems Design S t u d y A s s e s s m e n t Report; EG&G: Idaho Falls, ID, 1991; EGGWTD-9594. (11)Fed. Regist. 1991,56, 7134. (12) Fed. Regist. 1990,55,17862-921. (13) Fed. Regist. 1990,55, 17892. (14) Code of Federal Regulations, Title 40, Part 270, Section 32. (15) Code of Federal Regulations, Title 40, Part 60, 1971. (16) McIlvaine, R. W. 1.Air Waste Manage. ASSOC.2992, 42(3), 272-75. (17) Ujihara, A. M.; Gough, M. In Managing A s h from Municipal Waste Incinerators; Resources for the Future: Washington, DC, 1989. (18) Schofield, W. Presented at the 1991 Incineration Conference, Knoxville, TN, May 1991. (19) Levin A. et al. J. Air Waste Manage. ASSOC.1991,42(1), 20-30. (20) Oppelt, E. T. In Handbook of Incineration of Hazardous Wastes; Rickman, W. S., Ed.; CRC Press: Boca Raton, FL, 1991; pp. 3-57. ( 2 1 ) Bealer, R. C.; Crider, D. M. In Solid a n d Liquid Wastes: M a n a g e m e n t , Methods, and Socioeconomic Conside r a t i o n s ; Majumdar, S . ; Millers, E. W., Eds.; The Pennsylvania Academy of Science: Easton, PA, 1984. (22) Slavic, P.; Fischoff, B.; Lichtenstein, S . In Readings in Risk; Glickman, T. S . ; Gough, M., Eds.; Resources for the Future: Washington, DC, 1979. (23) Zeiss, C.; Atwater, J. journal of Urban Planning a n d Development 1989, 2 2 5, 64-79. (24) Morell, D. In Resolving Locational Conflict; Lake, R. W., Ed.; Center for Urban Policy Research: New Brunswick, NJ, 1987. (25) Lee, C. C. Presented at the 1991 Incineration Conference, Knoxville, TN, May 1991. (26) Oppelt, E. T. Presented at the 79th Air Pollution Control Association Annual Meeting, Minneapolis, M N , J u n e 1986. (27) “ A Profile of Existing Hazardous Waste Incineration Facilities and Manufacturers in the U.S.”; U.S. Env i r o n m e n t a l Protection Agency: Washington, DC, 1984; PB 84-157072. (2)
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me Journal of Organic Chemistry solicits manuscripts that address topics at the interface of organic chemistry and biology, hile such manuscripts should address fundamental problems in organic chemistry (structure, mechanism, synthesis), we encourage submission of manuscripts in which these problems are solved with the use of techniques not traditionally associated with organic chemistry (enzyme kinetics, enzyme isolation and purification, identification of active site residues, etc.). The Journal hopes to foster integrated publications in which the chemical aspects are not separated from the biological aspects.
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For manuscript format, see J. Org. Chem, 1990, 55 (1 ), 7A-1OA. Send manuscripts to: C. H. Heathcock, Editor-in-Chief,The Journal of Organic Chemisty, Department of Chemistry, University of California, Berkeley, CA 94720 For subscription information American Chemical Society Sales and Distribution A!!1155 Sixteenth Street, N.W., Washington, D.C. 20036 (202) 872-4363 Toll Free, 1-800-227-5558 1814 Environ. Sci. Technol., Vol. 25, No. 1 1 , 1991
