Published in J. Environ. Qual. 33:1556-1561 (2004).
© ASA, CSSA, SSSA
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SHORT COMMUNICATIONS
Analysis of Volatile and Semivolatile Hydrocarbons Recovered from Steam-Classified Municipal Solid Waste
Joseph G. Leahy*,
Thomas E. Carrington and
Michael H. Eley
Department of Biological Sciences, University of Alabama in Huntsville, Huntsville, AL 35899
* Corresponding author (leahyj{at}uah.edu).
Received for publication May 9, 2003.
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ABSTRACT
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Hazardous household wastes comprise a significant proportion of municipal solid waste (MSW), and therefore serve as the source of many toxic or carcinogenic organic chemicals that are released in the environment through landfill gases or leachates. In the present study, we demonstrate the utility of the steam classification process in removing hazardous semivolatile organic compounds (SVOCs) and volatile organic compounds (VOCs) from MSW. Steam classification is a patented technology that involves the treatment of MSW with steam under pressure to yield a cellulosic biomass product that can be used as a fuel or in building materials. The SVOCs and VOCs from the waste off-gases are collected in the steam condensate and in an effluent charcoal filter. The results of this study show that at least two SVOCs and at least 17 VOCs can be removed from the waste. The most commonly identified compounds were diethylphthalate, styrene, 1,4-dichlorobenzene, and toluene in the condensates, and styrene, 1,1,1-trichloroethane, and toluene in the charcoal filters. On a weight basis, aromatic hydrocarbons were primarily recovered in the condensates, while the chloroaliphatic hydrocarbons were recovered almost exclusively from the charcoal filters. 1,3-Dichlorobenzene, 1,4-dichlorobenzene, and chloroform together comprised nearly 50% of the 4470 µg kg–1 average mass of SVOCs and VOCs recovered from about 454 kg of MSW in these experiments. Toxicity characteristic leaching procedure (TCLP) analyses showed that steam classification recovered at least 75 to 91% of tested analytes. Overall, these results suggest that steam classification represents an effective technology for a significant reduction or the removal of hazardous organics from the waste stream, and, consequently, in reducing the extent of environmental contamination associated with landfill leachates and gases.
Abbreviations: DCA, dichloroethane • DCE, dichloroethene • MSW, municipal solid waste • SVOC, semivolatile organic compound • TCA, trichloroethane • TCE, trichloroethene • TCLP, toxicity characteristic leaching procedure • VOC, volatile organic compound
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INTRODUCTION
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IT HAS BEEN ESTIMATED that 1.6 million Mg of household hazardous wastes are generated annually in the United States (USEPA, 1993). These wastes, which include household cleaners, automotive products, home maintenance products, and lawn and garden products, can contain many synthetic organic compounds that are toxic, carcinogenic, or otherwise hazardous (Reinhart, 1993). When disposed of in landfills, the constituents of such wastes can be released into both landfill leachate and landfill gas, posing a significant risk to both human health and natural ecosystems. Several studies have documented the contamination of aquifers with leachates containing dozens of VOCs and SVOCs (Barker, 1987; Chen and Zoltek, 1995; DeWalle and Chian, 1981). Landfill gases are the source of more than 500000 Mg of VOC emissions annually, including toluene, methylene chloride, trichloroethene (TCE), benzene, and vinyl chloride (USEPA, 1991). While the USEPA has promulgated a number of regulations and initiatives aimed at reducing the release of toxics associated with landfills, such facilities are likely to continue to serve as a source of air, soil, and water contamination as long as the disposal of hazardous household waste is unrestricted.
Steam classification is a patented process for MSW treatment that converts paper, food, and soft yard wastes into a cellulosic biomass or pulp fraction that can be used as a fuel or in a variety of building materials (Eley, 1994, 2001; Eley and Guinn, 1994; Eley et al., 1995). When coupled with post-treatment separation, recovery, and recycling operations, steam classification can reduce the amount of MSW requiring landfill disposal by more than 80% by weight and 90% by volume (Eley, 1994). In this process, MSW is exposed to pressurized steam and agitated. As a result, substantial quantities of organic compounds can be evaporated within the process unit and subsequently recovered either in liquid form in the steam condensate or by adsorption of noncondensable VOCs onto an activated charcoal filter (Eley, 2001). The organic compounds that are recovered include regulated pollutants that are derived from hazardous household waste. Once recovered, these compounds can be removed or their concentrations reduced through physical means, such as thermal oxidation units for off-gases, or through some form of biological treatment utilizing hydrocarbon-degrading bacteria, as we have demonstrated previously (Leahy et al., 2003a, 2003b). In this light, steam classification represents not only a technology for MSW volume reduction and resource recovery, but also for ameliorating the earlier-described risks associated with hazardous household wastes in the waste stream.
The purpose of the present study was to demonstrate removal of toxic VOCs and SVOCs from municipal solid waste by the steam classification process. For this work, several large batches of MSW were treated by steam classification. The resultant condensates and charcoal media were extracted by USEPA Methods 8260 and 8270, and the cellulosic biomass extracted by USEPA Method 1311 (USEPA, 1983). All extracts were analyzed for the presence of regulated VOC and SVOC pollutants by gas chromatography–mass spectrometry (GC–MS). Our results show that steam classification is capable of effectively removing at least 19 hazardous compounds from MSW, leaving a biomass fraction that is safe for use in a variety of commercial applications.
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Materials and Methods
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Collection of Municipal Solid Waste
Municipal solid waste was obtained from dumpsters at a local apartment complex without any organized curbside or drop-off recycling program. Only waste contained in closed plastic bags was collected, and the contents of the bags were unknown before processing. Five batches of MSW were collected at different times over a period of several months to ensure variability, and approximately 454 kg of each batch were processed. The waste was processed without any presorting or pretreatment by simply loading the unopened bags directly into the process unit.
Operation of Steam Classification Unit
The steam classification process unit was designed to meet process specifications and fabricated under American Society of Mechanical Engineers (ASME) codes for pressure and vacuum. A schematic diagram of the process unit is depicted in Fig. 1. The unit was operated as a batch system that was closed pressure tight from the boiler to the exit from the charcoal filter, such that all gases were released through the filter. The three condensers were connected in series so that vapors leaving the process unit passed through Condenser 1, 2, and 3, in that order, before exiting through the charcoal filter. Each condenser has a drain valve to ensure recovery of all hold-up condensate in the condensers as well as that in the condensate tank. The unit was operated as follows (for further details refer to Eley, 1994, 2001; Eley and Guinn, 1994; Eley et al., 1995). In the initial purge step, the exhaust valve was left open as steam was injected into the unit with continuous agitation. This continued until the gases escaping the unit reached 102°C, indicating that the contents had been heated to the boiling point of water and that the void spaces in the unit were filled with saturated steam. At this point, the exhaust valve was closed, sealing the process unit from the vapor collection system. The drain valves of the condensers and condensate tank were opened to collect the purge condensate and then reclosed. The charcoal filter was removed to collect the purge noncondensables and replaced with a new filter. Further injection of steam into the sealed process unit increased the pressure inside the unit to approximately 350 kPa and the temperature to 150°C. The waste within the process unit was continuously mixed by rotating the unit, which has a helical flighting attached to the inside wall of the unit contacting with the waste, while maintaining the above temperature and pressure for a period of 30 to 45 min, after which the unit was depressurized and cooled. The drain valves of the condensers and condensate tank were opened to collect the depressurization condensate, and the charcoal filter was removed to collect the depressurization noncondensables.
Chemical Analyses
Volatile and semivolatile hydrocarbons were extracted from the condensates and from activated charcoal using methanol. The VOCs in methanol extracts were analyzed by a purge and trap technique using USEPA Method 8260 (USEPA, 1983). These analyses were performed using a Tekmar (Mason, OH) LSC 2000 purge and trap concentrator containing a Carbopack/Carbosieve trap, a HP 5890 Series II gas chromatograph fitted with a DB-VRX column (60 m x 0.25 mm x 1.4 µm; Hewlett-Packard, Palo Alto, CA), and a HP 5970 mass selective detector. The SVOCs in methanol extracts were analyzed by USEPA Method 8270 (USEPA, 1983) using direct injection of samples into a HP 5890 Series II gas chromatograph fitted with a DB-5 column (30 m x 0.25 mm x 0.25 µm; Hewlett-Packard), and a HP 5988 quadrapole mass spectroscopy detector. Helium was used as a carrier gas for all analyses. The instrument detection limits were 5 and 100 µg L–1 for VOC and SVOC analyses, respectively.
The biomass fraction of the steam-classified material was analyzed by using the TCLP, USEPA Method 1311 (USEPA, 1983). Volatile and semivolatile analytes were analyzed in extracts as before, and metals were quantitated using a Leeman (Hudson, NH) 1000 ICP emission spectrophotometer.
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Results and Discussion
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Tables 1 and 2 depict the results of chemical analyses for the condensates derived from the purge and depressurization steps of steam classification, respectively. Two SVOCs were detected in both condensates, while 10 VOCs were identified in the purge condensates and 12 in the depressurization condensates. The most commonly occurring compounds were the SVOC diethylphthalate and the VOCs styrene, 1,4-dichlorobenzene, and toluene, which were detected in at least four of the five batches of MSW that were tested. Other compounds detected included the other isomers of dichlorobenzene, naphthalene, ethylbenzene, xylenes, trimethylbenzenes, and the chloroaliphatics chloroform, TCE, and 1,1,1-trichloroethane (TCA). Many of these compounds are constituents of hazardous household wastes or are derived from plastics (Reinhart, 1993), and virtually all have been detected in landfill leachates or landfill gases (see references cited earlier). Chemical concentrations in the condensates were quite high in some cases, with averages ranging from 100 to 5900 µg L–1. The highest concentrations were observed for aromatic compounds, including the dichlorobenzenes, xylenes, and styrene. Analyte concentrations were, for the most part, of the same orders of magnitude in the purge and depressurization condensates. However, since the purge step involves the introduction of only small quantities of steam into the process unit, relative to the depressurization step, a much lower volume of purge condensate (<5 L) than depressurization condensate (>50 L) was produced. Consequently, far greater quantities of SVOCs and VOCs were recovered in depressurization condensate (50–820 µg kg–1, compared with 0.5–6.9 µg kg–1 in the purge condensate).
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Table 1. Concentrations of semivolatile and volatile compounds in condensate following the purge step of steam classification.
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Table 2. Concentrations of semivolatile and volatile compounds in condensate following the depressurization step of steam classification.
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Most of the same VOCs that were identified in the condensates were also detected in methanol extracts from the charcoal filter, as seen in Tables 3 and 4 for the results of purge and depressurization steps, respectively. Significantly, however, the filter was also found to contain additional chloroaliphatic compounds that were too volatile to be retained by the condensate, including methylene chloride, 1,1-dichloroethane (DCA), and 1,1-dichloroethene (DCE). The most commonly detected compounds were styrene in the purge filter and toluene and 1,1,1-TCA in the depressurization filter. On a weight basis, the charcoal filters retained primarily chloroaliphatic compounds, of which chloroform, 1,1-DCE, 1,1,1-TCA, and methylene chloride were predominant. As with the condensate, the quantity of VOCs recovered was many times greater in the charcoal filter after depressurization (2.8–310 µg kg–1) than after the purge step (0.8–13 µg kg–1).
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Table 3. Amounts of volatile compounds in activated carbon filter following the purge step of steam classification.
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Table 4. Amounts of volatile compounds in activated carbon filter following the depressurization step of steam classification.
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The total quantity of each SVOC and VOC recovered from MSW by steam classification, together with the fractions recovered in the purge and depressurization condensates and charcoal filters, is depicted in Table 5. Interestingly, 1,3- and 1,4-dichlorobenzene and chloroform account for nearly 50% by weight of all of the hazardous compounds recovered from the MSW. Aromatic compounds were recovered primarily in the condensates, while the more volatile chloroaliphatics, with the exception of chloroform and TCE, were found predominantly in the filter media. Chloroform and TCE were recovered in significant quantities from both condensates and charcoal filter media. The average total concentration of hazardous SVOCs and VOCs recovered from MSW was 4470 µg kg–1.
Toxicity characteristic leaching procedure analyses of the steam classified biomass were performed to establish the effectiveness of steam classification in removing VOCs from MSW. The results (Table 6) indicate that all analytes were below the detectable limits of the method and instrumentation, and that the concentrations of all analytes were well below the regulatory concentrations. Due to the differences in the analyses for USEPA Methods 1311 and 8260, only three VOCs (chloroform, 1,4-dichlorobenzene, and 1,1-dichloroethene) are common to both methods. These three VOCs are used to show the effectiveness of steam classification in removing VOCs from MSW. From the data in Table 5, the total amount of 1,4-dichlorobenzene recovered in the vapor phase from 454 kg of MSW is 395 mg. From the data in Table 6, the concentration of this compound in the cellulosic biomass is below the detection limit of the method, and although the exact concentration is unknown, the total amount in the cellulosic biomass is less than 35 mg. Since the actual concentration of the 1,4-dichlorobenzene is below the detection limit, we can only calculate the minimum recovery, which is 91% for this compound, but the actual recovery would be greater than 91%. Similar results were obtained for chloroform with the minimum recovery at 90% of the total, but the actual recovery would again be greater, since the actual concentration of chloroform was also below the detection level. The results with 1,1-DCE were somewhat lower with the minimum recovery being 75% of the total, but this recovery would also actually be greater than this minimum, since the concentration of 1,1-DCE was also below detection levels. Consequently, while it is difficult to obtain an exact quantification of the recovery of VOCs in the vapor phase, it is clear that the recovery is at least 75% for one of the VOCs and at least 90% for the two other VOCs.
The results of this study indicate that significant quantities of hazardous SVOCs and VOCs derived from household hazardous waste can be recovered from MSW using the steam classification process. Similar results were obtained in a previous study (Eley et al., 1995) in which the concentration of all VOCs and the total phenols present in the biomass were also below the detection limit of the method and instrumentation. The experimental design of both the present work and previous studies did not allow for an analytical determination of whether the combination of the condensate and charcoal filters trapped 100% of the these organic compounds. Optimization of the process and proper scaling of condensate volumes and filter dimensions can, however, be addressed in future work. It is clear from the data presented that steam classification represents a novel and effective mechanism for removing pollutants from the waste stream, and that integration of this process with existing physical and/or biological technologies for waste treatment could dramatically reduce the release of hazardous chemicals from MSW to the environment.
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ACKNOWLEDGMENTS
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This work was supported by a grant from the Alabama Department of Public Health and Alabama Legacy for Environmental Trust. The technical assistance of Kathy Screws is gratefully acknowledged.
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REFERENCES
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- Leahy, J.G., K.D. Tracy, and M.H. Eley. 2003a. Degradation of mixtures of aromatic and chloroaliphatic hydrocarbons by aromatic hydrocarbon-degrading bacteria. FEMS Microbiol. Ecol. 43:271–276.
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- Reinhart, D.R. 1993. A review of recent studies on the sources of hazardous compound emitted from solid waste landfills: A U.S. experience. Waste Manage. Res. 11:257–268.[Abstract/Free Full Text]
- USEPA. 1983. Test methods for evaluating solid wastes. SW-846. 3rd. ed. USEPA, Washington, DC.
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- USEPA. 1993. Household hazardous waste: Steps to safe management. USEPA, Washington, DC.
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ABSTRACT
INTRODUCTION
