Environ. Sci. Technol. 1997, 31, 1054-1061
Trace Organic Compounds in Landfill Gas at Seven U.K. Waste Disposal Sites MATTHEW R. ALLEN, ALAN BRAITHWAITE,* AND CHRIS C. HILLS Department of Chemistry and Physics, The Nottingham Trent University, Clifton Lane, Clifton, Nottingham NG11 8NS, U.K.
The trace volatile organic compounds (VOCs) in landfill gas were examined at seven U.K. waste disposal facilities. Over 140 compounds were identified, of which more than 90 were common to all seven sites. The groups of compounds and concentrations observed were alkanes, 3021543 mg m-3; aromatic compounds, 94-1906 mg m-3; cycloalkanes, 80-487 mg m-3; terpenes, 35-652 mg m-3; alcohols and ketones, 2-2069 mg m-3; and halogenated compounds, 327-1239 mg m-3. The observed variations in landfill gas composition were largely attributed to differences in the waste composition and the stage reached in the decomposition processes at each of the sites. Three sites were found to have total chlorine concentrations, derived from the organochlorine compounds in the gas, of above 250 mg m-3. Chlorine contents of this level were considered to be potentially damaging to landfill gas fueled engines used for electricity generation. Chloroethene (>0.1-87 mg m-3) was identified as the most abundant toxic component. Chloroethene levels in the landfill gases from two of the sites studied were found in excess of the U.K. maximum exposure limit by a factor of 5 and 3. Total VOCs emissions from four of the seven sites studied were estimated to be of the order of 104 kg/yr.
Introduction Landfill gas is a product of the natural biological decomposition of organic material contained in wastes deposited in landfills. The latter includes paper, animal and vegetable matter and garden wastes (1). The production of the principal landfill gas components occurs in four more or less sequential phases, with the final phase being characterized by the constant production of methane (60%) and carbon dioxide (40%) (2). The latter gases, which are assigned the generic description of “landfill gas”, continue to be produced until the majority of the organic material in the waste has been degraded. Emissions from municipal landfill sites are potentially detrimental to both local and global air quality. For example, the global emissions of methane, which is an important greenhouse gas, are estimated to be between 20 and 70 Tg/yr (3). Perhaps of greater concern, however, are the emissions of trace volatile organic compounds (VOCs) (4). The trace VOCs arise from the volatilization of compounds contained within the waste and those formed during the decomposition * Corresponding author telephone: 0044 115 9418418, ext 3345; fax: 0044 115 9486636; e-mail:
[email protected].
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of the wastes. Keller (5) postulated that all landfill gases contain the following six classes of compounds: saturated and unsaturated hydrocarbons, acidic hydrocarbons and organic alcohols, aromatic hydrocarbons, halogenated compounds, sulfur compounds such as carbon disulfide and mercaptans, and inorganic compounds. Although the trace VOCs account for less than 1% of the total gaseous emissions, they exert a disproportionate environmental burden because of their physical and chemical properties. Dent et al. (6) reported significant levels of a range of chlorofluorocarbons that, if allowed to enter the atmosphere, could potentially contribute to both stratospheric ozone depletion and the greenhouse effect (7). In terms of local air quality, the undesirable odor and perceived health risks associated with landfill gas have often led to widespread complaints from residents living near landfill sites. Young and Parker (8) identified the two groups of compounds responsible for the odor as being alkyl benzenes and limonene and certain esters and organosulfur compounds. In some cases, they reported that a 106 dilution of the gas would be required to reduce the concentration of some of the trace species below their odor threshold. Concern over the health effects associated with prolonged exposure to landfill gas, particularly for landfill operatives, appears to be supported by a number of authors, who reported elevated levels of benzene, chloroethene, trichloroethene, and tetrachloroethene (6-13). The use of landfill gas as a fuel for electricity generation has also been affected by the levels of certain trace VOCs. Gases containing organosulfur or organochlorine compounds react with oxygen and water during the combustion process forming H2SO4 and HCl, which both contribute to the corrosion of the surfaces within the combustion chamber. The latter point was clearly illustrated at the Braunschweigs landfill site in Germany where, after 900-1000 engine hours, the three landfill gas fueled engines were found to be seriously damaged by corrosion. The corrosion was attributed to relatively high concentrations of chlorinated compounds (ca. 600 mg m-3 ) present in the gas (14). Despite the obvious importance and interest in the analysis of the trace volatile organic compounds in landfill gas, few authors have published complete trace component analyses due to inefficiencies in the analytical techniques used. The aim of this study was to identify all of the trace VOCs at seven U.K. waste disposal sites, four of which had landfill gas utilization schemes, and then to assess their potential corrosive and toxic effects. Estimates of the annual flux of trace VOCs are also calculated for the four landfill sites with gas utilization schemes.
Experimental Section Study Sites. The sites studied are representative of the majority of modern municipal waste disposal facilities in the U.K. and similar in both methods of operation and construction to those in other industrialized countries. Each of the sites accepts both domestic and trade waste. The latter consists of a combination of building waste and waste with a similar organic content to that of domestic waste. The exact proportions of each were unknown. None of the sites were licensed to accept toxic or industrial waste. Geographically, sites A-E are located in the Midlands, and sites F and G are located in south-east England. A brief description of each landfill site is given in Table 1. Sampling Procedure. The first step in the analytical procedure is preconcentration of the VOCs. The sample tubes developed for this work were prepared using a 1:1:1 ratio by
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TABLE 1. Summary of Seven Landfill Sites Studied site description
waste types
gas end use
in operation since 1984; liner system mainly consists of compacted colliery shale valley infill; in operation since 1920; liner system mainly consists of compacted colliery shale former brick pit; in operation since 1990; liner system consists of in-situ compacted marls and clays located at clay extraction site; in operation since 1990; liner system consists of in-situ compacted marl and shales quarry infill; in operation since 1965; liner system consists of in-situ compacted clay
trade waste (33%) and domestic waste (67%), deposited to a height of 15 m trade waste (54%) and domestic waste (46%) deposited to depths of 30 m
gas extraction wells installed in 1992; ∼300 m3 h-1 gas extracted to fuel an engine for electricity generation ∼1000 m3 h-1 landfill gas extracted to fuel three engines for electricity generation
trade waste (43%) and domestic waste (57%) deposited to depths of 20 m
gas extraction system and flare stack installed in 1992
trade waste (49%) and domestic waste (51%) deposited to depths of 20 m
gas extraction system and flare stack installed in 1993
trade waste (52%) and domestic waste (48%) deposited to depths of 30 m
site F
quarry infill; in operation since 1986
accepts both domestic (80%) and trade waste (20%)
site G
former clay extraction site; in operation since 1987
accepts both domestic (90%) and trade waste (10%)
at the present time there are no gas extraction facilities; large gas emissions have occured during the last two decades ∼480 m3 h-1 landfill gas extracted to fuel three engines for electricity generation ∼350 m3 h-1 landfill gas extracted to fuel two engines for electricity generation
site A
site B
site C
site D
site E
volume of the following adsorbents packed in series: Tenax TA (80/100mesh), Chromosorb 102 (80/100mesh), and Carbosieve SIII (60/80mesh). The adsorbents were arranged in order of increasing adsorptive properties, which enabled a single sample tube to be used to trap VOCs with a wide range of boiling points and volatility. A blank run was carried out on all the tubes prior to sampling or calibration, and sample tracking was achieved by using adsorption tubes with unique laser etched numbers. A 500-mL sample of landfill gas was drawn through the sampling tubes at 50 mL min-1 using a Gillian personal air sampler fitted with a constant low-flow module. Each sampling tube was then immediately sealed with Swagelok end-caps fitted with PTFE ferrules and stored at 4 °C until analyzed, usually within 24 h. A second analysis of all the sample tubes was carried out to ensure that complete desorption of all the retained components had been achieved. The gas samples were obtained from monitoring points on the existing gas extraction systems at each site. In the case of site E, the gas samples were taken from a trial gas well that had been installed by the landfill operators to assess gas production rates. Sites B-G were visited once, with each sample being taken in triplicate. In order to ascertain the significance of a single analysis data set, site A was monitored at monthly intervals from August 1994 (day 0) to November 1995 (day 450). Initially, three samples were taken in succession at each visit to establish the precision of the method and thereafter at regular intervals as a quality assurance check. Instrumental Analysis. Tube desorption and VOCs analysis was carried out using a Perkin Elmer automated thermal desorption system (ATD 50) interfaced to a Hewlett Packard 5890 gas chromatograph fitted with a HP5970 mass selective detector. The GC capillary column used for all the analyses was a 60-m Restek RTX-1 (100% dimethyl polysiloxane) column, 1.5 µm film thickness, supplied by Thames Chromatography. The above system was controlled with a Hewlett Packard 9000 workstation running HPMS-Chemstation software under UNIX. The analytical conditions employed for the ATD 50 were as follows: primary tube
desorption was performed at 250 °C for 15 min, the transfer line was held at 150 °C, the secondary trap adsorbent was Tenax TA, and the maximum and minimum operating temperatures were 300 °C and -30 °C, respectively. The overall split ratio on the ATD 50 was 200:1. Following desorption, the column temperature was maintained at 35 °C for 5 min and then programmed to ramp at 5 °C min-1 to 180 °C, which was then maintained for 15 min. The detector temperature and mass range were 250 °C and 20-250 amu, respectively. Fragmentation pattern identification was achieved by software comparison with the Wiley/NBS database of mass spectra and external reference compounds. Qualitative and Quantitative Analysis. Quantification of the VOCs was achieved using a 12-component external standard. Halocarbons were determined with reference to dichloromethane as were alcohols to ethanol; substituted aromatics to p-xylene; cyclic compounds to cyclohexane; pinenes to 1-limonene; and the alkanes to hexane, heptane, nonane, decane, and dodecane. A stock mixture containing 0.5 mL of each calibrant was prepared on a daily basis using high-purity HPLC grade chemicals obtained from Fisons. The calibrated tubes were prepared as follows: the tubes were clamped in an upright position with a plug of silanized glass wool inserted into the top. Known volumes (0.05, 0.10, 0.20, 0.35, and 0.5 µL) of the standard mixture were injected directly onto the silanized glass wool while drawing a 50 mL min-1 flow of clean air through the tube for a period of 10 min. Using this method and the analytical conditions described previously, we were able to achieve detection limits of between 0.10 (dichloromethane) and 0.02 mg m-3 (decane). The precision (% RSD, n ) 6) of the analytical method was less than 10%. Relative response factors were used for most of the major components that were not directly quantified. The retention times of all the analytes were checked to ensure they were within the expected time window, and the relevant spectral data were cross-referenced to the library data.
Results and Discussion An example of a chromatogram obtained from the analysis of a landfill gas sample is given in Figure 1 together with a list of the major trace components. Over 140 compounds
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FIGURE 1. Annotated chromatogram of VOCs in a landfill gas sample. 1, air; 2, chlorodifluoromethane; 3, propane; 4, 1-chloro-1,1-difluoroethane; 5, butane; 6, dichlorofluoromethane; 7, 2-methylbutane; 8, pentane; 9, 2-butanone; 10, hexane; 11, methylcyclopentane; 12, cyclohexane; 13, 2-methylhexane; 14, 3-methylhexane; 15, 4-methyl-1-hexene; 16, heptane; 17, methylcyclohexane; 18, toluene; 19, 2-methylheptane; 20, 3-methylheptane; 21, octane; 22, 2,6-dimethylheptane; 23, ethylcyclohexane; 24, 1,1,3-trimethylcyclohexane; 25, ethylbenzene; 26, 2,3-dimethylheptane; 27, 1,4-dimethylbenzene; 28, 3-methyloctane; 29, 1,2-dimethylbenzene; 30, 1-ethyl-4-methylcyclohexane; 31, nonane; 32, 1-ethyl-2-methylcyclohexane; 33, unidentified C9 alkane; 34, 3,5-dimethyloctane; 35, unidentified C10 alkane; 36, 2,6-dimethyloctane; 37, r-pinene; 38, 1-ethyl-2-methylbenzene; 39, 4-methylnonane; 40, 2-methylnonane; 41, 3-methylnonane; 42, β-pinene; 43, trimethylbenzene; 44, decane; 45, γ-terpinene; 46, 1-methyl-4-(methylethyl)benzene; 47, 2,6-dimethylnonane; 48, 1-limonene; 49, 2-methylpropylcyclohexane; 50, 1-methyl-2-propylbenzene; 51, undecane. were identified, of which over 90 were present in each of the samples taken. In order to ascertain the significance of a single analysis data set, site A was monitored at monthly intervals from August 1994 to November 1995. The results from this study are summarized in Figure 2. The observed fluctuations are thought to be due to a number of factors such as changes in atmospheric pressure and both ambient temperature and the temperature of the landfill itself. Given that this work was primarily concerned with the qualitative and quantitative analysis of the trace VOCs, a thorough examination of the effects of the aforementioned parameters was not undertaken. The extreme changes in both bulk and trace gas components at approximately 150, 225, and 300 days are thought to be due to changes in gas extraction rates at the gas well studied. Allowing for the latter point, the concentrations of VOCs in the landfill gas appear to be reasonably consistent over the time scale studied and directly related to the levels of methane. A summary of the total concentrations of VOCs detected at each of the seven sites is given in Table 2. The values quoted for sites B-G refer to the range of concentrations found in triplicate samples, obtained at regular intervals over a period of 1 h. The ranges quoted for site A summarize the variations in concentrations observed over a 15-month period. Although the majority of the components identified were present in each of the gases analyzed, the relative proportions of certain classes of compounds varied considerably between sites. The latter point probably reflects differences in waste composition and the rates and mechanisms by which the waste is decomposed. For example, the relative proportions of acid esters and alcohols and ketones (which included
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ethanol, propanol, butanol, pentanol, hexanol, propanone, and pentanone) were 2-3 orders of magnitude higher at sites F and G than at sites A-E. Knox (15), who examined the variation in the composition of landfill gas with the age of waste material, observed that levels of alcohols were greatest in fresh refuse. This observation was supported by Young and Heasman (13), who also noted that elevated levels of esters and alcohols were characteristic of sites with high gas production rates. Dent et al. (6) reported that the presence of alcohols arises from the fermentation of putrescible materials such as fruit and vegetables. The latter point may be of particular significance to sites F and G as a large amount of fruit is grown in southeast England. It is likely that the majority of the fruit that is not suitable for sale is disposed of in landfills. Given the lack of detailed records on waste inputs and the fact that the landfill gas sampled was extracted from areas of differing waste ages, it is impossible to identify the exact reasons for the observed levels without further detailed investigation. The majority of the alkanes, aromatic compounds, and cyclic compounds are produced during the waste degradation processes, and as a consequence, their levels in landfill gas are dependent upon both the waste composition and the stage reached in the decomposition process. Knox (15) noted that the predominance of trace levels of alkanes and aromatic compounds in landfill gas is usually associated with older refuse, together with a corresponding decrease in the levels of halogenated and oxygenated compounds such as alcohols. Interestingly, elevated levels of all the aforementioned groups of compounds were observed at site F. This suggests that the waste is particularly heterogeneous at this site, with the landfill
FIGURE 2. Changes in the levels of trace volatile organic components in landfill gas at site A over a 15-month period.
TABLE 2. Summary of Total Concentrations of VOCs Detected at Seven Sitesa landfill site compounds
A
B
C
D
E
F
G
alkanes C2-C5 alkanes C6-C12 alkanes alkenes alcohols and ketones chlorinated compds cyclic compds aromatic compds terpenes acid esters
302-503 140-237 150-345 8-31 6-51 327-739 80-208 94-330 74-152
