Environ. Sci. Technol. 2006, 40, 5984-5991
Release of Trace Organic Compounds during the Decomposition of Municipal Solid Waste Components BRYAN F. STALEY,† FANGXIANG XU,† S T E V E N J . C O W I E , †,‡ M O R T O N A . B A R L A Z , * ,† A N D GARY R. HATER§ Department of Civil, Construction, and Environmental Engineering, Box 7908, North Carolina State University, Raleigh, North Carolina 27695-7908, and Joyce Engineering, Inc., Suite 203 Henderson Building, 2301 W. Meadowview Road, Greensboro, North Carolina 27407, and Bioreactor & BioSites Technology, Waste Management Inc., 2956 Montana Avenue, Cincinnati, Ohio 45211
Landfill gas contains numerous speciated organic compounds (SOCs) including alkanes, aromatics, chlorinated aliphatic hydrocarbons, alcohols, ketones, terpenes, chlorofluoro compounds, and siloxanes. The source, rate and extent of release of these compounds are poorly understood. The objective of this study was to characterize the release of SOCs and the regulated parameter, nonmethane organic compounds (NMOCs) during the decomposition of residential refuse and its major biodegradable components [paper (P), yard waste (YW), food waste (FW)]. Work was conducted under anaerobic conditions in 8-L reactors operated to maximize decomposition. Refuse and YW were also tested under aerobic conditions. NMOC release during anaerobic decomposition of refuse, P, YW, and FW was 0.151, 0.016, 0.038, and 0.221 mg-C dry g-1, respectively, while release during aerobic decomposition of refuse and YW was 0.282 and 0.236 mg-C dry g-1, respectively. The highest NMOC release was measured under abiotic conditions (3.01 mg-C dry g-1), suggesting the importance of gas stripping. NMOC release was faster than CH4 production in all treatments. Terpenes and ketones accounted for 32-96% of SOC release in each treatment, while volatile fatty acids were not a significant contributor. Release in aerobic systems points to the potential importance of composting plants as an emissions source.
Introduction Approximately 55% of the 236 million tons of municipal solid waste (MSW) generated in 2003 were disposed of in U.S. landfills (1). While CH4 and CO2 are the major endproducts of waste decomposition in landfills (2, 3), landfill gas (LFG) also contains speciated organic compounds (SOCs) at pptv to ppmv levels including alkanes, aromatics, chlorinated aliphatic hydrocarbons, alcohols, ketones, terpenes, chlo* Corresponding author phone: 919-515-7676; fax: 919-515-7908; e-mail:
[email protected]. † North Carolina State University. ‡ Joyce Engineering, Inc. § Waste Management Inc. 5984
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rofluoro compounds, and siloxanes (4-6). However, the source of SOCs, and their rate and extent of production and/ or release, are poorly understood. Potential sources of SOCs in LFG include household hazardous waste (HHW) (e.g., toluene in furniture treatments), plastic packaging, anaerobic decomposition intermediates (e.g., carboxylic acids, alcohols), and chemicals present in biogenic compounds (e.g., terpenes in yard waste). Mechanisms controlling the presence of SOCs in LFG will vary based on the compound source. The release of a volatile organic will be controlled by volatilization and gas sparging, while release of decomposition intermediates will be governed by their accumulation in a landfill which is a function of the health of the microbial ecosystem. The U.S. Environmental Protection Agency’s (EPA) LANDGEM model is often used to estimate CH4 production from landfills, and the governing equation is given in eq 1:
G ) WLoke-kt
(1)
where G ) CH4 production rate (m3 yr-1), W ) annual waste acceptance rate (ton), Lo ) ultimate yield (m3 CH4 ton-1) and k ) decay rate (yr-1) (7). LFG production is calculated by assuming a CH4 concentration, and SOC release is estimated by multiplying the LFG production rate by the concentration of an individual compound. A common source for SOC concentration data is EPA’s AP-42 database (8). Equation 1 indicates that SOC release will increase with Lo, which is inconsistent with the factors that influence release. The objective of this research was to characterize SOC release during decomposition of the major biodegradable components of MSW. An ultimate yield for non-methane organic compounds (NMOCs) and 158 SOCs was measured for a paper mixture (P), food waste (FW), yard waste (YW), and residential MSW. NMOCs are a regulated parameter used to characterize LFG in which all non-CH4 organics are aggregated and reported as carbon (9).
Materials and Methods Experimental Design. NMOC and SOC release from residential MSW and individual refuse components were measured in 8-L reactors operated to maximize waste decomposition. The study was divided into two sets of experiments (phases I and II). The major biodegradable components of MSW, including FW, YW, and P, were tested under anaerobic conditions in phase I. Residential MSW was tested under both anaerobic conditions (MSW-an) and under conditions to simulate a landfill operating strategy in which waste is initially aerated to facilitate biodegradation of rapidly fermentable substrates and then allowed to turn anaerobic (MSW-aa). Phase II experiments were conducted to confirm certain phase I results, to evaluate SOC release from aerobically decomposed YW as would occur during composting (YW-ae), and to characterize SOC release from MSW under abiotic conditions (MSW-ab) in which gas sparging would be the only release mechanism. Abiotic conditions were achieved by sodium azide addition (65 g/L). Treatments are summarized in Table 1. All treatments except MSW-aa and MSW-ab were seeded with methanogenic leachate to initiate CH4 production. Treatments were tested in triplicate, incubated at 37 °C, and operated with leachate neutralization and recirculation to accelerate decomposition. In phase I, a control containing 500 mL of the leachate seed was monitored to evaluate background NMOC and SOC release. Background was so low that a leachate control was not run with phase II. Biologically 10.1021/es060786m CCC: $33.50
2006 American Chemical Society Published on Web 08/22/2006
TABLE 1. Treatments to Measure Release from Waste Components treatment residential MSW
mass condition abbreviation phase reactorsa (dry g)b anaerobic MSW-an aerobic/ MSW-aa anaerobic abiotic
MSW-ab
I
1-2
823.7
II I
3-5 6-8
853.5 740.2
II II
9-11 796.9 12-14 742.5
yard wastec
anaerobic YW-an aerobic YW-ae
I II
1-2 3-5
444.8 497.2
food waste
anaerobic FW
I II
1-3 4-5
421.4 251.2
mixed paperd anaerobic P
I
1-3
953.3
Leachate anaerobic C control used to seed reactors
I
1-3
500 (mL)
a When two reactors are reported, it denotes that data for one reactor was discarded due to suspected leakage. b Average mass used to fill reactors in a treatment set. c Yard waste includes 25% leaves, 25% branches, and 50% grass on a wet weight basis (10). d Mixed paper includes 20% newsprint, 42% corrugated containers, 15% office paper, 7% magazines, and 16% third class mail (10).
active reactors were monitored until degradation was virtually complete. Criteria for termination of monitoring are given in the Supporting Information (SI). Materials. Separate batches of FW for phases I and II were collected from a residential kitchen over several weeks and stored frozen prior to use. Fresh branches and leaves were obtained from the North Carolina State University (NCSU) compost facility 1-d prior to use. Freshly cut grass was obtained from an athletic field 1-d prior to use. Paper components were obtained from the NCSU facilities department. The mixed paper composition was developed by normalizing concentrations of the major paper components in MSW to 100% based on waste characterization data (Table 1) (10). Separate batches of residential MSW were obtained from a transfer station 1-3-d prior to use. The paper, branches, and MSW were shredded in a slow speed, high torque shredder to a particle size of ∼2 × 4 cm. All waste components were stored at 4 °C prior to use except P (25 °C) and FW (-20 °C). The leachate seed was obtained from a 208-L drum containing well-decomposed methanogenic refuse incubated at 37 °C. Reactor Construction. Eight-L reactors were constructed from 20.3 cm diameter aluminum cylinders and fitted to circular aluminum top and bottom plates as described (Figure S1 of the SI) (11). All surfaces exposed to wastes were coated with kynar (Electro Chemical, Emmaus, PA) to minimize sorption of SOCs to the reactor. For aerobically operated reactors, an additional 3.2 cm hole was added to the bottom plate for air supply. Reactors containing FW had an additional 3.2 cm hole in the top to facilitate waste additions. Gas was collected in 20-L bags made from 2 mil Tedlar inner linings and aluminized outer linings (Pollution Measurement Corp., Oak Park, IL). Reactor Filling. Fiberglass and 2 cm of washed gravel were placed on the bottom of each reactor to minimize outlet clogging. Wastes were compacted in 12.7 cm layers to ∼5 cm from the top. Fiberglass and gravel were placed over the refuse to promote distribution of recycled leachate. Siliconebased high vacuum grease (Dow Corning, Midland, MI) was applied to Teflon-coated viton o-rings in the top and bottom plates to ensure a gastight seal. Prior to gas bag attachment,
each reactor was leak tested (12). Methanogenic leachate, or deionized (DI) water for aerobic and abiotic treatments, was added to the leachate collection vessels and recycled to initiate the experiment. Leachate or water (1-2-L) was added to each reactor to ensure at least 500 mL of leachate was generated for recirculation. The FW reactors were operated in fed-batch mode to prevent an inhibitory acid accumulation and pH decrease. An initial seed averaging 100 dry g of FW was allowed to decompose until gas production was not measurable, after which each reactor received 5-7 approximately weekly additions of ∼40 dry g of FW. Reactor Operation and Monitoring. Leachate was recirculated every 1-2 d. Leachate was neutralized to pH 6.87.8 with 1 or 5 M NaOH for the first 10-30 d of reactor operation, after which the pH was stable. Leachate was sampled every 7-14 d for chemical oxygen demand (COD) analysis. To prevent refuse from drying, DI water (0.5-L) was added to the aerobic and abiotic reactors whenever leachate volumes decreased below 0.2-L. The internal reactor temperature of the aerobic reactors was monitored to ensure that they remained below 40 °C so to not inhibit microbial activity. For the anaerobic treatments, gas bags were replaced as they filled. Gas volume was measured by evacuation of the gas bags into a cylinder of known volume (11). For treatments that required air, house air was filtered through two Koby Junior King filters (Scientific Instrument Services, Ringoes, NJ) in series to remove SOCs followed by two CO2 traps (5 N KOH). Air flow was maintained between 30 and 60 mL min-1 per reactor. This flow range was identified in preliminary work as sufficient to ensure the presence of at least 1-2% O2 in the exit gas. Inlet air flow to reactors was measured with a digital bubble flow meter five times daily. The continuous flow of air through the aerobic and abiotic reactors precluded collection of all gas. Thus, the outlet flow rate was measured five times a day using a bubble meter to calculate daily gas volume. Similarly, grab samples were collected five times a day for 10 min to obtain a composite sample. Gas bags were divided into multiple sub-samples for analysis. First, a few mL were used for analysis of CH4, CO2, O2, and N2. For NMOC analysis, 2-5 L were transferred into a 10-L gas bag using a VAC-U-Chamber (SKC, Eighty Four, PA). For volatile fatty acid (VFA) analysis, 1-L was pumped (100 mL min-1) through an orbo tube 70 (Supelco, Bellefonte, PA) with two sections of 5% sodium carbonate on Chromosorb P (30/60). VFA samples were frozen prior to analysis. The remaining gas sample was used for SOC analysis. Analytical Methods. Leachate COD was measured using a Hach Kit (Hach Co., Loveland, CO). CH4, CO2, O2, and N2 were analyzed by using a GOW-MAC 580 gas chromatograph (GC) equipped with a thermal conductivity detector (TCD) (13). Total NMOCs were analyzed by Triangle Environmental Services (RTP, NC) by EPA method 25C (14). VFAs were analyzed by extraction of the orbo tubes with water, followed by acidification of samples, adsorption from the headspace by solid-phase microextraction (SPME), and analysis by GC/MS. Additional detail is presented in the SI. In preliminary work, a known mass of each acid was analyzed after addition to a gas bag. Average recoveries for acetic, propionic, isobutyric, n-butyric, 2-methylbutyric, isovaleric, valeric, isocaproic, caproic, and heptanoic acids ranged from 83 to 107.5% and are given in Table S1. SOCs were analyzed by GC-MS with an HP G1800A GCD equipped with a Tekmar Autocan sampler (14-ACAN-000, Spokane, WA). Prior to analysis, gas bags were maintained at 37 °C in a wooden box equipped with a space heater. Compounds were separated with a Petrocol DH column (Supelco) using He as the carrier gas (0.71 mL min-1). VOL. 40, NO. 19, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. Average Gas and NMOC Yield for Each Treatment
TABLE 3. NMOC and CH4 Yields for Anaerobic Treatments at 50% of Operating Time
gas yield treatmenta
phase
CH4 (mL dry g-1)b
MSW-an MSW-an MSW-aa MSW-aa MSW-ab YW-an YW-ae FW FW P C
I II I II II I II I II I I
97.9 (0.9) 127.8 (23.3) 2.0 (3.4) 0 (0.0) 0 (0.0) 156.0 (39.6) 0 (0.0) 152.9 (7.3) 207.0 (8.3) 113.5 (8.5) 0 (0.0)c
a
CO2 (mL dry g-1)b
NMOC yieldb (mg C dry g-1)
86.0 (3.5) 112.3 (19.2) 290.9 (32.9) 446.5 (25.9) 3.4 (3.5) 141.6 (25.7) 560.4 (66.4) 120.0 (15.9) 173.6 (1.1) 129.3 (7.8) 13.7 (4.0)c
0.113 (0.002) 0.177 (0.029) 0.233 (0.04) 0.330 (0.181) 3.01 (1.16) 0.038 (0.007) 0.236 (0.031) 0.347 (0.082) 0.031 (0.003) 0.016 (0.004) 0.044 (0.032)c
b
See Table 1 for nomenclature. The standard deviation is given in parentheses. c Gas yield for control reactors is mL L-1 of leachate. NMOC yield for control reactors is mg C L-1 leachate.
treatment
% of CH4 cumulative yielda
% of NMOC cumulative yielda
MSW-an1 MSW-an2 MSW-an3 MSW-an4 MSW-an5 YW-an1 YW-an2 P1 P2 P3
86.1 73.1 84.7 61.0 95.3 84.9 78.8 68.1 76.9 79.8
96.8 92.1 98.4 89.6 98.9 97.0 96.3 97.6 95.9 93.5
a Data represent the percentage of the cumulative yield that had been measured after 50% of the operating time of each treatment.
Standard curves were prepared for 158 SOCs using separate standard solutions for (1) terpenes, (2) cyclic hydrocarbons, (3) polar analytes, (4) ketones, and (5) a mixture consisting primarily of alkanes, alkenes, and cyclic hydrocarbons. Additional detail is presented in the SI. Cellulose analyses were performed as described in ref 15. Pectin analyses of FW were performed by the Complex Carbohydrate Research Center (Athens, GA) and reported as galacturonic acid (16-18). Data Analysis. CH4 production rates and yields are reported at STP. For FW, gas production at each time point was normalized to the cumulative FW mass added to a reactor at a time point. Total NMOC and SOC masses were corrected for background release from the leachate seed by eq 2.
NMOC or SOC yield ) g organic compound
((
)
g organic compound seed mL leachate seed × added to reactor mL leachate seed dry g waste
)
(2) Statistical comparisons were made in Microsoft Excel using a Student’s t-test with unequal variances.
Results and Discussion Gas Yields. Average CH4 and CO2 yields for each treatment are presented in Table 2, and CH4 production rate curves are presented in Figure S2. Some measurable CO2 was generated during the first 7-10 d for the MSW-ab reactors after which no measurable CO2 was detected. In addition, there was no cellulose loss in the MSW-ab reactors (-0.1 ( 16.3%). Comparable cellulose losses for phase II MSW-an and MSWaa treatments were 85.3% ((4.7%) and 88.2% ((1.9%), respectively. In the MSW-aa treatment, once the air supply was terminated, CH4 was detected in all reactors. As evidenced by the CO2 and CH4 yields, essentially all biodegradation occurred during the aerobic phase, which was 48 and 68 d in phases I and II, respectively (Table 2). In an earlier study, CH4 yields for residential MSW, grass, leaves, branches, and FW were 92.0, 136.0, 30.6, 62.6, and 300.7 mL dry g-1, respectively (2). The lower FW yields reported here provide some measure of this substrate’s variability. Using the individual yields for grass, leaves, and branches, a YW yield of 69.8 mL CH4 dry g-1 would be predicted, though it is not clear whether the higher yield measured here can be attributed to synergistic effects of a mixture, substrate variability, or both. 5986
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FIGURE 1. CH4 production and NMOC release rates for phase I MSW-an reactors. NMOC Yields. Average NMOC yields for each treatment are presented in Table 2 and individual reactor yields are presented in Table S2. As NMOC release in the abiotic treatment was greater than that in any other treatment, the reported yields refer to compound release. It is not possible to differentiate between production and release from the available data. The relatively low yield for paper may be explained by the narrow range of chemicals in paper compared to YW, FW, and MSW. Paper is predominantly comprised of cellulose, hemicellulose, and lignin with some nonvolatile additives such as sizing agents and ink. The phase I NMOC yield was higher in MSW-aa compared to MSW-an (p < 0.05), a difference that was not significant in phase II due to an NMOC yield that was substantially lower in 1 of 3 MSW-aa reactors (MSW-aa10 in Table S2). The average NMOC yield for YW-an was significantly less than YW-ae (p < 0.05), although different batches of yard waste were used for phases I and II. While phase II experiments were conducted in part to confirm the phase I results, the phase II FW NMOC yield was about 10% of the phase I yield, although the phase II CH4 yield was ∼35% higher (Table 2). Potential explanations are presented in the Terpenes section. To evaluate relative rates of NMOC and CH4 release in the anaerobic treatments, NMOC and CH4 yields after 50% of the operating time for each treatment are compared in Table 3. The corresponding rates curves are compared in Figures 1 (for MSW-an) and S2. The FW reactors were not included in this analysis because they were operated in fed-batch mode. In general, NMOC release was more rapid than CH4 production. NMOC release was roughly two times higher in MSW-aa versus MSW-an treatments. Aerobic biodegradation of
FIGURE 2. Percent contribution of speciated organic compounds. The other category consists of diethyl ether and ethyl acetate. The sum of all compounds is given at the top of each column in ng dry g-1. selected SOCs in microcosms containing landfill cover soil, as well as in landfill covers has been reported (19, 20). Since aerobic biodegradation is typically more rapid than anaerobic biodegradation, lower NMOC release under aerobic conditions might be expected. The higher NMOC levels measured in the aerobic treatments suggest that sparging of NMOCs dominated over biodegradation. Despite this, NMOC release is substantially lower in biologically active treatments (aa and an) compared to abiotic conditions, implying that biodegradation plays an important role in reducing NMOC release. The elevated NMOC yield in MSW-ab also supports the significance of gas sparging, the primary release mechanism for this treatment. Many SOCs reported in the following section are susceptible to anaerobic biodegradation. Speciated Organic Compounds. The contribution for eight categories of organics to the total SOC yield is presented in Figure 2. Compounds yields for individual reactors are presented in Table S3. As yields for three SOC categories were relatively low (alkenes, chlorinated, and other), they are discussed in the SI. There are several factors to consider in interpreting SOC yields. In general, release in the abiotic treatment was higher than in the live treatments. Thus, the data do not provide evidence for biological production. In addition, all substrates were exposed to aerobic conditions prior to testing. Thus, more oxidized compounds may have been produced prior to the onset of anaerobic conditions. In general, terpenes and/or ketones were dominant for all treatments except P and FW-II, both of which also contained sizable fractions of aromatics (Figure 2). Contributions from compound categories were similar between phase I and II MSW-an treatments, while MSW-aa and FW treatments exhibited considerable variability between phases. Seasonal shifts in waste composition could explain FW variability, but this explanation does not hold for MSW-aa because the MSWan did not exhibit similar variability. From a seasonal perspective, phase I MSW-aa and FW both had high terpene contributions relative to phase II, which also suggests a seasonal shift. Profiles for phase II MSW-aa and MSW-ab are similar, though the total yield is much greater for MSW-ab. The dominant compounds for each treatment account for 74.8-99.8% of all compounds quantified (Table 4). Of the 158 compounds, 46 were always below the quantitation limits (Table S3). Among the MSW treatments, individual SOC yields were generally in the order MSW-ab > MSW-aa > MSW-an although with a few exceptions, mean compound yields were not significantly different (p > 0.05) when compared by individual t-tests across treatments (Table 4). However, a paired t-test, in which trends among all SOCs are considered concurrently, confirmed this relative order (p < 0.05). One potential explanation for the higher yield in MSW-ab
compared to other treatments is the lack of microbial degradation. A paired t-test also showed that YW-ae SOC yields were greater than YW-an (p < 0.05). The higher release of compounds in aerobic treatments relative to anaerobic treatments suggests the importance of gas sparging. Gas flow through the aerobic systems was much greater (Table S2). While more complete biodegradation should occur in aerobic systems, biodegradation, and gas stripping would occur concurrently, and the effect of stripping appears dominant. SOC release from plastic packaging likely contributed to their presence in MSW and FW treatments. The presence of ketones, alkanes, alkenes, aromatics, and alcohols in plastic packaging has been reported (21-23). Common ketones in polyethylene include 2-butanone, 2-pentanone, and 2-hexanone, while n-butanol and n-propanol were dominant alcohols (21). Dominant aromatics and alkanes include styrene, decane, dodecane, hexane, and BTEX, depending on the polymer type (23). Alkanes. Pentane was the dominant alkane in all MSW treatments and its presence in gas from anaerobically decomposing refuse has been reported (24). Pentane biodegradation has been reported under aerobic conditions (25). Although not specific to pentane, methanogenic degradation has been reported for C6 and longer chain alkanes (26). In FW, decane and pentane accounted for 67 and 20% of the alkanes, respectively. Decane has been implicated as a volatile odor compound in foods (27), possibly due to its release from plastic packaging (23). Pentane is a thermal degradation product of partially hydrogenated soybean oil used in cooking (28). Aromatics. Members of the BTEX (benzene, toluene, ethylbenzene, and xylenes) family plus styrene were the dominant aromatics. These compounds have been documented in LFG under aerobic (29, 30) and anaerobic conditions (24, 31, 32). Toluene was most prevalent in phase II MSW, while ethylbenzene or styrene dominated other treatments. Both aerobic and anaerobic toluene biodegradation has been documented which is consistent with lower yields in MSW-aa and MSW-an relative to MSW-ab. Surprisingly, toluene was also dominant in YW treatments. Although it has been shown that toluene sequestration can occur in vegetative matter, the degree of partitioning into plants is limited by its aqueous solubility (33). No benzene was detected in YW treatments which eliminates deposition from vehicle emissions and petroleum (e.g., motor oil) as potential sources of toluene since benzene would also be present. Further investigation revealed that turf marking paint containing 4-20% toluene (Lesco, Strongville, OH) was being used at the time of grass collection. VOL. 40, NO. 19, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 4. Dominant Speciated Organic Compounds in Each Treatmenta,b (ng Compound Dry g-1) phase I compound alkanes pentane cyclohexane n-undecane dodecane 2,4-dimethylhexane 2,3-dimethylhexane octane decane totalc alkenes 1-octene 2-methyl-1-butene 2-methyl-1,3-butadiene totalc aromatics benzene toluene styrene ethylbenzene m & p xylene 1-methyl-3-isopropylbenzene 1-methyl-4-isopropylbenzene totalc alcohols 2-propanol 1-propanol 1-butanol totalc ketones acetone 2-butanone 2-pentanone Totalc terpenes limonene alpha-pinene beta-pinene totalc chlorinateds none totalc other ethyl acetate totalc total dominant compounds percent of SOC yielde
phase II
MSW-an MSW- aa MSW-and MSW-aad
MSW- abd
YW-an
5317 0 455 0 140 83 225 6 9102
6328 0 176 0 0 0 0 0 7213
6799 2174 872 756 42 43 162 258 12 568
13 835 5609 1713 4003 137 377 512 80 28 683
18 054 7475 2096 4295 6676 8202 1881 904 71 741
283 0 39 0 10 3 394 0 1080
0 0 285 0 352 645 72 82 1525
285 0 15 13 0 0 85 3093 3692
619 108 41 0 0 0 112 0 893
100 0 55 6 60 54 13 0 458
79 12 29 174
0 0 51 52
37 131 100 481
1464 9 580 2088
9648 174 0 12 919
14 42 29 232
1867 0 0 2112
0 498 234 1030
0 65 70 165
40 34 5 138
3 332 79 652 310 613 1047 3250
0 0 33 13 55 822 1217 2487
19 2854 117 583 154 312 675 4858
1 13 868 1875 10 139 5593 4350 10 441 46 808
38 112 679 4004 11 551 12 589 25 978 40 202 210 597
0 815 19 123 336 248 555 2146
0 83 543 1079 9961 6492 1630 59 554 162 337
0 300 0 12 56 12 245 17 788 33 863
2 1727 20 189 276 510 7119 9946
2 407 9 22 6 916 1024 2406
1033 0 1781 2818
540 1084 7434 9057
2555 2469 1516 7720
6860 17 626 18 970 43 945
29 452 11 554 45 060 93 719
2040 0 1528 3573
266 323 13 311 13 900
83 438 1177 1698
756 1509 2674 4940
10 49 944 1003
1117 14 698 1971 18 303
0 26 400 703 27 103
2866 10 721 1568 15 710
39 068 89 094 34 738 163 311
110 628 337 397 36 506 494 025
2058 11 205 3647 17 851
68 540 294 051 26 507 389 098
0 5185 351 5538
5741 0 1169 6910
49 862 122 1052
6000 1408 1003 8611
114 752 3900 1017 120 014
18 489 485 320x 19 445x,y
64 208 803 788y 65 798x
164 795 1790 2916x,y 170 199y
4286 8557 2416 16 678
120 281 934 619 121 857
166 524 14 111 11 370 196 361
1805 1403 323 3677
2491 60 20 2585
217
265
4
76
227
99
0
40
1100
46
666 666 37 669 87.3
12 789 12 789 173 416 96.9
2006 2010 58 621 93.4
11 856 11 856 357 794 98.7
19 637 19 737 1 024 437 95.5
35 35 31 194 74.8
0 0 689 484 99.8
356 356 221 009 91.1
309 309 25 291 90.5
52 52 7353 95.0
YW-ae
FW-I
FW-II
paper
a Compound masses are given in all treatments (i.e., MSW, YW, FW, P) for completeness even if compounds were not dominant across treatments. Dominant was defined as greater than 10% of the total mass of a class of compounds in a treatment. b Numbers in bold represent statistically significant differences (p < 0.05) based on comparisons of the same waste and the same phase. c Total of all dominant and nondominant speciated compounds for each compound class. d The different superscripts in the phase II MSW treatments indicate significant differences (p < 0.05). e Percent of SOC yield attributable to dominant compounds.
The dominant aromatic in FW and P treatments was 1-methyl 4-isopropylbenzene. Its presence in FW could be due to contact with food containers (34); however, no information has been published on this compound in paper. Also of interest is the presence of several aromatics in FW. In a review of 234 foods, higher aromatic concentrations were found in higher fat content foods (35). The four foods containing the highest concentrations of SOCs; sandwich cookies, margarine, butter, and cake doughnuts, were reported to contain styrene and toluene. Styrene has been linked to migration from polymer packaging materials, especially for higher fat foods due to styrene’s high lipid solubility (23, 36). Alcohols. Alcohol contribution to SOCs was 5-12% for MSW treatments, 2-9% for YW, 0.7-18% for FW and 13% for P. 2-propanol, 1-propanol, and 1-butanol were dominant across treatments, though there was not a consistent trend 5988
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between the aerobic and anaerobic treatments. Alcohol levels in MSW-ab were greater than other MSW treatments which suggests their presence in refuse upon reactor initiation and their degradation in biologically active treatments. Microbial production of alcohols under anaerobic conditions is enhanced by low pH (37) and could have occurred between refuse collection and placement in reactors. The production of 2-propanol, 2-pentanol, and ethanol during anaerobic waste decomposition has been reported (38). Members of the Clostridia genus have been reported to produce alcohols and ketones at low pH and substrate concentration as well as at high H2 partial pressures (37, 39), and the presence of Clostridia in both municipal landfill leachate (40) and solid waste (41) has been reported. Evidence of Clostridia in the reactor seed material used for this experiment was found via terminal restriction fragment length polymorphism on DNA amplified from the 16S gene
(unpublished data). Although the prominent source of alcohols and ketones is likely its initial presence in MSW, microbial production of both alcohols and ketones in anaerobic treatments cannot be ruled out. Ketones. Ketone yields in MSW-ab were greater than other MSW treatments which, similar to alcohols, suggests their presence in refuse prior to reactor filling. Ketones can be produced under anaerobic conditions as described for alcohols, and as intermediates of aerobic metabolism (29). Acetone and 2-butanone were the dominant ketones in all treatments and among the largest individual contributors to total SOCs. In MSW-aa-II and YW-ae, 2-butanone accounted for 25% and 43% of all SOCs, respectively. The release of several ketones, including acetone and butanone, during short term (8 h) aerobic drying of plant materials has been reported (42). This is consistent with their identification as major contributors to VOC emissions during composting (29, 43). 2-butanone can also be released from plastic packaging (21). Acetone and 2-butanone were released throughout the experiment, which suggests ongoing volatilization and/or biological production. Terpenes. Terpenes were a major contributor to SOC yields for each treatment, comprising from 13% in Phase II FW to 81% in Phase I FW. Limonene accounted for 81% of terpenes when averaged across all treatments while other terpenes contributed less than 15% to total terpenes in YW and FW and were not detected in any MSW treatment. Terpenes comprised 18% and 40% of SOCs in YW-ae and YW-an, respectively, and their production by plants is well documented (44). For example, terpenes are present in deciduous tree leaf litter emissions (45). In addition, limonene is a significant compound in coniferous yellow pine species in the southeastern U.S (46). Although terpene analysis on individual YW components was not performed, a large fraction of the branches collected for this experiment were visually identified as pine, suggesting that this could be a source of limonene in YW. Terpenes and other volatile compounds are also emitted from fruits, vegetables, and table-ready foods, though dominant compounds vary depending on the product (47-51). For MSW treatments, the data are consistent with previous findings of limonene as a large contributor to SOCs associated with both aerobic and anaerobic decomposition (29, 32, 43). There was a large difference in SOC yield and composition between FW-I and FW-II, and the phase II terpene yield was 1.9% of the phase I yield. Limonene comprised over 68% of total SOCs in FW-I but only 6.5% in FW-II. Large amounts of citrus fruit peels were visually identified in FW-I, whereas citrus waste content in FW-II was lower. As terpenes are the dominant class of SOCs in citrus peels and over 93% of citrus oil, primarily located in the peel, is comprised of limonene, we wondered whether differences in citrus fruit content could explain the difference in terpene yields (52, 53). As citrus peels contain approximately 30% pectin (53, 54), and since pectin is typically higher in citrus relative to other fruits and vegetables (55), the FW pectin content was analyzed. Pectin contents of 18.5 and 10.0 mg dry g-1 as galacturonic acid were determined for FW-I and FW-II, respectively. For pure citrus peels, a ratio of 19.7 mg limonene g pectin-1 was calculated from published concentrations of limonene and pectin (as galacturonic acid) (53). This ratio was 9.0 and 0.18 for FW-I and FW-II, respectively. While circumstantial, the lower limonene/pectin ratio in FW-II supports the idea that the elevated terpene yield in FW-I can be attributed to citrus peels. The differences in SOC yields reflects the inherent variability in FW, which will vary with season and geographic location. This variability in FW will also affect SOC release from MSW containing FW. Volatile Fatty Acids. Selected gas samples were analyzed for VFAs, with an initial emphasis on samples collected when
TABLE 5. Contribution of Volatile Fatty Acids in High COD/Low pH Samplesa treatment
phase
% contribution of VFAs to NMOCsb
MSW-an MSW-an MSW-aa MSW-aa MSW-ab YW-an YW-ae FW FW P
I II I II II I II I II I
0.5 (0.4) 0.4 (0.3) 1.8 (2.0) 7.8 (8.0) 2.9 (2.3) 4.1 (1.7) 39.0 (47.9) 0.4 (0.4) 0.2 (0.2) 4.4 (4.6)
a VFAs represent the sum of ten acids listed in the Materials and Methods. The data represent the average of one or two gas samples per reactor. The standard deviation is given parenthetically. The gas sample(s) analyzed represents the time when the pH was at a minimum and the COD was at a maximum. In some cases these times were the same and only one gas sample was analyzed. b The % of the measured NMOC that could be accounted for by the measured VFAs in the high COD/low pH sample(s) analyzed.
leachate pH was lowest and leachate COD was highest, as these conditions should result in maximum gas-phase VFAs. The average contribution of VFAs to the total NMOC in selected high COD/low pH samples is summarized in Table 5. Based on the results in Table 5, VFA concentrations in all samples were measured for YW-ae and MSW-aa, and rate curves for these treatments are presented in Figure S3. Despite the high VFA contribution in selected samples, the VFA contributions to the cumulative NMOC yield were 0.8 and 3.3% for MSW-aa phases I and II, respectively, and 3.3% for YW-ae. Thus, VFAs were not a significant contributor to the mass of SOCs under any condition evaluated in this study. The results suggest that landfill management practice can have a large effect on NMOC release. The higher release from aerobic systems reported here is consistent with a shortterm field study that showed increased SOC emissions when a landfill was aerated (30). This is noteworthy because, in general, neither yard waste composting facilities nor the few U.S. landfills that are operated under aerobic conditions are subject to gas control requirements. Methane yields are specified by the U.S. EPA in both AP42, which presents a typical value (100 L CH4 wet kg-1), and in the Clean Air Act New Source Performance Standards (NSPS), which presents a value used to calculate whether a landfill is subject to collection and control requirements (170 L CH4 wet kg-1) (8, 9). NMOC yields of 0.48 and 5.48 mg C dry g-1 were calculated from these CH4 yields by assuming that LFG contains 50% CH4 and refuse contains 20% moisture (wet weight basis), and using the default NMOC concentrations under AP-42 and NSPS of 595 and 4000 ppmv as hexane, respectively. These calculated NMOC yields are well above the measured values (Table 2). This suggests that either nonresidential waste is the major contributor to NMOC emissions from landfills, the values used by EPA represent an overestimate, and/or using a constant NMOC concentration over the entire decomposition cycle is not reasonable. In this study, NMOC concentrations were not correlated to methane production and decreased with time (data not shown). While the NSPS values may be useful to determine which landfills are required to collect and control gas emissions, they should not be used as an estimate of NMOC generation. Finally, this study shows that NMOC release occurs while gas production is still increasing. Thus, strategies to reduce NMOC emissions must include the early installation of gas collection systems.
Acknowledgments This study was supported by Waste Management Inc. Pectin analysis was performed by the Department of Energy funded VOL. 40, NO. 19, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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Center for Plant and Microbial Complex Carbohydrates at the University of Georgia. B.F.S. was partially supported by a fellowship from the Environmental Research and Education Foundation. We appreciate the constructive comments of the anonymous reviewers and assistance with the GC/MS analysis by Dr. Detlef Knappe.
Supporting Information Available Details of experimental design; analytical methods for VFAs and SOCs, NMOC release rates in MSW-ab reactors and SOC results for alkenes, chlorinated, and other compounds; VFA yield data, reactor design, gas, and NMOC rate curves; VFA recovery data, gas, and NMOC yields by reactor; and individual SOC yields. This material is available free of charge via the Internet at http://pubs.acs.org.
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Received for review April 2, 2006. Revised manuscript received July 8, 2006. Accepted July 20, 2006. ES060786M
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