Environ. Sci. Techno/. 1995, 29, 896-902
Introduction
BRIAN D. EITZER Department of Analytical Chemistry, The Connecticut Agricultural Experiment Station, P.O. Box 1106, New Haven, Connecticut 06504
A study of targeted volatile organic chemicals (VOCs) from solid waste composting facilities is reported. The study was conducted using portable sampling tubes and therm a I desorption g a s c hromato graphy/m ass spectrometty. The data indicate that most VOCs are emitted at early stages of processing. Emissions are concentrated at the tipping floors where the waste is dropped off, at the shredder, and at the initial active composting region where the temperatures rise to normal composting conditions ('55 "C). There are indications that ketones may be produced during the composting process. The results show that VOC concentrations remain below workplace permissible exposure limits.
The proper disposal of municipal solid wastes (MSW) is a problem in many communities. These communities are faced with rapidly filling landfills and public opposition to the siting of new landfills or solid waste incinerators. Therefore, both large and small municipalities are looking for ways to reduce the volume of wastes to be disposed. One solution addressing part of the problem is the recycling of solid wastes such as glass, newspapers, and aluminum cans (1). A second option for much of the solid waste stream is composting. Disposable items such as newspapers, cardboard, food wastes, and yard waste can all be composted. In fact, in the United States, 65% of municipal solid wastes by weight are materials that are potentially compostable (2). The United States Environmental Protection Agency (EPA) actually considers composting a recovery option rather than a disposal option, because composting recycles wastes into a usable soil amendment (3) f
There is, however, a community concern about the emission of chemicals from large composting facilities that needs to be resolved before composting can reach its full potential. This project was designed to address that concern, focusing on the emission of volatile organic chemicals (VOCs) from MSW composting facilities. While it is realized that the emission of odors is also a community concern related to compostingand thatVOCs are potentially odorous, the potential odors from the VOCs studied here were not examined. Odors were not reported because they can be synergistic in nature and several major odorant compounds that could be present in compost would not be measured by the methods used in this study. Composting is the aerobic degradation of organic materials. In a solid waste facility, materials must be shredded, mixed, exposed to air, watered, and be given sufficienttime for the aerobic digestion to occur. The major differences between facilitiesis the techniques used to keep the compost piles well mixed and aerobic. These techniques can be as simple as forming windrows of the mixed waste materials and turning them on a regular basis to keep them aerobic. They can also be complex; for example, materials can be loaded into a vessel fitted with blowers to provide aeration and a device which continually mixes the sample. Obviously, the amount of time necessary for the degradation to occur is dependent on the type of composting system used. This project was designed as a survey of eight different composting facilities that employ different composting techniques and feedstocks. The goal was the identification ofVOCs emitted during composting and their approximate concentrations in air at different locations within the facilities. This project provides the first data on VOC emissions from composting facilities. These data should prove useful in the design of future facilitiesand as a starting point for future projects on atmospheric loadings or odorous emissions.
Experimental Section Sampling Sites. Eight facilities were surveyed during this study. Table 1 summarizes pertinent information about these facilities. The facilities span a size range from 5-10
896 1 ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 4,1995
0013-936x/95/0929-0896$09.00/0
D 1995 American Chemical Society
TABLE 1
Summary of Conrliions at Faeilities Visited no.
size T P P (design/operating)
technology
1 2 3 4 5
2501160 100150 1015 6601600 10001420
aerated static pile aerated in-vessel open windrow aerated static pile in-vessel digestion
6
60/60
7 8
50150 50/50
digestion, aerated in-vessel open windrow aerated in-vessel
a
recycling at facility
feedstock
some curbside mandatory curbside mandatory source newspaper curbside some curbside
mechanicallhand sort magnetidhand sort hand sort mechanicallhand sort mechanicallhand sort
mandatory curbside
drop off area
MSWb MSW MSW MSW MSWlsewage sludge (85%115%) MSW
some newspaper wet bag demonstration
magnetidhand pick
recycling in area
MSW MSWIyard waste (75%125%)
Tons per day (TPD). Municpal Solid Waste (MSW).
to 660 tlday design capacity. Facility types range from invessel systems with forced aeration to open windrows with no aeration other than weekly turning. Some facilities use only source-separated waste, while others have extensive waste separating on site, and another co-composts sewage sludge. Several of the facilities use biofilters to reduce the odorous (and VOCs) emissions. This set of eight facilities, therefore, represents a cross section of the many possible approaches to MSW composting and provides a first estimate of the types and quantities of VOCs that MSW composting facilities should be concerned with. To draw general conclusions about composting facilities required a classification of different sample types within the facilities. Samples from compost piles were grouped by "age of compost", with age being relative to the time normally required to turn waste to compost at a facility. Samples were classified and coded by location within the facility as follows with: K, field blank A, air background; T, tipping pile or floor; S, air by shredder; I, indoor air at facility;D, back end of digester; F, fresh from newly formed active compost; M, mid-aged from one-fifth to four-fifths of the way through the active composting region; 0, old if it was at the end of the active composting region; C, curing if it was still on site but out of the active composting area. Sampling Protocol. Sampling cartridges containing either Carbotrap-300 [cartridges purchased from Supelco (Bellefonte, PA) with 425 mg of Carbotrap C, 500 mg of Carbotrap B, and 350 mg of Carbosieve S-1111 or 600 mg of Tenax GC (purchased from Supelcoand packed in a stainless steel tube in the laboratory) were precleaned in the laboratory by thermal desorption until no traces of VOCs were detected by visual inspection of a gas chromatography/ mass spectrometry (GClMS)total ion chromatogram (TIC). The cartridges were then sealed in glass containers until used. The two types of cartridges were compared during the development of methods for this project. Three sets of duplicate samples were collected. Results showed duplicate samples with different cartridges to have results with a 20% average difference for 15 of the target VOCs with neither cartridge being consistently high or low. The lack of major differences between these two cartridges is consistent with the data reported by De Bortoli et al. ( 4 ) . It was, therefore, decided to use the two cartridges interchangeably, and the protocol remained the same for both. Samples were taken by using small portable sampling pumps, SKC Model 224-PCXR3 (SKC, Eightyfour, PA), to draw air at a rate of 0.6 Llmin through the cartridge. Two types of samples were collected. In the first type of sample,
the cartridgeswere placed in the side of a bottomless plastic carboy that was set into the area to be sampled. This isolated the air so that the sample could be correlated with the specific location within the compost pile as opposed to the air blowing across the pile. Air samples were taken over a 10-20-min time period, encompassing one to two carboy volumes; therefore, there was some potential dilution of the air sample via mixing with air above the sampling location as the carboys were put into place. For the second type of samples, the cartridges were placed as close as possible to the selected region of the facility. This approach was used for locations in the facility where use of the sampling carboy was impractical, such as air samples by the shredder. After sampling, the cartridges were returned to the glass containers until analysis. In the field, these containers were stored at ambient temperature; in the laboratory, they were refrigerated at 4 "C. Field blanks (exposed to air without pumping air through the cartridge) and background air samples were also obtained. Sample concentrations were not corrected for field blank concentrations. Instrumentalhalysis. In the laboratory,the tubes were analyzed by thermal desorption GUMS utilizing a thermal desorber accessory to a Tekmar LSC-2000 purge and trap (Tekmar,Cincinnati,OH), interfaced to a Perkin-Elmer8500 Series GUMS (ion trap) (Perkin-Elmer, Nonvalk, CT). A 2-min dry purge to remove excess water in the cartridge was followed by thermal desorbtion at 250 "C for 4 min. The VOCs were cryogenically focused on a 30-M J&WDB624,0.53mm i.d. capillary column U&WScientific, Folsom, CA) at - 10 "C. The GC oven was temperature programmed as follows -10 "C for 2 min, 5 "Clmin to 180 "C, 10 "Clmin to 220 "C hold for 4 min. The ion trap was operated with automatic gain control on, background 39 amu, scanning from mlz = 40-400 at one scan per second. The focus of the project was VOCs which might impact human health; therefore, a set of 67 target compounds was developed, which included most of the volatile priority pollutants listed in EPA Method 8240 (5)as well as several other VOCs for which standards were readily available. Standards of all target compounds were analyzed to determine retention times and response factors, based on the most intense and/or most distinctive ion in the fullscan MS for each compound. Target compounds were identified based on peaks in single ion chromatograms (two ions per compound) at the proper retention time as indicated by the authentic standards. The detection limit for this study was approximately 10 ng per cartridge, which VOL. 29, NO. 4, 1995 / ENVIRONMENTAL SCIENCE &TECHNOLOGY
897
is equivalent to 1 pg/m3 given typical sample volumes. Approximately half of the targeted compounds were not detected in any sample. These compounds were removed from the data set prior to any further statistical analyses. In addition to the initially targeted compounds, a group of compounds that produced large peaks in the total ion chromatograms were identified as terpenes by computer searching of the full-scan mass spectra. It is difficult to identify which specific terpenes are present in a sample due to the large number of terpenes (with many structural and positional isomers) that produce very similar mass spectra. Therefore, an average terpene response for the mlz = 93 ion, present in almost all terpene mass spectra, was determined using five different authentic standards (a-pinene, camphene, 3-carene, D-limonene,a-terpineol). This average response was then used to determine the terpene content of individual peaks identified by retention time (or name if the authentic standard was used). The target compound approach allows the determination of many compounds that are not present in great enough concentration to generate peaks in the total ion chromatogram. The drawback is that other VOCs, though present, are not quantified. The data in this report are only for the initially targeted compounds and one group of identified compounds (terpenes). It is likely that there are a number of other unidentified VOCs present at composting facilities (such as aldehydes, organic acids, organic sulfur compounds, etc.),but these compounds were not included on the target list (many of these compounds could not be determined with the chosen methodology). Reported total concentrations (TVOCJ are the sum of identified compounds and, therefore, are lower than actual VOCs total concentrations. Quality Assurance. It should be noted that there are several potential error sources which limit the data to a first estimation on concentrations. All samples were quantified using the same set of response factors. Possible changes in response factors over the 6-month sampling period of the survey were tested as follows: Standards analyzed in duplicate on the same date differed by less than 10%for most compounds. Freshlyprepared standards analyzed 6 months apart remained within 30% for most compounds, but several changed by a factor of 3-5 and one changed by a factor of 10. These changes are likely the result of changes in instrumental conditions over the course of the study and could lead to errors in the concentration data. Volume calibration of the sampling pumps was performed on a Gilian Gilabrator primary standard airflow calibrator (Gilian Instrument Corp., Wayne, NJ). Calibrations on the pumps did not change during the 6-month sampling period of this study. Sample breakthrough on the samplers was not assessed (the sampling pumps were not of sufficient power to pull air through two cartridges stacked in series) but, given the short sampling durations, it was not thought to be a problem. The reactivity of target VOCs with the adsorbents was not assessed. Hazard and Brown have shown good stability for several of the target compounds on Carbotrap300 tubes held at ambient temperature for 14 days (a,while Rothweiler et al. have shown good recovery of most compounds from tubes held for 12 h at 4 "C ( 7 ) . Rothweiler et al. did show some conversion of a-pinene to other terpenes on Carbotrap and poor recovery of some polar compounds on both Carbotrap and Tenax-TA (7). Samples were analyzed within 2 weeks of their collection to reduce these potential losses. 898 = ENVIRONMENTAL SCIENCE 81 TECHNOLOGY I VOL. 29, NO. 4, 1995
TABLE 2
Maximum OLtsnrd Cmcentratb~aa# ThreslPeM Limit Value-Time Weigbted Average (TW-TWA) for Workplace Air (in p5/m3) maximum observed Concentration a l l samples
trichlorofluoromethane acetone carbon disulfide methylene chloride 1,I-dichloroethane 2-butanone chloroform 1,I,I -trichloroethane carbon tetrachloride 1,2-dichloroethane benzene trichloroethene 2-hexanone toluene tetrachloroethene 4-methyl-2-pentanone chlorobenzene ethyl benzene rn, 0-xyl e ne pxylene styrene isopropyl benzene n-propyl benzene 4-chlorotoluene 1,3,5-trimethylbenzene 1,2,4-trimethylbenzene seobutyl benzene 1,3-dichlorobenzene 1,4-dichlorobenzene pisopropyl toluene 1,2-dichlorobenzene n-butylbenzene 1,2,4-trichlorobenzene naphthalene hexachlorobutadiene 1,2,3-trichlorobenzene
915 000 166 000 150 260 1 320 000 54 15 000 290 2 700 1300 6 600 66 000 5 600 16 000 29 178 000 15 000 6 900 6 100 370 1200 240 2 200 1 000 220 2 90 4 800 1 210 9 1 400 4 6
non-carboy TLV-TWA' 49 000 31 000 0 12 0 38 000 54 4 400 290 0 560 40 6 600 36 000 360 7 500 5 78 000 9 800 5 100 460 120 370 220 1 500 880 2 0 90 340 1 92 0 1400 0 0
5 620 000 1 800 000 31 000 174 000 400 000 590 000 49 000 1 900 000 31 000 40 000 32 000 270 000 20 000 188 000 339 000 205 000 46 000 434 000 434 OOOb 434 OOOb 213 000 246 000 *C
* 123 OOOb 123 OOOb
*
*
451 000
*
150 000
*
123 OOOb 52 000 210
*
aTLV-lTVAvaluesfrom ref8. bValueforsumofall isomersofeach compound, CAnasterisk ( * ) indicates that the value is not listed.
It should be noted that the focus of the project was to survey the VOC emissions from different composting facilities. Facilities were spread around the country, and most could only be visited for a single day. Our emphasis was on collecting enough samples during that day to get a snapshot of the facility; our goal was not the detailed characterization of emissions at each facility. Therefore, reported results for a facility may not be indicative of longterm conditions at that facility. However, by sampling these eight facilities with the methodology and quality assurance used in this study, a first-order estimation on the scope of the problem of VOC emissions from waste composting facilities could be obtained.
Results Occupational Health and Safety. Table 2 lists the maximum observed concentration for each chemical found in at least one sample, along with workplace exposure limits for that chemical as listed by the American Conference of Governmental Industrial Hygienists (ACGIH) (8). It is important to remember that not a l l potentially hazardous VOCs are included in this study. Also workplace exposure limits are higher than outdoor ambient air exposure limits. There are two columns for maximum observed concentration in this table. The first lists the maximum concentration observed in any sample, while the second only lists the
TABLE 3
Total VOCs Concentration of Samples: Number of Samples of Characteristic Code in Specified TVOC~ (mum3) Concentration Rangea code
K A T
S I D F M 0 C
4 0 6 10 3 2 13 7 6
20-50
50-100
100-150
>150
-
-
-
1
-
1
2 2 1 2 7 4 3 1
1 1 1
-
2 1 2 3
10-20
1 5 16 3 4
3 5 -
1 1 -
1 1
aThe total VOCs concentration (TVOCJis broken down into six concentration rangesfrom less than 10 to greater than 150 mg/m3.The number of samples of each characteristic code, which had its total VOCs concentration within the specified range, is shown.
maximum concentration observed in samples takenwithout the carboy sampling device. This subset of samples is more likely to parallel potential worker exposure. The non-carboy subset usually has lower maximum observed concentrations than the total samples, indicating that the highest concentrations are observed in air samples taken directly from compost piles before any dilution with room air has occurred. It is important to note that even the highest concentrations from the compost piles remain well below the exposure guidelines, in most cases by several orders of magnitude. Also, these maximum concentrations represent worst cases (particularly when all samples are considered). Samples were taken directly from waste and compost piles so that the extremely high concentration might be localized,while workers would be exposed to a more mixed air sample (non-carboy samples) and, therefore, lower average concentrations. For example, a sample which came from a tipped pile of solid waste had an ethylbenzene concentration of 180 mg/m3; while a sample from the indoor air in the building in which the pile was situated had an ethylbenzene concentration of only 3 mg/m3. Total VOC concentrations in these two samples were 430 and 85 mg/m3, respectively. A second example of localization is seen in a different sample taken from a solid waste pile on a tipping floor. This sample had the highest toluene and xylene concentrations of any sample (66 and 22 mg/m3, respectively). The pile this sample came from was one at which an employee mentioned that someone had just dumped a load of paint. The fact that these worst cases still remain below the workplace air exposure limits indicates that these compounds, while present and of concern, do not pose a major health risk to workers at these facilities. Total Concentrations. TotalVOC concentrations (TVOC = sum of all identified compounds) range from less than 10 mg/m3 to greater than 150 mg/m3. The number of samples showing a particular total VOC concentration (separated into six concentration ranges) for a characteristic code is listed in Table 3. It is observed that for a particular characteristic code there are two to three concentration ranges in which the majority of the samples occur. This similarity in concentration for a sample class occurs even though these samples have come from facilities with differing operating conditions. Also, note how samples taken from the tipping floors, near the shredders, and in the freshest active composting regions (codes T, S, and F,
respectively) tend to have the highest concentrations, while more mature compost samples have lower concentrations (codes M, 0, and C). This trend will be discussed in more detail later. There are no other reports onVOC emissions from MSW composting facilities, but it is interesting to compare the concentrations determined in these samples with those based on theoretical considerations as determined by Kissel et al. (9). They used the literature on VOC emissions from MSW and landfills and a fugacity-based model to estimate emissions for a 165 tlday MSW facility receiving wastes which had 0.05% VOCs (a typical percentage in MSW literature). The model resulted in a steady-state totalVOC concentration ranging from 20 to 150 mg/m3 depending on where in the facilityVOCs were released. These numbers are quite consistent with the data reported in Table 4. The TVOQfor most sample codes fall within or just below this range (T, S, I, F, and M codes); the highest localized concentrations above this range (individual T, F, and M samples); and samples of more mature compost and curing compost (whichhave likely lost their VOCs content) below this range (codes 0 and C). Principal Components Analysis. The total ion chromatograms of all samples showed great similarities;therefore, the relative amount of different VOCs in the samples was compared more formally by the use of principal components analysis. The computation was performed on data normalized to TVOQon a personal computer using the SYSTATfor Windows software package (SYSTAT, Inc., Evanston, IL). The results of this analysis indicated that the first two principal components explained only 14%and 10%of the total variance within the data set, respectively. This low variance for the two main principal components indicates that there is no outstanding single cause ofvariabilitywithin the data. Overall, there appeared to be more variability in the sample’s Characteristic code, representing where in the facility it came from, than in which facility it came from. This would seem to indicate that theVOCs which are present in MSW are fairly consistent throughout the country, and therefore, emissions of those VOCs from composting will be consistent regardless of the method used to do the composting. The examination of VOC emissions should focus more closely on the location of the VOC emissions within the facilities. Given the similarities between the facilities relative concentration data and the consistency of the total concentration data, it is reasonable to examine VOC emissions within the facilities by averaging all of the data for each sample characteristic type. These averages are listed in Table 4. Average Concentrations. An examination of Tables 3 and 4 shows that the highest concentrations for mostVOCs (both xenobiotic and natural compounds such as terpenes) are observed in the tipping piles, near the shredders, and in the fresh active composting region (codes T, S, and F, respectively). This means that these chemicals are emitted early in the composting process. These VOCs are present in the waste as it is dropped off at the facility; remember the highest observed toluene and xylene concentrations were from a pile of tipped waste that had a load of paint dropped off, a point referred to earlier. Shredding of the waste to prepare it for composting increases the surface area and exposes the surfaces to the atmosphere, allowing these compounds to volatilize rapidly. Thus, air from above VOL. 29, NO. 4, 1995 / ENVIRONMENTAL SCIENCE &TECHNOLOGY
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TABLE 4 Part A Average Concentrations of Targeted Compounds for Each Characteristic Code (in pg/m3) sample type characteristic code no. of samples compound trichlorofluoromethane acetone carbon disulfide methylene chloride 1,l -dichloroethane 2-butanone chloroform l,l,l-trichloroethane carbon tetrachloride 1,2-dichloroethane benzene trichloroethene 2-hexanone toluene tetrachloroethene 4-methyl-2-pentanone chlorobenzene ethylbenzene m,o-xylene pxylene styrene isopropyl benzene n-propyl benzene 4-ch lorotol uene 1,3,5-trimethylbenzene 1,2,4-trimethyl benzene sec-butylbenzene 1,3-dichlorobenzene 1,4-dichIorobenzene pisopropyl toluene 1,2-dichlorobenzene n-butylbenzene 1,2,4-trichlorobenzene naphthalene hexachlorobutadiene 1,2,3-trichlorobenzene total targeted compds
field blank
K 6 240 250 0 0 0 3 0 0.2 0 0 2 0.5 0 0.3 0 43 0 0.7 0 0 0 0
bkg air A 11
tipping T 9
shredder S 5
indoor
digester
I
D
F
M
4
2
17
41
3 200 390 0 1 0 130 5 10 5 0 34 0 3 9 2 22 0 11 2 1 0 0 0 0 2 2 0 0 0
3500 0 0 2 0 1400 7 1100 58 0 8 16 1700 11 500 130 1 500 1 23400 3 700 1600 230 44 130 0 610 390
2500 7800 0 0 0 9 400 1 410 3 0 150 5 1000 1100 23 62 0 2900 250 110 250 51 42 0 180 160
21 000 0 0
0 0
0
0
36 000 0 2 300 0 0 130 21 1200 860 360 0 1 2 900 420 250 150 21 23 0 180 190 0 0 6 64 0 36 0 29 0 0
64 300 9 200 5 1 0 4 300 1 1 500 1 0 35 98 540 3 900 210 900 1 3 100 520 250 260 45 69 14 320 200 15 0 3 280 0 27 0 92 0 0
26500
66 100
90 200
0
0
12 600 6 100 0 2 0 25 500 6 1200 6 0 12 4 96 8 800 170 28 0 38 100 2 600 910 63 2 65 25 310 250 0 0 0 13 0 47 0 22 0 0
540
3 900
96 900
0 0
0 0 0 0 0 0 0 0 0 0
0 0 0 0 0
0
0
0 10 0 35 0 340 0 0 51 400
0 0 44 0 17 0 55 0
fresh
0 0
Part 6: Average Concentrations of Individual Terpenes for Each Characteristic terpenes by retention time (scan no.) 1157 0 0 0 0 0 0 1212 0 0 5 0 0 0 1241 0 0 38 16 0 3 120 1259 (a-pinene) 0 1 230 2 100 660 1304 (camphene) 19 0 1 110 780 460 130 1361 0 0 220 1800 680 1406 640 190 78 0 0 260 1438 (3-carene) 0 0 59 390 320 39 0 1455 0 0 2 20 0 1485 (D-limonene) 1.7 3 970 10100 2800 1500 1545 0 0 25 380 33 30 0 0 1573 0 0 0 80 46 0 1605 0 0 19 130 0 0 1627 0 0 0 0 0 0 1662 0 0 0 0 0 0 1709 0 0 19 0 0 0 1785 0 0 0 0 1801 0 0 0 0 0 0
old
curing
0 15
11
3500 9 500 9 0 0 8700 2 49 2 0 22 3 1000 700 37 1100 0 940 86 76 190 9 47 0 130 95 0 0 1 190 0 11 1 98 0.1 0.1
4700 6 100 8 0 0 14 500 0 99 3 0 50 3 700 400 78 830 2 610 55 31 120 10 7 0 46 66 0 0 3 66 0 13 0 45 0 0
3 700 2 300 6 25 0 1 400 0 30 4 0 120 2 130 88 4 60 0 780 6 4 49 17 2 0 7 6 0 0 4 3 0 1 0 8 0 0
26500
28500
8 800
C
Code (in pg/m3) 27 0 370 2100 1200 2 200 920 570 4 6100 160 13 78 4 9 16 1 9
20 0 52 820 410 630 200 240 8 2900 67 1 47 6 11 8 0 13
0 0 2 400 120 240 29 48 1 1200 7 0 10 1 1 2 0 3
0 1 0 24 7 13 1 9 1 50 0 0 0
0 0 0
0 0
1836
0
0
0
0
0
0
5
12
2
0
1872 (terpineol) 1983 2017
0 0 0
0 0 0
13 0 0 1940 98800
81 0 23 16600 68000
31 0 35 3 900 30400
22 0 3 3 200 69300
52 11 24
59 18 17
2 1 0
13 900 104 100
5 500 32000
24 4 5 2 100
110
30 600
8 900
total terpenes total VOCs
1.7 540
5 3900
the newly shredded material has some of the highest average concentrations for many chemicals as compared to the other sample types (the possibility of artifacts caused by small particles in the sampled air by shredders was not assessed). For example, the maximum average concentration for xenobiotics such as trimethylbenzenes, xylenes, 900
mid-aged
ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 4 , 1 9 9 5
toluene, and carbon tetrachloride or terpenes such as a-pinene and D-limonene all occurred in these shredder air samples (code SI. Those VOCs that remain after the shredding occurs continue to volatilize rapidly as the compost pile comes up to its operating temperature (approximately'5 "C)in the
100
75
50
25
0
0Fresh RSSl Middle Age
Old
€3
Curing
FIGURE 1. Concentrationof selected compounds in different sample types as a percent of the highest average concentration of that compound in any type of sample.
active composting region (fresh compost, F). The drop off in concentration between the fresh and mid-aged compost (F to M) is somewhat greater than the drop in concentration from the mid-aged to old compost (M to 0)for xenobiotics. For example, the average concentrations of l,l,l-trichloroethane, toluene, and ethylbenzene fall by 97%,82%, and 70%,respectively,from fresh to mid-aged but show a 100% increase and 43% and 35% decreases between mid-aged and old. Conversely,terpenes seem to show a slower initial but more steady drop off; for a-pinene, 3-carene, and D-limonene, the rates are 61%,58%, and 52% from codes F to M and 51%, 80%, and 59% drop offs from codes M to 0. This would seem to indicate that the xenobiotic VOCs remaining after shreddinghave an easilyvolatilized fraction and a more recalcitrant fraction; perhaps this recalcitrant fraction is bound to some of the organic matter and, therefore, is not so easily volatilized. Another possibility is that part of this fraction is locked inside some of the clumps of compost. The slower but steadier release of terpenes might be indicative that these compounds are more a part of the structure of the organic matter and are, therefore, not as easily released, or that there is some production of terpenes as volatile byproducts when the larger lignins and celluloses are metabolized. Finally, there is another drop off in concentration from the old compost to the curing compost. It is possible that this drop off is caused by the final screening. The screening process would once again expose all of the surfaces to air and possibly increase surface area, which would allow for volatilization of the biogenic and xenobiotic VOCs that had
remained in the old compost. For example, there is a 94% drop off in a-pinene and a 78%drop off in toluene between these two stages. In fact, the average concentrations in the curing compost are quite similar to the background air concentrations except for a few selected chemicals (primarily ketones, discussed below), indicating the slight amount of VOC emissions at this point. The above scenario is illustrated in Figure 1, showing normalized average concentrations of five typical VOCs (trichloroethane,toluene, tetrachloroethene, p-xylene, and D-limonene)for the six sample types that characterize waste as it passes through the composting facility; waste is dropped off at the tipping floor, shredded, formed into compost which matures (three stages shown), and is then cured. Each VOC is normalized to the highest average of that VOC so that the bars are in percent of maximum average concentration. Note, the great difference in the height of the first three bars of any VOC with the height of the last three bars in this figure. There is one group of compounds whose concentrations runcounter to this trend. This group consists of the ketones (acetone,2-butanone, 2-hexanone,4-methyl-2-pentanone) and is illustrated in Figure 2, with the normalized average concentrations. This group shows some increases in concentration from the shredder to the fresh compost or from the fresh to mid-aged compost; for example, 2-butanone goes from an average concentration of 1400pg/m3 at the shredders to 4300 pg/m3 in fresh compost to 8700 pg/m3in mid-age compost and 14500pg/m3in old compost. When the compounds in this group show a decrease in VOL. 29, NO. 4, 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 190’1
100
80
60
40
20
0
e'
0Fresh Middle Age
Old
R Curing
FIGURE 2. Concentration of selected ketones in different sample types as a percent of the highest average concentration of that ketone in any type of sample.
concentration, the decrease is much less than that for the other VOCs; compare the size of the mid-aged and old compost bars in Figure 2 with the bars for those compartments in Figure 1. One likely explanation for this type of concentration pattern would be that these compounds are being produced as volatile byproducts during the microbial aerobic digestion of large organic compounds. Thus, as the compost ages and the large bioorganic compounds such as lignins and proteins are being turned into humus, there is a concomitant production of some smaller compounds of possible metabolic origin such as the ketones. Other possible metabolic products, such as aldehydes, were not determinable by the chosen sampling methodology.
Summary This project has examined for the first time VOCs emissions at solid waste composting facilities. The results show that the observed concentrations of VOCs are below guidelines for workplace air. The results also indicate great similarities between facilities operatingunder differing conditions. The primary locations ofVOC emissions would appear to be on the tipping floors, by shredders, and in areas where the compost first comes to the designed operating temperatures. Knowledge of the location of these emissions should be helpful in designing future facilities with regard to reduced VOC emissions.
Ackaewleljnmts The work reported here was supported in part by a grant from the Composting Council. The author wishes to thank 902
ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 4,1995
Ten Misenti and William Berger for technical assistance and Mary Jane Incorvia Mattina for helpful comments and discussion.
Literature Cited (1) Arrandale, T. Governing 1992, 5 (9), 33. (2) Harrison, E. Z.; Richard, T. L. Biomass Bioenergy 1992,3 (3-4), 127.
(3) United States Environmental Protection Agency. Characterization ofMunicipalSolid Waste in the Unitedstates: 1992 Update, Executive Summaly; U.S. EPA Washington, DC, 1992; EPAl 530-S-92-019. (4) DeBortoli, M.; Knoppel,H.; Pecchio, E. Schauenburg, H.;Vissers, H. Indoor Air, 1992,2, 216-224.
Test Methods for Evaluating Solid Waste; US. EPA Washington, DC, 1986. (6) Hazard, S.; Brown, J. The Supelco Reporter 1993, 12 (3), 9. (7) Rothweiler, H.; Wager, P. A.; Schlatter, C. Atmos. Enuiron. 1991, (5) United States Environmental Protection Agency.
25B (21, 231. (8)American Conference of Governmental Industrial Hygienists. 1992-1993 ThresholdLimit Valuesfor ChemicalSubstancesand
Physical Agents and Biological Exposure Indices; ACGIH: Cincinnati, OH 1992. (9) Kissel,J. C.; Henry, C. L.; Harrison, R. B. BiomassBioenergy 1992, 3 (3-41, 181.
Received f o r review M a y 9, 1994. Revised manuscript received December 28, 1994. Accepted December 30, 1994.@
ES940280G @
Abstract published in Advance ACS Abstracts, February 1, 1995.
