mechanical separation at a centralized facility. Data from both ... Keywords-Municipal solid waste, cornposting, separation, heavy metals, contaminants. 1.
Biomassnnd Bioenergy Vol. 3. Nos 3-4, pp. 195-2 1 I, I992 Printed in Great Britain. All rights reserved
0961-9534,/92 $5.00 + 0.00 ‘C 1992 Pergamon Press L.td
THE IMPACT OF SEPARATION ON HEAVY METAL CONTAMINANTS IN MUNICIPAL SOLID WASTE COMPOSTS TOM L. RICHARD*
and
PETER B. W~~DBURY~
*Department of Agricultural and Biological Engineering, Cornell University, Ithaca, NY 14853. U.S.A. TBoyce Thompson Institute for Plant Research, Tower Road, Ithaca. NY 14853, U.S.A. (Received 22 May 1992; accepted 2 July 1992)
Abstract-Increased cornposting of mixed municipal solid wastes has been accompanied by heightened concern about contaminants in compost products. This paper addresses the impacts of different separation strategies on trace metal concentrations. Separation strategies considered include source separation of either compostables or contaminants prior to collection, wet/dry collection schemes, and manual or mechanical separation at a centralized facility. Data from both experimental trials and operating facilities indicate that the lowest levels of contaminants are achieved by source separation of compostables. Wet/dry systems produce variable quality, depending on whether separation of recyclables or compostables is emphasized. Centralized separation can achieve moderate reductions in metal levels, and some evidence suggests this separation is most effective at early stages of processing. Further research is needed to document the potential for source separation of critical contaminants such as lead, and the effectiveness of various centralized separation technologies (individually or in combination) at minimizing contaminant levels. Keywords-Municipal
solid waste, cornposting, separation, heavy metals, contaminants.
1. INTRODUCTION
During the last few years cornposting has gained wide acceptance as a key component of integrated solid waste management. However, there continues to be vigorous debate over what materials should be composted, and in particular whether composting should be limited to source separated organic wastes or applied more broadly to mixed Municipal Solid Waste (MSW). There are several important trade-offs between these approaches: quantity, quality, and cost. This paper focuses on the quality issues and specifically examines the impact of various approaches to contaminant separation on levels of key metals in finished composts. Many potential compost users are concerned about contaminants in composts made from mixed refuse and need to be confident that the composts are both physically and chemically safe. Physical contaminants are an obvious concern. Bits of glass, plastic, and metal are often visible in MSW composts,” although screening and separation technologies can help reduce that problem. The chemical contaminants in MSW include both toxic organic chemicals and heavy metals. Some of these contaminants are highly diffuse,
being present at dilute concentrations throughout much of our waste. Others are concentrated in a limited number of products, and are thus more amenable to separation approaches. Many organic chemicals volatilize or degrade during the composting process, but during waste processing they may expose compost workers to potential risks. ‘v3While some organic chemicals such as PCBs and dioxins may be a concern in finished composts, there are not enough data available to assess the impacts of separation on this class of contaminants. Heavy metals are the other class of chemical contaminants in the finished products, and will be the focus of the analysis in this paper. Metals and other inert contaminants do not degrade, but are instead concentrated during the composting process as the organic matter which dilutes them gradually degrades4%’ The metals of greatest concern are those which tend to bioaccumulate, causing short- or long-term toxic effects to organisms in the environment. Those most commonly regulated include cadmium (Cd), chromium (Cr), copper (Cu), mercury (Hg), lead (Pb), nickel (IQ), and zinc (Zn).” Crawford’ provides an overview of the toxicity and environmental fate of these metals. 195
T. L.
196
RICHARD
and P. B. WOODBURY
Table I. Proposed NOAEL/APL limits for sludges and sludge composts Element Cd Cr CU Hg Ni Pb Zll
NOAEL/APL sludge (pg g-‘) 25 3000 1200 20 500 300 2700
In most MSW composts the levels of these potentially toxic metals are low relative to many existing and proposed regulatory standards.6 A table of the proposed No Observed Adverse Effect Level (NOAEL) or Alternative Pollutant Limit (APL) for these metals in sludge and sludge composts is listed in Table 1.8 These levels were developed by a peer review of the United States Environmental Protection Agency’s (EPA) risk pathway analysis.’ These levels have been viewed as a possible basis for regulating MSW composts,‘0 although potential differences in solubility and bioavailability between sludges and MSW composts have not yet been explored.3,* While MSW composts can usually meet these limits for most metals (with the exception of lead), there is an increasing interest in achieving lower levels,” Several European countries and Canadian provinces have proposed or enacted much lower standards, which are sometimes based on background levels in the soi1.6.‘2m’s Whatever standards are ultimately established, solid waste managers and compost facility operators need to understand the effectiveness of different options to reduce contaminant levels. While there are a wide variety of possible approaches, most can be placed in one of five different categories, which are summarized in Table 2. This list can be viewed as a kind of hierarchy, with options at the top of the list holding greater potential for contaminant minimization than those lower down. For example, if no products Table 2. Options for reducing contaminant levels in MSW composts (1) (2) (3) (4) (5)
Reduce or eliminate contaminant levels in products destined to become MSW Source separate clean organic materials for separate collection and composting Source separate contaminants for separate collection and disposal Separate contaminants from MSW at a centralized facility prior to cornposting Separate contaminants from MSW compost at a centralized facility after composting
containing high concentrations of lead were ever manufactured, there would be little concern with lead in MSW compost. Unfortunately, this first option is beyond the control of the local governments usually responsible for solid waste management, and is therefore too often neglected, despite evidence that product design changes can greatly broaden our waste management options. 16.”For example, metal contaminants have been dramatically reduced in waste paper, with the virtual elimination of metalbased inks over the last decade, making this material much more attractive for recycling, livestock bedding, cornposting, and incineration. California has recently banned the use of lead foil seals on wine bottles, which if widely adopted may eliminate that as a source of lead. Similar trends with other materials may have long term implications for the selection of separation technologies. However, because such changes decrease the potential for toxicity with all disposal methods and separation strategies, this option will not be discussed further in this paper. The remaining four contaminant reduction options can all be implemented on the local level, creating choices which are widely debated by solid waste managers and concerned citizens. As the data summarized in this paper will demonstrate, descent through this hierarchy normally results in increasing contaminant levels. A rational procedure for selecting among these options should also consider cost, convenience, and the quantities of material recovered for cornposting or recycling. Comparative studies on how these other trade-offs affect MSW cornposting are generally not available. 2. SOURCES OF CONTAMINANTS WASTE STREAM
IN THE
Any attempt to reduce contaminants must begin with knowledge of the sources of the contaminants. Although waste streams vary throughout the world, information about contaminant sources in industrial countries suggest similarities which can be of use in designing separation strategies. Batteries are a particularly significant source of metal contaminants. Even after 80% recovery for recycling, the remaining 20% of lead-acid automobile batteries were estimated to contribute 66% of the lead in MSW in the United States in 1986.18Recovery rates for spent lead-acid batteries fluctuate significantly with
Heavy
metal contaminants
lead prices, with estimates ranging from 9094% in 1980 to 6066% in 1985.19 Franklin Associates estimated that 52% of cadmium in U.S. MSW could be attributed to nickelcadmium batteries.” A French study estimated household batteries contributed about 45% of cadmium, 93% of mercury, 20% of nickel, 1% of lead, and 45% of zinc in MSW.” This French estimate of mercury contributions is very close to the 89% attributed to batteries in a waste composition study in New Jersey.” Consumer electronics, ceramics, and some glass can also be significant sources of metals.22 Franklin Associates estimated that 27% of lead and 9% of cadmium are contributed by electronic goods, including video-cassette recorders, radios and other electronic items.” Ceramics, glazing and vitreous enamels may use metal oxides and silicates in their production.” Leaded glass, which is used for TV tubes, optical and automobile glass, crystal, and radiation shielding electrical glass can contain extremely high concentrations of lead,” although this lead may be less bioavailable than that from some other sources. There are a variety of other items which are known to contain lead, including light bulbs and bulb sockets, lead foils such as wine bottle closures, wheel weights, house dust and paint chips, and used motor oil and filters.” These items are not adequately measured in most waste characterization studies, so the quantities of contaminants contributed remain subject to speculation. However, any attempt to separate contaminants from the waste stream should consider targeting these items. Plastics are a more diffuse but significant source of some metals, particularly cadmium. Cadmium is used in pigments and stabilizers in some plastic products.‘9,23 Plastics are estimated to contribute between 21%22 and 28%‘* of the cadmium in U.S. MSW. In France, plastics are estimated to contribute 38% of cadmium and 25% of nickel in the solid waste stream.20 Of the combustible fraction of MSW (excluding metal, glass, and ceramics), plastics can account for 71% of the lead and 88% of the cadmium remaining.24 Although waste paper and paper inks were a significant source of contaminants in the past, over the last decade metal levels have dropped as evidenced by a number of studies summarized by Rugg. 22 Analysis of 12 representative samples of mixed waste paper in Washington State indicated average levels of 0.55 ppm Cd,
in MSW composts
197
6.48 ppm Cr, 18.12 ppm Cu, 0.05 ppm Hg, 6.92 ppm Ni, 7.51 ppm Pb, and 146 ppm Zn.*’ These levels are extremely low, even allowing for some concentration of metals during the cornposting process. 3. SEPARATION
STRATEGIES
During the last several decades there has been considerable evolution in the methods used for contaminant separation. The first generation of MSW composting facilities depended on screening after cornposting as the primary means of separation,26 sometimes combined with magnetic separation to recover ferrous materials. Over time a range of technologies were developed to separate inorganic and inert contaminants during earlier waste processing steps. Most modern MSW cornposting facilities include some front-end separation of noncompostable wastes.27.28 This can include manual picking lines, size separation, magnetic or eddy-current metal recovery, air classification, and other mechanical approaches.” Separation processes can be applied at several points in a single facility: front-end separation may be followed by additional separations after an initial decomposition period and again at the end of the process. However, the mechanical approaches available rarely target the specific sources of contaminants identified above, many of which are not particularly amenable to these centralized separation approaches. Several factors combine to challenge efforts to separate contaminants from a mixed waste stream. Some, such as motor oil and oil in filters, are liquid and therefore subject to spillage from the time they leave the house. Abrasion during waste handling can break off bits of lead from foil or weights, and fine dust or paint chips can attach to otherwise clean organic wastes, Van Roosmalen et a1.30 speculate that even seemingly sturdy materials, such as consumer electronics, can contaminate neighboring organic materials through leaching and direct contact prior to, and during, collection. Although increasing numbers of communities have household hazardous waste (HHW) collection programs, these programs rarely target the metal contaminants of concern in MSW composts. A study in Germany evaluated options for collection of the fine metal particles which are particularly problematic3 Of the estimated 30 g per resident per year of small lead particles (e.g. wine bottle caps, solder, shot pellets, and
198
T. L. RICHARDand P. B. WOODBURY
fishing weights), a HHW mobile unit drop-off program only collected 0.01 g per resident per year. Switching to a curbside HHW bag collection program achieved a dramatic 150-fold increase in lead particle collection. While this was still only 1.5 g per resident per year, further improvement by a factor of 10 was considered achievable and would then capture approximately 50% of this particular contaminant.” In discussing separation strategies it is often useful to differentiate positive from negative sorts. Each of the methods described above, from targeted HHW collection to centralized separation, represents a negative sort (subtraction) from the stream destined to become compost. Items which have been identified as problems are removed from the waste stream, with varying degrees of success. The default, if separation does not occur, is for a waste material to remain in the stream destined to become compost products. In contrast, a positive sort would specifically select material for composting, and anything not selected would remain in the mixed waste stream, destined for disposal or some other processing option. A positive sort for organic waste requires a conscious decision that a particular material is compostable, and can therefore be expected to minimize contaminant levels. Source separation of compostables, sometimes also referred to as biowaste or green waste composting, represents the positive sort approach to MSW cornposting. By defining source separation as a positive sort, a community that source separates for both compostables and recyclables will generate a minimum of three streams, including one to handle those materials which are neither compostable nor recyclable but are destined for environmentally sound disposal (in some cases as hazardous waste). A variety of organic waste source separation programs have been developed in recent years. This approach has largely been initiated in Europe, where concerns about metal contamination in soil have resulted in increasingly strict regulations which sometimes prohibit mixed waste collection.32,33 In North America, yard waste cornposting has become common in many communities, and some facilities have added other segregated streams, including cafeteria, restaurant, supermarket, produce market, food processing, and paper/ cardboard residues.3k37 A few are following the lead of communities in Europe, collecting a fuller range of source separated organic materials from residential and commercial
generators.38*39Source separation strategies typically target yard trimmings and food scraps, but an increasing number also target non-recyclable paper, which appears to be the key to high diversion rates. A few communities have selected a strategy which is intermediate between mixed and source separated organic waste collection. Commonly referred to as wet/dry collection, this approach segregates the waste stream into two components: a wet fraction containing food, yard, soiled packaging and paper waste, diapers, and pet waste; and a dry fraction containing materials destined for recycling. Items destined for disposal may be directed to either stream depending on the community’s focus. In a pilotscale comparison of wet/dry separation with complete source separation, the city of Guelph, Ontario recovered 96% of all organic and recyclable waste with a two stream wet/dry collection, vs. 85% with a three stream source separation program. 38The specific strategies of wet/dry separation programs vary, and can be described as either a positive or negative sort for organics depending on whether recyclables or compostables are emphasized, and which stream gets the rejects for disposal. Such gradations in emphasis and educational message are reflected in corresponding variations in the quality of MSW cornposting feedstocks. 4. COMPARATJVE STUDIES OF THE IMPACTS OF SEPARATION
The range of separation strategies described above can be expected to result in varying degrees of contaminant separation, There are two basic approaches to evaluating the impact of separation on compost quality; (1) comparative studies evaluating the effectiveness of specific separation strategies and (2) compilations of literature data to assess broad-scale trends. Each of these approaches is reviewed below. The ideal comparative evaluation of separation efficiencies would use an identical waste stream for each alternative. Because there is no such standardized waste available, all such studies are approximate at best. Some studies work with large volumes of waste over a long time period, while others focus on detailed measurement over a shorter time or with a smaller volume. Despite such variations in experimental approach, results from the published studies indicate similar trends.
Heavy metal contaminants in MSW composts
199
Table 3. Comparative trials in The Netherlands Cd Separation strategy
Cr
Cu
Hg
Ni
Pb
Zn Reference number
(mg kg-’ dry weight basis)
Source separation of organic fraction Source separation (produce and yard waste) Source separation (produce and resid. food and yd waste) Source separation Wet/dry-focus on recyclables: paper, tins, glass Wet/dry-focus on recy.: plastics, tins, glass Wet/dry-focus on recy.: pap., plas., metals, glass Centralized pre-separation Central separation: trommel/mag. Sep., air class. Central separation: trammel/magnetic sep. Central separation: sieve/magnetic separation Central Sep.: sieve/mag. sep./compost fines only Central separation: shred/separate inerts Final product screening only (no pre-separation) Final product screening only Final product screening only
0.80 0.97
43 36
35 28
0.55 1.00 2.50 1.80 1.60 3.33 1.70 1.90 1.80 2.50 1.80 6.40 8.50 7.00
32 30 95 60 70 113 20 59 60 70 40 171 140 180
23 50 270 250 130 268 70 220 240 270 100 693 530 600
Table 3 presents the data from several studies in The Netherlands. The first of these studies was reported by de Hann and Lubbers.40 They demonstrated the advantages of pre-separation at a centralized facility over simple screening of the finished compost product, with significant reductions in all metals tested. The most comprehensive study was by van Roosmalen et al. in 1987.” They examined a wide range of separation options, including end product screening, centralized separation, a wet/dry collection of various recyclables (emphasizing materials recovery, not cornposting), and source separation of compostables. Centralized separation and recyclables collection had roughly similar results, somewhat better than endproduct screening but significantly worse than source separation. Looking specifically at the data for lead, end product screening resulted in 83Opg g--’Pb. With that value as a starting point, centralized pre-separation resulted in
0.08
7
0.06
10 IO
1.37
219
2.90
35 25 235 110
130 42
195 170
30 41
38 160 580 400 680 591 670 600 530 700 420 875 830 800
135 230 880 720 460 666 540 670 710 800 520 1400 1600 1700
41 12 30 30 30 40 30 30 30 12 12 40 30 12
2040% reductions (to 53&67Opg gg’ Pb), wet/dry collection resulted in 2&50% reductions (to 400-680 pg g-’ Pb), and source separation of compostables achieved an 80% reduction, to 130 pugg-’ Pb. These low contaminant levels for source separation were also confirmed in a later study.4’ In another study, Oosthoek and Smit” describe a comparison of four MSW cornposting systems. Their first process composted the entire mixed waste stream, with screening of the finished product only. They also evaluated pre-screening and magnetic separation with cornposting of the fines, shredding and centralized separation of inerts and residential source separation of compostables. In each of these studies, consistent decreases in metal contaminant levels are seen with higher (as per the hierarchy proposed in Table 2) or earlier levels of separation. Another cluster of studies was performed in Germany, and is summarized in Table 4.
Table 4. Comparative trials in Germany Cd
Cu
Hg
Ni
Pb
Zn Reference number
(mg kg-’ dry weight basis)
Separation strategy Source separation Source separation w/out waste paper Source separation with waste paper Waste paper (not composted) Source sep./shred/reject > 10 mm screen/air classif. Rejects from air classif. after source sep. (above) Wet/dry-no processing Wet/dry-l2 mm screen/mag. sep./shred Wet/dry-8 mm screen/mag. sep./shred Central Sep. shred/screen/mag. sep., W&loch Central Sep.: reject < 12 mm screen/mag. sep./shred Central Sep.: reject < 12 mm screen only Central seoaration
Cr
1.00 0.50 0.40 0.15 1.20 1.40 1.57 3.06 2.75 2.35 2.44 1.76 5.50
36 55 40 3 32 43 25 25 25 24 17 19 71
33 47 39 24 60 92 158 116 127 93 140 163 274
0.50 0.30 0.06
2.40
29 14 1I 1 16 20 29 45 41 44 36 24 45
133 62 52 12 136 184 215 253 247 359 172 183 513
408 198 178 40 331 366 787 932 1001 690 697 628 1570
42, 43 32 32 32 44 44 44 44 44 44 44 44 42, 43
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L’Hermite,” also in Golueke and Diaz,43 provides data comparing source separation and centralized separation. Franke’* reported on a study by Albrecht et ul. that compared source separation of food and yard waste with and without the addition of wastepaper. Their data on mixed waste paper is included to indicate the low metal levels of this particular stream. Bidlingmaier er a1.44examined a wide range of source and centralized separation options. Although much of their original data was normalized on a fixed solids or ash basis to account for varying degrees of decomposition, we have converted those normalized values to the more commonly reported dry solids basis by using their standard level of decomposition of compost from the Wiesloch facility. The focus of the evaluation by Bidlingmaier et al. was on an innovative combination of screening, magnetic separation, and shredding. However, various combinations of these centralized separation processes resulted in limited reductions and some apparent increases in metal contaminant levels over the Wiesloch or unprocessed controls. Lead levels were in a narrower range than in the other studies, possibly indicating a difference in the incoming waste stream. Metal levels in composts from mixed and wet/dry collection systems were roughly comparable, although somewhat higher values in the wet stream compost were attributed to the loss of clean paper and other uncontaminated material to the recyclable dry stream. In this wet/dry system, the positive sort was apparently for the dry recyclables, and the wet compostable stream acquired much of the miscellaneous residue. Contaminants in both wet and mixed waste composts were significantly higher than in composts from source separated collection. Two other comparative reports warrant brief mention. De Baere4’ presented compost metal data from two source separation, one wet/dry, and one mixed waste collection. The source separated composts were roughly similar, and comparable to the values from a wet/dry collection which emphasized recyclables. For the critical element of lead, source separation averaged an 88% reduction over mixed waste compost values. Canarutto et a1.46used sequential extraction procedure to evaluate changes in metal availability over time from two MSW composts. One facility separated contaminants after 223 days of digestion in a rotating bioreactor, while the second facility used separation prior to 7 days in a horizontal bioreactor.
Both systems then provided a 60 day maturation period. While the first system had higher organic recovery levels and a more stabilized product, extractable metal levels were consistently higher than in the system with preseparation. The results of these comparative studies of the impacts of separation on MSW compost quality are consistent with the hierarchy presented in Table 2. The earlier that sorting occurs during the collection and cornposting process, the lower the heavy metal content in the finished compost. While these studies are not conclusive, in combination they present a strong case for source separation where high standards of compost quality are required. This thesis is further supported by data from operating plants, as collected and analyzed from an extensive review of literature sources. 5.
EFFECTS OF SEPARATION FACILITIES
AT OPERATING
A number of previous authors have developed summaries of contaminant data from existing plants. Such analyses are of necessity far from comprehensive, given the hundreds of facilities in operation across the world. O’Donnell et ~1.~’(summarized in Walker and 0’Donnel14*) confined their analysis to the United States, which has a limited number of facilities and very few which practice source separation. O’Donnell ef a1.47and Taylori both relied on data provided by facility operators, leaving open the opportunity for selective reporting. Epstein’o,49 compared mixed waste compost data from the United States with source separated compost data from Europe, making it difficult to isolate any differences due to waste stream characteristics from those resulting from separation strategies. Three studies proved particularly useful in developing the present database. Minnesota” collected a variety of literature data as well as samples from several facilities which they analyzed independently. Environment 0ntario5’ and GallardoLara and Nogales” each provided a moderate amount of data, citing several sources from a variety of countries. We have attempted to improve upon these previous assessments and hope to avoid some of the obvious flaws. One major challenge to this literature assessment is unavoidable-the quality of the data itself. Samples from a single facility may be subject to seasonal or spatial variability, neither
Heavy
metal contaminants
of which has been adequately addressed. Sources rarely document their sampling and analysis protocols or indicate whether standards were used or quality assurance/quality control mechanisms were in place. Differences in digestion technique can lead to significant differences in levels of “total metal” reported, as metals in compost tend to be very tightly bound to any remaining organic matter, and few digestions are absolutely complete. These issues can lead to serious discrepancies between results for total metal analysis from different labs testing the same compost. 53Nonetheless, with a large data set some of these errors may cancel each other out, and other potential problems will affect all categories of data, and thus may not interfere with our comparison of separation methods. 6. OPERATING
FACILITY
DATA:
METHODS
Our compilation of data is the result of a comprehensive search of the scientific literature, using both computerized data bases and informal networks. Nearly all of the references are available in scientific journals or published conference proceedings. In a few cases data from secondary sources were used, although every effort was made to obtain original sources where possible. We limited the search geographically to North America, Europe, and Japan, eliminating data from regions which are less industrialized and which therefore generate a dramatically different waste stream.” A complete listing of the data located through this search (with references) is provided in Appendix 1. All available tabular data are listed, but data originally presented graphically could not be used, since the exact values could not be determined. Where data from multiple samples were reported in an original reference, a calculated mean for that facility, process, or country has been listed to conserve space in the appendix, We used individual facilities as the primary unit of analysis. All sources were carefully reviewed to avoid presenting the same data from different sources. Some data sets were eliminated from the analysis, either to avoid possible overlap or because the specific facility was not identified. Anonymous data sets were particularly problematic, making it difficult to utilize several extensive national surveys. While these data may be useful, we felt that it was more conservative not to use them in order to avoid duplication of reporting, and lack of clarity as to how many samples were taken from each
in MSW composts
201
facility and also how mean values were calculated. Data from Japan were not included since there were data from only four facilities in that country. The data selected for analysis based on these criteria are marked with a Y in the appendix, while those we eliminated are marked with an N. The data were divided into categories based on location (North America and Europe), feedstock, and separation methodology. Feedstocks were categorized as MSW or MSW and sewage sludge. Some facilities may add sewage sludge only at certain times, and details of the feedstocks were not always reported. Thus a few of the feedstocks categorized as MSW may have contained some sludge. Separation methods were categorized as either source separation (indicating a positive sort for compostables by the waste generator), or central separation of a mixed waste stream. Source separation facilities which only process yard waste were not included, but those that include food wastes and possibly other organic materials were. Many of the facilities classified under central separation have source separation programs for cans, bottles, etc., but compostables and disposables are collected in a mixed stream. Wet/dry collection programs have been treated cautiously, and are listed separately and not used in the comparative analysis unless there was clear evidence whether the positive sort was for compostables or recyclables. Pilot studies with central separation technologies are listed separately and were eliminated from the analysis, since the conditions may not represent those of commercial facilities. Data from residential source separation pilot programs in North America were included because there were so few data from full scale facilities. In order to avoid bias due to extensive sampling at a single facility, the mean concentration of each metal at each facility was calculated. In cases where data were listed as less than a detection limit, that detection limit was used in calculating the mean. In one case a value reported as zero was eliminated, because no detection limit was reported. No attempt was made to adjust for different numbers of samples reported for a single compost, since it was not possible to determine whether multiple samples came from single or numerous piles. Finally, mean values were calculated for each of the different feedstocks and sorting methodologies. These means were calculated separately for North America and for Europe.
T. L. RICHARDand P. B. WOODBURY
202
Table 5. Metal contaminants in compost produced from centrally separated MSW mixed with sewage sludge in Europe and North America Cd
Cr
CU
Hg
Ni
Pb
Zn
5.4 f 0.8* 11.7 + 5.ot
lIO+ 16 159+72
376 + 68 248+71
5.8 + 1.9 2.4 + 1.6
71 rf: 11 170 If: 82
640 k 108 480 + 85
2318+939 1075 + 795
Continent Europe North America
*Mean and standard error (based on unequal variance t-tests used to compare regions, see text for details). tNo significant differences were found between the regions for any metal (p-value >O.lO, based on unequal-variance t-tests, see text for details). All data are expressed as pg g-’ on a dry weight basis.
independent samples t-tests were used to assess the effects of location (North America vs. Europe) and sorting method (source vs. centralized). All statistical analyses were conducted using SAS software (version 6.04, SAS Inc., Cary, NC). Unequal
variance
7. OPERATING
FACILITY DATA: RESULTS
One issue that has been raised but not resolved is whether MSW composts differ significantly in metal content from one region to another. Based on this literature survey, some useful comparisons can be made between MSW composts in Europe and in North America. For MSW composts that include sewage sludge, the mean value for cadmium, chromium, and nickel was higher in North America than in Europe (Table 5). However, due to extreme variability, none of these differences were statistically significant (p > 0.10). The high variability in composts which contain sewage sludge may be an indication of local rather than regional geographical variation. Sewage sludge contamination is strongly influenced by local industrial effluent, and individual facilities in close proximity may have very different metal levels.5s For MSW composts without sludge, regional trends were more apparent. With centralized separation the mean content of cadmium, chro-
mium, copper, zinc, mercury, lead, and nickel was higher in Europe than in North America (Table 6). However, these differences were statistically significant only for chromium (p = O.OOOl), lead (p = 0.0043), and nickel (p = 0.0458). For these three metals, the differences were substantial. For example, the mean lead content was 565 + 46 pg g-’ in Europe, but only 324 If: 55 pg gg’ in North America (Table 6). Interestingly, the lead content was similarly low for source separated composts from both regions (Table 6). Although not analyzed statistically, metal levels from the few facilities in Japan were close to and often lower than the North American means for both categories of separation. The differences in metal contents from different regions could be due to differences in central separation methodologies, or to differences in the waste streams, or both. The effects of feedstock and feedstock sorting strategy were also evaluated. In most cases, the metal content of MSW composts was highest when the feedstock included sewage sludge, and lowest with source separation. The values for centrally separated MSW composts were intermediate. For example, the mean lead content of composts made from MSW and sludge in Europe was 640 pg g-l (Table 5). Central separation of MSW without sludge resulted in a mean lead value of 565 pg g-’ (Table 6). Finally,
Table 6. Effect of different contaminant separation methods on the heavy metal content of MSW composts in Europe and North America Europe Sorting method Metal Cadmium Chromium Copper Mercury Nickel Lead Zinc
North America
Central
Source
Effect of sorting (p-value)
3.9 * 0.4* 117+ 18 354 + 53 2.6 + 0.3 63k 14 565 + 46 864k83
1.2kO.2 39+9 53+ 11 0.7 & 0.5 25 k 6 98+ 13 282 & 53
0.0001 0.0003 0.0001 0.0095 0.0093 0.0001 0.0001
Sorting method Central
Source
3.7 + 0.4 29 & 7 349 & 67 1.6kO.4 31 +6 324 + 55 771 f 141
1.1 * 0.2 15*5 64 F 20
1.o* 0.0 8+2 74 + 28 292 i 131
Effect of sorting (p-value) 0.0003 0.0696 0.0023 0.1630 0.0119 0.0015 0.0151
Effect of region ( p-value) Sorting method Central
Source
nst 0.0001 ns
O.onqsl9 ns
0.:58 0.0043 ns
oz95 ns ns
*Mean and standard error (based on unequal variance r-tests used to compare sorting methods, see text for details). tNo significant difference between regions (p-value > 0.10). All data are expressed as pg g-’ on a dry weight basis.
Heavy metal contaminants
source separation of MSW produced a lead content of only 98 pg g-’ (Table 6). A similar trend can be seen for North America. For MSW with sludge, the mean lead value was 480 pg g-’ (Table 5) while centrally-separated MSW resulted in 324 pg g- ’Pb and source separation resulted in only 74 pg g-’ Pb (Table 6). For other metals, similar trends can be seen, although the magnitude of differences is not as great. The effect of adding sludge will vary with the metal content of the specific sludge, and some sewage sludges can help reduce the levels of some metals (including lead) in a centrally separated MSW compost. Generally, the differences between MSW with and without sludge are not as great as the differences between centrally separated MSW and source separated MSW. Source separation resulted in lower levels of heavy metals both in Europe and North America. For cadmium, copper, and lead, these differences were highly significant (p < 0.01) in both regions (Table 6). For nickel and zinc these differences were highly significant for the European data, and significant (p < 0.05) for the North American data. For mercury, the difference was highly significant in Europe but not in North America. Finally, for chromium, these differences were highly significant for the European data, but only a trend was noted for the North American data (p = 0.0696, Table 6). Several important assumptions must be made if the results of our analysis are to be used to infer that source separation significantly reduces contaminant levels in MSW compost. We must assume that samples were taken from random locations within piles or windrows of finished compost, that the data are independent, and that the data are approximately normally distributed. We must also assume that no systematic biases occurred in chemical analyses, time of year of sampling, age of piles, etc. Although there is no way to directly verify these assumptions from the literature, there is no reason to assume that these assumptions were violated enough to invalidate the results. As discussed previously, we were conservative in selection of the data, in treating each facility equally, and in using unequal-variance r-tests to examine differences. These data from operating MSW cornposting facilities closely parallel the results reported in the comparative studies from the Netherlands and Germany (Tables 3 and 4). Facilities that compost a source separated MSW stream achieve significantly lower levels of metal con-
in MSW composts
203
tamination than those processing a mixed waste stream. This observation may be particularly important for lead, since the mean lead content in all categories of centrally separated composts exceeded the proposed NOAEL/APL standard (3OOpg gg’), while all categories of source separated composts averaged less than 100 lug g-’ (Tables 1 and 6). 8.
SUMMARY
AND IMPLICATIONS
The MSW composting industry in North America is achieving rapid growth in a changing regulatory environment. Some existing or proposed compost quality standards are difficult if not impossible to meet using traditional approaches, and compost users may demand even higher standards of quality than the regulators. Effective separation of contaminants is therefore likely to be increasingly important for successful marketing of MSW compost products. In this paper we evaluate a range of strategies for contaminant separation, ranging from a positive sort or source separation of compostables to various types of negative sorts of contaminants from a mixed waste stream. Evidence from both experimental separation trials and operating facilities provide similar results. The lowest levels of heavy metals were consistently achieved by source separation of compostables. Evidence regarding the effectiveness of contaminant source separation is limited because most recycling and HHW collection programs fail to specifically target sources of metals such as lead, but this approach may also have the potential for significant reductions. Once contaminants are mixed with compostables they are increasingly difficult to recover. Size reduction, abrasion, mixing and leaching disperse contaminants, and separation becomes less effective with time or intensive processing. While these results indicate that source separation significantly reduces metal contamination compared with current centralized separation approaches, it would be premature to conclude that centralized separation can never achieve similar levels. Reducing overall MSW contaminant levels and separating any remaining concentrated contaminants at the source (Options 1 and 3, Table 2) would narrow the differences between source and centralized separation of compostables. It is possible that source separation of a few contaminants such as lead-acid batteries and TV picture tubes could make an important difference.
T. L. RICHARD and P. B. WOODBURY
204
While reductions in heavy metals and other contaminants are possible in a centralized separation facility, there is at present essentially no information on the effectiveness or efficiency of specific separation processes, nor on the optimum way to combine unit processes in an integrated separation system. Research exploring the entire range of opportunities for separation of contaminants will be critical to the ultimate success of MSW composting. Acknowledgemenls-Many people assisted with the extensive literature search used in this paper and provided comments on earlier drafts. In particular, we would like to thank Vincent Breslin, Steven Crawford, Steven Ebbs, Harry Hoitink, Richard Kashmanian, and Mack Rugg. This research was partially supported by funds provided by Clark Engineers and Associates. This paper has not been formally reviewed by the aforementioned corporation and should not be construed to represent their policies
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APPENDIX 1 Literature Data from Cornposting Facilities Cd
Cr
Cu
Used MSW-CentraI Separation Austria (Siggerswiesen) Belgium (anonymous) Belgium (anonymous) Belgium (Cuesmes-18 smpl. mean) Belgium (Dendermonde-22 smpl. mean) Belgium (Gand-19 smpl. mean) Elelgium (Habaysmpl. mean) Belgium (Hoeselt-22 smpl. mean) Belgium (Tenneville-1 1 smpl. mean) Belgium (Tenneville and Habay) Belgium (Tenneville and Habay: 99104 smpls) France (61 smpl. mean) France France France France (Cotes-du-Nord, 2 smpl. mean) Great Britain (Stafford-Nusoil) Italy MSW (7 smpl. mean, 67 anonymous facilities) Italy (Cr is Cr III only) Italy (Cambiano, Turin) Italy (Perugia) Italy (Perugia) Italy (Piacenza) Italy (Rome) Italy (Udine) (3 smpl. mean) Germany (28-90 smpl. mean, 7&75) Germany (207 smpl. mean, 76-81) Germany Japan (Yokohama)
Y
Hg
Ni
Reference no.
2.00
45
125 89
3.00 2.95
3.90 3.83 3.60 6.33
67 152 148 153
205 203 367 210
N N
3.30 4.10
109 130
N
3.90 8.50 3.20 7.00 3.35
109 3 162 270 53
N N Y
7.00
350
3.70 4.55
Y N N
Y Y Y Y
Y Y N N N
N
Zn
ocg g-l) 500 178 87 154 143
N
Pb
67 4.80 9.00 2.50 3.70 3.10 4.20 3.55 3.70 5.50 3.20 2.30
120 380 109 67 126 108 64 71 48
59 38
250 491 375 724 595
830 1702 1100 1101 1081
56 57 57 58 58
2.15 4.33 2.50 4.53
44 39 62 38
750 828 795 910
994 1257 950 1352
58 58 58 58
3.20
48 48
216 760
1308 1090
59 58
89 190 42
447 599 527 600 735
1054 196 886 1000
31, 51 52 42, 51 26 4
610 198
140 55
800 476
1490 894
60 61, 62
400 800 150 160 329 475 146 266 274 173 189
68 320 38 27 79 33 25
656 420 530 411 511 324 298 229 513 463 173
558 600 427 282 693 964 694 1000 1570 902 760
63 64 31 65 66 31 67 68 69, 51 51 70
322 357 127 250
3.90 3.40 3.70 4.00 2.10
2.40 3.10 2.00 2.40 3.80 2.60
45 24 32
T. L. RICHARDand P. B. WOODBURY
208 Appendix I-Continued
Cd
Cr
Cu
Used Japan (Tokyo) (mean) Japan (11 smpl. mean) Japan (anonymous) Netherlands (Amersfoort) (wet/dry, negative sort) Netherlands (Groningen) (wet/dry, negative sort) Netherlands (Wijster, VAM) (1967-78) Netherlands (Wijster, VAM) (1971-79 mean) Netherlands (Wijster, VAM) (28 smpl. mean) Netherlands (Woerden) (wet/dry, negative sort) Netherlands Spain (anonymous site, vermicompost) Spain (Gava) Spain (Granada) (fines) (Cd, Cr, Pb, Ni suspect) Spain (Ma&) Spain (Puerto Real) (fines) (Cd, Cr, Pb, Ni suspect) Spain (Sevilla) (fines) (Cd, Cr, Pb, Ni suspect) Spain (Vilafranca) Switzerland (35-92 smpl. mean, 10 facilities) Switzerland (1979 avg.) Switzerland (anonymous) Switzerland (Kruchtal) Switzerland (SchatThausen) U.S.A. (Agripost, FL:22 smpl. mean) U.S.A. (Agripost, FL-6 smpl. mean) U.S.A. (Lake of the Woods, MN) U.S.A. (Pennington Co., MN-2 smpl. mean) U.S.A. (Lodi, WI) U.S.A. (St Cloud, MN-10-30 smpl. mean) U.S.A. (St Cloud, MN-1985) U.S.A. (St Cloud, MN-1986j U.S.A. (St Cloud. MN-11/89) U.S.A. (St Cloud; MN-5/90)’ U.S.A. (St Cloud, MN) U.S.A. (St Cloud, MN-agric. grade, 3 smpl. mean) U.S.A. (St Cloud, MN-hot?. grade, 3 smpl. mean) U.S.A. (St Cloud, MN-market grade, 3 smpl. mean) U.S.A. (St Cloud, MN-9/91, 10 smpl. mean) U.S.A. (Sumter Co., FL) U.S.A. (Sumter Co., FL) U.S.A. (Thief River Falls, MN-2 smpl. mean) U.S.A. (anonymous MSW) U.S.A. (anonymous MSW) U.S.A. (anonymous MSW) U.S.A. (anonvmous MSW) U.S.A. (anonymous MSW’and yard wastes) U.S.A. (anonymous MSW and yard wastes)
N N
Hg
400
720
680
460
13
110
900
1650
26
130
1280
2420
73
22
641
751
30
580
880
13
46 25
522 193
1020 7s
74 75
738 200
12 1
796 11
972 500
76 77
146 2
225 180
80 1
422 12
350 4000
76 77
0.04
2
200
1
9
700
77
4.00 11.00
63 179
145 715
36 90
442 1460
394 2200
76 78
90
36 58 21
780 720 90 1090 246
24 20 34
1570 1500 286 395 124
2330 2200 646 1430 607
26 79 50 50 47
54
464
2250
53
60
250
Y
1.60
70
130
2.00
N
6.00
220
630
5.00
Y
8.00
230
14s
4.00
Y
1.89
43
230
Y
2.50
95
270
N
2.10 3.00
50
101 430
6.00 0.09
77 2
3.00 0.09
N
Y N
Reference no. 792 676
Y
Y
Zn
119 238
150 1.83 0.50 1.10
Y
Pb
&g g-‘) 1.70 2.94 0.90 1.80
N
Ni
12.00 12.00 2.00 2.00 4.10
333 189
34 29
0.90
7.00 7.00
2.40
1080
70 71 72, 51 13
3.40 2.70
16 17
75 306
1.36
12 20
56 311
318 574
14 14
;::
55 45
419 197
0.92
29
548 263
1100 487
50 80
60
106 341 110 250 186 592
20 36 21 68
130 310 140 230 220 514
436 699 310 470 457 1010
50 50 47 47 14 53
2.00 4.00 1.30 3.00 2.71
E 44 29
0.68 1.20 0.99
Y
545
47
525
845
53
Y
441
31
348
614
53
Y
760
77
607
1190
53
27 38
290 370 390
580 870 580
47, 39 14, 39 50
Y Y
Y
5.00 1.80 3.50
22
250 300 333
1.67 2.49 1.01 1.76
