Heavy metals in source-separated compost and digestates

Waste Management 34 (2014) 867–874

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Heavy metals in source-separated compost and digestates Thomas Kupper a,⇑, Diane Bürge b, Hans Jörg Bachmann b, Sabine Güsewell a, Jochen Mayer b a b

School of Agricultural, Forest and Food Sciences, CH-3052 Zollikofen, Switzerland Agroscope, Institute for Sustainability Sciences ISS, CH-8046 Zürich, Switzerland

a r t i c l e

i n f o

Article history: Received 31 July 2013 Accepted 11 February 2014 Available online 7 March 2014 Keywords: Bio-waste Green waste Mass loads Anaerobic digestion Fertilizer Heavy metals

a b s t r a c t The production of compost and digestate from source-separated organic residues is well established in Europe. However, these products may be a source of pollutants when applied to soils. In order to assess this issue, composts, solid and liquid digestates from Switzerland were analyzed for heavy metals (Cd, Co, Cr, Cu, Ni, Pb and Zn) addressing factors which may influence the concentration levels: the treatment process, the composition, origin, particle size and impurity content of input materials, the season of input materials collection or the degree of organic matter degradation. Composts (n = 81) showed mean contents being at 60% or less of the legal threshold values. Solid digestates (n = 20) had 20–50% lower values for Cd, Co, Pb and Zn but similar values for Cr, Cu and Ni. Liquid digestates (n = 5) exhibited mean concentrations which were approximately twice the values measured in compost for most elements. Statistical analyses did not reveal clear relationships between influencing factors and heavy metal contents. This suggests that the contamination was rather driven by factors not addressed in the present study. According to mass balance calculations related to Switzerland, the annual loads to agricultural soils resulting from the application of compost and digestates ranged between 2% (Cd) and 22% (Pb) of total heavy metal loads. At regional scale, composts and digestates are therefore minor sources of pollution compared to manure (Co, Cu, Ni, Zn), mineral fertilizer (Cd, Cr) and aerial deposition (Pb). However, for individual fields, fertilization with compost or digestates results in higher heavy metal loads than application of equivalent nutrient inputs through manure or mineral fertilizer. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction Composting (i.e. aerobic degradation) and anaerobic digestion of source-separated organic residues represent well established processes for waste management in Europe. By 2009, 45  106 tons (t) of municipal waste were composted which corresponds to 18% of the total amount of municipal waste collected and a 2.4-fold increase since 1995 for the 27 EU countries (Blumenthal, 2011). This development was mainly the result of a diversion of biodegradable municipal waste from landfill in order to reduce emissions of greenhouse gases (European Community, 1999). The application of organic amendments such as compost and digestate to agricultural land has a number of beneficial effects (Diacono and Montemurro, 2010). The first one is the supply of plant nutrients. This is particularly important for phosphorus, whose global reserves may be depleted in 50–100 years (Cordell et al., 2009). Compost and digestate may contribute to the pool of soil organic carbon (Fortuna et al., 2003) and exert positive effects on soil ⇑ Corresponding author. Tel.: +41 31 910 21 17; fax: +41 31 910 22 99. E-mail address: [email protected] (T. Kupper). http://dx.doi.org/10.1016/j.wasman.2014.02.007 0956-053X/Ó 2014 Elsevier Ltd. All rights reserved.

physical properties (Giusquiani et al., 1995) as on soil microbial activity (Odlare et al., 2011). However, compost and digestate may contain pollutants such as heavy metals (Genevini et al., 1997) and organic pollutants (Brändli et al., 2005). This issue was often related to municipal solid waste as an input material. Accordingly, source separated collection of the organic fraction of household waste was promoted over the last years in order to minimize the contamination level of the resulting products (Hargreaves et al., 2008; Huerta-Pujol et al., 2011). Moreover, anaerobic digestion of organic residues (i.e. anaerobic treatment with or without a subsequent aerobic process) evolved in recent years in several European countries as a way to combine recycling of organic materials with the production of renewable energy (Körner et al., 2013). It therefore seems likely that the properties of input materials used currently differ significantly from those used in the past, and that the properties of the final products have also changed. Additionally, several studies demonstrated that the heavy metal burden of the atmospheric deposition decreased over the last years due to policies aiming at reducing pollution (BAFU, 2012; Cercasov and Wulfmeyer, 2008; Harmens et al., 2010; Heimburger et al., 2010; Torseth et al.,

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2012). It can therefore be expected that the heavy metal contents in compost and digestate have declined as well since it is likely that aerial deposition is closely linked with the contamination of the environment by heavy metals in general (Working Group on Effects, 2004). However, recent studies which document these changes are sparse. Heavy metal contents in compost and digestates are known to vary widely (Tavazzi et al., 2013). The comprehension of the reasons for this variability may contribute to the reduction of the contamination level. Several factors might be relevant. First, different types of input materials have been reported to differ in their contamination levels (Amlinger et al., 2004; Veeken and Hamelers, 2002). They are usually classified as green waste and bio-waste. Green waste denotes organic waste from private gardens and public green areas (Brändli et al., 2005). Bio-waste includes green waste and food and kitchen waste from households, restaurants, caterers and retail premises and comparable waste from food processing plants (European Union, 2008). Second, the origin of input material could be important because the aerial deposition of heavy metals, which may influence the contamination of input materials, is usually higher in urban environments than in rural regions (BAFU, 2012). The season of input material collection might play a role because aerial deposition of heavy metals tends to peak during the winter (Cercasov and Wulfmeyer, 2008). Other possibly relevant factors include the treatment process itself (composting or digestion), the grain size, which is determined by sieving the product before delivering (Paradelo et al., 2011; Veeken and Hamelers, 2002) and the amount of impurities (Amlinger et al., 2004; Brändli et al., 2005). The present study thus aims at (i) providing an overview on the heavy metal content of compost and digestates in Switzerland and comparing the measured data to literature and legislative values, (ii) assessing the factors that influence heavy metal concentrations in compost and digestate and (iii) determining heavy metal loads to soil by compost and digestate application relative to other input pathways. The present paper comprises analyses of heavy metals for most of the samples investigated in previous studies on organic pollutants (Brändli et al., 2007a,b). Therefore, the data presented complete the data on hazardous compounds in compost and digestate thus providing an integrated basis to balance potential detrimental impacts against beneficial effects of their application to soil.

2. Materials and methods 2.1. Experimental design 106 samples of compost, solid and liquid digestate were collected from 51 commercial composting and anaerobic digestion plants in Switzerland (total number of Swiss plants: ca. 300). Six facilities were anaerobic digestion plants using continuous dry fermentation operated at thermophilic conditions and two digestion plants processed liquid substrates under mesophilic process temperatures. The plants using continuous dry fermentation produced solid and liquid digestates. These two fractions resulted from solid–liquid separation of the fermenters output which was performed by means of a centrifuge. The remaining 43 facilities were composting units. All plants processed exclusively source separated bio-waste or green waste. The amounts of treated input material per plant were at least 100 t and up to some 10,000 t per year. The number of samples per plant was between one and four, taken at different seasons over the year. Large facilities were sampled more frequently than smaller ones. A thorough description of the samples and of the composting and digestion units is given in the supplementary information 1,

Table 1. The composts and digestates were used as fertilizers, horticultural substrates or soil conditioners, mostly in agriculture. The selection of sampling locations and sampling times accounted for the variables mentioned above: treatment process (anaerobic digestion/composting), composition of input materials (bio-waste, i.e. the bio-waste fraction containing food and kitchen waste from households, restaurants, caterers and retail premises; green waste, i.e. biodegradable waste from gardens and parks), origin of input material (urban and rural regions) and season of input material collection (spring, summer, autumn, winter). 34 samples containing bio-waste and 6 samples containing green waste included wastes of industrial origin (i.e. organic residues from food or paper processing). Their proportion was below 20% in 34 samples and ranged between 20% and 30% in 6 samples. 2.2. Sampling and analytical methods Samples were collected in 2004 and 2005 within the mandatory campaign related to the Chemical Risk Reduction Ordinance, ORRChem (Swiss Federal Council, 2013). Several subsamples were collected from a single lot of compost ready for use and combined according to the guidelines of FAC (1995). The samples transported to the laboratory comprised a volume of 60 L. The entire amount was thoroughly homogenized in a commercial concrete mixer and filled into a recipient. Subsamples were collected therefrom for subsequent analyses. More information on the sampling procedure is provided by Brändli et al. (2006). 2.3. Analytical methods Cadmium (Cd), cobalt (Co), chromium (Cr), copper (Cu), nickel (Ni), lead (Pb) and zinc (Zn) were analyzed as target elements. Additionally, for characterizing of the samples, the following parameters were determined: dry weight (dw), organic matter, pH, conductivity, dissolved organic carbon (DOC), humic substances, nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), iron (Fe), manganese (Mn) and sodium (Na). General analyses: dried (40 °C) and finely ground samples were used to determine the total carbon and the total nitrogen content with an elemental analyzer in duplicate (Euro EA, Hekatech, Wegberg, Germany). Organic matter was calculated by total carbon x 2. Inorganic carbon contents were small and not considered. Dry weight of compost samples was measured gravimetrically after drying 300 g fresh sample for 24 h at 105 °C (ART, 1998). For the salt content, 40 g fresh sample material (wet basis) was extracted in duplicate with 400 ml deionized water for 1 h at room temperature and filtered (0790½, Schleicher & Schuell, Dassel, Germany). Electrical conductivity was measured immediately after filtration. The data were expressed as mg KCl 100 g 1 fresh matter at 20 °C (BGK, 2006). The pH was determined in polyethylene bottles (1000 ml) filled with 400 ml deionized water in duplicate. Compost material was added up to a volume of 600 ml and extracted for 1 h. Immediately after extraction the pH was determined in the substrate-water suspension (Metrohm, Herisau, Switzerland) (ART, 1996). DOC was determined by water extraction at room temperature and membrane filtration. Samples were sieved (mesh <10 mm) immediately after sampling. A triplicate sample of 40 g fresh matter was extracted with 400 ml deionized water for 1 h and vacuum membrane filtered (Whatman, CA, 0.45 lm). Total organic C (TOC) in extracts was determined by infrared spectrometry after acidification of extracts and combustion at 850 °C (DIMA-TOC 100, Dimatec, Essen, Germany). Humic acid analysis was carried out according to Gerzabek et al. (1996) based on alkaline extraction with 0.1 M sodium pyrophosphate solution and precipitation with HCl (37%). Based on gravimetric determination, humic acid

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contents were calculated from photometrically measured optical densities and referred to organic dry weight. Extractable humic acids served as a parameter to describe progressing humification processes in composts. Elemental analysis: 2.5 g of dried and grinded (2 mm) material were calcinated at 450 °C. The ashes were taken up with 5 mL HCl (concentration 6 M) and filled up with water to 50 ml. The suspensions were filtrated and the 1:40 diluted extracts were measured by ICP-OES (inductively coupled plasma – optical emission spectroscopy). The instrument was the Vista Pro from Varian equipped with a concentric nebulizer, a cyclonic spray chamber and an axial plasma. The wavelengths used for the measurement are given in Table 1. Up to four subsamples of each compost sample were extracted and analyzed individually as described above. The reason for analyzing several subsamples was abnormal or peculiar concentrations found in preceding measurements. The extract from the first subsample was re-analyzed along with each of the other replicates to make sure that differences between subsamples were not due to analytical artefacts. The variability of the concentrations measured after the different extractions was low for most of the elements except for Cd for which only 19% of the samples exhibited a coefficient of variation (standard deviation relative to mean) of less than 10%. However, the concentration levels of Cd were rather low. For the other elements, the coefficients of variation of the results from the different extractions were less than 10% in most cases (range from 81% (Pb) up to 95% (Co) of the samples analyzed; more information is given in the supplementary information 1, Table 1). Samples with high concentrations showed low variations for all metals except for Cu and Pb. For these two elements, the variations at high concentration levels observed in the different extraction solutions were likely due to particulate contaminations in most cases. 2.4. Determination of mass loads to agricultural soils The mass loads of heavy metals to agricultural soils induced by application of compost, solid and liquid digestate can be compared to other input pathways. The most important ones related to the agricultural production are spreading of solid and liquid manure, mineral fertilizers, liming materials and pesticides. Aerial deposition has to be considered in addition. In order to determinate the mass loads two approaches were followed: (i) the total load to the entire Swiss agricultural area and (ii) the load per hectare following the application of a nutrients amount necessary to meet an average demand in nutrients for agricultural crops (140 kg N ha 1 y 1, 28 kg P ha 1 y 1, 116 kg K ha 1 y 1; Flisch et al., 2009). It has to be noted that P is the element determining the application rates for compost, digestates and manures. The

Table 1 Wavelengths used for the measurement of heavy metals and nutritive elements. Element Heavy metals Cd Co Cr Cu Ni Pb Zn Nutritive elements P K Ca Mg Fe Mn Na

Wavelength (nm) 214.439 228.615 205.560 224.700 216.555 182.143 202.548 177.434 766.491 315.887 279.079 238.204 257.610 589.592

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procedure and the basic data for calculation are given in the supplementary information 2.

2.5. Data processing and statistics The general description of the contamination level (Section 3.1) was based on all samples. For the analysis of possible influential factors (Section 3.2), 22 samples had to be excluded because they could not be characterized sufficiently with respect to one or more of these factors (see supplementary information 1, Table 1). A multiple regression model was used to relate the concentration of each individual element to the following factors: treatment process (anaerobic/aerobic), composition of input materials (green waste/ bio-waste and green waste), origin of input materials (urban/rural) and season of input material collection (spring/summer/autumn/ winter), the level of organic matter degradation (humic acids, DOC), the particle size of the samples (i.e. sieved to 2 cm or sieved to >2 cm particle size at the compost or digestion plant/unsieved material) and impurity content. The influence of each individual factor was also analyzed with a two-sample t-test or one-way ANOVA. Associations among elements were determined through cluster analysis (minimum variance algorithm) based on a Euclidean distance matrix calculated from log-transformed, standardized variables. Data were processed using the statistical software R, version 2.15.2 (R Core Team, 2012).

3. Results and discussion 3.1. Overview on concentrations An overview on the contents of heavy metals is given in Fig. 1 and in the (supplementary information 1, Tables 1 and 2). Mean values for compost (n = 81) ranged from 0.13 mg kg 1 dw (Cd) to 155 mg kg 1 dw (Zn). This corresponds to 60% or less of the threshold values according to the ORRChem (Swiss Federal Council, 2013). Cd, Co, Pb and Zn in solid digestates (n = 20) were lower by 20% to 50% compared to compost and values of Cr, Cu and Ni were similar, respectively (Fig. 1A). The lower concentrations can be explained by the level of organic matter which is higher for solid digestate (51% dw) compared to compost (40% dw). Probably, the separation of the liquid fraction might contribute to diminish the metal concentration of the solid digestate which is supported by liquid digestates showing contents that are considerably higher than the levels found in solid digestate for most heavy metals. This is in line with the results of Popovic et al. (2012) who found higher concentrations of Cu and Zn in the liquid fraction of separated pig slurry than in the solids. It can be explained by the higher partition of metals onto fine particles as was reflected by both a higher portion of Cu and Zn in particles <250 lm of the liquids and metal concentrations therein. In general, the variability of the element contents in the different products investigated was similar. The coefficient of variation (standard deviation relative to mean) exhibited numbers being lower than 50%, except for Cd (89%), Cu (73%) and Pb (203%) in compost and Cd (144%), Cu (54%) and Pb (149%) in solid digestate, respectively. Threshold values (Swiss Federal Council, 2013; supplementary information 1, Table 3) were exceeded by 12 samples which correspond to 11% of the total. In all of these samples, only one element was above the limit values: Cu in 5 samples (4 composts, 1 solid digestate), Ni in 4 samples (4 liquid digestates) and Pb in 3 samples Pb (2 composts, 1 solid digestate), respectively. The exceeding of threshold values seemed to occur at random. Four digestion plants and five composting units were concerned. Three plants exhibited two and six plants one incidence of exceedance. They were moder-

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Fig. 1. Heavy metal contents in compost, solid and liquid digestate in mg kg 1 dw (the y-axis is a logarithmic scale). The line within the box denotes the median, the box ranges from the lower to the upper quartile, whiskers extend up to the minimal and maximal values within 1.5 interquartile ranges of the box, and dots indicate outliers. All single analytical data are reported in the supplementary information 1, Table 1.

ate for Ni (up to a factor of 1.3) but considerable for Cu and Pb (exceedance by up to nine fold) in some cases. Saveyn and Eder (2014) provided an extensive overview on heavy metal concentrations in compost and digestate from analyses carried out over the last years (Supplementary information 1,

Table 4). The numbers for composts are comparable except for Cd where the present study exhibits lower values. Similarly, concentrations in compost reported by Dimambro et al. (2007), Riedel (2010) and Schmutz and Bono (2012) largely coincide with those measured in this study. Tavazzi et al. (2013) and Schmutz

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Table 2 Regression coefficients from multiple regressions relating log-transformed heavy metal concentrations to the influencing factors: treatment process (anaerobic/aerobic), composition of input materials (green waste/bio-waste and green waste), origin of input materials (urban/rural), the level of organic matter degradation (humic acids, DOC), the particle size of the samples (i.e. sieved to 2 cm or sieved to >2 cm particle size/unsieved material) and impurity content. Significant effects (p < 0.05) are indicated by bold font. The overall fit of the model (r-squared) is also given. Bold font indicates that the entire model explains a significant part of variation in heavy metal concentrations. Treatment process N P K Ca Mg Mn Na Fe Cd Co Cr Cu Ni Pb Zn

0.08 0.14 0.12 0.23 0.31 0.06 0.63 0.05 0.06 0.16 0.05 0.15 0.18 0.38 0.25

Composition of input materials

Origin of input materials

0.11 0.11 0.21 0.09 0.03 0.02 0.42 0.02 0.03 0.00 0.14 0.24 0.13 0.04 0.03

Humic acids

0.03 0.07 0.26 0.07 0.13 0.25 0.16 0.02 0.04 0.06 0.16 0.19 0.11 0.12 0.02

0.77 1.06 1.46 0.07 1.39 1.41 0.51 0.96 0.05 1.09 1.06 1.04 0.91 1.25 0.96

DOC 0.03 0.02 0.06 0.01 0.02 0.01 0.03 0.02 0.00 0.01 0.01 0.00 0.01 0.01 0.00

Particle size

Log impurities

0.02 0.12 0.04 0.08 0.02 0.03 0.10 0.15 0.06 0.08 0.13 0.06 0.16 0.25 0.15

Table 3 Mean heavy metal loads and the proportion of the total and the range (minimum, maximum) into agricultural soils in Switzerland (t y digestates and other inputs (manure, mineral fertilizer, liming materials, plant protection products, aerial deposition). Cd

Min

1

) due to application of compost and

Max

Co

Max

Cr

Max

Cu

0.03 0.02 0.00 0.01

2% 1% 0% 1%

0.01 0.00 0.00 0.01

– – – –

0.1 0.07 0.02 0.02

1.0 0.5 0.2 0.3

15% 8% 3% 4%

0.4 0.1 0.06 0.2

– – – –

1.5 0.8 0.3 0.3

5.7 2.5 1.2 2.0

17% 7% 4% 6%

2.3 0.3 0.5 1.6

– – – –

10 5.1 2.2 2.3

14 7.6 3.3 3.4

7% 4% 2% 2%

7.1 2.7 1.3 3.1

– – – –

56 42 10 4.2

Total other inputs Manure Mineral fertilizer Liming materials Plant protection products Aerial deposition

1.6 0.5 0.5 0.02

98% 31% 33% 1%

0.4 0.2 0.02 0.01

– – – –

10 5.5 2.6 0.2

5.3 3.3 0.1 0.6

85% 52% 2% 9%

2.5 1.2 0.1 0.5

– – – –

11 7.5 0.2 0.6

29 12 15 1.5

83% 34% 42% 4%

5.4 1.5 2.8 0.7

– – – –

138 94 40 2.5

0.5

33%

0.2

–

1.4

1.3

21%

0.7

–

2.7

1.1

3%

0.3

–

1.9

187 101 1.7 1.4 61 22

93% 50% 1% 1% 30% 11%

73 28 0.3 0.8 31 13

– – – – – –

507 359 12 3.3 92 41

Total

1.7

100%

0.4

–

10

6.3

100%

2.9

–

12

34

100%

7.7

–

147

202

100%

80

–

Min

Max

Pb

Min

0.37 0.38 0.48 0.19 0.27 0.33 0.57 0.33 0.33 0.33 0.30 0.11 0.32 0.20 0.38

Total compost and digestates Compost Solid digestate Liquid digestate

Ni

Min

R-squared of model

0.01 0.00 0.04 0.04 0.03 0.01 0.04 0.01 0.02 0.03 0.01 0.01 0.03 0.03 0.00

Min

Max

Zn

Min

Max

Min

564 Max

Total compost and digestates Compost Solid digestate Liquid digestate

4.4 2.0 0.8 1.5

14% 7% 3% 5%

1.8 0.4 0.3 1.1

– – – –

6.7 3.4 1.5 1.7

12 6.9 2.4 2.3

22% 13% 5% 5%

3.2 1.1 0.3 1.8

– – – –

149 129 17 3.1

38 19 7.4 11

6% 3% 1% 2%

22 9.1 3.7 9.6

– – – –

62 34 13 14

Total other inputs Manure Mineral fertilizer Liming materials Plant protection products Aerial deposition

27 17 2.1 0.7

86% 55% 7% 2%

12 7.9 0.2 0.5

– – – –

72 50 8.7 1.3

40 10 1.5 0.7

78% 20% 3% 1%

10 2.6 0.1 0.4

– – – –

154 86 15 1.5

603 461 16 3.9

94% 72% 2% 1%

277 227 0.9 2.7

– – – –

1491 1186 97 12

6.8

22%

3.5

–

13

28

53%

6.6

–

51

122

19%

47

–

196

Total

31

100%

14

–

79

52

100%

13

–

303

641

100%

300

–

1553

and Bono (2012) found lower contents in composts for Pb and higher values for Cd and Ni, respectively, compared to the present study. For digestates, a comparison of the present study and values reported by Riedel (2010), Saveyn and Eder (2014) and Tavazzi et al. (2013) is difficult. In some cases, data on digestates obtained from these studies included samples which contained manure or energy crops. This might explain a part of the discrepancies since manure or plant material originating from energy crops can differ from bio-waste with respect to heavy metal levels. It also has to be noted that the number of analytical data for digestates available from the literature is limited. 3.2. Factors influencing heavy metal contents The treatment process had no influence (Fig. 1A, see above), which is not surprising, since heavy metals are recalcitrant towards both the aerobic and the anaerobic degradation process.

To some extent, the factors ‘‘treatment process’’ and ‘‘composition of the input material’’ are correlated since anaerobic digestion plants preferentially process materials with low dry matter contents such as kitchen waste or lawn clippings which might cause odor problems at composting plants. The type of input materials possibly had a small influence on the heavy metal contents of composts and digestates. Products derived from green waste exhibited somewhat higher contents of Cd, Co, Ni, Pb and Zn than the ones based on bio-waste, while the opposite applied for Cr and Cu (Fig. 1B). However, the differences were statistically not significant. This is largely in line with data from the literature. Zethner et al. (2000) and Riedel (2010) did not find significant differences between composts derived from green waste and bio-waste, while Tavazzi et al. (2013) found slightly higher levels for most elements in bio-waste composts compared to green waste composts. Amlinger et al. (2004) concluded based on the evaluation of a series of studies that bio-waste composts tend to have higher concentrations of heavy metals than green waste composts.

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Fig. 2. Surface specific heavy metal loads (minimum, mean, maximum in g ha 1 y 1) following application of compost, digestates, manure and mineral fertilizer to meet average nutrient demands of crops (140 kg N ha 1 y 1, 28 kg P ha 1 y 1, 116 kg ha 1 y 1) and other inputs (liming materials, plant protection products, aerial deposition).

In contrast with the hypothesis, products from input materials collected in rural regions exhibited slightly higher levels of heavy metals (except for Cu) than those from urban areas (Fig. 1C), but the differences were statistically not significant. This seems to be supported by Zethner et al. (2000) who did not find a clear distinction between composts from urban and from rural regions except for Ni (higher level in rural regions). Some studies compiled by Amlinger et al. (2004) reported higher levels in urban composts. An important point to note is that differences between urban and rural regions in Switzerland are moderate due to the absence of major cities. Rural areas where bio-waste is collected separately are relatively densely populated owing to the overall small surface area and the vicinity of urban and rural regions. Samples collected in spring and/or summer had slightly higher concentrations for all metals except for Cr and Zn than autumn and winter samples (Fig. 1D). However, the differences were statistically not significant. These findings are mostly in agreement with data from the literature. Breuer et al. (1997) and Zethner et al. (2000) did not find correlations between compost contamination levels and the season of input material collection. Hanc et al. (2011) observed higher amounts of heavy metals in spring and

summer than in autumn and winter for bio-waste from gardens of family houses as well as from urban settlements. This might be due to a seasonal variation of feedstock composition (e.g. higher proportion of grass in summer and woody material in winter, respectively, for bio-waste from gardens of family houses). Materials sieved to less than 20 mm tended to have higher contents of heavy metals compared to unsieved materials (Fig. 1E). This is in line with the studies of Paradelo et al. (2011) and Veeken and Hamelers (2002) who showed that finer fractions in compost exhibited higher concentrations of heavy metals. However, the differences in the present study were statistically not significant. This might be due to the relatively large particle size of both sieved and non-sieved products (i.e. 620 mm versus >20 mm) as used in practice. The influence of impurities (average: 0.2% dw; supplementary information 1, Table 1) on heavy metal contents was negligible. This contrasts to the findings of Gronauer et al. (1997) who found increasing metal levels at elevated occurrences of impurities. The difference might be explained by much higher contents of impurities in that study, ranging between 1% and 10% w/w. When analyzed with multiple regression, all heavy metals (except for Cu) were significantly related to one or several factors, but the relevant factors differed for each metal. Moreover, the fraction of variation explained by the models (r2 = 0.11–0.38) was generally low (Table 2, last column). This indicates that heavy metal concentrations are not strongly related to the hypothesized influencing factors and mainly depend on other sources of variation than those considered here. This can be due to a relatively low environmental contamination level, as is reflected by bioindicators such as mosses (Harmens et al., 2010). Thus, traces of metals which end up coincidentally in the feedstock might significantly influence the content of the final product. The lack of clear relationships between influencing factors and the heavy metal content in compost and digestates combined with the apparent unsystematic occurrence of high metal levels suggests that the contamination of compost and digestates by heavy metals is mainly driven by impurities containing the metals, single lots of feedstock which are highly contaminated by particles or some hot spots in the input materials enriched with metals. This hypothesis is supported by the high variability in concentration (mainly Cu, Pb) found when analyzing several subsamples from the same samples (see Section 2.3). Further support might be given by the occurrence of two heavy metals groups found in the cluster analysis, whose concentrations tend to be positively correlated: (i) Cd, Cu, Pb, Zn (r = 0.18–0.67); (ii) Co, Cr, Ni (and Fe; r = 0.54–0.82). Cd is known to be associated with Zn in minerals (Fleischer et al., 1974). Fe, Cr and Ni and in some cases Co are used in alloys. The cluster could be a consequence of residues due to abrasion of materials which end up in the environment or occur at the plants during processing or transport operations.

3.3. Heavy metal loads to agricultural soils following compost and digestate application relative to other inputs The calculated total metal loads included in compost and digestates, manure, mineral fertilizers, plant protection products and aerial deposition onto agricultural soils are given in Table 3. For Co, Cu, Ni and Zn, manure was the main source (52%, 50%, 55% and 72% of the total for Co, Cu, Ni and Zn, respectively). Mineral fertilizer contributed the major part of the Cd and the Cr loads (33% and 42% of the total for Cd and Cr, respectively) and aerial deposition of the Pb load (53% of the total). The contribution to the total heavy metals input into agricultural soils of Switzerland associated to compost and digestates was between 2% (Cd) and 22% (Pb).

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An average application rate of compost, solid or liquid digestate induced metal loads between 0.5 g ha 1 y 1 (Cd) and 1371 g ha 1 y 1 (Zn). Apart from Cd loads originating from manure, mineral fertilizers and aerial deposition, and Cu loads from plant protection products, respectively, composts and digestates exhibited on average considerably higher metals loads into the soils than the other input pathways (Fig. 2). This is because of the higher nutrient to metal ratios of manures and mineral fertilizers compared to compost and digestates which can be explained by bio-waste consisting of materials which are poorer in nutritive elements than e.g. manures and by the metal accumulation during the treatment processes. However, the variability of the surface specific loads is large. Loads from manures can lie within the range of compost and digestates. Composts and digestates with particularly low contamination levels can be below the average values of manures. It has to be noted that parts of the calculated mass loads represent internal fluxes of farming systems (metals incorporated in roughages ingested by cattle and returned to agricultural soils via manures). Therefore, the mass loads as calculated here cannot be associated to an accumulation of heavy metals in soil. In addition, changes in soil composition were not considered in this approach (Moolenaar et al., 1997). Moreover, high mass loads do not necessarily imply adverse effects since matrices such as composts or digestates which induce a rise in soil pH or present many sorption sites (e.g. organic matter, clay), may immobilize heavy metals in the soil matrix and thus counteract toxic effects even if they contribute to heavy metal inputs (Kumpiene et al., 2008; Smolders et al., 2012). 4. Conclusions The heavy metal concentrations found in compost and digestates were mostly below threshold values in Switzerland (Swiss Federal Council, 2013) but occasionally much higher. The contamination level did not correlate with the treatment process, the composition and origin of input materials, the season of input materials collection, the particle size or the amount of impurities. The sporadic and apparently random occurrence of high metal contents suggests that these are due to impurities, individual contaminated lots or hot spots enriched with metals within input materials. Such factors are challenging to allocate. It seems advisable to further reduce contamination levels because high surface specific loads are induced by compost or digestate application compared to other fertilizers and amendments. However, it is difficult to propose solutions towards a reduction. Research activities should focus on the detection of highly contaminated lots and evaluate other factors influencing the metal levels than investigated in the present study. Since heavy metal inputs induced by application of compost and digestates do not necessarily correlate with adverse effects to the soil environment, it seems likely, however, that the various beneficial effects due to the agricultural utilization of these amendments outweigh potential risks related to heavy metals. Acknowledgements The Federal Office for the Environment and the Swiss Federal Office of Energy are acknowledged for financial support. We thank Konrad Schleiss (UMWEKO GmbH), Ena Smidt (University of Natural Resources and Life Sciences, Vienna), Petra Köppe (Agroscope, Zurich) and Rahel Brändli for their support to the present study. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.wasman.2014.02. 007.

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References Amlinger, F., Pollack, M., Favoino, E., 2004. Heavy metals and organic compounds from wastes used as organic fertilisers. Directorate-General for the Environment of the European Commission. (17.07.13). ART, 1996. pH-Wert im Volumenextrakt mit Wasser (H2OGH-pH). Reference methods of the Swiss Agroscope Reserach Stations. Agroscope ReckenholzTänikon Research Station ART, Zurich. ART, 1998. Bestimmung der Trockensubstanz und des Wassergehaltes in Düngerproben (D-TS). Reference methods of the Swiss Agroscope Reserach Stations. Agroscope Reckenholz-Tänikon Research Station ART, Zurich. BAFU. 2012. Nabel, Luftbelastung 2011. Messresultate des Nationalen Beobachtungsnetzes für Luftfremdstoffe (NABEL). Umwelt-Zustand Nr. 1118. Bundesamt für Umwelt (BAFU). BGK, 2006. C 2 Salzgehalt, In: Kehres, B. (Ed.), Methodenbuch zur Analyse organischer Düngemittel, Bodenverbesserungsmittel und Substrate 5. Auflage. Bundesgütegemeinschaft Kompost (BGK). Blumenthal, K., 2011. Generation and treatment of municipal waste. Statistics in focus, 31/11. (10.08.12). Brändli, R.C., Bucheli, T.D., Kupper, T., Furrer, G., Stadelmann, F.X., Tarradellas, J., 2005. Persistent organic pollutants in source-separated compost and its input materials – a review of field studies. J. Environ. Qual. 34, 735–760. Brändli, R.C., Bucheli, T.D., Kupper, T., Furrer, R., Stahel, W., Stadelmann, F.X., Tarradellas, J., 2007a. Organic pollutants in Swiss compost and digestate. 1. Polychlorinated biphenyls, polycyclic aromatic hydrocarbons and molecular markers, determinant processes, and source apportionment. J. Environ. Monit. 9, 456–464. Brändli, R.C., Bucheli, T.D., Kupper, T., Stadelmann, F.X., Tarradellas, J., 2006. Optimized accelerated solvent extraction of PCBs and PAHs from compost. Int. J. Environ. Anal. Chem. 86, 505–525. Brändli, R.C., Kupper, T., Bucheli, T.D., Zennegg, M., Huber, S., Ortelli, D., Müller, J., Schaffner, C., Iozza, S., Schmid, P., Berger, U., Edder, P., Oehme, M., Stadelmann, F.X., Tarradellas, J., 2007b. Organic pollutants in Swiss compost and digestate; 2. Polychlorinated Dibenzo-p-dioxins, and -furans, dioxin-like polychlorinated biphenyls, brominated flame retardants, perfluorinated alkyl substances, pesticides, and other compounds. J. Environ. Monit. 9, 465–472. Breuer, J., Drescher, G., Schenkel, H., Schwadorf, K., 1997. Hohe Kompostqualität ist möglich. Räumliche und zeitliche Variabilität von Kompostinhaltsstoffen. Begleituntersuchungen zum Kompostierungserlass des Landes BadenWürttemberg. Ministerium für Umwelt und Verkehr, Stuttgart, Germany. Cercasov, V., Wulfmeyer, V., 2008. Trends in airborne particulates in Stuttgart, Germany: 1972–2005. Environ. Pollut. 152, 304–313. Cordell, D., Drangert, J.O., White, S., 2009. The story of phosphorus: global food security and food for thought. Global Environ. Change 19, 292–305. Diacono, M., Montemurro, F., 2010. Long-term effects of organic amendments on soil fertility. A review. Agron. Sustain. Dev. 30, 401–422. Dimambro, M.E., Lillywhite, R.D., Rahn, C.R., 2007. The physical, chemical and microbial characteristics of biodegradable municipal waste derived composts. Compost Sci. Util. 15, 243–252. European Union, 2008. Directive 2008/98/EC of the European Parliament and the Council of 19 November 2008 on Waste and Repealing Certain Directives. Official Journal of the European Union, 22/11/2008. (22.01.13). European Community, 1999. Council Directive 1999/31/EC of 26 April 1999 on the landfill of waste. (22.01.13). FAC, 1995. Kompost und Klärschlamm, Weisungen und Empfehlungen der Eidg. Forschungsanstalt für Agrikulturchemie und Umwelthygiene (FAC) im Bereich der Abfalldünger, (EDMZ-Art. Nr. 730.920.d). EDMZ, 3000 Bern. Fleischer, M., Sarofim, A.F., Fassett, D.W., Hammond, P., Shacklette, H.T., Nisbet, I.C., Epstein, S., 1974. Environmental impact of cadmium: a review by the Panel on Hazardous Trace Substances. Environ. Health Perspect. 7, 253–323. Flisch, R., Sinaj, S., Charles, R., Richner, W., 2009. Grundlagen für die Düngung im Acker- und Futterbau – Kapitel 3–6. Agrarforschung 16, 6–31. Fortuna, A., Harwood, R., Kizilkaya, K., Paul, E.A., 2003. Optimizing nutrient availability and potential carbon sequestration. Soil Biol. Biochem. 35, 1005– 1013. Genevini, P.L., Adani, F., Borio, D., Tambone, F., 1997. Heavy metal content in selected European commercial composts. Compost Sci. Util. 5, 31–39. Gerzabek, M.H., Danneberg, O.H., Kandeler, E., 1996. Humification. In: Schinner, F., Öhlinger, R., Kandeler, E., Margesin, M. (Eds.), Methods in Soil Biology. Springer, Berlin, pp. 116–119. Giusquiani, P.L., Pagliai, M., Gigliotti, G., Businelli, D., Benetti, A., 1995. Urban waste compost: effects on physical, chemical and biochemical soil properties. J. Environ. Qual. 24, 175–182. Gronauer, A., Claassen, N., Ebertseder, T., Fischer, P., Gutser, R., Helm, M., Popp, L., Schön, H., 1997. Bioabfallkompostierung, Verfahren und Verwertung; Bayerisches Landesamt für Umweltschutz; Schriftenreihe Heft 139; München. Hanc, A., Novak, P., Dvorak, M., Habart, J., Svehla, P., 2011. Composition and parameters of household bio-waste in four seasons. Waste Manage. 31, 1450– 1460. Hargreaves, J.C., Adl, M.S., Warman, P.R., 2008. A review of the use of composted municipal solid waste in agriculture. Agric. Ecosyst. Environ. 123, 1–14.

874

T. Kupper et al. / Waste Management 34 (2014) 867–874

Harmens, H., Norris, D.A., Steinnes, E., Kubin, E., Piispanen, J., Alber, R., Aleksiayenak, Y., Blum, O., Coskun, M., Dam, M., De Temmerman, L., Fernandez, J.A., Frolova, M., Frontasyeva, M., Gonzalez-Miqueo, L., Grodzinska, K., Jeran, Z., Korzekwa, S., Krmar, M., Kvietkusr, K., Leblond, S., Liiv, S., Magnusson, S.H., Mankovska, B., Pesch, R., Ruehling, A., Santamaria, J.M., Schroder, W., Spiric, Z., Suchara, I., Thoni, L., Urumov, V., Yurukova, L., Zechmeister, H.G., 2010. Mosses as biomonitors of atmospheric heavy metal deposition: spatial patterns and temporal trends in Europe. Environ. Pollut. 158, 3144–3156. Heimburger, L.E., Migon, C., Dufour, A., Chiffoleau, J.F., Cossa, D., 2010. Trace metal concentrations in the North-western Mediterranean atmospheric aerosol between 1986 and 2008: Seasonal patterns and decadal trends. Sci. Total Environ. 408, 2629–2638. Huerta-Pujol, O., Gallart, M., Soliva, M., Martinez-Farre, F.X., Lopez, M., 2011. Effect of collection system on mineral content of biowaste. Resour. Conserv. Recycl. 55, 1095–1099. Körner, I., Amon, B., Amon, T., Balsari, P., Bioteau, T., Dach, J., De Buisonje, F., Deipser, A., Fabbri, C., Kupper, T., Malico, I., Marques, I.P., Mata, J., Roshani, E., Schnurer, A., Scholwin, F., Soldano, M., Verdoes, N., Ward, A., 2013. A survey system to present the status of anaerobic digestion in Europe in different sectors – Agriculture, industrial, urban waste and waste water sector. In: RAMIRAN 2013 –Recycling of Organic Residues for Agriculture: From Waste Management to Ecosystem Services, 15th-International Conference, Versailles, France. Kumpiene, J., Lagerkvist, A., Maurice, C., 2008. Stabilization of As, Cr, Cu, Pb and Zn in soil using amendments – a review. Waste Manage. 28, 215–225. Moolenaar, S.W., Lexmond, T.M., van der Zee, S.E.A.T.M., 1997. Calculating heavy metal accumulation in soil: a comparison of methods illustrated by a case-study on compost application. Agric. Ecosyst. Environ. 66, 71–82. Odlare, M., Arthurson, V., Pell, M., Svensson, K., Nehrenheim, E., Abubaker, J., 2011. Land application of organic waste – effects on the soil ecosystem. Appl. Energy 88, 2210–2218. Paradelo, R., Villada, A., Devesa-Rey, R., Moldes, A.B., Dominguez, M., Patino, J., Barral, M.T., 2011. Distribution and availability of trace elements in municipal solid waste composts. J. Environ. Monit. 13, 201–211. Popovic, O., Hjorth, M., Jensen, L.S., 2012. Phosphorus, copper and zinc in solid and liquid fractions from full-scale and laboratory-separated pig slurry. Environ. Technol. 33, 2119–2131. R Core Team, 2012. R: A Language and Environment for Statistical Computing. R Foundation for Statistical Computing, Vienna, Austria, ISBN 3-900051-07-0, (15.01.14).

Riedel, H., 2010. Verwertung biogener Abfälle: Rückstände und Schadstoffgehalte. Augsburg: Bayerisches Landesamt für Umwelt, Augsburg. (22.05.13). Saveyn, H., Eder, P., 2014. End-of-waste criteria for biodegradable waste subjected to biological treatment (compost & digestate): Technical proposals. Final Report. December 2013. European Commission, Joint Research Centre, Institute for Prospective Technological Studies, Seville (Spain). (14.01.14). Schmutz, D., Bono, R., 2012. Interkantonale Marktkontrolle Dünger 2011 Teil Recyclingdünger (Kompost, Gärgut) im Kt. Basel-Landschaft, Qualitätskontrolle. Amt für Umweltschutz und Energie, Kanton. Basel-Landschaft, CH-Liestal. Smolders, E., Oorts, K., Lombi, E., Schoeters, I., Ma, Y.B., Zrna, S., McLaughlin, M.J., 2012. The availability of copper in soils historically amended with sewage sludge, manure, and compost. J. Environ. Qual. 41, 506–514. Swiss Federal Council. 2013. Ordinance of 18 May 2005 on the reduction of risks relating to the use of certain particularly dangerous substances, preparations and articles, status as at 3 January 2013 (Chemical Risk Reduction Ordinance, ORRChem). Swiss Federal Council, Berne. Tavazzi, S., Locoro, G., Comero, S., Sobiecka, E., Loos, R., Eder, P., Saveyn, H., Blaha, L., Benisek, M., Gans, O., Ghiani, M., Umlauf, G., Mariani, G., Suurkuusk, G., Paracchini, B., Cristache, C., Fissiaux, I., Alonso-Ruiz, A., Gawlik, B.M., 2013. Occurrence and levels of selected compounds in European compost samples. Results of a Pan European Screening exercise FATE-COMES. European Commission Joint Research Centre, Institute for Environment and Sustainability, Ispra, Italy. (14.01.14). Torseth, K., Aas, W., Breivik, K., Fjaeraa, A.M., Fiebig, M., Hjellbrekke, A.G., Myhre, C.L., Solberg, S., Yttri, K.E., 2012. Introduction to the European monitoring and evaluation programme (EMEP) and observed atmospheric composition change during 1972–2009. Atmos. Chem. Phys. 12, 5447–5481. Veeken, A., Hamelers, B., 2002. Sources of Cd, Cu, Pb and Zn in biowaste. Sci. Total Environ. 300, 87–98. Working Group on Effects, 2004. Review and assessment of air pollution effects and their recorded trends. Working Group on Effects, Convention on Long-range Transboundary Air Pollution. Natural Environment Research Council, UK. (15.01.14). Zethner, G., Götz, B., Amlinger, F., 2000. Qualität von Kompost aus der getrennten Sammlung. Monographien; Band 133. Federal Environment Agency – Austria, Wien.