Source separate collection of recyclables reduces chromium and nickel content in municipal solid
waste compost
Rafael López1,*, Pilar Burgos1, Fernando Madrid1, and Ignacio Camuña2
1
Instituto de Recursos Naturales y Agrobiología de Sevilla, IRNAS-CSIC, Sevilla, Spain
2
EDIFESA, Avda. de la Innovación, edif. Convención, Sevilla, Spain. icamuna@edifema.com
Correspondence: R. López, Instituto de Recursos Naturales y Agrobiología de Sevilla, IRNAS-CSIC,
P.O. Box 1052, 41080-Sevilla, Spain
e-mail: rafael.lopez@csic.es
Running title: Separate collection reduces heavy metals in MSW compost
Abbreviations: GHG, greenhouse gas; MSW, municipal solid waste; MBT, mechanical biological
treatment; OM, organic matter; PCA, principal component analysis
Keywords: Collection strategies; Composting process; Dry recyclables, Heavy metals; Mechanical
biological treatment
Abstract
The composting process is a widespread option for municipal solid waste (MSW) treatment however the
low rate of separate waste collection leads to poor quality composts. The evolution of heavy metal content
in composts as a separate collection of dry recyclables became gradually implemented in the metropolitan
area of Seville city (1 million inhabitants, SW Spain) is hereby studied. During the last twelve years, Cr,
Ni, and Pb contents in compost were reduced by 60, 39 and 31%, respectively, whilst contents of Zn and
Cd increased by 20 and 108%. During the same period Cu remained unchanged. The metal content
reductions can be related to the separate collection of paper-cardboard, glass and package waste from
MSW, though materials separately collected were limited to 6.7% of raw MSW production. Extending the
source collection of recyclables to separate metallic components and performing slight changes in the
mechanical biological treatment would lead to additional reductions in other heavy metals whilst
implementing the separate recovery of the organic fraction.
Preprint submitted to Wiley
1
Introduction
In developed countries, municipal solid waste (MSW) represents an important percentage in waste
generation. In Spain, 23.6 million Mg of municipal solid wastes were collected during 2010; equivalent to
535 kg/person per year [1]. In the same year, Europe’s (EU-27) municipal waste generation was 219
million Mg; equivalent to 436 kg/person per year [2]. The huge amount of MSW generation is not only an
environmental threat, but also a cause of major social handicap throughout the world. Therefore, proper
management of MSW is of primary concern [3]. The best way to reduce the impact of MSW is to
minimize its production at its source, but despite efforts, success has been minimal. It is necessary to find
alternatives for the correct management of MSW. Biological treatments (aerobic composting and
anaerobic digestion) are the most environmentally acceptable options to treat putrescible residues because
both technologies maximize recycling and recovery of waste components [4-6]. In addition, composting
and mechanical biological treatment (MBT)-composting contributes very little to greenhouse gas (GHG)
emissions [7, 8], and emissions could be reduced by the introduction of gas recovery and increasing rates
of waste minimization and recycling [9]. In this way, composting is an environmentally clean process to
obtain a usable, secure and marketable product: compost.
Composts derived from MSW can generate income streams in the beginning (MSW disposal costs) and at
the end of the process (compost sales). There are also important results related to the final uses of the
compost; the compost is increasingly used because of its nutrient value as a low intensity fertilizer, its
ability to rebuild soil organic matter and improve soil physical properties as a soil conditioner, and also
for its capacity to suppress plant diseases [10-12].
During 2009, composting represented 18% of MSW treatment in Europe (EU27). During 2010, around
18% of municipal waste was treated by composting in Spain, and this percentage has been rising during
the last few years [1]. The number of composting facilities and the amount of source-separated and
composted MSW has been increasing in many countries of Europe and in the United States [3].
However, the agronomical properties of compost are dependent on several factors wherein pollutant
content becomes the most restrictive factor for its use. In this respect, the separate collection of specific
MSW components could affect final compost characteristics and two extreme categories depending on the
collection of MSW can be established: compost from source separated organic wastes or from mixed
MSW. Mixed MSW contains materials such as plastics, waste wood products preserved with chromated
copper arsenate and metals, which contributed several contaminants, mainly trace metals (heavy metals),
to MSW. On the contrary, compost prepared from source-separated organics, which is the intended way
for MSW collection in Europe, had lower contents of heavy metals [13, 14]. Between these edge
positions, a range of intermediate options in which some specific recyclables are source separated can be
found worldwide. In addition to well-known ‘point sources’ of heavy metals (e.g., batteries), other
materials such as paints, electronics, ceramics, plastics and inks/dyes can contribute to the heavy metal
burden of MSW. Overall, however, information on the sources, quantities and behavior of heavy metals
and other hazardous substances in MSW is lacking [15]. This lack of knowledge limits our understanding
of policy making on separation, collection and sustainable MSW management, and considerable effort
has thus been expended to better understand the heavy metal sources in MSW [16].
In Spain, the separate collection of dry recyclables (paper, cardboard, glass, and package waste) has been
increasingly adopted during the last years, while the rest of the generated MSW is collected on mass and
treated by mechanical separation at the beginning of the composting process.
This paper studies the evolution of MSW compost characteristics (mainly heavy metals) as separate
collection of dry recyclables was implemented to find general evolution trends. The study was carried out
during more than a decade in the metropolitan area of Seville city (1 million inhabitants, SW Spain).
Observed trends could help to implement adequate MSW collection strategies under different urban
conditions.
2 Materials and Methods
2.1
Waste generation data
Data about generation, composition and treatment, referring to MSW and its different fractions in
Andalusia region (SW Spain) were obtained from the environmental yearly reports elaborated by the
regional government [17]. When available, data corresponding to Seville province (~2 million
inhabitants) in addition to the entire region (~8 million inhabitants) were also used. The collection
strategy adopted in southern Spain (Andalusia Autonomous Community) and in the majority of Spanish
municipalities is based on source separate collection of dry recyclables (paper, glass, and packaging
waste) and the collection of the rest fraction (gray waste bin). Due to separate collection of the organic
components (food and kitchen waste) has not been implemented to date, the rest fraction including the
organic components contains a significant amount of impurities; The average composition of MSW in
Seville city is quite similar to that shown in Fig. 1 corresponding to Andalusia region. The organic
fraction in Seville MSW reaches 53%, a significant value but below the 60-67% indicated for an Asian
city [14].
2.2
Compost sampling
All compost samples considered in this study were collected in the composting plant ‘MontemartaCónica’ located in Alcalá de Guadaira, Seville, SW Spain. Compost samples were taken from the piles as
ready for use. At least six sampling points per pile were used to form a 2 kg composite sample. Except in
2005 when no samples were taken, 4 to 22 compost samples were taken each year in the period from
2000 to 2012, to a total of 168 samples. Independent samples were taken in the case that different particle
size composts were available.
2.3
Composting process
Montemarta-Cónica composting plant treats 1 500 Mg day-1 (450 000 Mg year-1) of MSW from ~1
million inhabitants living in 40 municipalities in the metropolitan area of Seville city (SW Spain). MSW
is treated by using MBT including the following steps: i) Preprocessing: bag opening in a rotary drum and
selection of mechanically sorted organic residuals (screen cut off of <11 cm); ii) First stage high rate
windrow composting (pile size: 40 m length, 10 m width, 4 m high) with intensive turning during two
weeks; iii) trommel screening to <3.5 cm and magnetic separation of metals; iv) maturation phase for at
least six months in static piles; v) compost final screening to <20 or <10 mm and glass separation by
using a gravity table.
2.4
Chemical analysis
Compost samples were analyzed following standard procedures for soil improvers and growing media of
the European Committee for Standardization. Samples were oven dried at 103°C and the moisture content
was determined. After careful mixing, the samples were divided by quartering. The selected portion was
sieved by 2 mm and impurities and stones in the coarser fraction were hand separated and weighed. The
two fractions (>2mm and <2 mm) were mixed again and the sample was grinded to pass a 0.5 mm sieve.
Organic matter (OM) was determined by dry combustion at 450°C [18] and N (organic-N + ammoniumN) was determined by distillation after Kjeldahl digestion. Total contents of mineral nutrients and trace
elements were determined after aqua regia digestion [19] in a microwave oven by ICP-OES (Thermo
Fisher Scientific, model IRIS Advantage). Contents of nutrients and heavy metals were reported on an
‘oven dry’ and ‘free of impurities’ matter basis. The pH and the electric conductivity were determined in
1:5 (weight) compost/water extracts [20].
Compost samples from the Wageningen Evaluating Programmes for Analytical Laboratories [21] were
also analyzed for quality control of analytical procedures. The obtained results for these samples agreed
±5% with the certified results.
2.5
Statistical analysis
Using the whole dataset, relations amongst variables were studied by using linear Pearson's coefficients
and factor analysis by using principal components (PCA) as extraction method and Quartimax rotation.
Factor analysis is a statistical technique that can be applied to a set of variables in order to reduce their
dimensionality. PCA has been widely used as an exploratory tool to identify major sources of
environmental pollutant emissions. The great advantage of using PCA is that there is no need for a prior
knowledge of emission inventories [22]. The Kaiser-Meyer-Olkin test was used as a measure of sampling
adequacy. All statistical analyses were carried out with SPSS 19 [23].
3. Results and Discussion
3.1
Compost characteristics
Average compost characteristics are shown in Table 1, considering the samples separated before and after
2006. Before this time, compost batches were subjected to final refining to separate stones, glass and light
plastics depending on the predicted type of use. In general terms, heavy metal contents of Seville
composts surpassed the usual contents found in source separated materials [13, 24]. They also were
higher than reported values in a recent paper corresponding to an Asian city using mixed organic and
inorganic MSW [14] although they were similar to that obtained in MBT composts from USA and Europe
[13, 24]. It is known that heavy metal content in compost are influenced by the quality of source
separation, the technological and socio-economic development and even local conditions (e.g. different
background soil heavy metal contents or agricultural practices)
In 2005, the Spanish national regulation concerning compost became stricter [25] and a new and more
efficient process for the refining of compost was used to treat all compost destined to agriculture. In
general terms, the compost showed adequate characteristics in the parameters related to organic fraction
composition and stabilization (OM, N, P, C/N, pH), giving evidence of the high organic matter content in
raw MSW (Fig. 1). Evidencing the lack of separate collection for the organic fraction, the current average
content for some metals surpass (Zn, Cd) or come close (Cu, Pb) to the non-restricted usage limits stated
in the 2005 Spanish regulation (Tab. 1). For this reason, some batches of compost have had to be applied
in reduced rates (<5 Mg Ha-1) or even discarded in landfill. The average contents of OM, N, Cu, Zn and
Pb during the period 2006-2012 were very similar to the average contents reported by Huerta Pujol et al.
[26] in the 63 analyzed samples taken from 36 facilities in Spain which treat mechanical-sorted organic
fraction of MSW. For the same period Cr and Ni contents were half of the average Spanish contents
indicated by Huerta Pujol et al. [26], although in the case of Cd, the average content in Seville compost
doubled the average Spanish value. During the period 2006-2014 average contents of Cu, Zn, Cd and Pb
in Seville composts surpassed the proposed European maximum limits [24] and only Cr and Ni achieved
them. Several researchers [27, 28] indicated that MBT plants show an inadequate separation of inert
waste in biodegradable (before biological treatment) and stabilized (final compost) fractions which
exhibit high levels of improper materials (like paper, plastics, glass fragments and batteries) and as a
consequence, Cu, Pb, Ni and Zn contents are quite high [27].
Supposedly as a result of compost de-stoning, several compost characteristics were different before and
after 2005. In addition to reduced glass content (Table 1), the average composition in the period 20062012 showed an increase in OM, OC, N and P contents. This enhancement of the compost organic
fraction after 2006 can hardly be explained as merely the concentration effect due to the released glass
and dense material during the de-stoning process. Metal content also changed after 2006. Contents of Cr
and Ni were noticeably reduced and Fe and Pb contents were reduced in a lesser extent. The content of
Cu remained relatively unaltered. On the contrary, Cd was markedly increased and Zn increased about
100 mg kg-1.
3.2
Metal contents relationships
Pearson’s coefficients among chemical parameters determined in compost are shown in Table 2.
Significant linear correlations among several chemical constituents of the compost are apparent from
Pearson’s coefficients. Highest coefficients can be observed for Cu which correlated with Mn, Fe, Zn and
Pb; Zn correlated with P, Cu and Cd; Cd conversely correlated with Cr and the glass content; Cr and Ni
were highly correlated among themselves and also they both correlated with the glass content. Singularly,
the significant correlations amongst the glass content and the contents of the metals Cd (inverse
relationship), Ni, Cr, Pb and Cu were surprising because glass particles are highly resistant to chemical
action and do not dissolve under the chemical extraction method (aqua regia) used for the chemical
analysis. To clarify the relationships controlling compost chemical properties, and particularly their metal
contents, compost dataset was subjected to factor analysis, obtaining the rotated component plot shown in
Fig. 2. The two first principal components obtained, component 1 and 2, explained respectively 22.4 and
21.4% of the data variance. Component 1 is characterized by the group of parameters glass-Cr-Ni, which
had an inverse effect to OM-Na-EC (organic matter and salinity). The clustering OM-Na-EC can be
related to food and kitchen waste. Their grouping is due to cooking salt use. On the other hand,
component 2 included more than one group of metals, showing that several effects or materials not
independent among themselves are responsible for the changes in compost composition. The clusters of
metals Fe-Mn-Pb-Cu, Mg-Ca and Zn-P can be observed in Fig. 2. These clusters could be related with
dust and soil (soils in the area contain great amounts of iron oxides and lime) but also with usual metallic
components in waste, for instance from cans. The elements Mn and Zn could likely be released from
spent batteries [29]. Studying the factors responsible for heavy metal content of air particulates, Karar et
al. [22] found a factor including Pb-Mn, which they assigned to vehicular traffic with the influence of
road dust. The pair Zn-P could derive from Zinc phosphate (Zn3(PO4)2), an inorganic chemical compound
used as a corrosion resistant coating on metal surfaces and metal food containers.
In the clustering glass-Cr-Ni, the relation between these metals and the glass content is not evident, as
stated previously. This clustering in component 1, as opposite to OM, should be related to the non-organic
materials in the MSW, consisting of package waste and other recyclable wastes, and not only to the glass
presence in compost. Recyclable materials subject to separate collection include glass, paper, cardboard,
and packaging waste. Starting in 1998, the separate collection of recyclables has been continuously
increasing in Andalusia to date (Fig. 3). The rising of the separate collection for the three considered
components has been parallel. In 2010, separate collection in Andalusia amounted to 6.7% of total MSW
production [17]. As can be seen in Fig. 3, the dropping of Cr (and also Ni) content was inverse to
selective collection rise. Chromium is a critical metal used in dozens of products that we rely on every
day. The most common application is in alloys, consuming 90% of virgin Cr. The addition of Cr and Ni
adds oxidation resistance to metals, making stainless steel. However, in the present study, these two
metals were not related with Fe, which is included in the component 2 of the factor analysis (Fig. 2).
Chromium was also found in colored newsprint and mixed paper, plastic film, textiles and footwear [30],
which are some of the materials subjected to separate MSW collection. Likely, the separate collection of
recyclables brought about the reduction in Cr and Ni contents.
Content decreases, but in a lesser extent, were also observed for the metals Pb and Cu (Table 1) clearly
associated to Fe (Fig. 2). The source separation, or the magnetic separation system used in the waste
treatment plant, for the metallic components of MSW, such as cans and metal caps, could have
contributed to the slight reduction in the contents of those metals.
Taking into account that paper, cardboard, glass, plastics and metals add up to 41% of MSW (Fig. 1) and
source separated recyclables were 6.7% of total MSW production in 2010, there is still some way to
obtain additional reductions in the metal contents by improving and extending source separate collection
of recyclables.
Otherwise, behavior of metals Zn and Cd differed from the previous ones, as they increased over time
(Table 1). These metals are highly ubiquitous; cadmium has been found in the particulate material of air
at residential sites, suggesting that its origin was not primarily from localized activity, but it has been
carried to soil particles by the action of wind from industrial emissions [22]. In general terms, the
solubility of both metal compounds were higher than that of the others. They were probably released from
a variety of non-separated metallic components contained in MSW during the initial acidophilic phase of
composting, and their content would be reduced by carrying out the separation of metallic components
before the start of the composting process. Particularly, these metals could be leached out from spent
household batteries [29] which were frequently found in MBT feedstock and compost in Spain [28].
Studying MSW from a nearby city, Rosal et al. [31] indicated that Zn, Cu, and Pb of the fractions of
particle size >50 mm seemed to be transferred to the fractions of particle size <50 mm during composting,
and they attributed this phenomenon to the formation of stable complexes between metal and humic
substances that appear during the composting process. A recent study [32] clearly demonstrated that
copper wires, galvanized nails and low quality alkaline batteries released significant levels of Cu, Zn, As,
Pb, and Co to the compost.
3.3
The current situation
In July 2011, a new regulation [33] made stricter the requirements for compost production and use,
setting as necessary a condition for the separate collection of the organic fraction of wastes. As a result,
most of the compost produced in Spain lacked legal status. In addition, intended EU regulation aimed to
reduce heavy metal upper limits [24]. Proposed EU limits (see Table 1) will be exceeded unless suitable
selective recovery strategies will be used.
In some circumstances it could be difficult for the municipalities to implement the necessary changes to
separately collect and treat the organic fraction. Now in Spain the practical result is that the destiny of a
lot of composts is the landfill, which leads to a loss of resources, and significant increases in landfill
costs. In a recent report about the treatment of MSW in Spain, Almasi and Milios [34] indicated that more
efforts are needed to achieve the targets on recycling and landfilling established by the European
directives. Due to the many sources of heavy metals within household waste, potentially passing through
mechanical screens designed to remove non-biodegradable components in MBT plants, a significant
reduction of heavy metal levels in stabilized waste can hardly be found [27] but extending the separate
collection to target MSW materials could help to reduce heavy metal contents.
According to compost use, the composts considered in this study have been usually spread on lime soils
with good agronomic results [35, 36]. Though the accumulation of some heavy metals was observed
when the compost was applied to greenhouse sandy soils for several consecutive seasons [35], in general
uses, only slight metal increases were detected in soils and plants. Phytotoxic or toxic levels were never
registered, even under heavy compost application rates [37, 38]. Even in different vegetables growing in a
dumping site, Karak et al. [39] found that accumulation of heavy metals did not exceed the recommended
maximum intake though they were a significant additional source in human diet.
Assuming that source separate collection of the MSW organic fraction will be the best way to obtain high
quality compost, this requirement should be adopted in a step-by-step process. Under some socioeconomic circumstances the required investment and public concern would slow down the process of
adapting current waste management and existing treatment facilities to a new legislation. Those particular
circumstances would prevail in many cities worldwide. Compost from MBT with similar quality of the
compost described in this research should be authorized in arable, lime soils, which are predominant in
Spain. This would avoid expensive extra costs in landfilling, and would extend their useful life.
4
Concluding remarks
The separate collection of dry-recyclables (paper-cardboard, glass, package waste) from MSW
implemented during the last twelve years, though limited to <7% of the total MSW production,
contributed to the reduction of certain heavy metals. The content of Cr, Ni, and Pb was reduced, while Zn
and Cd contents increased. Research is needed to find Zn and Cd sources. Extending the source collection
of recyclables to separate metallic components and doing slight changes in the mechanical biological
treatment would lead to obtain additional reductions in target heavy metals while implementing the
organic fraction separate recovery.
Prevailing socio-economic circumstances and required public concern would slow down the required
process to establish separate collection of the MSW organic fraction. Those particular circumstances
would prevail in many cities worldwide, and a step-by step process which progressively introduced dryrecyclables separate collection, careful MBT-composting process and adequate compost use could lead to
obtain a target compost quality.
Acknowledgements
This work was partially supported by the Operative Program FEDER and the Junta de Andalucía (PAIDIAGR108). The authors wish to thank Mr. Jerome Lock-Wah-Hoon for the English revision of the
manuscript.
The authors have declared no conflict of interest.
5
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Fig. 1. MSW composition in Andalusian region (Southern Spain) in the 2004 [17]
Fig. 2. Component plot in rotated space obtained from factor analysis applied to MSW compost samples.
Fig. 3. Evolution of Cr content in Sevilla MSW compost (line), and evolution in Andalusia (bars) of the
amount of source separated recyclables (glass, paper-cardboard, and packaging waste).
Table 1 Characterization of MSW compost from Seville treatment plant
Glass
2000-2004
2006-2012
2005
2013
Unita
Mean ± SDb
Mean ± SDc
Limitd
Limit e
g kg-1
148 ± 9
26 ± 6
6.74 ± 0.44
6.93 ± 0.53
pH
E.C.
f
dS m
7.93 ± 2.21
9.33 ± 1.89
OM
g
-1
g kg
421 ± 73
470 ± 99
g kg-1
244 ± 43
273 ± 58
12.7 ± 2.3
15.1 ± 2.0
19.8 ± 4.7
18.1 ± 3.6
OCh
TKN
i
-1
-1
g kg
C/N
P
gP2O5 kg-1
8.51 ± 1.61
9.74 ± 2.42
K
-1
6.37 ± 1.35
6.54 ± 1.38
-1
g K2O kg
Ca
g CaO kg
100.8 ± 20.0
98.7 ± 23.5
Mg
g MgO kg-1
Na
8.69 ± 2.89
8.73 ± 2.44
-1
5.38 ± 1.11
5.58 ± 1.73
-1
g kg
Fe
g kg
13.7 ± 4.3
11.1 ± 3.3
Cu
mg kg-1
288 ± 86
252 ± 75
Mn
mg kg-1
167 ± 26
145 ± 24
-1
<30j
<5i
>350
>150
<20
300
100
Zn
mg kg
465 ± 141
555 ± 151
500
400
Cd
mg kg-1
0.88 ± 0.52
1.83 ± 0.82
2
1.5
Cr
mg kg-1
111 ± 47
44.9 ± 34.2
250
100
Ni
-1
60.0 ± 18.2
36.5 ± 15.9
90
50
-1
216 ± 93
148 ± 40
150
120
Pb
mg kg
mg kg
a) Results expressed as total contents on over dry-free of impurities basis;
b) Standard deviation, n = 92;
c) Standard deviation, n = 76;
d) Limits for class B non-restricted use-compost, Spanish Royal decree 824/2005;
e) Proposed end-of-waste criteria in European Union;
f) E.C.: electrical conductivity;
g) OM: organic matter;
h) OC = organic carbon;
i) TKN, total Kjeldahl nitrogen (ammonium-N + organic-N);
j) Limit for impurities including glass, plastic and metal particles >2mm
Table 2 Pearson correlations between selected total metal contents and other constituents in Seville
compost samples (n = 166) analyzed during 2000 to 2012.
Cu
Zn
0.239**
-0.120
0.414**
0.444**
-0.513**
0.346**
0.035
0.177*
0.016
0.109
0.202*
-0.026
-0.101
-0.130
-0.197*
-0.249**
0.076
-0.208*
0.208**
0.338**
-0.458*
-0.334**
0.364**
0.091
P
0.386**
0.479**
-0.334**
-0.244**
0.410**
0.232**
Ca
0.431**
0.414**
-0.113
0.027
0.143
0.393**
Mn
0.510**
0.281**
0.345**
0.311**
-0.187*
0.532**
Fe
0.606**
0.430**
0.204**
0.294**
0.018
0.411**
Cu
1
0.527**
0.105
0.242**
0.060
0.656**
Zn
0.527**
1
-0.267**
-0.083
0.538**
0.267**
Cr
0.105
-0.267**
1
0.897**
-0.531**
0.262**
Ni
0.242**
-0.083
0.897**
1
-0.409**
0.307**
Cd
0.060
0.538**
-0.531**
-0.409**
1
-0.243**
Pb
0.656**
0.267**
0.262
0.307**
-0.243**
1
Glass
pH
OM
a
TKN
b
Cr
Ni
Cd
Pb
a) OM, organic matter;
b) TKN, total Kjeldahl nitrogen (ammonium-N + organic-N);
**Correlation is significant at the 0.01 level (2-tailed); *Correlation is significant at the 0.05 level (2tailed)
Fig. 1. MSW composition in Andalusian region (Southern Spain) in the year 2004 (data
from Yearly Reports on the Environment, Andalucía Regional Government).
Fig. 2. Component plot in rotated space obtained from Factor Analysis applied to MSW
compost samples.
Fig. 3. Evolution of Cr content in Sevilla MSW compost (line), and evolution in
Andalusia (bars) of the amount of source separated recyclables (glass, paper-cardboard,
and packaging waste).