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, Spain2 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 ‘Montemarta-

Cónica’ 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 + ammonium-

N) 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 2006-

2012 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 socio-

economic 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 dry-

recyclables 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 (PAIDI-

AGR108). 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.

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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

2000-2004 2006-2012 2005 2013

Unita Mean ± SDb Mean ± SDc Limitd Limit e

Glass g kg-1 148 ± 9 26 ± 6 <30j <5i

pH 6.74 ± 0.44 6.93 ± 0.53

E.C. f dS m-1 7.93 ± 2.21 9.33 ± 1.89

OM g g kg-1 421 ± 73 470 ± 99 >350 >150

OCh g kg-1 244 ± 43 273 ± 58

TKNi g kg-1 12.7 ± 2.3 15.1 ± 2.0

C/N 19.8 ± 4.7 18.1 ± 3.6 <20

P gP2O5 kg-1 8.51 ± 1.61 9.74 ± 2.42

K g K2O kg-1 6.37 ± 1.35 6.54 ± 1.38

Ca g CaO kg-1 100.8 ± 20.0 98.7 ± 23.5

Mg g MgO kg-1 8.69 ± 2.89 8.73 ± 2.44

Na g kg-1 5.38 ± 1.11 5.58 ± 1.73

Fe g kg-1 13.7 ± 4.3 11.1 ± 3.3

Cu mg kg-1 288 ± 86 252 ± 75 300 100

Mn mg kg-1 167 ± 26 145 ± 24

Zn mg kg-1 465 ± 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 mg kg-1 60.0 ± 18.2 36.5 ± 15.9 90 50

Pb mg kg-1 216 ± 93 148 ± 40 150 120

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 Cr Ni Cd Pb

Glass 0.239** -0.120 0.414** 0.444** -0.513** 0.346**

pH 0.035 0.177* 0.016 0.109 0.202* -0.026

OM a -0.101 -0.130 -0.197* -0.249** 0.076 -0.208*

TKN b 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

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 (2-

tailed)

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).