E

E
ENVIRONMENTAL ASSESSMENT OF
MUNICIPAL SOLID WASTE INCINERATOR
BOTTOM ASH IN ROAD CONSTRUCTIONS
Susanna Olsson
September 2005
TRITA-LWR.LIC 2030
ISSN 1650-8629
ISRN KTH/LWR/LIC 2030-SE
ISBN 91-7178-151-X
Susanna Olsson
TRITA-LWR.LIC 2030
ii
Environmental assessment of municipal solid waste incinerator bottom ash in road constructions
A CKNOWLEDG EMENT
This thesis has been conducted at the Department of Land and Water Resources Engineering at
the Royal Institute of Technology in Stockholm. The project was financed by FORMAS (Swedish
Research Council for Environment, Agricultural Sciences and Spatial Planning), J Gust Richert
Foundation and Värmeforsk.
I wish to thank Jon Petter Gustafsson, my main supervisor, for guidance, constructive comments
and encouragement. Thanks also to my co-supervisors Erik Kärrman, for advices and inspiring
discussions on environmental systems analyses, and Dan Berggren Kleja, for guidance and support during the chemical part of this work (and for giving me my very first insight into soil chemistry many years ago). I would also like to thank the department of soil science, SLU, for good
cooperation, for letting me use their facilities and for helping me in different ways with my experiments. Further, I am grateful to my reference groups, for comments on my work and for
providing information and contacts, and to Ecoloop for fruitful cooperation.
Several other people, including researchers, representatives from companies or authorities,
friends and family, have supported me in different ways throughout this work. Although I do not
name them all, their contribution has been very valuable and they are all gratefully acknowledged.
Thanks also to my colleagues at the Department of Land and Water Resources Engineering for
creating such a nice and enjoyable atmosphere.
Susanna Olsson
Stockholm, September 2005
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Susanna Olsson
TRITA-LWR.LIC 2030
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Environmental assessment of municipal solid waste incinerator bottom ash in road constructions
A BSTRACT
There are several incentives for using bottom ash from municipal solid waste incineration (MSWI
bottom ash) as a construction material, such as for road construction. These incentives include
decreased disposal of material on landfills and a reduced amount of raw material extracted for
road building purposes. However, one of the main obstacles to utilising the material is uncertainties regarding its environmental properties. The overall objective of this thesis is to describe the
potential environmental impacts of utilising MSWI bottom ash in constructions and to improve
the tools for environmental assessments.
An environmental systems analysis (ESA) approach based on a life cycle perspective was outlined
and used in a case study, with the aim of describing the differences in resource use and emissions
that can be expected if crushed rock in the sub-base of a road in the Stockholm region in Sweden
were to be substituted by MSWI bottom ash. The whole life cycle of the road was taken into
account and the alternative disposal of the bottom ash was included. It was found that the studied alternatives would cause different types of potential environmental impact; whereas the conventional alternative with only crushed rock in the road’s sub-base would lead to larger use of
energy and natural resources, the alternative with MSWI bottom ash in the sub-base would lead
to larger contaminant leaching. It was concluded that a life cycle approach is needed in order to
include both resource use and emissions in the comparison between the two alternative scenarios. The leaching of metals turned out to be the most important environmental aspect for the
comparison and in particular the difference in copper (Cu) leaching was shown to be large.
However, a large amount of Cu may not pose an environmental threat if the Cu is strongly
bound to dissolved organic carbon (DOC). In order to improve the basis for toxicity estimates
and environmental risk assessments, and thereby provide better input values for ESAs, the speciation of Cu to DOC in MSWI bottom ash leachate was studied. It was found that Cu to a large
extent was bound to DOC, which is consistent with previous research. The results also suggest
that the hydrophilic fraction of the MSWI bottom ash DOC is important for Cu complexation
and that the pH-dependence for Cu complexation to MSWI bottom ash DOC is smaller than for
natural DOC. This implies that models calibrated for natural DOC may give inconsistent simulations of Cu-DOC complexation in MSWI bottom ash leachate.
Keywords: bottom ash; environmental impact; LCA; waste management; leaching; copper; dissolved organic carbon
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Susanna Olsson
TRITA-LWR.LIC 2030
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Environmental assessment of municipal solid waste incinerator bottom ash in road constructions
T ABLE OF C ONTENT
ACKNOWLEDGEMENT .............................................................................................................................. III
ABSTRACT........................................................................................................................................................ V
INTRODUCTION............................................................................................................................................. 1
Objectives .......................................................................................................................................................... 2
METHODS.........................................................................................................................................................3
Literature review: Environmental assessment on different system levels.............................................................. 3
Environmental systems analysis .......................................................................................................................... 4
Copper speciation in MSWI bottom ash leachate and in soil solution.................................................................. 5
LITERATURE REVIEW: ENVIRONMENTAL ASSESSMENT..................................................................6
Environmental assessment on the material level.................................................................................................. 6
Characterisation of MSWI bottom ash ..................................................................................................................................... 7
Methods for characterisation...................................................................................................................................................... 8
Chemical mechanisms of trace metal leaching ............................................................................................................................. 9
Improvement of environmental properties.................................................................................................................................. 10
Environmental assessment concerning the material in its surroundings............................................................. 10
Environmental aspects considered ............................................................................................................................................ 10
Framework for risk assessments and critical limit definition ..................................................................................................... 11
Environmental assessment on an LCA level ..................................................................................................... 12
Environmental assessment on an industrial system level ................................................................................... 12
Information demand and supply for decision situations .................................................................................... 13
A LIFE CYCLE PERSPECTIVE ON THE USE OF MSWI BOTTOM ASH ............................................. 15
SPECIATION OF COPPER ........................................................................................................................... 15
DISCUSSION ................................................................................................................................................... 16
System levels used in this thesis ........................................................................................................................ 16
Applicability and interpretation of results on the LCA level .............................................................................. 17
Possibilities for improvement of environmental assessments ........................................................................... 18
CONCLUSIONS .............................................................................................................................................. 20
FURTHER RESEARCH ................................................................................................................................. 20
REFERENCES ................................................................................................................................................ 21
L IST OF P APERS
I. Olsson, S., Kärrman, E. & Gustafsson, J. P. Environmental systems analysis of the use of
bottom ash from incineration of municipal waste for road construction. Submitted June
2004 to Journal of Resources Conservation and Recycling.
II. Olsson, S., van Schaik, J., Gustafsson, J.P., Berggren Kleja, D. & van Hees, P. Speciation of
Copper in MSWI bottom ash and soil leachates -the role of hydrophobic and hydrophilic
compounds. Manuscript.
L IST OF APPENDICES
Appendix 1: Produktion av aska från storskalig förbränning av fasta bränslen i Stockholms län.
Appendix 2: Input data for environmental systems analysis.
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Susanna Olsson
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Environmental assessment of municipal solid waste incinerator bottom ash in road constructions
gravel for this purpose was around 4.75 million tonnes during year 2002. The annual
production of MSWI bottom ash and industrial waste incineration bottom ash in the
Stockholm region in 2002 was almost 50 000
tonnes dry substance, constituting the major
ash type produced in the region (Appendix
1), (Fig. 1). In the same year, the total production of MSWI bottom ash in Sweden was
up to 400 000 tonnes (Bjurström 2002).
So far, bottom ash has not been used for
road construction to any great extent in Sweden, even though previous studies have
shown that it is technically suitable for this
purpose (Hartlén et al. 1999, Vägverket 1999,
Arm 2003). One of the main obstacles to the
use of recycled materials in constructions in
Sweden is that the laws and regulations are
ambiguous (Kärrman et al. 2004). Because
the environmental risks from using MSWI
bottom ash in constructions are not yet fully
understood, the precautionary principle that
is practised in Sweden often makes the authorities restrictive to the utilisation of the
material. There is also a lack of national established norms regarding the materials that
can be used in a particular construction,
which often causes decisions to be unpredictable (Wilhelmsson et al. 2003). This
situation may be a result of the uncertainties
that still remain regarding the potential environmental impact from the utilisation. A
further obstacle is the lack of methods for
showing the benefits of recycling (Kärrman
et al. 2004).
MSWI bottom ash is a highly heterogeneous
material, predominantly composed of oxides
I NTRODUCTION
Today about 70 million tonnes of waste are
produced annually in Sweden. Disposal of
waste is resource-consuming. In addition, the
space at existing landfills is limited and it is
hard to find new suitable areas for waste
disposal. To stimulate the reuse of wastes,
society has therefore issued a variety of directives (fees on deposited waste, ban on the
disposal of certain types of waste, etc.). This
means that there are strong incentives to find
ways to utilise the wastes. Waste from households, municipal solid waste, is increasingly
being combusted for heat extraction. Previous research has shown that in Sweden it is
generally environmentally advantageous to
combust this kind of waste instead of disposing of it at landfills (Björklund 2000, Sundqvist et al. 2002). However, incineration of
waste results in the formation of other waste
products, such as different ash types. Management of this material in a sustainable way
is now an important issue.
The possibility of utilising municipal solid
waste incineration (MSWI) bottom ash for
construction purposes, such as for road construction, is a topic that has been thoroughly
discussed during the last ten years. The utilisation of waste material for construction
purposes is motivated by reduced management costs, insufficient local raw material
supply, inadequate local disposal capacity and
a desire to preserve natural resources
(Hartlén 1996). There is great demand for
road building material in Sweden. In the
Stockholm region alone, the use of rock and
Ash production
(tonnes of dry substance per year)
60000
50000
Bottom ash
40000
Fly ash
30000
20000
10000
0
Coal
Municipal solid
waste
Industrial
waste
Wood
products
1
Paper industry
waste
Figure 1. Production of
the most common ash
types (>10 000 tonnes/year) from different
fuels in Stockholm
county, 2002.
Susanna Olsson
TRITA-LWR.LIC 2030
and aluminosilicate minerals of Fe, Na, K
and Ca (Abbas et al. 2001, RVF 2002). The
material also contains a relatively large
amount of heavy metals that may be released
during utilisation (Chandler et al. 1997). To
assess environmental risks, leaching tests are
often used to calculate the release of contaminants and extensive research has been
carried out on this topic. However, there are
still some uncertainties remaining that limit
the possibilities for risk assessments. There
is, for example, an ongoing discussion on
whether the total concentration of a substance in the leachate, or only its toxic forms,
should be considered. This is an important
issue concerning copper (Cu), the toxicity of
which to aquatic organisms is reduced by
complex formation with dissolved organic
carbon (DOC) (Zitko et al. 1973, Sunda &
Guillard 1976, Borgmann 1983, Pagenkopf
1983). It has been found that more than 95%
of the dissolved Cu in MSWI bottom ash
leachate is bound to DOC (Meima et al.
1999). In order to decrease the uncertainties
regarding the potential environmental impact
from MSWI bottom ash utilisation, the effects of DOC on the leaching and toxicity of
metal contaminants need to be further investigated.
However, improved assessments of the risks
and effects from leaching are not sufficient,
since these assessments fail to consider other
types of environmental impact, such as resource use and pressures relevant on a regional or global scale (Roth & Eklund 2003,
Roth 2005). While much emphasis has been
placed on risks for contaminant leaching, less
attention has been paid to these types of
impacts, which should be equally important
according to the Swedish Code of Statues
(1998) for strategic decisions on MSWI bottom ash management. The utilisation of
MSWI bottom ash in constructions would,
for example, enable both decreased disposal
of bottom ash and reduced amounts of natural aggregates extracted for road building
purposes. If those aspects are neglected there
is a risk that non-optimal solutions are chosen. To enable the inclusion of impacts other
than leaching, such as resource use and emissions to air, the assessment would have to
expand and a life cycle perspective may be
useful.
Objectives
The overall objective of this thesis was to
describe the potential environmental impacts
of utilising MSWI bottom ash in road constructions and to improve the tools for environmental assessments. Specific aims were
to:
•
Investigate ways to perform environmental assessments of the use of MSWI
bottom ash in constructions and identify
topics on which more knowledge is
needed.
•
Develop an approach, based on a life
cycle perspective, for environmental assessment of the use of MSWI bottom ash
in road constructions.
•
Describe the differences in resource use
and emissions to air and water that can
be expected from utilising MSWI bottom
ash in a road construction compared to a
conventional material.
•
Investigate the metal binding properties
of the dissolved organic carbon in MSWI
bottom ash leachate.
•
Improve the basis for leaching and toxicity estimations of selected metals.
A literature review was performed in order to
organise previous research on the environmental impacts from using MSWI bottom
ash in constructions, to gain an increased
understanding of how such environmental
assessments may be performed and to identify data gaps. The potential environmental
impact from using MSWI bottom ash instead
of crushed rock in a road construction was
then studied (Paper I). An approach based on
a life cycle perspective was developed for this
purpose in order to include a broader perspective than that commonly used on the
utilisation of MSWI bottom ash. Not only
contaminant leaching, but also resource use
and emissions to air were included. Based on
the results from that study and from the
literature review, leaching of Cu and dissolved organic carbon (DOC) from MSWI
2
Environmental assessment of municipal solid waste incinerator bottom ash in road constructions
bottom ash was chosen as the focus for a
second study (Paper II).
the narrow life cycle level includes environmental pressures from the life cycle of the
materials, the industrial system level adds a
cross-sectoral dimension and addresses the
generation and substitution of material in the
whole industrial system. Furthermore, the
authors argue that the different systems perspectives are appropriate for addressing different questions and they suggest that environmental assessment of by-products in road
constructions should be complemented by
using wider system boundaries than only the
currently used leaching tests.
In this literature review, these system levels
were used as a basis for the organisation of
previous scientific work. All scientific papers
concerning the environmental aspects of
using MSWI bottom ash in constructions
were sought in different databases, resulting
in a great number of published works. As a
complement, some reports that were not
scientifically published concerning Swedish
conditions were also included. The focus was
only on the environmental aspects of using
the material in constructions, since the technical aspects of different applications have
been discussed elsewhere (Izquierdo et al.
2002, Arm 2003). The aim was to analyse the
previous studies on the environmental im-
M ETHODS
Literature review: Environmental assessment on different system levels
To utilise bottom ash from municipal solid
waste incineration in constructions, it has to
be proven that the material is technically and
environmentally suitable for this purpose.
How to define and evaluate the environmental impact is therefore an important
issue. Environmental assessment of using
MSWI bottom ash in road constructions can
be carreid out in several ways. Different types
of environmental impact may be considered,
and those may be estimated by separate
methods. There have been several previous
studies in this area of research. They have
addressed a wide range of aspects and provide extensive information on the environmental impact associated with the utilisation
of the material.
By defining different levels of systems perspective on which environmental assessment
of by-products in road constructions can be
made, Roth and Eklund (2003) provide a
useful basis for organising and interpreting
the outcome of those previous studies. They
describe four different levels on which environmental assessment of reuse of by-products in road constructions can be made and
some of the tools that can be used (Fig. 2).
These levels are:
1. Material
2. Road environment
3. Narrow life cycle level
4. Industrial system level
The material level addresses the properties
and content of the material and can be studied by leaching tests. Studies on the road
environment level deal with the material in its
spatial context and for this purpose substance
flow analysis can be used. The narrow life
cycle level (hereafter referred to as the LCAlevel) and the industrial system level require
that the system boundaries be expanded to
include more parts in the construction’s life
cycle than only the operations stage. While
MSWI bottom ash
MSWI bottom ash used
in a road construction
The life cycle of the road in which
the MSWI bottom ash is used
The life cycle of the road in its
surrounding industrial system
Figure 2. Different system levels on which
environmental assessment of the reuse of
by-products such as MSWI bottom ash in
constructions can be made (adapted from
Roth 2005).
3
Susanna Olsson
TRITA-LWR.LIC 2030
pacts from the use of MSWI bottom ash in
constructions in terms of the systems perspective used in the study, the type of environmental impact considered and the information gained. This also included a discussion of whether the studies reviewed provide
sufficient information for environmental
assessment of using MSWI bottom ash in
constructions to be made for decision purposes.
or to determine the parts of the life cycle that
are most critical to the system’s overall environmental impact.
Based on the concepts and methodology
used in LCA, an ESA approach was developed to study the differences between two
alternative scenarios where crushed rock
(Alternative 1) or MSWI bottom ash (Alternative 2) was used as sub-base material in a
road. The approach included the definition of
system boundaries and production of a conceptual model of the system, an inventory of
flows (energy, material and emissions, hereafter referred to as aspects) to, from and within
the system and, finally, processing of this
information and an assessment of the system’s environmental impact.
The definition of system boundaries is in
many ways a crucial part of an LCA, since it
may affect the results to a large extent
(Tillman et al. 1993). In this study, all life
cycle stages significant for the chosen aspects
of those system components that would be
different in the different alternatives were
included. Aspects were chosen based on
available data and on their potential to affect
any of the impact categories described by
SETAC-Europe (1999). The life cycles of
products used in the system were limited to
include the production of fuel and electricity
from raw material (Fig. 3). The system boundaries were also expanded to include the
effects of less landfilling, implying that the
disposal of MSWI bottom ash was consid-
Environmental systems analysis
In order to study the potential environmental
impacts of using MSWI bottom ash instead
of crushed rock in a road construction, an
environmental systems analysis (ESA) based
on a life cycle perspective was performed
(Paper I). The aim was to quantitatively include not only the emissions to water but
also emissions to air and the potential positive effects, such as saved resources. It was
assumed that the resource use and the emissions would occur at different stages of the
road’s life cycle (Mäkelä & Höynälä 2000).
Hence, a life cycle perspective would be
needed to include them in a comparison.
Life cycle analysis (LCA) is a common tool
for evaluating the environmental burdens
from the total life cycle of a product, ‘from
cradle to grave’, i.e. from the extraction of
basic resources, through production and
transportation, to use and disposal of the
product (ISO 1997). LCA is often used to
compare products with equivalent functions
Extraction and
crushing of rock
Sorting and sieving
of bottom ash
Fuel
Raw
material
Electricity
Raw
material
Loading material
Transport of
material
Use and
maintenance
SO2
NOx
CO2
CO
HC
CH4
VOC
N2O
Particles
COD
N-tot
Oil
Phenol
As
Cd
Cr
Cu
Ni
Pb
Zn
Disposal of bottom
ash at landfill
Figure 3. Resources and emissions investigated for the life cycle stages of the road studied
(Paper I).
4
Environmental assessment of municipal solid waste incinerator bottom ash in road constructions
ered in case the ash was not used in road
construction. Thus, the functional unit, to
which all aspects were related, consisted of
both the road and the management of a
certain amount of MSWI bottom ash. The
demolition stage of the life cycle was excluded from the analysis due to lack of data
and the assumption that there would be no
significant demolition differences between
the alternatives. To obtain the relative magnitude of each aspect studied, the result was
normalised by relating each substance or
energy flow to the same kind of flow on a
national basis. The outlined approach was
used in a case study, where information from
the test road Törringevägen (Hartlén et al.
1999, Arm 2003) in southern Sweden was
applied on a theoretical road, with given
dimensions, in the Stockholm region. The
technical performance of the materials was
assumed to be the same. Case-specific parameters and assumptions can be found in
Paper I and in Appendix 2.
Hydrophobic acids
NaOH
XAD-8
Cation exchanged MSWI bottom ash leachate
Hydrophilic acids
and neutrals
Cation
exchange
Anion
exchange
Cation
exchange
Hydrophilic acids,
neutrals and bases
Hydrophilic neutrals
Figure 4. Fractionation of DOC in MSWI
bottom ash leachate based on Leenheer
(1981).
umn containing Amberlite® XAD-8 and
exposing it to cation and anion exchange, the
hydrophobic acids and neutrals and the hydrophilic acids, bases and neutrals were estimated (Fig. 4). The hydrophobic acids, which
are retained in the XAD-8 column and then
recovered by using 0.01 M NaOH are often
referred to as ‘fulvic acids’ (FA) in literature
(Ritchie & Purdue 2003). A simplified fractionation procedure, where the hydrophilic
components were isolated from the hydrophobic components by the Amberlite®
XAD-8 containing column, was used to
produce subsamples for the titration experiments.
Titrations were made on subsamples of the
MSWI bottom ash leachate and the soil solution in order to measure the Cu2+ activity at
different pH values. The Cu2+ activity was
measured potentiometrically using a Cu-ion
selective electrode. Prior to titration, the
subsamples were exposed to different treatments (Table 1) in order to adjust pH or the
Cu to DOC ratio. The copper titrations were
performed with NaOH on 40 ml sample
solution that was constantly stirred and
purged with N2 (g) (Fig. 5). After each addition of base, the pH and the Cu2+ concentration in solution were measured. The amount
Copper speciation in MSWI bottom ash
leachate and in soil solution
In order to understand the speciation of Cu
in MSWI bottom ash leachate and the contribution from DOC to Cu leaching, the
DOC in a MSWI bottom ash leachate was
characterised and the Cu-binding properties
of its hydrophilic and hydrophobic components were studied (Paper II). As a reference,
the Cu-binding properties of the hydrophilic
and hydrophobic components in a soil leachate were also studied. MSWI bottom ash
leachate (BA1) was produced by leaching of
stored (>6 months) and sorted bottom ash
(<10 mm) with deionised H2O for 24 hours
in an end-over-end rotator (L/S 5). Soil solutions (S1 and S2) were collected from lysimeters in an untreated Oa horizon from a Haplic Podzol. After filtration through a 0.2 μm
membrane, pH, cations, anions, DOC and
low molecular weight organic acids
(LMWOA) were measured in the MSWI
bottom ash leachate and in the soil solution.
The MSWI bottom ash leachate was fractionated according to Leenheer (1981) in order to
determine its content of different DOC fractions. By passing the leachate through a col5
Susanna Olsson
TRITA-LWR.LIC 2030
Table 1. Subsamples for titration. Additions were made in order to adjust pH or
the Cu to DOC ratio.
Sample
BA1-a
BA1-b
BA1-c
Column
treatment
Untreated
Cation column
L ITER ATURE REVIEW :
E NVIRONMENTAL ASSESSMENT
OF MSWI BOTTOM ASH IN ROAD
Additions
CONSTRUCTIONS
HCl
Environmental assessment on the material level
Studies on the material level were by far the
most common among the studies found.
Typically, on this level a specific application
is not considered and therefore this category
includes studies of the properties of MSWI
bottom ash that are related to its environmental performance, regardless of the intended use (though different types of mixed
materials with other major components than
bottom ash have not been considered).
Leaching was found to be the main focus
when studying the environmental impact
from MSWI bottom ash utilisation on the
material level. Commonly, the authors refer
to the national policies as an argument for
studying leaching properties of the ash, or to
leaching as an important factor in the assessment of the potential hazards associated
with the use of the material in constructions.
The studies found on the material level may
be divided into four subgroups based on
their main focus: Material characterisation;
methods for material characterisation; chemical mechanisms of trace metal leaching; and
innovations to make the material more suitable for a certain purpose. Besides these,
NaOH, Cu(NO3) 2
NaOH, Cu(NO3) 2
S1-a
Cation coumn,
XAD-8 colmn
Untreated
S2-c
XAD-8 coumn
HNO3, Cu(NO3) 2
HNO3, Cu(NO3) 2
of complex-bound Cu(II) was estimated as
the difference between total amount and the
calculated concentration of Cu2+ ions (as obtained from the measured Cu2+ activity).
The results from the titrations were compared with speciation calculations made using
Visual MINTEQ (http://www.lwr.kth.se/
English/OurSoftware/vminteq/index.htm),
which uses submodels for organic matter
such as NICA-Donnan (Kinniburgh et al.
1999, Milne et al. 2003) and Stockholm Humic (SHM) models (Gustafsson 2001). When
applying the models, all inorganic and
LMWOA complexes as well as fulvic acid
were considered. It was initially assumed that
65% of the DOC was active, consisting of
fulvic acid, while the remaining 35% was
inert with respect to proton and metal binding, as found for natural DOC by Bryan et al.
(2002).
pH
Cu-selective
electrode
electrode
Burette
NaOH
Cu2+ activity
pH
Sample
6
Figure 5. Titration
experiment setup for
the measurement of
Cu-DOC complexes
at different pH values.
Environmental assessment of municipal solid waste incinerator bottom ash in road constructions
often small, so they are considered to be of
minor importance (Wiles 1996, RVF 2002).
The leaching potential for Cu, Pb, Cd, Zn
and Mo has been most frequently investigated.
Bottom ash contains relatively high amounts
of trace metals, especially Cu, compared to
natural aggregates such as sand or crushed
rock (Table 2). Such metals may pose an
environmental threat if they are leached into
the surroundings. The metal content in the
bottom ash may be controlled by the incineration temperature, since a higher temperature results in increased transfer of many
metals into the gaseous phase (Belevi &
Langmeier 2000). For most of the trace metals, however, there is not necessarily a correlation between the total content and the
leached amounts or between the composition
of the incinerated waste and the leaching
from the bottom ash (van der Sloot et al.
2001). There may also be numerous other
factors, together with the total content of a
certain metal, determining its leachability in a
simultaneous and complex manner as has
been reported for lead (Jeong et al. 2005). To
make predictions of potential leaching from
different management alternatives it is therefore essential to understand the controlling
mechanisms for the leaching from the ash.
This information can then be used for
mathematical extrapolations in coupled mass
there are also numerous papers investigating
chemical reactions that are of a general
character and thus not directly linked to
MSWI bottom ash even though they might
be useful when discussing leaching mechanisms of the ash. Some of the major findings
on MSWI bottom ash that are relevant for
environmental assessment of the material are
described below.
Characterisation of MSWI bottom ash
The composition of MSWI bottom ash has
been reported for many different incineration
plants and countries (Hjelmar 1996, Wiles
1996, Chandler et al. 1997, Chimenos et al.
1999, Izquierdo et al. 2002, Forteza et al.
2004) showing that the bottom ash is a heterogenous material with a great variation in
content and leaching properties. This variation is due to different waste materials but
also different types of incineration processes.
Information on Swedish MSWI bottom ash
can be found in some doctoral theses
(Fällman 1997, Johansson 2003) and in other
publications such as RVF (2002) and Bjurström et al. (2004). Generally the main emphasis regarding the bottom ash has been on
trace metals rather than organic pollutants,
even though the organic content in ash has
been studied (Brunner et al. 1987, Dugenest
et al. 1999, Johansson et al. 2000, Johansson
2003, Kim & Osako 2004). The reason may
be that the amounts of organic pollutants are
Table 2. Data on average and maximum leaching from Swedish MSWI bottom ash, average
leaching from fresh crushed rock (Gabbro-diorite) and maximum leaching from crushed bedrock, moraine and gravel. All measurements are made at L/S 2, and the result is given in μg/l.
Background concentration for lakes in southern Sweden is reported as a reference.
Metal
1
2
3
Average
Maximum crushed
Average MSWI Maximum MSWI
bottom ash
crushed rock bedrock, moraine and Lake background
bottom ash
concentration3
leachate1
leachate 2
gravel leachate 2
leachate1
Cd
1.1
8.0
0.1
0.50
0.016
Cr
9.0
45
0.5
3.4
0.2
Cu
1212
6000
5.5
9.1
0.5
Ni
18
40
2.7
40
0.4
Pb
4.5
30
0.3
1.0
0.24
Zn
34
195
3.3
16
2.0
(RVF 2002)
(Tossavainen & Håkansson 1999)
(Naturvårdsverket 1999)
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Susanna Olsson
TRITA-LWR.LIC 2030
1. Basic characterisation test (several
parameters are determined, considering both short-term and longterm leaching properties of the material)
2. Compliance test (focusing on some
identified key parameters)
3. On-site verification test
In Sweden, environmental assessments of
construction materials are often based on
total analyses (the material is dissolved in
strong chemicals or melted), batch tests (extraction test), or column tests (dynamic test)
(Carling & Hjalmarsson 1998, RVF 2002,
Wadstein et al. 2002).
Extraction tests typically involve mixing a
sample with a specific amount of leaching
solution, which is not renewed during the
test. The mixing is performed with the aim of
reaching equilibrium conditions and is followed by filtration and analysis of the
leachate. Examples of extraction tests are the
NT ENVIR 003, SS-EN 12457 (1-4) and
prEN 14429 (Table 3). The availability test
NT ENVIR 003 determines the amounts of
different substances that are available for
leaching in a long time perspective (several
thousands of years). The SS-EN 12457 (1-4)
and the pH dependent test prEN 14429, on
the other hand, aim to describe leaching
during a shorter time perspective and therefore lower L/S ratio and coarser particles are
used. In dynamic tests, some aspects of
transport models for long-term leaching
predictions.
Methods for characterisation
By using specifically designed or standardised
leaching tests, extensive information can be
obtained on ash properties and the mechanisms controlling leaching. There are a number of relevant test methods for determining
leaching of inorganic components from
MSWI bottom ash on a laboratory scale,
either exploring the mechanisms of leaching
or simulating what would happen in a field
situation. However, it is essential to know
how to interpret the results from such tests.
There may be substantial differences between
laboratory and field conditions (Belevi et al.
1992, Fällman & Hartlén 1994, Fällman &
Aurell 1996, Fällman 1997, Fällman & Rosén
2001) and test conditions, such as acidity or
redox potential or leaching affecting different
trace metals differently (Bruder-Hubscher et
al. 2002). Furthermore, according to Quilici
(2004), there is not necessarily a correlation
between the results from physico-chemical
testing and the resulting ecotoxicity. In order
to understand how the material will develop
chemically over a long time in a field situation, the laboratory tests need to be combined with modelling tools and field verification tests (van der Sloot 1996, van der Sloot
et al. 2001). Three different levels of waste
characterisation have been suggested by CEN
(European Committee for Standardisation):
Table 3. Examples of common leaching tests for granular material.
Test
prEN 12457
(1-4)
Type
Extraction
test
prEN 14429
Extraction
test
NT ENVIR 003
(availability test)
Extraction
test
prEN 14405 (upflow
percolation
test)
Dynamic
test
Procedure
Sample is mixed with H2O and
HNO3 for 24 h, L/S 2-10. Particle
size is <10 or <4 mm.
Replicate samples are mixed with
H2O and HNO3 for 48 h at different pH values (4-12). The L/S ratio
is 10 and the particle size is <1mm.
Sample is mixed with H2O and
HNO3 in two steps, at pH 7 (3h)
and pH 4 (18h). The L/S ratio is
high and particle size is <125 μm.
Sequential flushing with H2O in a
column of the sample at increasing
L/S ratios (0.1-10). Particle size is
<10 or <4 mm.
8
Results
Estimates of leaching during a short
time perspective.
Information on the impact of pH on
leaching.
Estimates of leaching during a long
time perspective (several thousands
of years).
Information on how leaching progresses at different L/S ratios in a
situation that is more similar to field
conditions than in an extraction test.
Environmental assessment of municipal solid waste incinerator bottom ash in road constructions
than in the fresh ash. During weathering, the
bottom ash is oxidised and the oxides are
hydrolysed (Speiser et al. 2000), i.e. to Feand Al hydroxides. Carbonation then takes
place, in which CO2 is absorbed by the material, pH is decreased and calcite (CaCO3) is
formed until equilibrium with the atmospheric CO2 is reached (Goumans et al. 1994,
Chandler et al. 1997, Meima & Comans 1997)
(eq. 1). At the same time, amorphous aluminosilicates have been found to precipitate
(Goumans et al. 1994). Calcite works as an
important long-term buffer in the bottom
ash, keeping pH relatively high for several
thousands of years (Fällman et al. 1999,
Johnson & Furrer 2002).
Leachate
Column packed
with the material to
be tested
Deionised water
Figure 6. In dynamic tests, some aspects of
leaching in which time is an important
variable are typically addressed. One example of such a test is the upflow percolation
test prEN 14405, which is under approval
by CEN (European Committee for Standardisation) to become a European standard.
Equation 1.
CaO + CO2 → CaCO3
CO2 + H2O ↔ HCO3- + H+
The processes that dominate in controlling
the leaching of trace metals from weathered
bottom ash are not yet fully understood.
Precipitation of calcite and other carbonates
may be one of the controlling mechanisms
for the leaching of Cd and Pb (Johnson et al.
1996) or Cd and Zn (Meima & Comans
1999). Neoformed minerals such as Fe/Al(hydr)oxides have also been discussed for
controlling leaching as potentially important
sorbent minerals (Kersten et al. 1997, Meima
& Comans 1998, Meima & Comans 1999,
Dijkstra et al. 2002), but additional sorption
sites, which need to be identified, may also be
important (Meima et al. 2002). For Cu, some
indications have been found that sorption to
amorphous Al-minerals reduces the mobility
(Meima & Comans 1999, Meima et al. 2002),
but there have also been several indications
of Cu-DOC complex formation (Kersten et
al. 1997, Meima & Comans 1997) (Fig. 7).
Meima et al. (1999) found that more than
95% of the dissolved Cu was bound to DOC
in leachate from both fresh and weathered
bottom ash. Even though some research has
been carried out on the organic material in
the MSWI bottom ash and its possibility to
form complexes with Cu, there is a need for
further studies of the nature of DOC and the
role of organic complexation in the long term
leaching in which time is an important variable are typically addressed. The test material
and the leaching solution are mixed and the
leaching solution is periodically or continuously renewed. An example of a dynamic test
is the upflow percolation test prEN 14405,
which is based on NEN 7345 and NT
ENVIR 002 (Fig. 6).
Chemical mechanisms of trace metal leaching
The chemical mechanisms for the solubility
of trace metals from MSWI residues have
been the subject of discussion by several
authors. Weathering processes have been
found to have a significant effect on the
leaching of trace metals. Meima & Comans
(1997) distinguished between three different
weathering stages, based on pH, in which
different bottom ash types show largely similar leaching behaviour. In the most weathered
MSWI bottom ash (1.5 years old), the pH
was found to be 8-8.5. Later, the same authors concluded that leaching was lower in
the more weathered (carbonated) bottom ash
(Meima & Comans 1999). The decreased
leaching was explained as an effect of pH and
of the controlling processes being different
9
Susanna Olsson
Ash
particle
TRITA-LWR.LIC 2030
Cu2+
properties of MSWI bottom ash, there are
fewer studies that focus on the material in its
surroundings and that may thus be incorporated on the second system level. Most of
these studies do not specifically consider a
certain material but rather waste material
expressed as residues, secondary materials,
recycled materials and by-products. It is
assumed that these categories may include
MSWI bottom ash. On this system level,
studies are therefore included that do not
focus solely on MSWI bottom ash.
Cu-DOC
Figure 7. There are several indications that
Cu in the leachate from MSWI bottom ash
forms complexes with DOC. Hence, the
concentration of Cu2+ in solution is dependent on equilibrium with DOC complexes and the solid phase.
Environmental aspects considered
Few of the studies found on the second
system level mention that there may be other
types of environmental impact from the
utilisation of the material in the construction
apart from leaching during the use of the
construction. Instead, leaching is brought
forward as the main environmental impact,
often without any further explanation.
Hartlén (1996) addresses some other possible
environmental aspects in addition to leaching, such as energy use and emissions during
transportation. However, even though he
considers it important to take all types of
emissions into account, he focuses on the
leaching from the material and states that
leaching of hazardous substances to soil and
water is the main environmental impact from
re-use of secondary material without any
further arguments.
Most of the studies on the second system
level are method-focused, suggesting frameworks for assessing the leaching from different secondary materials in constructions
during the use of the construction (Nunes et
al. 1996, Kosson et al. 2002, Mroueh &
Wahlström 2002, Apul et al. 2003, Svedberg
2003, Petkovic et al. 2004), or discussing the
agreement between predicted and measured
release in the field (Kosson et al. 1996,
Schreurs et al. 2000). There are also some
studies that focus on case study results in
relation to a certain decision situation. It may
be argued that these studies may also be
categorised into the material level since they
do not expressly consider the local environment in which the road is built but rather
some kind of average environment for which
limit values exist. However, since the leaching
(Fällman 1997, Grøn et al. 2003, Johansson
2003, van Zomeren & Comans 2004).
Improvement of environmental properties
There are different techniques described in
the literature that may be used to improve the
environmental performance of MSWI bottom ash. One possibility is to reduce the
leachability of contaminants by chemical
stabilisation or addition of sorbing components. Crannell et al. (2000) found that divalent metal cations in MSWI bottom ash were
effectively stabilised by treatment with soluble PO43-, whereas Comans et al. (2000)
found addition of Fe(III) and Al(III) salts to
be a promising technique to reduce the leaching of metals in the bottom ash. Washing
(Stegemann et al. 1995) or carbonatisation
(Van Gerven et al. 2005) of the bottom ash
are other reported methods to reduce the
leaching of some substances.
Thermal treatment of MSWI ash is another
way to increase the feasibility of recycling
incinerator ash as construction materials
(Wang et al. 1998, Sakai & Hiraoka 2000,
Nishida et al. 2001, Wang et al. 2003). Generally, a material with good technical properties and low metal leachability could be produced by this treatment. Co-heating or
mixing of the bottom ash with other ash
fractions may be a further alternative to get a
product usable in construction (Abbas et al.
2001, Sorensen et al. 2001, Baun et al. 2004).
Environmental assessment concerning
the material in its surroundings
While a large amount of scientific work has
been devoted to characterising the chemical
10
Environmental assessment of municipal solid waste incinerator bottom ash in road constructions
from the material as incorporated into the
road body is considered, here they are included in the second system level. Comparisons between national regulations and leaching of MSWI bottom ash have been reported
based on measurements in the field (BruderHubscher et al. 2001) or the laboratory
(Izquierdo et al. 2002, Forteza et al. 2004).
Generally, these results indicate an acceptable
level of contaminant release from the material. Only Bruder-Hubscher et al. (2001) includes a discussion of the contaminant release from conventional materials. In Sweden, leaching of heavy metals from bottom
ash in road applications has been studied i.e.
at Törringevägen in Malmö (Hartlén et al.
1999), Linköping (Flyhammar & Bendz 2003)
and Dåva (Lind et al. 2005).
acceptable exposure level for human or ecological systems back to the source to determine the acceptable content within the
source. In Sweden, the former perspective is
advocated by Svedberg (2003) and the latter
by ongoing work at the Swedish Geotechnical Institute (SGI).
Both Petkovic et al. (2004) and Kosson et al.
(2002) base their frameworks on the measurement of intrinsic leaching properties of
the material in conjunction with mathematical modelling to estimate release under field
management scenarios. Kosson et al. (2002)
in particular devote great effort to describing
how to assess leaching and argue that intrinsic leaching parameters need to be assessed in
order to provide a sound basis for estimation
of release potential in a range of different
potential waste management scenarios. Four
steps are proposed to determine the potential
for toxic constituent release from a waste by
leaching under a selected management option. Firstly, scenarios and mechanisms are
defined. This is followed by measurement of
intrinsic leaching parameters for the material.
Release models are then used to incorporate
the measured leaching parameters to estimate
release fluxes and long-term cumulative release. Finally, the release estimates are compared to acceptance criteria. In order to measure the intrinsic leaching parameters,
Kosson et al. (2002) suggest a three-tiered
testing programme:
1. Screening-based assessment (availability)
2. Equilibrium-based assessment (over a
range of pH and L/S conditions)
3. Mass transfer-based assessment
The progression from tier 1 to tier 3 provides
increasingly more realistic and tailored and
less conservative estimates of release, but also
requires more extensive testing. For the
modelling of cumulative release, equations
are demonstrated for both percolation and
mass transfer-controlled scenarios.
The testing programme and the release modelling described by Kosson et al. (2002) are
used by Petkovic et al. (2004) as a basis for
modelling leaching from recycled materials in
road constructions. The latter authors pre-
Framework for risk assessments and critical
limit definition
To develop realistic estimates of constituent
release from MSWI bottom ash in a certain
application, a combination is required of
laboratory tests which measure fundamental
leaching parameters, mathematical modelling
to carry out extrapolation of laboratory results to field scenarios and field certification
of critical assumptions (Kosson et al. 1996).
The leaching may then be compared to some
kind of critical limit or acceptance level in
order to assess the environmental impact
caused by the material utilisation. The idea
that the contaminants should leach into the
surroundings at an environmentally acceptable rate has been expressed for disposal
strategies by Hjelmar (1996) as the ‘controlled contaminant release strategy’ and this
view seems to be the basis for the suggested
frameworks. However, the problem is to
know what degree of environmental risk
should be accepted and at which point the
critical limit is being exceeded for different
types of environments.
The suggested frameworks may be used as
valuable tools in defining such acceptance
levels or critical limits for contaminant release in different surroundings. Frameworks
for risk assessments and critical limit definition may be focused either on the source and
the possible transportation of the pollutants,
or on backtracking the pollution from an
11
Susanna Olsson
TRITA-LWR.LIC 2030
sent a combination of the European standard
for characterisation of waste, ENV 12920,
and the Norwegian guidelines for evaluating
impacts on health and ecosystems. In the
procedure, information on the amount of
contaminants released is combined with a
risk assessment, which includes source identification, transport pathway evaluation and
exposure assessment of target organisms. In
this way, threshold values for material input
data can be determined for different scenarios and acceptance criteria for the acceptance
or refusal of the recycled material for a given
application can be formulated.
Apul et al. (2003) add a further dimension to
such assessment by including guidance on
how to incorporate different levels of uncertainty in contaminant release estimates.
from the material are being extensively discussed within the project.
In Sweden, no scientifically reviewed publications from studies on the LCA level concerning MSWI bottom ash or other reused
material in road constructions were found.
However, a life cycle perspective on the
utilisation of waste material in road constructions was found in a report by Carling &
Hjalmarsson (1998).
Environmental assessment on an Industrial system level
There is a lack of studies on the widest system approach, the fourth system level, concerning reuse of waste material in constructions. Such research could include the issues
of natural resource substitution by reuse of
waste material and the pressures of the waste
material production. In the case of MSWI
bottom ash, the industrial system level may
for example imply the consideration of environmental burdens from all the processes
that are needed to form the ash, including the
products that become waste and the combustion process. A large amount of information
would need to be processed to make such an
analysis.
According to Roth & Eklund (2003), one
methodology that could possibly cope with
these issues is strategic environmental assessment (SEA). An SEA can be seen as a comprehensive process of evaluation of the environmental impact from a policy, plan or
programme. Another approach may be to
evaluate the conservation of material quality
on the basis of thermodynamics. Brown &
Buranakarn (2003) use the concept of emergy
to evaluate the environmental impact from
using different construction materials. One of
their conclusions is that investments in recycling materials yield very positive returns to
society compared with landfill alternatives.
Emergy is defined as the amount of energy of
one form that is required, directly or indirectly, to provide a product or service. This
might be a comprehensive way to include and
aggregate the large amount of information
that would result from including all the chain
of processes making up a certain waste product.
Environmental assessment on an LCA
level
The studies previously described have focused on the chemical properties of the material or the use of the material in a construction, thus neglecting environmental impacts
that occur in other stages of the construction’s life cycle than the use stage. There are,
however, a few examples of studies that take
a broader view and include some of the aspects connected with the whole life cycle of
the construction.
Mroueh et al. (2001) describe results from a
research project that includes the development of a life cycle impact assessment procedure for comparisons and evaluation of alternative road and earth constructions. The
procedure outlined was used in case studies.
It was found that the use of by-products as a
substitute for natural aggregates could reduce
the environmental impact for some of the
impact categories. However, the project did
not include MSWI bottom ash.
In Denmark, a model for life cycle analysis
(LCA) of road construction and disposal of
MSWI residues is under development
(Birgisdottir et al. 2003). The model includes
the total environmental impact from the
constructions, and assessments of certain
parameters such as water percolation through
the material and the leaching of trace metals
12
Environmental assessment of municipal solid waste incinerator bottom ash in road constructions
Information demand and supply for decision situations
A large amount of information on the environmental aspects associated with the use of
MSWI bottom ash or other waste materials in
constructions has been gained through previous scientific work. Roth & Eklund (2003)
conclude that the outcomes from an assessment at one level could differ from the others and that a question addressed at one level
could not always be discussed on other levels.
They also claim that pollution aspects are
best studied on the first or second system
level, while a higher system level is needed to
include resource aspects. Furthermore, these
authors argue that using wider system
boundaries would improve the basis for
decision-making.
From this literature review the same conclusions can be drawn regarding the differences
in outcome on different system levels. While
the material level contributed knowledge on
the chemical properties of the material, the
surrounding level was needed to answer
questions on the risk of emissions during the
use of the material in a construction. The
LCA level was needed in order to obtain
results on different types of environmental
impacts, not only leaching, from using the
material in a construction. This means that
different system levels could be appropriate
for different decision situations. A combination of results from different system levels,
obtained with the tools specific for these
levels, may probably be necessary in several
cases. By quantifying resource use and emissions, the LCA level takes one step further
towards the possibility of measuring a system’s sustainability as described by (Robert et
al. 2002). However, according to these authors, social aspects, such as the present and
future needs of all the people on whom we
have an impact, also need to be taken into
account in a sustainability assessment.
Of all the scientific papers found that were
concerned with the environmental aspects of
using MSWI bottom ash in constructions,
most were done on the material level (Table
4). Very few studies were made on the two
highest levels. It was also found that studies
made on a high system level were dependent
on, and referred to, results from the levels
below (Fig. 8). Without any studies on the
material level, there would be no data input
for risk assessment and the results from risk
assessments are needed on the LCA level.
There are few results reported from such risk
assessments, which may cause a problem.
There are also still relatively unexplored areas
on the material level and on the construction’s environment level that need to be
further investigated in order to improve the
leaching parameters in LCA level studies. For
example, the mechanisms controlling trace
metal leaching from weathered bottom ash in
field applications, such as the role of organic
complexation of Cu, remain uncertain. Furthermore, leaching needs to be assessed not
only for the use stage of the construction’s
life cycle but also for the construction and
the demolition stages.
Table 4. Results from literature review of scientific papers concerned with environmental aspects of using MSWI bottom ash in constructions.
Level
Information gained
Type of impact considered
Material
Material chemical properties, results and
method suggestions
Risk for emissions during the use of the
material in a construction, results and
method suggestions
Different kinds of environmental impacts
from using the material in a construction and
where in the life cycle these occur
-
Content and leaching, mainly
of inorganic substances
Leaching during the use of the
construction, mainly of inorganic substances
Resource use and emissions to
air and water
Construction
environment
LCA
Industrial
system
13
-
Number of
studies found
40-50
10-15
1-2
0
Susanna Olsson
TRITA-LWR.LIC 2030
Parameter information
Narrow system level
Wide system level
Identification of important parameters
Increased complexity
Increased uncertainty
Figure 8. Information from a narrow system level, i.e. the material level, is needed in studies on
wider system levels, i.e. in life cycle analyses. The information obtained on the wider system
level can then be used to identify important parameters for further investigations.
However, a flow of information in the opposite direction would probably also be beneficial. Without studies made on a high system
level, there would be no knowledge of the
particular parameters that are of importance
for studies on lower levels. In such cases,
there is a risk that resources are spent only on
investigating parameters that are easily determined, or have a high public interest
(Wrisberg et al. 2002). Generally, a higher
system level has been found to be associated
with more complexity and uncertainty, and
the results seem to be more difficult to
communicate. However, such a broad view
may produce more relevant results, depending on the question. A choice has therefore
to be made between relevance and precision.
More effort needs to be devoted to the LCAlevel and the industrial system level in order
to investigate the relative importance of different types of environmental impacts from
MSWI bottom ash utilisation and where in
the life cycle these would occur. For comparisons between different construction
materials, information is needed on all types
of potential environmental impact that may
be expected for each alternative. Such information would decrease the risk of nonoptimal decisions.
Today, there is a gap between the societal
environmental ambitions of reuse and how
reuse in general is environmentally assessed.
The view of leaching as the most important
parameter commonly found without any
explanation or criteria for this prioritisation
may be the result of a lack of studies on high
system levels. It may also be a result of the
decision process, favouring such a choice in
one way or another, or the policies and regulations for the utilisation of waste material.
Several studies on the second system level
aimed to cover the environmental impacts
from utilisation of waste material in constructions, but in the end they only covered the
leaching parameter during the use of the
construction. A frequent explanation was that
leaching was considered to be the most important factor limiting the potential use of
the material due to national regulations and
policies. However, the fact that this parameter seems to be the most important environmental parameter in regulations does not
necessarily make it the most important one
from an environmental point of view. The
decision process for the use of waste material
in constructions would need to be further
investigated in order to create a more relevant basis for the choice of system bounda-
14
Environmental assessment of municipal solid waste incinerator bottom ash in road constructions
ries and thus improve the information basis
for the decisions.
road. A sensitivity analysis showed that the
alternative where crushed rock was used in
the sub-base would use more energy than the
alternative with MSWI bottom ash in the
sub-base only as long as the transportation
distance for MSWI bottom ash was less than
140 km (all other parameters remaining the
same).
The magnitude of each flow (energy, material
or emission) in the case study depended on
the specific assumptions made and should
therefore be interpreted carefully. However,
the time frame used for leaching estimates
and the assumed transport distance were the
only parameters with the potential to change
the major results. Changes in other parameters would not be able to affect the relative importance of the flows and their location in the life cycle stages for the alternatives
in the case study.
A LIFE CYC LE PERSPEC TIVE ON
THE USE OF MSWI BOTTOM ASH
IN ROAD CONSTRUCTIONS
The approach outlined described the resource use and the emissions to air and water
for the two alternatives that were compared
in the case study (Paper I). The aspects associated with the alternatives included the use
of natural resources (crushed rock) and energy (mainly fossil fuels), emissions to air
from the fossil fuel combustion and emissions to water from the road material.
It was found that the alternatives would
cause different kinds of potential environmental impact; whereas the conventional
alternative with only crushed rock in the road
sub-base would lead to larger use of energy
and natural resources, the alternative with
MSWI bottom ash in the sub-base would
lead to greater contaminant leaching. Since
these flows of resources and emissions would
occur in different stages of the road’s life
cycle, it is essential to include those stages if
comparisons are to be made between the
alternatives concerning both resource use and
emissions. The exclusion of the construction
stage would lead to exclusion of the aspect of
natural material and energy use, while exclusion of the use and maintenance stage would
lead to contaminant leaching not being considered.
The extent of the contaminant leaching of
Cd, Cr and Cu was important for the comparison, according to the normalisation results. However, leaching estimates are uncertain and they depend on hydrology, leaching
mechanisms and the time frame used for
leaching scenarios. Different leaching assumptions may change the results. Another
parameter with the potential to change the
results was the assumed transport distance
for the material from the extraction to the
S PECIATION O F C OPPER
The fractionation of MSWI bottom ash
leachate showed that it contained a large
proportion of hydrophilic components, 75%,
of which the major proportion were hydrophilic acids (Fig. 9). Simplified fractionations
of the soil leachates showed that these contained a much smaller proportion of hydrophilic components, only around 30-40%, and
a full fractionation of a soil leachate from the
same location as S1 and S2, performed by
Fröberg et al. (2003), showed that the hydrophilic acids comprised around 30% of the
DOC. These results are in agreement with
previous research. Generally the DOC in
natural waters has been shown to consist of
around 20% hydrophilic acids (Leenheer &
Huffman 1976, Malcolm & MacCarthy 1992,
Martin-Mousset et al. 1997) while for MSWI
bottom ash, the hydrophilic components
have been shown to make up more than 80%
of the total DOC (van Zomeren & Comans
2004).
15
Susanna Olsson
TRITA-LWR.LIC 2030
The Cu-ISE gave consistent results between
different measurements in all subsamples of
the MSWI bottom ash leachate and the soil
leachate. For the MSWI bottom ash leachate,
however, the high amount of chloride was
found to affect the result at pH values below
5. This interference has been found and
explained in previous studies (Westall et al.
1979, Rivera-Duarte & Zirino 2004). For all
subsamples of the MSWI bottom ash
leachate and the soil leachate the titration
results showed a decreasing Cu2+ activity with
an increasing pH (Paper II). These results
could not be explained by complexation with
inorganic ligands or with low-molecular
weight organic acids (LMWOA) alone and all
solutions were undersaturated with respect to
Cu(OH)2. The decrease in Cu2+ activity was
therefore interpreted as an increase in CuDOC complex formation.
One important finding was that removal of
the fulvic acids (which are generally considered to constitute the metal binding fraction
of DOC) had little effect on the Cu binding.
This was found both for the MSWI bottom
ash leachate and for the soil leachate. Hence,
the hydrophilic component of DOC in these
leachates appears to possess metal binding
properties fairly similar to fulvic acids. This
effect is important to consider when predicting Cu speciation in the MSWI bottom ash
leachate, in which the hydrophilic acids constitute the major part of the DOC.
When the SHM and NICA-Donnan models
were applied to the results, it was found that
neither of the models could capture the pH
dependence of the Cu2+ activity in the MSWI
bottom ash leachate correctly. The agreement
between predicted and obtained values was
improved by assuming a lower amount of
active DOC, but the pH dependence for Cu
complexation obtained was still lower than
expected from the modelling results. This
suggests that the Cu complexation properties
of the ash leachate DOC may not be strictly
comparable to that of natural DOC. For the
soil leachate, on the other hand, the model
predictions of Cu speciation were in good
agreement with the titration results. The best
agreement was obtained when the SHM
model was applied and when 65-100% of the
DOC was assumed to be acting as active
fulvic acids.
D ISCUSSION
System levels used in this thesis
Environmental assessments of the use of
MSWI bottom ash in constructions can be
made on several different system levels, as
shown in the literature review. While studies
using narrow system boundaries lead to detailed information about well-defined issues,
wide system boundaries provide information
about larger systems. In this thesis, two perspectives were used. First, the LCA level was
used to describe the resource use and emis-
100%
90%
80%
70%
60%
Hydrophilic bases
50%
Hydrophilic neutrals
40%
Hydrophilic acids
30%
20%
Hydrophobic neutrals
10%
Hydrophobic acids
0%
MSWI bottom
ash leachate
O horizon
leachate
16
Figure 9. DOC fractions in leachate
(L/S 5, native pH)
from >6months old
MSWI bottom ash
sample and data
from Fröberg et al.
(2003) on DOC fractions in soil leachate
from an O-horizon
in a Haplic Podzol.
Environmental assessment of municipal solid waste incinerator bottom ash in road constructions
sions from two alternative scenarios and to
identify the most important aspects and
parameters. The reason for choosing such a
broad perspective was to include aspects that
have so far been neglected and thereby improve the basis for decisions concerning the
management of MSWI bottom ash. It was
assumed that a life cycle perspective was
needed in order to include resource use as
well as emissions to air and water, something
which was confirmed by the fact that these
flows were found to occur at different life
cycle stages.
Leaching turned out to be the dominant
aspects for the environmental performance
of the system and, in particular, Cu leaching
was shown to be large. This metal has been
commonly included in previous studies,
possibly since it was considered as important
by those authors too. A large amount of Cu
in the leachate, however, may not pose an
environmental threat if the Cu is strongly
bound to DOC. In order to decrease the
uncertainties regarding the potential environmental impact from MSWI bottom ash
utilisation, the effects of DOC from MSWI
bottom ash on the leaching and toxicity of
Cu from the ash were chosen as the focus for
the second study. This study was made with
narrow system boundaries, not including the
construction and its surroundings but only
the material itself.
the individual companies and the changes
require the engagement of governmental
stakeholders. In that context the results from
studies on the LCA-level could be used to
incorporate different aspects in a comparison
of alternatives and thus decrease the risk for
suboptimisations. The LCA-based ESA approach outlined in this thesis may be used as
an example and may also be further developed to solve other types of questions regarding the environmental performance of
waste material in constructions.
The results from the approach outlined revealed that there are advantages and disadvantages with both the scenarios studied. A
decision maker will need to weigh the potential local toxicity impact on human health and
ecology against the resource use and the
more global impact potentially arising from
energy use. Since no alternative was free from
potential negative environmental impacts,
there is no easy decision for which alternative
to choose. Different weighting methods have
been developed within the LCA framework
to help in such trade-off situations (Nordiska
ministerrådet 1995, ISO 1997) (Table 5). In
these methods, a one-dimensional value for
the resource use and emissions is set, so that
the system’s total environmental impact can
be calculated. The flows of resources and
emissions may for example be weighted
based on economics (as in the EPS method),
on politically or scientifically determined
targets (as in the ECO-scarcity method), or
by experts or expert groups. The latter
method has been used in an LCA of road
constructions by Mroueh et al. (2000). Erlandsson (2002) suggests a method in which
environmental impact categories are compared to an acceptable environmental load
per person. This acceptable environmental
load is calculated from the Swedish environmental objectives (Gov. Bill 1998, Gov. Bill
2001) and the evaluation may therefore be
seen as based on ‘distance to target’, similar
to the ECO-scarcity method.
Applicability and interpretation of results
on the LCA level
Utilisation of bottom ash in road construction solves both the problem of waste management and the problem of natural resource
extraction. There are two main sectors involved, the construction companies that need
material and the energy producers that produce the ash. However, a third important
actor is the national authorities whose goal is
to ensure long-term sustainability. If both
recycling of resources and emission aspects
are to be considered, the scope goes beyond
17
Susanna Olsson
TRITA-LWR.LIC 2030
Instead of using weighting methods, the
relative importance of the different flows of
resources and emissions can also be demonstrated by relating them to a common basis
(normalisation). The flows may for example
be divided with data on national anthropogenic resource use and emissions (Kärrman
& Jönsson 2001), or related to what occurs
‘naturally’, i.e. the release of metals by weathering (Bergbäck et al. 1994). In this thesis,
weighting was not used. It was considered to
aggregate information to an unnecessary high
degree and to decrease the transparency of
the study. Instead, the results on the different
flows were normalised by division with the
national flow of each kind per person in
Sweden.
Research) has been constructed in order to
study the management of organic waste in
urban areas (Sonesson et al. 1997, Björklund
et al. 1999, Finnveden et al. 2005). Integrating
those results with the results on the use of
MSWI bottom ash would lead the research
on the beneficial use of MSWI bottom ash
towards the industrial system level. Before
doing this, the LCA perspective used in this
thesis would need to be slightly different; the
focus would have to be changed from the
road construction to the management of a
certain amount of waste. The results could
contribute to the environmental assessment
of the waste management system and reuse
of by-products in a wide sense.
Furthermore, the outlined approach could
not consider the site- and time-specific effects of contaminant release. This has also
been evident in previous LCA-studies of
landfills, in which leaching is an important
aspect (Finnveden et al. 1995). Concerning
this problem, a study using more narrow
system boundaries would be appropriate as a
complement to the LCA results. Several studies have been made on methods or frameworks for studying the leaching from a road
to its surroundings and to define proper
acceptance levels for different substances.
Commonly, these include the use of geochemical modelling, field verification and
toxicity estimations. There are, however, few
scientifically reported results. There were also
uncertainties found concerning some of the
Possibilities for improvement of environmental assessments
The outlined ESA approach proved to be
useful in integrating resource use and emissions in the comparison of two alternative
scenarios, but some limitations were identified. It could not, for example, answer the
question whether MSW should be incinerated
at all. To answer this, the system boundaries
would have to expand to include further
dimensions of the society, such as alternative
energy production and the effects of waste
recycling. An LCA approach has previously
been applied to other parts of the waste
management system in Sweden and the simulation model ORWARE (Organic Waste
Table 5. Examples of methods for determining the relative importance of different flows or
impact categories.
Method
Procedure
Basis for evaluation
Panel or expert
Different flows or the environmental impact Depend on the
groups
categories are weighted by a panel on an indi- members
vidual or a group level.
EPS
Flows are weighted based on their impact on Willingness to pay
five objects through the willingness to pay for
not degrading these objects.
ECO-scarcity
Flows are weighted based on the difference Distance to target
between the actual amount and a critical limit.
Method developed by Environmental impact categories are normalThe Swedish enviErlandsson (2002)
ised based on ‘distance to target’ for reaching
ronmental objectives
the Swedish environmental objectives.
18
Environmental assessment of municipal solid waste incinerator bottom ash in road constructions
leaching parameters. One example is the Cu
speciation in MSWI bottom ash leachate, on
which information is needed in order to
predict Cu release and to estimate its toxicity
in the leachate.
The information gained in this thesis of Cuspeciation in MSWI bottom ash leachate
(Paper II) may be seen as one step towards
better leaching and toxicity estimates. The
results confirmed previous knowledge that
Cu in MSWI bottom ash leachate is to a large
extent bound to DOC, which implies that
only a minor part of the Cu in the leachate is
bioavailable. This is an important issue not
only in risk assessments, but also when aggregating and interpreting the results of studies on an LCA level. In the United States, the
quality criteria recommendations for Cu in
water, formulated by the Environmental
Protection Agency (EPA), are being updated
to include the fact that not all of the Cu in
the water contributes directly to toxicity
(United States Environmental Protection
Agency 2003). The biotic ligand model
(BLM), which is a recently developed metal
bioavailability model (Di Toro et al. 2001), is
used for developing these new criteria. In
Sweden, however, water quality criteria for
Cu pollution are still usually based on the
total Cu concentration (i.e. Naturvårdsverket
1999).
It was also found that not only the fulvic
acids, as previously believed, take part in the
complex formation, but also that the hydrophilic fraction seems to be important. Furthermore, the results suggest that the Cu
complexation properties of the ash leachate
DOC may not be strictly comparable to
those of natural DOC. This needs to be
considered when using geochemical modelling to predict Cu-DOC complex formation.
However, in order to optimise parameters for
geochemical speciation modelling of MSWI
bottom ash DOC, more detailed information
is needed on the composition of the DOC
and its ability to complex Cu under different
conditions.
Besides the complementary use of other methods to consider time- and site-specific
effects, it should be possible to increase the
certainty of the leaching estimations used as
input values in an LCA approach. This
should include the use of risk assessment,
since the environmental impact from the
metal leaching is probably dependent on
concentrations on certain occasions, rather
than average concentration during a long
time period. For example, the risk for cracks
in the road’s surface and the risk for flooding
or reduced drainage conditions of the subbase needs to be considered.
19
Susanna Olsson
TRITA-LWR.LIC 2030
C ONC LUSIONS
F URTHER RESEARCH
•
An environmental systems analysis with a
life cycle perspective gives information
about waste management that cannot be
gained through assessments of leaching
only.
•
A life cycle perspective is needed to include both resource use and emissions to
air and water when comparing different
road construction materials.
•
In the case studied, the use of crushed
rock in a road sub-base would lead to larger use of energy and natural resources,
whereas the use of MSWI bottom ash
would lead to larger contaminant leaching.
•
For the utilisation of MSWI bottom ash
in road constructions, the leaching predictions of contaminants, together with
the transport distance, are the most important parameters for the overall environmental impact.
•
Toxicity estimates of MSWI bottom ash
leachate and the definition of acceptance
levels for contaminant release should
consider the fact that Cu is to a large extent bound to DOC, which reduces its
toxicity.
In order to further decrease the uncertainties
on the potential environmental impact from
MSWI bottom ash utilisation, there are still
several issues for future research in environmental assessments on the different system
levels. For example, the processes that dominate in controlling leaching of trace metals
from weathered bottom ash are not yet fully
understood, and parameters for geochemical
speciation modelling need to be optimised
and validated. Methods for contaminant release predictions in the field and the definition of acceptance criteria need to be adjusted for different Swedish conditions and
verified in field experiments. Furthermore,
the LCA-approach should be developed to
include more types of materials or applications and the system boundaries used may be
expanded. However, even if improved, no
single assessment method can probably obtain all types of information needed. The big
challenge for the future will therefore be to
integrate the information gained on different
system levels in order to create a relevant and
reliable basis for decisions on how we should
manage MSWI bottom ash.
•
The pH-dependence for Cu complexation to MSWI bottom ash DOC is
smaller than for natural DOC, implying
that models calibrated for natural DOC
may give inconsistent simulations of CuDOC complexation in MSWI bottom ash
leachate.
•
The hydrophilic fraction of the MSWI
bottom ash DOC is important for Cu
complexation.
20
Environmental assessment of municipal solid waste incinerator bottom ash in road constructions
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