Characteristics, treatment and utilization of

residues from municipal waste incineration

H.A. van der Sloot

a,

*, D.S. Kosson

b

, O. Hjelmar

c

a

ECN Soil & Waste Research, PO Box 1, 1755 ZG Petten, The Netherlands

b

Vanderbilt University, Box 1831 Station B, Nashville, TN 37235, USA

c

DHI, Agernallee 11, 2960 DK Hørsholm, Denmark

Received 28 February 2000; received in revised form 23 December 2000; accepted 2 January 2001

Abstract

Beneficial utilization of residues from municipal solid waste incineration is an important objective for integrated waste manage-

ment in many jurisdictions. When residues are to be used as an aggregate substitute in construction applications, the release of
constituents of concern to soils and water through leaching is an important environmental consideration. In this paper, residue
characteristics that control constituent leaching and testing approaches for evaluating leaching are discussed. Quality control and
potential improvement in case of beneficial application are addressed. # 2001 Elsevier Science Ltd. All rights reserved.

Keywords:

Leaching; Treatment; Waste incineration; Residues; Environmental impact; Ash

1. Introduction

From waste-to-energy conversion of municipal solid

waste, in which high standards of emission control are
reached, solid residues remain, including bottom ash, fly
ash and air pollution control residues, that need to be
dealt with in an environmentally acceptable manner.
Advanced air pollution control measures in incinerators
shift constituents of concern from air emissions to solid
residues, which potentially may lead to soil and water
pollution. The evaluation of the environmental quality
of such residues is necessary before decisions can be taken
on the utilization, treatment or disposal of the residues.
The quality of the residues from waste-to-energy conver-
sion is very diverse, as has been detailed in the Interna-
tional Ash Working Group’s (IAWG) book ‘‘Municipal
Solid Waste Incinerator (MSWI) Residues

’’[1].

Management practices for incinerator residues are

very different in different jurisdictions. As a result of
recent developments in waste management, considera-
tion is given to recycling and reuse of residues in con-
struction. This necessitates a judgement on the short

and long-term environmental acceptability of such uti-
lization scenarios. In addition to the environmental
aspects, the new material will have to meet technical
specifications similar to those of natural materials tra-
ditionally used for the same purpose. In Europe, utili-
zation of MSWI bottom ash is either practised (e.g. the
Netherlands, Denmark, Germany, France) or increas-
ingly considered as a viable option (e.g. Belgium).
National legislation has been implemented to regulate
utilization of MSWI bottom ash in the Netherlands [2]
and in France [3]. In the Netherlands, bottom ash is
placed in a special category, because bottom ash, as
currently produced, does not always meet the regulatory
requirements. It is anticipated that improvement in ash
quality will bring the material within the regulatory
specifications [2]. Quality control programmes and cer-
tification of bottom ash are in progress to ensure pro-
duction of a marketable product [4].

In this paper, recent developments in characterization

of environmental properties of MSWI residues are dis-
cussed. Leaching data and relevant information from
field studies are interpreted to reach conclusions on how
to predict long-term environmental impact. Since
MSWI residues as produced often do not meet envir-
onmental criteria, treatment options are discussed. One
of the options is to control the input through waste

0956-053X/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved.
P I I : S 0 9 5 6 - 0 5 3 X ( 0 1 ) 0 0 0 0 9 - 5

Waste Management 21 (2001) 753–765

www.elsevier.com/locate/wasman

* Corresponding author. Tel.: [phone]; fax: +31-224-

563-163.

E-mail address:

[e-mail] (H.A. van der Sloot).

acceptance criteria. For this latter aspect, the relation-
ships between input and ultimate leaching behaviour of
residues are needed.

2. Materials and methods

The work mainly deals with MSWI residues. Some

earlier published data [1] have been used as reference for
more recent results. The sampling of MSWI residues
was carried out in accordance with the recommenda-
tions of the IAWG [1]. The leaching test methods
applied are preliminary versions of the pH dependence
leaching test [5] and the percolation test [6] now stan-
dardized in CEN TC 292 Characterization of waste —
Working Group 6.

3. Environmental quality of residues

The main leaching character of MSWI residues has

been largely outlined in the IAWG book [1]. Since the
time of publication, more detailed information has been
generated following this same basis of characterization
by using the pH dependence leaching test and percola-
tion tests [5,6]. Also a stronger emphasis has been
placed on modelling leaching behaviour to identify the
solubility controlling phases and binding mechan-
isms. In the framework of the European Standardi-
zation Committee (CEN) Technical Committee (TC)
292 Characterization of Waste, the two leaching
procedures — pH dependence test and percolation up-
flow test — identified as forming the basis for char-

acterization of MSWI residues are now being standar-
dized (Working Group 6). In the framework of this
Working Group a methodology guideline (ENV 12920)
has been described for the assessment of long-term
leaching behaviour [7]. It advocates evaluation of utili-
zation and disposal options based on the specific man-
agement scenario. This aspect will be addressed in more
detail later.

Comparison of leaching behaviour of different MSWI

residues and treated residues is possible through the
information generated by the pH dependence test. Fig. 1
presents the leaching behaviour of Cd from different
MSWI residue streams. The different leaching curves of
Cd as a function of pH are largely related to the chlo-
ride content in the residue for pH greater than 7. The
amount released at pH less than 5 usually reflects the
total content of Cd in the residue stream, as almost all
Cd present is leachable. Refuse Derived Fuel (RDF) ash
and vitrified MSWI fly ash are included for comparison.
The danger of indiscriminate use of an extraction test,
which provides data at only one condition, is illustrated
through the results of a single extraction test at the own
pH of the material. Clearly, this very limited evaluation
could result in erroneous classification of a material

1

. If

local equilibrium is assumed, the actual release can be
quantified from the pH dependence leach curves of Cd
with varying Cl levels. Fig. 1 illustrates that, if the pH
would decrease over the long-term as a result of external
influences, the leachability of Cd from RDF ash and
MSWI fly ash may increase dramatically. A compen-

Fig. 1. Leaching behaviour of Cd from MSWI residues (bottom ash, fly ash [11]) in comparison with Refuse derived Fuel (RDF) ash [1] and vitrified
MSWI fly ash [1]). All leaching tese were carried out at L/S=10 1/kg. Data illustrate role of increasing Cl-concentration on Cd leachability from
MSWI and related residues.

1

The ‘‘own pH’’ of a material is the final pH of the leaching solu-

tion when the material is extracted with deionized water.

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H.A. van der Sloot et al. / Waste Management 21 (2001) 753–765

sating factor for this dramatic increase may occur if Cl

is released under high pH conditions when Cd is not
leachable, and by the time the pH decreases no Cl

is

left to mobilize the Cd. In this case, the dynamics of
leaching can lead to a substantially lower release in
practice. Such processes illustrate the need to carry out
more detailed characterization tests such as percolation
tests for some cases. Batch leaching tests will not always
clarify such interrelated mechanisms. However, no
extent of characterization will result in a perfect predic-
tion of future environmental performance and a balance
must be maintained to obtain information necessary for
a sufficiently, but not overly, conservative decision and
excessive testing requirements should be avoided.

The consistency of data from different sources both in

time and place is illustrated in Figs. 2 and 3. For the ele-
ments Cd, Cr, Mo and Zn, the characteristic behaviour of
MSWI bottom ash and MSWI fly ash is clear. In case of
Cd the leaching from fly ash is similar in shape but shifted

due to the Cl

concentration and total Cd content in the

material (note that the plateau is at pH<7 for high Cl
concentrations but pH<5 for low Cl concentrations).
For Crm the difference in leaching behaviour of bottom
ash and fly ash at pH>5 is attributed to the presence of
Cr as chromate in flue gas cleaning wastes, whereas in
bottom ash Cr is present as Cr III due to its initial slightly
reducing properties. In case of Mo, the leaching beha-
viour as a function of pH is typical for an oxyanion [8].

In fly ash, the Mo level is increased relative to bottom

ash but the shape of the curve is similar, indicating no
significant difference in chemical speciation between the
two types of residues. The difference between different
MSWI residues is not very marked at pH 8–12 in spite of
large differences in total composition. At pH>7 the level
available for leaching is reached (plateau) with a difference
of only a factor of 2–3 between fly ash and bottom ash.

The pH dependence leaching test provides a very

good means of mutual comparison of MSWI residues as

Fig. 2. Characteristic leaching behaviour of Cd and Cr from MSWI residues, in particular MSWI, bottom ash and MSWI fly ash [1,11,28]. Leaching
experiments carried out at L/S=10 1/kg. The smooth lines reflect the generic leaching behaviour of bottom ash (two data clusters) and fly ash.
BA=bottom ash; DTL=detection limit.

H.A. van der Sloot et al. / Waste Management 21 (2001) 753–765

755

well as a comparison of leaching behaviour within one
and the same type of MSWI residue.

4. Parameter settings in pHdependence and column tests

In the studies carried out to provide a basis for para-

meter setting in these standards worked out by CEN TC
292 Working Group 6, MSWI bottom ash has been
used as a reference material, which owing to its hetero-
geneous nature cannot be considered a simple choice.
Aspects such as particle size, contact time and percola-
tion rate have been addressed. In Fig. 4 the aspects
percolation rate and contact time are highlighted.
Results of column tests performed at different flow
velocities (particle size: 95% < 4 mm) and pH depen-
dence tests at different contact times (particle size: 95%
< 2 mm) are shown in Figs. 4 and 5, respectively. These
results indicate that the hydraulic retention time within
the column is not a critical parameter within the range
of retention times evaluated.

From V and Mo leachability as a function of pH and

as a function of contact time, further conclusions can be
drawn with respect to mechanisms of release, which in
turn can be used to guide activities to improve ash
quality and in assessing short- and long-term environ-
mental impact. Mo is leached as molybdate (oxyanion)
and its mobile fraction is almost completely washed out
within L/S=2 l/kg. Molybdenum is therefor an example
of availability controlled leaching. When the percola-
tion test data are plotted in the pH dependence graph
(test carried out at L/S=10 l/kg) the cumulative leached
amounts approach the pH stat curve (determined at L/
S=10 l/kg) at the relevant pH from L/S=2 l/kg onwards.
This indicates consistency between the two fully indepen-
dent leaching tests. Obviously, when concentration data
from the column test at low L/S are plotted in compar-
ison to the pH dependence curve, greater concentrations
are observed from the column test. However, this is also
relevant information, as it will indicate what concentra-
tions may be expected in leachate in the short-term or
even long-term, depending on the level of infiltration.

Fig. 3. Characteristic leaching behaviour of Mo and Zn from MSWI residues, in particular MSWI, bottom ash and MSWI fly ash [1,11,28]. Leaching
experiments carried out at L/S=10 1/kg. The smooth lines reflect the generic leaching behaviour of bottom ash (two data clusters) and fly ash.

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H.A. van der Sloot et al. / Waste Management 21 (2001) 753–765

The relation between L/S (in l/kg) and time is given

by [9]:

L=S ¼ I  t=   h

ð

Þ

with I is the net infiltration of precipitation in mm/year,
t

the time in years,  the dry bulk density of the material

in kg/m

3

and h the height of the application in m. For

open applications with a height of 0.5 m and an infil-
tration rate of 200 mm per year, an L/S of about 10 l/kg
is reached in about 100 years, whereas in a situation
with top cover for a height of 5 m and a substantially
reduced infiltration, an L/S of 1 l/kg may only be
reached in more than 1000 years.

In the case of V, solubility controls release, which is

reflected by the horizontal line in the plot of concentra-
tion in percolate versus L/S and the slope of about 1

(dotted line in the middle figure in Fig. 4) in the cumu-
lative release versus L/S curve [1]. Residence times
(shortened to RT in the graphs) have been calculated as,
respectively, 2.7, 15.6 and 40.5 h for the fast, normal
and slow flow rates in the percolation test. The perco-
lation rate is apparently not a very critical factor as the
percolation rates differ significantly between the three
experiments, whereas the differences between element
concentrations are not as large [10].

The pH may also be a factor, which should not be

forgotten in this type of comparison. The pH ranges in
the fast, normal and slow column are respectively
10.43–10.94, 9.88–10.88 and 8.75–10.05. Clearly, the pH
tends to become lower as the percolation time increases.
Whether this is caused by reactions in the material
matrix itself or is caused by external influences (atmo-
sphere) is at present not clear.

Fig. 4. Leaching of V and Mo from MSWI bottom ash in a pH dependence test (left) and in a percolation test (middle: cumulative release and right:
eluate concentrations). Dotted line in middle graph points at solubility control [28].

H.A. van der Sloot et al. / Waste Management 21 (2001) 753–765

757

The contact time has been varied in the pH depen-

dence test. In Fig. 5 the data are given for V and Mo,
that indicates under specific pH conditions (Mo at pH 4
and 6) reactions are continuing after 48 h, which is now
identified as the standard condition for testing. The
bottom ash is still reactive at pH 4 and 6 as indicated by
the acid addition needed to maintain the respective pH
levels constant (Fig. 6). It is therefore not surprising to
see that Mo is still reacting. For Mo and V, it is likely
that slow sorption reactions (iron oxide phases) are
responsible for the observed decrease with time, because
otherwise, one would expect an increase in Mo leach-
ability as a result of more acid consumption.

The more critical pH values (4 and 6), in terms of

reaching stable conditions, are relevant from a leaching
behaviour and modelling point of view. These condi-
tions are less important for the evaluation of environ-
mental impact as due to carbonation (calcite buffer
formation) in MSWI bottom ash, it is not likely that
these pH values will be reached under field exposure
conditions.

5. Impact evaluation from leaching data

In the framework of the network harmonization of

leaching/extraction tests interrelations between different
test methods have been addressed [11,12]. Through the
acid neutralization capacity (ANC) and base neu-
tralization capacity (BNC) information generated in the
pH dependence test, the external stresses on a material
in a given environmental setting can be addressed.
These include acidification resulting from, for instance,
degradation of organic matter, sulphide oxidation, buf-
fering capacity of natural waters, acid rain and atmo-

spheric CO

2

. In Fig. 7, a typical ANC curve is

illustrated. As can be concluded from this graph, it
takes about 0.3 mol/kg of acid equivalent to reach pH
around 8. This can come from the uptake of CO

2

from

the atmosphere as well as from degradation of organic
matter generating CO

2

.

Based on this information and conditions specific to

the application which can be site-specific, the relevant
pH boundaries for the application can be identified. In
the case of MSWI bottom ash, the lower boundary is set
by the calcite buffering around 8. In Fig. 8 the pH
domain relevant for application of granular MSWI
bottom ash in road base or embankment is given. This
can be set against hypothetical regulatory criteria at a
specified L/S ratio as indicated with the dotted lines. For
Cr, no problems are expected to occur as the actual
leaching level is well below the critical value. In case of Zn,
the leachability may become critical, when the pH drops
to below 7.5, which is, however, unlikely under natural
conditions. This type of evaluation is helpful in deciding
which elements need to be addressed in quality control
and for which elements improvement or additional con-
trols may be needed. It helps to define the relevant range
of pH as a result of external stresses and internal chan-
ges (mineralization, organic matter degradation).

To create durable material improvements in ash

quality it is essential to base measures on understanding
of the controlling mechanisms by using characterization
data for evaluation of treatment. Therefore, it is impor-
tant to relate changes to the input, changes to the con-
version process and changes after treatment to the basic
characterization as described before. Firstly, to avoid
solving one problem and creating a new one, and sec-
ondly, to ensure that a reduced leaching level is durable
and not counteracted by exposure to the environment.

Fig. 5. Leaching of V and Mo as function of contact time in the pH dependence test (pH static mode L/S =10 1/kg) [28].

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H.A. van der Sloot et al. / Waste Management 21 (2001) 753–765

6. Field studies

Several field verification studies have been carried out

[13]. Meima [14], in particular, has provided information
on cores taken from a 20-year-old MSWI bottom ash
deposit. Results from field work (8 year old road base of
MSWI bottom ash [13] and 20-year-old landfill [14])
indicate that MSWI bottom ash is neutralized under
unsaturated conditions. This observation leads to the
question of which testing conditions are most relevant
for MSWI bottom ash: the high pH resulting from
crushing fresh ash or the longer term stable condition of
carbonated and neutralized material (which can be
mimicked by pH=8 control in testing). Upon weathering
new phases may be formed [15]. Such effects are included
in the field weathered samples that were tested. Thus age-

ing of MSWI bottom ash leads primarily to a significant
pH change with its inherent changes in leachability.

Field verification studies [13] have revealed that

leachable Cu is fractionated between a labile dissolved
organic matter (DOC; likely low molecular weight
organic acids) and a more stable DOC complex (more
humic matter like) in MSWI bottom ash, as roughly
40% of the leachable Cu is retained in the soil directly
underneath an application of MSWI bottom ash as a
road base stabilization layer. The labile Cu complex is
either degraded or destroyed though exchange with soil
organic matter. About 60% has been transported to the
groundwater as a water soluble organic complex. This
type of Cu fractionation in MSWI bottom ash has also
been identified in laboratory studies [16]. Johnson et al.
[17], Baranger et al. [18] have also modelled MSWI
bottom ash leaching, thus providing an increasingly
better understanding of controlling factors under field
conditions.

7. Quality control

Once sufficiently well defined data are obtained

through characterization, reduction of the testing effort
can be achieved by limiting the number of leaching steps
that need to be measured and by limiting the effort to
constituents that are really relevant for the material
being evaluated. Fig. 9 shows quality control data rela-
tive to characterization data providing clues for action.
For example, reductions in Mo may be possible through
control of the waste sources and Cu leaching may be
controlled through reduction in residual organic matter
content. In contrast, the behaviour of Pb and Zn leach-
ing from different facilities appears to be controlled only
by the material’s own pH.

Fig. 7. Acid Neutralization Capacity of MSWI bottom ash obtained from acid/base consumption in the pH dependence test [28].

Fig. 6. Acid consumption in pH dependence test on MSWI bottom
ash illustrating continuation of dissolution reations at pH 6 and 4. A
and B denote acid or base addition [28].

H.A. van der Sloot et al. / Waste Management 21 (2001) 753–765

759

This type of approach provides a basis for QA/QC

testing and eventually certification, when proven to be
sufficiently stable. By presenting the QA/QC data in
perspective to previous characterization, the power of
decision based on a single data point increases drasti-
cally. This aspect is grossly overlooked in current prac-
tice. In several cases, pH, a crucial parameter is not even
measured, which makes such regulatory leaching data
useless. New developments in this area provide a sound
basis for quality control [19,20], in which no more test-
ing is done than needed. However, as soon as results
appear to be out of specification, the level of testing
needs to be increased.

7.1. Treatment of MSWI residues

MSWI bottom ash does not always comply with reg-

ulatory criteria for utilization. This implies that treat-
ment prior to utilization may be required. A commonly
applied method to stabilize material properties is a
minimum sample storage period of several weeks up to
a few months to age the material. However, this treat-
ment may not be sufficient and additional treatment to
remove critical components, e.g. through washing of
salts, Cu and Mo may prove necessary.

The main issue in relation to MSWI fly ash and Air

Pollution Control (APC) residues is the content of
soluble salts [1,27]. Several options have been evaluated
to treat these residues. Controlled containment of the
stabilized salt containing residues is an option for treat-
ment. Removal of the soluble salts makes the remaining
inorganic residue much more manageable. However, it
is unlikely to lead to a marketable product. Vitrification
for fly ash has been tested and is practised on a small

scale. The process is more costly than other solutions
and also requires further treatment of salt concentrates
(salts cannot be vitrified). In many cases impact assess-
ments carried out in accordance with the environmental
assessment principle specified in the Methodology
Guideline ENV 12920 [7] will show that MSWI residues
cannot be utilised or even landfilled without prior
treatment. Various treatment options exists, several of
which are already part of the common MSWI manage-
ment practice. Nearly all the available treatment meth-
ods are based on one or more of three treatment
principles: physical or chemical separation, stabiliza-
tion/solidification and thermal treatment. These treat-
ment principles and a number of the corresponding
treatment processes or unit operations are listed in
Table 1, which also provides an evaluation of the
potential applicability of the various methods. Before
selecting a treatment process for a given residue and a
given scenario, it is important to set target properties,
which are in harmony with the short- and long-term
conditions expected for the scenario. When evaluating a
given treatment option it is also important to take all
new waste streams created by the process into account.
In most cases, a particular treatment process will consist
of a combination of several of the unit operations
shown in Table 1. For instance, it will be very difficult to
stabilize acid gas cleaning residues without prior or
simultaneous removal of the high content of readily
soluble salts, e.g. by aqueous extraction (and sub-
sequent treatment and/or discharge of the extract).

In addition to the treatment of residues from tradi-

tional mass burn incinerators, new incineration tech-
nologies have been developed in which e.g. thermal
treatment of the residues are integrated [21]. To judge

Fig. 8. Leaching behaviour of Cr and Zn from MSWI bottom ash as a function of pH as a means of evaluating long-term environmental impact and
the role of external stresses on release [12]. Dotted line represents a hypothetical limit value based on BMD Cat I limit values in the Dutch Building
Materials Decree [2]. The box reflects the relevent pH domain for a given application. All data obtained for L/S=10 1/kg.

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H.A. van der Sloot et al. / Waste Management 21 (2001) 753–765

Fig. 9. Compliance leaching test data for MSWI bottom ash from different Dutch MSW incinerators [19] in relation to characterization data (pH
stat-solid line) for MSWI bottom ash as QC for application in road constructions. Extraction conditions were LS=20, 24 h, leachant demineralized
water. Each symbol fill pattern represents a different facility. BMD Cat I and BMD CatII [2] are limit values from the Building Materials Decree
converted from mg/kg to microgram per litre at L/S=10.

H.A. van der Sloot et al. / Waste Management 21 (2001) 753–765

761

the performance of treatment options, a compliance test
(single extraction method) does not generally suffice. A
characterization of the changes in leaching behaviour
caused by the treatment can be visualized by using the
pH dependence test, as it provides a direct measure for
such changes in leachability.

8. Waste input measures and mass balance

An important question posed by government and

municipal solid waste incinerator (MSWI) operators is
how the quality of MSWI residues is influenced by the
composition of the waste input to the incinerator. Also
operating conditions, which are not discussed here, have
been identified as important controlling factors [22,23].
A question linked to the input characteristics is to what
extent specific (often smaller) waste streams are respon-
sible for the final quality of the residues. This forms
another option to improve the quality of the residues.
Besides the chemical composition of the residues, a
main aspect for the evaluation of MSWI residue quality
is the leaching behaviour. This has been shown to be
unrelated to the bulk chemical composition for most
elements of interest. Several studies have been carried
out to address this question [1,19,23–25]. Characteriza-
tion of the waste input to the incinerator into separate
sub-streams for their chemical composition is an essen-
tial element in answering such a question. Back in 1973,
the National Public Health Institute (RIVM) conducted
a separation of curb collected domestic waste in 11 dif-
ferent sub-streams to be able to quantify both the mag-
nitude of the different contributions to the total waste

input as well as the chemical composition of the differ-
ent streams [25]. An inherent limitation of the study was
that the sorting of the waste was carried on integral
collected waste. This resulted in a situation where the
composition of the wet organic fraction was sig-
nificantly biased by other non-organic sub-streams in
integral waste, such as vacuum cleaner dust and other
fine particulate contributions not originally belonging to
the organic fraction. The organic fraction analysed by
the sorting of integrally collected waste is therefore sig-
nificantly different from the organic fraction for sepa-
rately collected organic waste. By comparing the
composition of the organic fraction as obtained from
sorting integral waste with composition data derived
from analysis of pure organic matter the level of con-
tamination can be clearly identified. This is particularly
relevant for the elements Cr, Pb, Cu, Cd.

At present, the leaching behaviour of MSWI residues,

in particular, MSWI bottom ash, forms a limitation for
the beneficial use of such residues in construction
applications. Based on knowledge gained on how the
quality of the residues is affected by specific materials or
constituents in the input stream, the input to the incin-
erator could be managed in a manner in which such
inputs are minimized or eliminated. Such contaminating
waste streams may form a minor fraction of the total
throughput, but greatly affect the overall quality.

Before such measures will lead to measurable effects it

is important to relate the leaching behaviour of MSWI
residues to the waste input quality. Based on work car-
ried out in the framework of the IAWG and other stu-
dies

[1,8,11],

the

lack

of

correlation

between

composition and leaching has been unambiguously

Table 1
Overview of principles and methods for treatment of MSWI residues (after [20])

a

Treatment principle

Examples of processes
and unit operations

Bottom
ash

Fly
ash

Acid gas cleaning residues
(with or without fly ash)

Separation

Washing and extraction, e.g. at various pH values

a

a/b

a/b

Chemical precipitation

a/b

a/b

Crystallization/evaporation

b

Ion exchange

b

Density and particle size based separation

a

b

b

Distillation

b

b

Elektrolysis

c

Elektrokinetic separation

c

Magnetic separation

a

Eddy current separation

a

Stabilization and/or solidification

Addition of hydraulic binders

a

a/c

a/c

Addition of pore-filling additives

a/b

a

Chemical stabilization

a

a

b

Thermal treatment

Sintering

a

a/c

c

Melting/vitrification

a/c

a/c

c

a

a, Part of existing and proven treatment technology; b, have shown promising results, may be expected to be included in future treatment sys-

tems; c, currently under investigation or have been investigated and not found technically and/or economically feasible.

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H.A. van der Sloot et al. / Waste Management 21 (2001) 753–765

established. The elements Na, K, Mo, Br and Cl form
an exception, as these do show a direct relation between
input and leachability due to absence of matrix interac-
tion, and thus are leached within L/S=2 l/kg. A control
measure at the input for these elements will lead to a
direct and proportional improvement of quality in terms
of leaching. However, these elements are also readily
removed by washing after the residue is produced. In
Fig. 10 the positive correlation between total composi-
tion and leachability is illustrated for Mo.

For elements (e.g. metals) controlled by solubility,

measures at the input side are meaningless. There may
be other reasons to separate metals from the waste stream,
but not from an environmental impact evaluation. Only
when leachability is controlled by availability, can mea-
sures at the input side of the installation be useful.

To be able to quantify the contribution of different

waste streams to the residue composition, a spreadsheet
has been developed, which uses the composition of the
separate sub-streams identified in the input to the
incinerator, their moisture content, their loss on ignition
and their fractional contribution to the total waste

stream [25]. Based on these data, the ash composition
of, respectively, bottom ash and fly ash is quantified by
assigning the ash resulting from a specific sub-stream in
the waste input entirely or partially to either bottom ash
or fly ash. The rationale behind this concept is that
during combustion, materials with a very low ash con-
tent release elements almost completely to the gaseous
phase which will be carried over to the fly ash. This also
applies to elements considered to be non-volatile. Ele-
ments from sub-streams with a high ash content are lar-
gely or completely assigned to bottom ash. A few
materials will be distributed between the two. The result-
ing predicted compositions can be verified against the
average Dutch bottom ash and fly ash composition [19].
The advantage of such a model is that a decision can be
taken on the acceptability of new or increased inputs of
certain waste streams beforehand. In addition, it points
at the gaps in the present information on sub-streams.

In Table 2, a first comparison of predicted bottom ash

and fly ash composition with the average bottom ash and
fly ash composition in the Netherlands is made. With
this type of comparison a main drawback is the limited

Fig. 10. Relation between total Mo in Bottom ash and leached Mo from ash at L/S=10 1/kg (pH range 10.5–12; n=64 [19].

Table 2
Comparison of calculated composition for bottom ash (BA) and fly ash (FA) with average composition of Dutch MSWI bottom ash and fly ash (all
data in mg/kg)

Cd

Cu

Mo

Pb

Sb

Zn

As

Cr

Cr

a

Bottom ash
Calculated BA from input

7.5

1278

6.2

252

14

545

5.2

178

160

Average BA Netherlands

4.0

2200

11.0

1270

34

1976

6.6

117

Fly ash
Calculated FA from input

208

2505

82

2948

504

4451

36

1174

479

Average FA Netherlands

345

1175

69

8975

736

28 245

60

192

600

a

Cr based on adjusted values for the organic fraction.

H.A. van der Sloot et al. / Waste Management 21 (2001) 753–765

763

number of elements for which composition data are
available. This applies in particular for major elements.

The Cr content reported for MSWI fly ash is low,

which may be related to the type of sample destruction
(aqua regia). Data from the ‘‘Mammoet’’ project [26]
point at higher average Cr contents in fly ash (complete
matrix dissolution with HF). In view of the raw nature of
the data, the agreement between the prediction and the
measured data is quite good for Cd, Mo, Cu, Sb and As.
The Zn and Pb levels in bottom and fly ash are clearly
underestimated, which may be attributed to an insuffi-
ciently described composition of electronic parts in the
feed stock. The assessment is particularly interesting to
assess the qualities of the fly ash and APC residues as a
result of variations in low ash materials in the feedstock.

9. Conclusions

9.1. General

To guide measures in acceptance of waste, in the

conversion process and in treatment of residues, a more
thorough understanding of factors controlling the
leachability of MSWI residues is needed. Waste titration
to beat a single extraction test is not an acceptable
approach, as it will generally lead to undesirable envir-
onmental impact. Instead, the emphasis should be on an
assessment of the long-term behaviour of the materials
in their service life and ultimate fate (‘‘end of life’’). The
tools and associated modelling to make such assess-
ments have grown substantially in recent years and are
now subject of standardization. Leaching behaviour of
MSWI residues has been shown to be very systematic
and has been modelled with a great deal of success in
identifying solubility controlling phases. The under-
standing of controlling factors helps greatly in focusing
the quality improvement activities and in defining the
necessary quality control for certification purposes. Field
verification assists in keeping a balance between labora-
tory practice and realistic field conditions. Release
modelling has already shown potential for prediction pur-
poses. As the understanding grows, such assessments
may have consequences for the regulatory framework
and level setting. In addition, the process of evaluation
of environmental properties of materials for recycling
and treatment for utilization is not unique to MSWI
residues, but also applies to other industrial residue
streams.

9.2. Specific

The pH dependence leaching test provides a very

good means of mutual comparison of MSWI residues as
well as a comparison of leaching behaviour within one
and the same type of MSWI residue. Ageing of MSWI

bottom ash leads primarily to a significant pH change
with its inherent changes in leachability. The more cri-
tical pH values (4 and 6) in terms of reaching stable
conditions are relevant from a leaching behaviour and
modelling point of view. These conditions are less impor-
tant for the evaluation of environmental impact of MSWI
bottom ash. Due to carbonation (calcite buffer forma-
tion) in MSWI bottom ash, it is not likely that these pH
values will be reached under field exposure conditions.

The percolation rate within the range now used does

not appaer to be very sensitive. The type of release
mechanism (solubility, wash out, other) can be identi-
fied readily. Defining the relevant range of pH for a
scenario based on an evaluation of material behaviour
(mineralization, organic matter degradation) and exter-
nal stresses is helpful in deciding which elements need to
be addressed in quality control and for which elements
improvement or additional controls may be needed.

To create durable material improvements in ash

quality it is essential to base treatment on understanding
of the controlling mechanisms by using characterization
data for evaluation of treatment. Therefore, it is impor-
tant to relate changes to the input to the process, chan-
ges to the conversion process and changes after
treatment to the basic characterization using at least
time and pH dependent leaching data. Firstly, to avoid
solving one problem and creating a new one, and sec-
ondly, to ensure that a reduced leaching level is durable
and not counteracted by exposure to external influences.

By presenting quality control data in perspective to

previous characterization data the power of decision
based on a single data point increases drastically. The
potential of this mode of data interpretation is grossly
overlooked in current practice. For elements (e.g. metals)
controlled by solubility, measures at the input side to
reduce environmental impact are meaningless. Only when
leachability of elements is correlated with total compo-
sition or availability, can measures at the input side of
the installation be useful. Based on the assumption of
complete transfer of inorganic elements from low ash
content materials to the flue gas predictions of MSWI
bottom ash composition and fly ash composition have
been made. The agreement between the prediction and
the measured composition data is quite good for Cu,
Cd, Mo, Sb and As given the level of inherent variability
in MSWI bottom ash composition. The Zn and Pb levels
in bottom and fly ash are clearly underestimated.

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