Leachate Composition from MSWI Bottom Ash
P. Mostbauer
Institute of Waste Management, BOKU – University of Natural Resources and Applied Life
Sciences, Vienna, Austria
Abstract
A literature review about leachate composition from MSWI bottom ash was supplemented by
newer data from fully carbonated MSWI bottom ash from the Rautenweg landfill. Based on an
interpretation of the weathering state and of the chemical environments of ash monofills (pH,
Eh), pretreatment / artificial weathering of incineration residues was recommended.
Furthermore, additional monitoring of deposits of fresh, non-weathered MSWI bottom ash was
proposed.
1
Influence of weathering processes
The fundamental weathering processes, and the exothermal reactions accompanying these
processes have been described previously, e.g. by other presentations presented in this
conference proceedings (e.g. by Holger Ecke). The main question regarding the long-term
behavior of incineration residues in landfills is to what extent inorganic pollutants such as heavy
metals are “bound” or “fixed” by weathering. It is often assumed that the decrease of the pH
value is responsible for the reduction of leachability. On the other hand, sorption and the
formation of solid solutions also has an enormous effect on the leaching of some metals,
especially Pb, Ni, Cd and Cu. Secondary mineral phases formed during weathering are of high
surface area and therefore tend to bind divalent and trivalent elements.
Figure 1 illustrates the effects observed during artificial weathering of MSWI bottom ash. The
pH obtained by the German standard leaching test DIN 38.414 S4 (batch test, L/S=10) of the
original bottom ash was about 12,6. By aeration with air, the pH dropped to 11,9. With the use
of 3% CO2 or 30% CO2 in synthetic air (humidity: 100%) pH values in the range between 6,5
and 8,5 are obtained after one to three months of treatment at room temperature. Experimental
data shown in Figure 1 were obtained by Jaros and Huber (1997), those shown in Figure 2 also
by Sabbas et al. (2001).
A comparison with geochemical calculations performed with the code PHREEQC-2
demonstrates that the reduction of leachability was often stronger than expected by equilibrium
of pure mineral phases. For example, the very low Pb leachability in the pH-range between 7
and 8 cannot be explained only by a decrease of the pH. Beside the precipitation of pure Pbhydroxide and Pb-carbonates it has to be assumed that Pb is sorbed, coprecipitated or bound
in secondary solid solutions formed during the weathering process (see Figure 2).
10
5,9 mg Pb/l
Cu, Zn, Pb (mg/l)
Step 1
1
Cu
Zn
Pb
0,1
Step 3:
CO2
excess
Step 2
0,01
6
7
8
9
10
11
12
13
14
pH-value
Figure 1: Change of leachability during artificial weathering of MSWI bottom ash in laboratory
1000
Pb(OH)2
100
Cerrusite
PbCO3
Pb (mg/l)
10
Hydrocerrusite
PbCO3*Pb(OH)2
1
0,1
Anglesite
PbSO4
0,01
0,001
0,0001
3
4
5
6
7
8
9
10
11
12
13
pH value
Figure 2: Comparison of geochemical calculation – obtained Pb leachability
Fresh or artificially
weathered MSWI
bottom ash
(SABBAS et al.,
2001)
Fresh or artificially
weathered MSWI
bottom ash (JAROS
& HUBER, 1997)
Another possible explanation (derived from BCR sequential extraction) for the sometimes
extraordinary high reduction of leachability of Pb and Cd is sulfide formation (Zhang et al.,
2005).
2
Leachate from outdoor test cells – Rautenweg cells for weathered ashes
Leachate composition from field lysimeters is very dependent on the weathering state of the
material. If fresh MSWI bottom ash is covered with clayey soil in order to avoid access of CO 2
and to reduce the leachate amount, the leachate remains strongly alkaline (pH>12,0) for up to
three years (Higuchi et al., 1995). Uncovered test cells enable CO 2 access from the atmosphere
and the deposited material tends to carbonate “through and through” after a prolonged
observation period. Some results are still controversial (see Table 1), even if the leachate
samples are collected in a closed system to avoid carbonation during on-site storage of the
samples.
Height and cover
1,74 m, covered with
Results regarding pH in leachate
References
12,0 < pH < 12,8 in the first 3 years
Higuchi et al., 1995
Alkaline for 400d, then pH drops down rapidly to
Higuchi et al., 1995
clayey soil
1,74 m, uncovered
about 7 to 8,5
0,75 m, uncovered
Alkaline for about 80d
Higuchi et al., 1995
0,42 m, uncovered
9,9 < pH < 11,36, still alkaline after 2 years
Todorovic, 2004
Partially weathered,
pH about 9,5 to 10 within 200d
Weigand et al., 2003
Partially weathered,
7,9 < pH < 11,2, medium pH=10,4, no decrease
Sabbas et al., 2001
1,1 m, uncovered
during 270d
1,2 m, uncovered
Table 1: Field lysimeters and road test cells constructed with fresh or partially weathered MSWI
bottom ash.
In contrast to this, the leaching behavior of MSWI bottom ash deposited above a large amount
of untreated MSW is characterized by a complete carbonation of the surface of the grains.
During the project „IMMO1“ (Sabbas et al., 2001) bottom ash and ash/filter ash mixtures were
excavated from the Rautenweg landfill
in Vienna. These ash samples had never been in
contact with leachate from the untreated MSW deposited below them, but were fully carbonated
due to the access of landfill gas, which had lasted between 11 and 14 years. Between 10 and 20
t of the excavated ash material have been used to fill outdoor lysimeter test cells in the year
1999, when the project „IMMO1“ started. Meanwhile, 4 of these test cells are still in operation
within the project „IMMO2“. Leachate composition data from these fully carbonated materials
are shown in Table 2 and the figures below. The following abbreviations have been used:
RAU-S....MSWI bottom ash from the Rautenweg landfill containing some other ashes. Age: 11a
RAU-S2..MSWI bottom ash from the Rautenweg landfill. Age: 11 to 14a.
RAU-A....Mixture of ashes from hazardous waste incineration and sewage sludge combustion containing
some MSWI bottom ash. Age: about 11a
Medium value (if not
Material
Material
Material
Austrian drinking
otherwise stated)
RAU-A
RAU-S
RAU-S2
water standards
Period
1999 - 2003
1999 - 2003
1999 - 2003
2004
Samples (n)
133
122
151
pH, Minimum
6,92
6,97
7,24
pH, Medium
7,51
7,60
7,77
pH, Maximum
8,20
8,19
8,28
Cond.(mS/cm)
4,0
4,9
5,7
Al (mg/l)
0,067
0,043
0,026
0,200
Sb (mg/l)
0,045
0,029
0,021
0,005
B (mg/l)
3,8
4,2
1,8
1,0
< 9,5
Table 2: Leachate from weathered incineration residues (Rautenweg test cells)
0,50
7
Zn
Mo
L/S
Zn, Mo in leachate of RAU-A (mg/l)
6
0,40
5
0,30
4
3
0,20
2
0,10
1
0,00
0
0,0
0,5
1,0
1,5
2,0
Time (a)
Figure 3: Leachate from test cell RAU-A
2,5
3,0
3,5
4,0
1,00
7
Zn
Mo
L/S
5
0,60
4
3
0,40
2
Liquid-to-solid ratio (L/S)
Zn, Mo in leachate of RAU-S (mg/l)
6
0,80
0,20
1
0,00
0
0,0
0,5
1,0
1,5
2,0
2,5
3,0
3,5
4,0
Time (a)
Figure 4: Leachate from test cell RAU-S
1,00
7
5
0,60
4
Zn
Mo
L/S
3
0,40
2
0,20
1
0,00
0
0,0
0,5
1,0
1,5
2,0
Time (a)
Figure 5: Leachate from test cell RAU-S2
2,5
3,0
3,5
4,0
Liquid-to-solid ratio (L/S)
Zn, Mo in leachate of RAU-S2 (mg/l)
6
0,80
Within the IMMO1 and IMMO2 projects, calculations with PHREEQC-2 were used to interpret
the leachate composition. Without going into detail, the conclusion from this glance into the
chemistry of ashes was:

For some main components, like Ca, Mg, Ba and Al, the solubility-controlling mineral phases
were identified, e.g. calcite, dolomite, Mg(OH)2, amorphous alumina, gibbsite, gypsum and
barite

Sorption, coprecipitation and formation of solid solutions play an important role in the fixation
of heavy metals

Fe and Mn were mobile to some extent during extremely wet climatic periods or after
artificial irrigation, but were removed from the leachate again after sufficient access of
atmospheric oxygen during dry weather. This points to the significance of redox processes
for the leaching behavior of Fe, Mn, V and trace compounds that are fixed by Fe-IIIprecipitates.
Furthermore, physical retention of soluble chemical species like K, Na, sulfate and molybdate
also occurred. For example, the first campaign of artificial irrigation for the test cell RAU-S
resulted in a remarkable increase of Mo concentration at a L/S-ratio of 1,9 (see Figure 4). It can
be assumed that stagnant water zones were attained after build-up of temporary water horizons
inside the test cell. In the future, stagnant water models (so-called dual porosity models) could
be used to estimate the release of Mo, Sb and K during snowmelt or intensive rain.
3
Leachate from ash landfills (monofills)
The actual composition of leachate is only available for a few monofills containing MSWI bottom
ash, e.g.:

Vestskoven, Denmark

Woodburn, USA

Hartford, USA

Lostorf, Switzerland
The Danish landfill Vestskoven is a monofill for bottom ash (85%) and fly ash (15%). Sampling
of the leachate from this landfill has already been performed over the last 21 years (Hjelmar,
1995)
The Lostorf landfill (Switzerland) consists of several compartments, in which MSWI bottom ash
has been deposited between 1992 and 1996. The hydrology and leaching behavior in one of
these compartments is very well documented. MSWI bottom ash has been deposited here after
a natural weathering in piles after 6 to 9 months (Johnson et al., 1999)
Hartford and Woodburn are American monofills of bottom ash and fly ash. The exact origin of
the deposited ash wastes is unknown (Kosson et al, 1995, Hjelmar, 1996).
The development of the pH-values in the leachate was reported, but the graphs used for
illustration in literature are in most cases of weak performance. The published literature data
about pH are summarized in Table 3. The redox state (e.g. Eh value) of the landfills is welldocumented only for the Vestskoven landfill and a landfill near Copenhagen.
Landfill
Development of the pH in the leachate
Redox state of the leachate
Vestskoven
First available value: 10,5
Eh value fluctuating, between
During the first two years: 8,8 to 10,1. Afterwards
-300 and –10 mV
the pH increases slowly to a mean value of about
10,0 (Minimum: 8,9)
Woodburn
Hartford
Mostly between 6,5 and 7,5, occasionally lower
Rather reducing (much
(Minimum: 5,7)
dissolved Fe in leachate)
Values of the first two years missing, afterwards:
---
6,1 to 7,0
Lostorf
Values of the first three years missing,
Oxidizing. All leachate samples
afterwards: Fluctuating between 8,7 and 11,3
contained dissolved oxygen.
(Age of waste: 3 to 4 years)
Table 3: Development of pH and Eh in leachate of ash monofills
Hjelmar (1996) suggests the unburned portion in the MSWI bottom ash could be higher in
Woodburn compared to Vestskoven. TOC- and Fe-values of the leachate support this
suggestion. By biological degradation of unburned organics, and perhaps also by oxidation of
Fe-II, CO2 is formed, which leads to a faster decrease of the pH.
Dry weather runoff from MSWI bottom ash at Lostorf is on an average more alkaline (pH=11,26)
than leachate, even after rain (minimum: 8,68). The leachate composition is affected by
preferential flow (Johnson et al., 1998). The average alkalinity measured by titration with acid
was 1,62 mmoles during dry periods, which is lower than the total average (1,39 mmoles,
Johnson et al., 1999). Similar observations were made for conductivity, Na, K, Cl and sulfate.
Obviously, the micropore leachate of the landfill is diluted by macropore leachate.
pH-values between 7,5 and 9,9 were observed by Kieser et al.(1995) in bore hole samples of
the Offenbach bottom ash landfill. However, these pH values have probably been distorted by
the sample preparation procedure, because the samples were dried in laboratory before the
leaching test.
As already mentioned in chapter 2 (Table 1), field test cells at a height of 1,74 m, constructed
with fresh MSWI bottom ash, resulted in alkaline leachate. In the case of a low-permeability soil
cover the pH ranged between 12,0 and 12,8 in the first three years (Higuchi et al., 1995). After
about three years the pH starts to decrease slowly. The alkaline time horizon is reduced
dramatically, if the height of test cell is only 0,75 m and the bottom ash is uncovered. Under
these circumstances, the carbonation proceeds faster and a chemically neutral leachate is
observed after >80 days. Contradictory to this, leachate from a road test cell (height: 0,42 m)
was still alkaline in the second year of investigation as reported by Todorovic, 2004. Within two
years, pH values between 9,9 and 11,56 were observed in the runoff of the road test cell, and
Pb concentrations were quite low (from ˜0,002 mg/l to 0,007 mg/l). Todorovic suggests that lead
was fixed by secondary mineral phases like ettringite and CSH-minerals.
4
Conclusion

Initially, leachate from fresh MSWI bottom ash is strongly alkaline (even pH>12) and saline.

The time horizon until a carbonate buffer system is reached depends on many factors, such
as geometry of the landfill, characteristics and mass portion of incineration residues,
hydrology of the landfill etc.

If the landfill is built on rapidly, no full carbonation will occur. The leachate is assumed to
remain alkaline and highly saline.

The quality of landfill leachate reported until now points to a partial weathering (step 1 of
carbonation, Lostorf, Vestskoven) or full carbonation (Woodburn, Rautenweg) of the
deposited waste. Regarding the Rautenweg landfill, the full carbonation (step 2 of
carbonation) is caused by long-lasting access of landfill gas from the adjacently deposited
untreated MSW.

With further CO2 access (step 3 of carbonation), a decrease of the pH down to pH=5,7 to
˜6,5 and mobilization of some elements (Zn, Cd, Cu, Ni) is expected.

Although a massive mobilization of Pb and Zn in the alkaline environment is not observed in
the Lostorf and Vestskoven landfills, eluate analysis from the Waldering bore hole samples,
geochemical modeling, as well as test cells operated by Higuchi et al.(1995) indicate that
there is a danger of a severe release of Pb up to about one to two decades after deposition
of unweathered MSWI bottom ash.

Moreover, high temperatures and the formation of considerable amounts of hydrogen are
expected in bottom ash monofills. No repair system exists for liners, which are eventually
damaged in landfills by the evolving high temperatures.

As a consequence, artificial weathering of MSWI bottom ash or other treatment before
deposition is recommended.

Existing landfills in Austria, which deposit untreated MSWI bottom ash in spite of this
recommendation, need additional monitoring systems regarding the development of pH,
temperature and gas formation in the landfill. In the case of separate collection of leachate,
the water threshold , pore gas composition and the composition of the leachate should be
documented in a way that these data are available also for geochemical interpretation. In
case no separate leachate collection system is foreseen, we suggest that it could be useful
to construct large-scale field test cells for the documentation of the emission behavior of the
specific waste.

According to the Austrian environmental legislation, Pb-containing alkaline leachate has to
be treated before discharge into the sewer or rivers.

Regarding monitoring of leachate emissions and testing of waste acceptance criteria,
carbonation of leachate samples and alkaline bottom ash solid samples should be avoided
on-site as well as in laboratory.

For scenarios where there is much CO2 access to the deposited MSWI bottom ash or CO2
production in the landfill, specific validated tests like the MPWLP test (Mostbauer et al.,
2003) should be applied for the evaluation of the long-term behavior.
5
some other points
5.1
and more articles
5.1.1
und more specific
References
Heuss-Assbichler, S. (2004): Salt horizons – as heavy metal barriers within the monofill for MSWI
residues. Applied Mineralogy, 2004, pp. 431-434.
Higuchi, S. et al. (1995): Dissolution process of landfilled incineration residue. In: Proceedings Sardinia 95,
5th International Landfill Symposium.
Hjelmar, O. (1995): Incineration of MSW: Implications for landfill strategy and leachate management. In:
Proceedings Sardinia 95, 5th International Landfill Symposium.
Hjelmar, O. (1996): Disposal strategies for municipal solid waste incineration residues. Journal of
Hazardous Materials 47, pp. 345-368.
Jaros, M. and Huber, H. (1997): Emissionsverhalten von MVA-Schlacke nach forcierter Alterung. In:
Lechner (Editor) Waste Reports, No. 6, Institute of Waste Management, BOKU University, Vienna.
Johnson, A.C., Richner, G.A., Vitvar, T., Schittli, N. and Eberhard, M. (1998): Hydrological and
geochemical factors affecting leachate composition in municipal solid waste incinerator bottom ash.
Part I: The hydrology of landfill Lostorf, Switzerland. Journal of Contaminant Hydrology, Vol. 33, pp.
361-376.
Johnson, A.C., Kaeppeli, M., Brandenberger, S., Ulrich, A. and Baumann W. (1999): Hydrological and
geochemical factors affecting leachate composition in municipal solid waste incinerator bottom ash.
Part II: The geochemistry of leachate from landfill Lostorf, Switzerland. Journal of Contaminant
Hydrology, Vol. 40, pp. 239-259.
Kieser, J., Begand, S. and Hanke, W. (1995): Untersuchung kausaler Zusammenhänge zwischen
Stoffwandlungsvorgängen und Temperaturverhältnissen in der MVA-Schlackedeponie Offenbach/M.
Umweltplanung, Arbeits- und Umweltschutz, No. 204. Editor: Hessische Landesanstalt für Umwelt.
Klein, R., Baumann, T., Kahapka, E. and Niessner, R. (2001): Temperature development in a modern
municipal solid waste incineration (MSWI) landfill with regard to a sustainable waste management. In:
Journal for Hazardous Materials, pp. 265-280.
Kosson, D.S., Van der Sloot, H.A. and Eighmy, T.T. (1996): An approach for estimation of contaminant
release during utilization and disposal of municipal waste combustion residues. Journal of Hazardous
Materials, Vol. 47, pp. 43-75.
Magel, G., Heuss-Assbichler, S. and Fehr, K.T. (2001): Abiotic H2-Generation Processes in a MSWI
Monofill, in: Proceedings of the 8th International Waste Management & Landfill Symposium, Sardinia,
pp. 489-498.
Meima, J.A. (1997): Geochemical modelling and identification of leaching processes in MSWI bottom ash.
PhD, University Utrecht, NL.
Mostbauer, P., Riegler, H., Lechner, P. (2003): Artificial Weathering of Inorganic Waste – Evaluation of
Laboratory Weathering and Leaching Methods, in: Proceedings of the 9th International Waste
Management & Landfill Symposium, Sardinien, C12: pp. 288-289.
Sabbas, T. et al. (2001): Evaluierung von Testmethoden und Modellen für die Bewertung des mittel- bis
langfristigen
Emissionsverhaltens
schadstoffhältiger
anorganischer
Abfälle
und
Immobilisierungsprodukte, im Auftrag der Stadt Wien, BMLFUW, BMBWK und INTERREG.
Todorovic, J. (2004): Diffusion tests for assessing leaching from incineration residues. Dissertation
Nr.14/2004, Lulea University of Technology, Lulea, Sweden.
Weigand, H., Müller, S. and Marb, C. (2003): Seepage water emission of inorganic contaminants from
MSWI bottom ash: Results from lab-, pilot-, and field-scale experiments. In: Proceedings of the 9th
International Waste Management & Landfill Symposium, Sardinia, pp. 489-498.