This study elucidates the behaviour of heavy metals in slag produced from four different sewage sludge ashes mixed with municipal
solid waste incinerator fly ash and then co-melted. Experimental results indicate that sewage sludge ashes consisted of SiO2,
CaO, and Al2O3. Fly ash consisted of CaO, Na2O and SO3. The speciation of sewage sludge ashes indicates that the ashes contained
quartz and AlPO4. The speciation in fly ash consisted of anhydrite, microcline, calcium chloride, sylvite and halite. The
leaching behaviours of sewage sludge ashes met the Taiwan Environmental Protection Administration’s regulatory standards.
The fly ash had high concentrations of Zn and Pb; however, the leaching of these metals was low. The major components of synthetic
slags were SiO2 (33.5–54.0%), CaO (21.4–36.7%), and Al2O3 (8.1–15.7%). The X-ray diffraction patterns of co-melted
slags demonstrate that the slags contained significant amounts of glass. Most heavy metals can be fixed in a net-like structure;
thus, they can not be extracted easily. The toxicity characteristic leaching procedure (TCLP) leaching concentrations for target
metals in all slags met the Taiwan Environmental Protection Administration’s regulatory standards.
Keywords: Sewage sludge ash, fly ash, heavy metal, leaching concentration, vitrification
Introduction
With the spread of sewage systems and the increase in the volume
of sewage to be treated, sewage sludge production has
increased annually. More than 500 000 tonnes of sewage sludge
ash are generated in Taiwan annually. In Europe, heavy metals,
as well as phosphorus, from sewage sludge ash are recycled
(Pettersson et al. 2008). In Taiwan, however, locations deemed
appropriate for disposal of sludge are limited and its treatment/
disposal and environmental impact has become major public
concern. Additionally, according to statistics from the Taiwan
Environmental Protection Administration, by 2010, municipal
solid waste incinerators (MSWIs) islandwide will discharge
more than 1 000 000 tonnes of incinerator residues (including
bottom ash, fly ash, and scrubber ash) annually. The fly ash
(including cyclone ash and scrubber ash) is an especially problematic
residue as it contains high concentrations of heavy metals
(Jakob et al. 1995) and trace amounts of polychlordibenzodioxins
and -furans (Wang et al. 2006, Shih et al. 2006).
Several new technologies, such as the immobilization of fly
ash with cement (Kuo et al. 2006), acid neutralization (Johnson
et al. 1995), wet chemical treatment (Arickx et al. 2006),
and thermal treatment, are in the development process. The
goal is to decontaminate toxic residues and render them
Corresponding author: Kae-Long Lin, Department of Environmental Engineering, National Ilan University, 1, Sec.1, Shen Lung RD., I-Lan, Taiwan,
26047, R.O.C.
inert such that they can be reused or deposited without risk.
Of significant promise are the thermal processes that melt
residues at temperatures of roughly 1300–1400 °C, which
produces a relatively inert glass. However, scrubber ash, the
largest component (about 80% by weight) in MSWI fly ash,
which is a mixture of boiler ash, scrubber ash and cyclone
ash, makes melting MSWI fly ash alone difficult due to the
high pouring point, which is roughly 2570 °C of the scrubber
ash component CaO (11–35% by weight). The major components
in sewage sludge ash and MSWI fly ash, which includes
cyclone ash and scrubber ash (referred to as fly ash herein),
are silica, aluminum, and calcium (Iori & Balg 1995). The
principal advantages of the sludge melting process are that
most hazardous materials, such as heavy metals, are tightly
fixed in a solid phase, and the slag generated by this process
can be used as construction material (Sakai et al. 1990).
However, the basicity of the main components of MSWI fly
ash and its fluxes must be modified by mixing fly ash with
sludge ash from sewage treatment plants. This step is necessary
to reduce the pouring point of MSWI fly ash mixes during
co-melting treatment (Lin 2006). Glassy slag can be generated
by melting waste at temperatures exceeding 1300 °C,
after which the molten ash is water-quenched or air-cooled.
The volume of the resulting slag can be reduced and the slag
stabilized such that heavy metals become immobilized in a
glassy Si–O matrix; thus, leaching behaviour is improved
(Young & Jong 2002, Lin 2006).
The effects of heavy metals leaching from slags comprised
of co-melted sewage sludge ash and MSWI fly ash have seldom
been investigated and so the present study was performed
to attempt to fill in this gap (Johnson et al. 1996, Kida
et al. 1996). The study, in which sewage sludge ash and MSWI
fly ash were co-melted, focused on the co-melting conditions
under which the slag was generated and so the behaviour of
heavy metals in slags produced from four different sewage
sludge ashes mixed with MSWI fly ash and then co-melted
was characterized.
Materials and methods
Preparation of sewage sludge ash and MSWI fly ash
Dewatered sewage sludge was collected from the Ba-Li Sewage
Treatment Plant, Chung-Chou Sewage Treatment Plant,
Chung-Hsing-Hsin-Tsun Sewage Treatment Plant and Nei-
Hu Sewage Treatment Plant. In total, 300 kg of sewage
sludge was obtained from the I-lan Plant, an incineration
plant. The sludge was then heated in a brick-fired kiln to a
temperature of 900 °C for 1 h. Table 1 lists the properties of
the four sewage sludge ash types, namely Ba-Li sewage sludge
ash (BL-Ash), Chung-Chou sewage sludge ash (JJ-Ash),
Chung-Hsing-Hsin-Tsun sewage sludge ash (SC-Ash) and Nei-
Hu sewage sludge ash (NH-Ash). The fly ash was collected
from the cyclone of a mass-burning incinerator located in
northern Taiwan. The incinerator, which is capable of processing
1350 metric tonnes of municipal solid waste per day, is
equipped with air-pollution-control devices, consisting of a
cyclone, semidry scrubber system and a fabric baghouse filter.
In total, 200 kg of fly ash was obtained from the incineration
plant. The MSWI fly ash and sewage sludge ashes were homogenized
and oven dried at 105 °C for 24 h. The chemical composition
was then characterized. In summary, the co-melted slags
were produced according to the following treatment steps.
Fly ash → (+four sewage sludge ash types)
→ (co-melting treatment) → (water quenching)
→ (milling) → modified slag (1)
Preparation of slag from MSWI fly ash and sludge ashes by
co-melting treatment
In this study, test samples were obtained by first grinding
MSWI fly ash and sewage sludge ashes; varying composite fractions
were then utilized. As defined by Murakami, basicity =
CaO/SiO2 and so the pouring point was affected by the content
of CaO and SiO2 in the mixtures. Experimental results suggest
that the pouring point of the mixture increased as basicity
increased and that the pouring point was lowest when basicity
was 1. Therefore, the experimental design of basicity (CaO/
SiO2) was 0.4–1.06. The fractions in MSWI fly ash and sewage
sludge ashes are shown in a ternary phase diagram (Figure 1).
Table 2 shows the different proportions used in the co-melting
treatment. Twelve synthetic slag samples were produced. The
molten slag was water-quenched to produce a fine slag, which
was then further pulverized in a ballmill until the particles
could pass through a #200 mesh sieve. The resultant pulverized
slag was desiccated prior to testing.
Analyses
The following chemical and physical analyses of the ash and
the slag samples were conducted.
Table 1: Properties of sewage sludge.
Type of
sludge ash
Sewer system
Wastewater treatment
process
Sludge treatment
process
Conditioning process
BL-Ash Combined sewer system Primary treatment – Polymer-conditioned sludge
JJ-Ash Combined sewer system Primary treatment Anaerobic
digestion
Polymer-conditioned sludge
SC-Ash Separated sewer system Secondary treatment Anaerobic
digestion
Polymer-conditioned sludge
NH-Ash Separated sewer system Secondary treatment Aerobic
treatment
Polymer-conditioned and
lime-stabilized sludge
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K.-L. Lin, W.-J. Huang, J.-D. Chow, K.-C. Chen, H.-J. Chen
662
1. Toxicity characteristic leaching procedure (TCLP): SW846-
1311. The extraction procedure requires the preliminary
evaluation of the pH characteristic of the sample to
determine the proper extraction fluid necessary for the
experiment. After testing, extraction fluid #B (pH = 2.88
± 0.05) was selected for the TCLP analysis. This fluid was
prepared by diluting 5.7 mL acetic acid to a volume of 1 L.
A 25-g sample was placed in a 1-L Erlenmeyer flask, and
500 mL of extraction fluid was added to each Erlenmeyer
flask. The samples were then agitated for 18 h with an
electric vibrator. The slurry was filtered using 6–8 μm pore
size Millipore filter paper. The leachates were preserved
in 2% HNO3.
2. Total heavy metal concentration. The total heavy metal
concentrations in the MSWI ash and slag samples were
confirmed by inductively coupled plasma atomic emission
spectroscopy (ICP-AES). The samples were crushed, and
the heavy metals were extracted by acid (HF : HClO4 :
HNO3 = 2 : 1 : 1). The data were the average of three
replicates throughout the experiments in order to obtain
as good reproducibility as possible.
3. Pouring point. The pouring point of the ash was determined
by the ASTM D1857 method (ASTM D1857 2008).
4. Chemical composition. X-ray fluorescence (XRF) was
performed with an automated RIX 2000 spectrometer.
The specimens were prepared for XRF analysis by mixing
0.4 g of the sample and 4 g of 100 Spectroflux, at a dilution
ratio of 1 : 10. Homogenized mixtures were placed in Pt–
Au crucibles, before being treated for 1 h at 1000 °C in an
electrical furnace. The homogeneous melted sample was
recast into glass beads 2 mm thick and 32 mm in diameter.
5. Mineralogy. The X-ray diffraction (XRD) analysis was carried
out using a Siemens D-5000 X-ray diffractometer with
CuKα radiation and 2θ scanning, ranging between 5 and
70° (2θ). The XRD scans were run in 0.05° steps, with a 1 s
counting time.
Results and discussion
Characterization of sewage sludge ashes and MSWI fly ash
Table 3 lists the chemical compositions of sludge ashes. The
major components of the sludge ashes were SiO2 (44.6–
76.5%), Fe2O3 (4.6–8.1%) and Al2O3 (10.3–17.2%). The next
Fig. 1: Sewage sludge ashes and co-melted slags in a ternary phase diagram. Temperature in the diagram is the melting point of slag.
Table 2: Proportions of raw materials used for preparation of synthetic slags.
BL3 BL4 BL5 JJ3 JJ4 JJ5 SC3 SC4 SC5 NH4 NH5 NH6
Fly ash (%) 70 60 50 70 60 50 70 60 50 60 50 40
Sludge ash (%) 30 40 50 30 40 50 30 40 50 40 50 60
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Heavy metals immobilization by co-melting sewage sludge ash and MSW incinerator fly ash
663
most abundant components were CaO (0.5–6.8%), P2O5 (0.8–
10.2%) and Na2O (0.01–0.92%). According to XRF analysis,
the major components in the fly ash were CaO (38.4%),
Na2O (5.9%) and SO3 (4.7%). The CaO existed in large
amounts in fly ash due to the injection of a lime solution to
remove acidic gas. The next most abundant components
were Fe2O3 (0.6%), SiO2 (2.3%) and K2O (4.4%). Figure 2
presents the speciation of sewage sludge ashes. Speciation
results indicate that the major components were quartz
(SiO2) and AlPO4.
Total heavy metals and leaching concentration of sewage
sludge ashes and MSWI fly ash
For fly ash, most heavy metal concentrations were higher than
those in sewage sludge ash (Table 4). This can be explained
by the fact that vapourized metals condensed and aggregated
on the surface of fly ash due to the low temperature in the
flue gas cooling system.
Table 5 lists the leaching concentrations obtained by TCLP
testing. All leaching concentrations of sewage sludge ashes
complied with the regulatory thresholds of Taiwan’s Environmental
Protection Administration. The fly ash had higher Pb,
Cd and Cr leaching concentrations than the sludge ashes. The
analytical results of metal content analysis, as well as those
for the leaching behaviour of heavy metals, show that the fly
ash had high concentrations of Zn and Pb, and little leaching
of these metals. In particular, the Cr concentration leaching
from fly ash was 5.05 mg L–1, which exceeded the current Taiwan
Environmental Protection Administration’s regulatory
standard; thus, the fly ash was classified as a hazardous waste.
The leaching characteristics of heterogeneous MSWI ash
were difficult to determine because leaching was dependent
on numerous physicochemical interactions (Johnson et al.
1996). However, the study results remain worthwhile as they
show the irregularity of characteristics of heavy metal leaching
from ash collected from different municipal solid waste
incinerators.
Composition characterization of co-melted slag
Table 6 summarizes the XRF analysis results obtained for different
slags. Table 6 lists the major components of co-melted
slag used in this study. The major components in synthetic
slags were SiO2 (33.5–54.0%), CaO (21.4–36.7%), and Al2O3
(8.1–15.7%). The next most abundant components were
Fe2O3 (2.9–5.6%), MgO (1.1–2.0%), P2O5 (1.0–8.2%) and
K2O (0.2–1.5%). Figure 3 shows the XRD patterns of comelted
slag. The slags contained large amounts of glass. The
XRD pattern results indicate that sludge ashes (Figure 2)
and slag structures significantly differed (Figure 3); the
former having a more crystalline structure whereas the latter
had an amorphous glassy matrix.
Total heavy metals after co-melting treatment
Table 7 shows the total heavy metals in co-melted slags and
residual fractions of heavy metals in co-melted slags. The residual
fractions of heavy metals, defined as the ratio of heavy
metal mass in a slag sample to that in original ash. Several
heavy metals that are known to be volatile were immediately
vapourized during the melting process. Such metals easily
become volatile when co-melting sewage sludge ashes and fly
ashes. Lead has a low elemental melting point of 327 °C and
boiling point of 1740 °C. Cadmium has a low elemental melting
point of 321 °C and boiling point of 765 °C (Lees et al.
1995). Chromium has a high elemental melting point of
1900 °C and boiling point of 2642 °C. High vapour pressure
makes the retention of such metals as Cd, Pb, and Zn in the
melt and slag difficult because they are emitted during the
co-melting treatment. Owing to its volatility as a metal, Cd has
a low solubility in molten glass-like slag and a high vapour
pressure in the vitrification temperature range; thus, it is typically
completely removed from ash via vapourization. The
high loss of Pb can be attributed to volatilization, with large
concentrations existing in the off-gas system (Cortez et al.
1996). This study demonstrates that the emission behaviours
of volatile metals during co-melting varied with ash composition.
Another group of metals, which are classified as semivolatile
or volatile metals, includes Zn, which showed values
of approximately 48.2–64.5% in the slag. Notably, Zn was
partially vapourized when temperature increased to 1200 °C
(Fukui et al. 1994). However, the collection rate for Cu indicated
that roughly 7.6–18.6% remained in the slag. It is
thought that Cu had been converted to Cu2O, which has a
relatively high boiling point, indicating that the amount of
this volatile compound was relatively low (Songa et al. 2004).
Of the non-volatile metal group, about 92–100% of Cr was
retained in the slag due to its low volatility. Notably, Cr was
converted into CrO3, a relatively stable compound, during
high temperature oxidation (Chang & Biswas 1993). The
major chromium species in fly ash were chromium chloride
(boiling point, roughly 1200–1500 °C) and chromium oxide
(boiling point, roughly 1900 °C); that is, both have high boiling
and co-melting points.
Leaching concentrations of co-melted slags
To assess environmental implications, co-melting products
were tested in accordance with the Taiwan Environmental
Protection Administration’s TCLP. Table 8 provides a summary
of leaching concentrations of co-melted slags. The leaching
concentrations of vitrified products and metals were
Fig. 3: XRD patterns of co-melted slags.
below Taiwan Environmental Protection Administration limits
for each of the four regulated metals. The maximum
allowable leachate concentrations were 5, 1, 15 and 5 mg L–1
for Pb, Cd, Cu and Cr, respectively. Co-melting decreased
leaching concentrations in the slag samples (Table
. The
TCLPs of Cu and Zn were increased relative to fly ash alone,
and for Zn, also relative to sewage sludge ashes. Leaching
concentrations of heavy metals in these slags were lower than
those in fly ash due to the large amount of SiO2 with a netlike
structure in sludge ashes. The fundamental structure of
silicates consists of four O2– at the apices of a regular tetrahedron
surrounding and coordinated by one Si4+ at the centre.
The strength of any single Si–O bond is only equal to one-half
of total bonding energy available in the oxygen atom. Typically,
most heavy metals can be fixed in this net-like structure;
therefore, they cannot be extracted easily. The leaching concentrations
of vitrified products and heavy metals all met the
Taiwan Environmental Protection Administration’s standards.
Based on these experimental results, all TCLP leaching concentrations
for target metals in all co-melted slag samples met
the Taiwan Environmental Protection Administration’s regulatory
standards. The low leachability of metals presumably
resulted from the fact that heavy metal ions were replaced by
parent ions (Al3+ and Ca2+) and enclosed in a silicate framework.
Conclusions
1. Sludge ashes consisted of SiO2, CaO, and Al2O3. Fly ash
contained CaO, Na2O and SO3. The speciation of sewage
sludge ashes demonstrated that they contained quartz
and AlPO4. The speciation of fly ash showed that it contained
anhydrite, microcline, calcium chloride, sylvite
and halite.
2. All leaching concentrations of metals in sewage sludge
ashes met the Taiwan Environmental Protection Administration’s
regulatory thresholds. Fly ash contained high
concentrations of Zn and Pb; however, the leaching ratios
of these metals were low.
3. The major components in synthetic slags were SiO2 (33.5–
54.0%), CaO (21.4–36.7%) and Al2O3 (8.1–15.7%). The
XRD patterns of co-melted slag showed that the slags
contained large amounts of glass.
4. Most heavy metals were fixed in a net-like structure,
and, thus, not easily extracted. All TCLP leaching concentrations
for the target metals in slags met the Taiwan
Environmental Protection Administration’s regulatory
thresholds.
5. Co-melting increased the TCLP of Cu and Zn. Additionally,
heavy metals vapourized and were emitted into air
during co-melting; thus, an adequate means of preventing
their leaching is needed. Recovering and recycling heavy
metals should be explored in future work.
Acknowledgements
The authors would like to thank the National Science
Council of the Republic of China, Taiwan, for financially
supporting this research under Contract No. NSC-95-2221-
E-197-016. Ted Knoy is appreciated for his editorial assistance.
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