Evolution of heavy metals in municipal solid waste during bio-drying and implications of their subsequent transfer during combustion

Waste Management 31 (2011) 1790–1796

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Evolution of heavy metals in municipal solid waste during bio-drying and implications of their subsequent transfer during combustion Dong-Qing Zhang, Hua Zhang ⇑, Chang-Lin Wu, Li-Ming Shao, Pin-Jing He ⇑ State Key Laboratory of Pollution Control and Resources Reuse, College of Environmental Science and Engineering, Tongji University, 1239 Siping Road, Shanghai 200092, PR China

a r t i c l e

i n f o

Article history: Received 26 September 2010 Accepted 4 April 2011 Available online 4 May 2011 Keywords: Bio-drying Combustion Heavy metal speciation Heavy metal transfer Low heating value Municipal solid waste

a b s t r a c t Bio-drying has been applied to improve the heating value of municipal solid waste (MSW) prior to combustion. In the present study, evolution of heavy metals in MSW during bio-drying and subsequent combustion was studied using one aerobic and two combined hydrolytic–aerobic scenarios. Heavy metals were concentrated during bio-drying and transformed between different metal fractions, namely the exchangeable, carbonate-bound, iron- and manganese-oxides-bound, organic-matter-bound and residual fractions. The amounts of heavy metals per kg of bio-dried MSW transferred into combustion flue gas increased with bio-drying time, primarily due to metals enrichment from organics degradation. Because of their volatility, the partitioning ratios of As and Hg in flue gas remained stable so that bio-drying and heavy metal speciation had little effect on their transfer and partitioning during combustion. In contrast, the partitioning ratios of Pb, Zn and Cu tended to increase after bio-drying, which likely enhanced their release potential during combustion. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction The recent municipal solid waste (MSW) management strategies advocate material recycling, energy recovery, reduction and stabilization of MSW before landfill disposal (Adani et al., 2002). Therefore, the combustion and biological processes yielding thermal power, refuse-derived fuel, compost, and stabilized product, have drawn increasing interest. Combustion is an effective MSW treatment option that contributes to waste stabilization and maximum reduction of waste volume, as well as to sanitation and energy recovery (Liu and Liu, 2005). Due to a high proportion of food waste (>60%), MSW in many developing countries such as China, India and Chile has a high water content (Bezama et al., 2007; Hazra and Goel, 2009; SIDREE, 2009), which lowers the recovery of recoverables and increases the operation cost of combustion. Composting and other biostabilization processes result in the complete degradation of easily degradable organic matter, whereas bio-drying dries the waste while increases its heating value by reducing its water content (Bezama et al., 2007; Rada et al., 2007; Sugni et al., 2005; Zhang et al., 2008a). When there are high temperatures and adequate ventilation, the biological processes enhance the evaporation of water via heat produced by the degradation of organics. The combined hydrolytic–aerobic process consists of a hydrolytic stage and a subsequent aerobic biodegradation stage. The former runs on limited oxygen and ⇑ Corresponding authors. Tel./fax: +86 21 6598 6104. E-mail addresses: [email protected] (H. Zhang), [email protected] (P.-J. He). 0956-053X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.wasman.2011.04.006

polymer hydrolysis, resulting in mass decrease of the organic materials in MSW and leachate release, which favors water evaporation during the latter stage due to a low water-to-organics ratio (Zhang et al., 2008a). Bio-drying involves waste aeration and thus resembles composting. It leads to the biodegradation of organics and causes significant variations in water content, pH, ammonia, dissolved organic carbon, and humic substances. These variations again affect the speciation of heavy metals (As, Hg, Pb, Zn, and Cu) in MSW and their subsequent release and partitioning during combustion. Currently, it is not clear how heavy metal concentrations and speciation vary in MSW bio-drying, although the formation of heavy metals in MSW or sludge composting has been described (Farrell and Jones, 2009; He et al., 2009). Heavy metal concentrations increased during composting due to organics degradation and moisture loss. However, the changes in heavy metal speciation were not consistent for different trace elements, because of differences in metal species, pH, and content of water, ammonia, dissolved organic carbon, and humic substances in the composted materials. The transfer of heavy metals during combustion has primarily been studied in terms of their distribution in combusted wastes, physicochemical combustion conditions such as turbulence and time, and the parameters affecting combustion kinetics (Zhang et al., 2008b). However, there is little information available on whether and how heavy metal speciation affects its transfer and partitioning in combustion flue gas. The transfer of toxic heavy metals to flue gas is a major environmental concern because it determines metal concentrations in residues collected in air pollution control (APC) devices or in flue gas if

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the devices fail. In this study, one aerobic and two combined hydrolytic–aerobic bio-drying scenarios were used to examine variations in heavy metal concentrations and speciation during MSW bio-drying and their effect on the transfer of these metals to flue gas during the combustion of bio-dried MSW. 2. Materials and methods 2.1. Characteristics of the MSW feedstock Raw MSW was sampled from a residential area of Shanghai, China during spring. The mixed collected MSW sample had an initial water content of 68% (w/w, in wet weight). The volatile solid (VS) content of the sampled MSW after hand sorting to remove the plastics was 0.88 g/g-total solid (in dry weight). No batteries were found in the sampled waste in this research.

Fig. 1. Schematic diagram of the apparatus (Zhang et al., 2009).

2.2. Experimental treatments Three treatments as shown in Table 1, including one aerobic and two combined hydrolytic–aerobic scenarios, were implemented to evaluate the bio-drying performance. The process conditions were determined based on the previous experiments in which the optimal parameters for MSW bio-drying were studied with regard to water removal (Zhang et al., 2008a; Zhang, 2009). Lab-scale columns were used in the experiments. Briefly, the MSW was mixed adequately, after which 28 kg were loaded into each column, as shown in Fig. 1 (Zhang et al., 2009). The samples were then treated for 16 days and the air-inflow rate was fixed at 0.056 m3 per kg of wet waste per hour throughout the entire process. For the aerobic bio-drying scenario (hereafter referred to as ‘‘Aerobic’’), the ventilation procedure was fixed at 7 min run/23 min stop and the fed waste was manually turned every 2 days. For the combined hydrolytic–aerobic bio-drying scenarios, during the hydrolytic stage (0–4th day) the aeration strategies were adopted as non-forced aeration (hereafter referred to as ‘‘Combined 1’’) and a micro-aeration with a ventilation procedure at 10 min run/230 min stop (hereafter referred to as ‘‘Combined 2’’). During the aerobic stage (5th–16th day), the aeration and turning operations were the same as those for the Aerobic scenario. 2.3. Equipment for bio-drying Each column was made up of PVC plastic and had a height of 1200 mm and an internal diameter of 400 mm, as shown in Fig. 1 (Zhang et al., 2009). Each column was wrapped with hollow cotton with a thickness of 100 mm to provide thermal insulation. At the bottom of each column there was a 100-mm layer of round stones with a diameter of about 5 mm to enable leachate drainage and air distribution. Above this stone layer, there was a perforated baffle (2-mm mesh) to support waste and facilitate aeration. Straw and cotton cushions were placed above the waste to avoid heat loss and vapor condensation. Whirlpool pumps (XGB-8, Penghu Co., China) and gas-flow meters (LZB-10, Shanghai Instrument Co., China) were used for aeration. 2.4. Combustion experiment In a combustion reactor tube (40 mm i.d. and 710 mm length) as shown in Fig. 2, about 20 g of the ground waste (<0.5 mm)

Fig. 2. Combustion experimental setup.

was added to each ceramic boat and heated to 850 °C at a rate of 20 °C/min under an air flow of 100 mL/min, where it was maintained for 30 min. During combustion, the gaseous emissions were absorbed into an aqueous solution of 5% HNO3/10% H2O2 (analyzed for all metals) and an acidic solution of 5% KMnO4 (for Hg). The sampling train and solutions were submerged in ice bath.

2.5. Sampling and analysis Samples weighing about 150 g were collected from the top, middle and bottom of each column every 4 days. The collected waste was mixed and then divided into three parts. One part was used to analyze the lower heating value (LHV). Another part was dried at 70 °C to a constant weight (Larney et al., 2000) and then ground to size U < 0.5 mm for subsequent use in the combustion experiment as well as analysis of water, chlorine, VS, and heavy metal contents. The last part was separated into three fractions, plastics, organic fraction of MSW (OFMSW), and other materials (glasses, metals, etc.). Then the plastics and OFMSW fraction were ground to a size less than 0.5 mm. For the plastics fraction, the heavy metal concentrations were measured and followed combustion

Table 1 Experimental condition for the three bio-drying processes. Process

Aerobic

Combined 1

Combined 2

0–4th day 5–16th day

7 min run/23 min stop, plus turning every 2 days 7 min run/23 min stop, plus turning every 2 days

No ventilation, no turning 7 min run/23 min stop, plus turning every 2 days

10 min run/230 min stop, no turning 7 min run/23 min stop, plus turning every 2 days

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experiment was conducted to investigate the transfer behavior of the heavy metals. For the OFMSW fraction, the speciation of heavy metals was determined. All analyses were conducted in triplicate, with standard deviations less than 10%. The water content was determined based on the weight loss of the waste dried at 105 °C to a constant weight. The VS content was determined based on the loss on combustion at 550 °C after 6 h. The LHV was determined using a calorimeter (6100, PARR, USA). To measure the inorganic chlorides and organic chlorines, the samples were extracted in distilled water with a solid to liquid ratio of 100 mL/g at 100 °C for 6 h (Guo et al., 2001; Jakob et al., 1996), after which the inorganic chlorides and organic chlorines were measured by AgNO3 titration (APHA et al., 1998) and an organic halide analyzer (AQF-100/ICS-1500 system, Dionex, USA), respectively. With the exception of Hg, heavy metal concentrations in the waste samples were analyzed by ICP (ICP-optima 2001DV, Perkin–Elmer, USA) after acid digestion using HCl/HNO3/HF/HClO4. Hg was analyzed using a XGY1012 atomic fluorescence spectrophotometer (AFS, Institute of Geophysical and Geochemical Exploration of the Chinese Academy of Sciences, China) after HCl/HNO3/ H2O2 digestion. The speciation of heavy metals was conducted using the sequential extraction procedure proposed by Tessier et al. (1979). Briefly, heavy metals were classified into the following five fractions: (1) exchangeable (EXCH); (2) carbonate-bound (CARB); (3) iron- and manganese-oxides-bound (FeMnOX); (4) organic-matter-bound (OMB); and (5) residual (RESI), by sequential extraction of 1-g OFMSW using (1) 20 mL of 1 mol/L CH3COONa (pH 8.2, agitated continuously for 1 h at room temperature); (2) 20 mL of 1 mol/L CH3COONa (adjusted to pH 5 with acetic acid, agitated continuously for 5 h at room temperature); (3) 20 mL of 0.04 mol/L NH2OH–HCl in 25% (v/v) acetic acid (agitated occasionally for 5 h at 96 ± 3 °C); (4) 3 mL of 0.02 mol/L HNO3 and 5 mL of 30% H2O2 (adjusted to pH 2 with HNO3, agitated occasionally for 2 h at 85 ± 2 °C), and a second 3 mL of 30% H2O2 (adjusted to pH 2 with HNO3, agitated occasionally for 3 h at 85 ± 2 °C), and then 5 mL of 3.2 mol/L CH3COONH4 in 20% (v/v) HNO3 (agitated continuously for 30 min at room temperature after dilution to 20 mL with distilled water); (5) HCl/HNO3/HF/HClO4 digestion. The heavy metal concentrations in the extraction solutions, leachates produced

during bio-drying, and absorption solutions produced during combustion were measured by ICP (As, Pb, Zn, Cu) and AFS (Hg). 2.6. Statistical analysis All statistical analyses were performed using SPSS 16.0 (SPSS, Inc., Chicago, USA). Pearson’s correlation coefficient was used to evaluate the linear correlation between two parameters. Partial correlation analysis was used to evaluate the correlation between two parameters when controlling the linear influences of other variables. The correlations were considered statistically significant at a confidence level of 99%. 3. Results and discussion 3.1. Organics degradation, water contents, and heating values during bio-drying The extent of organics degradation (EX) was calculated using Eq. (1):

EXð%Þ ¼

DM0  VS0  DM t  VSt  100% DM0  VS0

ð1Þ

where DM0 (kg) and DMt (kg) are the dry weight of MSW in the column at time 0 and t, and VS0 (%) and VSt (%) are the VS content of MSW at time 0 and at t, respectively. During bio-drying, the Aerobic treatment showed the highest organics degradation, followed by Combined 2 and Combined 1 (Fig. 3a). All three treatments resulted in similar final water contents of 50–55% (Fig. 3a), while more organics and water were lost in the Aerobic treatment, due to the higher temperature and ventilation rate. LHVs increased from an initial 3560 kJ/kg to 7540 (Combined 1), 8590 (Combined 2), and 8260 kJ/kg (Aerobic), respectively (Fig. 3b), and were negatively correlated with the water content (R2 = 0.92). The reason was that, LHV took into account water vaporization during combustion and indicated the heat produced from combustion of wet materials.

Fig. 3. Variation of the water content and extent of organics degradation (a) as well as heating value (b) of the MSW during bio-drying.

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3.2. Variation of heavy metal concentrations and their transformation during bio-drying According to the related pollution control standards for MSW incineration, landfill, etc., the regulated heavy metals include As, Cd, Cu, Cr, Hg, Ni, Pb, and Zn. Based on which, the heavy metals in this study were chosen. In the MSW sample, some heavy metals, such as Cr and Cd were not detected. Therefore, only the data of As, Hg, Pb, Cu, and Zn were discussed. Fig. 4a presents the evolution of As, Hg, Pb, Zn, and Cu concentrations in the MSW samples (in dry weight) during bio-drying. Mass flow analysis was adopted to estimate the partitioning of heavy metals in the waste and leachate (Fig. 4b), and is described by Eq. (2):

C x0  DM0 ¼ C xt-DM  DMt þ

t X

C xt-leachate  V t

ð2Þ

0

where C x0 (mg/kg) and C xt-DM (mg/kg) are the concentration of heavy metal x in the MSW sample in dry weight at time 0 and t. C xt-leachate (mg/L) is the concentration of heavy metal x in leachate at time t; V t (L) is the leachate volume produced at time t. As expected, the heavy metals in the waste were concentrated during bio-drying (Fig. 4a) due to organics degradation. The metal concentrations increased more rapidly to higher levels during the

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Aerobic treatment than the two combined treatments, because the former had a greater extent of organics degradation. For Combined 1 and 2, the heavy metal concentrations remained stable in the initial hydrolytic stage (0–4 days), and then increased slightly with the degradation of waste. In addition, some metals were removed with the acidic leachate (pH 4.09–5.28) in the first 4 days in Combined 1 and 2 (about 5% for As, 15.9–17% for Hg, 5.8–6.2% for Pb, about 4% for Zn and 7.6–8.6% for Cu), while no leachate was released in the Aerobic treatment because the water was mainly vaporized at higher temperature and aeration rates. The heavy metal speciation in OFMSW during bio-drying is presented in Fig. 5. The amount of heavy metals in each fraction was calculated based on the total OFMSW input in the columns. As, Pb, and Zn dominated the RESI fractions in the OFMSW. Hg was evenly distributed in the fractions of EXCH, CARB, FeMnOX, and OMB. Cu had a high affinity for organic matters and was directly bound to two or more organic functional groups, primarily carboxylic, carbonyl, and phenolic groups (Coquery and Welbourn, 1995; Nomeda et al., 2008). Thus, Cu was mainly immobilized as complexes of OMB and RESI. There was no obvious variation of As and Zn speciation during bio-drying for any treatment, indicating that bio-drying would not cause a significant transformation of As and Zn. The easilydegradable organics in the OFMSW were converted to more stable humic organic ligands by the microbial community during

Fig. 4. Variation in the occurrence of heavy metals in the MSW during bio-drying. The lines reflect the variation in heavy metal concentrations in the bio-dried sample in dry weight and the columns indicate the total amounts of heavy metals in the waste and leachate fractions.

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bio-drying (Amir et al., 2005; Zhang et al., 2008a; Zheng et al., 2007) and increased the contents of Hg–OMB, Pb–OMB, and Cu– OMB (Fig. 4b) due to the stronger complexibility of humic substances with heavy metal cation (Van Riemsdijk et al., 2006). The insignificant variation of Zn–OMB might be explained by the sequence of metals reacting with humic organic ligands, which was Zn < Cu < Pb (He et al., 2009; Qiao and Ho, 1997). 3.3. Transfer of heavy metals from the bio-dried MSW into the combustion flue gas Fig. 6 shows the transfer of heavy metals from the bio-dried sample to the flue gas during combustion. Similar to the evolution trends in heavy metal concentrations, the transfer amounts of heavy metals from the waste (mg of heavy metals per kg of bio-dried sample in dry weight) increased during bio-drying, which would cause their enhanced concentrations in APC residues from MSW combustion due to the high removal efficiency of modern APC devices. Organics degradation played an important role in the

increasing transfer amounts of heavy metals when the matrix reduction was considered (correlation coefficients for As, Hg, Pb, Zn, and Cu were 0.96, 0.82, 0.97, 0.96, and 0.96, respectively). As a result, the Aerobic treatment had higher transfer amounts of heavy metals than Combined 1 and 2. The partitioning ratios were calculated based on the percentage of heavy metals transferred into the flue gas from the waste (mg/ kg). The constant partitioning ratios of As and Hg suggesting that the rising transfer amounts of the two heavy metals were due to their increased concentrations in the bio-dried waste. On the other hand, the partitioning ratios of Pb, Zn, and Cu was enhanced during bio-drying, which indicated that the increased partitioning ratios of the heavy metals were attributed to not only their increased concentration in MSW but also to variation of metal speciation. The total amounts of Pb, Zn, and Cu transferred into the flue gas rose by 2.9, 0.8, and 1.3 folds for Combined 1, by 3.5, 0.9, and 1.5-folds for Combined 2, by 5.1, 1.1, and 1.6-folds for Aerobic (as shown in Table 2), respectively after 16 days of bio-drying, which were possibly attributed to variation of metal speciation.

Fig. 5. Variation in heavy metal speciation in the OFMSW during bio-drying based on the total OFMSW input.

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Fig. 6. Amounts of heavy metals transferred to combustion flue gas and their partitioning ratios during bio-drying. The lines reflect the amounts of heavy metals transferred to combustion flue gas and the columns indicate their partitioning ratios.

Aerobic showed the largest, and Combined 1 showed the least increase of metal transfer. It has been reported that the chlorine concentrations would influence heavy metal transformation between the stable metallic oxides and volatile metallic chlorides (Abanades et al., 2002; Durlak et al., 1997). At the combustion temperature, heavy metals tended to form metallic chlorides spontaneously, rather than oxides (Fernández et al., 1992). The boiling points of these metallic chlorides were lower than or approached the combustion temperature; as a result, they could be transferred to the gas phase. Therefore, the effects of chlorine on the Pb, Zn, and Cu transfer during combustion were investigated using correlation analysis. The result showed that the transfer amounts of Pb, Zn, and Cu were correlated with the extent of organics degradation, heavy metal concentrations, inorganic chlorides and organic chlorines in the bio-dried MSW (correlation coefficients: 0.97–0.98 for Pb, 0.96–0.97 for Zn, and 0.91–0.96 for Cu, respectively). However, each of these correlations may have been caused by the organics degradation and MSW reduction. To determine if this were the case, partial correlation analysis was adopted to elucidate the

intrinsic interactions of these variables while excluding the effects of organics degradation (Table 3). The result showed that inorganic chlorides and organic chlorines did not determine the transfer amounts of Pb, Zn, and Cu. Indeed, the only significant correlation was observed between Pb transfer amounts and Pb concentration in the bio-dried samples (correlation coefficient, 0.74). The effects of heavy metal speciation in OFMSW on metal transfer during combustion were also evaluated. The partitioning potential of As and Hg was relatively stable (>85%) during bio-drying (Fig. 6), indicating that their speciation had the least influence (Table 4). However, for Pb, Zn, and Cu, significant correlations were observed between speciation and the partitioning ratio. As shown in Fig. 5, there was a large amount of FeMnOX and OMB fractions for Pb, Zn, and Cu in OFMSW. In addition, among the fractions of heavy metals, FeMnOX oxides were thermodynamically unstable (Tessier et al., 1979) and the organometallic compounds had a low decomposition temperature (approximately 250 °C). Therefore, during combustion, metals bound in the FeMnOX and OMB fractions would be easily released due to decomposition of these complexes, after which they would form metallic chlorides and

Table 2 Total amounts of heavy metals transferred into the combustion flue gas. Time

As (mg)

Hg (mg)

Pb (mg)

Zn (mg)

Cu (mg)

Combined Combined Aerobic Combined Combined Aerobic Combined Combined Aerobic Combined Combined Aerobic Combined Combined Aerobic 1 2 1 2 1 2 1 2 1 2 0 day 4 day 8 day 12 day 16 day

3.17 3.18 3.16 3.07 3.23

3.17 3.09 3.17 3.06 3.25

3.17 3.08 2.98 2.90 3.20

2.21 2.07 2.15 1.91 1.90

2.21 2.11 2.05 1.89 1.95

2.21 1.97 1.77 1.73 1.76

0.99 1.11 1.86 3.59 3.87

0.99 1.29 2.33 3.61 4.46

0.99 2.75 4.46 5.29 5.99

70.9 66.3 105 120 127

70.9 67.2 102 111 132

70.9 71.8 120 139 146

9.15 9.13 11.0 18.4 21.4

9.15 9.35 13.8 17.4 22.6

9.15 14.1 16.5 20.6 23.9

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Table 3 Partial correlation matrix for transfer amounts and concentration of heavy metals, inorganic chlorides, and organic chlorines.* Concentrations in the bio-dried MSW Pb Transfer amount Pb 0.74*** Zn Cu * ** ***

Zn

Cu

Inorganic chlorides

Organic chlorines

0.15

0.62** 0.37 0.03

0.53** 0.45 0.08

0.44

*

This work was financially supported by the National Basic Research Program of China (973 Program No. 2011CB201500), the National Key Technology R&D Program of China (2006BA C06B04), the Key Special Program on the S&T for the Pollution Control and Treatment of Water Bodies (No. 2008ZX07101-006), and National Natural Science Foundation of China (20807031). References

Controlling the extent of organics degradation. Correlation is significant at the 0.05 level. Correlation is significant at the 0.01 level.

Table 4 Correlations between partitioning ratios of heavy metals in combustion flue gas and their speciation in OFMSW.

**

Acknowledgements

Partitioning in combustion flue gas (%)

Speciation of heavy metals in OFMSW EXCH (%)

CARB (%)

FeMnOX (%)

OMB (%)

RESI (%)

As Hg Pb Zn Cu

0.28 0.28 0.19 0.69** 0.90**

0.05 0.41 0.84** 0.44 0.86**

0.19 0.43 0.97** 0.81** 0.79**

0.33 0.42 0.84** 0.70** 0.80**

0.28 0.13 0.47 0.82** 0.59*

Correlation is significant at the 0.05 level. Correlation is significant at the 0.01 level.

oxides in situ. The partitioning of Pb in combustion flue gas was mainly determined by Pb–OMB, which had a positive correlation coefficient of 0.84. As for Zn, the partitioning in the combustion flue gas was positively correlated with Zn–FeMnOx (correlation coefficient, 0.81). Cu partitioned in combustion flue gas varied positively, corresponding to both Cu–FeMnOX (correlation coefficient, 0.79) and Cu–OMB (correlation coefficient, 0.80). As shown in Table 4, several negative relationships between the partitioning of Pb, Zn, and Cu in combustion flue gas and their distribution in the EXCH, CARB, and RESI fractions were also observed since these fractions would cause variations in the FeMnOX and/or OMB fractions. These results indicated that bio-drying resulted in speciation variation of Pb, Zn, and Cu, which then affected their transfer and partitioning behavior during waste combustion. The enhanced transfer of Pb, Zn, and Cu into flue gas from the bio-dried waste indicated the increasing air pollution risk, which should be of a concern to this technology. 4. Conclusions Bio-drying significantly increased the LHV of MSW and decreased its water content. Organics degradation led to the concentration of heavy metals in MSW during bio-drying and the increasing amounts of heavy metals per kg of bio-dried MSW transferred into combustion flue gas, especially in the Aerobic treatment which obtained the greatest organics degradation and least leachate production. Nevertheless, bio-drying and metal speciation had little effect on the transfer and partitioning of As and Hg during combustion due to their volatility, and their partitioning ratios in flue gas remained constant. In contrast, those of Pb, Zn, and Cu tended to increase during bio-drying, contributing to their speciation variation (Pb–OMB, Zn–FeMnOX, Cu–OMB, and Cu– FeMnOX) in OFMSW during bio-drying. This would further affect their transfer and partitioning behavior during combustion. Aerobic and Combined 1 showed the largest and the least increase of metal transfer, respectively.

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