Characterization of Polychlorinated Dibenzo-p-dioxins and Dibenzofurans of the Flue Gases , Fly Ash and Bottom Ash in a Municipal Solid Waste Incinerator

The emissions factors of PCDD/Fs from the stack flue gas, bottom ash and fly ash of a municipal solid waste incinerator (MSWI) were analyzed in this study. The congner profiles of PCDD/F mass were dominant in OCDD and OCDF; however, those of PCDD/Fs-TEQ were mainly 2,3,4,7,8-PeCDF and 1,2,3,7,8-PeCDD in all samples. The PCDD/F emission factors of MSWI per metric ton of waste incinerated from the stack flue gas, bottom ash and fly ash were at an averaged of 0.0919, 7.20 and 12.8 μg PCDD/Fs-WHO2005-TEQ ton, respectively. Futhermore, the emission factor of MSWI in the unit of electricity produced averaged 0.185, 14.5 and 26.3 μg PCDD/Fs-WHO2005-TEQ (MWh), respectively. As the results shown in this study, the majority of total PCDD/Fs-WHO2005-TEQ were mainly in both bottom ash and fly ashes. From long-term perspective, the disposal of both bottom and fly ashes should pay more attention to this issue. The results of this study provide useful information for both further studies and environmental control strategies aimed at persistent organic compounds (POPs).


INTRODUCTION
With the development of the economy and the growth of industry, incineration has become one of the major pathway for treatment of municipal solid wastes, due to its advantages in terms of volume reduction, energy recovery, pathogen elimination and chemical-toxicity destruction (Dempsey and Oppelt, 1993).Since polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/Fs) were first discovered in the flue gases and fly ash of municipal solid waste incinerators (MSWIs) in 1977 (Olie et al., 1977), PCDD/Fs have been of wide concern due to their extreme toxicity and adverse implications for human health.The characteristics of being semi-volatile and hydrophobic enhance the PCDD/Fs ability to accumulate in the environment (Chi et al., 2016), especially in organic carbonrich media such as soil and sediment (Schuhmacher et al., 1997;Chao et al., 2007).Pollution issues related to the incineration process have drawn much more attention, even though the wastes have been reduced and stabilized.The chlorine content of the wastes and the temperature of the combustion process usually play a major role in the formation of PCDD/Fs (Lee et al., 2003;Wang et al., 2003;Li et al., 2007;Wang et al., 2010).Human activities are the major sources of PCDD/F emissions, including industrial and heat-treatment processes (Oh et al., 1999;Baker and Hites, 2000;Tame et al., 2007;Xu et al., 2009;Cheruiyot et al., 2015;Cheruiyot et al., 2016;Redfern et al., 2017).Previous studies suggested that there are processes in the post-furnace stage which by de novo synthesis or precursor synthesis can generate lots of PCDD/Fs, and the operating conditions include the flue gas temperature profile, air supply, carbon and metal contents in fly ash and so on, which are the major factors that affect the PCDD/Fs formation rates (Dickson et al., 1992;Stieglitz et al., 1993;Gullett et al., 1994;Addink and Olie, 1995a, b;Stieglitz, 1998;Chang and Huang, 2000).Combustion experments suggest that most of the PCDDs and PCDFs are generated at temperatures higher than 650°C, but the process of condensation of CPhs (Chlorinated phenols) usuall produce more PCDDs, and generally more PCDFs are produced by a low-or nonchlorinated precursor followed by further chlorination reactions (Wikström and Marklund, 2000).MSWIs are a significant source for the transformation of PCDD/Fs to the atmosphere.In recent years, the levels of PCDD/Fs emitted by MSWIs and into the ambient air have often been used to evaluate the atmospheric concentrations of these pollutants near such facilities (Lorber et al., 1996;Abad et al., 1997;Koblantz et al., 1997;Fielder, 1999a, b;Oh et al., 1999).PCDD/F emissions result in subsequent aerial deposition onto soil and vegetation.
For MSWIs, as the incineration temperature increase, the concentrations of PCDD/Fs from flue gases will also rise (Han et al., 2017).Fly ash has the highest dioxin concentration produced by MSWIs, and it acts as a heterogeneous catalyst that is important for the synthesis of PCDD/Fs in the MSWIs (Karasek and Dickson, 1987;Goldfarb, 1989;Altwicker et al., 1992;Huang and Buekens, 1995;Lin et al., 2008).Previous studies have suggested that the PCDD/F emissions during start-up are a large source of emissions from MSWIs (Neuer-Etscheidt et al., 2006).PCDD/Fs released from the MSWIs (include stack flue gas, bottom ash and fly ash) can spread through the media of air, soil and water, eventually enter animal and human bodies through atmospheric deposition (Du et al., 2011;Chandra Suryani et al., 2015;Chang et al., 2016;Chen et al., 2017;Zhu et al., 2017a, b).
Because of Taiwan's high population density, incineration has become the major method to treat municipal solid wastes in the country.At present, there are 23 large MSWIs in operation in Taiwan, which has a very strict PCDD/F emission standard (0.1 ng I-TEQ Nm -3 ) for MSWIs.
The congener profiles and emission factors of PCDD/Fs in the stack flue gas, bottom ash and fly ash produce when using unit waste and electricity generation, respectively, were investigated.This study provides useful information for the control strategies of MSWIs.

Sampling
From January 2010 to December 2015, a number of stack flue gas, bottom ash and fly ash samples were collected from an MSWI, located in southern Taiwan.The air pollution control devices (APCDs) which are commonly believed to be the most effective techniques for PCDD/Fs control are the most widely used such devices in MSWIs in Taiwan (Wang et al., 2009).
Flue gas samples from the stack were collected with an isokinetic sampler (KNJ, Korea) according to US EPA modified Method 23.XAD-2 resin was spiked with PCDD/F surrogate standards pre-labeled with isotopes before sampling, and the sampling lasted for ~3 h.The bottom ash samples were collected directly from each furnace.The fly ash samples were collected by the bag filters.To ensure the collected samples were free from contamination, one trip blank and one field blank were also taken during the field sampling.

Analysis of PCDD/Fs
The US EPA Modified Method 23 was used for the analyses of the stack flue gas samples, while the US EPA Method 1613B was used for the the analyses of bottom and fly ash samples (US EPA, 1994, 1996).
All chemical analyses were measured in the Super Micro Mass Research and Technology Centre of Cheng Shiu University, which has passed the international intercalibration standards test for PCDD/Fs in fly ash, sediment, mother's milk, human blood, and cod liver.A known amount of the internal standard was spiked in each sample, which was then extracted for 24 hours.The extract was treated with concentrated sulfuric acid, and then a series of sample cleanup and fractionation procedures.Sample cleanup was done using an acidic silica-gel column, an alumina column, and an activated carbon column.Consequently, the elution was concentrated to around 1 mL, and further concentrated to near dryness with a nitrogen stream.Before PCDD/Fs analysis, each sample was added to the standard solution to ensure recovery during the analysis process (Shih et al., 2006).We used high-resolution gas chromatographs/highresolution mass spectrometers (HR-GC/ HR-MS) for the PCDD/F analysis.The HRGC (Hewlett-Packard 6970 Series gas, CA) was equipped with a DB-5 fused silica capillary column (L = 60 m, ID = 0.25 mm, film thickness = 0.25 μm) (J&W Scientific, CA) with a splitless injection, while the HR-MS (Micromass Autospec Ultima, Manchester, UK) had a positive electron impact (EI+) source.Selected ion monitoring with the resolving power of 10,000 was used for the analyzer mode.The electron energy and source temperature were specified at 35 eV and 250°C, respectively.In addition, the oven temperature program was set as follows: initially at 150°C (held for 1 min), then increased by 30°C min -1 to 220°C (held for 12 min), and finally increased by 1.5 °C min -1 to 310°C (held for 20 min).Helium was used as the carrier gas.The analysis process strictly followed the protocol for quality analysis/quality control (Wang and Lee, 2010).

Basic Information of the MSWI
The basic information of the MSWI investigated in this study is presented in Tables 1 and 2, which shows the weight of monthly MSW treated from 2009 to 2015 is in the range between 19,917 and 22,173 ton month -1 and with an average of 20,886 ton month -1 .Table 2 present the monthly electric energy production per ton of MSW treated from 2009 to 2015, which ranged from 459 and 523 kWh ton -1 , and averaged 490 kWh ton -1 .
Furthermore, from 2010 to 2015, the monthly average gas flow rate in the stack flue gas ranged between 1225 and 1639 Nm 3 min -1 , and averaged 1460 Nm 3 min -1 .The operational time of MSWI ranged from 599 to 665 hr month -1 , and averaged 629 hr month -1 .From 2010 to 2015, the monthly generation rate of bottom ashes varied from 1360 to 7380 ton month -1 , and averaged 4700 ton month -1 ; as for the fly ash, ranged from 1270 to 2630 ton month -1 , and averaged 1830 ton month -1 .The mean PCDD/F mass contents in bottom ash were between 0.0455 and 4.85 ng g -1 , and averaged 1.48 ng g -1 , and those of fly ash were between 0.0121 and 19.69 ng g -1 , and averaged 2.56 ng g -1 , respectively, and the corresponding PCDD/Fs-WHO 2005 -TEQ levels for bottom ash ranged from 0.001 to 0.138 ng PCDD/Fs-WHO 2005 -TEQ g -1 and averaged 0.038 ng PCDD/Fs-WHO 2005 -TEQ g -1 , and those for fly ash were from 0.0002 to 0.900 ng PCDD/Fs-WHO 2005 -TEQ g -1 and averaged 0.111 ng PCDD/Fs-WHO 2005 -TEQ g -1 .The average value of PCDD/F content in the fly ash was approximately 2.92 times higher than that of bottom ash.The total PCDD/Fs-WHO 2005 -TEQ contents in both bottom and fly ashes were all lower than the regulated standard (1.0 ng WHO 2005 -TEQ g -1 ).However, the total PCDD/Fs-WHO 2005 -TEQ content in a certain fly ash sample was as high as up to 0.900 ng PCDD/Fs-WHO 2005 -TEQ g -1 , and thus needs more attention.The great variation in PCDD/F content in both the bottom and fly ashes indicated that the feeding wates, particularly those related to the chlorine content, may vary to a great extent.

Total PCDD/Fs-TEQ Concentration in the Stack Flue Gas
From 2010 to 2015, the total PCDD/Fs mass concentration in the stack flue gas ranged between 0.363 and 2.077 ng Nm -3 and averaged 0.917 ng Nm -3 ; and the corresponding total PCDD/Fs-WHO 2005 -TEQ values were between 0.00808 and 0.0711 ng PCDD/Fs-WHO 2005 -TEQ Nm -3 , and averaged 0.0343 ng PCDD/Fs-WHO 2005 -TEQ Nm -3 .These results show that the total PCDD/Fs-WHO 2005 -TEQ concentrations in the stack flue gas were all lower than the PCDD/F emission standard (0.1 ng PCDD/Fs-I-TEQ Nm -3 ) for MSWIs, as regulated by the Taiwan EPA.The average result found in this study was lower than those of previous studies, which the mean total PCDD/Fs-I-TEQ concentrations were 0.0593 ng PCDD/Fs-I-TEQ Nm -3 (Wang et al., 2005) and 0.0533 ng PCDD/Fs-I-TEQ Nm -3 (Lee et al., 2003), respectively.
The results indicated that the mass fraction of higher chlorinated PCDD/F congeners are higher than those of lower chlorinated ones.The congener profiles of PCDD/Fs-TEQ in all samples show that 2,3,4,7,8-PeCDF and 1,2,3,7,8-PeCDD are the most dominant congeners, due to the product of their relatively high toxic equivalency factor (TEF) and PCDD/F concentration.

Emission Factors
The emission factors are important information to establish a PCDD/F inventory and develop a control strategy.Figs. 4, 5 and 6 show the emission factors of total PCDD/Fs-TEQ by the unit ton of MSW treated from the stack flue gas, bottom ash and fly ash, respectively.
From 2010 to 2015, the PCDD/F emission factors of MSWI (EF) by the unit ton of MSW treated from stack flue gas are shown in Fig. 4, and these reveal that between 2011 and 2014, the EF ranged from 0.0583 to 0.0673 μg PCDD/Fs-WHO 2005 -TEQ ton -1 ; while the EF in 2010 (0.148 μg PCDD/Fs-WHO 2005 -TEQ ton -1 ) and 2015 (0.153 μg PCDD/Fs-WHO 2005 -TEQ ton -1 ) were similar, and much larger than those from 2011 to 2014.Overall, the EF of total PCDD/Fs-TEQ in the stack flue gas are in the range of 0.0583-0.153μg PCDD/Fs-WHO 2005 -TEQ ton -1 , with an average of 0.0919 μg PCDD/Fs-WHO 2005 -TEQ ton -1 .These values are similar to those in Wang et al. (2003), which ranged from 0.0475 to 0.187 μg PCDD/Fs-I-TEQ ton -1 and averaged 0.0939 μg PCDD/Fs-I-TEQ ton -1 ; however, they are lower than the results of Lin et al. (2010), which aveaged 0.149 and 0.220 μg PCDD/Fs-I-TEQ ton -1 in MSWI-A and MSWI-B, respectively.The variation of PCDD/Fs-TEQ EF may due to the variations in feeding wastes and the use of different air control devices (Wang et al. 2003).The above results reveal that the stack flue gas of MSWIs is one of the major emission sources of PCDD/Fs.
Bottom ash is a highly heterogeneous burnt-out mixture.The PCDD/Fs EF by the unit ton of MSW treated in bottom ash are presented in Fig. 5, which show the EF of total PCDD/Fs-WHO 2005 -TEQ in bottom ash ranged between 1.83 and 12.9 μg WHO 2005 -TEQ ton -1 and averaged 7.20 μg WHO 2005 -TEQ ton -1 (Fig. 5); that of 2014 (12.9 μg WHO 2005 -TEQ ton -1 ) was approximately 7.0 times greater  The PCDD/F EF from fly ash are presented in Fig. 9 in the unit of electricity produced (MWh), and the results are different to those for bottom ash.The EF from fly ash increased from 2010 to 2015, which ranged from 4.68 to 64.8 μg WHO 2005 -TEQ (MWh) -1 and averaged 26.3 μg WHO 2005 -TEQ (MWh) -1 .
From 2010 to 2015, the fraction of the PCDD/F mass contributed by stack flue gas was the lowest, which ranged from 0.19% to 2.08% and averaged 0.48% (Table 3); while the fraction of corresponding PCDD/Fs-WHO 2005 -TEQ contributed by the stack flue gas ranged between 0.24% and 2.06% and averaged 0.45% (Table 3).For the bottom ash, the fraction of PCDD/F mass contributed by it, in general, increased year by year from 2010 to 2015, which ranged from 26.6% (2010) to 72.8% (2014) and averaged 43.61%; while the fraction of corresponding PCDD/Fs-WHO 2005 -TEQ contributed by the bottom ash ranged from 8.68% (2011) to 78.0% (2014) and averaged 34.71%.When compared with the bottom ash, the opposite trend was found in the fly ash.The fraction of the PCDD/F mass contributed by fly ash decreased year by year from 2010 to 2015, and ranged from 26.3% (2015) to 73.1% (2010) and averaged 55.91%; while the fraction of corresponding PCDD/Fs-WHO 2005 -TEQ contributed by the fly ash ranged from 21.6% (2014) to 91.1% (2011) and averaged 64.84%.As the results show in this study, the majority of total PCDD/Fs-WHO 2005 -TEQ were mainly in both bottom ash and fly ash.Landfill is a major disposal method for the MSWI ashes in Taiwan.From a long-term perspective, such landfills will cause serious environmental problems, since the ashes (bottom and fly) contain very high amounts of POPs (Lee et al., 2003;Neuer-Etscheidt et al., 2006;Lin et al., 2010;Wang et al., 2010;Cheruiyot et al., 2016;Redfern et al., 2017).

Scenario Analysis
From 2010 to 2015, the mean annual content of the PCDD/Fs in stack flue gas was 0.34 ng WHO 2005 -TEQ Nm -3 .Actually, the contents of the PCDD/Fs in stack flue gas are usually underestimated.A simple model describing the accumulation of PCDD/Fs in stack flue gas was used in this study.The model is employed to predict the contribution of PCDD/Fs in stack flue gas to total PCDD/Fs from MSWI.Scenario A is the case when the total PCDD/Fs from bottom and fly ash are constant, while the total PCDD/Fs TEQ concentrations in the stack flue gas are 0.01, 0.05, 0.1, 0.5.1.0, 5.0, 10.0, 20.0, and 50.0 ng WHO 2005 -TEQ Nm -3 .The results of the modeled fraction of the PCDD/F contributed by stack flue gas, bottom ash and fly are presented in Table 4.The model suggests that the fractions of PCDD/Fs from stack flue gas are increase greatly, and ranged from 0.14%-87.25%.
Scenario B is the case when the total PCDD/Fs from stack flue gas and fly ash are constant, while the total PCDD/Fs TEQ concentration in the stack flue gas are 0.01, 0.05, 0.1, 0.5.1.0, and 4.8 ng WHO 2005 -TEQ Nm -3 .The results of the modeled fraction of the PCDD/F contributed by stack flue gas, bottom ash and fly are presented in Table 5.

CONCLUSION
1.The mass concentration of higher chlorinated PCDD/Fs congeners are higher than those of the lower chlorinated ones.The congener profiles of TEQ PCDD/Fs in all samples showed that 2,3,4,7,8-PeCDF and 1,2,3,7,8-PeCDD were the major congeners, due to the relatively high Toxic Equivalency Factor (TEF) and PCDD/Fs concentration.concentration from stack flue gas, bottom ash and fly ash range of 0.19-2.08%,26.60-72.8%and 26.28-73.07%,respectively.While the corresponding fraction of the WHO 2005 -TEQ PCDD/Fs concentration from stack flue gas, bottom ash and fly ash were in the ranges of 0.24%-2.06%,8.68%-78.0%and 21.6-91.1% and averaged 0.45 %, 34.71% and 64.84%, respectively.9.The fraction of PCDD/Fs from stack flue gas increased from 0.14% to 87.25% in scenario A. In scenario B, the fraction of PCDD/Fs from stack flue gas increased from 0.14% to 65.29%, while the fraction of PCDD/Fs from fly ash decreased from 65.16 to 0. 10.The results of this study provide valuable information for the emission factors of PCDD/Fs in stack flue gas, bottom ash and fly ash from MSWI.Which will be useful for the establishment of control strategies in the future.
2. The PCDD/F emission factors by the unit ton of MSW treated from the stack flue gas of MSWI ranged between 0.0583 and 0.1532 μg WHO 2005 -TEQ ton -1 and averaged 0.0919 μg WHO 2005 -TEQ ton -1 .3. The PCDD/F emission factors by the unit ton of MSW treated from the bottom ash of MSWI ranged between 1.83 and 12.9 μg WHO 2005 -TEQ ton -1 and averaged 7.20 μg WHO 2005 -TEQ ton -1 .4. The PCDD/Fs emission factors by the unit ton of MSW treated from the fly ash ranged from 2.41 to 31.2 μg WHO 2005 -TEQ ton -1 and averaged 12.8 μg WHO 2005 -TEQ ton -1 . 5. The emission factor of total-PCDD/Fs-WHO 2005 -TEQ from the stack flue gas in the unit of electricity produced ranged between 0.119 and 0.307 μg WHO 2005 -TEQ (MWh) -1 and averaged 0.185 μg WHO 2005 -TEQ (MWh) -1 .6.The emission factor of total-PCDD/Fs -WHO 2005 -TEQ from the bottom ash in the unit of electricity produced in bottom ash range between 3.76 and 24.52 μg WHO 2005 -TEQ (MWh) -1 , with an average of 14.5 μg WHO 2005 -TEQ (MWh) -1 .7. The emission factor of total-PCDD/Fs-WHO 2005 -TEQ from the fly ash in the unit of electricity produced in fly ash range between 4.68 and 64.8 μg WHO 2005 -TEQ (MWh) -1 , with an average of 26.3 μg WHO 2005 -TEQ (MWh) -1 .8. From 2010 to 2015, the fraction of the mass PCDD/Fs

Table 3 .
The fraction of PCDD/F mass and total-PCDD/Fs-WHO 2005 -TEQ contributed by bottom ash, fly ash and stack flue gas, respectively.

Table 4 .
The modeled fraction of the PCDD/F from stack flue gas, bottom ash and fly ash in scenario A.

Table 5 .
The modeled fraction of the PCDD/F from stack flue gas, bottom ash and fly ash in scenario B.