Science of the Total Environment 473–474 (2014) 459–464
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Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv
Investigation into the distribution of polycyclic aromatic hydrocarbons (PAHs) in wastewater sewage sludge and its resulting pyrolysis bio-oils Yanjun Hu a,⁎, Guojian Li b, Mi Yan a, Chuanjuan Ping a, Jianli Ren a a b
Institute of Thermal and Power Engineering, Zhejiang University of Technology, Chaowang Road 18#, 310024, HangZhou, China School of Civil Engineering & Architecture, Zhejiang Sci-tech University, Xiasha District, 310018 Hangzhou, China
H I G H L I G H T S • • • • •
We investigated 16 US EPA PAHs in sewage sludge and the resulting pyrolysis bio-oil. We examined the factors influencing the distribution of different rings of PAHs. Most of the 16 target PAHs were enriched in the sludge and its resulting pyrolysis oil. LMW PAHs in bio-oil are correlated with MMW PAHs in raw sewage sludge at best. Temperature and residence time in sludge pyrolysis evidently impact PAH emission.
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Article history: Received 30 August 2013 Received in revised form 10 December 2013 Accepted 10 December 2013 Available online xxxx Keywords: Sewage sludge Pyrolysis Bio-oil Polycyclic aromatic hydrocarbons
a b s t r a c t This study firstly investigated the distributions of 16 US EPA priority controlled polycyclic aromatic hydrocarbons (PAHs) in seven kinds of different wastewater sewage sludges and bio-oils from the sludge pyrolysis. A lab-scale tube furnace was used to simulate sludge pyrolysis and retrieve condensed oils. PAH determination was conducted with the extraction, concentration, and purification of PAHs in sludge samples and the resulting bio-oils, and then GC-MS analysis. Then, the factors influencing the distributions of different rings of PAHs in pyrolysis bio-oil, such as the chemical characteristics of raw sewage sludge and pyrolysis condition, were analyzed. It was noted that the total amount of PAHs in raw sludge is evidently varied with the sludge resource, with values ranging between 9.19 and 23.68 mg/kg. The middle molar weight (MMW) PAH distribution is dominant. PAH concentrations in sludge pyrolysis bio-oil were ranged from 13.72 to 48.9 mg/kg. The most abundant PAHs were the low molar weight (LMW) PAHs. It could be found that the concentration of LMW PAHs in bio-oil is correlated with MMW PAHs in raw sewage sludge at best, which the correlation coefficient is 0.607. For MMW and HMW (high molar weight) PAHs, they are significantly correlated with HMW PAHs in raw sewage sludge, which the correlation coefficients are 0.672 and 0.580, respectively. The concentration of LMW PAHs in bio-oil is also relatively significant and correlated with the volatile matter content of raw sludge. In addition, it was proved that final temperature and residence time have important influences on PAH generations during the pyrolysis of sewage sludge. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Sewage sludge is abundant in volatile matter and is being considered as a valuable bio-resource. Currently, there is an increasing interest in its reutilization through converting sewage sludge into a diversity of products such chemicals, fuels by using thermo-chemical processes (Fytili and Zabaniotou, 2008; Rulkens, 2008). Thermo-chemical conversion technologies that develop refuse derived fuels (RDF) from sewage sludge have been widely studied and developed in many countries (Kim and Parker, 2008; Shen and Zhang, 2003; Dominguez et al., ⁎ Corresponding author at: Chaowang Road 18#, 310024, Hangzhou, China. Tel.: +86 571 88320942. E-mail address: [email protected] (Y. Hu). 0048-9697/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.scitotenv.2013.12.051
2006a, 2006b). Some innovative technologies, such as sludge pyrolysis, have been applied industrially and have so far presented good economics and social benefits. In the past decade, pyrolysis of sewage sludge in an oxygen-free atmosphere has aroused a significant interest. Particularly, the interest is being developed in this process due to its high recovery of liquid oil. Investigations have indicated that the obtained oil from sludge pyrolysis can be applied in diesel fuelled engines and is comparable to low-grade petroleum distillates from commercial refineries (Dominguez et al., 2005; Dominguez et al., 2006a, 2006b). It was also noted that pyrolysis bio-oils were complex mixtures made up of a group of aromatic clusters with one to three aromatic rings connected by long straight chain hydrocarbons with hydroxyl groups (Shen and Zhang, 2003). As reported by some researchers, the composition of pyrolysis bio-oils from different urban sewage sludges consistently
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contained polycyclic aromatic hydrocarbons (PAHs) (Tsai et al., 2009a, 2009b). PAHs are a group of organic compounds with two or more fused aromatic rings, which appeared on the US EPA priority controlled pollutants list. Their release in air, soil or water would be a serious problem from the viewpoint of protecting the environment and human health. Most PAHs have strong toxicity and are carcinogenic, teratogenic, and mutagenic. It was reported that 70–90% of cancer diseases in humans and animals are caused by highly toxic chemicals in the environment. PAHs remain a widely distributed health, ecological and environmental concern, which makes it necessary to know both their residue levels and their distribution in the process of wastewater sewage sludge disposals. Studies of the long-term effects of municipal sewage sludge on the enrichment of PAHs in the environment have been reported by many authors (Anikwe and Nwobodo, 2002; Stevens et al., 2003; Trably and Patureau, 2006). The levels of PAHs in pyrolysis bio-oil from different food-processing sewage sludges were also reported by Tsai et al. (2009a, 2009b). However, studies regarding the distribution of PAHs in the resulting products from sludge pyrolysis were little reported, neither were the relevant influencing factors (Shen and Zhang, 2003; Tsai et al., 2009a, 2009b). In this study, the concentration distributions of 16 US EPA priority controlled PAHs in different sources of wastewater sewage sludge and in sludge pyrolysis bio-oils were firstly investigated. A lab-scale tube furnace was used to simulate sludge pyrolysis and retrieve condensed oils. A qualitative and quantitative analysis of PAHs was carried out by means of a series of experimental pretreatment including the extraction, concentration and purification of PAHs, and GC-MS analysis. Finally, the correlations between the distributions of different rings of PAHs and the chemical characteristics of raw sewage sludge, pyrolysis condition were analyzed.
2. Bio-oil sampling and PAH analysis 2.1. Materials Seven types of raw sewage sludge samples were collected after mechanical dewatering using a filter press. The raw sewage sludge were from industrial wastewater treatment systems in dying (S1 and S6), beer-brewing (S2), paper manufacturing (S3), and municipal wastewater treatment plants (S4, S5 and S7) containing domestic wastewater and industrial wastewater, respectively. Except for S6 and S7, the other sludge samples were sampled in the same city of Hangzhou, China. S6 was sampled in Huzhou, China, and S7 was sampled in Ningbo, China. S1, S2, and S6 were subjected to aerobic digestion treatment. All sludge samples were firstly dried in open air for 2 days to remove most moisture content, and then were dried in a lab-scale air convection oven for 8 h. The dried sludge cakes were smashed in a milling machine to provide a feed sample in the size range of smaller than 60 μm for labanalyses. The resulting sludge sample was then kept in an airtight container to prevent re-absorption of moisture before experimentation. The main chemical characteristics of seven kinds of sludge are presented in Table 1. The volatile matter content (VM), the amount of ash (A), and the moisture content were determined according to the National Table 1 the main chemical characteristics of different sewage sludges (dry base). Sample code
VM (%)
A (%)
FC (%)
M (%)
Field of oil (%)
S1 S2 S3 S4 S5 S6 S7
35.3 25.2 50.6 46.3 53.4 38.5 41.2
55.1 65.4 37.2 48.1 40.9 48.3 46.3
2.1 5.8 7.2 1.7 1.9 13.4 1.6
7.5 3.6 5.0 4.0 3.8 5.0 3.5
31 38 31 31 33 36 29
Testing Standard of Proximate Analysis of Coal (GB/T 212–2008). The fixed carbon content was calculated on the basis of the mass balance. 2.2. Sludge pyrolysis and recovery of the bio-oils Fig. 1 shows the schematic diagram of the sewage sludge pyrolysis to collect bio-oil samples. A lab-scale tube furnace with a heating area of 40 mm in diameter and 600 mm in length was used to simulate sludge pyrolysis. The furnace is constructed with quartz and is heated by a programmable temperature control, and can be adjusted from room temperature to 1200 °C. The sewage sludge pyrolysis tests were operated at atmospheric pressure and an inert atmosphere. A nitrogen flow rate of 80 mL/min was passed through the whole pyrolysis system for 20 min prior to the commencement of the sludge pyrolysis testing. The nitrogen flow was shut down during the sludge pyrolysis, which is good for high residence time of volatiles derived from sludges in the furnace and their secondary reactions. A sample of sludge around 50 g in weight was placed in the center of the hot zone of the furnace. The volatiles evolved from the sample passed through three consecutive condensers which were placed in an ice-water bath. The condensed oil was retrieved using dichloromethane and kept for further chemical analysis. The yield of pyrolysis oil was obtained by calculating an average percentage of the difference between weights of the used sludge sample and the retrieved oil to the weight of the used dry sludge sample. In order to investigate the correlation between concentration distribution of PAHs in bio-oils and chemical characteristics of raw sewage sludge, the bio-oil samples from the pyrolysis of seven kinds of sewage sludge were obtained. These pyrolysis experiments were carried out at standard of pyrolysis conditions, which are to heat from room temperature to final temperature of 750 °C with a 30 °C/min heating rate and 15 min of residence time. In addition, the changes in PAH concentration in bio-oils relating to pyrolysis conditions were investigated by controlling the final temperature and residence time. Final temperature was examined at the varied stages of 450, 550, 650, 750, and 850 °C. The varied residence times were 5, 10, 15, and 20 min. 2.3. PAH analysis 2.3.1. The extraction, concentration, and purification of PAHs The information about the quantitative and qualitative analyses of PAHs in bio-oil from the pyrolysis of sewage sludge was scarcely reported (Hu et al., 2013). Bio-oil has a complex chemical composition and contains high contents of residues and water. In order to reduce the influence of residues and other chemical compositions during detection of PAHs with GC-MS analysis, the pretreatment of bio-oil is quite necessary. Around 10 g of the condensable pyrolysis oil was sampled. The pyrolysis oil was first diluted with dichloromethane solvent with a volume ratio of 1:1 and then centrifuged in order to separate coarse pollutants from the mixture of organic and aqueous phases. Thereafter, the dichloromethane solvent and aqueous phase were separated from organic phase in the bio-oil sample by decantation, distillation at 50 °C, and addition of anhydrous sodium sulfate. The extracted organic phase was further concentrated in a rotary evaporator to a final volume of 1 mL. Finally, the condensate was purified with a silica column using 4:6 (v: v) n-pentane/dichloromethane as the eluent and a flow rate of 2 mL/min. The collected materials were re-concentrated by purging with ultra-pure nitrogen to a final volume of 1 mL prior to GC-MS analysis. The PAH analysis experiment for each of the bio-oil samples and sewage sludge samples was duplicated to ensure the accuracy of data. The obtained bio-oils were kept in brown vials and refrigerated at 4 °C. 2.3.2. GC-MS analysis of PAHs CP3800-Saturn 2000 of GC-MS was employed to detect 16 target PAHs in the purified bio-oil samples. The 16 US Environmental Protection Agency priority controlled pollutant PAHs were monitored. The
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1-Nitrogen;2-pressure gauge;3-flow meter;4-quartz tube ;5-furnace;6-quatz container;7-a-programmable temperature controller;8-b-programmable temperature controller;9-water tank;10-pump;11-condensed oil container;12-gas dryer;13-gas filter;14-flowmeter; 15-gas collecting bag Fig. 1. The schematic diagram of the sewage sludge pyrolysis to produce bio-oils. 1—Nitrogen; 2—pressure gage; 3—flow meter; 4—quartz tube; 5—furnace; 6—quartz container; 7-a— programmable temperature controller; 8-b—programmable temperature controller; 9—water tank; 10—pump; 11—condensed oil container; 12—gas dryer; 13—gas filter; 14—flowmeter; 15—gas collecting bag.
column used in the GC was a 30 m × 0.25 mm HP Ultra 2 with a thickness of 0.24 μm. The following temperature program was adopted: oven temperatures rose from 60 °C to 230 °C at a rate of 10 °C/min, where it was held for 5 min, and then increased from 230 °C to 300 °C at a rate of 3 °C/min, where it was held for 0.67 min. One microliter of the sample was injected into the capillary column with an auto-sampler. The injector and ion source temperatures were both set at 310 °C. The carrier gas was N2 of 99.9% purity and was provided with a flow rate of 1.0 mL/min. Qualitative analyses of 16 target PAHs were done by comparing the retention time of chromatographic peak in the samples with those of PAH standards run under the same conditions. The quantitative analyses were carried out using calibration data from PAH standards which were purchased from a merchant agent of Chem Services Co. (US) in China. Perylene-d12 was used as the internal standard substance was. More detailed information regarding the quality control of PAH analysis can be referred to in a previous publication (Hu et al., 2013). The total pretreatment method of bio-oil and GC-MS analysis method was verified for accuracy and repeatability by running blank measurement and recovery measurement with edible oil and PAH standards. The recovery efficiencies of the target PAHs were basically between 80% and 99%, except for Nap with a low recovery of 33%. In addition, principal data analysis was conducted with SPSS 11.0 and EXCEL 2007 software. The National Measurement Standard of Determination of PAHs in Soil was employed to determine 16 target PAH concentrations in the dried raw sewage sludge samples (SN/T 1877.3-2007; ISO-18287: 2006(E)). 3. Results and discussion 3.1. Characterization of the raw sewage sludge and the yields of pyrolysis bio-oil Table 1 shows the main chemical characteristics and the average yields of bio-oil from pyrolysis of seven kinds of sewage sludge. It presents an evident variation in the contents of volatile matter, ash, fixed carbon, and moisture. The variation is expected to potentially impact the chemical compositions of the resulting pyrolysis bio-oils. In particular, it was noted that all of them had a relatively high volatile content. The sludge S3 from a paper manufacturing plant has the highest value content of 50.6% and the sludge S2 from a beer-brewing plant having the lowest value content of 25.2%, due to their particular industrial production process. 3.2. PAH distributions in different raw sewage sludges Table 2 shows the average concentration distributions of 16 target PAHs in the selected seven kinds of sewage sludge. The average is based on two samples for each type. The procedures used in this
study, including sampling sewage sludge, pretreatment of sewage sludge and GC-MS analysis of PAHs, have proven effective since the experimental errors of the replicated tests are less than 10%, which are small enough to draw conclusions. The 16 target PAHs were classified based on their molar weights. The high molar weight (HMW) PAHs are five and six ring PAHs, including BaP, IND, DBA, and BghiP. The middle molar weight (MMW) PAHs are four ring PAHs, including CHR, BbF, BkF, BaA, Pyr and FL, and the low molar weight (LMW) PAHs are two and three ring PAHs, including Nap, AcPy, Acp, Flu, PA, and Ant. The total amount of PAHs is evidently varied with the resource of sewage sludge, with values ranging between 9.19 and 23.68 mg/kg. The high values obtained for the PAH concentrations in the sewage sludge samples are expected to be one of the indicative factors of the PAH distributions in the resulting pyrolysis products. Formation of PAHs in wastewater sewage sludge could be dependent on the resources of wastewater, phi-chemical properties of sewage sludge, and treatment methods of wastewater. It has been reported that sewage sludge from various industrial wastewater contained rich PAHs, such as foundry industry, wood and paper products processing, electrical production, and textile industry (Zhao and Zhu, 2010). In this study, the sewage sludge from paper manufacturing wastewater treatment system (S3) gave the highest PAH concentration of 23.68 mg/kg. In addition, as the results of the mixture of domestic wastewater and industrial wastewater (approximately 1:1 of volume ratio), the three municipal sewage sludge samples (S4, S5, and S7) also contributed large amounts of PAHs, which are 20.90%, 20.47% and 16.09%, respectively. The sewage sludges (S1 and S6) from dying wastewater gave the lowest amounts of PAHs. The reason for that may be that S1 and S6 samples are collected after the digestion treatment of wastewater sludge in a dying plant. A digestion processing was proved to be effective to degrade PAHs in sewage sludge (Zhao and Zhang, 1992). Except for the S6 and S7, it can be seen that the most abundant PAHs in sewage sludge were the MMW PAHs. Basically, the decreasing order of concentration for different rings of PAHs is MMW, LMW, and HMW. With the comparison of MMW PAHs, the LMW PAHs are easier to volatilize, which possibly results in a reduction of LMW PAH concentration in bio-oils. A very low concentration of HMW can also be noted in all of the identified sewage sludge. Some of the HMW PAHs, such as IND, DBA, and BghiP cannot be detected out in most of the sludge samples. In addition, it was reported that PAHs with more than four rings in sewage sludge are mainly derived from incomplete combustion of fossil fuels, and PAHs with two and three rings are mainly from oil pollution. Thus, one of the possible reasons for the formation of PAHs in the tested sewage sludge is the incomplete combustion of fossil fuels. The more complex factors influencing the formation of PAHs in sewage sludge are not discussed in this study.
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Table 2 Concentration distribution of 16 target PAHs in seven sludge samples (mg/kg). Rings
PAHs
S1
S2
S3
S4
S5
S6
S7
LMW
Naphthalene (Nap) Acenaphthene (Acp) Acenaphthylene (AcPy) Fluorene (Flu) Phenanthrene (PA) Anthracene (Ant) Fluoranthene [FL] Pyrene (Pyr) Benz[a]anthracene (BaA) Chrysene (CHR) Benzo[b]fluoranthene (BbF) Benzo[k]fluoranthene (BkF) Benzo[a]pyrene (BaP) Indeno[1,2,3-cd]pyrene(IND) Dibenz[ah]anthracene (DBA) Benzo[ghi]perylene (BghiP) Total content
1.11 0.13 0.76 0.25 1.4 0.06 0.31 2.53 1.64 0.6 0.15 0.09 0.11 0.01 0.04 – 9.19
1.81 0.96 0.43 0.73 – – 0.95 1.95 3.88 2.09 0.05 – 0.08 0.05 – – 12.98
2.1 0.51 0.41 0.93 3.92 0.48 1.63 9.7 2.36 1.26 0.18 0.06 0.08 0.05 0.01 – 23.68
0.78 0.11 0.33 0.35 2.43 0.18 0.75 5.72 3.13 1.66 0.3 0.11 0.12 0.01 0.05 0.06 16.09
2.04 0.30 0.61 0.51 2.55 0.23 1.26 7.71 3.23 1.98 0.21 0.1 0.13 0.03 0.01 – 20.90
2.01 0.23 0.49 1.14 2.23 0.16 0.51 3.01 0.81 0.48 0.07 0.02 0.02 – – 0.05 10.74
0.43 0.76 0.57 0.87 2.54 6 0.11 6.41 2.42 0.22 0.05 0.01 0.01 0.05 0.02 – 20.47
MMW
HMW
The concentrations of PAHs are presented as milligram of PAHs per kilogram of dry-base sludge. “–” means the concentrations of PAHs are below the detection limitation.
3.3. PAH distributions in bio-oils and the correlations with chemical characteristics of raw sewage sludge Table 3 shows the average concentration distributions of 16 target PAHs in the resulting bio-oils from the pyrolysis of the seven kinds of sludge. Likewise, the average is based on two samples for each type. The experimental errors of the concentrations of 16 target PAHs in the replicated tests are less than 10%. The most carcinogenic potency PAH compounds existed in the resulting bio-oils. It was observed that the bio-oil from the pyrolysis of the municipal sewage sludge sample S7 contained the highest concentration of PAHs, which reached 48.9 mg/kg. The municipal sewage sludge samples S4 and S5 also contributed high PAH concentrations in the obtained bio-oils, which are 21.72 mg/kg and 26.62 mg/kg, respectively. But the bio-oil from the pyrolysis of beer-brewing sewage sludge contributed the lowest content of PAHs, which was 13.72 mg/kg. These results are not consistent with the PAH concentration distributions in raw sewage sludge samples, such that the concentrations of PAHs in the bio oil samples from the pyrolysis of S1 and S6 are higher than that of S2. It was also noticeable that the most abundant PAHs were LMW PAHs, which ranged from 11.14 mg/kg in the bio-oil from the pyrolysis of S2 to 36.82 mg/kg in the bio-oil from the pyrolysis of S7. Some of the MMW and HMW PAHs were not detected. This is different from the results reported by a previous study, where the pyrolytic bio-oil from food-processing sewage
sludge contained a higher proportion of HMW PAHs than MMW and LMW PAHs (Tsai et al., 2009a, 2009b). The factors influencing the distribution of PAHs into bio-oil during the pyrolysis of sewage sludge are quite complex. The relative analyses could refer to the previous investigations regarding the emission of PAHs in the process of coal pyrolysis (Cheng, 2010). It was indicated that the chemical characteristic of coal is one of the important influences on PAH emissions during the pyrolysis of coal, especially ash and volatile matter contents. Furthermore, the PAHs contained in coal can have an important impact on the amount of PAH concentration in gas and liquid products from coal combustion and pyrolysis. The correlation between the distributions of LMW (Y1), MMW (Y2), and HMW (Y3) PAHs in bio-oils and the chemical characteristics of sewage sludge was analyzed using multiple regression analysis with SPSS 11.0 software. The discussed chemical characteristics were the concentrations of HMW (X1), MMW (X2), LMW (X3) PAHs, ash contents (X4), and volatile contents (X5) in raw sewage sludge. Table 4 shows the results of the multiple regression analysis. The significant correlations can be observed between PAHs in bio-oils and PAHs in raw sewage sludge. It could be found that the concentration of LMW PAHs in bio-oil is significantly correlated with MMW PAHs in raw sewage sludge compared with other studied factors; the correlation coefficient is 0.607. The MMW and HMW PAHs in bio-oils are significantly correlated with HMW PAHs in raw sewage sludge, with correlation coefficients of
Table 3 PAH concentrations in bio-oils from the pyrolysis of seven kinds of sewage sludge(mg/kg).
LMW
MMW
HMW
PAHs
S1
S2
S3
S4
S5
S6
S7
Naphthalene (Nap) Acenaphthene (Acp) Acenaphthylene (AcPy) Fluorene (Flu) Phenanthrene (PA) Anthracene (Ant) Fluoranthene [FL] Pyrene (Pyr) Benz[a]anthracene (BaA) Chrysene (CHR) Benzo[b]fluoranthene (BbF) Benzo[k]fluoranthene (BkF) Benzo[a]pyrene (BaP) Indeno[1,2,3-cd]pyrene(IND) Dibenz[ah]anthracene (DBA) Benzo[ghi]perylene (BghiP) Total content
3.91 5.31 0.21 0.03 1.12 0.99 1.11 0.69 0.05 0.05 0.38 0.09 0.08 0.02 0.06 – 14.10
3.95 1.75 1.88 2.09 1.05 0.42 0.75 1.03 0.66 0.02 0.02 – 0.05 – 0.05 – 13.72
4.93 2.25 0.69 2.25 2.68 3.71 0.72 2.41 – 0.21 0.14 0.15 – 0.28 0.24 0.06 21.72
8.25 5.35 5.65 1.14 1.02 1.61 2.11 0.49 – – 0.35 0.19 0.22 – 0.24 0.16 26.78
5.33 7.12 3.14 3.19 3.21 2.1 – 3.27 0.11 0.26 – – – 1.45 0.65 – 29.83
5.55 3.32 0.6 0.07 1.7 1.22 0.88 2.41 0.05 – 0.05 0.08 0.11 0.05 – 0.13 16.22
12.8 8.74 5.35 5.01 4.91 – – 3.23 – – 0.36 3.42 – 4.06 0.91 0.11 48.90
The concentrations of PAHs are presented as milligram of PAHs per kilogram of original bio-oil. “–” means the concentrations of PAHs are below the detection limitation.
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Some literatures indicated that the final temperature and residence time have important influences on PAH generation in the thermochemical conversion process of fuels, such as combustion of coal and fuel oil (Dominguez et al., 2005; Cheng, 2010). In order to better understand how the pyrolysis temperature influences the distribution of the target PAHs, municipal sewage sludge samples (S5) were sampled and then was pyrolyzed at a controlled heating rate of 30 °C/min to the final temperature of 450, 550, 650, 750, and 850 °C, and then the final temperature was maintained for 15 min. Fig. 2 shows the average concentration variation of HMW, MMW, and LMW PAHs in bio-oil samples with the function of the pyrolysis temperature. The average is based on two samples for each type. The experimental errors of the average concentrations of 16 target PAHs in the replicated tests are less than 10%. It was noted that the target PAH concentrations varied regularly as the final temperatures increased from 450 °C to 850 °C. Two peak values of HMW, MMW, and LMW PAH concentrations were obtained at 550 °C and 750 °C. The highest PAH content is present in the bio-oil sample at 750 °C, containing 7.1 mg/kg of LMW PAHs, 18.1 mg/kg of MMW PAHs, and 4.3 mg/g of HMW PAHs. From Fig. 2, the “bathtube” distribution pattern provides a clear insight into the complex pyrosynthetic mechanism, which is liable to form PAHs with MMW and LMW in the process of sewage sludge pyrolysis. This result is not in accordance with the results in which the pyrolysis of the sludge at high temperature using conventional methods yielded an oil rich in HMW as reported by Dominguez et al., 2005. In our work, it was observed that HMW PAH concentration is only 0.9 mg/kg in the oil sample at 850 °C, which is a lower concentration than that of the detected LMW and MMW PAHs. It is also noticeable that LMW PAH contents are lower than MMW
PAHs in all the studied bio-oil samples. The lower content of LMW PAHs may be due to the gaseous properties of LMW, and the generated LMW PAHs were discharged with the non-condensable gas, not entirely concentrated in the pyrolysis oil during the process of sewage sludge pyrolysis. As a high ring PAH is more thermodynamically stable, some of the HMW PAHs in this work could be from the raw sewage sludge sample which contains a low content of HMW PAHs. Furthermore, a Diels–Alder reaction mechanism has been proposed by some researchers for the formation of PAHs (Cunliffe and Williams, 1988; Williams and Besler, 1994). It is assumed that the pyrolysis of alkanes at high temperature generates alkenes and dienes via dehydrogenation, which by cyclization and subsequent aromatization produce the aromatic compound (Cypres, 1987; Fairburn et al., 1990). In the present work, the generation of the new PAHs might also occur via a Diels–Alder reaction. Some other routes of PAH formation during combustion or pyrolysis have also been suggested e.g. direct combination of aromatic ring and H2 abstraction–C2H2 addition, and secondary reaction of oxygenated compounds such as phenols, cresols and xylenols at moderate to high temperatures can produce PAHs via deoxygenating (Morf et al., 2002; Egsgaard and Larsen, 2000). The relative mechanisms are not further investigated with this study. In addition, the yields of the bio-oil were varied with the increased final temperature, which were 38%, 41%, 35%, 33%, and 31% at 450, 550, 650, 750, and 850 °C, respectively. It was found that the final temperature of 650 °C and 750 °C for sludge pyrolysis would make a higher contribution to the total amount of the PAHs in bio-oil. At the final temperature of 450 and 850 °C, the total amounts of the PAHs in bio-oil were decreased to the lowest. In this work, the residence time was counted after reaching the final pyrolysis temperature. The total amount of PAHs is increased with the increased residence time as shown in Fig. 3. The average concentrations of the total PAHs are 3.9, 4.9, 5.1, and 6.8 mg/kg at 5, 10, 15, and 20 min, respectively. The result is in accordance with a previous investigation that a long residence time would favor the formation of PAHs by increasing the number of possible secondary reactions between the volatile products (Sancheza et al., 2009). Fig. 3 also indicated the concentration variations of LMW, MMW, and HMW PAHs as the function of different residence times. The experimental errors of the average concentrations of 16 target PAHs in the replicated tests are less than 10%. It can be noted that only the concentration of MMW PAHs shows a regular increase as the residence time is increased. From the point of view of concentration distribution, at any residence time, the most important PAHs were MMW (3.7 mg/kg at 20 min). The LMW PAHs also yielded the highest concentrations of 1.9 mg/kg at 10 min, but a slight decrease of the amount of LMW PAHs was observed with increased residence time from 10 to 15 min. Likewise, the concentration of HMW PAHs is also slightly decreased from 5 to 10 min with increasing residence time. This could be attributed to the possible direct combination
Fig. 2. HMW, MMW, and LMW PAH concentrations in bio-oil samples generated from SS pyrolysis under different ending temperatures.
Fig. 3. HMW, MMW, and LMW PAH concentrations in bio-oil samples generated from SS pyrolysis under different residence times.
Table 4 R value for PAHs and industrial characteristics of sewage sludge.
X1 X2 X3 X4 X5
Y1
Y2
Y3
0.205 0.607 0.298 0.446 0.453
0.408 0.533 0.673 0.404 0.263
0.252 0.528 0.580 0.333 0.272
0.673 and 0.580, respectively. It is also noted that the correlation between the volatile content and the concentration of LMW PAHs in bio-oil is not evident as presented by the low correlation coefficient of 0.453. As expected, MMW and HMW PAHs in bio-oil are closely correlated with the MMW PAHs in raw sewage sludge, with correlation coefficients of 0.533 and 0.548.
3.4. Influence of pyrolysis conditions on PAH distributions in bio-oils
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of LMW PAHs or a secondary reaction of oxygenated compounds to produce new MMW or HMW PAHs. It was also noted that the residence time did not present an evident impact on the yields of the bio-oil. The yields of bio-oil were 31%, 31%, 33%, and 29% at 5, 10, 15, and 20 min, respectively. The highest yield could be found at the residence time of 15 min for sludge pyrolysis, which also contributed the highest total amount of PAHs in bio-oil. In order to better describe the influence of the residence time on the yield of bio-oil and its chemical composition, a large sample size would be helpful.
4. Conclusions The problem of wastewater sewage sludge disposal is currently one of the complex environmental issues. As an alternative, the application of sewage sludge pyrolysis is a promising method for converting sewage sludge into fuels. But the results in this work indicated that most of 16 US EPA PAHs were enriched in different resources of sewage sludge and the resulting pyrolysis bio-oils. The total amount of PAHs in the sampled sludge is evidently varied with the sludge resource, with values ranging between 9.19 and 23.68 mg/kg, which imply that the sludge is formed from different industrial processes and treatment methods. For instance, the sewage sludge from paper manufacturing wastewater treatment system gave the highest PAH concentration. As the results of the mixture of domestic wastewater and industrial wastewater, the three municipal sewage sludge samples also contributed high amounts of PAHs, which are 20.26%, 19.85% and 15.76%, respectively. The sewage sludge from dying wastewater gave the lowest amounts of PAHs. Moreover, the digestion treatment of wastewater sludge is effective to degrade PAHs in sewage sludge. In addition, it is noted that a decreased order for the different rings of PAH concentrations in sludge is MMW, LMW, and HMW. PAH concentrations in the resulting bio-oils ranged from 13.72 to 48.9 mg/kg, in which the lowest and highest values were contributed by the bio-oils from the pyrolysis of the municipal sewage sludge sample and beer-brewing sewage sludge, respectively. The most abundant PAHs in bio-oils were the low molar weight (LMW) PAHs. The factors influencing the formations of different rings of PAHs into bio-oil during the pyrolysis of sewage sludge are quite complex. In this study, it could be proposed that the concentration of LMW PAHs in bio-oil is correlated at best with MMW PAHs in raw sewage sludge, due to the high correlation coefficient of 0.607. MMW and HMW PAHs in bio-oils are significantly correlated with HMW PAHs in raw sewage sludge, for which the correlation coefficients are 0.672 and 0.58, respectively. The concentration of LMW PAHs in bio-oil also showed relatively significant correlation with the volatile matter content of raw sludge. This work also indicated that the pyrolysis temperature and residence time varyingly influenced the generation of LMW, MMW, and HMW PAHs into the resulting sludge pyrolysis bio-oils. It was noted that the pyrolysis of the sludge at a high temperature does not necessarily yield the resulting PAHs rich in bio-oil. This is evident as a low PAH concentration was observed at 850 °C, though the highest PAH concentration was presented at 750 °C. The residence time also significantly influenced PAH generation during sludge pyrolysis. The formation of PAHs was believed to be via a Dields–Alder reaction and also via secondary reactions of oxygenated compounds such as phenols.
Acknowledgment The authors want to appreciate the projects of Zhejiang Provincial Natural Science Foundation (No. Y5100258) and the National Natural Science Foundation (Grant No. 51008279) for providing financial supports for this work. References Anikwe MA, Nwobodo KC. Long-term effect of municipal waste disposal on soil properties and productivity of sites used for urban agriculture in Abakaliki. Nigeria. Bioresour Technol 2002;83:241–50. Cheng Z. Study on the emission characteristic of polycyclic aromatic hydrocarbons from coal pyrolysis. Master thesisTaiYuan Sci-Tech University; 2010. Cunliffe AM, Williams PT. Composition of oils derived from the batch pyrolysis of tyres. J Anal Appl Pyrolysis 1988;44:131–52. Cypres R. Aromatic hydrocarbons formation during coal pyrolysis. Fuel Process Technol 1987;15:1–15. Dominguez A, Menendez JA, Inguanzo M, Pis JJ. Investigations into the characteristics of oils produced from microwave pyrolysis of sewage sludge. Fuel Process Technol 2005;86:1007–20. Dominguez A, Menendez JA, Pis JJ. Hydrogen rich fuel gas production from the pyrolysis of wet sewage sludge at high temperature. J Anal Appl Pyrolysis 2006a;77:127–32. Dominguez A, Menendez JA, Inguanzo M, Pis JJ. Production of bio-fuels by high temperature pyrolysis of sewage sludge using conventional and microwave heating. Bioresour Technol 2006b;97:1185–93. Egsgaard H, Larsen E. Thermal transformation of light tarspecific routes to aromatic aldehydes and PAH. 1st world conference on biomass for energy and industry, Sevilla, Spain; 2000. p. 1468–71. Fairburn JA, Behie LA, Svrcek WY. Ultrapyrolysis of nhexadecane in a novel micro-reactor. Fuel 1990;69:1537–45. Fytili D, Zabaniotou A. Utilization of sewage sludge in EU application of old and new methods—a review. Renew Sust Energ Rev 2008;12:116–40. GB/T 212–2008. The national testing standard of proximate analysis of coal of China; 2008. Hu YJ, Guan ZC, Zheng XY. Analysis on polycyclic aromatic hydrocarbons (PAHs) in pyrolysis oil produced from municipal wastewater sewage sludge. Chem Eng J 2013;33:67–72. ISO-18287: 2006(E). Solid quality—determination of PAH–gas chromatographic method with mass spectrometric detection (GC-MS). Switzerland: International standard; 2006. Kim Y, Parker W. A technical and economic evaluation of the pyrolysis of sewage sludge for the production of bio-oil. Bioresour Technol 2008;99:1409–16. Morf P, Hasler P, Nussbaumer T. Mechanism and kinetics of homogeneous secondary reactions of tar from continuous pyrolysis of wood chips. Fuel 2002;81:843–53. Rulkens W. Sewage sludge as a biomass resource for the production of energy: overview and assessment of the various options. Energy Fuels 2008;22:9–15. Sancheza ME, Menendezb JA, Dominguezb A, Pisb JJ, Martineza O, Calvoa LF, et al. Effect of pyrolysis temperature on the composition of the oils obtained from sewage sludge. Biomass Bioenergy 2009;33:933–40. Shen L, Zhang DK. An experimental study of oil recovery from sewage sludge by low-temperature pyrolysis in a fluidized-bed. Fuel 2003;82:1465–72. SN/T 1877.3–2007. Determination of PAHs in mineral oils. Industry standard for entry–exit inspection and quarantine of China; 2007. Stevens J, Northcott G, Stern G, Tomy G, Jones K. PAHs, PCBs, PCNs, organochlorine pesticides, synthetic musks, and polychlorinated n-alkanes in U.K. sewage sludge: survey results and implications. Environ Sci Technol 2003;37:462–7. Trably E, Patureau D. Successful treatment of low PAH-contaminated sewage sludge in aerobic bioreactors. Environ Sci Pollut Res 2006;13:170–6. Tsai WT, Lee MK, Chang JH, Su TY, Chang YM. Characterization of bio-oil from induction-heating pyrolysis of food-processing sewage sludges using chromatographic analysis. Bioresour Technol 2009a;100:2650–4. Tsai WT, Mi HH, Chang JH, Chang YM. Levels of polycyclic aromatic hydrocarbons in the bio-oils from induction-heating pyrolysis of food-processing sewage sludges. J Anal Appl Pyrolysis 2009b;86:364–8. Williams PT, Besler S. Polycyclic aromatic hydrocarbons in waste derived pyrolitic oils. J Anal Appl Pyrolysis 1994;30:17–33. Zhao JQ, Zhang XH. Studies of the effects of 34 organic toxicants on anaerobic digestion. China Envion Sci 1992;12:249–54. Zhao XL, Zhu W. Contents and characteristics of polycyclic aromatic hydrocarbons (PAHs) in municipal sewage sludge from cities in Yangtze river delta area. J Environ Sci Res 2010;23:1174–9.
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Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv
Investigation into the distribution of polycyclic aromatic hydrocarbons (PAHs) in wastewater sewage sludge and its resulting pyrolysis bio-oils Yanjun Hu a,⁎, Guojian Li b, Mi Yan a, Chuanjuan Ping a, Jianli Ren a a b
Institute of Thermal and Power Engineering, Zhejiang University of Technology, Chaowang Road 18#, 310024, HangZhou, China School of Civil Engineering & Architecture, Zhejiang Sci-tech University, Xiasha District, 310018 Hangzhou, China
H I G H L I G H T S • • • • •
We investigated 16 US EPA PAHs in sewage sludge and the resulting pyrolysis bio-oil. We examined the factors influencing the distribution of different rings of PAHs. Most of the 16 target PAHs were enriched in the sludge and its resulting pyrolysis oil. LMW PAHs in bio-oil are correlated with MMW PAHs in raw sewage sludge at best. Temperature and residence time in sludge pyrolysis evidently impact PAH emission.
a r t i c l e
i n f o
Article history: Received 30 August 2013 Received in revised form 10 December 2013 Accepted 10 December 2013 Available online xxxx Keywords: Sewage sludge Pyrolysis Bio-oil Polycyclic aromatic hydrocarbons
a b s t r a c t This study firstly investigated the distributions of 16 US EPA priority controlled polycyclic aromatic hydrocarbons (PAHs) in seven kinds of different wastewater sewage sludges and bio-oils from the sludge pyrolysis. A lab-scale tube furnace was used to simulate sludge pyrolysis and retrieve condensed oils. PAH determination was conducted with the extraction, concentration, and purification of PAHs in sludge samples and the resulting bio-oils, and then GC-MS analysis. Then, the factors influencing the distributions of different rings of PAHs in pyrolysis bio-oil, such as the chemical characteristics of raw sewage sludge and pyrolysis condition, were analyzed. It was noted that the total amount of PAHs in raw sludge is evidently varied with the sludge resource, with values ranging between 9.19 and 23.68 mg/kg. The middle molar weight (MMW) PAH distribution is dominant. PAH concentrations in sludge pyrolysis bio-oil were ranged from 13.72 to 48.9 mg/kg. The most abundant PAHs were the low molar weight (LMW) PAHs. It could be found that the concentration of LMW PAHs in bio-oil is correlated with MMW PAHs in raw sewage sludge at best, which the correlation coefficient is 0.607. For MMW and HMW (high molar weight) PAHs, they are significantly correlated with HMW PAHs in raw sewage sludge, which the correlation coefficients are 0.672 and 0.580, respectively. The concentration of LMW PAHs in bio-oil is also relatively significant and correlated with the volatile matter content of raw sludge. In addition, it was proved that final temperature and residence time have important influences on PAH generations during the pyrolysis of sewage sludge. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Sewage sludge is abundant in volatile matter and is being considered as a valuable bio-resource. Currently, there is an increasing interest in its reutilization through converting sewage sludge into a diversity of products such chemicals, fuels by using thermo-chemical processes (Fytili and Zabaniotou, 2008; Rulkens, 2008). Thermo-chemical conversion technologies that develop refuse derived fuels (RDF) from sewage sludge have been widely studied and developed in many countries (Kim and Parker, 2008; Shen and Zhang, 2003; Dominguez et al., ⁎ Corresponding author at: Chaowang Road 18#, 310024, Hangzhou, China. Tel.: +86 571 88320942. E-mail address: [email protected] (Y. Hu). 0048-9697/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.scitotenv.2013.12.051
2006a, 2006b). Some innovative technologies, such as sludge pyrolysis, have been applied industrially and have so far presented good economics and social benefits. In the past decade, pyrolysis of sewage sludge in an oxygen-free atmosphere has aroused a significant interest. Particularly, the interest is being developed in this process due to its high recovery of liquid oil. Investigations have indicated that the obtained oil from sludge pyrolysis can be applied in diesel fuelled engines and is comparable to low-grade petroleum distillates from commercial refineries (Dominguez et al., 2005; Dominguez et al., 2006a, 2006b). It was also noted that pyrolysis bio-oils were complex mixtures made up of a group of aromatic clusters with one to three aromatic rings connected by long straight chain hydrocarbons with hydroxyl groups (Shen and Zhang, 2003). As reported by some researchers, the composition of pyrolysis bio-oils from different urban sewage sludges consistently
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contained polycyclic aromatic hydrocarbons (PAHs) (Tsai et al., 2009a, 2009b). PAHs are a group of organic compounds with two or more fused aromatic rings, which appeared on the US EPA priority controlled pollutants list. Their release in air, soil or water would be a serious problem from the viewpoint of protecting the environment and human health. Most PAHs have strong toxicity and are carcinogenic, teratogenic, and mutagenic. It was reported that 70–90% of cancer diseases in humans and animals are caused by highly toxic chemicals in the environment. PAHs remain a widely distributed health, ecological and environmental concern, which makes it necessary to know both their residue levels and their distribution in the process of wastewater sewage sludge disposals. Studies of the long-term effects of municipal sewage sludge on the enrichment of PAHs in the environment have been reported by many authors (Anikwe and Nwobodo, 2002; Stevens et al., 2003; Trably and Patureau, 2006). The levels of PAHs in pyrolysis bio-oil from different food-processing sewage sludges were also reported by Tsai et al. (2009a, 2009b). However, studies regarding the distribution of PAHs in the resulting products from sludge pyrolysis were little reported, neither were the relevant influencing factors (Shen and Zhang, 2003; Tsai et al., 2009a, 2009b). In this study, the concentration distributions of 16 US EPA priority controlled PAHs in different sources of wastewater sewage sludge and in sludge pyrolysis bio-oils were firstly investigated. A lab-scale tube furnace was used to simulate sludge pyrolysis and retrieve condensed oils. A qualitative and quantitative analysis of PAHs was carried out by means of a series of experimental pretreatment including the extraction, concentration and purification of PAHs, and GC-MS analysis. Finally, the correlations between the distributions of different rings of PAHs and the chemical characteristics of raw sewage sludge, pyrolysis condition were analyzed.
2. Bio-oil sampling and PAH analysis 2.1. Materials Seven types of raw sewage sludge samples were collected after mechanical dewatering using a filter press. The raw sewage sludge were from industrial wastewater treatment systems in dying (S1 and S6), beer-brewing (S2), paper manufacturing (S3), and municipal wastewater treatment plants (S4, S5 and S7) containing domestic wastewater and industrial wastewater, respectively. Except for S6 and S7, the other sludge samples were sampled in the same city of Hangzhou, China. S6 was sampled in Huzhou, China, and S7 was sampled in Ningbo, China. S1, S2, and S6 were subjected to aerobic digestion treatment. All sludge samples were firstly dried in open air for 2 days to remove most moisture content, and then were dried in a lab-scale air convection oven for 8 h. The dried sludge cakes were smashed in a milling machine to provide a feed sample in the size range of smaller than 60 μm for labanalyses. The resulting sludge sample was then kept in an airtight container to prevent re-absorption of moisture before experimentation. The main chemical characteristics of seven kinds of sludge are presented in Table 1. The volatile matter content (VM), the amount of ash (A), and the moisture content were determined according to the National Table 1 the main chemical characteristics of different sewage sludges (dry base). Sample code
VM (%)
A (%)
FC (%)
M (%)
Field of oil (%)
S1 S2 S3 S4 S5 S6 S7
35.3 25.2 50.6 46.3 53.4 38.5 41.2
55.1 65.4 37.2 48.1 40.9 48.3 46.3
2.1 5.8 7.2 1.7 1.9 13.4 1.6
7.5 3.6 5.0 4.0 3.8 5.0 3.5
31 38 31 31 33 36 29
Testing Standard of Proximate Analysis of Coal (GB/T 212–2008). The fixed carbon content was calculated on the basis of the mass balance. 2.2. Sludge pyrolysis and recovery of the bio-oils Fig. 1 shows the schematic diagram of the sewage sludge pyrolysis to collect bio-oil samples. A lab-scale tube furnace with a heating area of 40 mm in diameter and 600 mm in length was used to simulate sludge pyrolysis. The furnace is constructed with quartz and is heated by a programmable temperature control, and can be adjusted from room temperature to 1200 °C. The sewage sludge pyrolysis tests were operated at atmospheric pressure and an inert atmosphere. A nitrogen flow rate of 80 mL/min was passed through the whole pyrolysis system for 20 min prior to the commencement of the sludge pyrolysis testing. The nitrogen flow was shut down during the sludge pyrolysis, which is good for high residence time of volatiles derived from sludges in the furnace and their secondary reactions. A sample of sludge around 50 g in weight was placed in the center of the hot zone of the furnace. The volatiles evolved from the sample passed through three consecutive condensers which were placed in an ice-water bath. The condensed oil was retrieved using dichloromethane and kept for further chemical analysis. The yield of pyrolysis oil was obtained by calculating an average percentage of the difference between weights of the used sludge sample and the retrieved oil to the weight of the used dry sludge sample. In order to investigate the correlation between concentration distribution of PAHs in bio-oils and chemical characteristics of raw sewage sludge, the bio-oil samples from the pyrolysis of seven kinds of sewage sludge were obtained. These pyrolysis experiments were carried out at standard of pyrolysis conditions, which are to heat from room temperature to final temperature of 750 °C with a 30 °C/min heating rate and 15 min of residence time. In addition, the changes in PAH concentration in bio-oils relating to pyrolysis conditions were investigated by controlling the final temperature and residence time. Final temperature was examined at the varied stages of 450, 550, 650, 750, and 850 °C. The varied residence times were 5, 10, 15, and 20 min. 2.3. PAH analysis 2.3.1. The extraction, concentration, and purification of PAHs The information about the quantitative and qualitative analyses of PAHs in bio-oil from the pyrolysis of sewage sludge was scarcely reported (Hu et al., 2013). Bio-oil has a complex chemical composition and contains high contents of residues and water. In order to reduce the influence of residues and other chemical compositions during detection of PAHs with GC-MS analysis, the pretreatment of bio-oil is quite necessary. Around 10 g of the condensable pyrolysis oil was sampled. The pyrolysis oil was first diluted with dichloromethane solvent with a volume ratio of 1:1 and then centrifuged in order to separate coarse pollutants from the mixture of organic and aqueous phases. Thereafter, the dichloromethane solvent and aqueous phase were separated from organic phase in the bio-oil sample by decantation, distillation at 50 °C, and addition of anhydrous sodium sulfate. The extracted organic phase was further concentrated in a rotary evaporator to a final volume of 1 mL. Finally, the condensate was purified with a silica column using 4:6 (v: v) n-pentane/dichloromethane as the eluent and a flow rate of 2 mL/min. The collected materials were re-concentrated by purging with ultra-pure nitrogen to a final volume of 1 mL prior to GC-MS analysis. The PAH analysis experiment for each of the bio-oil samples and sewage sludge samples was duplicated to ensure the accuracy of data. The obtained bio-oils were kept in brown vials and refrigerated at 4 °C. 2.3.2. GC-MS analysis of PAHs CP3800-Saturn 2000 of GC-MS was employed to detect 16 target PAHs in the purified bio-oil samples. The 16 US Environmental Protection Agency priority controlled pollutant PAHs were monitored. The
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1-Nitrogen;2-pressure gauge;3-flow meter;4-quartz tube ;5-furnace;6-quatz container;7-a-programmable temperature controller;8-b-programmable temperature controller;9-water tank;10-pump;11-condensed oil container;12-gas dryer;13-gas filter;14-flowmeter; 15-gas collecting bag Fig. 1. The schematic diagram of the sewage sludge pyrolysis to produce bio-oils. 1—Nitrogen; 2—pressure gage; 3—flow meter; 4—quartz tube; 5—furnace; 6—quartz container; 7-a— programmable temperature controller; 8-b—programmable temperature controller; 9—water tank; 10—pump; 11—condensed oil container; 12—gas dryer; 13—gas filter; 14—flowmeter; 15—gas collecting bag.
column used in the GC was a 30 m × 0.25 mm HP Ultra 2 with a thickness of 0.24 μm. The following temperature program was adopted: oven temperatures rose from 60 °C to 230 °C at a rate of 10 °C/min, where it was held for 5 min, and then increased from 230 °C to 300 °C at a rate of 3 °C/min, where it was held for 0.67 min. One microliter of the sample was injected into the capillary column with an auto-sampler. The injector and ion source temperatures were both set at 310 °C. The carrier gas was N2 of 99.9% purity and was provided with a flow rate of 1.0 mL/min. Qualitative analyses of 16 target PAHs were done by comparing the retention time of chromatographic peak in the samples with those of PAH standards run under the same conditions. The quantitative analyses were carried out using calibration data from PAH standards which were purchased from a merchant agent of Chem Services Co. (US) in China. Perylene-d12 was used as the internal standard substance was. More detailed information regarding the quality control of PAH analysis can be referred to in a previous publication (Hu et al., 2013). The total pretreatment method of bio-oil and GC-MS analysis method was verified for accuracy and repeatability by running blank measurement and recovery measurement with edible oil and PAH standards. The recovery efficiencies of the target PAHs were basically between 80% and 99%, except for Nap with a low recovery of 33%. In addition, principal data analysis was conducted with SPSS 11.0 and EXCEL 2007 software. The National Measurement Standard of Determination of PAHs in Soil was employed to determine 16 target PAH concentrations in the dried raw sewage sludge samples (SN/T 1877.3-2007; ISO-18287: 2006(E)). 3. Results and discussion 3.1. Characterization of the raw sewage sludge and the yields of pyrolysis bio-oil Table 1 shows the main chemical characteristics and the average yields of bio-oil from pyrolysis of seven kinds of sewage sludge. It presents an evident variation in the contents of volatile matter, ash, fixed carbon, and moisture. The variation is expected to potentially impact the chemical compositions of the resulting pyrolysis bio-oils. In particular, it was noted that all of them had a relatively high volatile content. The sludge S3 from a paper manufacturing plant has the highest value content of 50.6% and the sludge S2 from a beer-brewing plant having the lowest value content of 25.2%, due to their particular industrial production process. 3.2. PAH distributions in different raw sewage sludges Table 2 shows the average concentration distributions of 16 target PAHs in the selected seven kinds of sewage sludge. The average is based on two samples for each type. The procedures used in this
study, including sampling sewage sludge, pretreatment of sewage sludge and GC-MS analysis of PAHs, have proven effective since the experimental errors of the replicated tests are less than 10%, which are small enough to draw conclusions. The 16 target PAHs were classified based on their molar weights. The high molar weight (HMW) PAHs are five and six ring PAHs, including BaP, IND, DBA, and BghiP. The middle molar weight (MMW) PAHs are four ring PAHs, including CHR, BbF, BkF, BaA, Pyr and FL, and the low molar weight (LMW) PAHs are two and three ring PAHs, including Nap, AcPy, Acp, Flu, PA, and Ant. The total amount of PAHs is evidently varied with the resource of sewage sludge, with values ranging between 9.19 and 23.68 mg/kg. The high values obtained for the PAH concentrations in the sewage sludge samples are expected to be one of the indicative factors of the PAH distributions in the resulting pyrolysis products. Formation of PAHs in wastewater sewage sludge could be dependent on the resources of wastewater, phi-chemical properties of sewage sludge, and treatment methods of wastewater. It has been reported that sewage sludge from various industrial wastewater contained rich PAHs, such as foundry industry, wood and paper products processing, electrical production, and textile industry (Zhao and Zhu, 2010). In this study, the sewage sludge from paper manufacturing wastewater treatment system (S3) gave the highest PAH concentration of 23.68 mg/kg. In addition, as the results of the mixture of domestic wastewater and industrial wastewater (approximately 1:1 of volume ratio), the three municipal sewage sludge samples (S4, S5, and S7) also contributed large amounts of PAHs, which are 20.90%, 20.47% and 16.09%, respectively. The sewage sludges (S1 and S6) from dying wastewater gave the lowest amounts of PAHs. The reason for that may be that S1 and S6 samples are collected after the digestion treatment of wastewater sludge in a dying plant. A digestion processing was proved to be effective to degrade PAHs in sewage sludge (Zhao and Zhang, 1992). Except for the S6 and S7, it can be seen that the most abundant PAHs in sewage sludge were the MMW PAHs. Basically, the decreasing order of concentration for different rings of PAHs is MMW, LMW, and HMW. With the comparison of MMW PAHs, the LMW PAHs are easier to volatilize, which possibly results in a reduction of LMW PAH concentration in bio-oils. A very low concentration of HMW can also be noted in all of the identified sewage sludge. Some of the HMW PAHs, such as IND, DBA, and BghiP cannot be detected out in most of the sludge samples. In addition, it was reported that PAHs with more than four rings in sewage sludge are mainly derived from incomplete combustion of fossil fuels, and PAHs with two and three rings are mainly from oil pollution. Thus, one of the possible reasons for the formation of PAHs in the tested sewage sludge is the incomplete combustion of fossil fuels. The more complex factors influencing the formation of PAHs in sewage sludge are not discussed in this study.
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Table 2 Concentration distribution of 16 target PAHs in seven sludge samples (mg/kg). Rings
PAHs
S1
S2
S3
S4
S5
S6
S7
LMW
Naphthalene (Nap) Acenaphthene (Acp) Acenaphthylene (AcPy) Fluorene (Flu) Phenanthrene (PA) Anthracene (Ant) Fluoranthene [FL] Pyrene (Pyr) Benz[a]anthracene (BaA) Chrysene (CHR) Benzo[b]fluoranthene (BbF) Benzo[k]fluoranthene (BkF) Benzo[a]pyrene (BaP) Indeno[1,2,3-cd]pyrene(IND) Dibenz[ah]anthracene (DBA) Benzo[ghi]perylene (BghiP) Total content
1.11 0.13 0.76 0.25 1.4 0.06 0.31 2.53 1.64 0.6 0.15 0.09 0.11 0.01 0.04 – 9.19
1.81 0.96 0.43 0.73 – – 0.95 1.95 3.88 2.09 0.05 – 0.08 0.05 – – 12.98
2.1 0.51 0.41 0.93 3.92 0.48 1.63 9.7 2.36 1.26 0.18 0.06 0.08 0.05 0.01 – 23.68
0.78 0.11 0.33 0.35 2.43 0.18 0.75 5.72 3.13 1.66 0.3 0.11 0.12 0.01 0.05 0.06 16.09
2.04 0.30 0.61 0.51 2.55 0.23 1.26 7.71 3.23 1.98 0.21 0.1 0.13 0.03 0.01 – 20.90
2.01 0.23 0.49 1.14 2.23 0.16 0.51 3.01 0.81 0.48 0.07 0.02 0.02 – – 0.05 10.74
0.43 0.76 0.57 0.87 2.54 6 0.11 6.41 2.42 0.22 0.05 0.01 0.01 0.05 0.02 – 20.47
MMW
HMW
The concentrations of PAHs are presented as milligram of PAHs per kilogram of dry-base sludge. “–” means the concentrations of PAHs are below the detection limitation.
3.3. PAH distributions in bio-oils and the correlations with chemical characteristics of raw sewage sludge Table 3 shows the average concentration distributions of 16 target PAHs in the resulting bio-oils from the pyrolysis of the seven kinds of sludge. Likewise, the average is based on two samples for each type. The experimental errors of the concentrations of 16 target PAHs in the replicated tests are less than 10%. The most carcinogenic potency PAH compounds existed in the resulting bio-oils. It was observed that the bio-oil from the pyrolysis of the municipal sewage sludge sample S7 contained the highest concentration of PAHs, which reached 48.9 mg/kg. The municipal sewage sludge samples S4 and S5 also contributed high PAH concentrations in the obtained bio-oils, which are 21.72 mg/kg and 26.62 mg/kg, respectively. But the bio-oil from the pyrolysis of beer-brewing sewage sludge contributed the lowest content of PAHs, which was 13.72 mg/kg. These results are not consistent with the PAH concentration distributions in raw sewage sludge samples, such that the concentrations of PAHs in the bio oil samples from the pyrolysis of S1 and S6 are higher than that of S2. It was also noticeable that the most abundant PAHs were LMW PAHs, which ranged from 11.14 mg/kg in the bio-oil from the pyrolysis of S2 to 36.82 mg/kg in the bio-oil from the pyrolysis of S7. Some of the MMW and HMW PAHs were not detected. This is different from the results reported by a previous study, where the pyrolytic bio-oil from food-processing sewage
sludge contained a higher proportion of HMW PAHs than MMW and LMW PAHs (Tsai et al., 2009a, 2009b). The factors influencing the distribution of PAHs into bio-oil during the pyrolysis of sewage sludge are quite complex. The relative analyses could refer to the previous investigations regarding the emission of PAHs in the process of coal pyrolysis (Cheng, 2010). It was indicated that the chemical characteristic of coal is one of the important influences on PAH emissions during the pyrolysis of coal, especially ash and volatile matter contents. Furthermore, the PAHs contained in coal can have an important impact on the amount of PAH concentration in gas and liquid products from coal combustion and pyrolysis. The correlation between the distributions of LMW (Y1), MMW (Y2), and HMW (Y3) PAHs in bio-oils and the chemical characteristics of sewage sludge was analyzed using multiple regression analysis with SPSS 11.0 software. The discussed chemical characteristics were the concentrations of HMW (X1), MMW (X2), LMW (X3) PAHs, ash contents (X4), and volatile contents (X5) in raw sewage sludge. Table 4 shows the results of the multiple regression analysis. The significant correlations can be observed between PAHs in bio-oils and PAHs in raw sewage sludge. It could be found that the concentration of LMW PAHs in bio-oil is significantly correlated with MMW PAHs in raw sewage sludge compared with other studied factors; the correlation coefficient is 0.607. The MMW and HMW PAHs in bio-oils are significantly correlated with HMW PAHs in raw sewage sludge, with correlation coefficients of
Table 3 PAH concentrations in bio-oils from the pyrolysis of seven kinds of sewage sludge(mg/kg).
LMW
MMW
HMW
PAHs
S1
S2
S3
S4
S5
S6
S7
Naphthalene (Nap) Acenaphthene (Acp) Acenaphthylene (AcPy) Fluorene (Flu) Phenanthrene (PA) Anthracene (Ant) Fluoranthene [FL] Pyrene (Pyr) Benz[a]anthracene (BaA) Chrysene (CHR) Benzo[b]fluoranthene (BbF) Benzo[k]fluoranthene (BkF) Benzo[a]pyrene (BaP) Indeno[1,2,3-cd]pyrene(IND) Dibenz[ah]anthracene (DBA) Benzo[ghi]perylene (BghiP) Total content
3.91 5.31 0.21 0.03 1.12 0.99 1.11 0.69 0.05 0.05 0.38 0.09 0.08 0.02 0.06 – 14.10
3.95 1.75 1.88 2.09 1.05 0.42 0.75 1.03 0.66 0.02 0.02 – 0.05 – 0.05 – 13.72
4.93 2.25 0.69 2.25 2.68 3.71 0.72 2.41 – 0.21 0.14 0.15 – 0.28 0.24 0.06 21.72
8.25 5.35 5.65 1.14 1.02 1.61 2.11 0.49 – – 0.35 0.19 0.22 – 0.24 0.16 26.78
5.33 7.12 3.14 3.19 3.21 2.1 – 3.27 0.11 0.26 – – – 1.45 0.65 – 29.83
5.55 3.32 0.6 0.07 1.7 1.22 0.88 2.41 0.05 – 0.05 0.08 0.11 0.05 – 0.13 16.22
12.8 8.74 5.35 5.01 4.91 – – 3.23 – – 0.36 3.42 – 4.06 0.91 0.11 48.90
The concentrations of PAHs are presented as milligram of PAHs per kilogram of original bio-oil. “–” means the concentrations of PAHs are below the detection limitation.
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Some literatures indicated that the final temperature and residence time have important influences on PAH generation in the thermochemical conversion process of fuels, such as combustion of coal and fuel oil (Dominguez et al., 2005; Cheng, 2010). In order to better understand how the pyrolysis temperature influences the distribution of the target PAHs, municipal sewage sludge samples (S5) were sampled and then was pyrolyzed at a controlled heating rate of 30 °C/min to the final temperature of 450, 550, 650, 750, and 850 °C, and then the final temperature was maintained for 15 min. Fig. 2 shows the average concentration variation of HMW, MMW, and LMW PAHs in bio-oil samples with the function of the pyrolysis temperature. The average is based on two samples for each type. The experimental errors of the average concentrations of 16 target PAHs in the replicated tests are less than 10%. It was noted that the target PAH concentrations varied regularly as the final temperatures increased from 450 °C to 850 °C. Two peak values of HMW, MMW, and LMW PAH concentrations were obtained at 550 °C and 750 °C. The highest PAH content is present in the bio-oil sample at 750 °C, containing 7.1 mg/kg of LMW PAHs, 18.1 mg/kg of MMW PAHs, and 4.3 mg/g of HMW PAHs. From Fig. 2, the “bathtube” distribution pattern provides a clear insight into the complex pyrosynthetic mechanism, which is liable to form PAHs with MMW and LMW in the process of sewage sludge pyrolysis. This result is not in accordance with the results in which the pyrolysis of the sludge at high temperature using conventional methods yielded an oil rich in HMW as reported by Dominguez et al., 2005. In our work, it was observed that HMW PAH concentration is only 0.9 mg/kg in the oil sample at 850 °C, which is a lower concentration than that of the detected LMW and MMW PAHs. It is also noticeable that LMW PAH contents are lower than MMW
PAHs in all the studied bio-oil samples. The lower content of LMW PAHs may be due to the gaseous properties of LMW, and the generated LMW PAHs were discharged with the non-condensable gas, not entirely concentrated in the pyrolysis oil during the process of sewage sludge pyrolysis. As a high ring PAH is more thermodynamically stable, some of the HMW PAHs in this work could be from the raw sewage sludge sample which contains a low content of HMW PAHs. Furthermore, a Diels–Alder reaction mechanism has been proposed by some researchers for the formation of PAHs (Cunliffe and Williams, 1988; Williams and Besler, 1994). It is assumed that the pyrolysis of alkanes at high temperature generates alkenes and dienes via dehydrogenation, which by cyclization and subsequent aromatization produce the aromatic compound (Cypres, 1987; Fairburn et al., 1990). In the present work, the generation of the new PAHs might also occur via a Diels–Alder reaction. Some other routes of PAH formation during combustion or pyrolysis have also been suggested e.g. direct combination of aromatic ring and H2 abstraction–C2H2 addition, and secondary reaction of oxygenated compounds such as phenols, cresols and xylenols at moderate to high temperatures can produce PAHs via deoxygenating (Morf et al., 2002; Egsgaard and Larsen, 2000). The relative mechanisms are not further investigated with this study. In addition, the yields of the bio-oil were varied with the increased final temperature, which were 38%, 41%, 35%, 33%, and 31% at 450, 550, 650, 750, and 850 °C, respectively. It was found that the final temperature of 650 °C and 750 °C for sludge pyrolysis would make a higher contribution to the total amount of the PAHs in bio-oil. At the final temperature of 450 and 850 °C, the total amounts of the PAHs in bio-oil were decreased to the lowest. In this work, the residence time was counted after reaching the final pyrolysis temperature. The total amount of PAHs is increased with the increased residence time as shown in Fig. 3. The average concentrations of the total PAHs are 3.9, 4.9, 5.1, and 6.8 mg/kg at 5, 10, 15, and 20 min, respectively. The result is in accordance with a previous investigation that a long residence time would favor the formation of PAHs by increasing the number of possible secondary reactions between the volatile products (Sancheza et al., 2009). Fig. 3 also indicated the concentration variations of LMW, MMW, and HMW PAHs as the function of different residence times. The experimental errors of the average concentrations of 16 target PAHs in the replicated tests are less than 10%. It can be noted that only the concentration of MMW PAHs shows a regular increase as the residence time is increased. From the point of view of concentration distribution, at any residence time, the most important PAHs were MMW (3.7 mg/kg at 20 min). The LMW PAHs also yielded the highest concentrations of 1.9 mg/kg at 10 min, but a slight decrease of the amount of LMW PAHs was observed with increased residence time from 10 to 15 min. Likewise, the concentration of HMW PAHs is also slightly decreased from 5 to 10 min with increasing residence time. This could be attributed to the possible direct combination
Fig. 2. HMW, MMW, and LMW PAH concentrations in bio-oil samples generated from SS pyrolysis under different ending temperatures.
Fig. 3. HMW, MMW, and LMW PAH concentrations in bio-oil samples generated from SS pyrolysis under different residence times.
Table 4 R value for PAHs and industrial characteristics of sewage sludge.
X1 X2 X3 X4 X5
Y1
Y2
Y3
0.205 0.607 0.298 0.446 0.453
0.408 0.533 0.673 0.404 0.263
0.252 0.528 0.580 0.333 0.272
0.673 and 0.580, respectively. It is also noted that the correlation between the volatile content and the concentration of LMW PAHs in bio-oil is not evident as presented by the low correlation coefficient of 0.453. As expected, MMW and HMW PAHs in bio-oil are closely correlated with the MMW PAHs in raw sewage sludge, with correlation coefficients of 0.533 and 0.548.
3.4. Influence of pyrolysis conditions on PAH distributions in bio-oils
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of LMW PAHs or a secondary reaction of oxygenated compounds to produce new MMW or HMW PAHs. It was also noted that the residence time did not present an evident impact on the yields of the bio-oil. The yields of bio-oil were 31%, 31%, 33%, and 29% at 5, 10, 15, and 20 min, respectively. The highest yield could be found at the residence time of 15 min for sludge pyrolysis, which also contributed the highest total amount of PAHs in bio-oil. In order to better describe the influence of the residence time on the yield of bio-oil and its chemical composition, a large sample size would be helpful.
4. Conclusions The problem of wastewater sewage sludge disposal is currently one of the complex environmental issues. As an alternative, the application of sewage sludge pyrolysis is a promising method for converting sewage sludge into fuels. But the results in this work indicated that most of 16 US EPA PAHs were enriched in different resources of sewage sludge and the resulting pyrolysis bio-oils. The total amount of PAHs in the sampled sludge is evidently varied with the sludge resource, with values ranging between 9.19 and 23.68 mg/kg, which imply that the sludge is formed from different industrial processes and treatment methods. For instance, the sewage sludge from paper manufacturing wastewater treatment system gave the highest PAH concentration. As the results of the mixture of domestic wastewater and industrial wastewater, the three municipal sewage sludge samples also contributed high amounts of PAHs, which are 20.26%, 19.85% and 15.76%, respectively. The sewage sludge from dying wastewater gave the lowest amounts of PAHs. Moreover, the digestion treatment of wastewater sludge is effective to degrade PAHs in sewage sludge. In addition, it is noted that a decreased order for the different rings of PAH concentrations in sludge is MMW, LMW, and HMW. PAH concentrations in the resulting bio-oils ranged from 13.72 to 48.9 mg/kg, in which the lowest and highest values were contributed by the bio-oils from the pyrolysis of the municipal sewage sludge sample and beer-brewing sewage sludge, respectively. The most abundant PAHs in bio-oils were the low molar weight (LMW) PAHs. The factors influencing the formations of different rings of PAHs into bio-oil during the pyrolysis of sewage sludge are quite complex. In this study, it could be proposed that the concentration of LMW PAHs in bio-oil is correlated at best with MMW PAHs in raw sewage sludge, due to the high correlation coefficient of 0.607. MMW and HMW PAHs in bio-oils are significantly correlated with HMW PAHs in raw sewage sludge, for which the correlation coefficients are 0.672 and 0.58, respectively. The concentration of LMW PAHs in bio-oil also showed relatively significant correlation with the volatile matter content of raw sludge. This work also indicated that the pyrolysis temperature and residence time varyingly influenced the generation of LMW, MMW, and HMW PAHs into the resulting sludge pyrolysis bio-oils. It was noted that the pyrolysis of the sludge at a high temperature does not necessarily yield the resulting PAHs rich in bio-oil. This is evident as a low PAH concentration was observed at 850 °C, though the highest PAH concentration was presented at 750 °C. The residence time also significantly influenced PAH generation during sludge pyrolysis. The formation of PAHs was believed to be via a Dields–Alder reaction and also via secondary reactions of oxygenated compounds such as phenols.
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