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The effect of bioleaching on sewage sludge pyrolysis
Waste Management xxx (2015) xxx–xxx
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The effect of bioleaching on sewage sludge pyrolysis Zhihua Chen a, Mian Hu a,b,⇑, Baihui Cui c, Shiming Liu a, Dabin Guo a,b,⇑, Bo Xiao a,⇑ a
School of Environmental Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China SafeCleen Technologies Co. Ltd, Wuhan 430074, China c Wuhan Botanical Garden, Chinese Academy of Sciences, Wuhan 430074, China b
a r t i c l e
i n f o
Article history: Received 6 March 2015 Revised 22 September 2015 Accepted 1 October 2015 Available online xxxx Keywords: Sewage sludge Bioleaching Pyrolysis
a b s t r a c t The effects of bioleaching on sewage sludge pyrolysis were studied. Sewage sludge was treated by bioleaching with solid concentrations of 6% (w/v), 8% (w/v), 10% (w/v). Results showed that bioleaching treatment could modify the physicochemical properties of sewage sludge and enhance the metals removal. The optimum removal efficiencies of heavy metals were achieved with solid concentration of 6% (w/v) bioleaching treatment: Cu, 73.08%; Zn, 78.67%; Pb, 24.65%; Cd, 79.46%. The characterization results of thermogravimetric analysis (TGA) showed that the bioleached sewage sludge with a 6% (w/ v) solid concentration treatment was the easiest to decompose. Pyrolytic experiments of bioleached sewage sludge were performed in a laboratory-scale fixed bed reactor. Results indicated that bioleaching treatment greatly influenced the product yields and gas composition. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction With the widespread use of biological method in wastewater treatment field, a large amount of sewage sludge (SS) is produced as an unescapable by-product during the process of municipal sewage (MSW) treatment (Kang et al., 2012; Zhu et al., 2013). In China, it was reported that about 6,000,000 tons of dry sewage sludge was generated by more than 3080 MSW treatment plants annually (Liu et al., 2012). Moreover, numerous of sewage sludge is expected to be generated continuously with the rapid urbanization and stringent effluent standard implement for MSW treatment (Gu et al., 2013). The treatments and disposals of SS are becoming more and more severe to the environmental protection. Nowadays, the landfill, incineration and composting are the main methods for disposal of SS (Folgueras et al., 2013; Gasco and Lobo, 2007). Landfill is considered as an universal solution for sewage sludge disposal because of its low cost (Celary and Sobik-Szoltysek, 2014). However, landfill is limited to implement due to the shrinkage of land and leaching of toxic substances to environment (Lin et al., 2014). Composting is a good way to convert the decomposable organic materials into organic fertilizer for farmland application. But the odor, bioaerosol emissions and the heavy metals leaching cannot be avoided during composting.
⇑ Corresponding authors at: School of Environmental Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China. E-mail addresses: [email protected] (M. Hu), [email protected] (D. Guo), [email protected] (B. Xiao).
Incineration, which is effective for volume reduction, thorough stabilization, sanitation and energy generation, is more and more popular (Lin and Ma, 2012; Samolada and Zabaniotou, 2014). Nevertheless, the incineration also is disadvantageous to the environment with harmful fly ash produced during burning, which attributes to the high ash content of SS (Lin and Ma, 2012). Bioleaching, a technologically and economically feasible process, is considered as the most hopeful way to reduce heavy metals in sewage sludge (Anand et al., 2006). During bioleaching process, elemental or reduced sulfur compounds are oxidized to sulfuric acid by a variety of acidophilic and chemoautolithotrophic bacteria. The heavy metals can be dissolved from the sewage sludge as the increase of sulfuric acid concentration (i.e. the decrease of pH value) (Liu et al., 2008). After the bioleaching, there are still generous amount of bioleached SS ought to be further disposed. Such bioleached SS is in low pH value and could not be treated by landfill or composting. So far, pyrolysis is believed to be a considerable chemical procedure to convert organic waste into valuable oils and high calorific value combustible syngas (Bridgwater et al., 1999). Previous studies have shown that the pyrolysis behaviors of SS are greatly affected by the components (Gasco et al., 2005; Mendez et al., 2005). It was considered that the source of feedstock (Font et al., 2001), inorganic constituents (Yaman, 2004), and pretreatment methods could influence the decomposition temperature and product distributions of pyrolysis process. In order to improve the efficiency of waste resources utilization, many researchers have done extensive investigations on the pretreatment of feedstock (Fullana
http://dx.doi.org/10.1016/j.wasman.2015.10.002 0956-053X/Ó 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Chen, Z., et al. The effect of bioleaching on sewage sludge pyrolysis. Waste Management (2015), http://dx.doi.org/ 10.1016/j.wasman.2015.10.002
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et al., 2003). The structures and chemical compositions of feedstock can be altered by pretreatment. Such inwardness variations in the feedstock will lead to the change of reaction mechanism during pyrolysis process. Bioleaching is an environmentally-friendly, efficient and economical pretreatment method for the removal of heavy metals from sewage sludge. The generation of acid products and the metabolism of bacteria can remove or modify the compounds of sewage sludge during bioleaching process. It seems that the bioleaching can modify the pyrolysis behavior of sewage sludge. So far, to authors’ knowledge, the effects of bioleaching treatment on sewage sludge compositions, pyrolytic behaviors and pyrolytic products received little attention. The present study aimed to evaluate the variations in sewage sludge’s compositions, pyrolytic behaviors and pyrolytic products after bioleaching treatment. 2. Materials and methods 2.1. Materials The sewage sludge (SS) used in this study was obtained from an urban MSW treatment plant located in Wuhan, Hubei province, China. The dehydrated SS was collected and stored in refrigerator as the original material for bioleaching experiments. Initial pH value of sewage sludge and moisture content of sewage sludge was 8.6% and 80.4%, respectively. Heavy metal (Cu, Zn, Pb, and Cd) contents of sewage sludge were measured by using an Inductively Coupled Plasma Mass Spectrometry (ICP-MS) instrument (ELAN DRC-e, PerkinElmer) after dissolved by HNO3:HClO4:HF (3:1:1) solution. The heavy metal contents are 196.43 mg/kg (Cu), 269.72 mg/kg (Zn), 63.21 mg/kg (Pb) and 4.82 mg/kg (Cd). Acidithiobacillus ferrooxidans bacteria which were isolated from sewage sludge were used to perform the bioleaching experiments. The culture medium of 9 K broth (Xiao et al., 2013) was used for the cultivation of At. ferrooxidans bacteria. Such 9 K broth was prepared with 3 g/L (NH4)2SO4, 0.1 g/L KCl, 0.5 g/L K2HPO4, 0.5 g/L MgSO47H2O, 0.01 g/L Ca(NO3)2, 44.2 g/L FeSO47H2O (Xiao et al., 2013). The pH value was adjusted to 2.00 by H2SO4.
2.3. Sewage sludge characterization Sewage sludge samples were characterized according to the following parameters: total humic substances (THS) content, humic acid (HA) contents, fulvic acid (FA) contents, element contents and H/C, O/C, N/C ratios. THS were extracted firstly with 0.1 M NaOH and thereafter with 0.1 M Na4P2O7 (pH = 10). Suspensions were centrifuged for 0.5 h (3000 rpm). The obtained extracts were acidified with H2SO4 (pH = 1) after the supernate and coagulation were dialyzed. The coagulated HA was re-dissolved with 0.1 M NaOH for 3 times. The supernate was referred to fulvic acid (FA). The C contents of THS, HA and FA were determined by the Walkley–Black method (Nelson and Sommers, 1982). Ultimate analysis of the sewage sludge samples was measured by a CHNS/O analyzer (Vario Micro cube, Elementar). Such an analysis gave the weight percent of carbon, hydrogen, and nitrogen in the samples simultaneously. The weight percent of oxygen was determined by difference methods. The chemical functional groups in sewage sludge samples were investigated by using FT-IR technique. Sewage sludge samples were ground to fine particles and mixed with KBr powder. The mass ratio of samples to KBr powder was 1:100. The spectral resolution was set at 4 cm1. 2.4. Thermogravimetric analysis Thermogravimetric analysis (TGA) is considered as a powerful technique for pyrolysis behaviors characterization. By using TGA, the mass loss and mass loss rate are recorded as a function of temperature and time. The thermogravimetric analysis was carried out using Pyris1 TGA instrument (Perkin Elmer Co., Ltd). TG (%) denotes the normalized mass loss of the sample during the pyrolysis process. DTG (%/min), which denotes the mass loss rate of the sample, is obtained from the first derivative of TG (%) versus time (min). 3.5 ± 1.0 mg of sample was used in each experiment. Nitrogen with a flow rate of 100 mL/min was used as carrier gas. All the samples were heating from room temperature to 800 °C with a heating rate of 15 °C/min.
2.2. Bioleaching 2.5. Pyrolysis of sewage sludge samples Bioleaching experiments were carried out in 2.5 L flask agitated by stirring at 200 rpm. 1 L of aqueous solution with different sewage sludge solid concentrations (6%, 8% and 10% (w/v)) was added into the flasks. The solid concentration of the raw sewage sludge is approximately 26% w/v. 10 mL medium contained microorganism in log phase was incubated into the flask and maintained at 30 °C. 4 g FeSO47H2O was then fed into the flask. 150 mL distilled water was replenished for evaporation per 48 h. When the final pH value stabilized at 2 ± 0.1, the bioleaching experiment was held for 4 h. Heavy metals leaching efficiency was then investigated according to Eq. (1):
g ¼ 100
c0 c1 c0
ð1Þ
where g denotes the leaching efficiency, %; c0 (mg/kg) and c1 (mg/ kg) denote the heavy metal content of raw SS and bioleached SS, respectively. After the bioleaching, the flask was left standing for 2 h to separate the sediment from the liquid phase. The sediment was washed with excessive distilled water and then was dried at 105 °C for 24 h. The dried bioleached SS was then crushed and sieved through 2 mm size mesh. The bioleached sewage sludge sample with 6% (w/v), 8% (w/v) and 10% (w/v) was named as SS6, SS-8 and SS-10, respectively. And the raw sewage sludge sample was named as raw-SS.
The pyrolysis experiments were performed according to a laboratory-scale fixed bed pyrolysis system under atmospheric pressure. The schematic configuration of pyrolysis experiment is illustrated in Fig. 1. Before the experiments, nitrogen (with a constant flow rate of 50 mL/min) was injected into the reactor for 20 min to maintain an inert atmosphere. Subsequently, the reactor was heated in a heating rate of 35 °C/min to achieve the set-point temperature (800 °C). Then, 5.0 g sewage sludge sample was put into the stainless steel boat and placed into the middle of quartz tubular reactor. The set-point temperature was considered as the pyrolytic temperature which would be held for 20 min. During the pyrolysis process, the volatile flowed out of the reactor and was condensed. By means of such condensation process, the condensable volatiles (i.e. bio-oil) were captured in the collector. And the non-condensable (combustible) gas whose volume was measured by a gas flow meter was gathered in a Tedlar sample bag. The main compositions of non-condensable gas (i.e. H2, CO, CO2, CH4) were analyzed with a gas chromatography (9800T) by using thermal conductivity detector (TCD) and TDX-01 columns. At the end of the pyrolysis, the furnace was cooled down to room temperature and the mass of the solid char as well as bio-oil yields could be calculated. Each experiment was repeated two times in order to ensure the mass balance and data reliability. The data reported in this paper were the mean value of twice the data.
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Fig. 1. Schematic diagram of sewage sludges pyrolysis apparatus. 1. Temperature control; 2. nitrogen gas cylinder; 3. rotameter; 4. thermocouple; 5. stainless steel boat; 6. horizontal electric furnace; 7. condenser; 8. tar collection bottle; 9. cotton filter; 10. drying tube; 11. gas meter; 12. gas outlet.
3. Results and discussion 3.1. Effects of bioleaching on sewage sludge composition During the bioleaching process, the Fe2+ is oxidized into Fe3+ by At. ferrooxidans bacteria. The Fe3+ is leached in the liquid phase and the sulfuric acid is generated via following chemical reactions: At: ferrooxidans
2FeSO4 þ 0:5O2 þ H2 SO4 ! Fe2 ðSO4 Þ3 þ H2 O
ðR-1Þ
4Fe2 ðSO4 Þ3 þ 2MeS þ 4H2 O þ 2O2 ! 2M2þ þ 2SO2 4 þ 8FeSO4 þ 4H2 SO4
ðR-2Þ 2+
where MeS denotes the metal sulfide, and M denotes the soluble metallic ion. The production of sulfuric acid leads to the pH value decrease, which is beneficial to the solubilization of sludge-borne metals. Bioleaching process has been proven to be capable of sewage sludge digesting, dewatering ability improvement and organic compounds modifications. Table 1 gives the ultimate analysis results of sewage sludge samples. Variations in the C, H, O, N contents and H/C, O/C, N/C ratio demonstrate that the organic matter composition of sewage sludge has been changed by bioleaching treatment. Compared with the raw SS (i.e. raw-SS), the H/C and N/C ratios both decrease but the O/C ratio of sewage sludge increases after bioleaching. The H/C ratio decreases with the solid concentrations of bioleaching treatment, from 2.87 (raw-SS) to 2.51 (SS-10). Meanwhile, the H/C ratio decreases from 2.77 to 2.51 as the solid concentration increases from 6% to 10% (w/v). The phenomenon of H/C, N/C and O/C ratios variations suggests that the bioleached sewage sludge samples show more aromatic groups than raw-SS samples. It could be caused by the solubilization of light organic compounds during the bioleaching process (Gasco and Lobo, 2007). One feature is that the SS-6 exhibited higher H/C ratio than the other SS samples. This indicates that the bioleaching with 6% (w/v) solid concentrations shows the optimal performance for the presence of elevated aliphatic carbon and long chains (with CH2 groups) (Cao et al., 2011; Gasco et al., 2005). The higher aliphatic carbon content is beneficial to the generation of alkane gaseous or light aromatic hydrocarbon. The N/C ratio can be used to express the
polymerization degree of organic matter in sewage sludge. Higher polymerized organic material contains less nitrogenous functional groups (Gasco et al., 2005). Therefore, the higher polymerization degree of bioleached sewage sludge at 6% (w/v) sludge solid concentration suggests higher dewatering capacity. As can be seen in Table 1, the O/C ratios of SS-6, SS-8 and SS-10 are obviously higher than those of raw-SS. The O/C ratio is not an appropriate parameter for organic matter, indicating some oxygen can be transferred to oxygenous function groups or oxidized during the reaction. Table 2 lists the total humic substances (THS) content, humic acid (HA) contents, fulvic acid (FA) contents and HA/FA ratios of different sewage sludge samples. It can be observed that these parameters of sewage sludge varied significantly after bioleaching. HA and FA are the main fractions of humic substances. FA, which is in lower molecular weight and lower carbon content than HA, contains higher oxygen in the form of functional groups (Rodriguez et al., 2014). In addition, HA is soluble in alkali but insoluble in acid, while the FA is soluble in both alkali and acid. After bioleaching, the HA and FA contents of sewage sludge have increased. But the HA content decreases with the increase of bioleached solid concentrations from 0.79% (SS-6) to 0.49% (SS-10). It possibly attributes to the precipitation of HA under acid condition during the bioleaching process. Furthermore, the acidification rate of sewage sludge decreases as the solid concentration increases (high pH), which enhances the HA solubilization. The FA content in bioleached sewage sludge is a little higher than that of raw-SS. Meanwhile, the FA content decreases with the solid concentration increasing from 2.54% (SS-6) to 2.46% (SS-10), because the hydrolysis of polymerized organic matter generates more oxidized and light molecular compounds which are similar to the FA fraction under higher acidification rate (Gasco and Lobo, 2007). The HA/ FA ratio denotes the humic transformation degree. The HA/FA ratio inversely relates to the N/C ratio which can be used to compare the polymerization degree (Iakimenko et al., 1996). Table 3 gives heavy metal contents of the raw-SS and bioleached sewage sludge. It is obvious that the bioleaching treatment can reduce the heavy metal contents in sewage sludge. For these four heavy metals, the highest removal efficiencies (g) are all obtained at the solid concentration of 6% (w/v) and follow the order of Cd > Zn > Cu > Pb. Similar orders also can be found for the 8% (w/v) and 10% (w/v) solid concentration bioleaching
Table 1 Ultimate analysis of sewage sludge samples.
a b
Sewage sludge
C (wt%)
H (wt%)
Oa (wt%)
N (wt%)
H/Cb
O/Cb
N/Cb
Raw-SS SS-6 SS-8 SS-10
12.37 ± 0.17 13.00 ± 0.12 13.68 ± 0.23 14.26 ± 0.34
2.96 ± 0.09 3.01 ± 0.11 3.00 ± 0.15 2.99 ± 0.08
16.89 27.67 26.26 24.79
2.79 ± 0.14 1.91 ± 0.11 2.23 ± 0.07 2.47 ± 0.18
2.87 2.77 2.63 2.51
1.02 1.59 1.43 1.30
0.19 0.12 0.13 0.14
By difference. Molar ratio.
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Table 2 Properties of sewage sludge samples. Sewage sludge
CTHS (wt%)
CHA (wt%)
CFA (wt%)
HA/FA
Raw-SS SS-6 SS-8 SS-10
2.55 ± 0.09 3.24 ± 0.02 3.06 ± 0.10 2.95 ± 0.09
0.25 ± 0.06 0.70 ± 0.04 0.59 ± 0.02 0.49 ± 0.01
2.30 ± 0.02 2.54 ± 0.08 2.47 ± 0.10 2.46 ± 0.03
0.11 0.28 0.24 0.20
treatments. However, the removal efficiencies of these four heavy metals are strongly influenced by solid concentration. As can be seen in Table 3, with solid concentration increase, the contents of Cu, Zn, Pb and Cd in sewage sludge increase and the corresponding removal efficiencies decrease. The phenomenon may attribute to the final pH value of the bioleaching system. The higher solid concentration indicates more sewage sludge which has higher buffering capacity in the bioleaching system. And the pH value of bioleaching also can greatly affect the solubilization efficiencies of heavy metals during the bioleaching process (Chen and Lin, 2000; Sharifi and Renella, 2015). Another one feature is that the removal efficiencies of Cu, Zn and Cd are significantly higher than those of Pb at the same solid concentration. During the bioleaching process, the lower removal efficiency of Pb is possibly due to generation of PbSO4 (Ksp = 1.62 108) which has low solubility (Chen and Lin, 2000). The bioleaching is also found to be efficient in heavy metals removal at higher solid concentration than 6% (w/v). It is considered that the increase of solid concentration generally leads to a prolonged lag period (i.e. the required time for the microorganisms adapts to bioleaching medium), which decreases the bio-oxidation rate and the ultimate extent of oxidation (Ahmadi et al., 2015). In addition, the increase of sludge solid concentration can result a decrease in metal solubilization. Moreover, once more sewage sludge is treated, larger volume of reactor and treated time are needed and the cost of process will also increase certainly. FTIR spectra technique was used to qualitatively determine the chemical structures change in bioleached sewage sludge compared with the raw-SS. The FTIR spectra results (Fig. 2) demonstrate that the bonds of bioleached sewage sludge (SS-6, SS-8 and SS-10) are similar to the raw one (raw-SS). However, as for the SS-6 sample, the OAH (stretching vibrations at 3432 cm1), C@O (stretching vibrations at 1642 cm1), CAH (stretching vibrations at 1393 cm1) and CAO (stretching vibrations at 1038 cm1) have been greatly enhanced compared with the other samples. It implies that the quantities of these groups increase by breaking the macromolecule and release more free groups via bioleaching. As suggested by Zhang et al. (2011), the OAH indicates the existence of water, alcohol, phenol, or amine substance; C@O is compatible with the presence of acids and aldehydes; CAH is produced by ACH3 and ACH2 group which implies the sewage sludge contains a lot of fat hydrocarbon organic matter. CAO corresponds to the oxidizing substance of ether, ester, alcohol, and spenol. The FTIR spectra results (Fig. 2) clearly show that the oxygen functional
Fig. 2. FTIR spectra profiles of sewage sludge samples.
groups of sewage sludge have increased by bioleaching. Such increment of oxygen functional groups is possibly related to a higher content of FA, especially at 6% (w/v) solid concentration (Table 2). FTIR spectra results demonstrate that the bioleaching treatment leads to important changes in the characteristics organic matter of raw-SS. Compared to the other samples, SS-6 is proven to generate more oxygen functional groups which are conducive to the combustible gases (CO, H2, CO2) formation during pyrolysis. 3.2. Thermogravimetric characterization The thermogravimetric analysis (TGA) results (TG/DTG vs. temperature profiles) are shown in Figs. 3 and 4. Fig. 3 presents the weight loss of the sewage sludge samples. It could be observed that the weight loss of bioleached sewage sludge samples starts at lower temperature and finishes at higher temperature, especially for SS-6. The TG profile of SS-6 also shows significant different pyrolysis behaviors compared with the other bioleached SS (SS-8 and SS-10) and raw-SS. The alterations of pyrolysis behaviors attribute to the differences of the ingredient in their sewage sludge samples. As illustrated in the DTG profiles (Fig. 4), the pyrolysis process of sewage sludge samples could be divided into four stages. In the temperature ranges of room temperature to 120 °C, the DTG peaks are caused by the evaporation of physically absorbed water. For all the samples, the devolatilization stage which is accompanied by high weight loss rate (DTG) and attributed to the volatiles volatilization occurs between 120 °C and 600 °C. The weight loss at the temperature of 600–800 °C is caused by the decomposition of inorganic materials (mainly calcium carbonate) (Scott et al., 2006). However, the thermal decomposition behaviors of the devolatilization stage (120–600 °C) are quite complex and manifest as multiple peaks overlap. The peaks at
Table 3 Heavy metal contents (mg/kg) in sewage sludge (SS) and corresponding bioleaching efficiency (g, Eq. (1)).
a b
Cu
Zn
Pb
Cd
Raw SS Raw-SSa Bioleached SS
c0 196.43 ± 3.20 c1
g (%)
c0 269.72 ± 1.78 c1
g (%)
c0 63.21 ± 3.27 c1
g (%)
c0 4.82 ± 0.84 c1
g (%)
SS-6b SS-8b SS-10b
52.87 ± 2.48 87.42 ± 2.91 105.21 ± 2.83
73.08 55.50 46.44
57.53 ± 1.49 98.85 ± 0.88 129.11 ± 2.72
78.67 63.35 52.13
47.63 ± 3.01 51.12 ± 2.64 53.52 ± 2.82
24.65 19.13 15.33
0.99 ± 0.05 1.66 ± 0.04 2.01 ± 0.11
79.46 65.56 58.30
Heavy metal contents of raw SS, c0 in Eq. (1). Heavy metal contents of bioleached SS, c1 in Eq. (1).
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Fig. 3. TG (weight loss vs. temperature) curves of sewage sludge samples.
120–395 °C denote the preliminary decomposition stage of the organics in sewage sludge samples. In temperature range about 395–600 °C, obvious peaks also can be observed, which denotes the secondary decomposition stage of the organics. The weight loss rate of SS-6 in the secondary decomposition stage is higher than the rest samples at any same temperature point. It indicates that the organics of SS-6 is easier to decompose than the other samples. In addition, the DTG curves of bioleached sewage sludge (SS-6, SS-8 and SS-10) are more intensive to the temperature than the raw-SS.
alkanes and long fragment alkenes). Besides, the formed free radicals can be mutually combined freely with the hydrogen atom. Other bonds also start to break and generate more free radicals, which induces the polymerization and cyclization reactions. Therefore, the main primary pyrolysis products are char, small molecule gases (CO, CO2) and macromolecular condensable volatiles. As the temperature increases, the secondary reactions of tar cracking and shifting, such as decarboxylation, decarbonylation, dehydrogenation, cyclization, aromatization and polymerization, contribute to the thorough reaction of the pyrolysis (Li et al., 2007). Afterward, the light vapors underwent series reactions and are cracked to form H2, CO, CO2, alkanes, alkenes, and aromatic hydrocarbons. In this study, the pyrolysis experiments of sewage sludge were carried out in the reactor at temperature of 800 °C. The results of product distribution and gas compositions are depicted in Figs. 5 and 6. As shown in Fig. 5, the gas product yield of bioleached sewage sludge is larger and the char yield is less than that of the raw sewage sludge. Meanwhile, the tar yield also changes slightly. The variations in pyrolysis products (char, tar and gas) mainly attribute to the changes in the characteristics of sewage sludge after bioleaching, such as the compositions change (Tables 1 and 2) and the removal of heavy metals (Table 3). However, the gas yield gradually decreases with the increase of solid concentration. In addition, no remarkable change is observed for the tar yield.
60 SS-6 SS-8 SS-10 Raw SS
3.3. Pyrolysis products It is believed that the organic matter in sludge mainly consisted of fat, protein, carbohydrate, and cellulose (Kapanen et al., 2013). The main functional groups of these organic compounds include carboxylic acid, carbonyl, amide, amine, methyl and aromatic. With the primary thermochemical decomposition of sewage sludge, the instability C@O of macromolecular organic matter begins to break and generate CO and CO2. Then, the CAC and CAH bonds are ruptured to form the free radicals which are further recombined into low molecular compounds. However, the CAC bond could be easily broken since the bond energy of CAC (346.9 kJ) is lower than that of CAH bond (413.84 kJ) (Sorum et al., 2001). In general, the cleavage of alkanes which contains more carbon atoms will form new kinds of alkanes (short fragment
Percentage (wt.%)
50
40
30
20
10
0 Gas
Char
Tar
Product Fig. 5. Products percentages obtained by sewage sludge pyrolysis.
40
SS-6 SS-8 SS-10 raw-SS
Percentage (wt.%)
32
24
16
8
0
H2
CO
CO2
CH4
Gas composition Fig. 4. DTG curves (weight loss rate vs. temperature) of sewage sludge samples.
Fig. 6. Compositions of gas derived from sewage sludge samples pyrolysis.
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Reasons may lie in that: (1) After bioleaching, the contents of fulvic acid vary from 2.54% (SS-6) to 2.46% (SS-10) (Table 2). The fulvic acid is rich in oxygen content and its function groups could be easily ruptured to generate volatiles. Under different solid concentrations, the bioleached sewage sludge shows a gradually decreased H/C ratio (from 2.77 (SS-6) to 2.51 (SS-10)), which accordingly reduces the aliphatic carbon fractions. The higher aliphatic carbon fraction is advantageous to the generation of alkane gaseous and light aromatic hydrocarbon. (2) Meanwhile, as can be seen from the TG curves (Fig. 3), SS-6 sample exhibits a larger weight loss than the other samples in the whole pyrolysis temperature range. It demonstrates that the pyrolysis of SS-6 leads to a larger volatiles volume than the rest samples (SS-8, SS-10 and raw-SS), which may contribute to more gas production. The change of char yield relates to heavy metals dissolution efficiency. With the solid concentration increase, the buffering capacity of sludge also increases. And the dissolution of heavy metals is reduced by the pH reduction. Nonetheless, the contribution of bioleaching pretreatment on tar yield is inconspicuous. Bioleaching treatment has significant influences on the components of pyrolytic gas products. As can be seen in Fig. 6, H2 and CO fractions in pyrolytic gas products have been improved by bioleaching treatment, especially for the treatment with 6% (w/v) solid concentration. The generation of CO may be resulted from the reactions of carbonyl group cracking, oxygen function group rupture and dehydrogenation of hydroxyl group. Such reactions have been enhanced by bioleaching treatment. The decreasing trend of CO2 content may be due to the fact that the fractions of CO and CO2 are linked together according to the equilibrium of the Boudouard reaction under the experiment conditions. CH4 content presented is slightly changed, which possibly attributes to the variance of tar yield. 4. Conclusions During bioleaching process, heavy metals removal efficiency, organic compositions, pyrolytic behaviors, pyrolysis product yield and gas components were influenced by bioleaching treatments. Sewage sludge was treated by bioleaching with 6% (w/v), 8% (w/ v), and 10% (w/v) solid concentration. The bioleaching treatment could improve the removal of heavy metals, especially for the 6% (w/v) solid concentration treatment. The optimal removal efficiencies of Cu, Zn, Pb, Cd are 73.08%, 78.67%, 24.65% and 79.46%, respectively. The H/C, N/C ratios of sewage sludge decreased while the O/C ratios increased after bioleaching treatment. The humic acid, fulvic acid and total humic contents of bioleached sewage sludge increased compared with the raw sewage sludge. The thermogravimetric analysis characterizations indicated that the organics of bioleached sewage sludge was more intensive to the temperature than that of the raw sewage sludge. The bioleaching of sewage sludge with 6% (w/v) solid concentration showed the best performance on heavy metals removal, organic compositions transformation, pyrolytic behaviors, pyrolysis product yields and gas components. Acknowledgments The authors wish to acknowledge the financial supports of the National Nature High Technology Research and Development Program (863 Program) of China (No. 2012AA101809), National Natural Science Foundation of China (No. 21276100) and the Natural Science Foundation of Hubei Province, China (2014CFB411). The authors would also like to thank the Analytical and Testing Center of Huazhong University of Science and Technology for carrying out the analysis of the characterization of sewage sludge samples.
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Waste Management journal homepage: www.elsevier.com/locate/wasman
The effect of bioleaching on sewage sludge pyrolysis Zhihua Chen a, Mian Hu a,b,⇑, Baihui Cui c, Shiming Liu a, Dabin Guo a,b,⇑, Bo Xiao a,⇑ a
School of Environmental Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China SafeCleen Technologies Co. Ltd, Wuhan 430074, China c Wuhan Botanical Garden, Chinese Academy of Sciences, Wuhan 430074, China b
a r t i c l e
i n f o
Article history: Received 6 March 2015 Revised 22 September 2015 Accepted 1 October 2015 Available online xxxx Keywords: Sewage sludge Bioleaching Pyrolysis
a b s t r a c t The effects of bioleaching on sewage sludge pyrolysis were studied. Sewage sludge was treated by bioleaching with solid concentrations of 6% (w/v), 8% (w/v), 10% (w/v). Results showed that bioleaching treatment could modify the physicochemical properties of sewage sludge and enhance the metals removal. The optimum removal efficiencies of heavy metals were achieved with solid concentration of 6% (w/v) bioleaching treatment: Cu, 73.08%; Zn, 78.67%; Pb, 24.65%; Cd, 79.46%. The characterization results of thermogravimetric analysis (TGA) showed that the bioleached sewage sludge with a 6% (w/ v) solid concentration treatment was the easiest to decompose. Pyrolytic experiments of bioleached sewage sludge were performed in a laboratory-scale fixed bed reactor. Results indicated that bioleaching treatment greatly influenced the product yields and gas composition. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction With the widespread use of biological method in wastewater treatment field, a large amount of sewage sludge (SS) is produced as an unescapable by-product during the process of municipal sewage (MSW) treatment (Kang et al., 2012; Zhu et al., 2013). In China, it was reported that about 6,000,000 tons of dry sewage sludge was generated by more than 3080 MSW treatment plants annually (Liu et al., 2012). Moreover, numerous of sewage sludge is expected to be generated continuously with the rapid urbanization and stringent effluent standard implement for MSW treatment (Gu et al., 2013). The treatments and disposals of SS are becoming more and more severe to the environmental protection. Nowadays, the landfill, incineration and composting are the main methods for disposal of SS (Folgueras et al., 2013; Gasco and Lobo, 2007). Landfill is considered as an universal solution for sewage sludge disposal because of its low cost (Celary and Sobik-Szoltysek, 2014). However, landfill is limited to implement due to the shrinkage of land and leaching of toxic substances to environment (Lin et al., 2014). Composting is a good way to convert the decomposable organic materials into organic fertilizer for farmland application. But the odor, bioaerosol emissions and the heavy metals leaching cannot be avoided during composting.
⇑ Corresponding authors at: School of Environmental Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China. E-mail addresses: [email protected] (M. Hu), [email protected] (D. Guo), [email protected] (B. Xiao).
Incineration, which is effective for volume reduction, thorough stabilization, sanitation and energy generation, is more and more popular (Lin and Ma, 2012; Samolada and Zabaniotou, 2014). Nevertheless, the incineration also is disadvantageous to the environment with harmful fly ash produced during burning, which attributes to the high ash content of SS (Lin and Ma, 2012). Bioleaching, a technologically and economically feasible process, is considered as the most hopeful way to reduce heavy metals in sewage sludge (Anand et al., 2006). During bioleaching process, elemental or reduced sulfur compounds are oxidized to sulfuric acid by a variety of acidophilic and chemoautolithotrophic bacteria. The heavy metals can be dissolved from the sewage sludge as the increase of sulfuric acid concentration (i.e. the decrease of pH value) (Liu et al., 2008). After the bioleaching, there are still generous amount of bioleached SS ought to be further disposed. Such bioleached SS is in low pH value and could not be treated by landfill or composting. So far, pyrolysis is believed to be a considerable chemical procedure to convert organic waste into valuable oils and high calorific value combustible syngas (Bridgwater et al., 1999). Previous studies have shown that the pyrolysis behaviors of SS are greatly affected by the components (Gasco et al., 2005; Mendez et al., 2005). It was considered that the source of feedstock (Font et al., 2001), inorganic constituents (Yaman, 2004), and pretreatment methods could influence the decomposition temperature and product distributions of pyrolysis process. In order to improve the efficiency of waste resources utilization, many researchers have done extensive investigations on the pretreatment of feedstock (Fullana
http://dx.doi.org/10.1016/j.wasman.2015.10.002 0956-053X/Ó 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Chen, Z., et al. The effect of bioleaching on sewage sludge pyrolysis. Waste Management (2015), http://dx.doi.org/ 10.1016/j.wasman.2015.10.002
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et al., 2003). The structures and chemical compositions of feedstock can be altered by pretreatment. Such inwardness variations in the feedstock will lead to the change of reaction mechanism during pyrolysis process. Bioleaching is an environmentally-friendly, efficient and economical pretreatment method for the removal of heavy metals from sewage sludge. The generation of acid products and the metabolism of bacteria can remove or modify the compounds of sewage sludge during bioleaching process. It seems that the bioleaching can modify the pyrolysis behavior of sewage sludge. So far, to authors’ knowledge, the effects of bioleaching treatment on sewage sludge compositions, pyrolytic behaviors and pyrolytic products received little attention. The present study aimed to evaluate the variations in sewage sludge’s compositions, pyrolytic behaviors and pyrolytic products after bioleaching treatment. 2. Materials and methods 2.1. Materials The sewage sludge (SS) used in this study was obtained from an urban MSW treatment plant located in Wuhan, Hubei province, China. The dehydrated SS was collected and stored in refrigerator as the original material for bioleaching experiments. Initial pH value of sewage sludge and moisture content of sewage sludge was 8.6% and 80.4%, respectively. Heavy metal (Cu, Zn, Pb, and Cd) contents of sewage sludge were measured by using an Inductively Coupled Plasma Mass Spectrometry (ICP-MS) instrument (ELAN DRC-e, PerkinElmer) after dissolved by HNO3:HClO4:HF (3:1:1) solution. The heavy metal contents are 196.43 mg/kg (Cu), 269.72 mg/kg (Zn), 63.21 mg/kg (Pb) and 4.82 mg/kg (Cd). Acidithiobacillus ferrooxidans bacteria which were isolated from sewage sludge were used to perform the bioleaching experiments. The culture medium of 9 K broth (Xiao et al., 2013) was used for the cultivation of At. ferrooxidans bacteria. Such 9 K broth was prepared with 3 g/L (NH4)2SO4, 0.1 g/L KCl, 0.5 g/L K2HPO4, 0.5 g/L MgSO47H2O, 0.01 g/L Ca(NO3)2, 44.2 g/L FeSO47H2O (Xiao et al., 2013). The pH value was adjusted to 2.00 by H2SO4.
2.3. Sewage sludge characterization Sewage sludge samples were characterized according to the following parameters: total humic substances (THS) content, humic acid (HA) contents, fulvic acid (FA) contents, element contents and H/C, O/C, N/C ratios. THS were extracted firstly with 0.1 M NaOH and thereafter with 0.1 M Na4P2O7 (pH = 10). Suspensions were centrifuged for 0.5 h (3000 rpm). The obtained extracts were acidified with H2SO4 (pH = 1) after the supernate and coagulation were dialyzed. The coagulated HA was re-dissolved with 0.1 M NaOH for 3 times. The supernate was referred to fulvic acid (FA). The C contents of THS, HA and FA were determined by the Walkley–Black method (Nelson and Sommers, 1982). Ultimate analysis of the sewage sludge samples was measured by a CHNS/O analyzer (Vario Micro cube, Elementar). Such an analysis gave the weight percent of carbon, hydrogen, and nitrogen in the samples simultaneously. The weight percent of oxygen was determined by difference methods. The chemical functional groups in sewage sludge samples were investigated by using FT-IR technique. Sewage sludge samples were ground to fine particles and mixed with KBr powder. The mass ratio of samples to KBr powder was 1:100. The spectral resolution was set at 4 cm1. 2.4. Thermogravimetric analysis Thermogravimetric analysis (TGA) is considered as a powerful technique for pyrolysis behaviors characterization. By using TGA, the mass loss and mass loss rate are recorded as a function of temperature and time. The thermogravimetric analysis was carried out using Pyris1 TGA instrument (Perkin Elmer Co., Ltd). TG (%) denotes the normalized mass loss of the sample during the pyrolysis process. DTG (%/min), which denotes the mass loss rate of the sample, is obtained from the first derivative of TG (%) versus time (min). 3.5 ± 1.0 mg of sample was used in each experiment. Nitrogen with a flow rate of 100 mL/min was used as carrier gas. All the samples were heating from room temperature to 800 °C with a heating rate of 15 °C/min.
2.2. Bioleaching 2.5. Pyrolysis of sewage sludge samples Bioleaching experiments were carried out in 2.5 L flask agitated by stirring at 200 rpm. 1 L of aqueous solution with different sewage sludge solid concentrations (6%, 8% and 10% (w/v)) was added into the flasks. The solid concentration of the raw sewage sludge is approximately 26% w/v. 10 mL medium contained microorganism in log phase was incubated into the flask and maintained at 30 °C. 4 g FeSO47H2O was then fed into the flask. 150 mL distilled water was replenished for evaporation per 48 h. When the final pH value stabilized at 2 ± 0.1, the bioleaching experiment was held for 4 h. Heavy metals leaching efficiency was then investigated according to Eq. (1):
g ¼ 100
c0 c1 c0
ð1Þ
where g denotes the leaching efficiency, %; c0 (mg/kg) and c1 (mg/ kg) denote the heavy metal content of raw SS and bioleached SS, respectively. After the bioleaching, the flask was left standing for 2 h to separate the sediment from the liquid phase. The sediment was washed with excessive distilled water and then was dried at 105 °C for 24 h. The dried bioleached SS was then crushed and sieved through 2 mm size mesh. The bioleached sewage sludge sample with 6% (w/v), 8% (w/v) and 10% (w/v) was named as SS6, SS-8 and SS-10, respectively. And the raw sewage sludge sample was named as raw-SS.
The pyrolysis experiments were performed according to a laboratory-scale fixed bed pyrolysis system under atmospheric pressure. The schematic configuration of pyrolysis experiment is illustrated in Fig. 1. Before the experiments, nitrogen (with a constant flow rate of 50 mL/min) was injected into the reactor for 20 min to maintain an inert atmosphere. Subsequently, the reactor was heated in a heating rate of 35 °C/min to achieve the set-point temperature (800 °C). Then, 5.0 g sewage sludge sample was put into the stainless steel boat and placed into the middle of quartz tubular reactor. The set-point temperature was considered as the pyrolytic temperature which would be held for 20 min. During the pyrolysis process, the volatile flowed out of the reactor and was condensed. By means of such condensation process, the condensable volatiles (i.e. bio-oil) were captured in the collector. And the non-condensable (combustible) gas whose volume was measured by a gas flow meter was gathered in a Tedlar sample bag. The main compositions of non-condensable gas (i.e. H2, CO, CO2, CH4) were analyzed with a gas chromatography (9800T) by using thermal conductivity detector (TCD) and TDX-01 columns. At the end of the pyrolysis, the furnace was cooled down to room temperature and the mass of the solid char as well as bio-oil yields could be calculated. Each experiment was repeated two times in order to ensure the mass balance and data reliability. The data reported in this paper were the mean value of twice the data.
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Fig. 1. Schematic diagram of sewage sludges pyrolysis apparatus. 1. Temperature control; 2. nitrogen gas cylinder; 3. rotameter; 4. thermocouple; 5. stainless steel boat; 6. horizontal electric furnace; 7. condenser; 8. tar collection bottle; 9. cotton filter; 10. drying tube; 11. gas meter; 12. gas outlet.
3. Results and discussion 3.1. Effects of bioleaching on sewage sludge composition During the bioleaching process, the Fe2+ is oxidized into Fe3+ by At. ferrooxidans bacteria. The Fe3+ is leached in the liquid phase and the sulfuric acid is generated via following chemical reactions: At: ferrooxidans
2FeSO4 þ 0:5O2 þ H2 SO4 ! Fe2 ðSO4 Þ3 þ H2 O
ðR-1Þ
4Fe2 ðSO4 Þ3 þ 2MeS þ 4H2 O þ 2O2 ! 2M2þ þ 2SO2 4 þ 8FeSO4 þ 4H2 SO4
ðR-2Þ 2+
where MeS denotes the metal sulfide, and M denotes the soluble metallic ion. The production of sulfuric acid leads to the pH value decrease, which is beneficial to the solubilization of sludge-borne metals. Bioleaching process has been proven to be capable of sewage sludge digesting, dewatering ability improvement and organic compounds modifications. Table 1 gives the ultimate analysis results of sewage sludge samples. Variations in the C, H, O, N contents and H/C, O/C, N/C ratio demonstrate that the organic matter composition of sewage sludge has been changed by bioleaching treatment. Compared with the raw SS (i.e. raw-SS), the H/C and N/C ratios both decrease but the O/C ratio of sewage sludge increases after bioleaching. The H/C ratio decreases with the solid concentrations of bioleaching treatment, from 2.87 (raw-SS) to 2.51 (SS-10). Meanwhile, the H/C ratio decreases from 2.77 to 2.51 as the solid concentration increases from 6% to 10% (w/v). The phenomenon of H/C, N/C and O/C ratios variations suggests that the bioleached sewage sludge samples show more aromatic groups than raw-SS samples. It could be caused by the solubilization of light organic compounds during the bioleaching process (Gasco and Lobo, 2007). One feature is that the SS-6 exhibited higher H/C ratio than the other SS samples. This indicates that the bioleaching with 6% (w/v) solid concentrations shows the optimal performance for the presence of elevated aliphatic carbon and long chains (with CH2 groups) (Cao et al., 2011; Gasco et al., 2005). The higher aliphatic carbon content is beneficial to the generation of alkane gaseous or light aromatic hydrocarbon. The N/C ratio can be used to express the
polymerization degree of organic matter in sewage sludge. Higher polymerized organic material contains less nitrogenous functional groups (Gasco et al., 2005). Therefore, the higher polymerization degree of bioleached sewage sludge at 6% (w/v) sludge solid concentration suggests higher dewatering capacity. As can be seen in Table 1, the O/C ratios of SS-6, SS-8 and SS-10 are obviously higher than those of raw-SS. The O/C ratio is not an appropriate parameter for organic matter, indicating some oxygen can be transferred to oxygenous function groups or oxidized during the reaction. Table 2 lists the total humic substances (THS) content, humic acid (HA) contents, fulvic acid (FA) contents and HA/FA ratios of different sewage sludge samples. It can be observed that these parameters of sewage sludge varied significantly after bioleaching. HA and FA are the main fractions of humic substances. FA, which is in lower molecular weight and lower carbon content than HA, contains higher oxygen in the form of functional groups (Rodriguez et al., 2014). In addition, HA is soluble in alkali but insoluble in acid, while the FA is soluble in both alkali and acid. After bioleaching, the HA and FA contents of sewage sludge have increased. But the HA content decreases with the increase of bioleached solid concentrations from 0.79% (SS-6) to 0.49% (SS-10). It possibly attributes to the precipitation of HA under acid condition during the bioleaching process. Furthermore, the acidification rate of sewage sludge decreases as the solid concentration increases (high pH), which enhances the HA solubilization. The FA content in bioleached sewage sludge is a little higher than that of raw-SS. Meanwhile, the FA content decreases with the solid concentration increasing from 2.54% (SS-6) to 2.46% (SS-10), because the hydrolysis of polymerized organic matter generates more oxidized and light molecular compounds which are similar to the FA fraction under higher acidification rate (Gasco and Lobo, 2007). The HA/ FA ratio denotes the humic transformation degree. The HA/FA ratio inversely relates to the N/C ratio which can be used to compare the polymerization degree (Iakimenko et al., 1996). Table 3 gives heavy metal contents of the raw-SS and bioleached sewage sludge. It is obvious that the bioleaching treatment can reduce the heavy metal contents in sewage sludge. For these four heavy metals, the highest removal efficiencies (g) are all obtained at the solid concentration of 6% (w/v) and follow the order of Cd > Zn > Cu > Pb. Similar orders also can be found for the 8% (w/v) and 10% (w/v) solid concentration bioleaching
Table 1 Ultimate analysis of sewage sludge samples.
a b
Sewage sludge
C (wt%)
H (wt%)
Oa (wt%)
N (wt%)
H/Cb
O/Cb
N/Cb
Raw-SS SS-6 SS-8 SS-10
12.37 ± 0.17 13.00 ± 0.12 13.68 ± 0.23 14.26 ± 0.34
2.96 ± 0.09 3.01 ± 0.11 3.00 ± 0.15 2.99 ± 0.08
16.89 27.67 26.26 24.79
2.79 ± 0.14 1.91 ± 0.11 2.23 ± 0.07 2.47 ± 0.18
2.87 2.77 2.63 2.51
1.02 1.59 1.43 1.30
0.19 0.12 0.13 0.14
By difference. Molar ratio.
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Table 2 Properties of sewage sludge samples. Sewage sludge
CTHS (wt%)
CHA (wt%)
CFA (wt%)
HA/FA
Raw-SS SS-6 SS-8 SS-10
2.55 ± 0.09 3.24 ± 0.02 3.06 ± 0.10 2.95 ± 0.09
0.25 ± 0.06 0.70 ± 0.04 0.59 ± 0.02 0.49 ± 0.01
2.30 ± 0.02 2.54 ± 0.08 2.47 ± 0.10 2.46 ± 0.03
0.11 0.28 0.24 0.20
treatments. However, the removal efficiencies of these four heavy metals are strongly influenced by solid concentration. As can be seen in Table 3, with solid concentration increase, the contents of Cu, Zn, Pb and Cd in sewage sludge increase and the corresponding removal efficiencies decrease. The phenomenon may attribute to the final pH value of the bioleaching system. The higher solid concentration indicates more sewage sludge which has higher buffering capacity in the bioleaching system. And the pH value of bioleaching also can greatly affect the solubilization efficiencies of heavy metals during the bioleaching process (Chen and Lin, 2000; Sharifi and Renella, 2015). Another one feature is that the removal efficiencies of Cu, Zn and Cd are significantly higher than those of Pb at the same solid concentration. During the bioleaching process, the lower removal efficiency of Pb is possibly due to generation of PbSO4 (Ksp = 1.62 108) which has low solubility (Chen and Lin, 2000). The bioleaching is also found to be efficient in heavy metals removal at higher solid concentration than 6% (w/v). It is considered that the increase of solid concentration generally leads to a prolonged lag period (i.e. the required time for the microorganisms adapts to bioleaching medium), which decreases the bio-oxidation rate and the ultimate extent of oxidation (Ahmadi et al., 2015). In addition, the increase of sludge solid concentration can result a decrease in metal solubilization. Moreover, once more sewage sludge is treated, larger volume of reactor and treated time are needed and the cost of process will also increase certainly. FTIR spectra technique was used to qualitatively determine the chemical structures change in bioleached sewage sludge compared with the raw-SS. The FTIR spectra results (Fig. 2) demonstrate that the bonds of bioleached sewage sludge (SS-6, SS-8 and SS-10) are similar to the raw one (raw-SS). However, as for the SS-6 sample, the OAH (stretching vibrations at 3432 cm1), C@O (stretching vibrations at 1642 cm1), CAH (stretching vibrations at 1393 cm1) and CAO (stretching vibrations at 1038 cm1) have been greatly enhanced compared with the other samples. It implies that the quantities of these groups increase by breaking the macromolecule and release more free groups via bioleaching. As suggested by Zhang et al. (2011), the OAH indicates the existence of water, alcohol, phenol, or amine substance; C@O is compatible with the presence of acids and aldehydes; CAH is produced by ACH3 and ACH2 group which implies the sewage sludge contains a lot of fat hydrocarbon organic matter. CAO corresponds to the oxidizing substance of ether, ester, alcohol, and spenol. The FTIR spectra results (Fig. 2) clearly show that the oxygen functional
Fig. 2. FTIR spectra profiles of sewage sludge samples.
groups of sewage sludge have increased by bioleaching. Such increment of oxygen functional groups is possibly related to a higher content of FA, especially at 6% (w/v) solid concentration (Table 2). FTIR spectra results demonstrate that the bioleaching treatment leads to important changes in the characteristics organic matter of raw-SS. Compared to the other samples, SS-6 is proven to generate more oxygen functional groups which are conducive to the combustible gases (CO, H2, CO2) formation during pyrolysis. 3.2. Thermogravimetric characterization The thermogravimetric analysis (TGA) results (TG/DTG vs. temperature profiles) are shown in Figs. 3 and 4. Fig. 3 presents the weight loss of the sewage sludge samples. It could be observed that the weight loss of bioleached sewage sludge samples starts at lower temperature and finishes at higher temperature, especially for SS-6. The TG profile of SS-6 also shows significant different pyrolysis behaviors compared with the other bioleached SS (SS-8 and SS-10) and raw-SS. The alterations of pyrolysis behaviors attribute to the differences of the ingredient in their sewage sludge samples. As illustrated in the DTG profiles (Fig. 4), the pyrolysis process of sewage sludge samples could be divided into four stages. In the temperature ranges of room temperature to 120 °C, the DTG peaks are caused by the evaporation of physically absorbed water. For all the samples, the devolatilization stage which is accompanied by high weight loss rate (DTG) and attributed to the volatiles volatilization occurs between 120 °C and 600 °C. The weight loss at the temperature of 600–800 °C is caused by the decomposition of inorganic materials (mainly calcium carbonate) (Scott et al., 2006). However, the thermal decomposition behaviors of the devolatilization stage (120–600 °C) are quite complex and manifest as multiple peaks overlap. The peaks at
Table 3 Heavy metal contents (mg/kg) in sewage sludge (SS) and corresponding bioleaching efficiency (g, Eq. (1)).
a b
Cu
Zn
Pb
Cd
Raw SS Raw-SSa Bioleached SS
c0 196.43 ± 3.20 c1
g (%)
c0 269.72 ± 1.78 c1
g (%)
c0 63.21 ± 3.27 c1
g (%)
c0 4.82 ± 0.84 c1
g (%)
SS-6b SS-8b SS-10b
52.87 ± 2.48 87.42 ± 2.91 105.21 ± 2.83
73.08 55.50 46.44
57.53 ± 1.49 98.85 ± 0.88 129.11 ± 2.72
78.67 63.35 52.13
47.63 ± 3.01 51.12 ± 2.64 53.52 ± 2.82
24.65 19.13 15.33
0.99 ± 0.05 1.66 ± 0.04 2.01 ± 0.11
79.46 65.56 58.30
Heavy metal contents of raw SS, c0 in Eq. (1). Heavy metal contents of bioleached SS, c1 in Eq. (1).
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Fig. 3. TG (weight loss vs. temperature) curves of sewage sludge samples.
120–395 °C denote the preliminary decomposition stage of the organics in sewage sludge samples. In temperature range about 395–600 °C, obvious peaks also can be observed, which denotes the secondary decomposition stage of the organics. The weight loss rate of SS-6 in the secondary decomposition stage is higher than the rest samples at any same temperature point. It indicates that the organics of SS-6 is easier to decompose than the other samples. In addition, the DTG curves of bioleached sewage sludge (SS-6, SS-8 and SS-10) are more intensive to the temperature than the raw-SS.
alkanes and long fragment alkenes). Besides, the formed free radicals can be mutually combined freely with the hydrogen atom. Other bonds also start to break and generate more free radicals, which induces the polymerization and cyclization reactions. Therefore, the main primary pyrolysis products are char, small molecule gases (CO, CO2) and macromolecular condensable volatiles. As the temperature increases, the secondary reactions of tar cracking and shifting, such as decarboxylation, decarbonylation, dehydrogenation, cyclization, aromatization and polymerization, contribute to the thorough reaction of the pyrolysis (Li et al., 2007). Afterward, the light vapors underwent series reactions and are cracked to form H2, CO, CO2, alkanes, alkenes, and aromatic hydrocarbons. In this study, the pyrolysis experiments of sewage sludge were carried out in the reactor at temperature of 800 °C. The results of product distribution and gas compositions are depicted in Figs. 5 and 6. As shown in Fig. 5, the gas product yield of bioleached sewage sludge is larger and the char yield is less than that of the raw sewage sludge. Meanwhile, the tar yield also changes slightly. The variations in pyrolysis products (char, tar and gas) mainly attribute to the changes in the characteristics of sewage sludge after bioleaching, such as the compositions change (Tables 1 and 2) and the removal of heavy metals (Table 3). However, the gas yield gradually decreases with the increase of solid concentration. In addition, no remarkable change is observed for the tar yield.
60 SS-6 SS-8 SS-10 Raw SS
3.3. Pyrolysis products It is believed that the organic matter in sludge mainly consisted of fat, protein, carbohydrate, and cellulose (Kapanen et al., 2013). The main functional groups of these organic compounds include carboxylic acid, carbonyl, amide, amine, methyl and aromatic. With the primary thermochemical decomposition of sewage sludge, the instability C@O of macromolecular organic matter begins to break and generate CO and CO2. Then, the CAC and CAH bonds are ruptured to form the free radicals which are further recombined into low molecular compounds. However, the CAC bond could be easily broken since the bond energy of CAC (346.9 kJ) is lower than that of CAH bond (413.84 kJ) (Sorum et al., 2001). In general, the cleavage of alkanes which contains more carbon atoms will form new kinds of alkanes (short fragment
Percentage (wt.%)
50
40
30
20
10
0 Gas
Char
Tar
Product Fig. 5. Products percentages obtained by sewage sludge pyrolysis.
40
SS-6 SS-8 SS-10 raw-SS
Percentage (wt.%)
32
24
16
8
0
H2
CO
CO2
CH4
Gas composition Fig. 4. DTG curves (weight loss rate vs. temperature) of sewage sludge samples.
Fig. 6. Compositions of gas derived from sewage sludge samples pyrolysis.
Please cite this article in press as: Chen, Z., et al. The effect of bioleaching on sewage sludge pyrolysis. Waste Management (2015), http://dx.doi.org/ 10.1016/j.wasman.2015.10.002
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Reasons may lie in that: (1) After bioleaching, the contents of fulvic acid vary from 2.54% (SS-6) to 2.46% (SS-10) (Table 2). The fulvic acid is rich in oxygen content and its function groups could be easily ruptured to generate volatiles. Under different solid concentrations, the bioleached sewage sludge shows a gradually decreased H/C ratio (from 2.77 (SS-6) to 2.51 (SS-10)), which accordingly reduces the aliphatic carbon fractions. The higher aliphatic carbon fraction is advantageous to the generation of alkane gaseous and light aromatic hydrocarbon. (2) Meanwhile, as can be seen from the TG curves (Fig. 3), SS-6 sample exhibits a larger weight loss than the other samples in the whole pyrolysis temperature range. It demonstrates that the pyrolysis of SS-6 leads to a larger volatiles volume than the rest samples (SS-8, SS-10 and raw-SS), which may contribute to more gas production. The change of char yield relates to heavy metals dissolution efficiency. With the solid concentration increase, the buffering capacity of sludge also increases. And the dissolution of heavy metals is reduced by the pH reduction. Nonetheless, the contribution of bioleaching pretreatment on tar yield is inconspicuous. Bioleaching treatment has significant influences on the components of pyrolytic gas products. As can be seen in Fig. 6, H2 and CO fractions in pyrolytic gas products have been improved by bioleaching treatment, especially for the treatment with 6% (w/v) solid concentration. The generation of CO may be resulted from the reactions of carbonyl group cracking, oxygen function group rupture and dehydrogenation of hydroxyl group. Such reactions have been enhanced by bioleaching treatment. The decreasing trend of CO2 content may be due to the fact that the fractions of CO and CO2 are linked together according to the equilibrium of the Boudouard reaction under the experiment conditions. CH4 content presented is slightly changed, which possibly attributes to the variance of tar yield. 4. Conclusions During bioleaching process, heavy metals removal efficiency, organic compositions, pyrolytic behaviors, pyrolysis product yield and gas components were influenced by bioleaching treatments. Sewage sludge was treated by bioleaching with 6% (w/v), 8% (w/ v), and 10% (w/v) solid concentration. The bioleaching treatment could improve the removal of heavy metals, especially for the 6% (w/v) solid concentration treatment. The optimal removal efficiencies of Cu, Zn, Pb, Cd are 73.08%, 78.67%, 24.65% and 79.46%, respectively. The H/C, N/C ratios of sewage sludge decreased while the O/C ratios increased after bioleaching treatment. The humic acid, fulvic acid and total humic contents of bioleached sewage sludge increased compared with the raw sewage sludge. The thermogravimetric analysis characterizations indicated that the organics of bioleached sewage sludge was more intensive to the temperature than that of the raw sewage sludge. The bioleaching of sewage sludge with 6% (w/v) solid concentration showed the best performance on heavy metals removal, organic compositions transformation, pyrolytic behaviors, pyrolysis product yields and gas components. Acknowledgments The authors wish to acknowledge the financial supports of the National Nature High Technology Research and Development Program (863 Program) of China (No. 2012AA101809), National Natural Science Foundation of China (No. 21276100) and the Natural Science Foundation of Hubei Province, China (2014CFB411). The authors would also like to thank the Analytical and Testing Center of Huazhong University of Science and Technology for carrying out the analysis of the characterization of sewage sludge samples.
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Please cite this article in press as: Chen, Z., et al. The effect of bioleaching on sewage sludge pyrolysis. Waste Management (2015), http://dx.doi.org/ 10.1016/j.wasman.2015.10.002