Article pubs.acs.org/EF
Comparison of the Lipid Content and Biodiesel Production from Municipal Sludge Using Three Extraction Methods Fenfen Zhu,*,†,‡ Luyao Zhao,†,‡ Huimin Jiang,†,‡ Zhaolong Zhang,§ Yiqun Xiong,†,‡ Juanjuan Qi,†,‡ and Jiawei Wang∥ †
School of Environment and Natural Resources, Renmin University of China, Beijing 100872, People’s Republic of China Key Laboratory for Solid Waste Management and Environment Safety, Ministry of Education of China, Tsinghua University, Beijing 100084, People’s Republic of China § School of Resources Environment and Tourism, Capital Normal University, Beijing 100048, People’s Republic of China ∥ Beijing Drainage Group, No. 1 Gaobeidian, Chaoyang District, Beijing 100124, People’s Republic of China ‡
ABSTRACT: In light of the rapid development of society, wastewater treatment ratios and amounts as well as the amount of sewage sludge production have increased. High water content, heavy metal, and viral content of sewage sludge have resulted in a serious environmental problem. In this study, the performances of acid hydrolysis extraction, Soxhlet extraction, and the water bath shaking method were compared in terms of lipid extraction from sewage sludge. These methods are more cost-effective and easier to apply compared to supercritical extraction. The effects of different organic solvents, including ether, hexane, acetone, chloroform−methanol, and solvent combinations, on the extraction rate were also studied. More polar lipids were extracted by polar solvents (6.4%) than by nonpolar solvents (3.4%) from mixed sewage sludge. Soxhlet extraction, which extracted 2.5−10.3% lipids from dried sewage sludge, performed considerably better than acid hydrolysis extraction (2.2−7.5%) and the water bath shaking method (3.0−7.5%). The true yields (fatty acid methyl ester purity) of the biodiesel produced from lipids extracted by the acid hydrolysis method, Soxhlet method, and water bath shaking method by the optimal solvent were 1.30, 6.35, and 4.10%, respectively.
1. INTRODUCTION Today’s rapidly developing societies consume a large amount of energy, thereby giving rise to an energy crisis. To achieve sustainable development, alternative renewable and environmentally friendly energy sources have to be identified.1 Biodiesel, which is made from plants and animals, may be one such energy source. In comparison to traditional fossil fuels, biodiesel is safer and renewable and has good combustion performance.2,3 However, studies show that 80% of the cost of biodiesel production is spent on the process of obtaining raw materials, including cultivation and harvest. The high costs involved in harnessing biodiesel inhibit industrial production of biodiesel. To address this issue, cheaper raw materials are needed.4,5 Sewage sludge is considered a suitable substitute for raw biodiesel materials because of its two unique advantages: (1) it is widely and consistently available, and (2) it contains considerable amounts of organic compounds. In China, the average concentration of organic matter in sewage sludge is 38.4%, with carbohydrates accounting for 55%, proteins accounting for 20%, and lipids accounting for about 20%.6−8 In some large cities, such as Beijing, organic matter content [volatile solid (VS)] of sewage sludge has reached 60%, which is almost the same as the level in other countries. The total lipids extracted from sludge can reach about 12% of the dry weight of the material. Biodiesel producers can obtain sewage sludge at no monetary cost, and they can even generate money from wastewater treatment plants (WWTPs) through disposal of the sewage sludge. Therefore, lipid extraction from sewage sludge for biodiesel © 2014 American Chemical Society
production can bring about considerable economic and environmental benefits.9−11 In terms of sewage sludge treatment, the use of sewage sludge as a raw material for biodiesel production is reasonable. Sewage sludge contains substances, such as pathogens, heavy metals, and even some persistent organic pollutants (POPs), that are hazardous to human health and the environment.12 Several sludge treatment and disposal methods are available, including sanitary landfilling, incineration, anaerobic composting, pyrolysis, gasification, etc., each of which have their own special circumstances to be applied. For example, if land saving is the highest hierarchy and another purpose is to obtain some heat, incineration might be chosen, while if some fuel is about to achieve, then gasification or pyrolysis would have to be applied. Anaerobic composting can be applied under the condition of low heavy metal content and good market of fertilizer made from sewage sludge. A sanitary landfill can be cost-saving, and it is the final method to dispose of waste of reasonable and allowed water content and the residues produced from other treatment methods. As such, these methods have limited application. Recycling sewage sludge as raw material for biodiesel can be applied for sludge with both high and low heavy metal contents and for some place that is seriously scare of energy resources, especially in demand of Received: April 1, 2014 Revised: July 5, 2014 Published: July 30, 2014 5277
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Table 1. Sludge Composition item
VSS (%)
content item
52.5 S (%)
TC (ppm) 385.4 Mg (%)
content
2.36
0.85
Cl (ppm) 9.75 K (%)
Si (%) 5.06 Zn (%)
0.70
0.14
diesel.13,14 Lipid extraction, which is the most important step in producing biodiesel, will be discussed in this study. Complex sewage sources and many complicated wastewater treatment technologies produce both nonpolar and polar lipids in sewage sludge. If we aim to extract lipids from sewage sludge, the polarity of the extraction solution should be as close to that of the lipids as possible.15−20 However, lipid extraction technologies from sludge have been previously investigated seriously, and no national standard for lipid assessment for sewage sludge is yet available. Few studies have reported the extraction of lipids from sewage sludge. In this work, we referred to the national standard of quantitative lipid analysis from the food industry. Three methods were used to extract lipids from sludge, namely, acid hydrolysis extraction, Soxhlet extraction, and the water bath shaking method. These methods are more cost-effective and easier to apply than supercritical extraction. The effects of different solvents, including ethanol, ethyl ether, petroleum ether, hexane, chloroform−methanol, bromopropane, and hexane−ethanol,21−24 on extraction efficiency were also studied to determine the most suitable solvent or solvent combination for extracting lipids from sewage sludge. Subsequently, acid-catalyzed transesterification was employed to produce biodiesel from the lipids extracted by acid hydrolysis extraction, Soxhlet extraction, and the water bath shaking method. The quality and purity of the biodiesel were determined through gas chromatography−mass spectrometry (GC−MS).
Fe (%) 4.30 Ba (%)
Al (%)
P (%)
3.50 Mn (%)
2.72 Ca (%)
0.05
9.65
0.10
Table 2. Dipole Moment and Dielectric Constant of Some Solvents27 solvent
dipole moment (D)
dielectric constant (F/m)
water methanol ethanol acetone dichloromethane ether n-hexane bromopropane
1.85 1.70 1.69 2.88 1.60 1.10
80 33 25.3 21.01 8.93 4.23 1.89 9.46
2.21
was then heated at 105 °C and weighed using an electronic balance (0.1 mg). 2.4. Soxhlet Extraction (National Standard). A Soxhlet extraction device (Beijing Wan Cheng Yuan Xing Chemical Corporation), which comprises a 500 mL extraction flask, a condenser pipe, and an automatic siphon, was used to extract lipids. Sludge samples (5 g) were placed into a tube using filter paper and transferred to the automatic siphon. Approximately 20 mL of the extraction solvent was added to the extraction flask. The extraction solvents used in this experiment include ethanol, methanol, hexane, chloroform− methanol, ether−petroleum ether, and hexane−ethanol. The extraction was performed at a temperature that ranged from 70 to 80 °C. The total extraction run time ranged from 6 to 8 h. After extraction, the extraction solvent was recovered for reuse. The extracted mixture was then heated at 105 °C and weighed using an electronic balance. Three parallel sludge samples were used for each solvent. 2.5. Water Bath Shaking Method. The water bath shaking method is the third method used in this research to extract lipids from sludge. Preliminary experiments were performed to gain the optimum conditions to effectively extract lipids from sludge. The extraction temperature, stirring rate, and extraction time are three important factors that must be considered. In the preliminary experiments, the following conditions were tested: temperatures of 30, 40, 50, and 60 °C; stirring rates of 110 and 120 rpm; and extraction times of 2 and 3 days. Temperatures that range from 30 to 40 °C, a stirring rate of 120 rpm, and extraction time of 2 days were the best parameters for extracting lipids from sludge. Thus, the experiments were conducted as follows: Sludge samples (5 g) were placed in a tube made of filter paper, and the tube was placed into a 250 mL flask. Approximately 75 mL of the extraction solvent was then added to the flask. The extraction solvents used in this experiment include ethanol, methanol, acetone, chloroform−methanol, and ether−petroleum ether. The flask was sealed with a plastic film and placed in a water shaking bath at 30−40 °C with a magnetic stirring at 120 rpm for 2 days. After extraction, the extracted mixture was heated at 105 °C and weighed using an electronic balance. Three parallel experiments were conducted for each solvent. 2.6. Transesterification.24 Acid-catalyzed transesterification was used to produce biodiesel from lipids extracted from the sludge using the three methods. These methods were used to treat 10 g of dry sludge. The extracted lipids were treated with treatment combinations of reaction temperature (75 °C), 10 mL of H2SO4 concentration (5%, v/v), and methanol (mass ratio to dried sludge is 12:1). Then, 50 mL of hexane was added to improve lipid solubility in the reaction mixture. The mixture was then suspended in a solution using a mechanical stirring bar and heated to the set temperature using a hot water bath. A condenser was used to reduce methanol and hexane loss through evaporation. The reaction time was 8 h.
2. MATERIALS AND METHODS 2.1. Materials. Dewatered sludge was obtained from a WWTP in Beijing, China. The moisture content of the sludge sample was 79 ± 0.5%. The instruments used to determine the sludge components are as follows: Total carbon (TC) was detected using Jena multi N/C 2100S and Yena HT1300 instruments; Cl− was tested using ion chromatography (Dionex, ICS-900); and other elements were tested using an XRF-1800 instrument (Shimadzu, Kyoto, Japan). Table 1 shows the results of the tests. All of the chemicals used in this work were analytical-grade and used without further purification. The chemicals were purchased from Tianjin Guangfu Technology Development Co., Ltd. The solutions were prepared with water purified by a Millipore Milli-Q UV Plus system. 2.2. Preparation of Sewage Sludge. Dewatered sludge was dried at 105 °C for 12 h and then ground to a fine powder. The samples were stored in sealed plastic vials at 4 °C. 2.3. Acid Hydrolysis Extraction (National Standard). Sludge samples (2 g) were prepared using 10 mL of 1 N hydrochloric acid (36−38%) and 8 mL of distilled water. The mixture was heated at a temperature that ranged from 70 to 80 °C for about 40−50 min. After cooling, about 10 mL of ethanol (95%) was added to the solution. The mixture was filtered into an Erlenmeyer flask at room temperature. Afterward, 25 mL of the extraction solvent and 5 mL of ether− petroleum ether were added to the mixture. The extraction solvents used in this experiment include petroleum ether, hexane, chloroform− methanol, ether−petroleum ether, and hexane−ethanol. The mixtures were transferred to a separator funnel, and the remaining solution was washed 3 times using the extracted solvent. After the upper liquid became clear, the supernatant was removed from the flask. The mixture 5278
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Figure 1. (a) Lipid content by acid hydrolysis extraction, (b) lipid content by Soxhlet extraction, and (c) lipid content by water bath shaking method. When the mixture cooled after 8 h, the flask contents were transferred to a 500 mL bottle. Subsequently, 10 mL of saturated NaCl solution and 100 mL of hexane were added, and the bottle was shaken vigorously for 3 min. The mixture was centrifuged at 3000 rpm for 10 min. The supernatant hexane phase was withdrawn and transferred into a 250 mL separatory flask. The extraction procedure was repeated 3 times. The total volume of the collected supernatant was then washed with 20 mL of a 2% (w/v) potassium bicarbonate solution, and the aqueous phase was settled. The bottom layer was discarded, and the upper layer (hexane layer) was passed through a filter paper that contains anhydrous sodium sulfate and collected into a 1000 mL flask. A 10 mL aliquot of the hexane phase was pipetted into a test tube for fatty acid methyl ester (FAME) analysis, and the remainder was subjected to solvent removal under a vacuum using a rotary evaporator at 40 °C. After the hexane was completely
removed, the flask was flushed with nitrogen to remove any remaining hexane in the gas phase and then weighed to determine the residue weight. Three parallel experiments were conducted for each method. 2.7. FAME Analysis. FAMEs in the hexane phase were analyzed using Shimadzu GCMS-QP2010. A DB-5 ms, 30 m × 0.25 mm column was used for the analysis. The column temperature was programmed to start at 80 °C, maintained for 2 min, increased from 80 to 250 °C at 10 °C/min, and finally maintained at 250 °C for 20 min. The sample injection volume was 1.0 μL, with a split ratio of 10:1. The inlet line to mass spectrometry (MS) was maintained at 250 °C, whereas the MS source temperature was kept at 200 °C. Standard 70 eV electron ionization (EI) spectra were recorded from m/z 20 to 650. To calculate the purity of biodiesel, we just figured out the content of FAME and divided by the amount of crude biodiesel. 5279
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3. RESULTS 3.1. Acid Hydrolysis Extraction. The polarity of the solvents followed the following order: methanol > ethanol > acetone > bromopropane > dichloromethane > ether > n-hexane > petroleum ether. Table 2 shows the dipole moments and dielectric constants of the solvents. The results of the acid hydrolysis extraction are shown in Figure 1a. A total of 10 solvents or solvent combinations were used, and at least three parallel runs were performed for each solvent. Among the solvents, bromopropane extracted the highest amount of lipids from the sludge (7.5 ± 0.55%), whereas petroleum ether extracted the least amount (2.2 ± 0.38%). As shown in Table 2, the 14 solvents used in this study can be divided into three types according to their polarity: polar, nonpolar, and mixed solvents. Acetone, dichloromethane, methanol, and ethanol bromopropane are grouped under polar solvents, whereas ether, petroleum ether, and n-hexane are nonpolar solvents; the remaining solvents are mixed solvents. The average lipid content extracted by the polar, nonpolar, and mixed solvents through acid hydrolysis extraction were 6.6, 3.3, and 4.5%, respectively. The average total extraction capacity of acid hydrolysis extraction was 4.8%. Figure 1a shows that mixed solvents have higher extraction efficiency compared to single solvents. 3.2. Soxhlet Extraction. Figure 1b shows the results of Soxhlet extraction as well as the average values and error bars. Minimal differences in standard deviations were observed among the solvents used. Acetone and hexane−methanol− acetone extraction generated the maximum standard deviation (0.35). The average lipid contents extracted by polar, nonpolar, and mixed solvents through Soxhlet extraction were 7.3, 2.9, and 7.9%, respectively. The average total extraction capacity of the Soxhlet extraction method was 6.0%. As in the acid hydrolysis method, mixed solvents exhibited higher extraction efficiency than single solvents, while polar solvents extracted more polar lipids than nonpolar solvents. This result may be attributed to the fact that more polar lipids can be obtained from mixed sewage sludge than nonpolar lipids. Lipids in primary sludge are mostly nonpolar, because these lipids originate from organic compounds in influent. In contrast, lipids found in secondary sludge mainly originate from microbial cells, which contain polar phospholipids in their cell walls. As a result, the mixed solvent obtained a higher yield of lipid. Figure 1b shows the highest and lowest extraction rates from the different solvents. The highest amount of lipid extraction (10.3 ± 0.20%) was achieved using hexane−ethanol, whereas the lowest extraction (2.5 ± 0.10%) was obtained from petroleum ether. Extraction amounts obtained using Soxhlet extraction were much higher compared to the amounts obtained by acid hydrolysis. 3.3. Water Bath Shaking Method. Figure 1c shows the results of the water bath shaking method. The average lipid contents extracted by the polar, nonpolar, and mixed solvents through the water bath shaking method were 5.5, 4.0, and 5.8%, respectively. The average total extraction capacity of the water bath shaking method was 5.1%. Similar to the observations made from the Soxhlet extraction method, the standard deviations of all of the solvents were low for the water bath shaking method. The highest and lowest standard deviations (0.58 and 0.06) were obtained from methanol extraction and bromopropane−petroleum ether extraction, respectively. Furthermore, the highest (7.5 ± 0.06%) and lowest (3.0 ± 0.10%)
Figure 2. (a) Quality of FAMEs produced by acid hydrolysis extraction, (b) quality of FAMEs produced by Soxhlet extraction, and (c) quality of FAMEs produced by the water bath shaking method.
extraction amounts were obtained from hexane−ethanol and petroleum ether, respectively. For the water bath shaking method, solvents, such as ether and ether−petroleum ether, were not used because their boiling points are low, which means that they could not be recycled in the experiments. 3.4. Biodiesel Yield and Quality. The crude yields of biodiesel from lipids extracted by the acid hydrolysis method, Soxhlet method, and water bath shaking method were 1.33, 6.73, and 4.92%, respectively. The purity values, as determined through GC−MS, were 97.5, 94.3, and 83.3%, respectively. The FAMEs obtained when using lipids extracted from the three methods are shown in panels a−c of Figure 2. Results showed that, regardless of the extraction method used before transesterification, the most abundant species were methyl esters of palmitic acid (C16:0), stearic acid (C18:0), elaidic acid (C18:1n9t), and myristic acid (C14:0). 5280
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Figure 3. Comparison of the lipid content of the three methods.
Figure 4. Comparison of the lipid content of the three methods to every solvent.
However, species of FAMEs produced from Soxhlet extraction (14) and the water bath shaking method (14) were basically the same, which are greater in number than the FAMEs produced through the acid hydrolysis method (6).
using organic solvents with high polarity. However, the lipids extracted through Soxhlet extraction are crude fat, which may contain phospholipids, glycolipids, and lipoproteins depending upon the extraction solution employed.9 Although the principles of the water bath shaking method and Soxhlet extraction were similar, the solvents used in the water bath shaking method cannot be prevented from volatizing. In addition, the duration of the Soxhlet extraction method is longer than that of the two other methods. The concentration of lipids in the solution increases, while lipids are extracted, thus slowing the extraction speed. However, in Soxhlet extraction, the extraction solution circulated in the system several times. Every time, the extraction started from pure extraction solution, so that the extraction driving force is stronger than that in both acid hydrolysis extraction and water bath shaking.26 Moreover, the reflux condensation applied in the Soxhlet extraction
4. DISCUSSION Results shown in Figure 3 indicate several differences among the three methods used in extracting lipids from sludge. Figure 3 shows the average values of the total lipids extracted per method. Soxhlet extraction, which extracted 6.0% of the lipids in sludge, showed better extraction performance compared to acid hydrolysis extraction (4.8%) and the water bath shaking method (5.1%). In the acid hydrolysis extraction experiment, hydrochloric acid was used to decompose and transfer the combined fat into free fatty acids to obtain more pure glycerides, which is almost nonpolar and difficult to extract 5281
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Figure 5. Comparison of the lipid content of the different kinds of solvents.
Figure 6. Comparison of the yield and purity of biodiesel produced by the three methods.
method can limit the solvent volume to guarantee extraction efficiency. The standard deviation of acid hydrolysis extraction (0.83) was higher than that of the two other methods (Soxhlet extraction, 0.21; water bath shaking method, 0.21). Two reasons may explain these results. First, both Soxhlet and water bath shaking extraction contained setups that prevented the solvents from escaping to the air. For example, a condenser pipe was used during Soxhlet extraction, and a plastic film was used during water bath shaking to seal the outlet. Moreover, acid hydrolysis extraction required more manual operations, such as adding solutions before the extraction (refer to the instruction of methods), whereas Soxhlet extraction or the water bath shaking method employed automatable extraction. Figure 4 shows the amount of lipids extracted by different methods using the same extraction solvent. Different solvents correspond to different optimal extraction methods. For example, acid hydrolysis was the best extraction method for n-hexane or bromopropane. For petroleum ether, the water bath shaking method was the best method. The optimal method for all other solvents was Soxhlet extraction. Thus, to
enhance lipid recovery, an appropriate method must be selected for different extraction solvents. The results in Figure 5 show that polar solvents have the highest extraction performance for all results in all three methods. Polar solvents extracted more lipids (6.4%) than nonpolar solvents (3.4%). Mixed solvents extracted lipid amounts (6.1%) that are similar to those obtained by polar solvents. Therefore, more polar lipids than nonpolar lipids are found in sewage sludge.25 Figure 6 shows that, although the yields of biodiesel produced by sludge are not as high as other materials, the FAME purity is relatively high at 97.5, 94.3, and 83.3% for the acid hydrolysis method, Soxhlet method, and water bath shaking method, respectively. Overall, the values of the true yields (FAME purity) of the biodiesel that were obtained through the three methods are 1.30, 6.35, and 4.10%, respectively. According to the results, we can see that the yield of the biodiesel produced from the lipids extracted using the Soxhlet method was higher than that of the other two methods. It should be that Soxhlet extraction extracted 6.0% of the lipids from sludge, which showed better extraction performance compared to acid hydrolysis extraction 5282
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(9) Huynh, L. H.; Kasim, N. S.; Ju, Y. H. Extraction and analysis of neutral lipids from activated sludge with and without sub-critical water pre-treatment. Bioresour. Technol. 2010, 101, 8891−8896. (10) Huynh, L. H.; Nguyen, P. L. T.; Ho, Q. P.; Ju, Y. H. Catalystfree fatty acid methyl ester production from wet activated sludge under subcritical water and methanol condition. Bioresour. Technol. 2012, 123, 112−116. (11) Kwon, E. E.; Kim, S.; Jeon, Y. J.; Yi, H. Biodiesel production from sewage sludge: New paradigm for mining energy from municipal hazardous material. Environ. Sci. Technol. 2012, 46, 10222−10228. (12) Zhu, F.; Jiang, H.; Zhang, Z.; Zhao, L.; Wang, J.; Hu, J.; Zhang, H. Research on drying effect of different additives on sewage sludge. Procedia Environ. Sci. 2012, 16, 357−362. (13) Ke, Y.; Chen, Q.; Zhang, L. Study on the technique of utilization of municipal sewage sludge. China Resour. Compr. Util. 2008, 26 (8), 13−16. (14) Jarde, E.; Mansuy, L.; Faure, P. Organic markers in the lipidic fraction of sewage sludges. Water Res. 2005, 39, 1215−1232. (15) Palacios, L. E.; Wang, T. Egg-yolk lipid fractionation and lecithin in character fixation. J. Am. Oil Chem. Soc. 2005, 82 (8), 571−578. (16) Bligh, E. G.; Dyer, W. J. A rapid method of total lipid extraction and purification. Can. J. Biochem. Phys. 1959, 37, 911−917. (17) Folch, J.; Lees, M.; Sloane-Stanley, G. H. A simple method for the isolation and purification of total lipids from animal tissues. J. Biol. Chem. 1957, 226, 497−509. (18) Daugherty, C. E.; Lento, H. G. Chloroform−methanol extraction method for determination of fat in foods: Collaborative study. J.Assoc. Off. Anal. Chem. 1983, 66 (4), 927−932. (19) Chen, I. S.; Shen, C. S. J.; Sheppard, A. J. Comparison of methylene chloride and chloroform for the extraction of fats from food products. J.Assoc. Off. Anal. Chem. 1981, 58, 599−601. (20) St. Angelo, A. J.; James, C., Jr. Analysis of lipids from cooked beef by thin-layer chromatography with flame-ionization detection. J. AOAC Int. 1993, 70 (12), 1245−1250. (21) Dufreche, S.; Hernandez, R.; French, T.; Sparks, D.; Zappi, M.; Alley, E. Extraction of lipids from municipal wastewater plant microorganisms for production of biodiesel. J. Am. Oil Chem. Soc. 2007, 84, 181−187. (22) Olkiewicz, M.; Fortuny, A.; Stüber, F.; Fabregat, A.; Font, J.; Bengoa, Ch. Evaluation of different sludges from WWTP as a potential source for biodiesel production. Procedia Eng. 2012, 42, 634−643. (23) Pokoo. Aikins, G.; Heath, A.; Mentzer, R. A.; Mannan, M. S. A multi-criteria approach to screening alternatives for converting sewage sludge to biodiesel. J. Loss Prev. Process Ind. 2010, 23, 412−420. (24) Mondala, A.; Liang, K.; Toghiani, H.; Hernandez, R.; French, T. Biodiesel production by in situ transesterification of municipal primary and secondary sludges. Bioresour. Technol. 2009, 100, 1203−1210. (25) Zhao, C.; Xing, M.; Yang, J.; Lu, Y.; Lv, B. Microbial community structure and metabolic property of biofilms in vermifiltrationfor liquid-state sludge stabilization using PLFA profiles. Bioresour. Technol. 2014, 151, 340−346. (26) Manirakiza, P.; Covaci, A.; Schepens, P. Comparative study on total lipid determination using Soxhlet, Roese-Gottlieb, Bligh & Dyer, and modified Bligh & Dyer extraction methods. J. Food Compos. Anal. 2001, 14 (1), 93−100. (27) CRC Handbook of Chemistry and Physics, 94th ed.; Haynes, W. M., Lide, D. R., Bruno, T. J., Eds.; CRC Press (Taylor & Francis Group): Boca Raton, FL, 2013.
(4.8%) and the water bath shaking method (5.1%). However, the purity of the biodiesel produced from the lipids extracted using acid hydrolysis extraction (97.5%) was higher than that of the other two methods (94.3 and 83.3%). The water bath shaking method was simple and extracted once, which is so easy to lead to impurity of the extracted material. On the contrary, the yield of the biodiesel produced from the lipids extracted using acid hydrolysis extraction was low, while it is more pure. It might be that some lipid was decomposed and reacted into some fatty acid. It proved that the lipids extracted by the acid hydrolysis method was more pure.
5. CONCLUSION Three methods were used to extract lipids from dry sewage sludge. Soxhlet extraction can extract more lipids (6.0%) from sludge compared to acid hydrolysis extraction (4.8%) and the water bath shaking method (5.1%). Different solvents require different optimal extraction methods. Moreover, sewage sludge has higher polar lipid content than nonpolar lipids. The theoretical concentration of lipids in sewage sludge was about 12%. The maximum extraction rates of Soxhlet extraction, acid hydrolysis extraction, and the water bath shaking method were 85.8, 62.5, and 61.7%, respectively. The true yield of biodiesel produced from the lipids extracted using the Soxhlet method was higher than that of the other two methods.
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AUTHOR INFORMATION
Corresponding Author
*Telephone/Fax: +86-10-82502694. E-mail: zhufenfen@ruc. edu.cn. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research is from a project preliminary study of lipid profiles and characteristics in activated sludge (51308538). The study is supported by the National Natural Science Foundation of China and the Key Laboratory for Solid Waste Management and Environment Safety (SWMES 2011-12).
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REFERENCES
(1) Zhu, F.; Zhao, L.; Zhang, Z.; Jiang, H. Preliminary study at lipids extraction technology from municipal sludge by organic solvent. Procedia Environ. Sci. 2012, 16, 352−356. (2) Pérez, M. A.; Aracil, I.; Fullana, A. Biodiesel production by transesterification of sludge in supercritical conditions. Fuel 2013, 109, 427−431. (3) Angerbauer, C.; Siebenhofer, M.; Mittelbach, M.; Guebitz, G. M. Conversion of sewage sludge into lipids by Lipomyces starkeyi for biodiesel production. Bioresour. Technol. 2008, 99, 3051−3056. (4) Zhu, F.; Zhang, Z.; Jiang, H.; Zhao, L. The study of sewage sludge thermo-drying efficiency. Procedia Environ. Sci. 2012, 16, 363−367. (5) Kargbo, D. M. Biodiesel production from municipal sewage sludges. Energy Fuels 2010, 24, 2791−2794. (6) Li, Y.; Chen, T.; Luo, W.; Huang, Q.; Wu, J. Contents of organic matter and major nutrients and the ecological effect related to land application of sewage sludge in China. Acta Ecol. Sin. 2003, 23 (11), 2464−2474. (7) Seo, Y.; Lee, I.; Han, J. Cultivation and lipid production of yeast Cryptococcus curvatus using pretreated waste active sludge supernatant. Bioresour. Technol. 2013, 135, 304−308. (8) Siddiquee, M. N.; Rohani, S. Experimental analysis of lipid extraction and biodiesel production from wastewater sludge. Fuel Process. Technol. 2011, 92, 2241−2251. 5283
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