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Biodiesel production from wet municipal sludge: Evaluation of in situ transesterification using xylene as a cosolvent
Accepted Manuscript Biodiesel production from wet municipal sludge: Evaluation of in-situ transesterification using xylene as a cosolvent O.K. Choi, J.S. Song, D.K. Cha, J.W. Lee PII: DOI: Reference:
S0960-8524(14)00660-9 http://dx.doi.org/10.1016/j.biortech.2014.05.001 BITE 13409
To appear in:
Bioresource Technology
Received Date: Revised Date: Accepted Date:
26 February 2014 26 April 2014 2 May 2014
Please cite this article as: Choi, O.K., Song, J.S., Cha, D.K., Lee, J.W., Biodiesel production from wet municipal sludge: Evaluation of in-situ transesterification using xylene as a cosolvent, Bioresource Technology (2014), doi: http://dx.doi.org/10.1016/j.biortech.2014.05.001
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Biodiesel production from wet municipal sludge: Evaluation of in-situ transesterification using xylene as a cosolvent O. K. Choi1, J. S. Song2 *, D. K. Cha3, J. W. Lee1,2*
1
Program in Environmental Technology and Policy, Korea University, Sejong,
339-700, Korea 2
Department of Environmental Engineering, College of Science and Technology,
Korea University, Sejong, 339–700, Korea 3
Department of Civil and Environmental Engineering, University of Delaware, Newark,
DE, USA
* Corresponding author1: Tel) +82-44-860-1456; Fax) +82-44-860-1588; E-mail) [email protected] * Corresponding author2: Tel) +82- 44-860-1461; Fax) +82-44-860-1588; E-mail) [email protected]
1
Abstract This study proposes a method to produce biodiesel from wet wastewater sludge. Xylene was used as an alternative cosolvent to hexane for transesterification in order to enhance the biodiesel yield from wet wastewater sludge. The water present in the sludge could be separated during transesterification by employing xylene, which has a higher boiling point than water. Xylene enhanced the biodiesel yield up to 8.12%, which was 2.5 times higher than hexane. It was comparable to the maximum biodiesel yield of 9.68% obtained from dried sludge. Xylene could reduce either the reaction time or methanol consumption, when compared to hexane for a similar yield. The fatty acid methyl esters (FAMEs) content of the biodiesel increased approximately two fold by changing the cosolvent from hexane to xylene. The transesterification method using xylene as a cosolvent can be applied effectively and economically for biodiesel recovery from wet wastewater sludge without drying process.
Keywords Biodiesel, wet wastewater sludge, cosolvent, in-situ transesterification, xylene
2
1. Introduction Biodiesel is a well-known renewable fuel, which is commonly produced by the transesterification of fatty acids or triglycerides using alcohols such as methanol or ethanol, sometimes together with a catalyst. It has several environmental benefits over petroleum diesel including, higher biodegradability and lower harmful emissions (Ma and Hanna, 1999; Krawczyk, 1996). Currently, various types of vegetable oils and animal fats are utilized as biodiesel feedstock. However, their use is arguable due to the competition of these edible oils as a food source. Furthermore, the cost for feedstock is extremely high and accounts for more than 70% of the total cost of biodiesel production (Kargbo, 2010). Therefore, many researchers have placed their efforts into finding a cost-effective alternative (Chhetri et al., 2008; Zhou et al., 2013). Recently, wastewater sludge has been studied as an alternative feedstock and its relevant technologies have been developed (Hass et al., 2007; Hayyan et al., 2010; Huynh et al., 2012). Reusing wastewater sludge as an energy feedstock is urgent and popular in some countries that have small plantation area and high population density, such as many European and Asian countries. Moreover, interests in the development of these technologies are enlarged above the countries mentioned.
The amount of wastewater sludge production in the European Union (EU) and United States was approximately 10.1 and 7.1 million dry tons per year, respectively (Gendebien, 2010; Beech et al., 2007). Annual production of wastewater sludge in Korea was 3.0 million tons (m3) dewatered sludge cake in 2011 and it is expected to continue increasing into the future due to increased sewer services (Ministry of Environment in Korea, 2011). Ocean dumping has long been one of major disposal methods for wastewater sludge in
3
Korea; however, it was banned in 2012, according to London convention ’96 protocol. Therefore, biodiesel recovery from wastewater sludge could be a possible option of converting negative value waste into high-value product with concomitant elimination of wastewater sludge. Even though biodiesel production from wastewater sludge could be beneficial, there are still problems that need to be addressed, especially the high water content. Since the esterification process produces water as shown in equation 1, the reaction slows down and eventually stops if water is present in excess amount.
Fatty acid (RCOOH) + Alcohol (ROH) ↔ Alkyl ester (RCOOR) + Water (H2O)
(1)
Ma and Hanna (1999) suggested that the water content of the feedstock should be kept below 0.06% for biodiesel production in order to prevent deterioration of the catalyst, which adversely affect the
transesterification.
Direct application of in-situ
transesterification to wet sludge resulted in a lower biodiesel yield and high methanol consumption (Revellame et al., 2011). Biodiesel derived from sludge could be economical if the water content is reduced to at least 50%. Therefore, wastewater sludge needed to be dried to achieve efficient in-situ transesterification (Revellame et al., 2010; Mondala et al., 2009; Dufreche et al., 2007). However, drying wet sludge is a costly pretreatment step, which is energy intensive (Dufreche et al., 2007). Even though the water content in sludge is a crucial factor in biodiesel production, few technologies other than drying have been developed to address this problem. The overarching goal of this study was to develop a method that enhances the biodiesel yield from wet wastewater sludge through the modification of a conventional in-situ
4
transesterification method. In this study, hexane, which is typically used in in-situ transesterification, was replaced by xylene. Xylene is a cosolvent with a higher boiling point (138.5 °C) than water. This ensures that water is separated during the in-situ transesterification reaction. The specific objectives were: 1) to characterize biodiesel production from two different types of dried primary and secondary sludge, 2) to evaluate the productivity and quality of biodiesel from wet wastewater sludge when the cosolvent was changed from hexane to xylene, and 3) to estimate the cost effectiveness of the transesterification method using xylene. The productivity and quality were evaluated based on by biodiesel yield and fatty acid methyl esters (FAMEs) composition, respectively.
2. Materials and methods 2.1. Sludge sample and chemicals The dewatered wastewater sludge was obtained from Osong city wastewater treatment plant in Korea. It was a mixture of primary and secondary sludge collected following the centrifugal dewatering process. Alternatively, primary and secondary wastewater sludge was obtained separately from different wastewater treatment plants located at Daejeon city in Korea. The sludge was collected from each inlet pipe into an anaerobic digester. The sludge samples were stored in a refrigerated container during transportation to the laboratory. The sludge samples were immediately analyzed for total solids (TS), volatile solids (VS), and moisture content (MC), before being stored in a refrigerator at 4 °C. The primary and secondary sludge samples were dried in a furnace at 80 °C for 48 hours, in order to obtain the dried sludge samples. The dried sludge samples were ground and
5
sieved to pass through a 100 mesh. The dewatered sludge was under wet conditions with a high MC of 85%, whereas the dried sludge had < 5% MC. Commercial grade (99.8%) methyl alcohol anhydrous, laboratory grade (95%) sulfuric acid (H2SO4), 99.5% xylene, and commercial grade (95%) n-hexane were purchased from Samchun chemicals in Korea.
2.2. in-situ transesterification Two different types of sludge, dried and dewatered, were prepared for in-situ transesterification. Table 1 shows the reaction conditions for in-situ transesterification methods applied to these sludges. The dried sludge was treated using a conventional method, whereas the wet sludge was treated using a method developed for this study, which uses xylene as the cosolvent instead of hexane.
2.3. in-situ transesterification for dried wastewater sludge Figure 1(a) shows the soxhlet extractor used for the in-situ transesterification of the dried wastewater sludge. The method involved adding 22.5 g of dried sludge into a 500 mL reaction flask containing methanol and n-hexane. The methanol solution containing 5% H2SO4 and n-hexane were blended at 1:1 ratio (v/v), and the methanol solution was added at three different volumes of 45, 112.5, and 225 mL in order to determine optimal methanol dosage. The mixture in the flask was then well stirred for 8 hours of reaction. The flask was connected to a condenser and heated in a water bath at 55 °C. After the reaction ended, the mixture was cooled overnight at room temperature and then the supernatant phase was recovered by centrifugation at 300 rpm for 5 minutes. The collected supernatant was washed three times with 50 mL deionized water and then 1 g of
6
sodium sulfate (Na2SO4) was added. The samples were filtered to remove the water using Whatman GF/C glass fiber filters. The remaining n-hexane in the supernatant was vaporized using a vacuum and the final supernatant was collected as the biodiesel sample.
2.4. in-situ transesterification for dewatered (wet) wastewater sludge A 1-L flask fitted with a reflux condenser was used for this experiment. The condenser was designed to recirculate the condensed cosolvent continuously into the reaction flask after separation of cosolvent and solvent from water as shown in Figure 1 (b). This was followed by addition of 150 g of dewatered wastewater sludge into the flask containing 300 mL each of methanol solution and n-hexane or xylene. The methanol solution contained 15 mL of H2SO4, which acts as a catalyst. The mixture in the flask was then stirred at 100 rpm using a paddle-type agitator during the reaction. The reaction time varied from 1–8 h as shown in Table 1. The reaction temperatures were set at 55 and 105 °C for n-hexane and xylene, respectively. The dewatered-dried sludge was also used as a control and its transesterification condition is shown in Table 1. Methanol, water, and part of the xylene were vaporized when xylene was used as the cosolvent and both methanol and xylene were recirculated into the flask through the condenser. The condensed water phase was discarded to prevent it from flowing back into the flask during the reaction. Conversely, water did not vaporize and remained at the flask during the reaction that used hexane, which has a lower boiling point than water. The mixture was cooled overnight at room temperature after the reaction ended. Once the supernatant was collected separately from the flask, it was further treated by a series of polishing steps as described above, which included centrifugation, rinse, water removal, and vaporization followed by collection.
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2.5. Analyses for biodiesel Biodiesel samples were analyzed quantitatively and qualitatively to determine the biodiesel yield and FAME composition. The biodiesel yield from the sludge sample and FAME content of crude biodiesel was calculated based on the weight of the product as shown in equations 2 and 3 below:
Biodiesel yield (%) =
weight of the product (g) dried solids in the sludge (g)
ⅹ 100
(2)
ⅹ 100
(3)
Total FAMEs weight (g) FAME content (%) =
Biodiesel weight (g)
The FAME composition of the extracted biodiesel was analyzed using Gas Chromatograph (GC, Agilent 7890A, Agilent, USA) equipped with a mass selective detector (GC/MS, Agilent 5975C, Agilent, USA). The HP-5MS capillary column (30 m x 0.32 mm x 0.25 µm) was used for FAME analysis. Analytical conditions of GC/MS were as follows: the injector and detector temperatures were 250 and 280 °C, respectively; column program initial temperature of 50 °C was set for 1 minute; then ramped from 50 to 200 °C at 25 °C/min; and then ramped from 200 to 230 °C at 3 °C/min; and finally maintained at 230 °C for 18 minutes. The conditions were optimized by slightly modifying those of FAMEs analysis suggested by Lee et al., (2013). The injection sample volume was 1 µL and the split ratio was 1:50. Nitrogen was used as a carrier gas with a flow rate of 1 mL/min. Each hexane and xylene extract sample was diluted using hexane at 20 and 100 times, respectively. The FAME profiles and concentration was identified by
8
comparing it with a standard solution (37 comp. FAME mix, Supelco, USA) and NIST 2008 Libraries installed in GC/MS.
2.6. Statistical analyses Statistical analyses were performed using SPSS v12.0 for Windows software (SPSS v12.0 IBM Corporation, Somers, NY). All data were expressed as the mean ± standard deviation (SD). Any significant difference between the FAME profiles was determined by using two-way analysis of variance (ANOVA). The significance for all p values was 0.05 unless otherwise stated.
3. Results and discussion 3.1. Biodiesel production from dried wastewater sludge Biodiesel was produced from in-situ transesterification of primary, secondary dried wastewater sludge. Figure 2 shows the biodiesel yields obtained from these feedstocks depending on the methanol dosage used. Even though the biodiesel yield was not significantly different among the three sludge types, the secondary sludge showed a slightly higher yield except at 5 mL methanol/g dried sludge dosage. The biodiesel yield proportionally increased with the dosage of methanol to sludge up to 5 mL methanol/g dried sludge for both types of sludge. As methanol dosage further increased to 10 mL/g, the biodiesel yield continued increasing for the secondary sludge but not for the primary sludge. The maximum biodiesel yield was 9.68% when secondary sludge was used as the feedstock with the addition of 10 mL methanol/g dried sludge. It was similar to the maximum biodiesel yields of activated sludge presented in other studies (Dufreche et al., 2007; Mondala et al., 2009). This difference in biodiesel yield between the two types of
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sludge feedstock may be attributed to the types of lipids present in each. The major biodiesel source is phospholipid fatty acids in microbial membrane and triglyceride for secondary and primary wastewater sludge, respectively (Fukuda et al., 2001). Fatty acids are transformed into FAMEs via transesterification; however, glycerol remains as a byproduct when triglyceride is used as feedstock (Siddiquee and Rohani, 2011). The excess addition of methanol with hydroxyl groups results in more generation of emulsion with the remaining glycerol in the biodiesel phase in the transesterification step (Sun et al., 2008). Figure 3 shows weight percentage of the FAMEs identified in the biodiesel by conventional in-situ transesterification. GC/MS was used for identification of various FAMEs (data not shown). The abundance of individual FAME was calculated by using a standard solution as described above. Characterization of FAME constituents in biodiesel is important since the properties of biodiesel are strongly dependent on its FAME constituents (Knothe, 2005). In the crude biodiesel derived from the dried sludge, FAMEs between C12:0 and C24:0 were identified by GC/MS. Based on these FAMEs, methyl esters of palmitic acid (C16:0), palmitoleic acid (C16:1), stearic acid (C18:0), and oleic acid (C18:1n9c) were predominant components regardless of sludge type. The sum of these four FAMEs was 65.2 and 63.2% of the total FAMEs identified for primary and secondary wastewater sludge, respectively (Figure 3).
The total weight percent of FAMEs identified was 79.6 and 78.5% of the gravimetric biodiesel yield of the primary and secondary wastewater sludge, respectively. Even though overall profiles of the FAME components between the two dried sludges were not statistically different (p < 0.05), their structure appeared to be different. Biodiesel derived
10
from primary sludge contained saturated straight FAMEs as well as C16:0 and C18:0, while, biodiesel derived from secondary sludge had unsaturated FAMEs as well as C16:1 and C18:1. Saturated FAMEs have significantly better burning properties than unsaturated FAMEs (Ritz and Croudace, 2003; Knothe, 2005). Conversely, unsaturated FAMEs exhibited better fluidity under cold temperature condition than the saturated species (Kenesey and Ecker, 2003). Even though it is hard to evaluate the biodiesel property based upon only FAME results, the FAME profiles of biodiesel derived from both sludges are in good agreement with the previous studies (Mondala et al., 2009; Dufreche et al., 2007; Revellame et al., 2011). According to those previous studies, dried wastewater sludge containing triglyceride or phospholipids could be suitable feedstocks for biodiesel production (Revellame et al., 2010).
3.2. Biodiesel production from dewatered wastewater sludge Biodiesel was produced from in-situ transesterification of dewatered wastewater sludge by using two cosolvents: hexane or xylene. Figure 4 shows a comparison of the gravimetric yield between the two methods at various reaction times. These two methods were differentiated by the cosolvent type and reaction temperature. Xylene allowed a higher reaction temperature due to its higher boiling point (105 °C). Biodiesel yield from the dewatered sludge was limited with the use of hexane as the cosolvent. The maximum biodiesel yield from dewatered sludge was only 3.28%, which was less than half of the yield from dried sludge. However, the use of xylene as a cosolvent replacing hexane markedly enhanced the biodiesel yield. Maximum biodiesel yield with xylene was 8.12%, which was 2.5 times greater than that of the dewatered sludge and a little lower than that of dewatered-dried sludge when hexane was used as a cosolvent (Table 2).
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Wastewater sludge is produced in most wastewater treatment plants in the form of dewatered sludge prior to final disposal; however, its moisture content is still more than 70%. It was obvious that the presence of water caused detrimental effects on transesterification for biodiesel production. Ma and Hanna (1999) explained that presence of water might cause soap formation and lead to a decrease in the efficiency of the catalyst during the transesterification. According to Qian et al. (2009), transesterification efficiency of cottonseed oil to methyl ester increased from 80 to 98% when decreasing the moisture content from 8.7 to 1.9%. In this study, water was avoided during transesterification by employing the modified extraction system shown in Figure 1 (b) together with a high-boiling point cosolvent, xylene.
FAMEs in biodiesel obtained from 8 hours of transesterification were analyzed by GC/MS and the results are shown in Figure 5. The FAMEs profiles obtained from two in-situ transesterification methods depending on cosolvent were significantly different (p >0.05). The number of FAMEs identified was 13 and 7 FAMEs for hexane and xylene methods, respectively. Unlike xylene transesterification, the biodiesel obtained from hexane transesterification contains various components other than FAMEs, which might denote impurities in the biodiesel. Therefore, the use of xylene contributed to a better property of crude biodiesel, with higher FAME contents. Figure 6 shows the quantitative results of FAMEs identified. Total FAMEs content of xylene transesterification was 81.9% of the crude biodiesel. This value was twice the hexane transesterification (45.0% of the crude biodiesel) and was greater than biodiesels from dried sludge (Figure 6). The most dominant FAME in xylene transesterification was methyl esters of myristic acid
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(C14:0) with 23.85% in biodiesel (w/w) but the following FAMEs were also similarly noticeable: pentadecylic acid (C15:0), palmitic acid (C16:0), oleic acid (C18:1n9c), stearic acid (C18:0), and lignoceric (C24:0). Alternatively, FAMEs in hexane transesterification contained almost three major components of palmitoleic acid (C16:1), palmitic acid (C16:0), and oleic acid (C18:1n9c). Even though there were differences in FAMEs composition between the two transesterification methods, the major components identified are typically present in common biodiesels from wastewater sludge (Dufreche et al., 2007; Mondala et al., 2009; Revellame et al., 2010).
It was notable that the obtained FAME composition was very dependent on the transesterification method such as cosolvent type as well as the feedstock. When the same transesterification was applied, the FAME composition of the dewatered sludge was similar to that of dried wastewater sludge, particularly for the secondary dried sludge shown in Figure 2. However, it significantly shifted by adopting a different transesterification method using xylene as the cosolvent: the content of unsaturated fatty acids was lowered. The use of xylene enabled an increased reaction temperature, which enhanced the biodiesel burning property by producing significantly less unsaturated FAMEs. In this study, other biodiesel properties were not evaluated but this finding supported the validity of the assumption that biodiesel yield and quality were enhanced due to the avoidance of polymerization under high temperature conditions (Revellame et al., 2010). As the results suggest, the transesterification method using xylene was considered a more promising method by enhancing both the yield and quality of the biodiesel from wet wastewater sludge.
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3.3. Evaluation of applicability of modified transesterification with xylene for biodiesel production from wet sludge: technical and economic analysis Table 2 compares the maximum biodiesel yield depending on sludge type and cosolvent used. Data for biodiesel yield represent mean ± standard deviation for duplicate samples. Xylene was more the efficient cosolvent in production yield of biodiesel from wet sludge. Biodiesel yield from wet sludge was comparable to that of three different types of dried sludge. Xylene’s high-boiling point enabled an increase in the reaction temperature up to around 100 °C, which led to vaporization of water out of the reaction vessel through a specially designed unit, during the reaction. Therefore, xylene could boost the reaction kinetics and reduce consumption of methanol for the same biodiesel yield due to high temperature and avoidance of water during transesterification (Figure 4).
Table 3 shows the economic analysis for biodiesel production from both dried and wet wastewater sludge which were the mixture of primary and secondary sludge. The economic analysis was conducted to estimate the cost required for 1 L of biodiesel production from wastewater sludge based on the experimental results. The cost for dewatering was equally estimated assuming that a centrifugation process was applied (Dufreche et al., 2007). The estimated cost of drying process was based on the electrical power requirement (641.8 kWh/ton wet sludge) for dehydrating the dewatered sludge by reducing the moisture content from 85% to 5% (Gonçalves et al., 2007). Capital cost for the drying process was not included in this analysis. Chemical costs for methanol, hexane and xylene were estimated assuming recovery and reuse of those chemicals after
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transesterification for 40 batch cycles. The cost for the electricity was based on the total energy requirement for in situ transesterification reaction for 8 hr.
The use of xylene for biodiesel production from wet sludge resulted in great cost saving compared to hexane. Even though the use of xylene was the least costly method for wet sludge, cost saving was not significant when compared to dried sludge. This might be mainly attributed to the higher energy consumption, the unit price of which varies from country to country. However, the drying process is very costly with high capital and operational costs, thus it is not feasible in a wastewater treatment plant (WWTP). The operational cost for drying accounted for 50% of total cost necessary for biodiesel production from dried sludge. The in-situ transesterification using xylene suggested in this study will be a promising option that is applicable to real WWTPs because a similar productivity of biodiesel from wet sludge is achievable without the installation of additional unit processes.
4. Conclusions Biodiesel production from wet wastewater sludge was enhanced by replacing cosolvent from hexane to xylene showing a similar yield from dried wastewater sludge by conventional transesterification. FAME contents and the quality of biodiesel were also improved. In spite of the higher biodiesel yield, the total number of FAMEs was smaller but it included major indispensable components for high quality biodiesel. The use of xylene for biodiesel derived from wet sludge did not increase the cost compared to conventional transesterification using hexane for dried sludge, and this method will be more readily applicable to most of WWTP without the drying process.
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Acknowledgement This study was supported by a grant from the Korea University. The authors are grateful for useful assistance from Woon Oh Cha of TSK Water and Jung Bum Ahn of SK Chemicals.
References 1. Beech N., Crawford K., Goldstein N., Kester G., Lono-Batura M., dzieyk E., 2007. A national biosolids regulation, quality, end use and disposal survey: Final report. North East Biosolids and Residuals Association(NEBRA), Tamworth, NH. 2. Chhetri A. B., Tango M. S., Budge S. M., Watts K. C., Islam M. R., 2008. Non-edible plant oils as new sources for biodiesel production. Int. J. Mol. Sci. 9, 169-180. 3. Dufreche S., Hernandez R., French T., Sparks D., Zappi M., Alley E., 2007. Extraction of lipids from municipal wastewater plant microorganisms for production of biodiesel. J. Am. Oil Chem. Soc. 84, 181-187. 4. Fukuda, H., Kondo A., Noda H., 2001. Biodiesel fuel production by transesterification of oils. J. Biosci. Bioeng. 92, 405-416. 5. Gendebien A., 2010. Environmental, economic and social impacts of the use of sewage sludge on land: Final Report, Assessment of Existing Knowledge. Milieu Ltd and WRc for the European Commission. 6. Gonçalves R.F., Luduvice M., Sperling M.V., 2007. Sludge thickening and dewatering, in: Andreoli C.V., Sperling M.v., Fernandes F. (Eds.), Sludge Treatment and Disposal. IWA Publishing, London, pp. 76–119.
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7. Haas M. J., Scott K. M., Foglia T. A., Marmer W, N., 2007. The general applicability of in situ transesterification for the production of fatty acid esters from a variety of feedstocks. J. Am. Oil Chem. Soc. 84, 963-970. 8. Hayyan A., Alam M. Z., Mirghani M. E. S., Kabbashi N. A., Hakimi N. I. M. N., Siran Y. M., Tahiruddin S., 2010. Sludge palm oil as a renewable raw material for biodiesel production by two-step processes. Bioresour. Technol. 101, 7804-7811. 9. Huynh L. H., Phuong L. T. N., Ho Q. O., Ju Y. H., 2012. Catalyst-free fatty acid methyl ester production from wet activated sludge under subcritical water and methanol condition. Bioresour. Technol. 123, 112-116. 10. Kargbo D. M., 2010. Biodiesel Production from Municipal Sewage Sludges. Energy Fuels 24, 2791-2794. 11. Kenesey E., Ecker A., 2003. Oxygen connections to the improvement of the lubricity in fuels. Tribologie und Schmierungstechnik 50, 21-26. 12. Knothe G., 2005. Dependence of biodiesel fuel properties on the structure of fatty acid alkyl esters. Fuel Process. Technol. 86, 1059-1070. 13. Korea Electric Power Corporation, 2013. Electric Rates Table http://cyber.kepco.co.kr/ckepco/front/jsp/CY/E/E/CYEEHP00203.jsp accessed in March 16. 14. Krawczyk T., 1996. Biodiesel-alternative fuel makes inroads but hurdles remain. Inform 7, 800-815. 15. Lee K. H., Park K. Y., Khanal S. K., Lee J. W., 2013. Effects of household detergent on anaerobic fermentation of kitchen wastewater from food waste disposer. J. Hazard. Mater. 244, 39-45 16. Ma F. R., Hanna M. A., 1999. Biodiesel production: a review. Bioresour. Technol. 70,
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1-15. 17. Ministry of Environment in Korea, 2011. The state of waste generation and treatment in 2011. Seoul, Korea. 18. Mondala A., Liang K. W., Toghiani H., Hernandez R., French T., 2009. Biodiesel production by in situ transesterification of municipal primary and secondary sludges. Bioresour. Technol. 100, 1203-1210. 19. Qian J. F., Wang F., Liu S., Yun Z., 2008. In situ alkaline transesterification of cottonseed oil for production of biodiesel and nontoxic cottonseed meal. Bioresour. Technol. 99, 9009-9012. 20. Revellame E., Hernandez R., French W., Holmes W., Alley E., 2010. Biodiesel from activated sludge through in situ transesterification. J. Chem. Technol. Biot. 85, 614-620. 21. Revellame E., Hernandez R., French W., Holmes W., Alley E., Callahan R., 2011. Production of biodiesel from wet activated sludge. J. Chem. Technol. Biot. 86, 61-68. 22. Ritz G. P., Croudace M. C., 2003. Biodiesel/FAME analysis. Hydrocarb. Eng. 8, 83-84. 23. Siddiquee M. N. and Rohani S. 2011. Experimental analysis of lipid extraction and biodiesel production from wastewater sludge. Fuel Process. Technol. 92, 2241-2251. 24. Sun J., Ju J. X., Ji L., Zhang L. X., Xu N. P. 2008. Synthesis of biodiesel in capillary microreactors. Ind. Eng. Chem. Res. 47, 1398-1403. 25. Zhou X. P., Ge H. M., Xia L., Zhang D. L., Hu C. X., 2013. Evaluation of oil-producing algae as potential biodiesel feedstock. Bioresour. Technol. 134, 24-29.
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Figure Captions
Figure 1. Schematics of reactor system for in-situ transesterification for (a) dried sludge and (b) wet sludge. Figure 2. Gravimetric biodiesel yield as function of methanol to sludge ratio Figure 3. Weight percentage of FAMEs idenitified in biodiesel derived from each dried sludge. Figure 4. Effects of cosolvent on gravimetric biodiesel yield as a function of reaction time. Methanol to sludge dose was 2 mL/g. Figure 5. FAMEs profiles of the biodiesel produced from wet wastewater sludge through in-situ transesterification with 8 hr reaction by using (a) hexane and (b) xylene as a cosolvent. Transesterification with hexane and xylene was diluted with hexane by 20 and 100 times before GC analysis, respectively. Figure 6. Weight percentage of FAMEs components in biodiesel derived from dewatered sludge.
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Table 1. Co-solvent and reaction condition for in-situ transesterification depending on sludge types
Sludge Type
Reaction
Methanol
Time
Dosage
(hr)
(mL/g sludge)
Reaction
Moisture Cosolvent
Temp. (°C)
Content
Dried (primary)
< 5%
n-Hexane
55
8
2, 5, 10
Dried (secondary)
< 5%
n-Hexane
55
8
2, 5, 10
n-Hexane Dewatered*
55
1, 2, 8
2
85% Xylene
105
1, 2, 8
2
n-Hexane
55
8
10
Dewatered-Dried*
< 5%
* mixed with primary and secondary sludge
20
Table 2. Comparison of maximum biodiesel yield under different condition attempted in this study
Sludge Type
Methanol to Sludge
Biodiesel Yield
Ratio (mL/g)
(%)
Cosolvent
Primary Dried
n-Hexane
10
8.04 ± 0.50
Secondary Dried
n-Hexane
10
9.68 ± 0.39
n-Hexane
2
3.28 ± 0.40
Xylene
2
8.12 ± 0.11
n-Hexane
10
9.54 ± 0.39
Dewatered*
Dewatered-Dried **
* mixed with primary and secondary sludge ** dewatered sludge after drying
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Table 3. Economic analysis for in-situ transesterification of dried and wet activated sludge
Unit Price
Dried sludge
Wet sludge
A. Feedstock preparation 1. Centrifugation
0.11 a
2. Drying
($/L)
0.11
0.11
0.11
0.034b ($/kg)
2.03
-
-
1. Methanol
0.25 c
($/L)
0.58
2.14
0.86
2. Catalyst
0.046c ($/L)
0.21
0.79
0.32
n-Hexane
0.29 c
($/L)
0.68
2.50
-
Xylene
0.12 c
($/L)
-
1.01
0.05 d
($/kWh)
0.43
1.58
1.22
($/L)
4.05
7.12
3.52
B. Chemicals
3. Cosolvent
C. Electricity D. Total a
Dufreche et al., 2007 Gonçalves et al., 2007 c International Construction Information Society, 2013 (http://www. icis.com) d Korea Electric Power Corporation, 2013 b
22
(a)
(b) Figure 1
23
Figure 2
24
Figure 3
25
Figure 4
26
(a)
(b) Figure 5
27
Figure 6
28
Highlights A new method for biodiesel production from wet wastewater sludge was suggested. Biodiesel yield increased by 2.5 times when hexane was replaced with xyl ene. Both fatty acid methyl esters (FAMEs) contents and quality were enhance d. Drying process was not necessary as a pretreatment for biodiesel product ion.
29
S0960-8524(14)00660-9 http://dx.doi.org/10.1016/j.biortech.2014.05.001 BITE 13409
To appear in:
Bioresource Technology
Received Date: Revised Date: Accepted Date:
26 February 2014 26 April 2014 2 May 2014
Please cite this article as: Choi, O.K., Song, J.S., Cha, D.K., Lee, J.W., Biodiesel production from wet municipal sludge: Evaluation of in-situ transesterification using xylene as a cosolvent, Bioresource Technology (2014), doi: http://dx.doi.org/10.1016/j.biortech.2014.05.001
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Biodiesel production from wet municipal sludge: Evaluation of in-situ transesterification using xylene as a cosolvent O. K. Choi1, J. S. Song2 *, D. K. Cha3, J. W. Lee1,2*
1
Program in Environmental Technology and Policy, Korea University, Sejong,
339-700, Korea 2
Department of Environmental Engineering, College of Science and Technology,
Korea University, Sejong, 339–700, Korea 3
Department of Civil and Environmental Engineering, University of Delaware, Newark,
DE, USA
* Corresponding author1: Tel) +82-44-860-1456; Fax) +82-44-860-1588; E-mail) [email protected] * Corresponding author2: Tel) +82- 44-860-1461; Fax) +82-44-860-1588; E-mail) [email protected]
1
Abstract This study proposes a method to produce biodiesel from wet wastewater sludge. Xylene was used as an alternative cosolvent to hexane for transesterification in order to enhance the biodiesel yield from wet wastewater sludge. The water present in the sludge could be separated during transesterification by employing xylene, which has a higher boiling point than water. Xylene enhanced the biodiesel yield up to 8.12%, which was 2.5 times higher than hexane. It was comparable to the maximum biodiesel yield of 9.68% obtained from dried sludge. Xylene could reduce either the reaction time or methanol consumption, when compared to hexane for a similar yield. The fatty acid methyl esters (FAMEs) content of the biodiesel increased approximately two fold by changing the cosolvent from hexane to xylene. The transesterification method using xylene as a cosolvent can be applied effectively and economically for biodiesel recovery from wet wastewater sludge without drying process.
Keywords Biodiesel, wet wastewater sludge, cosolvent, in-situ transesterification, xylene
2
1. Introduction Biodiesel is a well-known renewable fuel, which is commonly produced by the transesterification of fatty acids or triglycerides using alcohols such as methanol or ethanol, sometimes together with a catalyst. It has several environmental benefits over petroleum diesel including, higher biodegradability and lower harmful emissions (Ma and Hanna, 1999; Krawczyk, 1996). Currently, various types of vegetable oils and animal fats are utilized as biodiesel feedstock. However, their use is arguable due to the competition of these edible oils as a food source. Furthermore, the cost for feedstock is extremely high and accounts for more than 70% of the total cost of biodiesel production (Kargbo, 2010). Therefore, many researchers have placed their efforts into finding a cost-effective alternative (Chhetri et al., 2008; Zhou et al., 2013). Recently, wastewater sludge has been studied as an alternative feedstock and its relevant technologies have been developed (Hass et al., 2007; Hayyan et al., 2010; Huynh et al., 2012). Reusing wastewater sludge as an energy feedstock is urgent and popular in some countries that have small plantation area and high population density, such as many European and Asian countries. Moreover, interests in the development of these technologies are enlarged above the countries mentioned.
The amount of wastewater sludge production in the European Union (EU) and United States was approximately 10.1 and 7.1 million dry tons per year, respectively (Gendebien, 2010; Beech et al., 2007). Annual production of wastewater sludge in Korea was 3.0 million tons (m3) dewatered sludge cake in 2011 and it is expected to continue increasing into the future due to increased sewer services (Ministry of Environment in Korea, 2011). Ocean dumping has long been one of major disposal methods for wastewater sludge in
3
Korea; however, it was banned in 2012, according to London convention ’96 protocol. Therefore, biodiesel recovery from wastewater sludge could be a possible option of converting negative value waste into high-value product with concomitant elimination of wastewater sludge. Even though biodiesel production from wastewater sludge could be beneficial, there are still problems that need to be addressed, especially the high water content. Since the esterification process produces water as shown in equation 1, the reaction slows down and eventually stops if water is present in excess amount.
Fatty acid (RCOOH) + Alcohol (ROH) ↔ Alkyl ester (RCOOR) + Water (H2O)
(1)
Ma and Hanna (1999) suggested that the water content of the feedstock should be kept below 0.06% for biodiesel production in order to prevent deterioration of the catalyst, which adversely affect the
transesterification.
Direct application of in-situ
transesterification to wet sludge resulted in a lower biodiesel yield and high methanol consumption (Revellame et al., 2011). Biodiesel derived from sludge could be economical if the water content is reduced to at least 50%. Therefore, wastewater sludge needed to be dried to achieve efficient in-situ transesterification (Revellame et al., 2010; Mondala et al., 2009; Dufreche et al., 2007). However, drying wet sludge is a costly pretreatment step, which is energy intensive (Dufreche et al., 2007). Even though the water content in sludge is a crucial factor in biodiesel production, few technologies other than drying have been developed to address this problem. The overarching goal of this study was to develop a method that enhances the biodiesel yield from wet wastewater sludge through the modification of a conventional in-situ
4
transesterification method. In this study, hexane, which is typically used in in-situ transesterification, was replaced by xylene. Xylene is a cosolvent with a higher boiling point (138.5 °C) than water. This ensures that water is separated during the in-situ transesterification reaction. The specific objectives were: 1) to characterize biodiesel production from two different types of dried primary and secondary sludge, 2) to evaluate the productivity and quality of biodiesel from wet wastewater sludge when the cosolvent was changed from hexane to xylene, and 3) to estimate the cost effectiveness of the transesterification method using xylene. The productivity and quality were evaluated based on by biodiesel yield and fatty acid methyl esters (FAMEs) composition, respectively.
2. Materials and methods 2.1. Sludge sample and chemicals The dewatered wastewater sludge was obtained from Osong city wastewater treatment plant in Korea. It was a mixture of primary and secondary sludge collected following the centrifugal dewatering process. Alternatively, primary and secondary wastewater sludge was obtained separately from different wastewater treatment plants located at Daejeon city in Korea. The sludge was collected from each inlet pipe into an anaerobic digester. The sludge samples were stored in a refrigerated container during transportation to the laboratory. The sludge samples were immediately analyzed for total solids (TS), volatile solids (VS), and moisture content (MC), before being stored in a refrigerator at 4 °C. The primary and secondary sludge samples were dried in a furnace at 80 °C for 48 hours, in order to obtain the dried sludge samples. The dried sludge samples were ground and
5
sieved to pass through a 100 mesh. The dewatered sludge was under wet conditions with a high MC of 85%, whereas the dried sludge had < 5% MC. Commercial grade (99.8%) methyl alcohol anhydrous, laboratory grade (95%) sulfuric acid (H2SO4), 99.5% xylene, and commercial grade (95%) n-hexane were purchased from Samchun chemicals in Korea.
2.2. in-situ transesterification Two different types of sludge, dried and dewatered, were prepared for in-situ transesterification. Table 1 shows the reaction conditions for in-situ transesterification methods applied to these sludges. The dried sludge was treated using a conventional method, whereas the wet sludge was treated using a method developed for this study, which uses xylene as the cosolvent instead of hexane.
2.3. in-situ transesterification for dried wastewater sludge Figure 1(a) shows the soxhlet extractor used for the in-situ transesterification of the dried wastewater sludge. The method involved adding 22.5 g of dried sludge into a 500 mL reaction flask containing methanol and n-hexane. The methanol solution containing 5% H2SO4 and n-hexane were blended at 1:1 ratio (v/v), and the methanol solution was added at three different volumes of 45, 112.5, and 225 mL in order to determine optimal methanol dosage. The mixture in the flask was then well stirred for 8 hours of reaction. The flask was connected to a condenser and heated in a water bath at 55 °C. After the reaction ended, the mixture was cooled overnight at room temperature and then the supernatant phase was recovered by centrifugation at 300 rpm for 5 minutes. The collected supernatant was washed three times with 50 mL deionized water and then 1 g of
6
sodium sulfate (Na2SO4) was added. The samples were filtered to remove the water using Whatman GF/C glass fiber filters. The remaining n-hexane in the supernatant was vaporized using a vacuum and the final supernatant was collected as the biodiesel sample.
2.4. in-situ transesterification for dewatered (wet) wastewater sludge A 1-L flask fitted with a reflux condenser was used for this experiment. The condenser was designed to recirculate the condensed cosolvent continuously into the reaction flask after separation of cosolvent and solvent from water as shown in Figure 1 (b). This was followed by addition of 150 g of dewatered wastewater sludge into the flask containing 300 mL each of methanol solution and n-hexane or xylene. The methanol solution contained 15 mL of H2SO4, which acts as a catalyst. The mixture in the flask was then stirred at 100 rpm using a paddle-type agitator during the reaction. The reaction time varied from 1–8 h as shown in Table 1. The reaction temperatures were set at 55 and 105 °C for n-hexane and xylene, respectively. The dewatered-dried sludge was also used as a control and its transesterification condition is shown in Table 1. Methanol, water, and part of the xylene were vaporized when xylene was used as the cosolvent and both methanol and xylene were recirculated into the flask through the condenser. The condensed water phase was discarded to prevent it from flowing back into the flask during the reaction. Conversely, water did not vaporize and remained at the flask during the reaction that used hexane, which has a lower boiling point than water. The mixture was cooled overnight at room temperature after the reaction ended. Once the supernatant was collected separately from the flask, it was further treated by a series of polishing steps as described above, which included centrifugation, rinse, water removal, and vaporization followed by collection.
7
2.5. Analyses for biodiesel Biodiesel samples were analyzed quantitatively and qualitatively to determine the biodiesel yield and FAME composition. The biodiesel yield from the sludge sample and FAME content of crude biodiesel was calculated based on the weight of the product as shown in equations 2 and 3 below:
Biodiesel yield (%) =
weight of the product (g) dried solids in the sludge (g)
ⅹ 100
(2)
ⅹ 100
(3)
Total FAMEs weight (g) FAME content (%) =
Biodiesel weight (g)
The FAME composition of the extracted biodiesel was analyzed using Gas Chromatograph (GC, Agilent 7890A, Agilent, USA) equipped with a mass selective detector (GC/MS, Agilent 5975C, Agilent, USA). The HP-5MS capillary column (30 m x 0.32 mm x 0.25 µm) was used for FAME analysis. Analytical conditions of GC/MS were as follows: the injector and detector temperatures were 250 and 280 °C, respectively; column program initial temperature of 50 °C was set for 1 minute; then ramped from 50 to 200 °C at 25 °C/min; and then ramped from 200 to 230 °C at 3 °C/min; and finally maintained at 230 °C for 18 minutes. The conditions were optimized by slightly modifying those of FAMEs analysis suggested by Lee et al., (2013). The injection sample volume was 1 µL and the split ratio was 1:50. Nitrogen was used as a carrier gas with a flow rate of 1 mL/min. Each hexane and xylene extract sample was diluted using hexane at 20 and 100 times, respectively. The FAME profiles and concentration was identified by
8
comparing it with a standard solution (37 comp. FAME mix, Supelco, USA) and NIST 2008 Libraries installed in GC/MS.
2.6. Statistical analyses Statistical analyses were performed using SPSS v12.0 for Windows software (SPSS v12.0 IBM Corporation, Somers, NY). All data were expressed as the mean ± standard deviation (SD). Any significant difference between the FAME profiles was determined by using two-way analysis of variance (ANOVA). The significance for all p values was 0.05 unless otherwise stated.
3. Results and discussion 3.1. Biodiesel production from dried wastewater sludge Biodiesel was produced from in-situ transesterification of primary, secondary dried wastewater sludge. Figure 2 shows the biodiesel yields obtained from these feedstocks depending on the methanol dosage used. Even though the biodiesel yield was not significantly different among the three sludge types, the secondary sludge showed a slightly higher yield except at 5 mL methanol/g dried sludge dosage. The biodiesel yield proportionally increased with the dosage of methanol to sludge up to 5 mL methanol/g dried sludge for both types of sludge. As methanol dosage further increased to 10 mL/g, the biodiesel yield continued increasing for the secondary sludge but not for the primary sludge. The maximum biodiesel yield was 9.68% when secondary sludge was used as the feedstock with the addition of 10 mL methanol/g dried sludge. It was similar to the maximum biodiesel yields of activated sludge presented in other studies (Dufreche et al., 2007; Mondala et al., 2009). This difference in biodiesel yield between the two types of
9
sludge feedstock may be attributed to the types of lipids present in each. The major biodiesel source is phospholipid fatty acids in microbial membrane and triglyceride for secondary and primary wastewater sludge, respectively (Fukuda et al., 2001). Fatty acids are transformed into FAMEs via transesterification; however, glycerol remains as a byproduct when triglyceride is used as feedstock (Siddiquee and Rohani, 2011). The excess addition of methanol with hydroxyl groups results in more generation of emulsion with the remaining glycerol in the biodiesel phase in the transesterification step (Sun et al., 2008). Figure 3 shows weight percentage of the FAMEs identified in the biodiesel by conventional in-situ transesterification. GC/MS was used for identification of various FAMEs (data not shown). The abundance of individual FAME was calculated by using a standard solution as described above. Characterization of FAME constituents in biodiesel is important since the properties of biodiesel are strongly dependent on its FAME constituents (Knothe, 2005). In the crude biodiesel derived from the dried sludge, FAMEs between C12:0 and C24:0 were identified by GC/MS. Based on these FAMEs, methyl esters of palmitic acid (C16:0), palmitoleic acid (C16:1), stearic acid (C18:0), and oleic acid (C18:1n9c) were predominant components regardless of sludge type. The sum of these four FAMEs was 65.2 and 63.2% of the total FAMEs identified for primary and secondary wastewater sludge, respectively (Figure 3).
The total weight percent of FAMEs identified was 79.6 and 78.5% of the gravimetric biodiesel yield of the primary and secondary wastewater sludge, respectively. Even though overall profiles of the FAME components between the two dried sludges were not statistically different (p < 0.05), their structure appeared to be different. Biodiesel derived
10
from primary sludge contained saturated straight FAMEs as well as C16:0 and C18:0, while, biodiesel derived from secondary sludge had unsaturated FAMEs as well as C16:1 and C18:1. Saturated FAMEs have significantly better burning properties than unsaturated FAMEs (Ritz and Croudace, 2003; Knothe, 2005). Conversely, unsaturated FAMEs exhibited better fluidity under cold temperature condition than the saturated species (Kenesey and Ecker, 2003). Even though it is hard to evaluate the biodiesel property based upon only FAME results, the FAME profiles of biodiesel derived from both sludges are in good agreement with the previous studies (Mondala et al., 2009; Dufreche et al., 2007; Revellame et al., 2011). According to those previous studies, dried wastewater sludge containing triglyceride or phospholipids could be suitable feedstocks for biodiesel production (Revellame et al., 2010).
3.2. Biodiesel production from dewatered wastewater sludge Biodiesel was produced from in-situ transesterification of dewatered wastewater sludge by using two cosolvents: hexane or xylene. Figure 4 shows a comparison of the gravimetric yield between the two methods at various reaction times. These two methods were differentiated by the cosolvent type and reaction temperature. Xylene allowed a higher reaction temperature due to its higher boiling point (105 °C). Biodiesel yield from the dewatered sludge was limited with the use of hexane as the cosolvent. The maximum biodiesel yield from dewatered sludge was only 3.28%, which was less than half of the yield from dried sludge. However, the use of xylene as a cosolvent replacing hexane markedly enhanced the biodiesel yield. Maximum biodiesel yield with xylene was 8.12%, which was 2.5 times greater than that of the dewatered sludge and a little lower than that of dewatered-dried sludge when hexane was used as a cosolvent (Table 2).
11
Wastewater sludge is produced in most wastewater treatment plants in the form of dewatered sludge prior to final disposal; however, its moisture content is still more than 70%. It was obvious that the presence of water caused detrimental effects on transesterification for biodiesel production. Ma and Hanna (1999) explained that presence of water might cause soap formation and lead to a decrease in the efficiency of the catalyst during the transesterification. According to Qian et al. (2009), transesterification efficiency of cottonseed oil to methyl ester increased from 80 to 98% when decreasing the moisture content from 8.7 to 1.9%. In this study, water was avoided during transesterification by employing the modified extraction system shown in Figure 1 (b) together with a high-boiling point cosolvent, xylene.
FAMEs in biodiesel obtained from 8 hours of transesterification were analyzed by GC/MS and the results are shown in Figure 5. The FAMEs profiles obtained from two in-situ transesterification methods depending on cosolvent were significantly different (p >0.05). The number of FAMEs identified was 13 and 7 FAMEs for hexane and xylene methods, respectively. Unlike xylene transesterification, the biodiesel obtained from hexane transesterification contains various components other than FAMEs, which might denote impurities in the biodiesel. Therefore, the use of xylene contributed to a better property of crude biodiesel, with higher FAME contents. Figure 6 shows the quantitative results of FAMEs identified. Total FAMEs content of xylene transesterification was 81.9% of the crude biodiesel. This value was twice the hexane transesterification (45.0% of the crude biodiesel) and was greater than biodiesels from dried sludge (Figure 6). The most dominant FAME in xylene transesterification was methyl esters of myristic acid
12
(C14:0) with 23.85% in biodiesel (w/w) but the following FAMEs were also similarly noticeable: pentadecylic acid (C15:0), palmitic acid (C16:0), oleic acid (C18:1n9c), stearic acid (C18:0), and lignoceric (C24:0). Alternatively, FAMEs in hexane transesterification contained almost three major components of palmitoleic acid (C16:1), palmitic acid (C16:0), and oleic acid (C18:1n9c). Even though there were differences in FAMEs composition between the two transesterification methods, the major components identified are typically present in common biodiesels from wastewater sludge (Dufreche et al., 2007; Mondala et al., 2009; Revellame et al., 2010).
It was notable that the obtained FAME composition was very dependent on the transesterification method such as cosolvent type as well as the feedstock. When the same transesterification was applied, the FAME composition of the dewatered sludge was similar to that of dried wastewater sludge, particularly for the secondary dried sludge shown in Figure 2. However, it significantly shifted by adopting a different transesterification method using xylene as the cosolvent: the content of unsaturated fatty acids was lowered. The use of xylene enabled an increased reaction temperature, which enhanced the biodiesel burning property by producing significantly less unsaturated FAMEs. In this study, other biodiesel properties were not evaluated but this finding supported the validity of the assumption that biodiesel yield and quality were enhanced due to the avoidance of polymerization under high temperature conditions (Revellame et al., 2010). As the results suggest, the transesterification method using xylene was considered a more promising method by enhancing both the yield and quality of the biodiesel from wet wastewater sludge.
13
3.3. Evaluation of applicability of modified transesterification with xylene for biodiesel production from wet sludge: technical and economic analysis Table 2 compares the maximum biodiesel yield depending on sludge type and cosolvent used. Data for biodiesel yield represent mean ± standard deviation for duplicate samples. Xylene was more the efficient cosolvent in production yield of biodiesel from wet sludge. Biodiesel yield from wet sludge was comparable to that of three different types of dried sludge. Xylene’s high-boiling point enabled an increase in the reaction temperature up to around 100 °C, which led to vaporization of water out of the reaction vessel through a specially designed unit, during the reaction. Therefore, xylene could boost the reaction kinetics and reduce consumption of methanol for the same biodiesel yield due to high temperature and avoidance of water during transesterification (Figure 4).
Table 3 shows the economic analysis for biodiesel production from both dried and wet wastewater sludge which were the mixture of primary and secondary sludge. The economic analysis was conducted to estimate the cost required for 1 L of biodiesel production from wastewater sludge based on the experimental results. The cost for dewatering was equally estimated assuming that a centrifugation process was applied (Dufreche et al., 2007). The estimated cost of drying process was based on the electrical power requirement (641.8 kWh/ton wet sludge) for dehydrating the dewatered sludge by reducing the moisture content from 85% to 5% (Gonçalves et al., 2007). Capital cost for the drying process was not included in this analysis. Chemical costs for methanol, hexane and xylene were estimated assuming recovery and reuse of those chemicals after
14
transesterification for 40 batch cycles. The cost for the electricity was based on the total energy requirement for in situ transesterification reaction for 8 hr.
The use of xylene for biodiesel production from wet sludge resulted in great cost saving compared to hexane. Even though the use of xylene was the least costly method for wet sludge, cost saving was not significant when compared to dried sludge. This might be mainly attributed to the higher energy consumption, the unit price of which varies from country to country. However, the drying process is very costly with high capital and operational costs, thus it is not feasible in a wastewater treatment plant (WWTP). The operational cost for drying accounted for 50% of total cost necessary for biodiesel production from dried sludge. The in-situ transesterification using xylene suggested in this study will be a promising option that is applicable to real WWTPs because a similar productivity of biodiesel from wet sludge is achievable without the installation of additional unit processes.
4. Conclusions Biodiesel production from wet wastewater sludge was enhanced by replacing cosolvent from hexane to xylene showing a similar yield from dried wastewater sludge by conventional transesterification. FAME contents and the quality of biodiesel were also improved. In spite of the higher biodiesel yield, the total number of FAMEs was smaller but it included major indispensable components for high quality biodiesel. The use of xylene for biodiesel derived from wet sludge did not increase the cost compared to conventional transesterification using hexane for dried sludge, and this method will be more readily applicable to most of WWTP without the drying process.
15
Acknowledgement This study was supported by a grant from the Korea University. The authors are grateful for useful assistance from Woon Oh Cha of TSK Water and Jung Bum Ahn of SK Chemicals.
References 1. Beech N., Crawford K., Goldstein N., Kester G., Lono-Batura M., dzieyk E., 2007. A national biosolids regulation, quality, end use and disposal survey: Final report. North East Biosolids and Residuals Association(NEBRA), Tamworth, NH. 2. Chhetri A. B., Tango M. S., Budge S. M., Watts K. C., Islam M. R., 2008. Non-edible plant oils as new sources for biodiesel production. Int. J. Mol. Sci. 9, 169-180. 3. Dufreche S., Hernandez R., French T., Sparks D., Zappi M., Alley E., 2007. Extraction of lipids from municipal wastewater plant microorganisms for production of biodiesel. J. Am. Oil Chem. Soc. 84, 181-187. 4. Fukuda, H., Kondo A., Noda H., 2001. Biodiesel fuel production by transesterification of oils. J. Biosci. Bioeng. 92, 405-416. 5. Gendebien A., 2010. Environmental, economic and social impacts of the use of sewage sludge on land: Final Report, Assessment of Existing Knowledge. Milieu Ltd and WRc for the European Commission. 6. Gonçalves R.F., Luduvice M., Sperling M.V., 2007. Sludge thickening and dewatering, in: Andreoli C.V., Sperling M.v., Fernandes F. (Eds.), Sludge Treatment and Disposal. IWA Publishing, London, pp. 76–119.
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7. Haas M. J., Scott K. M., Foglia T. A., Marmer W, N., 2007. The general applicability of in situ transesterification for the production of fatty acid esters from a variety of feedstocks. J. Am. Oil Chem. Soc. 84, 963-970. 8. Hayyan A., Alam M. Z., Mirghani M. E. S., Kabbashi N. A., Hakimi N. I. M. N., Siran Y. M., Tahiruddin S., 2010. Sludge palm oil as a renewable raw material for biodiesel production by two-step processes. Bioresour. Technol. 101, 7804-7811. 9. Huynh L. H., Phuong L. T. N., Ho Q. O., Ju Y. H., 2012. Catalyst-free fatty acid methyl ester production from wet activated sludge under subcritical water and methanol condition. Bioresour. Technol. 123, 112-116. 10. Kargbo D. M., 2010. Biodiesel Production from Municipal Sewage Sludges. Energy Fuels 24, 2791-2794. 11. Kenesey E., Ecker A., 2003. Oxygen connections to the improvement of the lubricity in fuels. Tribologie und Schmierungstechnik 50, 21-26. 12. Knothe G., 2005. Dependence of biodiesel fuel properties on the structure of fatty acid alkyl esters. Fuel Process. Technol. 86, 1059-1070. 13. Korea Electric Power Corporation, 2013. Electric Rates Table http://cyber.kepco.co.kr/ckepco/front/jsp/CY/E/E/CYEEHP00203.jsp accessed in March 16. 14. Krawczyk T., 1996. Biodiesel-alternative fuel makes inroads but hurdles remain. Inform 7, 800-815. 15. Lee K. H., Park K. Y., Khanal S. K., Lee J. W., 2013. Effects of household detergent on anaerobic fermentation of kitchen wastewater from food waste disposer. J. Hazard. Mater. 244, 39-45 16. Ma F. R., Hanna M. A., 1999. Biodiesel production: a review. Bioresour. Technol. 70,
17
1-15. 17. Ministry of Environment in Korea, 2011. The state of waste generation and treatment in 2011. Seoul, Korea. 18. Mondala A., Liang K. W., Toghiani H., Hernandez R., French T., 2009. Biodiesel production by in situ transesterification of municipal primary and secondary sludges. Bioresour. Technol. 100, 1203-1210. 19. Qian J. F., Wang F., Liu S., Yun Z., 2008. In situ alkaline transesterification of cottonseed oil for production of biodiesel and nontoxic cottonseed meal. Bioresour. Technol. 99, 9009-9012. 20. Revellame E., Hernandez R., French W., Holmes W., Alley E., 2010. Biodiesel from activated sludge through in situ transesterification. J. Chem. Technol. Biot. 85, 614-620. 21. Revellame E., Hernandez R., French W., Holmes W., Alley E., Callahan R., 2011. Production of biodiesel from wet activated sludge. J. Chem. Technol. Biot. 86, 61-68. 22. Ritz G. P., Croudace M. C., 2003. Biodiesel/FAME analysis. Hydrocarb. Eng. 8, 83-84. 23. Siddiquee M. N. and Rohani S. 2011. Experimental analysis of lipid extraction and biodiesel production from wastewater sludge. Fuel Process. Technol. 92, 2241-2251. 24. Sun J., Ju J. X., Ji L., Zhang L. X., Xu N. P. 2008. Synthesis of biodiesel in capillary microreactors. Ind. Eng. Chem. Res. 47, 1398-1403. 25. Zhou X. P., Ge H. M., Xia L., Zhang D. L., Hu C. X., 2013. Evaluation of oil-producing algae as potential biodiesel feedstock. Bioresour. Technol. 134, 24-29.
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Figure Captions
Figure 1. Schematics of reactor system for in-situ transesterification for (a) dried sludge and (b) wet sludge. Figure 2. Gravimetric biodiesel yield as function of methanol to sludge ratio Figure 3. Weight percentage of FAMEs idenitified in biodiesel derived from each dried sludge. Figure 4. Effects of cosolvent on gravimetric biodiesel yield as a function of reaction time. Methanol to sludge dose was 2 mL/g. Figure 5. FAMEs profiles of the biodiesel produced from wet wastewater sludge through in-situ transesterification with 8 hr reaction by using (a) hexane and (b) xylene as a cosolvent. Transesterification with hexane and xylene was diluted with hexane by 20 and 100 times before GC analysis, respectively. Figure 6. Weight percentage of FAMEs components in biodiesel derived from dewatered sludge.
19
Table 1. Co-solvent and reaction condition for in-situ transesterification depending on sludge types
Sludge Type
Reaction
Methanol
Time
Dosage
(hr)
(mL/g sludge)
Reaction
Moisture Cosolvent
Temp. (°C)
Content
Dried (primary)
< 5%
n-Hexane
55
8
2, 5, 10
Dried (secondary)
< 5%
n-Hexane
55
8
2, 5, 10
n-Hexane Dewatered*
55
1, 2, 8
2
85% Xylene
105
1, 2, 8
2
n-Hexane
55
8
10
Dewatered-Dried*
< 5%
* mixed with primary and secondary sludge
20
Table 2. Comparison of maximum biodiesel yield under different condition attempted in this study
Sludge Type
Methanol to Sludge
Biodiesel Yield
Ratio (mL/g)
(%)
Cosolvent
Primary Dried
n-Hexane
10
8.04 ± 0.50
Secondary Dried
n-Hexane
10
9.68 ± 0.39
n-Hexane
2
3.28 ± 0.40
Xylene
2
8.12 ± 0.11
n-Hexane
10
9.54 ± 0.39
Dewatered*
Dewatered-Dried **
* mixed with primary and secondary sludge ** dewatered sludge after drying
21
Table 3. Economic analysis for in-situ transesterification of dried and wet activated sludge
Unit Price
Dried sludge
Wet sludge
A. Feedstock preparation 1. Centrifugation
0.11 a
2. Drying
($/L)
0.11
0.11
0.11
0.034b ($/kg)
2.03
-
-
1. Methanol
0.25 c
($/L)
0.58
2.14
0.86
2. Catalyst
0.046c ($/L)
0.21
0.79
0.32
n-Hexane
0.29 c
($/L)
0.68
2.50
-
Xylene
0.12 c
($/L)
-
1.01
0.05 d
($/kWh)
0.43
1.58
1.22
($/L)
4.05
7.12
3.52
B. Chemicals
3. Cosolvent
C. Electricity D. Total a
Dufreche et al., 2007 Gonçalves et al., 2007 c International Construction Information Society, 2013 (http://www. icis.com) d Korea Electric Power Corporation, 2013 b
22
(a)
(b) Figure 1
23
Figure 2
24
Figure 3
25
Figure 4
26
(a)
(b) Figure 5
27
Figure 6
28
Highlights A new method for biodiesel production from wet wastewater sludge was suggested. Biodiesel yield increased by 2.5 times when hexane was replaced with xyl ene. Both fatty acid methyl esters (FAMEs) contents and quality were enhance d. Drying process was not necessary as a pretreatment for biodiesel product ion.
29