Biodiesel production from wet municipal sludge: Evaluation of in situ transesterification using xylene as a cosolvent
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 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 Eq. (1), the reaction slows down and eventually stops if water is present in excess amount.
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 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.
Section snippets
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
Biodiesel production from dried wastewater sludge
Biodiesel was produced from in situ transesterification of primary, secondary dried wastewater sludge. Fig. 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
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
Acknowledgements
This study was supported by a grant from the Korea University – South Korea. The authors are grateful for useful assistance from Woon Oh Cha of TSK Water and Jung Bum Ahn of SK Chemicals.
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