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Electropolar effects on anaerobic fermentation of lignocellulosic materials in novel single-electrode cells
Bioresource Technology 159 (2014) 88–94
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Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Electropolar effects on anaerobic fermentation of lignocellulosic materials in novel single-electrode cells Guangfei Qu, Weixia Qiu, Yuhuan Liu, Dongwei Zhong, Ping Ning ⇑ Faculty of Environmental Science and Engineering, Kunming University of Science & Technology, Kunming, Yunnan 650500, China
h i g h l i g h t s Anaerobic fermentation in single electrode-assisted fermenter was carried out. The effect of electric polarity and micro-voltage was investigated. The system promote biogas and methane yield, and enhance lignocellulose degradation. The effectively promotional intensity of electric polarity was: cathode > anode > blank. The positive effect of different cathodic micro-voltage was: 250 mV > 500 mV > 100 mV.
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Article history: Received 7 December 2013 Received in revised form 11 February 2014 Accepted 14 February 2014 Available online 24 February 2014 Keywords: Micro-voltage Methane Lignocellulose Single-electrode Anaerobic fermentation
a b s t r a c t As a promising renewable energy technology, anaerobic fermentation is consistently limited by low production and calorific value of biogas, along with the difficulty of lignocellulose degradation. The effects of polarity and micro-voltage on anaerobic fermentation from lignocellulosic materials were investigated in single-electrode fermenter to explore cost-efficient technology. The results illustrated that the biogas production and quality were significantly affected by electric polarity. And cathode-assisted fermentation led to more positive effects than anode-assisted. Compared with results in control group without electrode, the average biogas and methane yield under cathodic micro-voltage ( 250 mV) were astonishingly improved by 2.82 and 2.44 mL g 1 d 1 respectively. Meanwhile, the degradation ratios of lignin and cellulose were also improved by 23.11% and 19.46%. It demonstrated that single micro-voltage can not only promote lignocellulose degradation but biogas production and calorific value. These micro-voltage effects on fermentation process also provided great opportunity to breakthrough the present limitation of lignocellulosic materials fermentation. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Nowadays, energy consumption is rising rapidly due to industrialization and progress in the standard of living. Fossil fuel such as natural gas, petroleum and coal which are mentioned as the most important energy sources in the world are limited and transitory (Basha et al., 2009). Energy insecurity, fossil fuel depletion and global warming issues have convinced the academic society and government to research about new fuel resources (Hosseinin and Wahid, 2013). Biotechnology, marked by its environmental protection, low input and high output, has become a high priority of government concern world-wide. So the interest in biomass resource has been stimulated by the promise of clean energy production. Methane fermentation, as a low cost approach that ⇑ Corresponding author. Tel.: +86 13888528030. E-mail address: qgfl[email protected] (P. Ning). http://dx.doi.org/10.1016/j.biortech.2014.02.052 0960-8524/Ó 2014 Elsevier Ltd. All rights reserved.
produces little sludge and generates methane gas as a renewable energy resource (Forster-Carneiro et al., 2008), is also believed as one of the most socio-economically cost-efficient renewable energy technologies, using organic waste and plant biomass as feedstock. Lignocellulose is a kind of important renewable resource and the most abundant organic raw material on the earth, which is composed of hemicellulose, cellulose and lignin. Although the microbial decomposition of such lignocellulosic materials has been studied extensively, most of the studies showed that substrates with relatively simple composition and structure, such as xylan (Hui et al., 2013) could be degraded completely. However, the natural lignocellulose with a complex composition and structure, such as crop straws and other plant fibres (Kim et al., 2006) were hardly to break down. The barrier is the structure of lignocellulose which has evolved to resist degradation due to cross-linking between the polysaccharides (cellulose and hemicellulose) and the lignin via
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ester and ether linkages (Lin and Tanaka, 2006; Xiao et al., 2007; Weib et al., 2010) which is highly recalcitrant (Neves et al., 2006). Bioelectrochemical reactors (BERs) are now attracting attention in part because microbial activity in them can be altered or controlled using external electrochemical system (Thrash and Coates, 2008). In addition, reactors with a potential of 0.6 or 0.8 V showed greater methanogenesis than control reactors without electrochemical regulation (Sasaki et al., 2010). BERs have been used previously for redox dye degradation (Park et al., 1999). Recently, a BER was used in methane fermentation from a mixture of artificial garbage slurry and rice straw as a model of complex organic materials, and a cathodic reaction effectively promoted methane production (Sasaki et al., 2011). All those previous works have mainly focused on biogas production or methane yield rather than lignocellulose degradation by electrochemical system effects. The optimum carbon–nitrogen (C/N) ratio for anaerobic fermentation is in the range of 20–25 (Yen and Brune, 2007). Dried cow manure has high C/N ratio of 23.2 (Liao et al., 2004), which make them a suitable co-substrate for anaerobic digestion. The aim of this paper is to investigate the effects of electric polarity on cellulose and lignin degradation ratio, biogas production yield and methane content during anaerobic fermentation from lignocellulosic materials, taking the cow manure as raw material due to its high concentration in lignocellulose. A better understanding of the influences of electrodes and voltage on methane fermentation would be useful from both a scientific and an engineering point of view.
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oxygen and sealed to maintain anaerobic condition. The fermentation tank was incubated in thermostatic shaker at 35 ± 2 °C for up to 30 days. 2.3. Analytical methods The selected responses for analysis were biogas production and the content of methane indicating the quality of biogas. The complex substrate was subjected to fibre analysis to determine their concentration of lignin and cellulose (Molinuevo-Salces et al., 2013). Production of biogas was collected with gas pocket, and then the volume was measured according to the displacement method (Moosvi and Madamwar, 2007). The composition of biogas was analysed using gas chromatograph (Molinuevo-Salces et al., 2010). The chemical groups of the lignocellulosic materials were determined by Fourier Transform Infra-Red Spectrometer (FT-IR) over the range of 500–4000 cm 1. The crystallinity of cow manure was investigated by X-ray diffraction (XRD). Cu Ka was used as monochromatized radiation source at 40 kV and 200 mA at room temperature. The samples were scanned and intensity recorded in 2h over the range of 5–60° with a step size of 0.02° and step time of 0.15 s. In addition, scanning electron microscopy (SEM) had been applied to examine surface morphology and assess structural changes of lignocellulosic materials. Samples from the beginning and the end of experiments were analysed. 3. Results and discussion
2. Methods 3.1. The effects of electrode on fermentation 2.1. Feed materials All media and solutions were prepared using deionised water and analytical grade reagents without further purification. Cow manure was obtained from Dengchuan, Dali, Yunnan province, China. The cow manure were dewatered in drying oven to constant weight, then crushed by pulveriser and kept in a dry place for using in subsequent experiments as substrate. The chemical compositions of cow manure are shown in Table 1. The inoculum was collected from the mesophilic fermentation plant in Dengchuan, Dali. Expanding culture of inoculum was carried out in laboratory at 35 ± 2 °C. 2.2. Experimental process Experimental operation was carried out in self-designed anaerobic fermenter equipped with a single stainless steel electrode, and the fermenter was heated by an electric-heated thermostatic shaker (Fig. 1). In order to control the potential of the working electrode, each electrode was connected to a potentiostat. All the assays were carried out in duplicate. Batch tests were conducted in fermenter filled with a certain weight of substrate and a predetermined volume of inoculum in which the percentage of total solid (TS) is 8%. To investigate the influence of electrode and voltage on methane fermentation, the experimental schemes were designed and given in Table 2 in detail. The fermentation tanks were purged with nitrogen gas to replace the initial contained
Table 1 Chemical composition of cow manure. Composition
Ash (%)
Volatile solid (%)
Organic materials (%)
Caloric value (J g 1)
Lignocellulose (%)
Cow manure
12.7
73.32
62.46
14,780
35.52
3.1.1. Effect on biogas production The effect of three different electrodes (cathode, anode and blank) at 2500 mV on biogas production in anaerobic fermentation was evaluated in batch assays, while other operating parameters remained the same. The variation of cumulative biogas production with time was presented in Fig. 2A. It could be observed that the relationship between time and cumulative biogas production for cathodic fermentation approached linear approximately: The longer the retention time lasted, the more the biogas produced all through the fermentation process. The cumulative biogas production of anodic group was lower than the blank group without voltage assistance during the first 16 days in fermentation process, from then on, however, the biogas yield was contrary to what observed before, i.e. there was more biogas produced by anodic group than blank group. And it should be noted that these two groups had only minor differences in cumulative gas production from one another. The statistical analysis showed no significant difference between the quantity of produced biogas in cathodic and blank group during the first 12 days of the fermentation process, but after that, the producing rate of biogas in cathodic reactor kept going straight, while that in blank tended to slow down. The cumulative biogas production tendency of anodic and blank group came to their own equilibrium value earlier compared to cathodic group. When the biogas production rate was calculated on the basis of cumulative biogas production and the quantity of substrate, it was clear that the cumulative rate can be reached to 0.1686 L g 1 in cathodic group, which was much higher than that in anodic (0.1005 L g 1) and blank (0.0940 L g 1) group during whole fermentation process. The raw material conversion was computed according to the assumption that per gram of dried cow manure can theoretically generate 300 mL biogas. The data was 56.20%, 35.50% and 31.34% in cathodic, anodic assisted system and blank system without voltage assisted respectively. As far as cumulative biogas production concerned, these results indicated that electric-assisted system could improve fermentation process
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Fig. 1. Schematic diagram of reaction system (A); graphic model of fermenter (B). 1. Potentiostat; 2. valve; 3. electric-heated thermostatic shaker; 4. quick joints; 5. gas pocket 6. vacuum chamber;7. vacuum pump.
from cow manure and cathodic voltage showed a particularly significant effect. The phenomenon may be due to that the electric field assisted system was an effective strategy for stimulating metabolism and growth of correspondent microbe. It was reported by Cheng, H that the voltage needed for hydrogen derived from the electrolysis of water was theoretically 1210 mV at neutral pH. In practice, 1800–2000 mV was needed for water electrolysis (under alkaline solution conditions) due to overpotential at the electrodes (Cheng et al., 2002). If hydrogen was produced at the cathode derived from acetate, the potential required at least E0 = 410 mV (NHE) at pH 7.0. The open circuit potential of the anode was about 328 mV in a typical anaerobic condition, so hydrogen can theoretically be produced at the cathode by applying a circuit voltage greater than 82 mV (i.e., 410–328 mV) according to Liu et al. (2005). Another study has addressed that there was hydrogen observed at a 0.6 V applied voltage (Hu et al., 2008). The difference of needed voltage for hydrogen production may be due to the differences in the substrates, microbial communities and the structure of fermentation reactors. Actually, in the fermentation process the situation was quite different because there was no circuit produced by applying just single electrode. It kept unknown that how the cathodic and anodic field affect the anaerobic fermentation and further investigation was required to confirm the surmise.
3.1.2. Effect on methane content The low voltage effect on methane concentration of biogas in anaerobic fermentation was shown in Fig. 2B. It was illustrated that the biogas quality was best in cathodic system, then anodic, the blank in sequence. That is to say, the cathodic voltage had a significantly positive effect on methane content in fermentation biogas. We raise an assumption that the cathodic condition was more suitable for methane-producing bacteria because methanogens were negatively charged. As can be seen from Table 2, and 84% as the maximum methane content was achieved in the cathode-assisted system, which was an abnormal phenomenon. In addition, the maximum methane content was 80% in the anode-assisted system and 77% in the system without electrode-assisted respectively. In general, the concentration of methane in biogas from this substrate is lower than 70% v/v (Xu et al., 2013; Sasaki et al., 2011; Triolo et al., 2012). The average content of methane was 67.44% in control group which was in consistence with normal value. However, both 71.04% in anodic group and 77.94% in cathodic group clearly exceeded the usual value. As mentioned above, regardless of which electrode was used, these electrode-assisted conditions were helpful to improve the biogas production rate and quality, especially in cathodic condition. In order to investigate if there is the similar tendency and influence efficiency at lower voltage, the further experiments were carried out under the micro-voltage conditions.
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Fig. 2. Time-dependent changes in the gas production rate with different electrodes (A) or voltages (C). The distributive characters of methane with different electrodes (B) or voltages (D).
3.1.3. Effect on degradation of lignocellulose The lignocellulose accounts for approximately 35.52% of cow dung, which was difficult to be biodegraded by anaerobic microorganism, especially the sclerenchyma and xylem included in lignin tissues, resulting in low quality of biogas production and high quantity of biogas residue in anaerobic fermentation. Consequently, the biodegradation of lignocellulose was essential for the generation of biogas. So the main research goal of this paper was to investigate the efficiency of lignocellulose degradation in the process of electrode-assisted fermentation. Anaerobic digestion with cathode assistance presented the highest degradation ratio (up to 56.20%) of lignin, whereas 34.8% and 30.14% of degradation ratio were observed in anodic and blank group, respectively (Fig. 3A). On the other hand, the degradation ratio of cellulose was no significantly different in the system with electrode or not, however, it was still higher in electrode-assisted system than that without electrode. This effect could be explained by the promotion of electrodes in broking hydrogen-bonds and the crystal structure of lignocellulose. From the quantitative point of view, the degradation ratio of lignin was higher than that of cellulose in cathode-assisted system. While in other two fermentation systems, the degradation of lignin and cellulose was no significant difference. In other words, only the cathodic fermentation condition had positive effect on the degradation ratio of lignin. The results suggested that the microorganisms in cathodic fermentation system may be more inclined to degrade lignin than cellulose. On the other hand, the microorganisms and enzyme in cathodic condition became more reactive because most microbes are negatively charged. 3.2. The effects of cathodic micro-voltage on fermentation As discussed previously, it was obvious that the process of fermentation could be remarkably promoted by external low voltage,
especially cathodic voltage. But it was still uncertain whether there were effects of cathodic micro-voltage on fermentation, so further digestion experiments with cathodic assistance were carried out to investigate the effects of cathodic micro-voltages on anaerobic fermentation. 3.2.1. Effect on biogas production The biogas production with cathode electrode at different levels of micro-voltage ( 100 mV, 250 mV and 500 mV) was evaluated in batch assays, while other operating parameters remained the same. Cumulative biogas production for micro-voltages tested over time was shown in Fig. 2C. As expected, there was a significant positive effect of voltage on the biogas production. It could be observed that daily biogas production rate decreased over time due to the deteriorating condition of bacterial community, which caused by the lack of essential nutrition and bacteria’s own supersession. Although the growth trends of cumulative biogas production were similar, the rates of increase were notably different. It could be calculated that the cumulative biogas production could reach to 106.53, 174.77 and 134.80 mL g 1 at 100 mV, 250 mV and 500 mV respectively. In other words, the intensity of promotion in cumulative biogas production with cathodic micro-voltage increased as follows: 100 mV < 500 mV < 250 mV. The reason why 250 mV showed higher accumulation of biogas in comparison with 100 or 500 mV may be due to the following two aspects: firstly, the 250 mV potential created optimum condition for the growth and development of microorganism. Moreover, the 250 mV potential was much more favourable for the conversion of lignin and cellulose to small particles. So the utilization ratio of raw materials was enhanced. The average biogas production rate reached 0.1748 L g 1 at 250 mV, corresponding to 58.27% of raw material conversion. Meanwhile, the systems with 100 mV and 500 mV, showed lower gas production rate (0.1065 L g 1, 0.1348 L g 1), and lower raw material conversion
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methane and biogas yield, separately. The average methane content was also increased with cathodic micro-voltage. And these average dates were all higher than the common value without voltage assistance. These results strongly confirm the assumption that there was positive impact of cathodic micro-voltage on methane content through fermentation. As the most suitable redox potential for methanogens is from 150 mV to 400 mV. When the cathodic voltage was 250 mV, the micro-voltage perhaps was the closest value of the optimum redox potential for methanogens. Thus, under this level of voltage, the activity of methanogens was highest resulting in more stable of methane content in comparison with 100 mV and 500 mV. In addition, methane might be electrochemically generated from the reduction of CO2 via a cathodic reaction and hydrogenotrophic methanogens (Villano et al., 2010). What discussed above was just an assumption, the exactly mechanism of voltage effect on methane content needs further investigation.
Fig. 3. Degradation ratio profile of lignin and cellulose in cow manure after anaerobic fermentation with different electrodes (A) or different voltages (B).
(35.50%, 44.93%) than that of anaerobic fermentation system with 250 mV. 3.2.2. Effect on methane content The effect of different cathodic micro-voltage on methane content was shown in Fig. 2D. It was found that the cathodic microvoltage had a significantly positive effect on methane content in fermentation biogas. In addition, the effect of different micro-voltage on methane content was of great difference from each other. Just as the results of experiment illustrated in Table 2, the maximum content of methane in 100 mV, 250 mV and 500 mV cathodic field assisted system was 78%, 79% and 81%, respectively. The maximum methane content was increased with cathodic micro-voltage. And the average methane content of corresponding conditions was 72.39%, 76.67%, and 75.28% based on the average
3.2.3. Effect on degradation of lignocellulose As showed in Fig. 3B, the lignin and cellulose removal ratio was up to 53.25% and 50.80% in system with 250 mV cathodic field. Whereas 37.51% and 44.93% of lignin was removed in system with 100 mV and 500 mV, along with the cellulose removal increased from 32.02% to 45.14% when the cathodic voltage increased from 100 mV to 500 mV, respectively. The degradation ratio of lignin and cellulose at 250 mV was much higher than that at 100 mV and 500 mV, indicating that the voltage of 250 mV was more suitable for the reproduction of hydrolysis microbe or hydrolysis of functional enzyme. The more reactive of microorganisms became, the higher conversion ratio of raw materials would be got. Just because 250 mV was the closest micro-voltage to the optimum redox potential, the activity of microbes were stronger than that in other two systems with 100 mV or 500 mV. So the micro-voltage of 250 mV had more positive effect on degradation of lignin and cellulose in comparison with 100 or 500 mV. Although the exactly boosting mechanism of different micro-voltage on lignocellulose degradation was still unclearly, the positive effect of micro-voltage on the decomposition of lignocellulose was absolutely without doubt. 3.3. Characterization of cow manure before and after fermentation 3.3.1. FT-IR analysis The anaerobic fermentation assisted with electric field was analysed based on the FT-IR data of different stage. Just as the data showed, the stretching and bending vibrations observed in the spectra were of different intensities before and after fermentation. The most significant difference in spectra between undigested and digested with cathodic electrode-assisted system was the intensity of the peak at 2925 cm 1 and 3422 cm 1 which represented wagging vibrations in C–H and the stretching vibrations of H-bonded OH group in cellulose. The height of the peak at 2925 cm 1 and
Table 2 Specific experimental data for anaerobic fermentation. Substrate (g)
Inoculum (mL)
Electrode
Applied voltage (mV)
Maximum biogas yield (mL g 1 d 1)
Maximum methane content (%)
Average methane yield (mL g 1 d 1)
Average biogas yield (mL g 1 d 1)
Average methane content (%)
30 30 30 30 30 30
350 350 350 350 350 350
Cathode Anode Blank Cathode Cathode Cathode
2500 2500 – 100 250 500
8.17 12.08 10.88 9.47 15.67 11.33
84 80 77 78 79 81
4.38 2.38 2.03 2.57 4.47 3.38
5.62 3.35 3.01 3.55 5.83 4.49
77.94 71.04 67.44 72.39 76.67 75.28
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3422 cm 1 for digested samples was almost half of that for undigested sample. It could be inferred that those chemical bonds in cellulose had been degraded partially. A decrease in absorbance at 1645 cm 1 and increase in absorbance at 1047 cm 1, 469 cm 1 were observed, showing the decrease in the number of stretching vibrations in C–OH groups as well as increase in bending and wagging vibrations in C–O and organic halogenated hydrocarbon C–X. These results indicated the rupture of hydrogen bonds in lignocellulose. On the other hand, the absorbance at 1645 cm 1 still existed after fermentation, illustrating that the lignin was just partly degraded. 3.3.2. X-ray diffraction studies On the basis that two peaks presented at 2h of 22° and 35° correspond to two important crystals of cellulose (He et al., 2008). The results of X-ray diffraction patterns of materials before or after fermentation showed that the major peak at 2h of 22° was the main peak representing the presence of a highly organised crystalline region, whereas the smaller peak at 2h of 35° was the less organised region. The intensity of the peak at 2h of 22° for samples from electrode-assisted fermentation system was obviously weakened compared with that from none digestion. Then crystallinity of 11.18% was calculated in raw material by using Turley method. Just as expected, a great decrease in crystallinity (5.94%) was observed in the case of electrode-assisted fermentation, indicating structural changes occurred in the crystalline regions of cellulose, and that the decreased crystallinity of cellulose was very crucial for enzymatic hydrolysis. As it is well known that amorphous cellulose is more reactive, the XRD study was in line with the enhancement of degradation of lignin in the electrode-assisted fermentation system. 3.3.3. Microstructure analysis After fermentation, the morphology of substrate was of great difference from raw materials. According to SEM images, the rupture of lignocellulose structure and increase in porosity were believed in electrode-assisted fermentation, which was due to the hydrolysis of lignin, cellulose in cow manure and merging of smaller pores into large pores. The bigger pore size had contributed to fungal growth during fermentation (Hsu et al., 2010). Compared to raw material, the particles from electrode-assisted system were smaller and the distribution of grain sizes was much homogeneous after fermentation, indicating that electrode-assisted fermentation system had a beneficial effect on improving the degradation of lignocellulose. Because of degradation, lignocellulose become basic structure unit ramifications contained phenyl of lignin, some of which were decomposed to compound of small molecular weight that was beneficial for microorganism biodegradation. These differences might explain the effect of electric condition on compact regions of (crystalline zones) cellulose. Those results of characterization further confirmed the positive effect of single electrode-assisted system on anaerobic fermentation. The electrode-assisted fermentation system is not limited to biogas fermentation of cow manure. Theoretically speaking, it also inspires efficient decomposition of any other high-concentration lignocellulosic materials. Finally, it is necessary to analyze the effectiveness of the electrode-assisted fermentation on the basis of energy consumption and production. In the novel single-electrode fermenter, the energy consumed per day was so little duo to no circuit was produced that the energy consumption could be negligible. Even the applied voltage was much higher; the consumption energy was still low for the same reason. At the voltage of 250 mV, the amount of methane produced daily was 4.47 mL g 1 (Table 2) in average at experimental temperature 35 °C. According to reaction enthalpy (DrHhm ) of methane which is 890.36 kJ mol 1 under standard conditions, 0.157 J g 1 of
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energy as methane was obtained under standard conditions daily. However, the maximum 0.435 J g 1 of energy (methane production: 12.35 mL g 1) was stably obtained in 250 mV reactor. Thus, in this novel single-electrode fermenter, a large amount of methane energy was obtained with the consumption of a much lower electric energy. What is more, the degradation ratio of cellulose and lignin was significantly improved. Such effects may promise limitless application in methane fermentation industry. 4. Conclusions Electric polarity applied in anaerobic fermentation can not only influence biogas production and methane content but also affect lignocellulose degradation remarkably. Much more positive effects were obtained in cathode-assisted fermentation system than in anode ones. For the fermentation with different cathodic micro-voltage, the intensity of promotion was increased as 100 mV < 500 mV < 250 mV. Characterization results further confirm the effectiveness of single-electrode fermenter. Considering the wide influence of the single-electrode system or the polarity effects on efficient utilization of lignocellulosic materials, it is necessary to investigate the detailed mechanism of how singleelectrode assisted system promotes methane fermentation in further research. Acknowledgement The authors acknowledge the research grant provided by the National Major Projects of Science and Technology for Water Pollution Control and Management (Project No. 2008ZX07105-002). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.biortech.2014. 02.052. References Basha, Syed Ameer, Gopal, K. Raja, Jebaraj, S., 2009. A review on biodiesel production, combustion, emissions and performance. Renew. Sustain. Energy Rev. 13, 1628–1634. Cheng, H., Scott, K., Ramshaw, C., 2002. Intensification of water electrolysis in a centrifugal field. J. Electrochem. Soc. 149 (11), 172–177. Forster-Carneiro, T., Pérez, M., Romero, L.I., 2008. Thermophilic anaerobic digestion of source-sorted organic fraction of municipal solid waste. Bioresour. Technol. 99, 6763–6770. He, Yanfeng, Pang, Yunzhi, Liu, Yanping, Li, Xiujin, Wang, Kuisheng, 2008. Physiochemical characterization of rice straw pretreated with sodium hydroxide in the solid state for enhancing biogas production. Energy Fuels 24 (4), 2761–2766. Hu, Hongqiang, Fan, Yanzhen, Liu, Hong, 2008. Hydrogen production using singlechamber membrance-free microbial electrolysis cells. Water Res. 42, 4172– 4178. Hosseinin, Seyed Ehsan, Wahid, Mazlan Abdul, 2013. Feasibility study of biogas production and utilization as a source of renewable energy in Malaysia. Renew. Sustain. Energy Rev. 19, 454–462. Hsu, Teng-Chieh, Guo, Gia-Luen, Chen, Wen-Hua, Hwang, Wen-Song, 2010. Effect of dilute acid pretreatment of rice straw on structural properties and enzymatic hydrolysis. Bioresour. Technol. 101, 4907–4913. Hui, Wang, Jiajia, Li, Yucai, Lü, Peng, Guo, Xiaofen, Wang, Kazuhiro, Mochidzuki, Zongjun, Cui, 2013. Bioconversion of un-pretreated lignocellulosic materials by a microbial consortium XDC-2. Bioresour. Technol. 136, 481–487. Kim, Tae Hyun, Lee, Yoon Y., Sunwoo, Changshin, Kim, Jun Seok, 2006. Pretreatment of corn stover by low-liquid ammonia recycle percolation process. Appl. Biochem. Biotechnol. 133, 41–57. Liao, Wei, Liu, Yan, Liu, Chuanbi, Chen, Shulin, 2004. Optimizing dilute acid hydrolysis of hemicellulose in a nitrogen-rich cellulosic material-dairy manure. Bioresour. Technol. 94, 33–41. Lin, Yan, Tanaka, Shuzo, 2006. Ethanol fermentation from biomass resources: current state and prospects. Appl. Microbiol. Biotechnol. 69, 627–642. Liu, Hong, Grot, Stephen, Logan, Bruce E., 2005. Electrochemically assisted microbial production of hydrogen from acetate. Environ. Sci. Technol. 39, 4317–4320.
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Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Electropolar effects on anaerobic fermentation of lignocellulosic materials in novel single-electrode cells Guangfei Qu, Weixia Qiu, Yuhuan Liu, Dongwei Zhong, Ping Ning ⇑ Faculty of Environmental Science and Engineering, Kunming University of Science & Technology, Kunming, Yunnan 650500, China
h i g h l i g h t s Anaerobic fermentation in single electrode-assisted fermenter was carried out. The effect of electric polarity and micro-voltage was investigated. The system promote biogas and methane yield, and enhance lignocellulose degradation. The effectively promotional intensity of electric polarity was: cathode > anode > blank. The positive effect of different cathodic micro-voltage was: 250 mV > 500 mV > 100 mV.
a r t i c l e
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Article history: Received 7 December 2013 Received in revised form 11 February 2014 Accepted 14 February 2014 Available online 24 February 2014 Keywords: Micro-voltage Methane Lignocellulose Single-electrode Anaerobic fermentation
a b s t r a c t As a promising renewable energy technology, anaerobic fermentation is consistently limited by low production and calorific value of biogas, along with the difficulty of lignocellulose degradation. The effects of polarity and micro-voltage on anaerobic fermentation from lignocellulosic materials were investigated in single-electrode fermenter to explore cost-efficient technology. The results illustrated that the biogas production and quality were significantly affected by electric polarity. And cathode-assisted fermentation led to more positive effects than anode-assisted. Compared with results in control group without electrode, the average biogas and methane yield under cathodic micro-voltage ( 250 mV) were astonishingly improved by 2.82 and 2.44 mL g 1 d 1 respectively. Meanwhile, the degradation ratios of lignin and cellulose were also improved by 23.11% and 19.46%. It demonstrated that single micro-voltage can not only promote lignocellulose degradation but biogas production and calorific value. These micro-voltage effects on fermentation process also provided great opportunity to breakthrough the present limitation of lignocellulosic materials fermentation. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Nowadays, energy consumption is rising rapidly due to industrialization and progress in the standard of living. Fossil fuel such as natural gas, petroleum and coal which are mentioned as the most important energy sources in the world are limited and transitory (Basha et al., 2009). Energy insecurity, fossil fuel depletion and global warming issues have convinced the academic society and government to research about new fuel resources (Hosseinin and Wahid, 2013). Biotechnology, marked by its environmental protection, low input and high output, has become a high priority of government concern world-wide. So the interest in biomass resource has been stimulated by the promise of clean energy production. Methane fermentation, as a low cost approach that ⇑ Corresponding author. Tel.: +86 13888528030. E-mail address: qgfl[email protected] (P. Ning). http://dx.doi.org/10.1016/j.biortech.2014.02.052 0960-8524/Ó 2014 Elsevier Ltd. All rights reserved.
produces little sludge and generates methane gas as a renewable energy resource (Forster-Carneiro et al., 2008), is also believed as one of the most socio-economically cost-efficient renewable energy technologies, using organic waste and plant biomass as feedstock. Lignocellulose is a kind of important renewable resource and the most abundant organic raw material on the earth, which is composed of hemicellulose, cellulose and lignin. Although the microbial decomposition of such lignocellulosic materials has been studied extensively, most of the studies showed that substrates with relatively simple composition and structure, such as xylan (Hui et al., 2013) could be degraded completely. However, the natural lignocellulose with a complex composition and structure, such as crop straws and other plant fibres (Kim et al., 2006) were hardly to break down. The barrier is the structure of lignocellulose which has evolved to resist degradation due to cross-linking between the polysaccharides (cellulose and hemicellulose) and the lignin via
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ester and ether linkages (Lin and Tanaka, 2006; Xiao et al., 2007; Weib et al., 2010) which is highly recalcitrant (Neves et al., 2006). Bioelectrochemical reactors (BERs) are now attracting attention in part because microbial activity in them can be altered or controlled using external electrochemical system (Thrash and Coates, 2008). In addition, reactors with a potential of 0.6 or 0.8 V showed greater methanogenesis than control reactors without electrochemical regulation (Sasaki et al., 2010). BERs have been used previously for redox dye degradation (Park et al., 1999). Recently, a BER was used in methane fermentation from a mixture of artificial garbage slurry and rice straw as a model of complex organic materials, and a cathodic reaction effectively promoted methane production (Sasaki et al., 2011). All those previous works have mainly focused on biogas production or methane yield rather than lignocellulose degradation by electrochemical system effects. The optimum carbon–nitrogen (C/N) ratio for anaerobic fermentation is in the range of 20–25 (Yen and Brune, 2007). Dried cow manure has high C/N ratio of 23.2 (Liao et al., 2004), which make them a suitable co-substrate for anaerobic digestion. The aim of this paper is to investigate the effects of electric polarity on cellulose and lignin degradation ratio, biogas production yield and methane content during anaerobic fermentation from lignocellulosic materials, taking the cow manure as raw material due to its high concentration in lignocellulose. A better understanding of the influences of electrodes and voltage on methane fermentation would be useful from both a scientific and an engineering point of view.
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oxygen and sealed to maintain anaerobic condition. The fermentation tank was incubated in thermostatic shaker at 35 ± 2 °C for up to 30 days. 2.3. Analytical methods The selected responses for analysis were biogas production and the content of methane indicating the quality of biogas. The complex substrate was subjected to fibre analysis to determine their concentration of lignin and cellulose (Molinuevo-Salces et al., 2013). Production of biogas was collected with gas pocket, and then the volume was measured according to the displacement method (Moosvi and Madamwar, 2007). The composition of biogas was analysed using gas chromatograph (Molinuevo-Salces et al., 2010). The chemical groups of the lignocellulosic materials were determined by Fourier Transform Infra-Red Spectrometer (FT-IR) over the range of 500–4000 cm 1. The crystallinity of cow manure was investigated by X-ray diffraction (XRD). Cu Ka was used as monochromatized radiation source at 40 kV and 200 mA at room temperature. The samples were scanned and intensity recorded in 2h over the range of 5–60° with a step size of 0.02° and step time of 0.15 s. In addition, scanning electron microscopy (SEM) had been applied to examine surface morphology and assess structural changes of lignocellulosic materials. Samples from the beginning and the end of experiments were analysed. 3. Results and discussion
2. Methods 3.1. The effects of electrode on fermentation 2.1. Feed materials All media and solutions were prepared using deionised water and analytical grade reagents without further purification. Cow manure was obtained from Dengchuan, Dali, Yunnan province, China. The cow manure were dewatered in drying oven to constant weight, then crushed by pulveriser and kept in a dry place for using in subsequent experiments as substrate. The chemical compositions of cow manure are shown in Table 1. The inoculum was collected from the mesophilic fermentation plant in Dengchuan, Dali. Expanding culture of inoculum was carried out in laboratory at 35 ± 2 °C. 2.2. Experimental process Experimental operation was carried out in self-designed anaerobic fermenter equipped with a single stainless steel electrode, and the fermenter was heated by an electric-heated thermostatic shaker (Fig. 1). In order to control the potential of the working electrode, each electrode was connected to a potentiostat. All the assays were carried out in duplicate. Batch tests were conducted in fermenter filled with a certain weight of substrate and a predetermined volume of inoculum in which the percentage of total solid (TS) is 8%. To investigate the influence of electrode and voltage on methane fermentation, the experimental schemes were designed and given in Table 2 in detail. The fermentation tanks were purged with nitrogen gas to replace the initial contained
Table 1 Chemical composition of cow manure. Composition
Ash (%)
Volatile solid (%)
Organic materials (%)
Caloric value (J g 1)
Lignocellulose (%)
Cow manure
12.7
73.32
62.46
14,780
35.52
3.1.1. Effect on biogas production The effect of three different electrodes (cathode, anode and blank) at 2500 mV on biogas production in anaerobic fermentation was evaluated in batch assays, while other operating parameters remained the same. The variation of cumulative biogas production with time was presented in Fig. 2A. It could be observed that the relationship between time and cumulative biogas production for cathodic fermentation approached linear approximately: The longer the retention time lasted, the more the biogas produced all through the fermentation process. The cumulative biogas production of anodic group was lower than the blank group without voltage assistance during the first 16 days in fermentation process, from then on, however, the biogas yield was contrary to what observed before, i.e. there was more biogas produced by anodic group than blank group. And it should be noted that these two groups had only minor differences in cumulative gas production from one another. The statistical analysis showed no significant difference between the quantity of produced biogas in cathodic and blank group during the first 12 days of the fermentation process, but after that, the producing rate of biogas in cathodic reactor kept going straight, while that in blank tended to slow down. The cumulative biogas production tendency of anodic and blank group came to their own equilibrium value earlier compared to cathodic group. When the biogas production rate was calculated on the basis of cumulative biogas production and the quantity of substrate, it was clear that the cumulative rate can be reached to 0.1686 L g 1 in cathodic group, which was much higher than that in anodic (0.1005 L g 1) and blank (0.0940 L g 1) group during whole fermentation process. The raw material conversion was computed according to the assumption that per gram of dried cow manure can theoretically generate 300 mL biogas. The data was 56.20%, 35.50% and 31.34% in cathodic, anodic assisted system and blank system without voltage assisted respectively. As far as cumulative biogas production concerned, these results indicated that electric-assisted system could improve fermentation process
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Fig. 1. Schematic diagram of reaction system (A); graphic model of fermenter (B). 1. Potentiostat; 2. valve; 3. electric-heated thermostatic shaker; 4. quick joints; 5. gas pocket 6. vacuum chamber;7. vacuum pump.
from cow manure and cathodic voltage showed a particularly significant effect. The phenomenon may be due to that the electric field assisted system was an effective strategy for stimulating metabolism and growth of correspondent microbe. It was reported by Cheng, H that the voltage needed for hydrogen derived from the electrolysis of water was theoretically 1210 mV at neutral pH. In practice, 1800–2000 mV was needed for water electrolysis (under alkaline solution conditions) due to overpotential at the electrodes (Cheng et al., 2002). If hydrogen was produced at the cathode derived from acetate, the potential required at least E0 = 410 mV (NHE) at pH 7.0. The open circuit potential of the anode was about 328 mV in a typical anaerobic condition, so hydrogen can theoretically be produced at the cathode by applying a circuit voltage greater than 82 mV (i.e., 410–328 mV) according to Liu et al. (2005). Another study has addressed that there was hydrogen observed at a 0.6 V applied voltage (Hu et al., 2008). The difference of needed voltage for hydrogen production may be due to the differences in the substrates, microbial communities and the structure of fermentation reactors. Actually, in the fermentation process the situation was quite different because there was no circuit produced by applying just single electrode. It kept unknown that how the cathodic and anodic field affect the anaerobic fermentation and further investigation was required to confirm the surmise.
3.1.2. Effect on methane content The low voltage effect on methane concentration of biogas in anaerobic fermentation was shown in Fig. 2B. It was illustrated that the biogas quality was best in cathodic system, then anodic, the blank in sequence. That is to say, the cathodic voltage had a significantly positive effect on methane content in fermentation biogas. We raise an assumption that the cathodic condition was more suitable for methane-producing bacteria because methanogens were negatively charged. As can be seen from Table 2, and 84% as the maximum methane content was achieved in the cathode-assisted system, which was an abnormal phenomenon. In addition, the maximum methane content was 80% in the anode-assisted system and 77% in the system without electrode-assisted respectively. In general, the concentration of methane in biogas from this substrate is lower than 70% v/v (Xu et al., 2013; Sasaki et al., 2011; Triolo et al., 2012). The average content of methane was 67.44% in control group which was in consistence with normal value. However, both 71.04% in anodic group and 77.94% in cathodic group clearly exceeded the usual value. As mentioned above, regardless of which electrode was used, these electrode-assisted conditions were helpful to improve the biogas production rate and quality, especially in cathodic condition. In order to investigate if there is the similar tendency and influence efficiency at lower voltage, the further experiments were carried out under the micro-voltage conditions.
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Fig. 2. Time-dependent changes in the gas production rate with different electrodes (A) or voltages (C). The distributive characters of methane with different electrodes (B) or voltages (D).
3.1.3. Effect on degradation of lignocellulose The lignocellulose accounts for approximately 35.52% of cow dung, which was difficult to be biodegraded by anaerobic microorganism, especially the sclerenchyma and xylem included in lignin tissues, resulting in low quality of biogas production and high quantity of biogas residue in anaerobic fermentation. Consequently, the biodegradation of lignocellulose was essential for the generation of biogas. So the main research goal of this paper was to investigate the efficiency of lignocellulose degradation in the process of electrode-assisted fermentation. Anaerobic digestion with cathode assistance presented the highest degradation ratio (up to 56.20%) of lignin, whereas 34.8% and 30.14% of degradation ratio were observed in anodic and blank group, respectively (Fig. 3A). On the other hand, the degradation ratio of cellulose was no significantly different in the system with electrode or not, however, it was still higher in electrode-assisted system than that without electrode. This effect could be explained by the promotion of electrodes in broking hydrogen-bonds and the crystal structure of lignocellulose. From the quantitative point of view, the degradation ratio of lignin was higher than that of cellulose in cathode-assisted system. While in other two fermentation systems, the degradation of lignin and cellulose was no significant difference. In other words, only the cathodic fermentation condition had positive effect on the degradation ratio of lignin. The results suggested that the microorganisms in cathodic fermentation system may be more inclined to degrade lignin than cellulose. On the other hand, the microorganisms and enzyme in cathodic condition became more reactive because most microbes are negatively charged. 3.2. The effects of cathodic micro-voltage on fermentation As discussed previously, it was obvious that the process of fermentation could be remarkably promoted by external low voltage,
especially cathodic voltage. But it was still uncertain whether there were effects of cathodic micro-voltage on fermentation, so further digestion experiments with cathodic assistance were carried out to investigate the effects of cathodic micro-voltages on anaerobic fermentation. 3.2.1. Effect on biogas production The biogas production with cathode electrode at different levels of micro-voltage ( 100 mV, 250 mV and 500 mV) was evaluated in batch assays, while other operating parameters remained the same. Cumulative biogas production for micro-voltages tested over time was shown in Fig. 2C. As expected, there was a significant positive effect of voltage on the biogas production. It could be observed that daily biogas production rate decreased over time due to the deteriorating condition of bacterial community, which caused by the lack of essential nutrition and bacteria’s own supersession. Although the growth trends of cumulative biogas production were similar, the rates of increase were notably different. It could be calculated that the cumulative biogas production could reach to 106.53, 174.77 and 134.80 mL g 1 at 100 mV, 250 mV and 500 mV respectively. In other words, the intensity of promotion in cumulative biogas production with cathodic micro-voltage increased as follows: 100 mV < 500 mV < 250 mV. The reason why 250 mV showed higher accumulation of biogas in comparison with 100 or 500 mV may be due to the following two aspects: firstly, the 250 mV potential created optimum condition for the growth and development of microorganism. Moreover, the 250 mV potential was much more favourable for the conversion of lignin and cellulose to small particles. So the utilization ratio of raw materials was enhanced. The average biogas production rate reached 0.1748 L g 1 at 250 mV, corresponding to 58.27% of raw material conversion. Meanwhile, the systems with 100 mV and 500 mV, showed lower gas production rate (0.1065 L g 1, 0.1348 L g 1), and lower raw material conversion
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methane and biogas yield, separately. The average methane content was also increased with cathodic micro-voltage. And these average dates were all higher than the common value without voltage assistance. These results strongly confirm the assumption that there was positive impact of cathodic micro-voltage on methane content through fermentation. As the most suitable redox potential for methanogens is from 150 mV to 400 mV. When the cathodic voltage was 250 mV, the micro-voltage perhaps was the closest value of the optimum redox potential for methanogens. Thus, under this level of voltage, the activity of methanogens was highest resulting in more stable of methane content in comparison with 100 mV and 500 mV. In addition, methane might be electrochemically generated from the reduction of CO2 via a cathodic reaction and hydrogenotrophic methanogens (Villano et al., 2010). What discussed above was just an assumption, the exactly mechanism of voltage effect on methane content needs further investigation.
Fig. 3. Degradation ratio profile of lignin and cellulose in cow manure after anaerobic fermentation with different electrodes (A) or different voltages (B).
(35.50%, 44.93%) than that of anaerobic fermentation system with 250 mV. 3.2.2. Effect on methane content The effect of different cathodic micro-voltage on methane content was shown in Fig. 2D. It was found that the cathodic microvoltage had a significantly positive effect on methane content in fermentation biogas. In addition, the effect of different micro-voltage on methane content was of great difference from each other. Just as the results of experiment illustrated in Table 2, the maximum content of methane in 100 mV, 250 mV and 500 mV cathodic field assisted system was 78%, 79% and 81%, respectively. The maximum methane content was increased with cathodic micro-voltage. And the average methane content of corresponding conditions was 72.39%, 76.67%, and 75.28% based on the average
3.2.3. Effect on degradation of lignocellulose As showed in Fig. 3B, the lignin and cellulose removal ratio was up to 53.25% and 50.80% in system with 250 mV cathodic field. Whereas 37.51% and 44.93% of lignin was removed in system with 100 mV and 500 mV, along with the cellulose removal increased from 32.02% to 45.14% when the cathodic voltage increased from 100 mV to 500 mV, respectively. The degradation ratio of lignin and cellulose at 250 mV was much higher than that at 100 mV and 500 mV, indicating that the voltage of 250 mV was more suitable for the reproduction of hydrolysis microbe or hydrolysis of functional enzyme. The more reactive of microorganisms became, the higher conversion ratio of raw materials would be got. Just because 250 mV was the closest micro-voltage to the optimum redox potential, the activity of microbes were stronger than that in other two systems with 100 mV or 500 mV. So the micro-voltage of 250 mV had more positive effect on degradation of lignin and cellulose in comparison with 100 or 500 mV. Although the exactly boosting mechanism of different micro-voltage on lignocellulose degradation was still unclearly, the positive effect of micro-voltage on the decomposition of lignocellulose was absolutely without doubt. 3.3. Characterization of cow manure before and after fermentation 3.3.1. FT-IR analysis The anaerobic fermentation assisted with electric field was analysed based on the FT-IR data of different stage. Just as the data showed, the stretching and bending vibrations observed in the spectra were of different intensities before and after fermentation. The most significant difference in spectra between undigested and digested with cathodic electrode-assisted system was the intensity of the peak at 2925 cm 1 and 3422 cm 1 which represented wagging vibrations in C–H and the stretching vibrations of H-bonded OH group in cellulose. The height of the peak at 2925 cm 1 and
Table 2 Specific experimental data for anaerobic fermentation. Substrate (g)
Inoculum (mL)
Electrode
Applied voltage (mV)
Maximum biogas yield (mL g 1 d 1)
Maximum methane content (%)
Average methane yield (mL g 1 d 1)
Average biogas yield (mL g 1 d 1)
Average methane content (%)
30 30 30 30 30 30
350 350 350 350 350 350
Cathode Anode Blank Cathode Cathode Cathode
2500 2500 – 100 250 500
8.17 12.08 10.88 9.47 15.67 11.33
84 80 77 78 79 81
4.38 2.38 2.03 2.57 4.47 3.38
5.62 3.35 3.01 3.55 5.83 4.49
77.94 71.04 67.44 72.39 76.67 75.28
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3422 cm 1 for digested samples was almost half of that for undigested sample. It could be inferred that those chemical bonds in cellulose had been degraded partially. A decrease in absorbance at 1645 cm 1 and increase in absorbance at 1047 cm 1, 469 cm 1 were observed, showing the decrease in the number of stretching vibrations in C–OH groups as well as increase in bending and wagging vibrations in C–O and organic halogenated hydrocarbon C–X. These results indicated the rupture of hydrogen bonds in lignocellulose. On the other hand, the absorbance at 1645 cm 1 still existed after fermentation, illustrating that the lignin was just partly degraded. 3.3.2. X-ray diffraction studies On the basis that two peaks presented at 2h of 22° and 35° correspond to two important crystals of cellulose (He et al., 2008). The results of X-ray diffraction patterns of materials before or after fermentation showed that the major peak at 2h of 22° was the main peak representing the presence of a highly organised crystalline region, whereas the smaller peak at 2h of 35° was the less organised region. The intensity of the peak at 2h of 22° for samples from electrode-assisted fermentation system was obviously weakened compared with that from none digestion. Then crystallinity of 11.18% was calculated in raw material by using Turley method. Just as expected, a great decrease in crystallinity (5.94%) was observed in the case of electrode-assisted fermentation, indicating structural changes occurred in the crystalline regions of cellulose, and that the decreased crystallinity of cellulose was very crucial for enzymatic hydrolysis. As it is well known that amorphous cellulose is more reactive, the XRD study was in line with the enhancement of degradation of lignin in the electrode-assisted fermentation system. 3.3.3. Microstructure analysis After fermentation, the morphology of substrate was of great difference from raw materials. According to SEM images, the rupture of lignocellulose structure and increase in porosity were believed in electrode-assisted fermentation, which was due to the hydrolysis of lignin, cellulose in cow manure and merging of smaller pores into large pores. The bigger pore size had contributed to fungal growth during fermentation (Hsu et al., 2010). Compared to raw material, the particles from electrode-assisted system were smaller and the distribution of grain sizes was much homogeneous after fermentation, indicating that electrode-assisted fermentation system had a beneficial effect on improving the degradation of lignocellulose. Because of degradation, lignocellulose become basic structure unit ramifications contained phenyl of lignin, some of which were decomposed to compound of small molecular weight that was beneficial for microorganism biodegradation. These differences might explain the effect of electric condition on compact regions of (crystalline zones) cellulose. Those results of characterization further confirmed the positive effect of single electrode-assisted system on anaerobic fermentation. The electrode-assisted fermentation system is not limited to biogas fermentation of cow manure. Theoretically speaking, it also inspires efficient decomposition of any other high-concentration lignocellulosic materials. Finally, it is necessary to analyze the effectiveness of the electrode-assisted fermentation on the basis of energy consumption and production. In the novel single-electrode fermenter, the energy consumed per day was so little duo to no circuit was produced that the energy consumption could be negligible. Even the applied voltage was much higher; the consumption energy was still low for the same reason. At the voltage of 250 mV, the amount of methane produced daily was 4.47 mL g 1 (Table 2) in average at experimental temperature 35 °C. According to reaction enthalpy (DrHhm ) of methane which is 890.36 kJ mol 1 under standard conditions, 0.157 J g 1 of
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energy as methane was obtained under standard conditions daily. However, the maximum 0.435 J g 1 of energy (methane production: 12.35 mL g 1) was stably obtained in 250 mV reactor. Thus, in this novel single-electrode fermenter, a large amount of methane energy was obtained with the consumption of a much lower electric energy. What is more, the degradation ratio of cellulose and lignin was significantly improved. Such effects may promise limitless application in methane fermentation industry. 4. Conclusions Electric polarity applied in anaerobic fermentation can not only influence biogas production and methane content but also affect lignocellulose degradation remarkably. Much more positive effects were obtained in cathode-assisted fermentation system than in anode ones. For the fermentation with different cathodic micro-voltage, the intensity of promotion was increased as 100 mV < 500 mV < 250 mV. Characterization results further confirm the effectiveness of single-electrode fermenter. Considering the wide influence of the single-electrode system or the polarity effects on efficient utilization of lignocellulosic materials, it is necessary to investigate the detailed mechanism of how singleelectrode assisted system promotes methane fermentation in further research. Acknowledgement The authors acknowledge the research grant provided by the National Major Projects of Science and Technology for Water Pollution Control and Management (Project No. 2008ZX07105-002). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.biortech.2014. 02.052. References Basha, Syed Ameer, Gopal, K. Raja, Jebaraj, S., 2009. A review on biodiesel production, combustion, emissions and performance. Renew. Sustain. Energy Rev. 13, 1628–1634. Cheng, H., Scott, K., Ramshaw, C., 2002. Intensification of water electrolysis in a centrifugal field. J. Electrochem. Soc. 149 (11), 172–177. Forster-Carneiro, T., Pérez, M., Romero, L.I., 2008. Thermophilic anaerobic digestion of source-sorted organic fraction of municipal solid waste. Bioresour. Technol. 99, 6763–6770. He, Yanfeng, Pang, Yunzhi, Liu, Yanping, Li, Xiujin, Wang, Kuisheng, 2008. Physiochemical characterization of rice straw pretreated with sodium hydroxide in the solid state for enhancing biogas production. Energy Fuels 24 (4), 2761–2766. Hu, Hongqiang, Fan, Yanzhen, Liu, Hong, 2008. Hydrogen production using singlechamber membrance-free microbial electrolysis cells. Water Res. 42, 4172– 4178. Hosseinin, Seyed Ehsan, Wahid, Mazlan Abdul, 2013. Feasibility study of biogas production and utilization as a source of renewable energy in Malaysia. Renew. Sustain. Energy Rev. 19, 454–462. Hsu, Teng-Chieh, Guo, Gia-Luen, Chen, Wen-Hua, Hwang, Wen-Song, 2010. Effect of dilute acid pretreatment of rice straw on structural properties and enzymatic hydrolysis. Bioresour. Technol. 101, 4907–4913. Hui, Wang, Jiajia, Li, Yucai, Lü, Peng, Guo, Xiaofen, Wang, Kazuhiro, Mochidzuki, Zongjun, Cui, 2013. Bioconversion of un-pretreated lignocellulosic materials by a microbial consortium XDC-2. Bioresour. Technol. 136, 481–487. Kim, Tae Hyun, Lee, Yoon Y., Sunwoo, Changshin, Kim, Jun Seok, 2006. Pretreatment of corn stover by low-liquid ammonia recycle percolation process. Appl. Biochem. Biotechnol. 133, 41–57. Liao, Wei, Liu, Yan, Liu, Chuanbi, Chen, Shulin, 2004. Optimizing dilute acid hydrolysis of hemicellulose in a nitrogen-rich cellulosic material-dairy manure. Bioresour. Technol. 94, 33–41. Lin, Yan, Tanaka, Shuzo, 2006. Ethanol fermentation from biomass resources: current state and prospects. Appl. Microbiol. Biotechnol. 69, 627–642. Liu, Hong, Grot, Stephen, Logan, Bruce E., 2005. Electrochemically assisted microbial production of hydrogen from acetate. Environ. Sci. Technol. 39, 4317–4320.
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