Bioresource Technology 205 (2016) 104–110
Contents lists available at ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Electricity generation from food wastes and characteristics of organic matters in microbial fuel cell Hui Li b, Yu Tian a,b,⇑, Wei Zuo b, Jun Zhang b, Xiaoyue Pan c, Lipin Li b, Xinying Su d a
State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology (SKLUWRE, HIT), Harbin 150090, China School of Municipal and Environmental Engineering, Harbin Institute of Technology, Harbin 150090, China c Beijing Tsinghua Tongheng Urban Planning & Design Institute, Beijing 100085, China d School of Food Engineering, Harbin University of Commerce, Harbin 150076, China b
h i g h l i g h t s The MFC was used for food waste disposal and electricity generation. 3
The maximum power density of 5.6 W/m was achieved in the MFC. Characteristics of the food waste before and after MFC treatment were investigated. Biodegradation regularity of the food waste in MFC was discussed. The results may assist pre- and post-treatment choices for MFC fed with food waste.
a r t i c l e
i n f o
Article history: Received 6 October 2015 Received in revised form 9 January 2016 Accepted 11 January 2016 Available online 23 January 2016 Keywords: Microbial fuel cell Food waste Electricity generation Organic matter Fractionation
a b s t r a c t The microbial fuel cell (MFC) was evaluated as an alternative way to recover electricity from canteen based food waste. Characteristics of the organics in food waste before and after the MFC treatment were analyzed to investigate how the organic matters were biodegraded and transformed during the MFC treatment. A maximum power density of 5.6 W/m3 and an average output voltage of 0.51 V were obtained. During the MFC operation, the hydrophilic and acidic fractions were more readily degraded, compared to the neutral fractions. Additionally, aromatic compounds in the hydrophilic fraction were more preferentially removed than non-aromatic compounds. The MFC could easily remove the tryptophan protein-like substances in all fractions and aromatic proteins in hydrophilic and hydrophobic neutral fractions. Additionally, the hydrophobic amide-1 proteins and aliphatic components were readily hydrolyzed and biodegraded in the MFC. These findings may facilitate the pretreatment and posttreatment choices for MFC system fed with food waste. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Food waste is organic waste comes from various sources including households, restaurants, cafeterias and so forth, accounting for a considerable portion of the municipal solid waste (Uçkun Kiran et al., 2014). It was reported by the Food and Agricultural Organization that one third of the food produced in the world for human consumption (1.3 billion tons per year) is wasted (FAO, 2012). In China, about 60 million tons of food wastes were produced yearly, and the daily food waste generation in Beijing has reached approximately ⇑ Corresponding author at: State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology (SKLUWRE, HIT), Harbin 150090, China. Tel./fax: +86 451 8628 3077. E-mail address: [email protected] (Y. Tian). http://dx.doi.org/10.1016/j.biortech.2016.01.042 0960-8524/Ó 2016 Elsevier Ltd. All rights reserved.
1.6 thousand tons (Meng et al., 2014). With large amount of organic matters, high moisture and high salinity (Meng et al., 2015), the food waste would cause serious environmental contamination and health threat without proper treatment (Shen et al., 2013). Nowadays most of the food wastes are disposed by the conventional methods such as landfill, compost and incineration, which could lead to ground water contamination, vermin attraction and toxic gas emission (Goud and Mohan, 2011). Additionally, the conventional methods are unsustainable and uneconomical, as the valuable source of nutrients and energy in the food waste cannot be fully or efficiently used. Thus, exploiting the highly biodegradable waste as an alternative source for energy recovery with simultaneous treatment could be an attractive approach. Anaerobic digestion is increasingly accepted as an effective technology for organic waste treatment and bio-energy recovery,
H. Li et al. / Bioresource Technology 205 (2016) 104–110
and it has been considered as an attractive and sustainable approach to viably overcome the current global energy crisis (Huang et al., 2015). The food waste is inexhaustible resource with high energy potential and good biodegradability, and significant energy recovery could be achieved during the anaerobic digestion (Grimberg et al., 2015). As a promising anaerobic waste treatment device, the microbial fuel cell (MFC) can use microorganisms as catalyst to recover electricity from diverse organic wastes including domestic wastewater (Ahn et al., 2014), industrial wastewater (Feng et al., 2015) and excess sludge (Jiang et al., 2009). The MFC could efficiently achieve safe, clean and direct electricity production and simultaneous organic matter removal. Therefore, employing food waste as a substrate in MFC for electricity generation could be a promising approach for food waste treatment and energy recovery. Recently, some researches on the MFC which directly employ the food waste as substrate have been reported (Jia et al., 2013; Moqsud et al., 2014), however, how were the organic matters of the food waste chemically converted during the MFC treatment has not been explored. The organic matters are the major constituents of food wastes and mainly consist of various high molecular-weight polymers such as carbohydrate, cellulose, protein, and lipid. In the MFC fed with food waste, the organic matters are the energy sources of the electricigens for the electricity production and the characteristics of the organic matters significantly affect the electricity generation efficiency. Additionally, hydrolysis of the organic matters is commonly known as the rate limiting step for electricity generation (Ma et al., 2014). In order to accelerate hydrolysis of the substrate for more efficient electricity recovery, pretreatment processes were often required, such as sonication (More and Ghangrekar, 2010) and microwave-pretreatment (Yusoff et al., 2013). Additionally, to minimize the environmental pollution of the food waste, post-treatment process may be needed for the food waste after the MFC treatment. For appropriate selection of pretreatment and post-treatment method, it is necessary to research the characteristics and the biodegradation regularity of the organic matters in the food waste during the MFC treatment. In this study, the food wastes collected from canteen were exploited as anodic fuel for direct energy generation in a singlechamber MFC. The electrochemical performance of the MFC was studied. Additionally, the food waste organics before and after the treatment were fractionated into five different fractions by XAD resins, and the properties of the organic fractions were investigated. This research aims at revealing the transformation characteristics of the organic matters in the food waste during the MFC treatment and determined which fraction of the organic matters in the food waste could be preferentially degraded. Such knowledge might assist in the understanding of the compositions and chemical properties of organic matter in food waste and the biodegradation regularity of the organics during MFC treatment, and, in turn, may facilitate the pretreatment and post-treatment choices for MFC system fed with food waste.
2. Methods 2.1. Food wastes The food waste used as the substrate in the MFC was collected from the student canteen at Harbin Institute of Technology, Harbin, China. The food waste mainly comprises boiled rice, vegetables, fruit, cooked meat, bones, as well as plastics. In order to supply a suitable substrate for the MFC, the bones and plastics in the food waste were first removed, and then the left food waste was pulverised in an electrical pulverizer (JYL-C051, Joyoung, China) for two minutes. Avoiding the clogging problem, the pulverised food
105
waste was filtered through a stainless steel mesh with an average pore diameter of 2 mm to remove the coarse material. The oil in the food waste was separated by gravity. The COD of the pretreated food waste was 71,000–90,000 mg/L. Before fed into the MFC, the food waste was diluted to a COD concentration of 2700 ± 20 mg/ L. Finally, the pH of the diluted food waste was adjusted to 7 by NaOH solution (1 M). 2.2. MFC configuration and operation A single-chamber air cathode MFC with a working volume of 120 ml was constructed using ‘‘Perspex” as previous description (Sevda et al., 2013). The spacing between the anode and cathode placed on opposite sides was 4 cm. The anode was made of carbon cloth without any pretreatment. The carbon cloth based cathode containing 10% platinum and three diffusion layers was prepared according to the method reported previously (Cheng et al., 2006). Before making the diffusion layers, a carbon base layer was prepared by applying a mixture of carbon powder and 30 wt% PTFE solution onto one side of the carbon cloth, air-drying at room temperature for 2 h, followed by heating at 370 °C for 30 min. Additional diffusion layers were made by brushing a PTFE solution (60 wt%) onto the coating side, followed again by drying at room temperature and heating at 370 °C for 10 min. Diffusion layers were applied for 3 times. 10% Pt/C catalyst (0.5 mg Pt/cm2) was then applied to the other side (water-facing side) of the carbon cloth using Nafion as a binder. The MFC was inoculated with anaerobic sludge from the laboratory-scale anaerobic reactor and the prepared food waste was directly used as the substrate. The external resistance connected across the anode and cathode was 1000 X. The MFC was repeatedly filled with the inoculant and substrate until the constant output voltage was obtained. After about one month’s inoculation, the MFC achieved stable performance, and the MFC was changed to using food waste as substrate without anaerobic sludge. The MFC fed with food waste was operated in batch mode at room temperature and was refilled when the output voltage decreased to lower than 100 mV, which formed an operation cycle. 2.3. Fractionation procedure The food waste was fractionated into five fractions namely hydrophilic (HPI), hydrophobic acid (HPO-A), hydrophobic neutral (HPO-N), transphilic acid (TPI-A) and transphilic neutral (TPI-N) fractions using XAD-8/XAD-4 resins according to the reported method (Chow et al., 2006). Briefly, 2 L of the food waste sample was acidified to pH 2 using HCl and passed through the XAD-8 and XAD-4 resin columns in series. HPI was the organic matter not retained on either resin and existed in the XAD-4 effluent. The XAD-8 and XAD-4 resin columns were backward eluted using 200 ml NaOH (0.1 mol/L) and 50 ml Milli-Q water, and the eluate were HPO-A and TPI-A fractions, respectively. After eluation by the NaOH and Milli-Q water, the HPO-N and TPI-N still adsorbed on the XAD-8 and XAD-4, respectively. And the HPO-N and TPI-N were respectively desorbed by 75% acetonitrile +25% Milli-Q water solution from the XAD-8 and XAD-4. Finally, the acetonitrile existed in the HPO-N and TPI-N samples was removed using rotary evaporation. 2.4. Analysis The output voltage across the external resistance was monitored every 1 min using a paperless recorder (VX5100R/C2/U, Hangzhou, China). The polarization and power density curves were measured by varying the external resistance from 5000 to 50 X. Power (W) was calculated according to the formula P = U I.
106
H. Li et al. / Bioresource Technology 205 (2016) 104–110
Power density (W/m3) and current density (A/m3) were calculated by dividing the output power and current with the volume (m3) of the anode chamber. The TOC was measured by a Shimadzu TOC-5000 Total Organic Carbon analyzer. The Ultraviolent (UV) absorbance of samples was measured by a T6 ultraviolet spectrophotometer at 254 nm (UV-254) using a quartz cuvette having a path length of 1 cm. The specific ultraviolet absorbance (SUVA) was calculated according to the equation (UV-254/DOC)/100, where DOC is the dissolved organic carbon. The excitationemission matrix (EEM) spectra were monitored using a Jasco FP-6500 spectrofluorometer by scanning excitation spectra from 220 to 400 nm with increments of 5 nm, detecting the emission wavelength between 280 and 550 nm with 2 nm steps. The slits were both maintained at 5 nm for both excitation and emission and the scanning speed was 2000 nm/min. In addition, the samples for excitation–emission matrix (EEM) measurements were all diluted to 1 mg/L of TOC with distilled water. For fouriertransform infrared spectroscopy (FTIR) analysis, 5 mg lyophilized sample powder and 200 mg KBr were mixed and pressed into a pellet and measured immediately in an FTIR spectrophotometer (Spectrum One B, PerkinElmer, USA) scanning from 4000 to 400 cm 1.
Fig. 2. Variation of power density and polarization curves for the MFC fed with food waste.
the MFC could recover electricity from food waste with efficient removal of the organic matters.
3. Results and discussion
3.2. Treatment efficiency
3.1. Power generation of the food waste-MFC
In order to assess the biodegradabilities of the food waste applied to the MFC, the TOC concentrations of organic matters in food waste before and after the MFC treatment were investigated (Fig. 3). The XAD fractionation results showed that the HPI was the major fraction of the food waste, accounting for 44.9% of the bulk TOC, followed by the HPO-A (14.6%), HPO-N (13.9%), TPI-N (13.8%) and TPI-A (12.8%). From the Fig. 3, it could be observed that the bulk TOC of the food waste decreased from 385.6 mg/L to 74.2 mg/L after the MFC treatment, resulting in a removal efficiency of 80.8%, which demonstrated that the microorganism in the MFC could efficiently degrade the organic matters in the food waste. During the operation of the MFC, the highest TOC removal efficiency of 87.4% was for HPI, meaning that the hydrophilic fraction of the food waste could be readily removed by the MFC for electricity generation. The TOC removal efficiencies of the acidic fractions were 83.1% for HPO-A and 81.8% for TPI-A, while the degradation efficiencies of the neutral fractions were relatively low (73.7% for HPO-N and 65.3% for TPI-N). Additionally, the fractions of the HPO-N and TPI-N were significantly increased from 13.9% and 19.4% to 19.4% and 25.3%, respectively, which demonstrated that the HPO-N and TPI-N were resistant to the biodegradation of the bacteria in the MFC compared with HPI, HPO-A and TPI-A fractions (Jiang et al., 2010). However, during the operation, the different fractions may inter-transform in the
After a 30 days’ successful acclimation period, stable electricity generation was achieved in the MFC (Fig. 1a). In the following operation, the MFC was fed with food waste. As shown in Fig. 1b, after the food waste was fed into the MFC, the voltage rapidly increased to higher than 0.54 V in 6 h, and then remained stable at an average voltage of 0.51 V until the 116th hour when the voltage began to dramatically decrease. Additionally, the voltage remained higher than 0.1 V before the 202nd hour. The polarization curve was measured by varying the external resistance to determine the maximum output power density and the internal resistance (Fig. 2). It could be observed that the open circuit voltage of the MFC was 0.7 V, and the maximum power density of 5.6 W/m3 was obtained with a current density of 15.3 A/m3. The voltage and power outputs achieved in this research were lower than MFCs fed with simple substrates like glucose and acetate (Cheng and Logan, 2011), mainly attributed to that the complex organics in the food waste could not be readily degraded in MFC, which limited the electricity generation (Lee et al., 2008). However, the obtained voltage and power outputs were comparable with other single chamber MFCs using food waste as substrates (Jia et al., 2013). Additionally, an average COD removal efficiency of 90.3% was obtained in the MFC. These results demonstrated that
Fig. 1. Variations of voltage output with time for the MFC during inoculation period (a) and one operation cycle after food waste addition (b).
H. Li et al. / Bioresource Technology 205 (2016) 104–110
107
Fig. 3. TOC concentrations of the organic fractions in raw food waste and MFCtreated food waste.
MFC. Therefore, in order to further investigate the biodegradabilities of the five fractions in the food waste, each fraction was used as the sole substrate of the MFC, and the removal efficiency was evaluated after about 200 hours’ operation. Before fed into the MFC, each fraction was diluted to a TOC concentration of about 50 mg/L. After the MFC treatment, the removal efficiency of each fraction showed a decrease trend of HPI (92.4%) > HPO-A (80.6%) > TPI-A (77.5%) > HPO-N (72.9%) > TPI-N (64.4%) as shown in Table S1. This result indicated that the HPI exhibited the highest biodegradability among the five fraction, and the acidic fractions could be more readily biodegraded in the MFC compared to the neutral fractions, which was in accordance with the discussion above. In addition, compared with the MFC fed with raw food waste, the MFCs using each fraction as single substrate achieved a higher removal efficiency for HPI and lower removal efficiencies for other four fractions, inferring that biodegradations of hydrophobic and transphilic fractions might induce formations of hydrophilic substances.
3.3. Change in aromaticity of food waste It is generally accepted that the UV-254 can represent the aromaticity of organics (Her et al., 2003). The UV-254 for the original unfractionated raw food waste and organic fractions before and after MFC treatment are presented in Fig. 4a. In the raw food waste, the HPI was the principal component, representing 77% of the total UV-254. The remaining organic fractions were HPO-N, HPO-A, TPIA and TPI-N (11.5%, 6.5%, 2.7% and 2.3%, respectively). The results from Fig. 4a showed that the removal efficiency of the UV-254 for the bulk organics of the food waste was 81.7%, and the order of the UV-254 removal with respect to the five fractions was observed to be HPI (90.2%) > TPI-A (77.4%) > HPO-A (72.1%) > HPO-N (43.1%) > TPI-N (26.7%). These results indicated that the bulk UV-254 reduction of the food waste was dominated by the removal of the aromatic compounds in HPI, TPI-A and HPO-A. The HPI fraction was mainly consisted of polar compounds of low molecular weights, presenting the most readily degradable compounds (Namour and Muller, 1998). Thus, the aromatic compounds in the hydrophilic fraction (HPI) could be more readily degraded by the microbes in the MFC (Zhang et al., 2015) compared to the hydrophobic and transphilic fractions (HPO-A, HPON, TPI-A and TPI-N). Moreover, the aromatic intermediates generated during the biodegradation of the hydrophilic aromatic substances might transform into other fractions, inducing the lower content of aromatic compounds in HPI fraction of the MFC-
Fig. 4. UV-254 (a) and SUVA (b) values for the organic fractions of the raw food waste and MFC-treated food waste.
treated food waste. Higher UV-254 removal efficiencies of the HPO-A and TPI-A fractions compared to HPO-A and TPI-A fractions suggested that the aromatic compounds in acidic fractions were preferential for the microbe in the MFC to degrade. Westerhoff and Pinney (2000) demonstrated that biofilm could preferentially absorb and remove the organics with higher polarity. As reported by Henriksen et al. (2005), acidic organics have higher polarity than the neutral organics. Thus the biofilm attached on the electrode might more readily absorb and remove TPI-A and HPO-A fractions compared to the HPO-N and TPI-N fractions, inducing higher aromatic compounds removal efficiencies of acidic fractions compared to the neutral fractions. The specific ultraviolet light absorbance (SUVA) calculated as (UV-254/DOC)/100 represents the relative aromaticity of organics. The SUVA of HPI, HPO-N, HPO-A, TPI-A and TPI-N isolated from the raw food waste had average value of 1.4 L/m mg, 0.7 L/m mg, 0.4 L/m mg, 0.2 L/m mg and 0.1 L/m mg (Fig. 4b), respectively, suggesting that HPI contained a relatively high amount of aromatic structures. The SUVA for the bulk organics of the food waste was decreased by 5% after the MFC treatment. However, among the five fractions, only HPI showed a decrease (22.5%) in the SUVA after the MFC treatment. The remaining four fractions all represented an increase in aromaticity after the treatment of MFC. The aromaticity enhancement might be attributed to the preferential removal of non-aromatic compounds or formation of aromatic products by the biodegradation during the MFC operation (Xue et al., 2009).
108
H. Li et al. / Bioresource Technology 205 (2016) 104–110
3.4. Characteristics of organic matters in food waste 3.4.1. Fluorescence spectroscopic analysis Fluorescence EEMs were conducted on the organic fractions of the food waste before and after MFC treatment (Fig. 5). Four main peaks could be identified from the fluorescence spectra of the food waste corresponding to the regions reported by Chen et al. (2003). The first main peak was located at the excitation/emission wavelengths (Ex/Em) of 220–240/320–360 which was related to simple
aromatic proteins such as tyrosine (region I). The second peak was observed at excitation wavelengths shorter than 250 nm and emission wavelengths longer than 350 nm, which was described as the fulvic acid-like substances (region II). The third peak at the Ex/Em of 260–290/320–380 nm was associated with the tryptophan protein-like substances (region III). The fourth peak was located at longer excitation wavelengths (>280 nm) and emission wavelengths (>380 nm), correlated with humic acid-like organics (region IV).
Fig. 5. EEM fluorescence spectra of organic fractions in the raw food waste and MFC-treated food waste.
H. Li et al. / Bioresource Technology 205 (2016) 104–110
All of the five fractions in raw food waste represented obvious fluorescent peaks in region III, revealing that the organic matters in the raw food waste were dominated by the tryptophan protein-like substances. Additionally, the HPI, HPO-N and TPI-N also showed distinct aromatic protein peaks in region I and HPOA and TPI-A showed humic acid-like peaks in region. After the treatment of the MFC, the intensities of the peaks in region III for all organic fractions of the food waste were significantly decreased, implying the reduction effects of the tryptophan protein-like substances due to the MFC treatment. It could be observed that the aromatic protein peak intensities of the HPI and HPO-N were also distinctly reduced after the MFC operation. Conversely, the aromatic protein peak intensity was increased for the TPI-N following the MFC operation, demonstrating the possible formation of new soluble aromatic proteins via the hydrolysis of the food waste. The fluorescence peaks for the HPI were all substantially decreased during the MFC treatment, suggesting that the hydrophilic fluorescent substances could be easily degraded by the microbes in the MFC, which was accordant with the result reported by Jiang et al. (2010). Compared to the raw food waste, the intensities of the humic acid-like peaks for HPO-A and TPI-A of the MFCtreated food waste were both enhanced accompanied with the emergence of the new fluorescence peaks for fulvic acid-like substances. For the effluent HPO-N, two new fluorescence peaks of humic acid-like and fulvic acid-like compounds emerged, accompanying the disappearance of the aromatic protein and tryptophan protein-like peaks, although the fulvic acid-like peak was more like shoulder. These results demonstrated that the tryptophan proteinlike substances in all fractions and the aromatic protein in HPI and HPO-N could be preferentially degraded by the MFC, and the humic acid-like and fulvic acid-like compounds of HPO-A, HPO-N and TPIA were enriched following the MFC treatment. It has been widely reported that the humic substances mostly composed of humic acids and fulvic acids (FA) mainly exhibited relative hydrophobicity (Liu et al., 2012). Thus the humic substances mainly accumulated in hydrophobic and transphilic fractions rather than the hydrophilic fraction of the food waste after the MFC treatment. Humic substances contain relatively large amounts of aromatic carbon (Park et al., 2006), therefore, enrichment of these substances might be one main reason for the aromaticity increases of the hydrophobic and transphilic fractions. As shown in Fig. 5, the accumulation of humic substances in HPO-N was much higher than other fractions. Additionally, much more aromatic proteins were accumulated in TPI-N. This indicated that the aromatic intermediates accumulated in neutral fractions were much more than that of acidic fractions, which might be a reason for the lower aromatic compounds removal efficiencies in neutral fractions compared with the acidic fractions. 3.4.2. FT-IR spectroscopic analysis The possible mechanism of food waste degradation by the microbes in the MFC was investigated using the FTIR analysis in the range of 4000–400 cm 1. Fig. S1 shows the changes in the FTIR spectra of organic fractions in the food waste before and after MFC treatment. For the HPI, HPO-A, HPO-N, TPI-A, and TPI-N fractions in the raw food waste, the spectra all showed broad regions of adsorption at 3500–3300 cm 1, attributing to the O–H and N–H stretching vibrations of hydroxyl and amine functional groups (Kumar et al., 2006). Additionally, all of the five fractions presented obvious absorption bands due to aromatic C–H stretching at 3100– 3000 cm 1, demonstrating the presences of aromatic compounds in the five fractions. The remarkable bands at 2925 cm 1 resulted from aliphatic C–H, C-H2 and C-H3 stretching for the neutral fractions (HPO-N and TPI-N) were stronger than those for the acid fractions (HPO-A and TPI-A), indicating the higher content of aliphatic chains in the neutral fractions. The band (1659 cm 1) due to C@O
109
stretching of amide 1 groups was more pronounced in the HPON than other fractions. Furthermore, the HPO-A, HPO-N, TPI-A and TPI-N fractions all displayed strong aliphatic C–H deformation signals at 1440–1465 cm 1. Bands at 1150–1000 cm 1 were correlated with C–O stretching of alcohols, ethers and carbohydrates, and these C–O related substances mainly accumulated in the hydrophilic and transphilic fractions. The HPI of the raw food waste presented a more noticeable absorption band attributed to C–H bending vibration of aromatic rings at 910–730 cm 1 than the other fractions, demonstrating that the aromatic components were enriched in the HPI. After the treatment of the MFC, the aliphatics C–H peaks (1440– 1465 cm 1) for the HPO-A and HPO-N were distinctly decreased, which indicated that the aliphatics in the hydrophobic fractions could be readily hydrolyzed by the MFC. Conversely, the carboxylic (1412 cm 1) and alcohol peaks (1000–1150 cm 1) for the hydrophobic fractions became more pronounced, which might be attributed to byproducts generation during the hydrolysis and fermentation. The peak at 1659 cm 1 disappeared in the HPO-N after the MFC treatment, showing that the amide-1 protein was preferentially removed during the MFC operation. The alcohol peak (1000–1150 cm 1) was enhanced for the TPI-A after the MFC treatment. However, the alcohols in the TPI-N were effectively removed by the MFC, demonstrated by the decreased intensity of the alcoholic peak. The aromatic C–H stretching band at 3100–3000 cm 1 of HPI was effectively weakened, while those of HPO-A, HPO-N, TPI-A and TPI-N fractions were all enhanced. In addition, the absorption bands at 910–730 cm 1 related to C–H bending vibration of aromatic rings for the hydrophobic and transphilic fractions also became more pronounced. This further demonstrated the aromaticity reduction of hydrophilic fractions and aromaticity improvements of hydrophobic and transphilic fractions after MFC treatment, agreeing with the SUVA results. The FT-IR analysis results demonstrated that the amide-1 proteins and aliphatic components for the hydrophobic fractions could be easily removed, while the aromatic substances for the hydrophobic and transphilic fractions were resistant to the degradation of the microbes within the MFC. The enrichments of the carboxylic and alcohols groups in HPO-A, HPO-N and TPI-A of the food waste treated by the MFC were attributed to the anaerobic hydrolysis and fermentation of the high-molecular organic compounds in MFC. 4. Conclusions Electricity recovery was achieved with efficient organics biodegradation in the MFC using food waste as substrates. During the MFC treatment, the aromatic compounds of the HPI could be preferentially degraded compared to the non-aromatic compounds, leading to aromaticity reduction in HPI. Moreover, the tryptophan protein-like substances in all fractions and aromatic proteins in HPI and HPO-N could be well degraded. The aliphatic components and amide-1 proteins in the hydrophobic fractions could be efficiently removed by the MFC. The carboxylic and alcoholic groups in HPO-A, HPO-N and TPI-A increased due to the hydrolysis and fermentation of high-molecular organics in the MFC. Acknowledgements This study was supported by the Major Science and Technology Program for Water Pollution Control and Management of China (No. 2013ZX07201007), the State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology (No. 2014DX03) and the Science Fund for Distinguished Young Scholars of Heilongjiang Province (JC201303). The authors also appreciate
110
H. Li et al. / Bioresource Technology 205 (2016) 104–110
the Funds for Creative Research Groups of China (No. 51121062), the National Science Funds of China (No. 50978071) and the National Natural Science Foundation of China (No. 51408169). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.biortech.2016.01. 042. References Ahn, Y., Hatzell, M.C., Zhang, F., Logan, B.E., 2014. Different electrode configurations to optimize performance of multi-electrode microbial fuel cells for generating power or treating domestic wastewater. J. Power Sources 249, 440–445. Chen, W., Westerhoff, P., Leenheer, J.A., Booksh, K., 2003. Fluorescence excitation– emission matrix regional integration to quantify spectra for dissolved organic matter. Environ. Sci. Technol. 37, 5701–5710. Cheng, S., Logan, B.E., 2011. Increasing power generation for scaling up singlechamber air cathode microbial fuel cells. Bioresour. Technol. 102 (6), 4468–4473. Cheng, S., Liu, H., Logan, B.E., 2006. Increased performance of single-chamber microbial fuel cells using an improved cathode structure. Electrochem. Commun. 8 (3), 489–494. Chow, A.T., Guo, F., Gao, S., Breuer, R.S., 2006. Size and XAD fractionations of trihalomethane precursors from soils. Chemosphere 62 (10), 1636–1646. FAO, 2012. Towards the Future We Want: End Hunger and Make the Transition to Sustainable Agricultural and Food Systems. Food and Agriculture Organization, United Nations Rome. Feng, C., Huang, L., Yu, H., Yi, X., Wei, C., 2015. Simultaneous phenol removal, nitrification and denitrification using microbial fuel cell technology. Water Res. 76, 160–170. Goud, R.K., Mohan, S.V., 2011. Pre-fermentation of waste as a strategy to enhance the performance of single chambered microbial fuel cell (MFC). Int. J. Hydrogen Energy 36 (21), 13753–13762. Grimberg, S.J., Hilderbrandt, D., Kinnunen, M., Rogers, S., 2015. Anaerobic digestion of food waste through the operation of a mesophilic two-phase pilot scale digester–assessment of variable loadings on system performance. Bioresour. Technol. 178, 226–229. Henriksen, T., Juhler, R.K., Svensmark, B., Cech, N.B., 2005. The relative influences of acidity and polarity on responsiveness of small organic molecules to analysis with negative ion electrospray ionization mass spectrometry (ESI-MS). J. Am. Soc. Mass Spectrom. 16 (4), 446–455. Her, N., Amy, G., McKnight, D., Sohn, J., Yoon, Y., 2003. Characterization of DOM as a function of MW by fluorescence EEM and HPLC-SEC using UVA, DOC, and fluorescence detection. Water Res. 37 (17), 4295–4303. Huang, H., Qureshi, N., Chen, M.H., Liu, W., Singh, V., 2015. Ethanol production from food waste at high solids content with vacuum recovery technology. J. Agric. Food Chem. 63 (10), 2760–2766. Jia, J., Tang, Y., Liu, B., Wu, D., Ren, N., Xing, D., 2013. Electricity generation from food wastes and microbial community structure in microbial fuel cells. Bioresour. Technol. 144, 94–99. Jiang, J., Zhao, Q., Zhang, J., Zhang, G., Lee, D.J., 2009. Electricity generation from biotreatment of sewage sludge with microbial fuel cell. Bioresour. Technol. 100 (23), 5808–5812.
Jiang, J., Zhao, Q., Wei, L., Wang, K., 2010. Extracellular biological organic matters in microbial fuel cell using sewage sludge as fuel. Water Res. 44 (7), 2163–2170. Kumar, M., Adham, S.S., Pearce, W.R., 2006. Investigation of seawater reverse osmosis fouling and its relationship to pretreatment type. Environ. Sci. Technol. 40 (6), 2037–2044. Lee, H.S., Parameswaran, P., Kato-Marcus, A., Torres, C.I., Rittmann, B.E., 2008. Evaluation of energy-conversion efficiencies in microbial fuel cells (MFCs) utilizing fermentable and non-fermentable substrates. Water Res. 42 (6–7), 1501–1510. Liu, H., Jeong, J., Gray, H., Smith, S., Sedlak, D.L., 2012. Algal uptake of hydrophobic and hydrophilic dissolved organic nitrogen in effluent from biological nutrient removal municipal wastewater treatment systems. Environ. Sci. Technol. 46 (2), 713–721. Ma, J., Wang, Z., Li, X., Wang, Y., Wu, Z., 2014. Bioelectricity generation through microbial fuel cell using organic matters recovered from municipal wastewater. Environ. Prog. Sustainable Energy 33 (1), 290–297. Meng, Y., Shen, F., Yuan, H., Zou, D., Liu, Y., Zhu, B., Chufo, A., Jaffar, M., Li, X., 2014. Start-up and operation strategies on the liquefied food waste anaerobic digestion and a full-scale case application. Bioprocess Biosyst. Eng. 37 (11), 2333–2341. Meng, Y., Li, S., Yuan, H., Zou, D., Liu, Y., Zhu, B., Li, X., 2015. Effect of lipase addition on hydrolysis and biomethane production of Chinese food waste. Bioresour. Technol. 179, 452–459. Moqsud, M.A., Omine, K., Yasufuku, N., Bushra, Q.S., Hyodo, M., Nakata, Y., 2014. Bioelectricity from kitchen and bamboo waste in a microbial fuel cell. Waste Manage. Res. 32 (2), 124–130. More, T.T., Ghangrekar, M.M., 2010. Improving performance of microbial fuel cell with ultrasonication pre-treatment of mixed anaerobic inoculum sludge. Bioresour. Technol. 101 (2), 562–567. Namour, P., Muller, M.C., 1998. Fractionation of organic matter from wastewater treatment plants before and after a 21-day biodegradability test: a physicalchemical method for measurement of the refractory part of effluents. Water Res. 32 (7), 2224–2231. Park, N., Kwon, B., Kim, S.-D., Cho, J., 2006. Characterizations of the colloidal and microbial organic matters with respect to membrane foulants. J. Membr. Sci. 275 (1–2), 29–36. Sevda, S., Dominguez-Benetton, X., Vanbroekhoven, K., De Wever, H., Sreekrishnan, T.R., Pant, D., 2013. High strength wastewater treatment accompanied by power generation using air cathode microbial fuel cell. Appl. Energy 105, 194–206. Shen, F., Yuan, H., Pang, Y., Chen, S., Zhu, B., Zou, D., Liu, Y., Ma, J., Yu, L., Li, X., 2013. Performances of anaerobic co-digestion of fruit & vegetable waste (FVW) and food waste (FW): single-phase vs. two-phase. Bioresour. Technol. 144, 80–85. Uçkun Kiran, E., Trzcinski, A.P., Ng, W.J., Liu, Y., 2014. Bioconversion of food waste to energy: a review. Fuel 134, 389–399. Westerhoff, P., Pinney, M., 2000. Dissolved organic carbon transformations during laboratory-scale groundwater recharge using lagoon-treated wastewater. Waste Manage. 20, 75–83. Xue, S., Zhao, Q.L., Wei, L.L., Ren, N.Q., 2009. Behavior and characteristics of dissolved organic matter during column studies of soil aquifer treatment. Water Res. 43 (2), 499–507. Yusoff, M.Z., Hu, A., Feng, C., Maeda, T., Shirai, Y., Hassan, M.A., Yu, C.P., 2013. Influence of pretreated activated sludge for electricity generation in microbial fuel cell application. Bioresour. Technol. 145, 90–96. Zhang, G., Zhao, Q., Jiao, Y., Lee, D.J., 2015. Long-term operation of manure-microbial fuel cell. Bioresour. Technol. 180, 365–369.
Contents lists available at ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Electricity generation from food wastes and characteristics of organic matters in microbial fuel cell Hui Li b, Yu Tian a,b,⇑, Wei Zuo b, Jun Zhang b, Xiaoyue Pan c, Lipin Li b, Xinying Su d a
State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology (SKLUWRE, HIT), Harbin 150090, China School of Municipal and Environmental Engineering, Harbin Institute of Technology, Harbin 150090, China c Beijing Tsinghua Tongheng Urban Planning & Design Institute, Beijing 100085, China d School of Food Engineering, Harbin University of Commerce, Harbin 150076, China b
h i g h l i g h t s The MFC was used for food waste disposal and electricity generation. 3
The maximum power density of 5.6 W/m was achieved in the MFC. Characteristics of the food waste before and after MFC treatment were investigated. Biodegradation regularity of the food waste in MFC was discussed. The results may assist pre- and post-treatment choices for MFC fed with food waste.
a r t i c l e
i n f o
Article history: Received 6 October 2015 Received in revised form 9 January 2016 Accepted 11 January 2016 Available online 23 January 2016 Keywords: Microbial fuel cell Food waste Electricity generation Organic matter Fractionation
a b s t r a c t The microbial fuel cell (MFC) was evaluated as an alternative way to recover electricity from canteen based food waste. Characteristics of the organics in food waste before and after the MFC treatment were analyzed to investigate how the organic matters were biodegraded and transformed during the MFC treatment. A maximum power density of 5.6 W/m3 and an average output voltage of 0.51 V were obtained. During the MFC operation, the hydrophilic and acidic fractions were more readily degraded, compared to the neutral fractions. Additionally, aromatic compounds in the hydrophilic fraction were more preferentially removed than non-aromatic compounds. The MFC could easily remove the tryptophan protein-like substances in all fractions and aromatic proteins in hydrophilic and hydrophobic neutral fractions. Additionally, the hydrophobic amide-1 proteins and aliphatic components were readily hydrolyzed and biodegraded in the MFC. These findings may facilitate the pretreatment and posttreatment choices for MFC system fed with food waste. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Food waste is organic waste comes from various sources including households, restaurants, cafeterias and so forth, accounting for a considerable portion of the municipal solid waste (Uçkun Kiran et al., 2014). It was reported by the Food and Agricultural Organization that one third of the food produced in the world for human consumption (1.3 billion tons per year) is wasted (FAO, 2012). In China, about 60 million tons of food wastes were produced yearly, and the daily food waste generation in Beijing has reached approximately ⇑ Corresponding author at: State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology (SKLUWRE, HIT), Harbin 150090, China. Tel./fax: +86 451 8628 3077. E-mail address: [email protected] (Y. Tian). http://dx.doi.org/10.1016/j.biortech.2016.01.042 0960-8524/Ó 2016 Elsevier Ltd. All rights reserved.
1.6 thousand tons (Meng et al., 2014). With large amount of organic matters, high moisture and high salinity (Meng et al., 2015), the food waste would cause serious environmental contamination and health threat without proper treatment (Shen et al., 2013). Nowadays most of the food wastes are disposed by the conventional methods such as landfill, compost and incineration, which could lead to ground water contamination, vermin attraction and toxic gas emission (Goud and Mohan, 2011). Additionally, the conventional methods are unsustainable and uneconomical, as the valuable source of nutrients and energy in the food waste cannot be fully or efficiently used. Thus, exploiting the highly biodegradable waste as an alternative source for energy recovery with simultaneous treatment could be an attractive approach. Anaerobic digestion is increasingly accepted as an effective technology for organic waste treatment and bio-energy recovery,
H. Li et al. / Bioresource Technology 205 (2016) 104–110
and it has been considered as an attractive and sustainable approach to viably overcome the current global energy crisis (Huang et al., 2015). The food waste is inexhaustible resource with high energy potential and good biodegradability, and significant energy recovery could be achieved during the anaerobic digestion (Grimberg et al., 2015). As a promising anaerobic waste treatment device, the microbial fuel cell (MFC) can use microorganisms as catalyst to recover electricity from diverse organic wastes including domestic wastewater (Ahn et al., 2014), industrial wastewater (Feng et al., 2015) and excess sludge (Jiang et al., 2009). The MFC could efficiently achieve safe, clean and direct electricity production and simultaneous organic matter removal. Therefore, employing food waste as a substrate in MFC for electricity generation could be a promising approach for food waste treatment and energy recovery. Recently, some researches on the MFC which directly employ the food waste as substrate have been reported (Jia et al., 2013; Moqsud et al., 2014), however, how were the organic matters of the food waste chemically converted during the MFC treatment has not been explored. The organic matters are the major constituents of food wastes and mainly consist of various high molecular-weight polymers such as carbohydrate, cellulose, protein, and lipid. In the MFC fed with food waste, the organic matters are the energy sources of the electricigens for the electricity production and the characteristics of the organic matters significantly affect the electricity generation efficiency. Additionally, hydrolysis of the organic matters is commonly known as the rate limiting step for electricity generation (Ma et al., 2014). In order to accelerate hydrolysis of the substrate for more efficient electricity recovery, pretreatment processes were often required, such as sonication (More and Ghangrekar, 2010) and microwave-pretreatment (Yusoff et al., 2013). Additionally, to minimize the environmental pollution of the food waste, post-treatment process may be needed for the food waste after the MFC treatment. For appropriate selection of pretreatment and post-treatment method, it is necessary to research the characteristics and the biodegradation regularity of the organic matters in the food waste during the MFC treatment. In this study, the food wastes collected from canteen were exploited as anodic fuel for direct energy generation in a singlechamber MFC. The electrochemical performance of the MFC was studied. Additionally, the food waste organics before and after the treatment were fractionated into five different fractions by XAD resins, and the properties of the organic fractions were investigated. This research aims at revealing the transformation characteristics of the organic matters in the food waste during the MFC treatment and determined which fraction of the organic matters in the food waste could be preferentially degraded. Such knowledge might assist in the understanding of the compositions and chemical properties of organic matter in food waste and the biodegradation regularity of the organics during MFC treatment, and, in turn, may facilitate the pretreatment and post-treatment choices for MFC system fed with food waste.
2. Methods 2.1. Food wastes The food waste used as the substrate in the MFC was collected from the student canteen at Harbin Institute of Technology, Harbin, China. The food waste mainly comprises boiled rice, vegetables, fruit, cooked meat, bones, as well as plastics. In order to supply a suitable substrate for the MFC, the bones and plastics in the food waste were first removed, and then the left food waste was pulverised in an electrical pulverizer (JYL-C051, Joyoung, China) for two minutes. Avoiding the clogging problem, the pulverised food
105
waste was filtered through a stainless steel mesh with an average pore diameter of 2 mm to remove the coarse material. The oil in the food waste was separated by gravity. The COD of the pretreated food waste was 71,000–90,000 mg/L. Before fed into the MFC, the food waste was diluted to a COD concentration of 2700 ± 20 mg/ L. Finally, the pH of the diluted food waste was adjusted to 7 by NaOH solution (1 M). 2.2. MFC configuration and operation A single-chamber air cathode MFC with a working volume of 120 ml was constructed using ‘‘Perspex” as previous description (Sevda et al., 2013). The spacing between the anode and cathode placed on opposite sides was 4 cm. The anode was made of carbon cloth without any pretreatment. The carbon cloth based cathode containing 10% platinum and three diffusion layers was prepared according to the method reported previously (Cheng et al., 2006). Before making the diffusion layers, a carbon base layer was prepared by applying a mixture of carbon powder and 30 wt% PTFE solution onto one side of the carbon cloth, air-drying at room temperature for 2 h, followed by heating at 370 °C for 30 min. Additional diffusion layers were made by brushing a PTFE solution (60 wt%) onto the coating side, followed again by drying at room temperature and heating at 370 °C for 10 min. Diffusion layers were applied for 3 times. 10% Pt/C catalyst (0.5 mg Pt/cm2) was then applied to the other side (water-facing side) of the carbon cloth using Nafion as a binder. The MFC was inoculated with anaerobic sludge from the laboratory-scale anaerobic reactor and the prepared food waste was directly used as the substrate. The external resistance connected across the anode and cathode was 1000 X. The MFC was repeatedly filled with the inoculant and substrate until the constant output voltage was obtained. After about one month’s inoculation, the MFC achieved stable performance, and the MFC was changed to using food waste as substrate without anaerobic sludge. The MFC fed with food waste was operated in batch mode at room temperature and was refilled when the output voltage decreased to lower than 100 mV, which formed an operation cycle. 2.3. Fractionation procedure The food waste was fractionated into five fractions namely hydrophilic (HPI), hydrophobic acid (HPO-A), hydrophobic neutral (HPO-N), transphilic acid (TPI-A) and transphilic neutral (TPI-N) fractions using XAD-8/XAD-4 resins according to the reported method (Chow et al., 2006). Briefly, 2 L of the food waste sample was acidified to pH 2 using HCl and passed through the XAD-8 and XAD-4 resin columns in series. HPI was the organic matter not retained on either resin and existed in the XAD-4 effluent. The XAD-8 and XAD-4 resin columns were backward eluted using 200 ml NaOH (0.1 mol/L) and 50 ml Milli-Q water, and the eluate were HPO-A and TPI-A fractions, respectively. After eluation by the NaOH and Milli-Q water, the HPO-N and TPI-N still adsorbed on the XAD-8 and XAD-4, respectively. And the HPO-N and TPI-N were respectively desorbed by 75% acetonitrile +25% Milli-Q water solution from the XAD-8 and XAD-4. Finally, the acetonitrile existed in the HPO-N and TPI-N samples was removed using rotary evaporation. 2.4. Analysis The output voltage across the external resistance was monitored every 1 min using a paperless recorder (VX5100R/C2/U, Hangzhou, China). The polarization and power density curves were measured by varying the external resistance from 5000 to 50 X. Power (W) was calculated according to the formula P = U I.
106
H. Li et al. / Bioresource Technology 205 (2016) 104–110
Power density (W/m3) and current density (A/m3) were calculated by dividing the output power and current with the volume (m3) of the anode chamber. The TOC was measured by a Shimadzu TOC-5000 Total Organic Carbon analyzer. The Ultraviolent (UV) absorbance of samples was measured by a T6 ultraviolet spectrophotometer at 254 nm (UV-254) using a quartz cuvette having a path length of 1 cm. The specific ultraviolet absorbance (SUVA) was calculated according to the equation (UV-254/DOC)/100, where DOC is the dissolved organic carbon. The excitationemission matrix (EEM) spectra were monitored using a Jasco FP-6500 spectrofluorometer by scanning excitation spectra from 220 to 400 nm with increments of 5 nm, detecting the emission wavelength between 280 and 550 nm with 2 nm steps. The slits were both maintained at 5 nm for both excitation and emission and the scanning speed was 2000 nm/min. In addition, the samples for excitation–emission matrix (EEM) measurements were all diluted to 1 mg/L of TOC with distilled water. For fouriertransform infrared spectroscopy (FTIR) analysis, 5 mg lyophilized sample powder and 200 mg KBr were mixed and pressed into a pellet and measured immediately in an FTIR spectrophotometer (Spectrum One B, PerkinElmer, USA) scanning from 4000 to 400 cm 1.
Fig. 2. Variation of power density and polarization curves for the MFC fed with food waste.
the MFC could recover electricity from food waste with efficient removal of the organic matters.
3. Results and discussion
3.2. Treatment efficiency
3.1. Power generation of the food waste-MFC
In order to assess the biodegradabilities of the food waste applied to the MFC, the TOC concentrations of organic matters in food waste before and after the MFC treatment were investigated (Fig. 3). The XAD fractionation results showed that the HPI was the major fraction of the food waste, accounting for 44.9% of the bulk TOC, followed by the HPO-A (14.6%), HPO-N (13.9%), TPI-N (13.8%) and TPI-A (12.8%). From the Fig. 3, it could be observed that the bulk TOC of the food waste decreased from 385.6 mg/L to 74.2 mg/L after the MFC treatment, resulting in a removal efficiency of 80.8%, which demonstrated that the microorganism in the MFC could efficiently degrade the organic matters in the food waste. During the operation of the MFC, the highest TOC removal efficiency of 87.4% was for HPI, meaning that the hydrophilic fraction of the food waste could be readily removed by the MFC for electricity generation. The TOC removal efficiencies of the acidic fractions were 83.1% for HPO-A and 81.8% for TPI-A, while the degradation efficiencies of the neutral fractions were relatively low (73.7% for HPO-N and 65.3% for TPI-N). Additionally, the fractions of the HPO-N and TPI-N were significantly increased from 13.9% and 19.4% to 19.4% and 25.3%, respectively, which demonstrated that the HPO-N and TPI-N were resistant to the biodegradation of the bacteria in the MFC compared with HPI, HPO-A and TPI-A fractions (Jiang et al., 2010). However, during the operation, the different fractions may inter-transform in the
After a 30 days’ successful acclimation period, stable electricity generation was achieved in the MFC (Fig. 1a). In the following operation, the MFC was fed with food waste. As shown in Fig. 1b, after the food waste was fed into the MFC, the voltage rapidly increased to higher than 0.54 V in 6 h, and then remained stable at an average voltage of 0.51 V until the 116th hour when the voltage began to dramatically decrease. Additionally, the voltage remained higher than 0.1 V before the 202nd hour. The polarization curve was measured by varying the external resistance to determine the maximum output power density and the internal resistance (Fig. 2). It could be observed that the open circuit voltage of the MFC was 0.7 V, and the maximum power density of 5.6 W/m3 was obtained with a current density of 15.3 A/m3. The voltage and power outputs achieved in this research were lower than MFCs fed with simple substrates like glucose and acetate (Cheng and Logan, 2011), mainly attributed to that the complex organics in the food waste could not be readily degraded in MFC, which limited the electricity generation (Lee et al., 2008). However, the obtained voltage and power outputs were comparable with other single chamber MFCs using food waste as substrates (Jia et al., 2013). Additionally, an average COD removal efficiency of 90.3% was obtained in the MFC. These results demonstrated that
Fig. 1. Variations of voltage output with time for the MFC during inoculation period (a) and one operation cycle after food waste addition (b).
H. Li et al. / Bioresource Technology 205 (2016) 104–110
107
Fig. 3. TOC concentrations of the organic fractions in raw food waste and MFCtreated food waste.
MFC. Therefore, in order to further investigate the biodegradabilities of the five fractions in the food waste, each fraction was used as the sole substrate of the MFC, and the removal efficiency was evaluated after about 200 hours’ operation. Before fed into the MFC, each fraction was diluted to a TOC concentration of about 50 mg/L. After the MFC treatment, the removal efficiency of each fraction showed a decrease trend of HPI (92.4%) > HPO-A (80.6%) > TPI-A (77.5%) > HPO-N (72.9%) > TPI-N (64.4%) as shown in Table S1. This result indicated that the HPI exhibited the highest biodegradability among the five fraction, and the acidic fractions could be more readily biodegraded in the MFC compared to the neutral fractions, which was in accordance with the discussion above. In addition, compared with the MFC fed with raw food waste, the MFCs using each fraction as single substrate achieved a higher removal efficiency for HPI and lower removal efficiencies for other four fractions, inferring that biodegradations of hydrophobic and transphilic fractions might induce formations of hydrophilic substances.
3.3. Change in aromaticity of food waste It is generally accepted that the UV-254 can represent the aromaticity of organics (Her et al., 2003). The UV-254 for the original unfractionated raw food waste and organic fractions before and after MFC treatment are presented in Fig. 4a. In the raw food waste, the HPI was the principal component, representing 77% of the total UV-254. The remaining organic fractions were HPO-N, HPO-A, TPIA and TPI-N (11.5%, 6.5%, 2.7% and 2.3%, respectively). The results from Fig. 4a showed that the removal efficiency of the UV-254 for the bulk organics of the food waste was 81.7%, and the order of the UV-254 removal with respect to the five fractions was observed to be HPI (90.2%) > TPI-A (77.4%) > HPO-A (72.1%) > HPO-N (43.1%) > TPI-N (26.7%). These results indicated that the bulk UV-254 reduction of the food waste was dominated by the removal of the aromatic compounds in HPI, TPI-A and HPO-A. The HPI fraction was mainly consisted of polar compounds of low molecular weights, presenting the most readily degradable compounds (Namour and Muller, 1998). Thus, the aromatic compounds in the hydrophilic fraction (HPI) could be more readily degraded by the microbes in the MFC (Zhang et al., 2015) compared to the hydrophobic and transphilic fractions (HPO-A, HPON, TPI-A and TPI-N). Moreover, the aromatic intermediates generated during the biodegradation of the hydrophilic aromatic substances might transform into other fractions, inducing the lower content of aromatic compounds in HPI fraction of the MFC-
Fig. 4. UV-254 (a) and SUVA (b) values for the organic fractions of the raw food waste and MFC-treated food waste.
treated food waste. Higher UV-254 removal efficiencies of the HPO-A and TPI-A fractions compared to HPO-A and TPI-A fractions suggested that the aromatic compounds in acidic fractions were preferential for the microbe in the MFC to degrade. Westerhoff and Pinney (2000) demonstrated that biofilm could preferentially absorb and remove the organics with higher polarity. As reported by Henriksen et al. (2005), acidic organics have higher polarity than the neutral organics. Thus the biofilm attached on the electrode might more readily absorb and remove TPI-A and HPO-A fractions compared to the HPO-N and TPI-N fractions, inducing higher aromatic compounds removal efficiencies of acidic fractions compared to the neutral fractions. The specific ultraviolet light absorbance (SUVA) calculated as (UV-254/DOC)/100 represents the relative aromaticity of organics. The SUVA of HPI, HPO-N, HPO-A, TPI-A and TPI-N isolated from the raw food waste had average value of 1.4 L/m mg, 0.7 L/m mg, 0.4 L/m mg, 0.2 L/m mg and 0.1 L/m mg (Fig. 4b), respectively, suggesting that HPI contained a relatively high amount of aromatic structures. The SUVA for the bulk organics of the food waste was decreased by 5% after the MFC treatment. However, among the five fractions, only HPI showed a decrease (22.5%) in the SUVA after the MFC treatment. The remaining four fractions all represented an increase in aromaticity after the treatment of MFC. The aromaticity enhancement might be attributed to the preferential removal of non-aromatic compounds or formation of aromatic products by the biodegradation during the MFC operation (Xue et al., 2009).
108
H. Li et al. / Bioresource Technology 205 (2016) 104–110
3.4. Characteristics of organic matters in food waste 3.4.1. Fluorescence spectroscopic analysis Fluorescence EEMs were conducted on the organic fractions of the food waste before and after MFC treatment (Fig. 5). Four main peaks could be identified from the fluorescence spectra of the food waste corresponding to the regions reported by Chen et al. (2003). The first main peak was located at the excitation/emission wavelengths (Ex/Em) of 220–240/320–360 which was related to simple
aromatic proteins such as tyrosine (region I). The second peak was observed at excitation wavelengths shorter than 250 nm and emission wavelengths longer than 350 nm, which was described as the fulvic acid-like substances (region II). The third peak at the Ex/Em of 260–290/320–380 nm was associated with the tryptophan protein-like substances (region III). The fourth peak was located at longer excitation wavelengths (>280 nm) and emission wavelengths (>380 nm), correlated with humic acid-like organics (region IV).
Fig. 5. EEM fluorescence spectra of organic fractions in the raw food waste and MFC-treated food waste.
H. Li et al. / Bioresource Technology 205 (2016) 104–110
All of the five fractions in raw food waste represented obvious fluorescent peaks in region III, revealing that the organic matters in the raw food waste were dominated by the tryptophan protein-like substances. Additionally, the HPI, HPO-N and TPI-N also showed distinct aromatic protein peaks in region I and HPOA and TPI-A showed humic acid-like peaks in region. After the treatment of the MFC, the intensities of the peaks in region III for all organic fractions of the food waste were significantly decreased, implying the reduction effects of the tryptophan protein-like substances due to the MFC treatment. It could be observed that the aromatic protein peak intensities of the HPI and HPO-N were also distinctly reduced after the MFC operation. Conversely, the aromatic protein peak intensity was increased for the TPI-N following the MFC operation, demonstrating the possible formation of new soluble aromatic proteins via the hydrolysis of the food waste. The fluorescence peaks for the HPI were all substantially decreased during the MFC treatment, suggesting that the hydrophilic fluorescent substances could be easily degraded by the microbes in the MFC, which was accordant with the result reported by Jiang et al. (2010). Compared to the raw food waste, the intensities of the humic acid-like peaks for HPO-A and TPI-A of the MFCtreated food waste were both enhanced accompanied with the emergence of the new fluorescence peaks for fulvic acid-like substances. For the effluent HPO-N, two new fluorescence peaks of humic acid-like and fulvic acid-like compounds emerged, accompanying the disappearance of the aromatic protein and tryptophan protein-like peaks, although the fulvic acid-like peak was more like shoulder. These results demonstrated that the tryptophan proteinlike substances in all fractions and the aromatic protein in HPI and HPO-N could be preferentially degraded by the MFC, and the humic acid-like and fulvic acid-like compounds of HPO-A, HPO-N and TPIA were enriched following the MFC treatment. It has been widely reported that the humic substances mostly composed of humic acids and fulvic acids (FA) mainly exhibited relative hydrophobicity (Liu et al., 2012). Thus the humic substances mainly accumulated in hydrophobic and transphilic fractions rather than the hydrophilic fraction of the food waste after the MFC treatment. Humic substances contain relatively large amounts of aromatic carbon (Park et al., 2006), therefore, enrichment of these substances might be one main reason for the aromaticity increases of the hydrophobic and transphilic fractions. As shown in Fig. 5, the accumulation of humic substances in HPO-N was much higher than other fractions. Additionally, much more aromatic proteins were accumulated in TPI-N. This indicated that the aromatic intermediates accumulated in neutral fractions were much more than that of acidic fractions, which might be a reason for the lower aromatic compounds removal efficiencies in neutral fractions compared with the acidic fractions. 3.4.2. FT-IR spectroscopic analysis The possible mechanism of food waste degradation by the microbes in the MFC was investigated using the FTIR analysis in the range of 4000–400 cm 1. Fig. S1 shows the changes in the FTIR spectra of organic fractions in the food waste before and after MFC treatment. For the HPI, HPO-A, HPO-N, TPI-A, and TPI-N fractions in the raw food waste, the spectra all showed broad regions of adsorption at 3500–3300 cm 1, attributing to the O–H and N–H stretching vibrations of hydroxyl and amine functional groups (Kumar et al., 2006). Additionally, all of the five fractions presented obvious absorption bands due to aromatic C–H stretching at 3100– 3000 cm 1, demonstrating the presences of aromatic compounds in the five fractions. The remarkable bands at 2925 cm 1 resulted from aliphatic C–H, C-H2 and C-H3 stretching for the neutral fractions (HPO-N and TPI-N) were stronger than those for the acid fractions (HPO-A and TPI-A), indicating the higher content of aliphatic chains in the neutral fractions. The band (1659 cm 1) due to C@O
109
stretching of amide 1 groups was more pronounced in the HPON than other fractions. Furthermore, the HPO-A, HPO-N, TPI-A and TPI-N fractions all displayed strong aliphatic C–H deformation signals at 1440–1465 cm 1. Bands at 1150–1000 cm 1 were correlated with C–O stretching of alcohols, ethers and carbohydrates, and these C–O related substances mainly accumulated in the hydrophilic and transphilic fractions. The HPI of the raw food waste presented a more noticeable absorption band attributed to C–H bending vibration of aromatic rings at 910–730 cm 1 than the other fractions, demonstrating that the aromatic components were enriched in the HPI. After the treatment of the MFC, the aliphatics C–H peaks (1440– 1465 cm 1) for the HPO-A and HPO-N were distinctly decreased, which indicated that the aliphatics in the hydrophobic fractions could be readily hydrolyzed by the MFC. Conversely, the carboxylic (1412 cm 1) and alcohol peaks (1000–1150 cm 1) for the hydrophobic fractions became more pronounced, which might be attributed to byproducts generation during the hydrolysis and fermentation. The peak at 1659 cm 1 disappeared in the HPO-N after the MFC treatment, showing that the amide-1 protein was preferentially removed during the MFC operation. The alcohol peak (1000–1150 cm 1) was enhanced for the TPI-A after the MFC treatment. However, the alcohols in the TPI-N were effectively removed by the MFC, demonstrated by the decreased intensity of the alcoholic peak. The aromatic C–H stretching band at 3100–3000 cm 1 of HPI was effectively weakened, while those of HPO-A, HPO-N, TPI-A and TPI-N fractions were all enhanced. In addition, the absorption bands at 910–730 cm 1 related to C–H bending vibration of aromatic rings for the hydrophobic and transphilic fractions also became more pronounced. This further demonstrated the aromaticity reduction of hydrophilic fractions and aromaticity improvements of hydrophobic and transphilic fractions after MFC treatment, agreeing with the SUVA results. The FT-IR analysis results demonstrated that the amide-1 proteins and aliphatic components for the hydrophobic fractions could be easily removed, while the aromatic substances for the hydrophobic and transphilic fractions were resistant to the degradation of the microbes within the MFC. The enrichments of the carboxylic and alcohols groups in HPO-A, HPO-N and TPI-A of the food waste treated by the MFC were attributed to the anaerobic hydrolysis and fermentation of the high-molecular organic compounds in MFC. 4. Conclusions Electricity recovery was achieved with efficient organics biodegradation in the MFC using food waste as substrates. During the MFC treatment, the aromatic compounds of the HPI could be preferentially degraded compared to the non-aromatic compounds, leading to aromaticity reduction in HPI. Moreover, the tryptophan protein-like substances in all fractions and aromatic proteins in HPI and HPO-N could be well degraded. The aliphatic components and amide-1 proteins in the hydrophobic fractions could be efficiently removed by the MFC. The carboxylic and alcoholic groups in HPO-A, HPO-N and TPI-A increased due to the hydrolysis and fermentation of high-molecular organics in the MFC. Acknowledgements This study was supported by the Major Science and Technology Program for Water Pollution Control and Management of China (No. 2013ZX07201007), the State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology (No. 2014DX03) and the Science Fund for Distinguished Young Scholars of Heilongjiang Province (JC201303). The authors also appreciate
110
H. Li et al. / Bioresource Technology 205 (2016) 104–110
the Funds for Creative Research Groups of China (No. 51121062), the National Science Funds of China (No. 50978071) and the National Natural Science Foundation of China (No. 51408169). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.biortech.2016.01. 042. References Ahn, Y., Hatzell, M.C., Zhang, F., Logan, B.E., 2014. Different electrode configurations to optimize performance of multi-electrode microbial fuel cells for generating power or treating domestic wastewater. J. Power Sources 249, 440–445. Chen, W., Westerhoff, P., Leenheer, J.A., Booksh, K., 2003. Fluorescence excitation– emission matrix regional integration to quantify spectra for dissolved organic matter. Environ. Sci. Technol. 37, 5701–5710. Cheng, S., Logan, B.E., 2011. Increasing power generation for scaling up singlechamber air cathode microbial fuel cells. Bioresour. Technol. 102 (6), 4468–4473. Cheng, S., Liu, H., Logan, B.E., 2006. Increased performance of single-chamber microbial fuel cells using an improved cathode structure. Electrochem. Commun. 8 (3), 489–494. Chow, A.T., Guo, F., Gao, S., Breuer, R.S., 2006. Size and XAD fractionations of trihalomethane precursors from soils. Chemosphere 62 (10), 1636–1646. FAO, 2012. Towards the Future We Want: End Hunger and Make the Transition to Sustainable Agricultural and Food Systems. Food and Agriculture Organization, United Nations Rome. Feng, C., Huang, L., Yu, H., Yi, X., Wei, C., 2015. Simultaneous phenol removal, nitrification and denitrification using microbial fuel cell technology. Water Res. 76, 160–170. Goud, R.K., Mohan, S.V., 2011. Pre-fermentation of waste as a strategy to enhance the performance of single chambered microbial fuel cell (MFC). Int. J. Hydrogen Energy 36 (21), 13753–13762. Grimberg, S.J., Hilderbrandt, D., Kinnunen, M., Rogers, S., 2015. Anaerobic digestion of food waste through the operation of a mesophilic two-phase pilot scale digester–assessment of variable loadings on system performance. Bioresour. Technol. 178, 226–229. Henriksen, T., Juhler, R.K., Svensmark, B., Cech, N.B., 2005. The relative influences of acidity and polarity on responsiveness of small organic molecules to analysis with negative ion electrospray ionization mass spectrometry (ESI-MS). J. Am. Soc. Mass Spectrom. 16 (4), 446–455. Her, N., Amy, G., McKnight, D., Sohn, J., Yoon, Y., 2003. Characterization of DOM as a function of MW by fluorescence EEM and HPLC-SEC using UVA, DOC, and fluorescence detection. Water Res. 37 (17), 4295–4303. Huang, H., Qureshi, N., Chen, M.H., Liu, W., Singh, V., 2015. Ethanol production from food waste at high solids content with vacuum recovery technology. J. Agric. Food Chem. 63 (10), 2760–2766. Jia, J., Tang, Y., Liu, B., Wu, D., Ren, N., Xing, D., 2013. Electricity generation from food wastes and microbial community structure in microbial fuel cells. Bioresour. Technol. 144, 94–99. Jiang, J., Zhao, Q., Zhang, J., Zhang, G., Lee, D.J., 2009. Electricity generation from biotreatment of sewage sludge with microbial fuel cell. Bioresour. Technol. 100 (23), 5808–5812.
Jiang, J., Zhao, Q., Wei, L., Wang, K., 2010. Extracellular biological organic matters in microbial fuel cell using sewage sludge as fuel. Water Res. 44 (7), 2163–2170. Kumar, M., Adham, S.S., Pearce, W.R., 2006. Investigation of seawater reverse osmosis fouling and its relationship to pretreatment type. Environ. Sci. Technol. 40 (6), 2037–2044. Lee, H.S., Parameswaran, P., Kato-Marcus, A., Torres, C.I., Rittmann, B.E., 2008. Evaluation of energy-conversion efficiencies in microbial fuel cells (MFCs) utilizing fermentable and non-fermentable substrates. Water Res. 42 (6–7), 1501–1510. Liu, H., Jeong, J., Gray, H., Smith, S., Sedlak, D.L., 2012. Algal uptake of hydrophobic and hydrophilic dissolved organic nitrogen in effluent from biological nutrient removal municipal wastewater treatment systems. Environ. Sci. Technol. 46 (2), 713–721. Ma, J., Wang, Z., Li, X., Wang, Y., Wu, Z., 2014. Bioelectricity generation through microbial fuel cell using organic matters recovered from municipal wastewater. Environ. Prog. Sustainable Energy 33 (1), 290–297. Meng, Y., Shen, F., Yuan, H., Zou, D., Liu, Y., Zhu, B., Chufo, A., Jaffar, M., Li, X., 2014. Start-up and operation strategies on the liquefied food waste anaerobic digestion and a full-scale case application. Bioprocess Biosyst. Eng. 37 (11), 2333–2341. Meng, Y., Li, S., Yuan, H., Zou, D., Liu, Y., Zhu, B., Li, X., 2015. Effect of lipase addition on hydrolysis and biomethane production of Chinese food waste. Bioresour. Technol. 179, 452–459. Moqsud, M.A., Omine, K., Yasufuku, N., Bushra, Q.S., Hyodo, M., Nakata, Y., 2014. Bioelectricity from kitchen and bamboo waste in a microbial fuel cell. Waste Manage. Res. 32 (2), 124–130. More, T.T., Ghangrekar, M.M., 2010. Improving performance of microbial fuel cell with ultrasonication pre-treatment of mixed anaerobic inoculum sludge. Bioresour. Technol. 101 (2), 562–567. Namour, P., Muller, M.C., 1998. Fractionation of organic matter from wastewater treatment plants before and after a 21-day biodegradability test: a physicalchemical method for measurement of the refractory part of effluents. Water Res. 32 (7), 2224–2231. Park, N., Kwon, B., Kim, S.-D., Cho, J., 2006. Characterizations of the colloidal and microbial organic matters with respect to membrane foulants. J. Membr. Sci. 275 (1–2), 29–36. Sevda, S., Dominguez-Benetton, X., Vanbroekhoven, K., De Wever, H., Sreekrishnan, T.R., Pant, D., 2013. High strength wastewater treatment accompanied by power generation using air cathode microbial fuel cell. Appl. Energy 105, 194–206. Shen, F., Yuan, H., Pang, Y., Chen, S., Zhu, B., Zou, D., Liu, Y., Ma, J., Yu, L., Li, X., 2013. Performances of anaerobic co-digestion of fruit & vegetable waste (FVW) and food waste (FW): single-phase vs. two-phase. Bioresour. Technol. 144, 80–85. Uçkun Kiran, E., Trzcinski, A.P., Ng, W.J., Liu, Y., 2014. Bioconversion of food waste to energy: a review. Fuel 134, 389–399. Westerhoff, P., Pinney, M., 2000. Dissolved organic carbon transformations during laboratory-scale groundwater recharge using lagoon-treated wastewater. Waste Manage. 20, 75–83. Xue, S., Zhao, Q.L., Wei, L.L., Ren, N.Q., 2009. Behavior and characteristics of dissolved organic matter during column studies of soil aquifer treatment. Water Res. 43 (2), 499–507. Yusoff, M.Z., Hu, A., Feng, C., Maeda, T., Shirai, Y., Hassan, M.A., Yu, C.P., 2013. Influence of pretreated activated sludge for electricity generation in microbial fuel cell application. Bioresour. Technol. 145, 90–96. Zhang, G., Zhao, Q., Jiao, Y., Lee, D.J., 2015. Long-term operation of manure-microbial fuel cell. Bioresour. Technol. 180, 365–369.
