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Co-generation of hydrogen and electricity from biodiesel process effluents
international journal of hydrogen energy xxx (xxxx) xxx
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Co-generation of hydrogen and electricity from biodiesel process effluents Sanath Kondaveeti 1, In-Won Kim 1, Sachin Otari, Sanjay K.S. Patel, Raviteja Pagolu, Venkatramana Losetty, Vipin Chandra Kalia, Jung-Kul Lee* Division of Chemical Engineering, Konkuk University, 1 Hwayang-Dong, Gwangjin-Gu, Seoul 05029, Republic of Korea
highlights Application of external voltage to a microbial fuel cell (MFC) during acclimatization improves bioelectrogenesis. Dark fermentation with synthetic wastewater yields high H2 production. Higher power generation is noted with crude glycerol (CG) dark fermentation effluent over CG. Enhanced performance of MFC is proved at an applied voltage of 0.8 V after 24 h acclimatization.
article info
abstract
Article history:
In this study, we apply a short-term voltage (0.2e0.8 V) to both crude glycerol (CG) and an
Received 20 April 2019
anaerobic digestion (AD) effluent in a single-chamber microbial fuel cell (MFC) for power
Received in revised form
production. This improves the bioelectrogenesis in both CG (in MFC-1) and the AD effluent
26 July 2019
(in MFC-2), but higher power generation is attained in MFC-2. The use of domestic and
Accepted 28 August 2019
synthetic wastewaters in the AD process leads to the generation of 195 and 350 mL H2/L-
Available online xxx
medium, respectively. MFC-2 performs better than MFC-1 in terms of both voltage generation and chemical oxygen demand (COD) reduction. The application of 0.8 V yields a
Keywords:
power density of 311 mW/m2 (1.94 times higher than that of the control (160 mW/m2)). In
Voltage supplementation
addition, MFC-2 exhibits a 70% COD removal at 0.8 V, which decreases to 56% at 0.2 V. Thus,
Microbial fuel cells
the application of a short-term voltage in MFC can stimulate both bioelectrogenesis and
Crude glycerol
COD removal.
Dark fermentation
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Cyclic voltammetry
Abbreviations: CG, Crude glycerol; MFCs, Microbial Fuel cells; H2, Hydrogen; COD, Chemical oxygen demand; CV, Cyclic voltammetry; EIS, Electrochemical impedance spectroscopy; VFAs, Volatile fatty acids; AD, Anaerobic digestion; PD, Power density; DC, Direct current; NE, Net energy; PBS, Phosphate buffer solution; DW, Domestic wastewater; SW, Synthetic wastewater; PW, Petroleum wastewater; I, Current; V, Voltage; W, Power; U, Resistance; Roh, Ohmic resistance; Rct, Charge transfer resistance; Rin, Internal resistance. * Corresponding author. E-mail address: [email protected] (J.-K. Lee). 1 These authors equally contributed to this work. https://doi.org/10.1016/j.ijhydene.2019.08.258 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Kondaveeti S et al., Co-generation of hydrogen and electricity from biodiesel process effluents, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.08.258
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Introduction
Materials and methods
Biodiesel is an effective alternative to fossil fuels and is commercially produced. Just in 2017, the European Union and the United States generated 21 and 32 million tons of biodiesel (http://www.ebb-eu.org/stats.php; https://www.afdc.energy. gov/data/10325). About 70e80% of biodiesel industrial effluents consist of glycerol; the remaining compounds include methanol, salts, and other organic compounds. For every 9 kg of biodiesel generated, about 1 kg of crude glycerol (CG) is produced as a by-product [1]. With the increase in biodiesel production, the price of CG is declining drastically and presently the focus has shifted from its utilization to its disposition. Hence, several researchers developed alternatives to convert CG to value-added products [2,3]. Biofermentation can allow the production of 0.34e6 mol H2/mol glycerol [3,4]. H2 is energetically efficient and can be used as biofuel. The heat of combustion and energy content of H2 were 285.8 kJ/mol and 142.9 kJ/g, respectively [5]. The production of H2 from CG is promising, considering its high calorific value and clean combustion [1]. However, the complete degradation of CG into H2 is difficult, perhaps due to the high concentration of organic compounds (e.g., volatile fatty acids, VFAs) in the effluent. These organic compounds should be treated, in order to improve energy production and treatment efficiency. Nevertheless, the treatment of the effluent in the initial stages can reduce the VFA content, resulting in enhanced energy generation. Therefore, the integration of a two-stage H2 production process (dark and photo fermentation) could allow the achievement of maximum energy recovery [6]. Still, the limitations imposed by scale up and light management problems should be considered [7]. The use of CG as a substrate for microbial fuel cells (MFC), with the help of Bacillus subtilis as an anodic biocatalyst, resulted in a maximum power density (PD) of 60 mW/m2 [8]; On the other hand, the use of CG and a dark fermentation effluent in an Htype double-chamber MFC provided maximum PD values of 31 and 90 mW/m2, respectively [9,10]. In these studies, PD and COD reduction values were lower than those reported in other studies on MFC, which might be due to poor bioelectrogenesis, leading to lower degradation of recalcitrant pollutants such as CG. Recent studies suggest that the application of acclimatization voltage can enhance the bioelectrogenic biofilm activity on an anode while simultaneously enhancing power generation [11,12]. However, the application of a short-term voltage in a single-chamber MFC using CG or dark fermentation effluents as substrates for electrochemical bacteria has not been reported yet. In the present study, the electricity-producing potential of CG was investigated through two routes: (i) directly, through MFC, and (ii) indirectly, based on the effluent generated by the conversion of CG to H2. The following benefits can be obtained from the proposed method: (i) the generation of H2, (ii) the generation of electricity, and (iii) a reduction in chemical oxygen demand (COD). We analyzed possible limitations in the use of dark fermentation effluents and CG in terms of power generation and by using electrochemical techniques (e.g., cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS)).
Enzymatic biodiesel production waste CG, employed as a substrate in our bioreactor, was collected from a laboratory scale system, which used olive oil and lipase for the production of biodiesel (Fig. 1). In that system, biodiesel was produced through a transesterification reaction (using olive oil as substrate, methanol as reactant, and lipase as catalyst [13];: lipase was derived from Termomyces lanuginosus (Lipozyme TL 100 L from Novozymes Inc., Franklinton, USA), while methanol and olive oil were purchased from SigmaAldrich Co. (St. Louis, MO, USA). The purity of CG was approximately 80%; its impurities included sodium and potassium salts (5%), methanol (4%), non-glycerol organic matter (6%), and water (5%) [14,15].
Bioreactor operation for dark fermentation The anaerobic digestion (AD) effluent, used as a source of inoculum for dark fermentation in this study, was collected from the Suwon Wastewater Treatment plant (Suwon-si, South Korea) and treated at 90 C for 1 h to suppress the activity of methanogens [16]. CG (15 g/L) was used as a carbon source for the generation of H2. The bioreactor consisted of a 120-mL bottle connected to a gas holder, containing 5 mL of pre-treated inoculum and 35 mL of growth media (MgSO4$7H2O, 3.0 g/l; MnSO4$H2O, 0.5 g/l; NaCl, 1.0 g/l; FeSO4$7H2O, 0.1 g/l; CoSO4$7H2O, 0.18 g/l; H3BO3, 0.01 g/l; Na2MoO4$2H2O, 0.01 g/l; NiCl2$6H2O, 0.03 g/l; Na2SeO3$5H2O, 0.30 mg/l). The reactors were incubated at 35 C for 56 h, for the generation of H2. The growth medium, used to encourage bacterial growth, was prepared as described in Ref. [9]. The initial pH of the growth medium was adjusted to 7 and the bioreactors were flushed with nitrogen gas for 15 min to create anaerobic conditions. After 56 h, 5 mL of bioreactor culture medium was transferred into a new medium, and the total working volume was made up to 40 mL. This mixed medium was further operated under similar conditions, before measuring the concentration of gas in the head space. Liquid samples were collected from the bioreactor at the end of the operation (after 56 h) to analyze pH, COD, and VFAs. In this study, we calculated the average values of H2 generation, pH, COD, and VFAs based on the results of three experiments.
Microbial fuel cell construction and operation Two single-chamber air cathode MFCs (working volume ¼ 150 mL; total volume ¼ 200 mL) were constructed using Perspex sheet. A graphite-felt anode was pre-treated (heat-treated) as described previously [17], while a platinum coated carbon paper (0.5 mg/cm2) was used as air cathode (4.9 cm2). Both MFCs were operated in fed-batch mode and at room temperature (30 ± 2 C) without mixing, except during liquid sampling. An electroactive biofilm was developed on the anode, by feeding the MFCs with the effluents of a separate, double chamber MFC (0.435 mV and 1000 U) that employed glucose as a substrate. The stability of voltage generation was checked using an external resistance (500 U);
Please cite this article as: Kondaveeti S et al., Co-generation of hydrogen and electricity from biodiesel process effluents, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.08.258
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Fig. 1 e Electricity production using (A) MFC from crude glycerol and (B) the AD effluent generated by anaerobic digestion process for hydrogen production from glycerol.
then, the electrolyte solutions contained in the MFCs were replaced with a 50 mM phosphate buffer solution (PBS), as well as with substrate, mineral and vitamin solutions [18]. We designated the MFC operating with approximately 10 g/L of CG substrate and PBS as MFC-1, while the MFC operating with the AD effluent (CG dark fermentation effluent) was termed MFC2. The three initial cycles of both MFCs, carried out without applying any external voltage, are referred to as “controls”. An initial voltage of 0.2 V was applied for 24 h to both MFCs (acclimatization period); next, the MFCs were operated to generate energy (experimental period) and perform a simultaneous organics reduction. Afterwards, other two different voltages were applied (0.5 and 0.8 V) and their correspondent electric current generation was registered during several cycles. A short-term voltage (0.2, 0.5 and 0.8 V) was applied to the MFC during the acclimatization period (during initial 24 h), by connecting positive and negative leads of DC (direct current) power supply to the anode and cathode, respectively. After
24 h, the power supply was disconnected from the MFC and a 500 U external resistance was connected instead, in order to close the MFC circuit and generate electric current. The pH of the dark fermentation process effluent was adjusted to 7 before feeding it to MFC-2. The MFCs operating without the application of any external voltage were regarded as controls. To maintain anaerobic conditions, the anodes of the MFCs were purged with nitrogen gas for 15e20 min. The values presented in the paper are based are averages calculated from the results of three experiments.
Analysis and calculation The voltage, regarded as the potential difference between the anode and the cathode, was measured across an external resistance (500 U) at 15 min intervals, using digital multimeter. The voltage data were converted to electric current (I) and power (W) data applying Ohm's law. The maximum power
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density was obtained from a polarization analysis; this was based on the variation in external resistance (5 kUe5 U) starting from open-circuit voltage (OCV) [19]. The electric power and current densities were calculated from the cathode surface area (4.9 cm2), while the internal resistance (Rin) was calculated from the slope of the IeV curve [20]. Moreover, we calculated the net energy generated at different voltages based on the substrate characteristics, following [12]. An EIS analysis was performed to measure the Ohmic (Roh) and charge-transfer resistance (Rct). Both an CV and the EIS analyses were carried out in the three electrode systems by using a BioLogic potentiostat (VSP-150, France). The EIS analysis was carried out within a frequency range of 10 kHze10 mHz, at a voltage amplitude of 10 mV: the ohmic resistance and the charge-transfer resistance are measured from the high and the low frequency regions, respectively [21,22]. On the other hand, the CV was carried out within a potential window of 0.6 to 1.0 versus Ag/AgCl, and at a scan rate of 5 mV/s. The anode of the MFC was used as a working electrode, while the Ag/AgCl electrode and the cathode served as a reference and as a counter electrode, respectively [23,24]. The production of H2 was measured in the head space during the dark fermentation, using a gas chromatograph (Younglin Instruments, Korea) equipped with TCD detector; the correspondent analysis was done following [25]. The COD was measured in calorimetry mode, according to the manufacture's protocol (HACH Co., USA). The glycerol to H2 efficiency was calculated as described previously [9,26].
Glycerol to H2 conversion efficiency ¼
examined using CLSM. The high and low bacterial growth activities were distinguished with green and red color, respectively. Likewise, the biomass measurements of anode biofilms were recorded using the Coomassie Brilliant Blue method mentioned in previous studies on MFC [28,29]. Briefly, each sample of the anode with biofilm was weighed and aggressively mixed with glass beads for 5 min in 0.2 M NaOH (2 mL). The total protein concentration was measured against bovine serum albumin standard. Five random samples of anode were evaluated for biomass, and their average was considered the final anode biomass.
Results and discussion CG is an organic pollutant. Since the disposition of CG is uneconomical, it is desirable to use it in the production of valueadded products [30]. An accessible option would be to use it as a carbon substrate, to produce bioenergy and, ultimately, electricity. In this work, we exploited CG through dark fermentation and MFC, by applying a short-term voltage. Previous studies employing AD effluents showed an incomplete conversion of the organics to H2 [9,14]. In order to convert a higher quantity of organics, we combined the use of an AD effluent and of an MFC with a short-term applied voltage. The proposed process provides a cleaner combustion, a higher calorific H2 value, and an easier operation/purification in comparison with other biological processes [14,31].
Yield of H2 combustion energy of H2 100% Combustion energy of glycerol
The VFAs present in the filtered liquid samples were analyzed using an ion chromatograph, fitted with an organic acid column (Metrosep 250/7.8, Metrohm, Swiss). The effluent and influent pH of the solutions were analyzed with a benchtop digital pH meter (Hanna Instruments, USA).
Electrogenic biofilm visualization and biomass analysis Bioelectrogenic microbial visualization and biomass analysis of anode were performed at the end of the MFC operation. Field-emission SEM (FE-SEM) was used for microbial visualization. A part of the anode (approximately 1 cm2) was cut and immersed in 2.5% glutaraldehyde solution for 30 min. The samples were cautiously rinsed thrice with PBS (pH 7.0, 50 mM) and twice with distilled water. The samples were then subjected to dehydration by using ethanol in series (25%, 50%, 75%, and 90%, for 15 min) and were dried under ambient conditions. SEM measurements were recorded on FE-SEM, Hitachi S-4800, Tokyo, Japan. The bioelectrogenic activity on the MFC anode was measured using CLSM (Olympus FV-1000, Tokyo, Japan), and the bacterial viability was calculated as described in a previous paper [27]. The anode biofilms were stained using LIVE/DEAD backlight staining kit (Molecular Probes, Invitrogen) and were
Biohydrogen generation by dark fermentation of crude glycerol The co-digestion of DW (domestic wastewater) and SW (synthetic wastewater) with CG under dark fermentation resulted in 195 and 350 mL H2/L, respectively, over a period of 57 h (the equivalent of 0.23 and 0.59 mol H2/mol glycerol; Fig. 2). H2 constituted 58.3e61.2% of the total biogas produced employing DW and SW, respectively. Based on the combustion heat of CG (1674 kJ/mol) and H2 (285 kJ/mol) [9], the energetic efficiencies reached using DW and SW were 3.91% and 10.00%, respectively. The H2 yield produced by the digestion of CG is similar to that reported previously [9,30,32e37]: 0.20e1.1 mol H2/mol glycerol (Table 1). The difference in the H2 yield noted in comparison with other studies might be due to the difference in the operational conditions, type of inoculum, and variation in pre-treatment of the inoculum [38,39]. The thorough optimization using response surface methodology, bacterial strain screening, and other strategies such as micro oxidation are in progress to improve the H2 yield. Because of H2 production, the COD reductions in DW and SW were 13.4 and 22.8%, respectively; the correspondent VFA productions were 15.1 and 22.8 mM, respectively (Table 2). The most abundant VFA was acetic acid (12.5 and 16.9 mM in DW
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Table 2 e Characterization of dark fermentation effluent with crude glycerol as a substrate. Characteristics
Maximum gas generation (ml H2/l) COD in influent (g/l) COD in effluent (g/l) COD removal (%) Acetic Acid (mM) Butyric Acid (mM) Propionic Acid (mM) Valeric Acid (mM) Caproic Acid (mM)
Fig. 2 e Cumulative dark fermentative H2 production from crude glycerol slurry (15 g/l) prepared in (i) synthetic wastewater (black filled square) and (ii) domestic wastewater (red filled circle). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
and SW, respectively). The presence of butyric acid in the effluent of SW, and the higher quantity of acetic acid, justify the enhanced H2 compared to that registered during the operation of AD using DW [40]. The level of residual COD in the effluent was high (77.2 and 86.6% for SW and DW, respectively); these values indicate that a further treatment was needed to achieve a complete degradation. A high residual COD could imply that the generation of H2 is a self-metabolic redox process, which does not need external electron acceptors like oxygen or nitrates [41,42].
Electric current generation from the effluent of fermented crude glycerol Fermentation of CG in two different types of wastewaters resulted in the generation of H2 and an effluent-rich organic matter. The generation of electric current potential of CG before and after its digestion was evaluated operating two MFC reactors (MFC-1 and MFC-2) each fed with one type of
Table 1 e Various studies on H2 production with CG as a substrate. Glycerol H2 volume Yield Operation Reference (g/L) (mL) (mol H2/mol type glycerol) 3 3 5 10 5 3
18 133 N/A 305 160 1310
0.20 0.31 0.19 0.55 0.45 0.26
10 2.5 10
N/A 11.9 352
0.60 1.10 0.59
Batch mode Batch mode Batch mode Batch mode Batch mode Continuous mode Batch mode Batch mode Batch mode
[29] [31] [32] [9] [33] [34] [35] [36] This study
Effluent Based on Synthetic wastewater (SW)
Distilled water (DW)
350
195
14.9 11.5 22.8 16.9 0.98 2.95 0.89 1.03
14.9 12.9 13.4 12.5 e 1.82 e 0.82
wastewater. An increase in electric current generation corresponded to the application of a short-term voltage for 24 h (Fig. S1). In MFC-1, a progressive increase in electric current generation was recorded from 0.52 mA in the control (no applied voltage) to 0.82 mA for an applied voltage of 0.8 V (Table 3). In contrast, the MFC-2 operation with applying a voltage of 0.5 V and 0.8 V resulted in a current generation of 0.59 mA and 0.68 mA, respectively. As seen in MFC-1, the electric current generation at 0.8 V (0.52 mA) was 1.57 times higher than that observed in the absence of voltage (Fig. S1A). These current generation values for MFC-1 are found to be lower than those compared with for MFC-2 operation with AD effluent. MFC-2 was fed with CG treated with AD: the by-products present in the residual material after the release of H2 had higher electric current generation capacity compared to those from the untreated CG (Table 3). The application of a shortterm voltage increased the electricity generation capacity in all cases by 18% (0.70 mA) at 0.2 V, 34% (0.79 mA) at 0.5 V, and 46% (0.86 mA) at 0.8 V (compared with control: 0.59 mA). The overall electric current generation was 1.46 (0.59 vs. 0.86 mA) times higher than that in the absence of voltage (control; Fig. S1B). These results suggest that the provision of acclimatization voltage can enhance the bioelectrogenesis of MFC, thereby resulting in its better performance. Lower performance is possibly caused by the direct use of CG in the MFC-1 control, which limits the biological activity at the anode. On basis of these outcomes, it is evident that the dark fermentation was beneficial in two aspects: (i) it stimulated the production of a higher quantity of H2 (350 ml H2/L), and (ii) caused an 8.1% increase in electric current generation at 0.8 V (MFC-1 vs MFC-2). This method enables a higher level of bioconversion and energy extraction from biowaste. Although both of the MFC reactors were operated at similar external resistances (~500 U), the quantity electric current generated was different, depending on the type of substrate and short-term applied voltage. Previous studies demonstrated that higher carbon chain compounds like CG cannot be easily reduced, in comparison to simple carbon compounds like acetate [43,44]. In our experiment, a high concentration of acetate (16.9 mM) in the AD effluent enhanced power generation in MFC-2. The present study is the first in which an acclimatization voltage was applied in the presence of a CG dark fermentation effluent, and in which CG was directly used in an MFC. The
Please cite this article as: Kondaveeti S et al., Co-generation of hydrogen and electricity from biodiesel process effluents, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.08.258
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Table 3 e Performance parameters of MFC operating with crude glycerol and dark fermentation effluents. MFC-1a
Current (mA) Power density (mW/cm2) COD in effluent (g/l) COD removal % CE (%) Ohmic Resistance (Roh, U) Charge transfer resistance (Rct, U) a b
MFC-2b
Control
0.2 V
0.5 V
0.8 V
Control
0.2 V
0.5 V
0.8 V
0.52 72.0 8.41 16.7 14.2 2.98 1050
0.59 111 7.50 25.5 15.6 e e
0.68 157 6.51 35.5 16.8 1.98 456
0.82 184 5.80 42.0 15.9 e e
0.59 160 5.71 48.3 25.6 2.67 613
0.70 231 4.82 56.1 28.9 e e
0.79 290 3.81 65.3 30.1 1.72 256
0.86 311 3.21 70.0 27.4 e e
10 g/L of COD with CG. 11.5 g/l of COD with CG dark fermentation effluent as a substrate.
electric current generation at 0.8 V (0.82 and 0.86 mA in MFC-1 and MFC-2, respectively) recorded during our experiments was higher than that reported in other studies regarding organic rich wastes. It suggests that the electrogenic acclimatization methodology can enhance energy (current) generation. A single-chamber MFC, operated with cereal and swine wastewaters, exhibited maximum electric current generations of 0.30 and 0.35 mA, respectively [44e46]. Similarly, using CG as a substrate in a single-chamber MFC [10], obtained maximum electric current generations of 0.16 and 0.49 mA. Our study demonstrates that the application of an acclimatization voltage can enhance power generation. The use of a raw CG dark fermentation effluent, without applying any voltage, resulted in a maximum electric current generation of 0.21 mA in Ref. [9], whereas in our study we could reach 0.86 mA. In conclusion, the acclimatization of MFC by the application of a short-term voltage can enhance the production of electric power, using complex substrates like CG.
Polarization analysis using a dark fermentation effluent and crude glycerol We analyzed the maximum power generation using CG and a dark fermentation effluent through a polarization analysis (Fig. 3). The maximum power densities and OCV of the MFCs were different depending on the operational conditions. MFC1 exhibited a 2.5-fold increase in maximum power density from 72.0 (control) to 184 mW/m2 (0.8 V)) compared to control. Upon its operation at 0.2 and 0.5 V a change in the maximum power density (from 111 to 157 mW/m2) was observed. Similarly, MFC-2 exhibited a variation in maximum power density as the operational conditions changed. The control of MFC-2 (160 mW/m2) exhibited a PD 1.94 times lower than at 0.8 V (311 mW/m2). Application of 0.2 and 0.5 V to MFC-2, resulted in a maximum PD of 231 and 290 mW/m2, respectively. At all operational conditions, MFC-2 exhibited higher power densities than MFC-1. A similar increase in power density (from 45 to 132 mW/m2) varying the short-term applied voltage was noted in other studies on MFCs, using petroleum wastewater (PW) [12]. MFC-2, operated at 0.5 V (290 mW/m2) with CG, generated a power density 2.2 times higher in comparison with the MFC operated with PW (132 mW/m2). On the other hand, MFC-1 operated at 0.5 V (157 mW/m2) exhibited a power density only 1.2 times higher than that of the MFC operated with PW. These results suggest that the use of CG as a
substrate, and its operation using AD and MFC, can increase its calorific value. Based on the IeV curve from the polarization analysis, we calculated the internal resistances of both MFCs (Fig. 3a and b). At all intermittent applied voltages, MFC-2 exhibited similar internal resistances (~256 U), while the control system without any acclimatized voltage exhibited an internal resistance of 613 U. At different applied voltages, MFC-1 had an internal resistance of ~456 U (lower than that of the control: 1050 U). The maximum power density at 0.8 V measured in the present study is higher than that reported in a previous study dealing with the treatment of MFC with 50% CG dark fermentation effluents (90 mW/m2) [9], and in another, where CG (60 mW/ m2) was used as a substrate in a single-chamber MFC [8]. The energy (power) consumption during the short-term voltage period (acclimatization period 24 h) was compared with the power generation. A maximum 8.5-fold higher energy generation (MFC-2) was observed during the experimental period (MFC mode) compared to power consumption for acclimatization. The NE was calculated on the basis of the difference between power generated during the acclimatization period of MFC (initial 24 h) and after the 24-h operation. The increase in energy (voltage) supplementation led to an increase in the bioelectrogenic activity in MFC. Throughout the acclimatization period, both MFC-1 and MFC-2 exhibited similar power consumption of 7.1 and 7.4 mW/h, respectively, for initial 24 h. However, during the MFC voltage generation mode, the NE of MFC-2 at 0.8 V (55 mW/h) was 3.5 times higher than that of MFC-1 (16 mW/h, Table 4). The smaller NE in MFC1 is possibly due to the difficulty of using CG as a substrate, thereby limiting the bioelectrogenic activity on its anode. Based on NE generation in MFC-1 (16 mW/h) and MFC-2 (55 mW/h), 1 kg/L CG (biodiesel industry waste) can yield NE up to 1.06 and 3.66 W/h, respectively. The power generation is high compared to that of single-chamber air cathode MFCs operating with PW [12].
Organic removal in the microbial fuel cell The bioelectrogenic oxidation of CG during power generation was analyzed in terms of COD removal. A similar trend was noted from both MFCs in terms of COD removal and power generation. The increase in COD removal due to the accelerated oxidation of the substrate led to higher power generation. COD removal in both MFCs was enhanced compared to the
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Fig. 3 e Changes in voltage and power density during the polarization analyses of MFC-1 (crude glycerol; A-B) and MFC-2 (dark fermentation effluent; C-D). Control (black filled square), 0.2 V (red filled circle), 0.5 V (blue filled triangle), and 0.8 V (magenta filled inverted triangle). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) control (Fig. 4; Table 3). COD removal in the control of MFC-1 reached only 16%. An increment in COD removal was noted at the application of short-term voltage: COD removal was equal to 25% at 0.2 V, 35% at 0.5 V, and 42% at 0.8 V. A 1.7-fold
increase in COD removal was observed from the control to the MFC operated at 0.8 V. MFC-2, without any applied voltage, exhibited a COD removal of 48%. Once short-term voltages were applied, the COD removal reached 56% at 0.2 V, 65% at
Table 4 e Comparative evaluation between energy consumed and generated during short term voltage application with crude glycerol and dark fermentation effluent. Applied Voltage (V)
Energy Consumed
Net Energy produced (mW/h)
Generated
Current (mA) Power (mW) Total (mW/h) Current (mA) Power (mW) Total (mW/h) Crude glycerol in MFC-1 Control N/A 0.2 0.11 0.5 0.32 0.8 0.37 Anaerobic digestion effluent in MFC-2 Control N/A 0.2 0.13 0.5 0.34 0.8 0.39
N/A 0.02 0.16 0.29
N/A 0.48 3.84 7.10
0.36 0.46 0.56 0.59
0.06 0.10 0.16 0.17
10.7 18.8 26.9 23.0
10.7 18.3 23.1 16.0
N/A 0.02 0.17 0.31
N/A 0.62 4.08 7.44
0.457 0.58 0.71 0.77
0.10 0.16 0.25 0.30
18.8 19.5 41.7 62.7
18.1 18.8 37.6 55.2
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2, the CEs were 25.6% (for the control), 28.9% (at 0.2 V), 30.1% (at 0.5 V), and 27.4% (at 0.8 V). The CE obtained in this study was higher compared to that reported in other MFC studies using CG dark fermentation effluent as a substrate (13.8%) [9] or starch processing wastewater (8%) [44].
Electrochemical analysis of the microbial fuel cell
Fig. 4 e COD concentration in the influent, effluent, and COD removal, in MFC-1 and MFC-2 (COD removal ¼ red filled circle). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
0.5 V, and 70% at 0.8 V. These values are higher than those reported in other studies on double chamber MFCs using CG dark fermentation effluents as substrate [9]. Overall, this suggests that a short-term applied voltage enhances the performance of MFC. The columbic efficiency (CE) was calculated based on the COD removal and on the electric current generation in both MFCs, for various operational conditions. MFC-1 exhibited CEs of 14.2% (for the control), 15.6% (at 0.2 V), 16.8% (at 0.5 V), and 15.9% (at 0.8 V). By using a dark fermentation effluent in MFC-
Electrochemical techniques (e.g., EIS and CV) can provide useful information on the electrochemical response of anode biofilms. Our analyses were performed under turnover conditions, in the presence of a substrate for both the control and the MFCs operating at 0.5 V (Fig. 5). The MFCs, both operating at 0.5 V, exhibited a notable difference in performance; therefore, 0.5 V was chosen a suitable voltage for comparing them with the control. Both MFCs showed higher oxidation and a reduction in electric current generation, compared with the control, after the application of a short-term voltage. MFC-1 generated a lower level of redox electric current compared to MFC-2. In MFC-1, the oxidative electric current increased with the application of 0.5 V (from 0.18 mA in the control, to 0.51 mA at 0.5 V), while the reductive electric current generation changed from 2.5 mA (in the control) to 4.1 mA (at 0.5 V). These quantities were higher in MFC-2: the oxidative electric current increased from 0.72 mA to 7.11 mA, while the reductive electric currents changed from 6.9 mA and 14.5 mA. The increase in electric current generation following the application of a short-term voltage in both MFCs, suggests an enhanced biological activity/growth on the anode electrode.
Fig. 5 e CV (AeB) and EIS (CeD) values measured for MFC-1 (A, C) and MFC-2 (B, D) under stable voltage generation. Control (black filled square), 0.5 V (red filled circle). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) Please cite this article as: Kondaveeti S et al., Co-generation of hydrogen and electricity from biodiesel process effluents, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.08.258
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The application of short-term voltage of 0.5 V in both MFCs was shown to enhance their performance compared to the control, as observed via EIS. The anode of both MFCs exhibited a difference in ohmic resistance (Roh) and charge transfer resistance (Rct) in comparison with control (no applied voltage). The control of MFC-1 exhibited a Roh and a Rct of 2.98 U and 1050 U, respectively. After the application of 0.5 V, the Roh and Rct decreased to 1.98 U and 456 U, respectively. The Rct of MFC-1 was higher compared to that of MFC-2. The use of an AD effluent in MFC-2 changed the Rct to 613 U in the case of no voltage, and to 256 U after the application 0.5 V. In MFC-2, H2 and electricity were co-generated; still, there was a residual COD of 28%, which could be exploited for the production of polyhydroxyalkanoates [1,45e47] and other products such as lipids, and alcohols [48e50]. Overall, in the present study we were able to extract value-added products from biodiesel industry waste, through a sustainable, renewable, and cleaner combustion process. Hence, the electrochemical method described in this paper can contribute to pollution reduction [51,52]. The PD generation and COD removal values, measured following the application of an intermittent voltage, were higher compared to those reported in previous studies [9].
9
Influence of supplemental voltage on bioelectrogenic growth The effect of the type of substrate (AD effluent vs. CG) and applied voltage (during acclimatization period) on the bioelectrogenic anode of the MFC was evaluated (Fig. 6). The microbial growth of MFCs (MFC-1 and MFC-2) was analyzed for control MFC and MFC supplied with an acclimatization voltage of 0.5 V. The latter exhibited a noteworthy performance; therefore, 0.5 V was chosen for comparison with control. The mode of attachment of the bioelectrogenic bacteria on the anode was analyzed using FE-SEM. The SEM images revealed that the electrogenic microbes were attached on the carbon fibers of anode by self-secretion of an extracellular polymeric substance (EPS) [53,54]. The bioelectrogens were more uniformly distributed when the MFC was supplemented with an external voltage, indicating that MFC operation supplied with acclimatization voltage can enhance biofilm growth and lead to an increase in power generation and COD reduction. The electrogenic bacterial viability of the anode biofilm was analyzed using CLSM (Fig. S2). In the absence of acclimatization voltage, a low bacterial viability of 60 ± 3% was
Fig. 6 e Microbial biofilm visualization on anode of MFC under various operational conditions: Pristine graphite felt (A), Control MFC-1 (B), Control MFC-2 (C), MFC-1 at 0.5 V (D), and MFC-2 at 0.5 V (E). The inset shows the attachment of bioelectrogens on the anode. Please cite this article as: Kondaveeti S et al., Co-generation of hydrogen and electricity from biodiesel process effluents, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.08.258
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noted in MFC-1 using CG as a substrate. However, this increased to 69 ± 5% when an acclimatization voltage of 0.5 V was supplied. MFC-2 (substrate: AD effluent) operation without and with acclimatization voltage of 0.5 V exhibited bacterial viabilities of 66 ± 4% and 77 ± 3%, respectively. These results suggest that organic-rich recalcitrant pollutants (e.g., CG) when used directly inhibit the bacterial activity and thereby lead to a decrease in power generation. In addition, the integration of an AD effluent with MFC can yield better performance in terms of power generation. In general, the bacterial viability of the anode biofilm of MFC varies from 20% to 95% depending on the substrate, exoelectrogens, and other operational parameters [51]. The biomass on the anode increased slightly from 17.5 ± 2 mg/g in control MFC-1 to 20.5 ± 2 mg/g in MFC-1 supplied with an acclimatization voltage of 0.5 V. In contrast, biomass on the anode in control MFC-2 and MFC-2 supplied with an acclimatization voltage of 0.5 V were 19.6 ± 3 and 23.1 ± 2 mg/g, respectively. As seen, a higher biomass was obtained when an AD effluent was used as the substrate. These results indicate that a higher calorific value can be obtained using an integrated approach (MFC-2) compared with the direct utilization of CG (MFC-1). The recent developments in the dark fermentation process has been proven to have an enormous potential for scalability of biohydrogen production from organic wastes. However, biohydrogen production is mostly studied at a laboratory scale, and there are limited studies on pilot scale operation. One of the major limitations in scaling-up of H2 production by dark fermentation is the low organic matter removal, thereby leading to low H2 yields [55,56]. Therefore, the integration of dark fermentation with other biological renewable systems such as MFC can enhance the overall yield and energy efficiency, with simultaneous reduction in the organics. In particular, the co-generative dark fermentation with MFC to generate bioelectricity can circumvent product purification, thereby leading to a decrease in the downstream cost.
Conclusions The application of a short-term voltage in a single-chamber air cathode MFC, improved the production of energy from CG and AD. The maximum H2 production rate (350 mL H2/L, yielding 0.59 mol H2/mol glycerol) was registered during a dark fermentation in synthetic wastewater. The feeding of AD into MFC-2 resulted in a higher maximum power density (311 mW/ m2; 0.8 V) than in the control, and in better performances under all operational conditions. Our study can be applied to the reduction of biodiesel pollution and opens new possibilities in the treatment of complex organics, through the application of short-term voltage. Corresponding supplementary material can be found in the online version of this paper.
Acknowledgements This paper was supported by Konkuk University in 2016.
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.08.258.
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Co-generation of hydrogen and electricity from biodiesel process effluents Sanath Kondaveeti 1, In-Won Kim 1, Sachin Otari, Sanjay K.S. Patel, Raviteja Pagolu, Venkatramana Losetty, Vipin Chandra Kalia, Jung-Kul Lee* Division of Chemical Engineering, Konkuk University, 1 Hwayang-Dong, Gwangjin-Gu, Seoul 05029, Republic of Korea
highlights Application of external voltage to a microbial fuel cell (MFC) during acclimatization improves bioelectrogenesis. Dark fermentation with synthetic wastewater yields high H2 production. Higher power generation is noted with crude glycerol (CG) dark fermentation effluent over CG. Enhanced performance of MFC is proved at an applied voltage of 0.8 V after 24 h acclimatization.
article info
abstract
Article history:
In this study, we apply a short-term voltage (0.2e0.8 V) to both crude glycerol (CG) and an
Received 20 April 2019
anaerobic digestion (AD) effluent in a single-chamber microbial fuel cell (MFC) for power
Received in revised form
production. This improves the bioelectrogenesis in both CG (in MFC-1) and the AD effluent
26 July 2019
(in MFC-2), but higher power generation is attained in MFC-2. The use of domestic and
Accepted 28 August 2019
synthetic wastewaters in the AD process leads to the generation of 195 and 350 mL H2/L-
Available online xxx
medium, respectively. MFC-2 performs better than MFC-1 in terms of both voltage generation and chemical oxygen demand (COD) reduction. The application of 0.8 V yields a
Keywords:
power density of 311 mW/m2 (1.94 times higher than that of the control (160 mW/m2)). In
Voltage supplementation
addition, MFC-2 exhibits a 70% COD removal at 0.8 V, which decreases to 56% at 0.2 V. Thus,
Microbial fuel cells
the application of a short-term voltage in MFC can stimulate both bioelectrogenesis and
Crude glycerol
COD removal.
Dark fermentation
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Cyclic voltammetry
Abbreviations: CG, Crude glycerol; MFCs, Microbial Fuel cells; H2, Hydrogen; COD, Chemical oxygen demand; CV, Cyclic voltammetry; EIS, Electrochemical impedance spectroscopy; VFAs, Volatile fatty acids; AD, Anaerobic digestion; PD, Power density; DC, Direct current; NE, Net energy; PBS, Phosphate buffer solution; DW, Domestic wastewater; SW, Synthetic wastewater; PW, Petroleum wastewater; I, Current; V, Voltage; W, Power; U, Resistance; Roh, Ohmic resistance; Rct, Charge transfer resistance; Rin, Internal resistance. * Corresponding author. E-mail address: [email protected] (J.-K. Lee). 1 These authors equally contributed to this work. https://doi.org/10.1016/j.ijhydene.2019.08.258 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Kondaveeti S et al., Co-generation of hydrogen and electricity from biodiesel process effluents, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.08.258
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Introduction
Materials and methods
Biodiesel is an effective alternative to fossil fuels and is commercially produced. Just in 2017, the European Union and the United States generated 21 and 32 million tons of biodiesel (http://www.ebb-eu.org/stats.php; https://www.afdc.energy. gov/data/10325). About 70e80% of biodiesel industrial effluents consist of glycerol; the remaining compounds include methanol, salts, and other organic compounds. For every 9 kg of biodiesel generated, about 1 kg of crude glycerol (CG) is produced as a by-product [1]. With the increase in biodiesel production, the price of CG is declining drastically and presently the focus has shifted from its utilization to its disposition. Hence, several researchers developed alternatives to convert CG to value-added products [2,3]. Biofermentation can allow the production of 0.34e6 mol H2/mol glycerol [3,4]. H2 is energetically efficient and can be used as biofuel. The heat of combustion and energy content of H2 were 285.8 kJ/mol and 142.9 kJ/g, respectively [5]. The production of H2 from CG is promising, considering its high calorific value and clean combustion [1]. However, the complete degradation of CG into H2 is difficult, perhaps due to the high concentration of organic compounds (e.g., volatile fatty acids, VFAs) in the effluent. These organic compounds should be treated, in order to improve energy production and treatment efficiency. Nevertheless, the treatment of the effluent in the initial stages can reduce the VFA content, resulting in enhanced energy generation. Therefore, the integration of a two-stage H2 production process (dark and photo fermentation) could allow the achievement of maximum energy recovery [6]. Still, the limitations imposed by scale up and light management problems should be considered [7]. The use of CG as a substrate for microbial fuel cells (MFC), with the help of Bacillus subtilis as an anodic biocatalyst, resulted in a maximum power density (PD) of 60 mW/m2 [8]; On the other hand, the use of CG and a dark fermentation effluent in an Htype double-chamber MFC provided maximum PD values of 31 and 90 mW/m2, respectively [9,10]. In these studies, PD and COD reduction values were lower than those reported in other studies on MFC, which might be due to poor bioelectrogenesis, leading to lower degradation of recalcitrant pollutants such as CG. Recent studies suggest that the application of acclimatization voltage can enhance the bioelectrogenic biofilm activity on an anode while simultaneously enhancing power generation [11,12]. However, the application of a short-term voltage in a single-chamber MFC using CG or dark fermentation effluents as substrates for electrochemical bacteria has not been reported yet. In the present study, the electricity-producing potential of CG was investigated through two routes: (i) directly, through MFC, and (ii) indirectly, based on the effluent generated by the conversion of CG to H2. The following benefits can be obtained from the proposed method: (i) the generation of H2, (ii) the generation of electricity, and (iii) a reduction in chemical oxygen demand (COD). We analyzed possible limitations in the use of dark fermentation effluents and CG in terms of power generation and by using electrochemical techniques (e.g., cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS)).
Enzymatic biodiesel production waste CG, employed as a substrate in our bioreactor, was collected from a laboratory scale system, which used olive oil and lipase for the production of biodiesel (Fig. 1). In that system, biodiesel was produced through a transesterification reaction (using olive oil as substrate, methanol as reactant, and lipase as catalyst [13];: lipase was derived from Termomyces lanuginosus (Lipozyme TL 100 L from Novozymes Inc., Franklinton, USA), while methanol and olive oil were purchased from SigmaAldrich Co. (St. Louis, MO, USA). The purity of CG was approximately 80%; its impurities included sodium and potassium salts (5%), methanol (4%), non-glycerol organic matter (6%), and water (5%) [14,15].
Bioreactor operation for dark fermentation The anaerobic digestion (AD) effluent, used as a source of inoculum for dark fermentation in this study, was collected from the Suwon Wastewater Treatment plant (Suwon-si, South Korea) and treated at 90 C for 1 h to suppress the activity of methanogens [16]. CG (15 g/L) was used as a carbon source for the generation of H2. The bioreactor consisted of a 120-mL bottle connected to a gas holder, containing 5 mL of pre-treated inoculum and 35 mL of growth media (MgSO4$7H2O, 3.0 g/l; MnSO4$H2O, 0.5 g/l; NaCl, 1.0 g/l; FeSO4$7H2O, 0.1 g/l; CoSO4$7H2O, 0.18 g/l; H3BO3, 0.01 g/l; Na2MoO4$2H2O, 0.01 g/l; NiCl2$6H2O, 0.03 g/l; Na2SeO3$5H2O, 0.30 mg/l). The reactors were incubated at 35 C for 56 h, for the generation of H2. The growth medium, used to encourage bacterial growth, was prepared as described in Ref. [9]. The initial pH of the growth medium was adjusted to 7 and the bioreactors were flushed with nitrogen gas for 15 min to create anaerobic conditions. After 56 h, 5 mL of bioreactor culture medium was transferred into a new medium, and the total working volume was made up to 40 mL. This mixed medium was further operated under similar conditions, before measuring the concentration of gas in the head space. Liquid samples were collected from the bioreactor at the end of the operation (after 56 h) to analyze pH, COD, and VFAs. In this study, we calculated the average values of H2 generation, pH, COD, and VFAs based on the results of three experiments.
Microbial fuel cell construction and operation Two single-chamber air cathode MFCs (working volume ¼ 150 mL; total volume ¼ 200 mL) were constructed using Perspex sheet. A graphite-felt anode was pre-treated (heat-treated) as described previously [17], while a platinum coated carbon paper (0.5 mg/cm2) was used as air cathode (4.9 cm2). Both MFCs were operated in fed-batch mode and at room temperature (30 ± 2 C) without mixing, except during liquid sampling. An electroactive biofilm was developed on the anode, by feeding the MFCs with the effluents of a separate, double chamber MFC (0.435 mV and 1000 U) that employed glucose as a substrate. The stability of voltage generation was checked using an external resistance (500 U);
Please cite this article as: Kondaveeti S et al., Co-generation of hydrogen and electricity from biodiesel process effluents, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.08.258
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3
Fig. 1 e Electricity production using (A) MFC from crude glycerol and (B) the AD effluent generated by anaerobic digestion process for hydrogen production from glycerol.
then, the electrolyte solutions contained in the MFCs were replaced with a 50 mM phosphate buffer solution (PBS), as well as with substrate, mineral and vitamin solutions [18]. We designated the MFC operating with approximately 10 g/L of CG substrate and PBS as MFC-1, while the MFC operating with the AD effluent (CG dark fermentation effluent) was termed MFC2. The three initial cycles of both MFCs, carried out without applying any external voltage, are referred to as “controls”. An initial voltage of 0.2 V was applied for 24 h to both MFCs (acclimatization period); next, the MFCs were operated to generate energy (experimental period) and perform a simultaneous organics reduction. Afterwards, other two different voltages were applied (0.5 and 0.8 V) and their correspondent electric current generation was registered during several cycles. A short-term voltage (0.2, 0.5 and 0.8 V) was applied to the MFC during the acclimatization period (during initial 24 h), by connecting positive and negative leads of DC (direct current) power supply to the anode and cathode, respectively. After
24 h, the power supply was disconnected from the MFC and a 500 U external resistance was connected instead, in order to close the MFC circuit and generate electric current. The pH of the dark fermentation process effluent was adjusted to 7 before feeding it to MFC-2. The MFCs operating without the application of any external voltage were regarded as controls. To maintain anaerobic conditions, the anodes of the MFCs were purged with nitrogen gas for 15e20 min. The values presented in the paper are based are averages calculated from the results of three experiments.
Analysis and calculation The voltage, regarded as the potential difference between the anode and the cathode, was measured across an external resistance (500 U) at 15 min intervals, using digital multimeter. The voltage data were converted to electric current (I) and power (W) data applying Ohm's law. The maximum power
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density was obtained from a polarization analysis; this was based on the variation in external resistance (5 kUe5 U) starting from open-circuit voltage (OCV) [19]. The electric power and current densities were calculated from the cathode surface area (4.9 cm2), while the internal resistance (Rin) was calculated from the slope of the IeV curve [20]. Moreover, we calculated the net energy generated at different voltages based on the substrate characteristics, following [12]. An EIS analysis was performed to measure the Ohmic (Roh) and charge-transfer resistance (Rct). Both an CV and the EIS analyses were carried out in the three electrode systems by using a BioLogic potentiostat (VSP-150, France). The EIS analysis was carried out within a frequency range of 10 kHze10 mHz, at a voltage amplitude of 10 mV: the ohmic resistance and the charge-transfer resistance are measured from the high and the low frequency regions, respectively [21,22]. On the other hand, the CV was carried out within a potential window of 0.6 to 1.0 versus Ag/AgCl, and at a scan rate of 5 mV/s. The anode of the MFC was used as a working electrode, while the Ag/AgCl electrode and the cathode served as a reference and as a counter electrode, respectively [23,24]. The production of H2 was measured in the head space during the dark fermentation, using a gas chromatograph (Younglin Instruments, Korea) equipped with TCD detector; the correspondent analysis was done following [25]. The COD was measured in calorimetry mode, according to the manufacture's protocol (HACH Co., USA). The glycerol to H2 efficiency was calculated as described previously [9,26].
Glycerol to H2 conversion efficiency ¼
examined using CLSM. The high and low bacterial growth activities were distinguished with green and red color, respectively. Likewise, the biomass measurements of anode biofilms were recorded using the Coomassie Brilliant Blue method mentioned in previous studies on MFC [28,29]. Briefly, each sample of the anode with biofilm was weighed and aggressively mixed with glass beads for 5 min in 0.2 M NaOH (2 mL). The total protein concentration was measured against bovine serum albumin standard. Five random samples of anode were evaluated for biomass, and their average was considered the final anode biomass.
Results and discussion CG is an organic pollutant. Since the disposition of CG is uneconomical, it is desirable to use it in the production of valueadded products [30]. An accessible option would be to use it as a carbon substrate, to produce bioenergy and, ultimately, electricity. In this work, we exploited CG through dark fermentation and MFC, by applying a short-term voltage. Previous studies employing AD effluents showed an incomplete conversion of the organics to H2 [9,14]. In order to convert a higher quantity of organics, we combined the use of an AD effluent and of an MFC with a short-term applied voltage. The proposed process provides a cleaner combustion, a higher calorific H2 value, and an easier operation/purification in comparison with other biological processes [14,31].
Yield of H2 combustion energy of H2 100% Combustion energy of glycerol
The VFAs present in the filtered liquid samples were analyzed using an ion chromatograph, fitted with an organic acid column (Metrosep 250/7.8, Metrohm, Swiss). The effluent and influent pH of the solutions were analyzed with a benchtop digital pH meter (Hanna Instruments, USA).
Electrogenic biofilm visualization and biomass analysis Bioelectrogenic microbial visualization and biomass analysis of anode were performed at the end of the MFC operation. Field-emission SEM (FE-SEM) was used for microbial visualization. A part of the anode (approximately 1 cm2) was cut and immersed in 2.5% glutaraldehyde solution for 30 min. The samples were cautiously rinsed thrice with PBS (pH 7.0, 50 mM) and twice with distilled water. The samples were then subjected to dehydration by using ethanol in series (25%, 50%, 75%, and 90%, for 15 min) and were dried under ambient conditions. SEM measurements were recorded on FE-SEM, Hitachi S-4800, Tokyo, Japan. The bioelectrogenic activity on the MFC anode was measured using CLSM (Olympus FV-1000, Tokyo, Japan), and the bacterial viability was calculated as described in a previous paper [27]. The anode biofilms were stained using LIVE/DEAD backlight staining kit (Molecular Probes, Invitrogen) and were
Biohydrogen generation by dark fermentation of crude glycerol The co-digestion of DW (domestic wastewater) and SW (synthetic wastewater) with CG under dark fermentation resulted in 195 and 350 mL H2/L, respectively, over a period of 57 h (the equivalent of 0.23 and 0.59 mol H2/mol glycerol; Fig. 2). H2 constituted 58.3e61.2% of the total biogas produced employing DW and SW, respectively. Based on the combustion heat of CG (1674 kJ/mol) and H2 (285 kJ/mol) [9], the energetic efficiencies reached using DW and SW were 3.91% and 10.00%, respectively. The H2 yield produced by the digestion of CG is similar to that reported previously [9,30,32e37]: 0.20e1.1 mol H2/mol glycerol (Table 1). The difference in the H2 yield noted in comparison with other studies might be due to the difference in the operational conditions, type of inoculum, and variation in pre-treatment of the inoculum [38,39]. The thorough optimization using response surface methodology, bacterial strain screening, and other strategies such as micro oxidation are in progress to improve the H2 yield. Because of H2 production, the COD reductions in DW and SW were 13.4 and 22.8%, respectively; the correspondent VFA productions were 15.1 and 22.8 mM, respectively (Table 2). The most abundant VFA was acetic acid (12.5 and 16.9 mM in DW
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Table 2 e Characterization of dark fermentation effluent with crude glycerol as a substrate. Characteristics
Maximum gas generation (ml H2/l) COD in influent (g/l) COD in effluent (g/l) COD removal (%) Acetic Acid (mM) Butyric Acid (mM) Propionic Acid (mM) Valeric Acid (mM) Caproic Acid (mM)
Fig. 2 e Cumulative dark fermentative H2 production from crude glycerol slurry (15 g/l) prepared in (i) synthetic wastewater (black filled square) and (ii) domestic wastewater (red filled circle). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
and SW, respectively). The presence of butyric acid in the effluent of SW, and the higher quantity of acetic acid, justify the enhanced H2 compared to that registered during the operation of AD using DW [40]. The level of residual COD in the effluent was high (77.2 and 86.6% for SW and DW, respectively); these values indicate that a further treatment was needed to achieve a complete degradation. A high residual COD could imply that the generation of H2 is a self-metabolic redox process, which does not need external electron acceptors like oxygen or nitrates [41,42].
Electric current generation from the effluent of fermented crude glycerol Fermentation of CG in two different types of wastewaters resulted in the generation of H2 and an effluent-rich organic matter. The generation of electric current potential of CG before and after its digestion was evaluated operating two MFC reactors (MFC-1 and MFC-2) each fed with one type of
Table 1 e Various studies on H2 production with CG as a substrate. Glycerol H2 volume Yield Operation Reference (g/L) (mL) (mol H2/mol type glycerol) 3 3 5 10 5 3
18 133 N/A 305 160 1310
0.20 0.31 0.19 0.55 0.45 0.26
10 2.5 10
N/A 11.9 352
0.60 1.10 0.59
Batch mode Batch mode Batch mode Batch mode Batch mode Continuous mode Batch mode Batch mode Batch mode
[29] [31] [32] [9] [33] [34] [35] [36] This study
Effluent Based on Synthetic wastewater (SW)
Distilled water (DW)
350
195
14.9 11.5 22.8 16.9 0.98 2.95 0.89 1.03
14.9 12.9 13.4 12.5 e 1.82 e 0.82
wastewater. An increase in electric current generation corresponded to the application of a short-term voltage for 24 h (Fig. S1). In MFC-1, a progressive increase in electric current generation was recorded from 0.52 mA in the control (no applied voltage) to 0.82 mA for an applied voltage of 0.8 V (Table 3). In contrast, the MFC-2 operation with applying a voltage of 0.5 V and 0.8 V resulted in a current generation of 0.59 mA and 0.68 mA, respectively. As seen in MFC-1, the electric current generation at 0.8 V (0.52 mA) was 1.57 times higher than that observed in the absence of voltage (Fig. S1A). These current generation values for MFC-1 are found to be lower than those compared with for MFC-2 operation with AD effluent. MFC-2 was fed with CG treated with AD: the by-products present in the residual material after the release of H2 had higher electric current generation capacity compared to those from the untreated CG (Table 3). The application of a shortterm voltage increased the electricity generation capacity in all cases by 18% (0.70 mA) at 0.2 V, 34% (0.79 mA) at 0.5 V, and 46% (0.86 mA) at 0.8 V (compared with control: 0.59 mA). The overall electric current generation was 1.46 (0.59 vs. 0.86 mA) times higher than that in the absence of voltage (control; Fig. S1B). These results suggest that the provision of acclimatization voltage can enhance the bioelectrogenesis of MFC, thereby resulting in its better performance. Lower performance is possibly caused by the direct use of CG in the MFC-1 control, which limits the biological activity at the anode. On basis of these outcomes, it is evident that the dark fermentation was beneficial in two aspects: (i) it stimulated the production of a higher quantity of H2 (350 ml H2/L), and (ii) caused an 8.1% increase in electric current generation at 0.8 V (MFC-1 vs MFC-2). This method enables a higher level of bioconversion and energy extraction from biowaste. Although both of the MFC reactors were operated at similar external resistances (~500 U), the quantity electric current generated was different, depending on the type of substrate and short-term applied voltage. Previous studies demonstrated that higher carbon chain compounds like CG cannot be easily reduced, in comparison to simple carbon compounds like acetate [43,44]. In our experiment, a high concentration of acetate (16.9 mM) in the AD effluent enhanced power generation in MFC-2. The present study is the first in which an acclimatization voltage was applied in the presence of a CG dark fermentation effluent, and in which CG was directly used in an MFC. The
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Table 3 e Performance parameters of MFC operating with crude glycerol and dark fermentation effluents. MFC-1a
Current (mA) Power density (mW/cm2) COD in effluent (g/l) COD removal % CE (%) Ohmic Resistance (Roh, U) Charge transfer resistance (Rct, U) a b
MFC-2b
Control
0.2 V
0.5 V
0.8 V
Control
0.2 V
0.5 V
0.8 V
0.52 72.0 8.41 16.7 14.2 2.98 1050
0.59 111 7.50 25.5 15.6 e e
0.68 157 6.51 35.5 16.8 1.98 456
0.82 184 5.80 42.0 15.9 e e
0.59 160 5.71 48.3 25.6 2.67 613
0.70 231 4.82 56.1 28.9 e e
0.79 290 3.81 65.3 30.1 1.72 256
0.86 311 3.21 70.0 27.4 e e
10 g/L of COD with CG. 11.5 g/l of COD with CG dark fermentation effluent as a substrate.
electric current generation at 0.8 V (0.82 and 0.86 mA in MFC-1 and MFC-2, respectively) recorded during our experiments was higher than that reported in other studies regarding organic rich wastes. It suggests that the electrogenic acclimatization methodology can enhance energy (current) generation. A single-chamber MFC, operated with cereal and swine wastewaters, exhibited maximum electric current generations of 0.30 and 0.35 mA, respectively [44e46]. Similarly, using CG as a substrate in a single-chamber MFC [10], obtained maximum electric current generations of 0.16 and 0.49 mA. Our study demonstrates that the application of an acclimatization voltage can enhance power generation. The use of a raw CG dark fermentation effluent, without applying any voltage, resulted in a maximum electric current generation of 0.21 mA in Ref. [9], whereas in our study we could reach 0.86 mA. In conclusion, the acclimatization of MFC by the application of a short-term voltage can enhance the production of electric power, using complex substrates like CG.
Polarization analysis using a dark fermentation effluent and crude glycerol We analyzed the maximum power generation using CG and a dark fermentation effluent through a polarization analysis (Fig. 3). The maximum power densities and OCV of the MFCs were different depending on the operational conditions. MFC1 exhibited a 2.5-fold increase in maximum power density from 72.0 (control) to 184 mW/m2 (0.8 V)) compared to control. Upon its operation at 0.2 and 0.5 V a change in the maximum power density (from 111 to 157 mW/m2) was observed. Similarly, MFC-2 exhibited a variation in maximum power density as the operational conditions changed. The control of MFC-2 (160 mW/m2) exhibited a PD 1.94 times lower than at 0.8 V (311 mW/m2). Application of 0.2 and 0.5 V to MFC-2, resulted in a maximum PD of 231 and 290 mW/m2, respectively. At all operational conditions, MFC-2 exhibited higher power densities than MFC-1. A similar increase in power density (from 45 to 132 mW/m2) varying the short-term applied voltage was noted in other studies on MFCs, using petroleum wastewater (PW) [12]. MFC-2, operated at 0.5 V (290 mW/m2) with CG, generated a power density 2.2 times higher in comparison with the MFC operated with PW (132 mW/m2). On the other hand, MFC-1 operated at 0.5 V (157 mW/m2) exhibited a power density only 1.2 times higher than that of the MFC operated with PW. These results suggest that the use of CG as a
substrate, and its operation using AD and MFC, can increase its calorific value. Based on the IeV curve from the polarization analysis, we calculated the internal resistances of both MFCs (Fig. 3a and b). At all intermittent applied voltages, MFC-2 exhibited similar internal resistances (~256 U), while the control system without any acclimatized voltage exhibited an internal resistance of 613 U. At different applied voltages, MFC-1 had an internal resistance of ~456 U (lower than that of the control: 1050 U). The maximum power density at 0.8 V measured in the present study is higher than that reported in a previous study dealing with the treatment of MFC with 50% CG dark fermentation effluents (90 mW/m2) [9], and in another, where CG (60 mW/ m2) was used as a substrate in a single-chamber MFC [8]. The energy (power) consumption during the short-term voltage period (acclimatization period 24 h) was compared with the power generation. A maximum 8.5-fold higher energy generation (MFC-2) was observed during the experimental period (MFC mode) compared to power consumption for acclimatization. The NE was calculated on the basis of the difference between power generated during the acclimatization period of MFC (initial 24 h) and after the 24-h operation. The increase in energy (voltage) supplementation led to an increase in the bioelectrogenic activity in MFC. Throughout the acclimatization period, both MFC-1 and MFC-2 exhibited similar power consumption of 7.1 and 7.4 mW/h, respectively, for initial 24 h. However, during the MFC voltage generation mode, the NE of MFC-2 at 0.8 V (55 mW/h) was 3.5 times higher than that of MFC-1 (16 mW/h, Table 4). The smaller NE in MFC1 is possibly due to the difficulty of using CG as a substrate, thereby limiting the bioelectrogenic activity on its anode. Based on NE generation in MFC-1 (16 mW/h) and MFC-2 (55 mW/h), 1 kg/L CG (biodiesel industry waste) can yield NE up to 1.06 and 3.66 W/h, respectively. The power generation is high compared to that of single-chamber air cathode MFCs operating with PW [12].
Organic removal in the microbial fuel cell The bioelectrogenic oxidation of CG during power generation was analyzed in terms of COD removal. A similar trend was noted from both MFCs in terms of COD removal and power generation. The increase in COD removal due to the accelerated oxidation of the substrate led to higher power generation. COD removal in both MFCs was enhanced compared to the
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Fig. 3 e Changes in voltage and power density during the polarization analyses of MFC-1 (crude glycerol; A-B) and MFC-2 (dark fermentation effluent; C-D). Control (black filled square), 0.2 V (red filled circle), 0.5 V (blue filled triangle), and 0.8 V (magenta filled inverted triangle). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) control (Fig. 4; Table 3). COD removal in the control of MFC-1 reached only 16%. An increment in COD removal was noted at the application of short-term voltage: COD removal was equal to 25% at 0.2 V, 35% at 0.5 V, and 42% at 0.8 V. A 1.7-fold
increase in COD removal was observed from the control to the MFC operated at 0.8 V. MFC-2, without any applied voltage, exhibited a COD removal of 48%. Once short-term voltages were applied, the COD removal reached 56% at 0.2 V, 65% at
Table 4 e Comparative evaluation between energy consumed and generated during short term voltage application with crude glycerol and dark fermentation effluent. Applied Voltage (V)
Energy Consumed
Net Energy produced (mW/h)
Generated
Current (mA) Power (mW) Total (mW/h) Current (mA) Power (mW) Total (mW/h) Crude glycerol in MFC-1 Control N/A 0.2 0.11 0.5 0.32 0.8 0.37 Anaerobic digestion effluent in MFC-2 Control N/A 0.2 0.13 0.5 0.34 0.8 0.39
N/A 0.02 0.16 0.29
N/A 0.48 3.84 7.10
0.36 0.46 0.56 0.59
0.06 0.10 0.16 0.17
10.7 18.8 26.9 23.0
10.7 18.3 23.1 16.0
N/A 0.02 0.17 0.31
N/A 0.62 4.08 7.44
0.457 0.58 0.71 0.77
0.10 0.16 0.25 0.30
18.8 19.5 41.7 62.7
18.1 18.8 37.6 55.2
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2, the CEs were 25.6% (for the control), 28.9% (at 0.2 V), 30.1% (at 0.5 V), and 27.4% (at 0.8 V). The CE obtained in this study was higher compared to that reported in other MFC studies using CG dark fermentation effluent as a substrate (13.8%) [9] or starch processing wastewater (8%) [44].
Electrochemical analysis of the microbial fuel cell
Fig. 4 e COD concentration in the influent, effluent, and COD removal, in MFC-1 and MFC-2 (COD removal ¼ red filled circle). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
0.5 V, and 70% at 0.8 V. These values are higher than those reported in other studies on double chamber MFCs using CG dark fermentation effluents as substrate [9]. Overall, this suggests that a short-term applied voltage enhances the performance of MFC. The columbic efficiency (CE) was calculated based on the COD removal and on the electric current generation in both MFCs, for various operational conditions. MFC-1 exhibited CEs of 14.2% (for the control), 15.6% (at 0.2 V), 16.8% (at 0.5 V), and 15.9% (at 0.8 V). By using a dark fermentation effluent in MFC-
Electrochemical techniques (e.g., EIS and CV) can provide useful information on the electrochemical response of anode biofilms. Our analyses were performed under turnover conditions, in the presence of a substrate for both the control and the MFCs operating at 0.5 V (Fig. 5). The MFCs, both operating at 0.5 V, exhibited a notable difference in performance; therefore, 0.5 V was chosen a suitable voltage for comparing them with the control. Both MFCs showed higher oxidation and a reduction in electric current generation, compared with the control, after the application of a short-term voltage. MFC-1 generated a lower level of redox electric current compared to MFC-2. In MFC-1, the oxidative electric current increased with the application of 0.5 V (from 0.18 mA in the control, to 0.51 mA at 0.5 V), while the reductive electric current generation changed from 2.5 mA (in the control) to 4.1 mA (at 0.5 V). These quantities were higher in MFC-2: the oxidative electric current increased from 0.72 mA to 7.11 mA, while the reductive electric currents changed from 6.9 mA and 14.5 mA. The increase in electric current generation following the application of a short-term voltage in both MFCs, suggests an enhanced biological activity/growth on the anode electrode.
Fig. 5 e CV (AeB) and EIS (CeD) values measured for MFC-1 (A, C) and MFC-2 (B, D) under stable voltage generation. Control (black filled square), 0.5 V (red filled circle). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) Please cite this article as: Kondaveeti S et al., Co-generation of hydrogen and electricity from biodiesel process effluents, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.08.258
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The application of short-term voltage of 0.5 V in both MFCs was shown to enhance their performance compared to the control, as observed via EIS. The anode of both MFCs exhibited a difference in ohmic resistance (Roh) and charge transfer resistance (Rct) in comparison with control (no applied voltage). The control of MFC-1 exhibited a Roh and a Rct of 2.98 U and 1050 U, respectively. After the application of 0.5 V, the Roh and Rct decreased to 1.98 U and 456 U, respectively. The Rct of MFC-1 was higher compared to that of MFC-2. The use of an AD effluent in MFC-2 changed the Rct to 613 U in the case of no voltage, and to 256 U after the application 0.5 V. In MFC-2, H2 and electricity were co-generated; still, there was a residual COD of 28%, which could be exploited for the production of polyhydroxyalkanoates [1,45e47] and other products such as lipids, and alcohols [48e50]. Overall, in the present study we were able to extract value-added products from biodiesel industry waste, through a sustainable, renewable, and cleaner combustion process. Hence, the electrochemical method described in this paper can contribute to pollution reduction [51,52]. The PD generation and COD removal values, measured following the application of an intermittent voltage, were higher compared to those reported in previous studies [9].
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Influence of supplemental voltage on bioelectrogenic growth The effect of the type of substrate (AD effluent vs. CG) and applied voltage (during acclimatization period) on the bioelectrogenic anode of the MFC was evaluated (Fig. 6). The microbial growth of MFCs (MFC-1 and MFC-2) was analyzed for control MFC and MFC supplied with an acclimatization voltage of 0.5 V. The latter exhibited a noteworthy performance; therefore, 0.5 V was chosen for comparison with control. The mode of attachment of the bioelectrogenic bacteria on the anode was analyzed using FE-SEM. The SEM images revealed that the electrogenic microbes were attached on the carbon fibers of anode by self-secretion of an extracellular polymeric substance (EPS) [53,54]. The bioelectrogens were more uniformly distributed when the MFC was supplemented with an external voltage, indicating that MFC operation supplied with acclimatization voltage can enhance biofilm growth and lead to an increase in power generation and COD reduction. The electrogenic bacterial viability of the anode biofilm was analyzed using CLSM (Fig. S2). In the absence of acclimatization voltage, a low bacterial viability of 60 ± 3% was
Fig. 6 e Microbial biofilm visualization on anode of MFC under various operational conditions: Pristine graphite felt (A), Control MFC-1 (B), Control MFC-2 (C), MFC-1 at 0.5 V (D), and MFC-2 at 0.5 V (E). The inset shows the attachment of bioelectrogens on the anode. Please cite this article as: Kondaveeti S et al., Co-generation of hydrogen and electricity from biodiesel process effluents, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.08.258
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noted in MFC-1 using CG as a substrate. However, this increased to 69 ± 5% when an acclimatization voltage of 0.5 V was supplied. MFC-2 (substrate: AD effluent) operation without and with acclimatization voltage of 0.5 V exhibited bacterial viabilities of 66 ± 4% and 77 ± 3%, respectively. These results suggest that organic-rich recalcitrant pollutants (e.g., CG) when used directly inhibit the bacterial activity and thereby lead to a decrease in power generation. In addition, the integration of an AD effluent with MFC can yield better performance in terms of power generation. In general, the bacterial viability of the anode biofilm of MFC varies from 20% to 95% depending on the substrate, exoelectrogens, and other operational parameters [51]. The biomass on the anode increased slightly from 17.5 ± 2 mg/g in control MFC-1 to 20.5 ± 2 mg/g in MFC-1 supplied with an acclimatization voltage of 0.5 V. In contrast, biomass on the anode in control MFC-2 and MFC-2 supplied with an acclimatization voltage of 0.5 V were 19.6 ± 3 and 23.1 ± 2 mg/g, respectively. As seen, a higher biomass was obtained when an AD effluent was used as the substrate. These results indicate that a higher calorific value can be obtained using an integrated approach (MFC-2) compared with the direct utilization of CG (MFC-1). The recent developments in the dark fermentation process has been proven to have an enormous potential for scalability of biohydrogen production from organic wastes. However, biohydrogen production is mostly studied at a laboratory scale, and there are limited studies on pilot scale operation. One of the major limitations in scaling-up of H2 production by dark fermentation is the low organic matter removal, thereby leading to low H2 yields [55,56]. Therefore, the integration of dark fermentation with other biological renewable systems such as MFC can enhance the overall yield and energy efficiency, with simultaneous reduction in the organics. In particular, the co-generative dark fermentation with MFC to generate bioelectricity can circumvent product purification, thereby leading to a decrease in the downstream cost.
Conclusions The application of a short-term voltage in a single-chamber air cathode MFC, improved the production of energy from CG and AD. The maximum H2 production rate (350 mL H2/L, yielding 0.59 mol H2/mol glycerol) was registered during a dark fermentation in synthetic wastewater. The feeding of AD into MFC-2 resulted in a higher maximum power density (311 mW/ m2; 0.8 V) than in the control, and in better performances under all operational conditions. Our study can be applied to the reduction of biodiesel pollution and opens new possibilities in the treatment of complex organics, through the application of short-term voltage. Corresponding supplementary material can be found in the online version of this paper.
Acknowledgements This paper was supported by Konkuk University in 2016.
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.08.258.
references
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