Article pubs.acs.org/est
Conversion of Residual Organics in Corn Stover-Derived Biorefinery Stream to Bioenergy via a Microbial Fuel Cell Abhijeet P. Borole,†,‡,* Choo Y. Hamilton,‡ and Daniel J. Schell§ †
Biosciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States University of Tennessee, Knoxville, TN § National Renewable Energy Laboratory, Golden, CO ‡
S Supporting Information *
ABSTRACT: A biorefinery process typically uses about 4−10 times more water than the amount of biofuel generated. The wastewater produced in a biorefinery process contains residual sugars, 5-furfural, phenolics, and other pretreatment and fermentation byproducts. Treatment of the wastewater can reduce the need for fresh water and potentially add to the environmental benefits of the process. Use of microbial fuel cells (MFCs) for conversion of the complete range of phenolic compounds and furan aldehyde derivatives present in a postfermentation biorefinery stream is reported here. The consortium was capable of removing the molecules simultaneously with sugars, which were present at 2 orders of magnitude higher concentrations. Organic loading in a fed-batch MFC affected Coulombic efficiency, which decreased from 40% at 0.66 g/L loading to 1.8% at 66.4 g/L loading. Power density increased with loading reaching 1180 mW/m2 at 5.3 g/L (8% dilution), but decreased thereafter. Excessive loading leads to poor electrogenic performance; therefore, operation of an MFC at an intermediate loading using dilution and recirculation of the process stream can enable effective treatment with bioenergy recovery.
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INTRODUCTION Use of microbial fuel cells (MFCs) and microbial electrolysis cells (MECs) has been suggested for conversion of residual organics present in biorefinery process water after production and recovery of biofuel.1,2 The process water in a cellulosic biorefinery includes sugar- and lignin-degradation products such as furfural, phenolics as well as acetate, fermentation byproducts including other organic acids and unconverted carbohydrates.3,4 Conversion of contaminants including furfural, 5-hydroxymethylfurfural (HMF), acetate and some model phenolics to electricity using MFCs has been demonstrated.2 This can potentially lead to a process for recovery of energy from residual organics in the biorefinery using microbial fuel/electrolysis cells, while enabling water recycle.1 Treatment of fermentation inhibitors in wastewater is typically not addressed by existing technologies.5 The volume of the wastewater generated in biorefineries is significant. The stillage stream in biorefineries can range from 200 to 3500 million liters per year per plant for commercial-scale plants.6 Besides biofuel production, generation of bioproducts in biorefineries in future will increase the volume of wastewater generated. Anaerobic digestion is a mature technology which is presently being considered for wastewater treatment in a cellulosic biorefinery.4 While this is a viable option for water treatment, the emerging field of bioelectrochemical systems may be considered a potential alternative. The ability to use the residual organics to generate hydrogen via MECs offers © 2012 American Chemical Society
competitive advantages compared to anaerobic digestion under certain circumstances.7,8 The energy conversion efficiency of a biochemical conversion route for biofuel and energy production from lignocellulosic biomass can be potentially increased, if efficient bioelectrochemical systems can be designed and implemented. In addition, more than 80% of the water coming into a biorefinery process is lost to cooling towers, etc. in existing designs which use evaporators for stillage concentration and combustor-boiler for heat generation from residual organics.4 Ability of new process schemes, such as those using bioelectrochemical systems that operate at ambient temperature, can potentially reduce water losses and increase energy recovery. Recent work has revealed the importance of water recycle in biochemical conversion processes for biofuel production,9 demonstrating the need for process alternatives with ability to recycle water. The electrons extracted from the residual organics present in biorefinery wastewater using a bioanode can be used to either generate electricity, hydrogen or other value-added products in a biorefinery. The case of hydrogen production is particularly important due to the critical need for renewable hydrogen in a biorefinery. Among the various waste conversion options, MEC technology is one of the most attractive alternatives for application in a biorefinery considering the utility of hydrogen Received: February 29, 2012 Accepted: November 29, 2012 Published: November 29, 2012 642
dx.doi.org/10.1021/es3023495 | Environ. Sci. Technol. 2013, 47, 642−648
Environmental Science & Technology
Article
respectively. The inoculum from the MFC enriched on furan aldehyde and phenolic compounds was characterized previously and shown to contain novel organisms. The dominant members of the consortium were Desulfovibrio sp. (50% of the population) along with 28% β-proteobacteria and 14% αproteobacteria.2 The external resistance was reduced from 250 ohm (day 1) to 100 ohm on day 3 and then to 50 ohm on day 5 to maximize growth of exoelectrogenic microbes on the anode electrode. Reduction in external resistance has been reported to enhance current production in MFCs.14,16 A mixture of acetate and glucose (0.1 g/L each) was used as the carbon source for microbial growth for the first 3 days. Further enrichment was carried out with a mixture of 5 substrates: furfural, HMF, HB, HAP and VA over the next 5 days. The bioanode was operated over the next twelve days with individual substrates to verify electricity production from each individual substrate. The substrate concentration was 0.04 g/L in the individual substrate runs and 0.2 g/L total carbon source in the mixed substrate experiment. The substrates were added in a fed-batch mode during this period. Preparation of Representative Postfermentation Stream. Pretreated corn stover was produced in a 5 dry kg/ h continuous-horizontal-screw reactor located at National Renewable Energy Laboratory (NREL) in Golden, CO. Reactor operating conditions were 158 °C, 24 mg sulfuric acid/g dry biomass, 5 min residence time and approximately 30% total solids. A sample of the pretreated corn stover slurry was diluted to 20% total solids, pH adjusted to 5.0 with 29% (w/w) ammonium hydroxide and temperature adjusted to 50 °C. Then enzymatic cellulose hydrolysis was initiated by addition of cellulase at an approximate loading of 20 mg protein/g dry pretreated biomass. After four days, the pH of the slurry was raised to 5.8 with ammonium hydroxide and the temperature was lowered 33 °C. Nutrients (5 g/L yeast extract, 1 g/L KH2PO4, 0.5 g/L MgSO4•7H2O) were added as a concentrated 10× solution. Fermentation was then initiated by addition of a 10% (v/v) culture of the glucose-xylose fermenting bacterium, Zymomonas mobiliz 8b, that achieved an initial cell density of approximately 0.5 g dry cell mass/L.17 The fermentation was terminated after three days and the broth was autoclaved to kill live bacteria prior to shipment to Oak Ridge National Laboratory (ORNL). Sample Characterization. The sample generated from fermentation of pretreated corn stover (referred to as “stover process water (SPW) sample” henceforth) was characterized via high performance liquid chromatography (HPLC) equipped with a refractive index and a UV-vis detector (Hitachi LaChrome Elite System). Two different columns were used to separate the individual sugars, organic acids and phenolic molecules present in the water sample. An HPX-87H column was used for analysis of sugars and phenolics compounds using a mobile phase of Milli-Q water and 15% acetonitrile in 5 mM sulfuric acid, respectively. A HPX-87P column was used for organic acid analysis using a mobile phase of 5 mM sulfuric acid. Testing of MFC at Different Substrate Concentrations. On day 21, fed-batch addition of the SPW sample was initiated. The sample was diluted to 1%, 2%, 4%, 8%, 16%, 32%, and 64% using growth medium14 as a diluent and then used in the MFC. The conductivity of the growth medium was 7.2 mS/cm and that of SPW sample was 13.8 mS/cm. The pH of the resulting mixtures was adjusted to 7.2 at the start of each run. A run with 100% strength SPW sample was also conducted. The anode
in a biorefinery and potentially high efficiency of a bioelectrochemical conversion process.10 Estimates of the energy recovery from the pretreatment byproducts, residual sugars, etc. present in the stillage, that is, the stream after lignin separation via BESs has been reported and compared with other methods of energy recovery.7 The conversion of the contaminants present in the process water has not been evaluated previously in a bioanode, except for a few select model compounds including furfural, hydroxymethylfurfural, 4hydroxybenzaldehyde, vanillic acid, and 4-hydroxyacetophenone.2 Additionally, conversion of byproducts generated during fermentation of sugars to ethanol such as organic acids, aldehydes, etc. in a bioanode in the presence of molecules inhibitory to bioethanol production has also not been shown.11 One study which investigated the effect of such inhibitory molecules on glucose conversion in an MFC found that furfural, benzyl alcohol and acetophenone inhibited electricity production in an MFC at concentrations as low as 0.2 mM.12 Here, we report on the removal of pretreatment byproducts, fermentation byproducts, and residual sugars present in a single stream. A process sample obtained postfermentation from dilute-sulfuric-acid pretreated corn stover is used as a representative stream. Bioelectricity production in a MFC from a mixture of over 20 contaminant molecules present in this sample and the electrochemical performance under various organic loading conditions is reported. We also investigate the question: To what extent does an MFC anode remove organics in the biorefinery stream and at what Coulombic efficiencies? Since previous reports indicated that the conversion of sugars could not be achieved in the presence of certain inhibitory molecules, investigations targeting study of biorefinery wastewater treatment in the presence of such inhibitor molecules is necessary. An inoculum enriched on fermentation inhibitors as substrates is used in this study. Another question we address is the effect of organic loading on MFC performance. Batch operation with fed-batch substrate addition is a common practice in MFC studies. However, the effects of organic loading and fed batch addition over long-term operation of MFC are not well comprehended. We investigate these effects in this study. Although the study focused on fed-batch operation, the MFC used in this study can also be adapted to flow-through operation, as needed.13
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MATERIALS AND METHODS MFC Set Up, Operation and Anode Consortium Enrichment. The MFC used in this study was an air-cathode, one-chamber design as reported previously.14 Briefly, the MFC consisted a carbon-felt anode with minimal dead space in the anode chamber and a Pt-deposited carbon as the cathode, separated by a Nafion-115 membrane. A growth medium consisting of 50 mM phosphate buffer, mineral salts and vitamin solution was recirculated through the anode at a flow rate of 7 mL/min using a 200 mL external reservoir, sparged continuously with nitrogen.14 The anode chamber volume was 16 mL, with a projected surface area of 12.6 cm2. The microbial enrichment was carried out over a period of 8 days. The inoculum was a mixture of electrogenic cultures from two existing MFCs (a volume of 10 mL drawn from each MFC), one fed with acetate (0.1 g/L) and the other fed with a mixed substrate containing furfural, HMF, 4-hydroxybenzaldehyde (HB), 4-hydroxyacetophenone (HAP) and vanillic acid (VA) at 0.04 g/L.2,15 The latter three substrates were chosen as model compounds for phenolics aldehyde, ketone and acid, 643
dx.doi.org/10.1021/es3023495 | Environ. Sci. Technol. 2013, 47, 642−648
Environmental Science & Technology
Article
Table 1. Chemical Characterization of Corn Stover Hydrolysate before and after Fermentationa concentration, g/L
cellobiose
glucose
xylose
galactose
arabinose
mannose
EtOH
2-furfural
HMF
acetic acid
otherb
before fermentation after fermentation (SPW)
3.5 0.94
76.5 0.36
46.5 20.28
3.4 3.06
6.5 6.45
NA 0.35
4.5 25.98
2.15 0.04
0.25 0.009
7.5 8.7
NA 0.21
a
The fermented corn stover hydrolyzate was used as the substrate in this study (described as stover process water - SPW). bSee Supporting Information for determination of the concentration of “other” components in the SPW sample.
Figure 1. Changes in MFC power density with SPW loading. Increasing trend was observed in power density with loading up to 8%, while thereafter, the power density decreased.
fluid mixtures were deaerated by sparging with nitrogen and the pH adjusted to 7.2 prior to beginning flow through the anode. Experiments were conducted over a period of 24−72 h and 2− 4 samples were collected over the duration of each experiment. Electrochemical Analyses. The power density analysis was conducted by linear sweep voltammetry using a conventional three electrode potentiostat (Reference 3000, Gamry, Warminster, PA). Working electrode was anode and reference and counter electrode was cathode. Current density was also assessed by linear sweep voltammetry. Working electrode was anode, counter electrode was cathode and the reference electrode was Ag/AgCl electrode (BASi, Inc., West Lafeyette, IN). Cyclic voltammetry was also conducted over the range of −0.5 to +0.1 V at a scan rate of 1 mV per second.
sample via high pressure liquid chromatography (HPLC) with UV detection showed over 20 distinct peaks indicating presence of a large number of aromatic compounds besides furfural and HMF. Biocatalyst Growth. Growth of the anode microbial consortium was followed with increase in current production over time. The maximum current typically observed in the MFC system using the substrates glucose and acetate was reached within a 3-day period (See Figure S1 in the Supporting Information). Enrichment of the anode biocatalyst over an eight-day period, beginning day 2, using a mixture of sugar and lignin-degradation products showed steadily increasing current. Fed-batch addition of the mixed substrate was carried out to enrich microbes capable of conversion of these substrates, yielding a maximum current of 1.8 ± 0.2 mA upon addition of 0.2 g/L of the mixed substrate. Current Production with Model Substrates in the MFC. Prior to the use of the SPW sample in the MFC, conversion of model sugar- and lignin-degradation compounds was assessed via fed-batch addition of model substrates. This enabled continued growth and enrichment of the microbial consortium using these molecules as substrates. Current production was highest with furfural as the substrate reaching up to 0.8 mA at a loading of 0.04 g/L-day. The Coulombic efficiency (CE) was 52% and the current density was 0.64 A/ m2. Experiments with higher concentration of furfural (0.2 g/Lday) lead to a proportionally higher current of 3.6 ± 0.4 mA (2.87 A/m2). Use of other substrates: HMF, HB, HAP, and VA, resulted in current production, although the CE was much lower (
