Long-Term Performance of Liter-Scale Microbial Fuel Cells Treating ...

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Environmental Science & TechnologyFigure 5. Concentrations of nitrogen compounds in the MFCsdesigned for nitrogen removal: (A) TKN and (B) ammonium, nitrate,and nitrite. (Inset) Schematic of the MFC system consisting of adenitrifying MFC and the MFC-AC. PE, primary effluent; D-MFC-a,anode of the denitrifying MFC; MFC-a, anode of the MFC-AC; MFCc,cathode of the MFC-AC; and D-MFC-c, cathode of the denitrifyingMFC.MFC-Pt by analyzing the contributions from different sources,including electricity, methane, oxygen, sulfate, and otherunknown factors (Figure 6 and Table S2 of the SupportingFigure 6. Carbon balance based on TCOD obtained from the MFC-Pt.Information). Because carbon is an electron donor, this balancecould also represent an electron balance. Derived from CE, thecarbon distribution to electricity production was 13.2% (on thebasis of TCOD) or 22.8% (on the basis of SCOD).Surprisingly, sulfate consumed much more carbon thanelectricity production (37.2% TCOD or 64.0% SCOD). Theprimary effluent contained a sulfate concentration of 119.6 ±69.7 mg of SO 4 2‐ /L, and the anode removed 81.2 ± 17.2%sulfate, indicating an active sulfate reduction in the MFC anode.The primary effluent contained dissolved oxygen of 3.3 ± 1.3mg/L, which could consume 2.2% TCOD or 3.8% SCOD.4946ArticleOxygen flux through the CEM was estimated to contribute to11.7% TCOD or 20.2% SCOD. Methane production wasobserved in MFCs, 28 and thus, we examined both methane gasand the dissolved methane in the anode effluent. The averageconcentration of the dissolved methane was about 1 mg/L, andmethane gas production was ∼0.5 mL/g of SCOD, resulting incarbon consumption of 1.3% TCOD (or 2.2% SCOD) and0.04% TCOD (or 0.1% SCOD), respectively.There was 34.4% TCOD unaccounted for by thesecalculations, while there was 13.1% SCOD overly accounted(see Table S2 of the Supporting Information). The possiblemeasurement/analytic errors (e.g., collection of methane gas)might also lead to unknown carbon flow in the TCODcalculation. For the balance of SCOD, the total input coulombscould have been underestimated, because additional SCODcould be produced from the degradation of TCOD. Becausesulfate reduction was found to be a major contributor to CODremoval, it would be interesting to know whether inhibitingsulfate reduction could improve electricity production. We haveconducted the tests on inhibiting sulfate reduction, which canbe found in the Supporting Information. Because of thesignificant variation of wastewater quality and testing conditionsin the field that could strongly disturb the experimental results,we cannot draw any definitive conclusions on the effect ofsulfate reduction on electricity generation, and more laboratorytests under controlled conditions will be needed to further ourunderstanding of this issue.Perspectives. A proper understanding of the applicationniche and the benefits of the MFC technology is very importantto its development. It is critical to acknowledge that the primaryfunction of MFCs is wastewater treatment instead of energyproduction (which is an added benefit). MFCs are advantageousin several aspects. (1) Low energy consumption: A MFCsystem avoids the use of aeration, which consumes about 50%of the electric energy of an aerobic wastewater treatmentprocess. (2) Low sludge production: the anaerobic process inthe anodes of a MFC system accumulates little secondarysludge. This result has two potential benefits: minimizing theuse of a secondary clarifier to save both operation andinfrastructure expense and reducing the treatment of secondarysludge, which requires significant effort and energy. (3) Energyrecovery from wastewater: the electric energy produced by aMFC system could be used to offset the energy requirement bythe pumping system, thereby further reducing the energyconsumption of the wastewater treatment process.We have not observed satisfactory energy production fromhigh-strength/high-solid wastes in MFCs; 29 therefore, webelieve that MFCs cannot compete with anaerobic digestionfor the treatment of high-strength/high-solid wastes, 30 andMFCs are more suitable for medium- and low-strengthwastewater that is currently treated by aerobic processes.However, with the increased attention to anaerobic treatment[e.g., anaerobic membrane bioreactor (AnMBR)] of domesticwastewater, 31,32 there will be competition between conventionalanaerobic treatment and MFC technologies (if provenapplicable in the future). In comparison to conventionalanaerobic treatment of low-strength wastewater, the potentialadvantages of MFC technology may exist in several aspects: (1)better ability to handle temperature variation, especially at lowtemperatures, (2) no need of gas collection, treatment, andconverting system (for energy production) because of directelectricity production, (3) low dissolved methane in the treatedeffluent to avoid the release of greenhouse gas, and (4) nitrogendx.doi.org/10.1021/es400631r | Environ. Sci. Technol. 2013, 47, 4941−4948
Environmental Science & TechnologyArticleremoval with linking to specially designed MFC systems.Integrating membrane technology into MFCs can also producehigh-quality effluent, 33 such as that in AnMBRs. Thoseadvantages need to be further verified and demonstrated withmore work of larger scale MFCs treating actual wastewater.Although energy production may not be the most importantfeature of MFC technology, it is still beneficial to furtherimprove electricity production by optimizing MFC configuration,materials, and operation. For instance, we recently usedspiral spacers to double energy production in tubular MFCs. 34Nitrogen removal is certainly possible in the present MFCsystem, and carbon distribution should be carefully coordinatedfor■the purpose of energy production.ASSOCIATED CONTENT*S Supporting InformationCalculation of CE, energy consumption, and oxygen flux,details of the sulfate-reduction inhibition test, summary of thetreatment performance in Table S1, data of carbon balance inTable S2, and MFC schematic and data of the temperature,nutrients, coliform bacteria, and sulfate in Figures S1−S10. Thismaterial is available free of charge via the Internet at http://pubs.acs.org.■ AUTHOR INFORMATIONCorresponding Author*Telephone: (414) 229-5846. Fax: (414) 229-6958. E-mail:zhenhe@uwm.edu.NotesThe authors declare no competing financial interest.■ ACKNOWLEDGMENTSThis study was financially supported by a grant from VeoliaWater North America. The authors thank MilwaukeeMetropolitan Sewerage District (MMSD) for providing atesting site at the South Shore Water Reclamation Facility. Theauthors also thank Yann Moreau, Caroline Dale, KhristopherRadke, Scott Royer, Mark Swayne, and Ronan Treguer fromVeolia Water, Christopher Magruder from MMSD, and KyleJacobson from University of WisconsinMilwaukee for theirhelp with the project, Michelle Schoenecker from University ofWisconsinMilwaukee for proofreading the manuscript, andanonymous reviewers for their helpful comments.■ REFERENCES(1) Logan, B. Essential data and techniques for conducting microbialfuel cell and other types of bioelectrochemical system experiments.ChemSusChem 2012, 5 (6), 988−994.(2) Logan, B.; Rabaey, K. Conversion of wastes into bioelectricityand chemicals using microbial electrochemical technologies. Science2012, 337, 686−690.(3) Moon, H.; Chang, I. S.; Kim, B. H. Continuous electricitygeneration from artificial wastewater using a mediator-less microbialfuel cell. Bioresour. Technol. 2006, 97, 621−627.(4) Borole, A. P.; Aaron, D.; Hamilton, C. Y.; Tsouris, C.Understanding long-term changes in microbial fuel cell performanceusing electrochemical impedance spectroscopy. Environ. Sci. Technol.2010, 44 (7), 2740−2745.(5) Zhang, F.; Pant, D.; Logan, B. E. Long-term performance ofactivated carbon air cathodes with different diffusion layer porosities inmicrobial fuel cells. Biosens. Bioelectron. 2011, 30 (1), 49−55.(6) Zhang, G.; Wang, K.; Zhao, Q.; Jiao, Y.; Lee, D.-J. Effect ofcathode types on long-term performance and anode bacterial4947communities in microbial fuel cells. Bioresour. Technol. 2012, 118,249−256.(7) Li, X.; Zhu, N.; Wang, Y.; Li, P.; Wu, P.; Wu, J. Animal carcasswastewater treatment and bioelectricity generation in up-flow tubularmicrobial fuel cells: Effects of HRT and non-precious metallic catalyst.Bioresour. 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