Selection Of Anodophilic Microbes For Bioelectrochemical Systems

Bioelectrochemical systems (BESs) is an emerging technology in whichmicroorganisms function as catalysts to convert chemical energy into electricalenergy. More recently, BES has been tested for many potential applicationsbesides electricity generation, including biohydrogen prodution, metal reductionand recovery, desalination, organic products synthesis and treatment of variouswastewaters. However, no matter what kind of BES it is, the half reactions in theanode always rely on the electrongenic microorganisms. Thus, the optimization ofanodophilic microbes is one of key approaches to enhance BES performance. Thisdissertation is focused on optimization of anodophilic microbes by using selectiveenrichment of metal reducing microorganisms, enhancing syntrophic communitiesunder optimal redox potential, and isolating pure exoelectrogenic culture asinoculums to improve the BESs performances.A stable and efficient inoculum (Anodophilic consortium, AC) for BESs wasenriched using series dilution and incubation on the iron(III) reduction medium.Biofilm sample from anode of the BES, which was inoculated with activated sludge,was serially diluted and then incubated in an Fe(III)-acetate medium. Fe(III)reducing AC was obtained for the Fe(III)-reduction positive dilution at10-6. Theperformance of BES inoculated with the enriched AC was compared with thoseinoculated with the original biofilm and activated sludge from a wastewatertreatment plant. The power densities and Coulombic efficiencies of the BES withAC (226mW/m2and34%, respectively) were higher than those with the originalbiofilm and the activated sludge. The start-up period with AC was60h and shorterthan those with the other inoculums. This indicated that this approach caneffectively obtain high activity inoculum for improving BES performance and makestart-up period short.A syntropic consortium was developed to improve degradation of formic acid,a poorly substrate for electricity generation, in BESs by operating BESs at specificanode potentials. Three different anode potentials (–0.30,–0.15, and+0.15V; vs. astandard hydrogen electrode, SHE) were tested in this study relative to anon-controlled, high resistance system (1000external resistance). No current was generated at–0.30V, suggesting the lack of direct formic acid oxidation (standardreduction potential:–0.40V). More positive potentials that allowed for acetic acidutilization all produced current, with the best performance at–0.15V. The anodecommunity in the–0.15V reactor, based on16S rRNA clone libraries, wascomposed of58%Geobacter sulfurreducens and17%Acetobacterium, with lowerproportions of these genera found in the other two MFCs. Acetic acid was detectedin all MFCs, suggesting that current generation by G. sulfurreducens was dependenton acetic acid production by Acetobacterium. When all MFCs were subsequentlyoperated at an external resistance for maximum power production, they producedsimilar power densities. The microbial communities on the anode in all BESsconsisted of similar proportions of the two predominant genera (ca.52%G.sulfurreducens and22%Acetobacterium). These results show that syntrophicinteractions can be enhanced through long-term incubation at certain anodepotentials, which could significantly improved the performance of BESs withformic acid as substrate.An exoeletrogenic strain, which was designated strain SD-1, was isolated froman anode biofilm by using high throughput mine-MEC based ondilution-to-extinction methods. Strain SD-1was a facultative anaerobic,nonfermentative, nonmotile, gram-negative, rod-shaped bacterium. The16S rRNAsequence indicated that this organism belongs in the genus of Geobacter, and itsclosest known relative is G. sulfurreducens. However, the strain SD-1cannotreduce fumarate and sulfur with acetate as electron donor. This indicated that SD-1should not be identified as the species of G. sulfurreducens. The bioelectrochemicalactivity of strain SD-1in BESs was compared with two type strains of Geobacter i.e.G. sulfurreducens PCA and G. metallireducens GS-15, and mixed culture. StrianSD-1showed the highest electrochemical activity in current gerneration, cyclicvoltammetry and electrochemical impedance spectroscopy tests. Moreover, thebiomass of biofilm SD-1was also the higher than those of others three cultures.Besides the optimization of simple substrates fed BESs, the performance ofBESs fed with complex substracts fed BESs was also investigated. A coupliedhydrogen production system consisting of a fermentation reactor and BESs wasconstructed. In this system, the fermentaion reactor produced hydrogen and VFAsfrom cellulose, and BESs further recoverd energy from organics and VFAs in the fermentation effluent. Without the supplementation of external electrical source, thecouplied system produced more hydrogen by41%compared with fermentationalone.