Enhancing energy recovery from industrial wastewater using microbial fuel cells

Microbial fuel cells (MFCs) hold great promise for the simultaneous treatment of wastewater and electricity production. However, the electricity recovery needs improvement if MFCs are to compete with already established technologies e.g. anaerobic digestion. The aim of this study was to investigate ways of enhancing electricity recovery from (synthetic) industrial wastewater. Initial studies investigated the use of defined cocultures as a way of improving turnover of substrate and hence electricity produced by exploiting mutualistic relationships such as syntrophy or ability of facultative microoganisms (Saccharomyces cerevisiae) to consume residual oxygen from the anode. A coculture of Shewanella oneidensis and Clostridium beijerinckii, investigated here for the first time, gave a power production of 87 mW m-2 compared to 48 mW m-2 for S. oneidensis alone or 60 mW m-2 for C. beijerinckii alone. Substrate degradation was also improved significantly from 20% (S. oneidensis alone) to 67% using the coculture. Similar improvements were observed for novel cocultures of G. sulfurreducens, S. cerevisiae and C. beijerinckii as well as cocultures of C. beijerinckii, S. oneidensis and S. cerevisiae. To improve electricity recovery from MFCs, mechanisms of electron transfer need to be understood. The contribution of direct electron transfer mechanisms to overall electron transfer was investigated for the first time by restricting S. oneidensis cells close to or away from an anode electrode. A maximum power output of 114 mW m−2 was obtained when cells were retained close to the anode. This was 3.5 times more than when the cells were separated away from the anode. This result was corroborated by another study where S. oneidensis cells were entrapped in alginate gels. To further investigate the contribution of the c-type cytochromes forming the Mtr pathway to extracellular electron transfer, Rapid DNA Prototyping Assembly was used for the first time to assemble Mtr-pathway coding genes individually or as operons. The different constructs were overexpressed in S. oneidensis and heterologously expressed in E. coli and power production compared with the wild type strains. The best power generated was from the mtrAB S. oneidensis strain (144 mW m-2) and from the mtrCAB E. coli strain (24 mW m-2). Since electricity production is linked to exoelectrons forming a biofilm on the anode, ways of enhancing biofilm formation were sought. The quorum sensing molecule N (-3-oxodecanoyl)-L-homoserine lactone of different concentrations was for the first time exogenously added to MFCs and its effect on biofilm formation and power production determined. The results were compared with control experiments without N (-3-oxodecanoyl)-L-homoserine lactone. The results indicated that power production of 184 mW m-2 , the highest obtained of all approaches taken in this investigation, could be obtained when 10 uM of the chemical was added compared to 56 mW m-2 for the control, with significant increases in biofilm density. Taken together, these results highlight the importance of using defined cocultures (e.g. for bioaugmentation of working MFCs), direct electron transfer mechanisms, overexpression of the Mtr-pathway and need to increase biofilm density on anode surfaces, for enhancing electricity recovery in microbial fuel cells.