Performances of microbial fuel cells fed with rejected wastewater from BioCH₄ and BioH₂ processes treating molasses wastewater

An integrated process involving conventional anaerobic digestion and microbial fuel cells (MFCs) has attracted attention recently to produce sustainable energy and to treat wastewater efficiently. To evaluate the possibility of CH₄-producing process (BioCH₄)-MFC or H₂-producing process (BioH₂)-MFC integrating systems, the MFC performances were investigated using rejected wastewater from a BioCH₄ reactor (RW_CH4) or BioH₂ reactor (RW_H2) treating molasses wastewater. When RW_CH4 or RW_H2 was fed into a single-chamber MFC reactor (designated as AC-MFC_CH4 and AC-MFC_H2, respectively) at different hydraulic retention times (HRT) of 1-7 d, both MFC systems showed maximum electricity production efficiencies at a HRT of 3 d. In the AC-MFC_CH4 reactor, the average current density and average power density were 60.5 mA·m^-2 and 8.8 mW·m^-2, respectively. The AC-MFC_H2 reactor generated an average current density of 71.4 mA·m^-2 and an average power density of 12.0 mW·m^-2. The COD removal rates were 45.7% in the AC-MFC_CH4 reactor and 90.3% in the AC-MFC_H2 reactor. There were no significant differences of the eubacterial community structures between the MFC systems, where Proteobacteria was remarkably dominant in both MFC systems. However, the archaeal community structures were significantly different where Methanothrix (89.3%) was remarkably dominant in the AC-MFC_CH4 system, while Methanothrix (52.5%) and Methanosarcina (33.5%) were abundant in the AC-MFC_H2 system. These findings demonstrate that the utilization of MFCs after the BioCH₄ or BioH₂ process is advantageous for energy recovery as well as COD removal from molasses wastewater.