Vulnerable Methanogenic Community in Microbial Electrolysis Cells Alters Electron Allocation in Response to Community Coalescence

The integration of microbial electrochemical technologies (METs) and conventional wastewater treatment units has become a promising pathway to enhance energy recovery from wastewater treatment plants, but how the microbial communities from these two systems interact with and affect each other remains unknown. This study investigated the performance and microbial diversity under community coalescence by introducing activated sludge (AS) into typical microbial electrolysis cells (MECs). When coupled with AS, the electrochemical active surface area (ECSA) of the anode significantly increased from 1.67 to 2.97 mF/m(2), which could be ascribed to the retention of porous flocs on the electrode surface. However, the anodic Coulombic efficiency was stable under sludge introduction, even with stronger electron transfer capacity. The increased mcrA gene copies of anodic methanogens could indicate enhanced competition in electron allocation to methane rather than to the anode, thereby maintaining the anodic Coulombic efficiency. The energy efficiency and cathodic Coulombic efficiency were enhanced by improving the hydrogen production from 0.018 +/- 0.011 to 0.161 +/- 0.136 m(3)/m(3)d, which was likely due to the lower cathodic methanogenesis activity that promoted the electron allocation to hydrogen instead of methane. Isotope verification indicated that acetoclastic methanogenesis was the predominant pathway for methanogenesis throughout the operation, but a temporary increase in the hydrogenotrophic pathway was observed in response to community coalescence (alpha c = 0.44), indicating that acetoclastic methanogenesis could be inhibited. Overall, the methanogenic community could be more vulnerable during the integration of METs with traditional wastewater treatment technology; specifically, acetoclastic methanogenesis could be shocked in response to community coalescence. These findings provide an insight into the integration of METs with traditional wastewater treatment units from the microbial perspective.