Electrode Spacing Controls Electrochemical and Microbial Dynamics in Wastewater-Treating Microbial Electrochemical Systems

Microbial electrochemical systems (MESs) offer a promising sustainable approach for wastewater treatment, while the high solution resistance of low-conductivity wastewater limits the efficiency. Reducing the electrode spacing (ES) enhances the MES performance. However, the synergistic interactions of ES on the interdependent processes of current generation, COD degradation, and exoelectrogenic dynamics remain poorly understood. In this study, reducing ES in MES is systematically investigated in relation to the solution resistance and biofilm activity, and its performance in different types of media (acetate, acetate/wastewater, and wastewater) is evaluated. As a result, the highest COD removal and electricity generation capacity are obtained with the smallest electrode spacing of 2 cm under all media conditions. In particular, the COD removal efficiency at 2 cm increases by 306% compared with that at 6 cm after 1 h of operation, accompanied by an enhanced current density of 123% in real wastewater. The electrochemical tests prove that reduced ES promoted extracellular electron transport of exoelectrogens, which is conducive to the increase in biofilm activity and current density. Moreover, the exoelectrogens responsible for simultaneous pollutant degradation and electricity production are enriched and establish cooperative relationships. This study provides a theoretical framework for optimizing MES reactor configurations to enhance the efficiency of wastewater treatment.