Long-term impact of flat-plate architectures on the performance of microbial electrolysis cells

Microbial electrolysis cells (MECs) offer a promising approach for wastewater treatment while enabling the simultaneous production of hydrogen or other high-value compounds. Nonetheless, bioelectrochemical technologies, particularly microbial electrolysis cells, have not yet reached commercial viability. This study investigates the critical influence of architecture on flat-plate MEC performance by integrating long-term bioelectrochemical experiments with statistical and hydrodynamic analysis, including an evaluation based on Péclet numbers. To this end, two flat-plate MEC architectures (square and rectangular) were operated for 150 days using synthetic and real brewery wastewater under total recirculation and chemostat modes, the latter with hydraulic retention times ranging from 3 to 24 h. Electrochemical performance was assessed via cyclic voltammetry at various space velocities, from 0.18 to 0.67 min^-1. Additionally, abiotic residence time distribution experiments were conducted at different flow rates, recording tracer evolution to characterise flow patterns of both architectures and relate them to bioelectrochemical performance. The experiments revealed that the square MEC consistently produced 30-50 % lower current densities than the rectangular architecture, highlighting the critical role of hydrodynamics in MEC performance. The superior performance of the rectangular MEC was related to its higher Péclet number, lower dispersion (minimizing channelling and recirculation zones) and a more invariant flow pattern across flow rates, whereas the square architecture showed significant mass transport limitations. These results demonstrate that internal hydrodynamics, governed by reactor architecture, are crucial to maximise flat-plate MEC efficiency and should be a central focus in future design and scale-up strategies.