Computational Fluid Dynamics Reveals Mass Transfer Limitations in a Pilot-Scale Microbial Electrolysis Cell

The scalability of microbial electrochemical technologies (METs), particularly microbial electrolysis cells (MECs), is constrained by hydrodynamic and mass transfer limitations that hinder efficient resource recovery from wastewater. This study presents a comprehensive computational fluid dynamics (CFD) model of a pilot-scale MEC (1 m³), representing the largest MEC modeled to date and integrating fluid dynamics with bioelectrochemical substrate consumption. The model simulates the spatial distribution of the anolyte under various operational conditions to identify flow-induced limitations in substrate transport to anodic biofilms. Dead zones and preferential flow paths caused significant inefficiencies, resulting in poor acetate removal under laminar flow. When evaluating the influence of HRT, reaction kinetics, and diffusivity on MEC performance, simulations further underscore that reactor performance is predominantly governed by external mass transfer rather than intrinsic reaction kinetics. To mitigate transport limitations without reducing the volumetric treatment capacity, a recirculation strategy was implemented, enhancing acetate removal efficiency from 16% to 48%. Model predictions agreed well with experimental data from similar pilot-scale MECs, supporting the validity of the approach. This work is intended as a reduced-order framework to diagnose hydrodynamic and external mass transfer limitations in large-scale cassette-type MECs, offering practical insights for improving reactor design and operation.