Chemical perturbation in wastewater-fed microbial fuel cells: biofilm reconfiguration, electron redistribution, and electrochemical signatures.

Microbial fuel cells (MFCs) couple wastewater treatment with energy recovery, but their practical application is constrained by electrochemical instability when complex influents decouple substrate removal from electrode recovery. This review proposes a measurement-guided diagnostic framework integrating the Electron Redistribution Mechanism (ERM), the electron-recovery deficit ratio (De), and Electrochemical Perturbation Signatures (EPSig). Redox-active perturbations, including metals, nitrate, nitrite, sulfur species, and soluble redox mediators, are evaluated alongside non-redox perturbations, such as pH fluctuations, salinity, total ammonia nitrogen (TAN)/free ammonia stress, and organic overloading. Redox-active perturbations may establish competing electron-transfer pathways or redox cycling when the corresponding reactions are biologically and kinetically accessible. Non-redox perturbations alter microbial metabolism, the organization of biofilm extracellular polymeric substances (EPS), substrate and ion transport, electrolyte conductivity, and biofilm-electrode charge transfer, and may impose matrix-dependent transport limitations rather than serving as additional terminal electron acceptors. ERM provides an electron-equivalent accounting structure for quantifying removal-recovery divergence, whereas De expresses unrecovered electron equivalents relative to electrode recovery without identifying a specific sink. EPSig supports localization of charge-transfer, ohmic, mass-transfer, anodic, and cathodic limitations through electrochemical impedance spectroscopy (EIS), polarization analysis, electrode-resolved potentials, and time-resolved electrical responses. Application to published tofu-processing wastewater data indicates marked removal-recovery decoupling, but the absence of pathway-resolved measurements and resistance- and electrode-resolved electrochemical diagnostics prevents allocation of the recovery deficit and localization of the associated electrochemical limitation. The integrated ERM-EPSig framework provides a structured basis for distinguishing measured allocation to non-electrode pools from electrochemical limitations and for informing matrix-specific operational responses.