Is Electro-Stimulation Always Effective for Methanogenesis? A Holistic Assessment of Electric Potential and Current Effects in Highly Conductive Systems

Abstract

Electro-stimulated anaerobic digestion (EAD) typically relies on voltage-only regulation, neglecting the impacts of the associated response current, particularly in highly conductive systems. Herein, we investigated an EAD system for treating iron-containing sludge digestate. Unexpectedly, under appropriate applied voltages of 0.9 to 1.2 V, the high conductivity triggered excessive currents, which subsequently inhibited methanogenesis. By decoupling the dual potential–current effects, integrated with metatranscriptomics analysis, we revealed that excessive currents induced severe, localized oxidative stress on anodic biofilms (up to 2.3-fold of the CF group) and hindered carbon metabolism. Concurrently, lower cathodic potentials triggered microenvironmental alkalinization, accelerating calcite and ankerite deposition, which increased the mass transfer resistance by up to 1.4-fold. Consequently, the positive biological effects of electro-stimulation (e.g., Geobacter enrichment, upregulated methanogenic genes) were counterbalanced by these current-induced physical and biochemical barriers. This study highlights the necessity of comanaging electric potential and current when deploying electro-microbial technologies in highly conductive systems toward energy-efficient AD facilities.