Conductive Materials in Anaerobic Digestion: From Baseline Performance to Microbial Competition under Sulfate Inhibition

Anaerobic digestion is a microbiologically mediated process that offers a sustainable route to decouple energy production from carbon-intensive sources by converting waste streams into renewable bioenergy and recoverable nutrients, supporting the transition towards a circular bioeconomy. Despite its technological maturity, anaerobic digestion remains highly sensitive to environmental and operational disturbances, with performance often compromised by process instability, accumulation of inhibitory compounds, and competition between microbial guilds. In sulfate-rich systems, these pressures are amplified through the simultaneous introduction of microbial competition and biochemical toxicity. Though not inherently inhibitory, sulfate stimulates sulfate-reducing bacteria that compete with methanogens for shared substrates while producing hydrogen sulfide, lowering biogas quality and overall process efficiency.

This Ph.D. research investigates the potential of conductive materials as a strategy to mitigate these sulfate-induced challenges by altering microbial interactions and system conditions, thereby enhancing methanogenesis. Employing systematic experimentation with mechanistic and microbial analyses, this Ph.D. study examined how different conductive materials, both carbon-based such as biochar and powder-activated carbon, and non-carbon-based such as magnetite, influence anaerobic digestion performance and microbial dynamics across systems of varying complexity.

In the absence of sulfate, biochar served as a model material to establish a baseline understanding of how its physicochemical properties affect digestion performance. Results showed that biochar physicochemical traits alone do not inherently improve methane yield once the stoichiometric potential of the substrate is reached, with additional methane gains deriving only from leftover biodegradable content. Under sulfate stress, both biochar and powder-activated carbon were unable to counteract inhibition. The addition of biochar affected process kinetics without improving methane yield, whereas powder-activated carbon suppressed overall microbial growth over the experimental period. These findings indicate that the benefits of carbon-based conductive materials depend on the operating environment and may emerge only during long-term operation, allowing for microbial adaptation, aggregate formation, and microbial immobilization, conditions that foster syntrophic stability.

In contrast to the limited effectiveness of carbon-based materials, magnetite significantly enhanced methanogenesis under sulfate-stressed conditions. Using a dual mechanism, magnetite simultaneously precipitated toxic sulfide species and biologically stimulated co-operation via direct interspecies electron transfer. Genome-centric metagenomic analysis further revealed a distinctly reorganized microbial community, characterized by strengthened cooperative networks linking hydrogenotrophic methanogens, homoacetogens, and sulfate reducers, supporting more efficient carbon and electron flow. These findings highlight the innovative role of magnetite as a mediator capable of both mitigating chemical inhibition and fostering cooperative microbial interactions, thereby sustaining methanogenesis even under adverse sulfate conditions.

Overall, this research advances the mechanistic understanding of how conductive materials influence anaerobic processes when applied as additives, highlighting the context dependent nature of their effects. These findings provide a framework for the rational selection and sustainable implementation of conductive materials in biogas production, contributing to the development of more robust and efficient anaerobic digestion systems.