Industrial red mud establishes redox-active interfaces to steer metabolic pathways toward chain elongation in sludge anaerobic fermentation

Medium-chain fatty acids (MCFAs) production from waste activated sludge (WAS) provides a promising route for sludge valorization, but is often limited by inefficient hydrolysis and restricted interspecies electron transfer. This study evaluated industrial red mud (RM) as a conductive and alkaline regulator to enhance anaerobic chain elongation (CE). With 5 g/L RM addition, MCFAs yield reached 12.6 g COD/L, representing a 164% increase over the control. Spectroscopic analysis showed that the strong alkalinity of RM altered protein secondary structures, facilitating substrate hydrolysis while maintaining stable pH favorable for CE. Increased release of humic-like substances was observed, and electrochemical evidence suggested that the adsorption of these redox mediators onto the RM surface potentially facilitated the formation of redox-active interfaces, which contributed to the enhanced electron transfer capacity. Microbial network analysis demonstrated that RM acted as a topological hub, restructuring the community into a synchronized syntrophic consortium (hydrolysis–acidogenesis–CE) and highly enriching key CE bacteria. Metagenomic analysis revealed an increase in the abundance of genes encoding conductive membrane proteins (cytochromes and Mtr-associated), suggesting a potential enhancement in direct interspecies electron transfer. Meanwhile, RM increased the gene abundance of the CE key pathway (reverse β-oxidation pathway), thereby favoring the genetic potential for MCFAs accumulation. These findings establish a sustainable ’waste-treating-waste’ framework, utilizing RM-driven electron reservoirs to facilitate the high-value conversion of WAS in anaerobic systems.