Enhanced medium-chain fatty acids production from waste activated sludge via manganese-driven two-stage fermentation:...

Medium-chain fatty acid (MCFA) production from waste activated sludge through biological chain elongation (CE) is a promising strategy for jet fuel precursor production. However, MCFA production efficiency is restricted by the high thermodynamic barriers. Herein, zero-valent manganese (ZVMn) was introduced to break through the bottleneck by enhancing microbial energy conservation and promoting syntrophic metabolism. Results showed that ZVMn increased MCFA production and selectivity by 64.0% and 37.5%, respectively, with the highest MCFA concentration, MCFA production rate and MCFA selectivity reached 3360.4 mg/L, 3179.4 mg/L/day and 55%, respectively. Mechanistically, ZVMn loosened the extracellular polymeric substances matrix and led to 1.2-2.4-fold improvement in soluble chemical oxygen demand of extracellular polymeric substances (EPS) to increase the substrate availability. In parallel, ZVMn promoted both intercellular and intracellular electron transfer, as indicated by the increased current response, cytochrome c concentration, and intracellular NADH/NAD⁺ pool. Microbial community analysis suggested that ZVMn reshaped the microbial niche of waste activated sludge and particularly enriched the organic-degrading phyla Actinobacteriota and Chloroflexi. Moreover, ZVMn strengthened reverse β-oxidation and upregulated respiration-related pathways, thereby shifted reducing equivalents from competing sinks toward a metabolic configuration more conducive to CE. This study offers new insights into enhancing MCFA production via regulated energy metabolism by Mn additives.