Persulfate-driven sludge biorefinery toward value-added medium-chain fatty acids

Transforming waste activated sludge (WAS) into high-value biofuels is a key pathway toward sustainable waste management and carbon neutrality, yet the recalcitrance of extracellular polymeric substances (EPS) and microbial cell walls severely limits medium-chain fatty acids (MCFAs) production during anaerobic fermentation. Here, we propose a persulfate (PDS)-based pretreatment strategy that enhances MCFAs synthesis by driving sludge disintegration and substrate transformation. Treatment with 7.5 mM PDS increased MCFAs yield by ∼50%, reaching 13,341.4 mg COD/L. Mechanistic investigations reveal that SO 4 ·⁻ and ·OH radicals preferentially degrade tightly bound EPS, reducing protein and polysaccharide content by 38% and 46%, respectively, and increasing soluble chemical oxygen demand (SCOD) 5.05-fold. This transformation produces nitrogen-rich, low-molecular-weight dissolved organic matter (DOM). The resulting DOM exhibited high H/C ratios, low O/C ratios, and low aromaticity indices (AI mod ), significantly enhancing its bioavailability during anaerobic fermentation. Integrated metagenomic functional annotation and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) revealed that EPS-derived DOM reshaped the microbial metabolic network, stimulating glycolysis, amino acid metabolism, and carbon chain elongation. Moreover, the formation of unsaturated and aromatic-like fermentation products indicated enhanced DOM humification, which facilitated carbon chain elongation and microbial metabolic activity. Life cycle assessment and techno-economic analysis confirmed the environmental sustainability and economic feasibility of this radical-driven strategy. By elucidating the radical–EPS–DOM–metabolism cascade, this study provides mechanism-guided strategies for efficient sludge biorefinery, advancing the field from empirical operation toward targeted, high-efficiency design.