Breaking the Sludge Hydrolysis Barrier: EPS-Centered Accessibility in Pretreatment-Enhanced Anaerobic Digestion

Municipal sludge anaerobic digestion (AD) is often constrained by limited substrate accessibility due to extracellular polymeric substances (EPSs), microbial cell walls, and compact floc structures. This review integrates pretreatment technologies and EPS characteristics through a five-dimensional conceptual framework: cell disruption, EPS solubilization, particle-size reduction, soluble chemical oxygen demand (SCOD) release, and enhanced biodegradability. The framework connects structural changes with substrate accessibility and digestion performance. Physical, chemical, and biological pretreatment methods are critically compared using study-specific evidence and descriptive numerical ranges rather than pooled estimates. Thermal hydrolysis promotes cell disruption and organic matter solubilization, while other methods offer complementary mechanisms with distinct operational constraints. However, increased SCOD does not necessarily translate into improved biodegradability or methane recovery. Similarly, improvements in laboratory-scale biochemical methane potential (BMP) cannot be directly extrapolated to full-scale digesters. Pretreatment effectiveness should therefore be evaluated by considering biodegradability, net methane gain, energy and chemical inputs, and downstream environmental impacts. This framework supports the interpretation of heterogeneous evidence and the selection of context-specific pretreatment strategies.