Transforming heterogeneity into a structural advantage: multiscale synergistic mechanisms for converting full-component sludge hydrolysate into biopolymer films

The heterogeneous composition of sludge is often regarded as a barrier to high-value resource recovery. We propose a hydrolysis–self-assembly route that directly combines full-component sludge hydrothermal supernatant (SHS) with methylcellulose (MC) to fabricate mechanically reinforced composite films, converting compositional complexity into a multiscale structural advantage. The optimized SHS/MC film achieved a tensile strength of 20.21 MPa and an elongation at break of 151.13%. Multiscale characterization and simulation revealed that sludge proteins and polysaccharides formed hydrogen-bonding networks to balance rigidity and flexibility, while small-molecule-regulated phase separation generated uniformly dispersed nanodomains with spacings of 32.25–40.57 nm for stress dispersion. Inorganic mineral particles further strengthened organic–inorganic interfaces and promoted stress transfer. These molecular, nano-/microstructural, and interfacial mechanisms collectively enabled simultaneous strengthening and toughening, improving the strength–ductility balance of the film. Life cycle assessment (LCA) further demonstrated the environmental benefits of this pathway. This study establishes a mechanistic framework for designing SHS/MC composite biopolymer films and provides a promising strategy for sludge valorization into high-value materials.