Mechanisms of water-state transformations in low-temperature thermal dewatering of sludge: Synergistic control via extracellular protein α-helix conformation and water–solid interfacial affinity

Municipal sludge dewatering is non-negotiable for volume reduction and resource recovery. Existing thermal-dewatering methods face a paradox: to reduce energy use and volatile emissions, temperatures are kept low, yet low-temperature processes (<70 °C) suffer from inefficient removal of bound water. This bottleneck arises from insufficient understanding of how extracellular polymeric substances (EPS) influence water-state transformation. Herein, we clarified the molecular mechanisms of temperature-dependent water-state changes within the 30 °C-70 °C range by elucidating the transformation pathways of water states during sludge dewatering, resolving the aforementioned paradox. By integrating multi-scale approaches, namely macroscopic quantification of water fractions (TGA-DSC/LF-NMR), microscopic analysis of molecular conformations (FTIR/2D-FTIR/XRD) and response-mechanism modelling (SEM), this study provides a comprehensive understanding of the coupled processes of water-state transformation, protein conformational evolution and interfacial interactions. TGA-DSC analysis revealed a critical temperature threshold at 50 °C, below which (30 °C-50 °C), bound water was initially converted into interstitial water and subsequently into free water (or directly released as free water), resulting in a 10.5 % improvement in dewatering performance. This transformation is driven by a reduction in α-helix content and unfolding of peptide chains. However, within 50 °C-70 °C, interstitial or free water is re-adsorbed as bound water, resulting in a 42.8 % decline in dewatering performance. This reversal is attributed to reassembly of α-helix structures and formation of hydrophilic Maillard intermediates via amine-aldehyde condensation, which increase water-solid interfacial affinity. This study establishes quantitative relationships among protein secondary structure, interfacial hydrophilicity, and water-state transitions, and provides guidance for optimizing low-temperature thermal dewatering of sludge through EPS conditioning.