Fly ash-assisted low-temperature hydrothermal treatment enables deep sludge dewatering by restoring sludge cake water...

Low-temperature hydrothermal treatment (LHT) is an important pretreatment technology for sludge stabilization and resource recovery; however, the associated deterioration in mechanical dewaterability remains a major barrier to its wider application. To alleviate this limitation, this study developed a fly ash (FA)-assisted LHT process and evaluated its effects on sludge dewatering performance and filter cake structure. LHT alone at 120 °C markedly worsened sludge dewatering, whereas FA addition effectively improved the dewatering of LHT-treated sludge, with performance progressively improving as the FA dosages increased within the tested range. For example, compared with LHT alone, FA addition at 25% of sludge dry solids decreased the filter-cake water content (W_c) after pressure filtration from 83.19% to 58.30% and reduced the capillary suction time (CST) by 68.12%. Mechanistic analysis revealed that FA caused an obvious additional release of soluble components and left the net bound-water fraction estimated by differential scanning calorimetry (DSC) nearly unchanged. In contrast, the cake compressibility coefficient decreased from 1.10 to 0.50 and the total porosity resolved by X-ray computed tomography (X-CT) increased from 7.34% to 21.07%, while the proportion of connected pores within the identified pore volume increased from 0% to 52.46%. These findings associate the dewatering improvement with reduced cake compressibility and increased pore connectivity, without a corresponding decrease in the DSC-estimated net bound-water fraction. This study highlights the importance of restoring filter-cake structure of LHT-treated sludge and provides a practical route for integrating sludge conditioning with industrial solid-waste utilization.