The three-stage effect of hydrothermal treatment on sludge physical-chemical properties: Evolution of polymeric substances and their interaction with physicochemical properties

Hydrothermal treatment (HT) is effective for the deep dewatering of sewage sludge (SS); however, the effective temperature generally exceeds 180 degrees C, resulting in the production of refractory compounds in the sludge filtrates. To explore a new process based on HT, achieving ideal dehydration efficiency at lower temperatures, it is essential to identify the key sludge dewatering mechanism under different HT stages. In this study, the relationship between the properties of sludge polymeric substances (components and molecular structures) and the physical-chemical properties of sludge flocs during HT (120-260 degrees C) was investigated. The results indicated that the SS surface hydrophilicity/hydrophobicity was mainly responsible for sludge dewaterability in the solubilization (120 degrees C) and hydrolyzation stages (140-180 degrees C), while the mechanically bound water and capillary force were the main limiting factors of sludge dewaterability during the carbonization stage (200-260 degrees C). Moreover, in the solubilization stage (120 degrees C), a plenty of high-Mw (Mw > 70 kDa) polymeric substances with numerous hydrophilic functional groups and a compact structure were released from the intracellular region to the outer layer, which improved the hydrophilicity of sludge floc surface and deteriorated the sludge dewaterability. With the hydrolysis of the polymeric substances (140-180 degrees C, hydrolyzation stage), the destruction of proteins secondary structures and peptide chains exposed more hydrophobic groups, resulting in the release of bound water and improvement of sludge dewaterability. At HT temperatures of 200-260 degrees C (carbonization stage), dehydration and amine aldehyde condensation occurred, benefiting the formation of fixed carbon and smooth morphology structure of SS, reducing the capillary force-induced water-holding capacity of sludge flocs. The establishment of the three-stage influencing theory and the identification of key influencing factors are conducive to the further regulation and upgrading of HT.

Automated summary

Hydrothermal treatment (HT) is effective for deep dewatering of sewage sludge, but high temperatures often result in the production of refractory compounds. This study investigated the key sludge dewatering mechanism under different HT stages. The results showed that the surface hydrophilicity/hydrophobicity of the sludge plays a major role in solubilization and hydrolyzation stages, while mechanically bound water and capillary force are the main limiting factors during the carbonization stage. The release of high molecular weight polymeric substances in the solubilization stage worsens the sludge dewaterability, while the hydrolysis of polymeric substances improves it. Dehydration and amine aldehyde condensation in the carbonization stage reduce the water-holding capacity of sludge flocs.