Effects of hydrothermal treatment on organic compositions, structural properties, dewatering and biogas production of raw and digested sludge

The effects of hydrothermal treatment (HTT) under different temperatures and time (120 °C to 250 °C, 10 min to 60 min) on organic matter solubilization and structure changes of secondary sludge (SS) and digested sludge (DS), as well as downstream dewatering and anaerobic digestion were investigated. The results showed that organic matter solubilization increased significantly at 120 °C to 170 °C, then decreased at 200 °C to 250 °C. The organic matter solubilization during HTT showed no obvious difference for two sludge, but for the different organic components. The polysaccharides are easier to be dissolved than protein, which was manifested by the higher dissolution rate at low temperature. The protein was the main soluble component for both of hydrothermal SS and DS, which accounted for 44 % to 64 % of soluble chemical oxygen demand (SCOD). The decrease of residual extracellular polymeric substances (EPS) content and increase of N-acetylglucosamine and DNA concentrations indicated that sludge EPS and cell wall structure were damaged at 170 °C, which contributed to the high organic matter solubilization. Nitrogen balance and molecular weight distribution indicated the concentrations of soluble organic components were the combined result of dissolution and hydrolysis reaction. The hydrolysis and polymerization reaction were intensified at 170 °C to 250 °C, which was verified by the COD balance and molecular weight transformation. The hydrothermal time could further facilitate the organics dissolution and hydrolysis based on the effect of hydrothermal temperature. The EPS structure damage also contributed to the high percentage of free moisture, resulting in enhanced dewaterability. The highest methane production was 298.1 mL CH₄/g VS_add for DS hydrothermally treated at 170 °C, which were 125 % and 9.8 % higher than SS and SS-HTT, respectively. This study provided an insight into the general mechanism of HTT and the application of different HTT and AD configurations.