Effect of mixing on rheological and structural changes in sludge during thermal hydrolysis

Thermal hydrolysis (THP) is an established pretreatment method to enhance sludge biodegradability and dewaterability, yet the specific impact of mixing during THP on its efficiency remains unclear. This study systematically compared THP of waste activated sludge with and without mechanical mixing under identical thermal conditions (160 °C, 30 min) to evaluate sludge solubilization (dissolved solids and soluble COD release), disintegration degree, extracellular polymeric substance (EPS) degradation, sludge rheology, and dewaterability. The results showed that mixing enhanced thermal disruption, increasing soluble COD by ~60% and disintegration degree from 12.4% to 20.0%. Combined mixing and heating reduced viscosity and particle size, indicating enhanced floc fragmentation and improved fluidity. Surprisingly, however, this enhanced fragmentation did not translate into improved dewaterability. The centrifuged cake solids content was 12.7% for mixing and heating, which was slightly lower than the 13.7% for the heating-only treatment. These results demonstrate that mechanical shear synergistically enhances sludge disintegration during THP, yet mixing may hinder solid–liquid separation in the downstream treatment. Further evaluation under realistic THP operating conditions (≈16 % solids, including flash phase) is recommended to identify the optimal balance between disintegration and dewaterability.