Dewatering is important for activated sludge disposal. The dewaterability of activated
sludge was first deteriorated and then ameliorated when the temperature was raised from
100 to 200 C with a threshold temperature of 130 C under hydrothermal treatment.
Calcium chloride assisted hydrothermal treatment to improve the dewaterability of activated
sludge, and eliminated the threshold temperature at as less as 20 mg/g dry solid (DS).
An increase in temperature and dosage of CaCl2 till 60 mg/g DS allowed a continuous
improvement of dewaterability. It is found that the charge neutralization resulted from
biopolymers solubilization dominated the dewaterability evolution below 160 C, while the
decomposition of water-binding components played a more important role at higher
temperatures. The variation of molecular weight of soluble protein and polysaccharides
implies that CaCl2 interacted with the component of sludge and altered the constituent
during the hydrothermal treatment. The integration of soluble biopolymers into the floc
matrix by CaCl2 contributed to the compacted floc structure and thus improved the dewaterability.
This work presents an insight into the floc variation in both the composition
and structure associated with the dewaterability and offers a new understanding to the
role of temperature and CaCl2 in hydrothermal treatment on activated sludge dewatering.