Dewatering is very important for excess sludge treatment and disposal. Hydrothermal
treatment coupled with mechanical expression is a novel technology, in which a conventional
pressure dewatering is combined with hydrothermal effect to realize an
improved liquid/solids separation with low energy consumption. In this study, the
process was performed by way of that the excess sludge was hydrothermally treated first
and then the mechanical expression was employed immediately at increased temperature
in two separate cells respectively. The results demonstrated that the mechanical
expression employed at increased temperature showed a significant advantage than that
at room temperature, given a further reduction of 19e47% of the moisture content. The
dewatering process at room temperature was mostly depended on the effect of mechanical
expression. Hydrothermal process, more importantly than mechanical effect at
increased temperatures, seemed to govern the extent to which the dewatering process
occurred. The dewatering began to show a positive effect when the temperature was
exceeded the threshold temperature (between 120 and 150 C). The residence time of
30 min promoted a substantial conversion in the sludge surface properties. After dewatering
at temperatures of 180e210 C, the moisture content decreased from 52 to 20%
and the corresponding total water removal as filtrate was between 81 and 93%. It was
observed that the moisture content of filter cake correlated with surface charge
(Rp ¼ 0.93, p < 0.05) and relative hydrophobicity (Rp ¼ 0.99, p < 0.05). The calculated
energy balance suggested that no additional external energy input is needed to support
the dewatering process for excess sludge. The dewatering process needs an obviously
lower energy input compared to thermal drying and electro-dewatering to produce a
higher solids content cake.