The mechanical expression employed at increased temperature
showed a significant advantage than that at room temperature.
So a detailed investigation on the dewatering at
increased temperature for excess sludge was conducted. This
work presented an insight into the influence of the operating
conditions on excess sludge dewatering, under a variety of
processing parameters ranging from 90 to 210 C for
10e90 min and from 2.0 to 8.0 MPa for 20 min.
The dewatering process seemed to be governed by the
hydrothermal effect to a large extent. It is concluded that the
dewatering began to show a positive effect at the hydrothermal
temperature exceeding 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%.
The analysis of the correlation in this study showed that
the moisture content of cake correlated with the surface
charge (Rp ¼ 0.93, p < 0.05) and the relative hydrophobicity
(Rp ¼ 0.99, p < 0.05). The improvement of the surface properties
promoted the decrease of negative electrostatic repulsion
among flocs and destroyed the binding between particles
and water, setting the water free