Effects of thermal treatment on physical and expression dewatering characteristics of municipal sludge

• Thermal hydrolysis modified physical properties of municipal sludge remarkably.
• Thermal hydrolysis improved the filterability of municipal sludge.
• Thermal hydrolysis was benefit to the expression dewatering.
• Sludge viscoelasticity has a correlation to expression dewaterability.

The effect of thermal treatment on physical properties and dewatering characteristics of municipal sludge was further studied by series of thermal hydrolysis and dewatering (including filtration and expression) experiments. Bound water content, particle size and viscoelasticity of municipal sludge decreased, while specific surface area of sludge floc increased after thermal hydrolysis. The decreased softness and increased rigidity of treated sludge particles, contributed to forming a filter cake with higher permeability and lower compression coefficient during filtration process. Consequently, filtrate amount significantly increased by nearly 7 times during 60 min, and a much compacter and less elastic filter cake was built up. For expression stage, contribution of primary consolidation stage decreased from 0.071 to 0.016. Creep factor (η) increased by 11 times, suggesting that the mobility of particles in filter cake became easier, and the contribution of secondary consolidation stage also increased. In contrast, due to the markedly reduced bound water content, the contribution of tertiary consolidation stage decreased. With the increase of expression pressure, both effects of the primary and the second consolidation stages reduced slightly, while, the contribution of the tertiary consolidation stage had a slight increase. Rheology analysis indicated that viscoelasticity of sludge has a correlation to expression dewaterability in the range of our study.