Ultrahigh-pressure expression of activated sludge assisted with self-flocculation caused by ultrasonication

• Flocculant-free mechanical deliquoring method of activated sludge was proposed.
• Cell disruption caused by ultrasonication induced self-flocculation of sludge.
• Floc breakage induced by ultrasonication greatly facilitated removal of bound water.
• Expression assisted with ultrasonic pretreatment resulted in high-degree deliquoring.
• Cake moisture content was reduced to 38 wt% at expression pressure of 50 MPa.

The flocculant-free mechanical deliquoring method has been proposed for obtaining the compressed cake with an extremely low moisture content in expression of municipal excess activated sludge. The method is comprised of the mild ultrasonic pretreatment followed by ultrahigh-pressure expression. The floc breakage induced by ultrasonication greatly facilitated the removal of bound water during ultrahigh-pressure expression. Moreover, the cell disruption caused by ultrasonication induced self-flocculation of the sludge with the aid of the intracellular materials released from within the cells, preventing the souring of dewaterability. The moisture content of the compressed cake produced by expression of ultrasonicated sludge was dramatically reduced to as low as 38.0 wt% at the ultrahigh-pressure of 50 MPa, which exhibited the value much lower than that obtained for untreated sludge, due to the removal of bound water. Analysis of kinetics of ultrahigh-pressure expression for ultrasonicated and untreated sludge showed that the primary consolidation was followed by a three-stage creep phenomenon with different values of the creep constant. The primary consolidation based on the Terzaghi spring analogy accounted for the major portion of moisture removal in this method, and the effect increased with increasing pressure.