Effect of ultrasound, thermal and alkali treatments on the rheological profile and water distribution of waste activated sludge

•Waste activated sludge was subjected to ultrasound, thermal and alkali treatments.•The treatments reduced the elasticity and zero shear rate viscosity of the sludge.•The treatments reduced the steady state viscosity and thixotropy of the sludge.•The bound water content in the sludge increased when treatment intensity increased.•Despite increasing bound water, the treatments were able to improve sludge dewatering.

Waste activated sludge is difficult to dewater, which hinders handling operations, such as pumping, transport and disposal. An accurate measurement of the distribution of water in sludge may help identify dewatering problems. Bound water, which is a gross estimate of several states of water including vicinal water, water of hydration and some fraction of interstitial water, is considered one of the primary limiting factors affecting sludge dewatering. In the present study, waste activated sludge was subjected to ultrasound, thermal and alkali treatments based on the assumption that these treatments can partially disintegrate sludge by disrupting flocs and cells and releasing interstitial water and organic compounds in the liquid phase, thus improving sludge dewatering. The three aforementioned treatments resulted in the reduction of the steady state viscosity and the hysteresis area of the sludge, and this reduction was more significant as the treatment intensity was increased. Likewise, the treatments also reduced the sludge elasticity and zero shear rate viscosity. Conversely, the bound water content increased when the treatment intensities increased because the organic matter released after sludge floc disruption created extra surfaces for water binding. Nevertheless, the centrifugation tests revealed that the three conditions of the thermal treatment (60, 80 and 90 °C) and the higher intensities of ultrasound (27,000 kJ/kg TS) and alkali (157 g NaOH/kg TS) treatments improved sludge dewatering by releasing the interstitial water.