Structural model to study the influence of thermal treatment on the thixotropic behaviour of waste activated sludge

The present study proposed a thixotropic model to examine the variations in the thixotropic behaviour of waste activated sludge after the application of different thermal specific energies (ES). The model is based on the definition of the time dependence of a structural parameter, S, which quantifies the structural level of the internal structure at any time and shear rate, and the instantaneous variations in the alignment and deformation of flocs. The thixotropic behaviour was quantified based on the kinetic coefficients for the breakdown and build-up processes, which appeared to depend on the shear rate. The application of thermal ES significantly reduced the steady state viscosity and the kinetic coefficient for the breakdown process, suggesting that the thixotropy slightly increased. Complementarily, the sludge was further characterised to understand the variations in sludge rheology. The extracellular polymeric substances (EPSs) present in sludge were fractionated and subsequently analysed by gel permeation chromatography, which revealed that thermal treatment solubilised most of the peptides and proteins of low molecular weight but denatured the high molecular weight proteins. On the other hand, the capillary suction time increased at low thermal ES and decreased at high thermal ES. However, the water removed by centrifugation was increased even at low temperatures, despite the increase in the capillary suction time value.