The viscoelastic characterisation of thermally-treated waste activated sludge

•Treated sludge transitions smoother from solid-like to liquid-like behaviour.•Linear relationship between rsCOD and G′/G′0 or G″/G″0 during thermal treatment.•Long heat treatment at low temperature is similar to short one at high temperature.•Modified Cox-Merz rule determines sludge flow curve from viscoelastic properties. Compositional change in sewage sludge due to thermal treatment drives rheological changes in the sludge. In this study, the viscoelastic characteristics of waste activated sludge (WAS) was investigated by measuring viscoelastic properties of sludge, the storage (G′) and loss (G″) moduli over 1h thermal treatment at different temperatures ranging from 20 to 80°C as well as after treatment when sludge cooled down to 20°C (thermal history effect). The soluble chemical oxygen demands (COD) was also measured at the same thermal treatment conditions. The results showed a distinctive difference between thermally-treated sludge and raw sludge in transitions from solid-like to liquid-like behaviour. It was observed that thermally induced compositional change in sludge impacted on both viscoelastic properties (G′ and G″) and viscous properties of sludge. Modified Cox-Merz rule (or Rutgers-Delaware) was successfully used to determine the sludge flow curve from the linear viscoelastic measurements of sludge. It was also found that the effect of short-time thermal treatment at high temperature on viscoelastic properties of sludge is similar to the effect of long-time thermal treatment at low temperature. This indicates that the amount of supplied thermal energy was the key controlling parameter of the evolution of rheological properties and the degree of organic matter solubilisation. A linear proportionality was also established between a dimensionless form of the released soluble COD and a dimensionless form of both G′ and G″. This suggests a possibility of controlling thermal treatment processes via rheological measurements.