Dewatered sewage sludge textural and viscoelastic changes during its drying: Focus on the sticky phase

This study characterizes viscoelastic and textural evolution of dewatered sewage sludge (DSS) during thermal drying, with a particular focus on identifying the moisture ranges associated with its sticky-phase (S-P). DSS samples were collected from three wastewater treatment plants and analysed for total solids (TSC) and volatile solids (VS). Samples were dried in 100 g batches at 105 °C and tested at discrete moisture levels. Samples were homogenized and extruded into cylinders measuring 3 cm in height and 1.5 cm in diameter. The samples were analysed using texture profile analysis, rheology, penetrometry, and X-ray microtomography; tests were done by triplicate to ensure reproducibility. For all samples, the sticky phase was identified between 20% DS and 35% DS , a range consistently confirmed across methods, by a peak in the adhesiveness-to-cohesiveness (A/C) ratio in TPA and penetrometry, a stabilization of the storage modulus (G′) in rheology, and a drop in porosity by microtomography This convergence indicates that the sticky phase represents a critical transition between viscoelastic-pasty and viscoelastic-solid states, associated with internal structural rearrangement and moisture redistribution The proposed multi-technique methodology can be adapted to other sludge types to map their sticky phase and to support the design and optimization of drying operations.