Thermally Modified Drinking-Water Sludge as a Mineral Conditioner for Municipal Sludge Dewatering and Low-Temperature Drying

Sludge-conditioning strategies are commonly optimized for filtration performance, with less attention paid to the rheological and textural properties of concentrated sludge and their relevance to low-temperature drying. Here, thermally modified drinking-water sludge (HDWS) was evaluated as a waste-derived mineral conditioner for filtration dewatering and low-temperature drying. The specific resistance to filtration (SRF) was lowest at 30% DS, whereas the 60% DS treatment reduced the moisture content to 35.66% after drying at 60 °C for 30 min, indicating that the optimum filtration condition did not deliver the best drying performance. Increasing the HDWS dosage from 0 to 60% DS reduced the Jenike shear stress from approximately 565 to 490 Pa and the apparent yield stress from approximately 670 to 380 Pa, while also decreasing adhesiveness and cohesiveness. At higher dosages, the sludge retained relatively high small-strain stiffness but exhibited lower nonlinear elastic and viscous resistance. These responses were consistent with weakened macroscopic bonding and altered deformation-dependent energy dissipation after HDWS addition. A possible contribution from mineral-particle contacts is suggested, although the underlying microstructural mechanism was not directly resolved. The 60% DS treatment shortened the time required to reach 20% water content to approximately two-thirds of that for raw sludge but reduced the cake calorific value from approximately 9.6 to 4.3 kJ g−1. HDWS, therefore, exhibited process-specific trade-offs, and its dosage should be selected according to the targeted unit operation and downstream management route rather than regarded as a single overall optimum.