Implication of Rheological and Thermal Properties of Sludge for Energy Optimization in a Sludge Treatment Train Incorporating Thermal Hydrolysis and Anaerobic Digestion

Thermal hydrolysis process (THP), as an emerging pretreatment step for anaerobic digestion process (ADP), improves sludge biodegradability and thus solids volume reduction and energy recovery. However, a lack of integrated data on the changes in sludge properties across the entire treatment train hinders the design of key energy-efficient units. To address this data gap, this study reports the physical and chemical properties of sludge sampled over 12 months at five crucial locations within a sludge treatment train. Along with measurements at 20°C, dynamic viscosity and thermal conductivity were measured based on actual temperature range up to 100°C. Detailed rheological modeling was also conducted using the Herschel-Bulkley equation to assess yield stress and flow index across the THP, cooling heat exchangers (CHEs), and ADP. Results showed that THP reduces yield stress from 34.3 to 8.0 Pa through CHEs and ADP and increases thermal conductivity by disrupting flocs and releasing bound water. ADP further lowered viscosity, shaping sludge into a more fluid state. Notably, a critical divergence was observed at operational temperature (70°C) while dynamic viscosity decreased remarkably (20%-50%), and thermal conductivity remained stable. Overall, it was proven that property changes are governed not only by total solids content but also by the fundamental restructuring of the physical and chemical composition (suspended vs. dissolved solids, organic vs. inorganic fractions), which are the primary drivers altering the thermal and rheological properties of sludge, deviating significantly from water in a predictable and quantifiable manner. This study provides the first stage-resolved analysis linking the changes in the physicochemical properties to real operational temperature regimes across a full-scale sludge treatment train. The knowledge provides valuable insights for design and operation of sludge treatment systems to reduce energy demand and improve sustainability.