Application of COMSOL Multiphysics Model in Studying Effects of Straw Addition on Dewatering Performance of Residual Sludge During Freeze–Thaw Cycles

Freeze–thaw (F/T) technology is an environmentally friendly and efficient method for residual sludge treatment. This study investigates the enhancement of sludge dewatering performance through the addition of straw during F/T cycles. A mathematical model was established using the Box–Behnken central composite design and validated via COMSOL Multiphysics simulations. The optimal conditions were identified as freezing at −16 °C for 24 h, with 12.5 freeze–thaw cycles and a straw mixing ratio of 20%, reducing the sludge moisture content from 62.7% to 35.9%. The specific resistance to filtration (SRF) and cake moisture content decreased significantly with increasing straw addition, reaching a minimum SRF of 1.30 × 1012 m/kg at the optimal straw ratio. Straw conditioning also intensified the combustion stage of the sludge by increasing the maximum weight loss rate and elevating the thermal decomposition temperature. Numerical simulations confirmed the experimental results, demonstrating that straw addition significantly improves sludge dewaterability by modifying heat and mass transfer mechanisms.