Application of COMSOL Multiphysics Model in the Study of Straw Addition Effects 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 de-watering performance through the addition of straw during F/T cycles. A mathemati-cal model was established using the Box-Behnken central composite design and vali-dated via COMSOL Multiphysics simulations. Optimal conditions were identified as freezing at -16°C for 24 hours, 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 1.30×1012m/kg at the optimal straw ratio. Straw conditioning also intensified the combustion stage of 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.