Microwave drying is a promising strategy for sewage sludge (SS) reduction and low-carbon treatment, but the kinetic behavior of thick-layer SS drying remains insufficiently understood. The present work therefore aimed to elucidate the stage-dependent moisture-removal behavior and kinetic evolution of thick-layer SS under microwave irradiation, with particular emphasis on the coupled effects of microwave energy input and internal moisture transport. A thermogravimetric system for thick-layer SS microwave drying was designed to continuously monitor sample temperature and mass, and the effects of microwave power (400, 450, 500, 550, and 600 W) and sample loading (20, 25, 30, 35, and 40 g) on drying behavior and kinetic parameters were systematically investigated. SS with an initial moisture content of 82.3 wt.% was treated in a customized quartz reactor, and the staged kinetic parameters were determined using the Coats-Redfern method by screening 20 candidate reaction mechanism models. The results showed the principal drying process exhibited a distinct four-stage kinetic evolution, reflecting successive changes in moisture state and internal mass-transfer resistance. Increasing microwave power enhanced initial heating and water evaporation, but excessive power caused overheating above 100 °C during the quasi-isothermal stage. The activation energy of Stage III decreased to 104.03 kJ/mol at 500 W, but increased to 133.86 and 170.52 kJ/mol at 550 and 600 W, respectively, indicating intensified mass-transfer limitation under excessive power. These results show that excessive microwave input, although beneficial for early-stage free-water vaporization, can generate vapor faster than it can be transported through the thick sludge bed, thereby increasing the apparent resistance to subsequent bound-water removal. Sample loading also strongly affected the drying kinetics. The 20 g sample showed only three effective kinetic stages and incomplete drying, while the 40 g sample retained a high residual mass because of excessive bed thickness. The 30 g sample exhibited complete staged kinetic behavior, with activation energies of 110.08, 65.10, 104.03, and 46.92 kJ/mol for Stages I-IV, respectively, indicating a more balanced relationship between microwave energy absorption and moisture transport. Overall, under the investigated conditions, 500 W and 30 g provided the most favorable operating combination. More broadly, the results demonstrate that the performance of thick-layer microwave drying is governed not simply by the magnitude of microwave energy input, but by the balance between volumetric energy deposition and internal moisture transport. The stage-resolved kinetic framework established here provides a mechanistic basis for controlling overheating and mass-transfer limitations in the design and scale-up of energy-efficient thick-layer SS microwave drying processes.

Sewage sludge,Microwave,Kinetic analysis,Coats-Redfern method