Moisture diffusivity during microwave drying of wastewater sewage sludge

Microwave drying of wastewater sewage sludge was investigated in this study. The power level ranged from 360 to 600 W, and the initial mass of the samples varied from 30 to 50 g. The drying kinetics of sludge showed three distinct phases: a short adaptation period, a long constant drying rate period, and then a falling drying rate period. Dimensional measurements indicated that the initial volume shrank by up to 40% depending on the applied drying conditions. Mathematical modeling of microwave drying of wastewater sewage sludge was performed using Fick's law with introduction of shrinkage and considering finite dimensions of the sample. The diffusion coefficient was expressed as a function of the applied drying conditions (power level, initial thickness and initial mass of the sample). In this study, it was proved that neglecting shrinkage or considering an infinite shape leads to an overestimation of the diffusion coefficient.