This work compares two positive operations, back-mixing and sawdust addition, on the drying of wastewater sludges. The influences of back-mixing (with mass percentages of dried sludge of 40, 60, and 70% on a dry basis) and sawdust addition (40% on a dry basis) operations on the drying rates of sludge in a pilot-scale fixed bed were studied. X-ray tomography was used to monitor the evolution of structural characteristics of the samples during drying. When using the same mass percentage of dried materials (40%), the sawdust sludge mixture has a higher drying rate than the back-mixed sludge as a result of higher initial and final bed volumes together with higher total exchange surface areas. To reach a similar rate increase with a back-mixing operation, 60% dried sludge must be back-mixed.