Shrinkage is an important phenomenon during sludge drying, as a significant reduction in volume occurs during the loss of moisture. As the structure changes, the drying area decreases and the drying rate reduces accordingly. X-ray tomography is used to explore the structure evolution that a sludge bed undergoes in a convective pilot-scale fixed bed dryer. A high energy (420 kV), large-scale (max. 0.45 m in diameter and max. 4 m in height) X-ray tomograph with a spatial resolution of approximately 0.36 mm was chosen. Two types of sludge samples collected in two wastewater treatment plants (Grosses Battes WWTP and Oupeye WWTP, Belgium) were chosen for this study. Continuous and intermittent drying experiments were both carried out to study the drying kinetics and structure evolutions (volume, shrinkage, void, exchange surface). After experiments, 2D cross-sections and 3D images were obtained, which provide a convenient way to obtain global and quantitative information on the evolution of the sludge bed structure during the entire drying process. The results show that the volume, shrinkage, void, and exchange surface all substantially change during the drying process. Moreover, the structures of the two sludges evolve quite differently during the drying process. Image analysis of the X-ray tomograms is used to gain insight on the transport phenomena that occur during the drying process. (C) 2015 Elsevier Ltd. All rights reserved.