The aim of this work is to characterize the convective drying aptitude of residual urban sludge in isothermal conditions at atmospheric pressure. Three examples of biological sludge are studied, taken from various wastewater treatments, after mechanical dewatering. Total and volatile solids content, dryness and water content are quantified using standard methodologies. The consistency limits (liquidity and plasticity) are determined using standardized soil mechanics trials based on Atterberg's limits. A desorption unit is used to dehydrate the sludge under controlled humidity (RH ranging from 7% to 96%) at a temperature of 30 degrees C in an immobile drying atmosphere. The influence of relative humidity on dewatering and shrinkage is quantified. The coupled analysis of the dewatering and induced shrinkage curves shows correlations between the hydric and the textural characteristics. Emphasis is placed on the influence on these correlations of the nature of the sludge, characterized by their organic matter content. These results offer a description of the drying and shrinkage aptitudes of biological sludge for a controlled atmospheric drying process.