The aim of this study was to characterize gas flow during the composting, at a pilot scale, of sludge mixed with bulking agent, in order to model heat and mass transferts involved in the process. Thus, a closed 300-litre cylindrical pilot was fed with a mixture of wastewater treatment sludge and pine bark. Aeration was supplied from the bottom via an air blower and gases were collected at the top. Two gas flow rates were studied Substrate's. Temperature and oxygen consumption were monitored all along the trials. Data evolutions were discussed. Moreover, Retention Time Distribution curves were obtained by injecting a pulse of methane in the entering airflow and by analyzing the methane concentration in the exhaust gas, every two days during composting. Gas flow within the composting medium was characterized by a dispersion model, which is a deviation of the plug flow model. Dispersion coefficient was then determined and the dispersive effect of the flow was used to describe heat and mass transfers mathematical equations.