A one-dimensional numerical model was developed for an airtight composting reactor in which air was continuously blown from the bottom. The composting model was based on three main assumptions: (1) the organic matter was divided in three fractions (easily biodegradable, slowly biodegradable and inert); (2) the biodegradation kinetics of the organic fractions was written as a Monod-type model considering the microorganism metabolic kinetics; (3) the airflow, characterized as a dispersed-plug flow, induced vertical gradients. Ten outputs were modelled: the biomass content, the slowly biodegradable organic matter content, the easily biodegradable organic matter content, the inert organic matter content, the dry matter content and the substrate moisture, the oxygen content, the carbon dioxide content and the water content of the gaseous phase. The model was validated on the basis of experimental results with a maximum 20 % deviation between the simulated results and the experimental ones. The proposed model represents a useful tool that can already be used to evaluate the duration of the treatment, the best aeration strategy or to search the best initial characteristics of a waste mixture. However to obtain a reliable simulation of an industrial process, further studies will have to be done, particularly concerning the heat and mass transfers at the industrial scale.