A two-dimensional finite difference numerical model of composting was developed based on a two-component first-order kinetic model and heat and mass balance equations. Data for validation of the numerical model used results from four different pilot-scale composting systems built to investigate these aeration strategies. Results of temperature profiles for the simulations showed strong correlation with experimental data for layers 2, 3, and 4 (inner layers) in the system but indicated a need to strengthen the model's simulation of boundary condition effects (layers I and 5). Based on the simulations, one-directional airflow yielded the highest temperature gradients during composting, whereas air recirculation and air recirculation with reversed-direction airflow had the smallest temperature gradients. Model prediction of moisture content was highly accurate for the middle layers since they were less affected by boundary condition effects. Other layers were affected by boundary conditions and the physical phenomenon occurring in the headspace of the reactor Reversing the direction of airflow with and without air recirculation reduced moisture gradients. Moisture retention was increased with reversed-direction airflow. As a result of gradients in the process variables, decomposition gradients existed. The decomposition gradients were highly variable in the experimental studies and were not highly correlated with the simulation results.