The coupled two-dimensional heat and oxygen diffusion in a compost pile of sewage sludge obtained from domestic wastewater treatment was studied. The unsteady, coupled, non-linear mathematical model, solved through the finite volume method, includes the volumetric heat generation caused by the action of aerobic bacteria and by the oxidation of cellulose in a porous media. The numerical simulation allows the prediction for the pile shape and size effects on the heat generated and on oxygen consumption. Temperature and oxygen concentration transient distributions within compost piles are presented for different geometries. Heat (temperature) and mass transfer (oxygen) results indicate that the pile height has an important effect on the heating process and that keeping the compost pile height lower than 1.7 m the self-combustion of sewage sludge can be avoided. (C) 2009 Elsevier Ltd. All rights reserved.