This report describes development of a time-dependent computerized model for composting of wastewater treatment plant sludge with forced aeration of the pile. The work was undertaken because, in the past, development of the composting process for wastewater sludge has been almost wholly experimental. The model is two-dimensional because piles are long compared to dimensions of the trapezoidal cross-section. The cross-sectional area of the pile is divided into rectangular and triangular areal increments. The pressure source for the forced aeration is along the longitudinal plane of symmetry at ground level. The air flow regime is established first using an iterative solution to LaPlace's equation. Mass flow rates for water vapor, oxygen, and enthalpy are computed around each areal increment. The biological decomposition rate is treated as a function of temperature and moisture content of each areal increment. Properties such as temperature, moisture content, and oxygen concentration are found for each areal increment at each time point. A time interval of 15 minutes was found to give satisfactory results. Physical properties of compost were derived. Measurements made on mechanically aerated piles by the Los Angeles County Sanitation Districts were used to test the validity of the model.