In Saudi Arabia, the Riyadh Sewage Treatment Plant (RSTP) uses the activated sludge technology as the secondary treatment process for sewage. Due to the complex nature of the process, a rather simplified, yet practical, steady state model that captures the most important features of the RSTP was developed. Actual operating and design conditions were obtained from RSTP data bank. The monthly average plant data obtained in 1997 was used to calibrate the model by adjusting four parameters: mmax,H, mmax,A, bH and bA (seasonal variation of temperature are therefore embedded within these values) A computer program was developed to solve the resulting model equations. The predictive nature of the proposed model was verified (without further tuning) using five sets of plant data collected in 2003. Model predictions were found to be in excellent agreement with the plant data (within±5%). Simulation results revealed the sensitivity of model predictions to the values parameters.