Improving the operational performance of wastewater treatment plants can be effectively approached by means of model simulation. GPS-X model was used in this study. Calibration and validation of the model were carried out, with various sensitive parameters subject to modification, with results found within the prescribed parameters for R and RMSE. Sensitivity analysis then indicated that the most important factor for reducing nitrogen and phosphorous concentrations was the readily biodegradable fraction; thus, the IR, RAS ratio, DO, and WAS flows were reduced from 3% to 1%, from 100 to 20%, from 3.5 to 2 mg/L, and from 3,500 to 1,000 m3/d, respectively, producing an optimization that saved 688.4 Kw.h in energy and gave a sludge reduction of 32%. These results showed that an IR percentage of 3% is not appropriate. Decreased rbCOD thus necessitates a chemical upgrade, which was implemented in this case by means of adding an external carbon source, represented by acetic acid, propionic acid, methanol, and glycerol, with good results. These additions led to improvements in terms of reduced TN and TP by suitable ratios. The best external carbon source was thus determined to be methanol, while glycerol was less effective than the others. The process of pre-denitrification was compared with the post-denitrification process by means of the addition of methanol as an external carbon source, which gave good results for the reduction of TN in the post-denitrification process, by up to 80%; however, the effect on other pollutants was to increase concentrations.