Abstract : Models’ emission and fate of odorous hydrogen sulphide gas in full-scale plants anoxic/anaerobic/oxic A2/O using a TOXCHEM V4.1 model was used in Karbala sewage treatment plants (STP). Monthly influent and effluent samples were taken during a one year to assess the performance of Karbala STP. pH, COD, BOD5, TSS, NO3, NO2, NH4+, SO42-, H2S, oil and grease, and PO4-3 measurements. Degradation and emissions were two most important processes across the system. Model calibration and validation was applied to ensure that all data was calibrated and validated within the predefined limits, with the root mean square error (RMSE) and R2 are 0.06 and 0.9095 respectively within acceptable boundaries results showing the model to be close to an ideal fit. At 30 °C in summer and 20 °C in winter, 25% and 23.4% of all such gas respectively was released into the atmosphere, 0.01 and 0.04% were discharged with wastewater, 0.11 and 0.13% were absorbed by sludge, and 74.8 and 76.3% were degraded. Total emitted concentration of H2S at Karbala STP was 4.918 ppm in summer and 4.6325 ppm in winter. A sensitivity analysis was also performed to develop a better understanding of H2S fate and emission. Overall, the TOXCHEM V4.1 model performed well with regard to predicting the emission and fate of H2S gas in an A2/O compare with actual data system, and this provides accurate data on the location of the noxious gas emissions, allowing make decisions at any plant to make the right decisions.