Chlorination in wastewater treatment is regarded as a complicated process due to its ammonia and nitrite content. Chlorine added to such systems reacts with ammonia undergoing episodes of complex reactions resulting in the chlorination breakpoint behavior. Most of the available chlorination mechanistic models are not easily applied which has restricted their practical utilization in treatment plants. In this study, a new mechanistic model for the chlorination breakpoint in ammonia-nitrite systems is suggested with a user-friendly interface and to be applicable with conditions occurring in wastewater treatment plants. The model was validated against laboratory studies reported in the literature and was also applied to forecast chlorine residual in a wastewater treatment plant in the region. The model simulated both experimental and field data reasonably well.