Effect of chlorine demand on the ammonia breakpoint curve: Model development, validation with nitrite, and application to municipal wastewater

Chlorine added during wastewater disinfection may be consumed through reactions with chlorine-demanding chemical species. In this study, a mechanistically based kinetic model for chlorine demand in the presence of ammonia was developed and validated with laboratory studies on ammonia-nitrite systems, and then applied to breakpoint curves obtained with wastewater samples. The model is a modification of kinetic models for chlorine-ammonia systems to include hypochlorous acid-demand and monochloramine-demand reactions. The model accurately describes both laboratory-generated breakpoint curves with added nitrite and literature data. In a plant thought to be undergoing partial nitrification, breakpoint curves were consistent with high chlorine demand (i.e., small initial slopes and large doses to achieve the total chlorine maximum and breakpoint). A simplified kinetic model was also developed. Chlorine demand calculated from the simplified model was similar to chlorine demand from plant data. The simplified model was used to generate operating guidelines to calculate chlorine doses needed to overcome demand from nitrite or other sources.