Monochloramine dissipation in storm sewer systems: field testing and model development

Monochloramine (NH2Cl), as the dominant disinfectant in drinking water chloramination, can provide long-term disinfection in distribution systems. However, NH2Cl can also be discharged into storm sewer systems and cause stormwater contamination through outdoor tap water uses. In storm sewer systems, NH2Cl dissipation can occur by three pathways: (i) auto-decomposition, (ii) chemical reaction with stormwater components, and (iii) biological dissipation. In this research, a field NH2Cl dissipation test was conducted with continuous tap water discharge into a storm sewer. The results showed a fast decrease of NH2Cl concentration from the discharge point to the sampling point at the beginning of the discharge period, while the rate of decrease decreased as time passed. Based on the various pathways involved in NH2Cl decay and the field testing results, a kinetic model was developed. To describe the variation of the NH2Cl dissipation rates during the field testing, a time coefficient fT was introduced, and the relationship between fT and time was determined. After calibration through the fT coefficient, the kinetic model described the field NH2Cl dissipation process well. The model developed in this research can assist in the regulation of tap water outdoor discharge and contribute to the protection of the aquatic environment.