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

Abstract

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 f_T was introduced, and the relationship between f_T and time was determined. After calibration through the f_T 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.