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

Monochloramine (NHCl), as the dominant disinfectant in drinking water chloramination, can provide long-term disinfection in distribution systems. However, NHCl can also be discharged into storm sewer systems and cause stormwater contamination through outdoor tap water uses. In storm sewer systems, NHCl dissipation can occur by three pathways: (i) auto-decomposition, (ii) chemical reaction with stormwater components, and (iii) biological dissipation. In this research, a field NHCl dissipation test was conducted with continuous tap water discharge into a storm sewer. The results showed a fast decrease of NHCl 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 NHCl decay and the field testing results, a kinetic model was developed. To describe the variation of the NHCl dissipation rates during the field testing, a time coefficient f was introduced, and the relationship between f and time was determined. After calibration through the f coefficient, the kinetic model described the field NHCl 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.