Monochloramine loss mechanisms and dissolved organic matter characterization in stormwater.

Monochloramine (NHCl) is widely used for secondary disinfection by water utilities. However, Edmonton field stormwater sampling results have shown that NHCl, because of its long-lasting property, can cause stormwater contamination through outdoor potable water uses during the summer season. To protect water sources, it is important to understand NHCl dissipation mechanisms in stormwater. Natural organic matter (NOM) is the dominant species that contributes to NHCl decay in stormwater. In this research, it is proposed that NOM reacted with both NHCl and free chlorine through rapid and long-term reactions during NHCl dissipation. Based on this assumption, a kinetic model was developed and applied to estimate the NHCl decay in real stormwater samples, and the modeling results matched experimental data well under all the conditions. Further, the stormwater dissolved organic matter (SWDOM) collected from different neighborhoods was analyzed by Fourier transform infrared (FTIR) and fluorescence excitation-emission matrix (EEM) techniques. Humic substances were found to be dominant in SWDOM, and the samples from different neighborhoods had similar organic constituents. After reaction with excess NHCl, 25%-41% SWDOM fluorophores converted to inorganic components, while most of DOM remained in organic form. Humic substances as the major components in SWDON, are the dominant precursors of disinfection by-products in chloramination. Therefore, the potential reaction products of stormwater humic substances with NHCl should also be of concern. This research provided a useful method to estimate the NHCl dissipation in stormwater, and the methodology can also be applied for stormwater NHCl decay studies in other cities. Further, it is believed the SWDOM analysis in this research will contribute to future studies of NHCl NOM reaction mechanisms in both storm sewers and drinking water distribution systems.