Impact of brominated amines on monochloramine stability during in-line and pre-formed chloramination assessed by kinetic modelling

Highlights: • Kinetic model predicts the stability of NH₂Cl in presence of iodide and bromide. • Kinetic model predicts the stability of NH₂Cl for both preformed and inline practice. • Bromide affects the stability of NH₂Cl by forming brominated-amines. • Loss of oxidant following NH₃ addition mainly due to reaction of NHBrCl and NHBr₂. In this study, a comprehensive kinetic model was developed and validated to predict the stability of monochloramine (NH₂Cl) in presence of iodide and bromide for both pre-formed and in-line chloramination application in absence of organic matter. pH had the greatest influence on the stability of NH₂Cl in waters containing bromide. For in-line chloramination, the NH₂Cl decay over 3 days was only 10% for pH 9 and 58% for pH 7 (400 μgBr⁻/L and 3 mgCl₂/L). Bromide also greatly affected the stability of NH₂Cl by influencing the formation and speciation of the halamines produced during chloramination. In-line chloramination is commonly used since the pre-chlorination oxidises iodide to the non-toxic iodate. During pre-chlorination, brominated organics are formed from reaction between bromine and dissolved organic matter (DOM). In the case of the Colorado River DOM, 26% of the bromine was sequestered in only 4 min, and therefore not available to form brominated amines during chloramination. Following ammonia addition, an immediate loss of oxidant was observed in water containing bromide at pH 7 and 8. This is due to the reaction between NHBrCl and NHBr₂, and the auto-decomposition of NHBr₂ formed from NH₂Br. Once NHBr₂ was consumed, NHBrCl accumulated and then slowly decayed. Thereafter, the total oxidant concentration decayed slowly due to the auto-decomposition of NHCl₂ and the reaction between NHBrCl and NHBr_2. In the presence of DOM, the CHBr₃ concentration increased, while the CHCl₃ concentration (formed during pre-chlorination) was constant during chloramination, indicating that brominated-amines may continue to form disinfection by-products (DBPs).