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

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 3days was only 10% for pH9 and 58% for pH7 (400μgBr-/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 4min, 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 pH7 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 NHBr2. 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).