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

Abstract

© 2017 Elsevier B.V. In this study, a comprehensive kinetic model was developed and validated to predict the stability of monochloramine (NH 2 Cl) 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 NH 2 Cl in waters containing bromide. For in-line chloramination, the NH 2 Cl decay over 3days was only 10% for pH9 and 58% for pH7 (400µgBr - /L and 3 mgCl 2 /L). Bromide also greatly affected the stability of NH 2 Cl 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 NHBr 2 , and the auto-decomposition of NHBr 2 formed from NH 2 Br. Once NHBr 2 was consumed, NHBrCl accumulated and then slowly decayed. Thereafter, the total oxidant concentration decayed slowly due to the auto-decomposition of NHCl 2 and the reaction between NHBrCl and NHBr 2. In the presence of DOM, the CHBr 3 concentration increased, while the CHCl 3 concentration (formed during pre-chlorination) was constant during chloramination, indicating that brominated-amines may continue to form disinfection by-products (DBPs).