Impact of brominated amines onmonochloramine 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 organicmatter. 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 mu gBr(-)/L and 3 mgCl(2)/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 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). (C) 2017 Elsevier B.V. All rights reserved.