NDERSTANDING BREAKPOINT CHLORINATION, CHLORINE/CHLORAMINE RESIDUALS ANALYTICAL METHODS IN HIGH pH WATERS

摘要

In natural waters containing ammonia, disinfection often requires breakpoint chlorination to establish a free chlorine residual. The breakpoint curve plots chlorine residual with changing chlorine to ammonia ratios. The breakpoint is the dose of chlorine which is required to consume or overcome any ammonia present in the water, and begin to establish residual free chlorine, which is required to meet disinfection requirements. In waters with elevated pH, however, the reactions between chlorine species are altered, and breakpoint chlorination chemistry is more difficult to predict. In addition, some methods of measuring free chlorine residual are subject to interferences from chloramines, and understanding when a “true” free chlorine residual is established is critical to effective disinfection. Two different treated waters were studied in the laboratory, with breakpoint curves conducted at varying pH targets (pH 9.0 – pH 10). Chlorine residuals were speciated to measure total residual chlorine, free chlorine, monochloramine, dichloramine, trichloramine, and organochloramines. The data was plotted to observe the change in chloramine speciation with varying chlorine doses and chlorine to ammonia ratios. In addition, four different methods were used to monitor the free chlorine residual: 1. Amperometric Titration (SM 4500-Cl D) – Phenylarsine oxide titration w/electrode 2. DPD Method (SM 4500-Cl G) – Colorimetric N,N,-diethyl-p-phenylenediamine 3. Indophenol Method (HACH 10171) – Cyanoferrate catalyst & substituted phenol 4. FACTS Method (SM 4500-Cl H) – Free Available Chlorine Test utilizing syringaldazine The different methods for measuring free chlorine residual were compared via statistical evaluation to determine the variation between the different methods. In distribution systems with elevated pH, breakpoint curves are often atypical, with changed distribution in chloramine species. Gaining an understanding of these chemical interactions is critical for designing and optimizing disinfection processes.