IHSS&IWA26 / BRNO / CZECHIA / 23–28 August 2026 BOOK OF ABSTRACTS 274 Poster Session / Technologies and Applications Hall A P5.11 Enhanced Nitrosamine Formation in PreChlorination with Post-Amination Schemes Yi-Hsueh Chuang1, Yen-Pao Chiang2 1 Institute of Environmental Engineering, National Yang Ming Chiao Tung University, Taiwan. 1001 University Rd., Hsinchu City, Taiwan 30010. yhchuang@nycu.edu.tw 2 Institute of Environmental Engineering, National Yang Ming Chiao Tung University, Taiwan Chloramination are widely practiced to balance microbial inactivation with the control of regulated chlorinated disinfection byproducts. In situ chloramination is achieved by reacting chlorine (as hypochlorite) with ammonia at an approximately 1:1 Cl2:NH4+ molar ratio to produce monochloramine (NH2Cl). In full-scale practice, chlorine and ammonia are typically dosed at separate locations such that an initial free-chlorine (HOCl) contact period precedes conversion to NH2Cl. Because ammonia is frequently present in drinking-water sources, pre-chlorination often requires chlorine doses exceeding the breakpoint, leaving NCl3 and free chlorine coexisting in treated water. When ammonia is subsequently added for chloramination, the system contains a mixture of NCl3 and NH2Cl. Previous work has indicated that NCl3–NH2Cl interactions can enhance nitrosamine formation when precursors are present [1], suggesting that sequential prechlorination followed by post-amination may promote nitrosamine formation, which is however a phenomenon that remains poorly understood. In this study, we systematically investigated the NCl3–NH2Cl reaction and its role in precursor activation and N-nitrosodimethylamine (NDMA) formation, using N-chloro-dimethylamine (Cl-DMA) as a model precursor. Chloramine stock solutions were prepared freshly, and experiments were conducted in 40-mL glass vials with minimum headspace at room temperature (25±2 °C). Samples were quenched with stoichiometric amount of ascorbic acid prior to solvent extraction. The solvent extracts were analyzed using a GC-MS for NDMA concentrations. Using Cl-DMA as a model precursor, we found that pre-chlorination followed by post-amination consistently increased NDMA yields, despite pre-chlorination or chloramination alone generating negligible NDMA. This enhancement was attributed to reactions occurring when excess ammonia is introduced into waters containing residual HOCl and accumulated NCl3. Batch experiments showed that NCl3 reacts with NH2Cl to produce HOCl and N2 as the major products, with a second-order rate constant of 2.8±0.3 M-1s-1 at pH 7. The HOCl generated subsequently converts NH2Cl to NHCl2, enabling the much faster NHCl2–NCl3 pathway that produces nitrosyl-chloride intermediates
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