The presence of ammonia causing breakpoint chlorination promoted the formation of trihaloacetaldehydes and trihalomethanes

The presence of ammonia can cause breakpoint chlorination when chlorination is applied for water disinfection. However, the formation of trihalomethanes (THMs) and trihaloacetaldehydes (THALs) which are typical disinfection byproducts during breakpoint chlorination is not fully understood. In this study, THMs and THALs formation during breakpoint chlorination was evaluated and compared with that during chlorination without ammonia. THMs formation was occasionally higher during breakpoint chlorination than during chlorination without ammonia for water containing natural organic matter (NOM), as breakpoint chlorination produced more THMs for amino acids but less THMs for phenols than chlorination without ammonia. However, breakpoint chlorination caused 16.2−40.2% higher THALs formation than chlorination without ammonia for NOM-containing samples and consistently higher THALs formation for model compounds. The roles of trichloramine (NCl₃) and hydroxyl radical (•OH) formed from chlorine-ammonia breakpoint reactions in THMs and THALs formation during breakpoint chlorination were evaluated. Scavenging •OH by tert-butanol reduced THALs formation by 11.5–27.6%, which was higher than the reduction of THMs formation, indicating a more important role of •OH in transforming THALs precursors than THMs precursors. NOM and amino acids formed more THMs and THALs when reacting with NCl₃ than with free chlorine, indicating a promotional effect of NCl₃ on THMs and THALs formation during breakpoint chlorination. The formation of THMs and THALs during breakpoint chlorination at various conditions confirmed that the presence of ammonia causing breakpoint chlorination could increase the formation of THMs and THALs and particularly the brominated species. This research expands the understanding of disinfection byproducts formation during breakpoint chlorination.