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 are 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 natural organic matter (NOM) and real water samples, as breakpoint chlorination produced more THMs for amino acids but less THMs for phenols. However, breakpoint chlorination caused 16.2−40.2% higher THALs formation than chlorination without ammonia for NOM and real water and consistently higher THALs formation for model compounds. The roles of trichloramine (NCl 3 ) 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. NOMs and amino acids formed more THMs and THALs when reacting with NCl 3 than with free chlorine, indicating a promotional effect of NCl 3 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. • Breakpoint chlorination formed more THALs than chlorination without ammonia. • •OH formed during breakpoint chlorination promoted TCM and TCAL formation. • NCl 3 produced more TCM and TCAL than free chlorine for NOM and amino acids. • Breakpoint chlorination preferred to promote brominated THALs formation.