Breakpoint Chlorination Enhances the Disinfection of Amoeba Spores and Their Intraspore Bacteria

Breakpoint chlorination often occurs during the chlorination of ammonia-containing water for disinfection. This study first reports that breakpoint chlorination significantly enhances the inactivation efficiencies of model amoeba (Dictyostelium discoideum) and their intraspore bacteria (Burkholderia), compared with chlorination or chloramination, in simulated and real drinking water. The inactivation rates of amoeba spores and their intraspore bacteria by breakpoint chlorination were 5.59- and 5.67-log10 within 30 min, respectively, at 0.49 mg/L (as N) ammonia and 5 mg/L (as Cl2) chlorine with a chlorine-to-ammonia molar ratio of 2:1, whereas those by chlorination were 4.05- and 2.80-log10, respectively. The enhanced inactivation of amoeba spores appeared to be driven by the generation of reactive nitrogen species (RNS) during breakpoint chlorination, as determined by radical scavenger and oxygen-free tests, which resulted in the damage of spore coats. The endogenic reactive oxygen species was likely involved in the inactivation of intraspore bacteria by changing their cell membrane integrities. This study demonstrated that chlorine-resistant pathogens can be effectively inactivated by breakpoint chlorination.