Disinfection by-products formation and precursors transformation during chlorination and chloramination of highly-polluted source water: Significance of ammonia

Abstract

Many studies have demonstrated the different trends of disinfection by-products (DBPs) formation between chlorination and chloramination. However, the reactions between precursors and disinfectants are widely assumed to be “black box” and the reasons for abovementioned difference are not well illustrated. This study focused on source water with high levels of natural organic matter (NOM) and bromide, and compared the transformation of NOM specific characteristics and the ratios of specific DBPs as an equivalent of chlorine to total organic halogen (TOX) among three disinfection scenarios of chlorination, chloramination and chlorine–chloramine sequential treatment (Cl2–NH2Cl process). A three-reaction-phrases model was proposed thereafter to illustrate the major reactions involved in, i.e., stage-I: rapid consumption of fast reactive sites (DOC1), which transformed to slow reactive sites (DOC2) and measured DBPs, i.e., trihalomethanes, haloacetic acids, etc; stage-II: oxidation and/or halogenation of DOC2 into unknown TOX (UTOX) intermediates; stage-III: oxidation of UTOX intermediates into measured DBPs. The effect of ammonia was also quantified. Ammonia is observed to inhibit the formation of measured DBPs by 68–92%, 94–99%, and 92–95% of that in chlorination in Stage-I, II, and III, respectively, and the formation of UTOX is reduced by 2–80%, 60–94%, and 82–93% accordingly. These effects lead to the steady accumulation of DBPs intermediates such as UTOX, and to the elevated UTOX/TOX during chloramination and Cl2–NH2Cl process thereafter. The results illustrate the mechanism of ammonia participating in DBPs formation, and are valuable to fill in the gap between the transformation of precursors and the formation of different DBPs.