I-THM Formation and Speciation: Preformed Monochloramine versus Prechlorination Followed by Ammonia Addition

Abstract

An increasing number of utilities in the United States have been switching from chlorination to chloramination practices to comply with the more stringent trihalomethane (THM) and haloacetic acid (HAA) regulations. This has important implications for disinfection byproduct (DBP) formation because the reactions of chlorine and monochloramine (NH(2)Cl) with natural organic matter (NOM) are not the same. In this study, iodinated trihalomethane (I-THM) formation from preformed NH(2)Cl and prechlorination (at two chlorine doses and contact times) followed by ammonia addition was compared. A representative bromide/iodide ratio of 10:1 was selected and four bromide/iodide levels (ambient, 50/5 or 100/10, 200/20, and 800/80 [μg/L/μg/L]) were evaluated. The results showed that I-THM formation was generally lower for prechlorination as compared to preformed NH(2)Cl due to the oxidation of iodide to iodate by chlorine. However, while prechlorination minimized iodoform (CHI(3)) formation, prechlorination sometimes formed more I-THMs as compared to preformed NH(2)Cl due to a large increase in the formation of brominated I-THM species, which were formed at much smaller amounts from preformed NH(2)Cl. I-THM concentrations and speciation for the two chloramination scenarios (i.e., preformed NH(2)Cl vs prechlorination followed by ammonia) depended on chlorine dose, contact time, bromide/iodide concentration, and NOM characteristics of the source water (SUVA(254)).