N-nitrosodimethylamine (NDMA), a probable human carcinogen, is a disinfection by-product that has been detected in chloraminated drinking water systems. Understanding its formation rate plays a key role in controlling NDMA levels in distribution systems. The main objectives of this study were to: (i) investigate the role of chloramine species in the formation of NDMA; (ii) examine the factors that affect chloramine decomposition (i.e., pH, sulfate and natural organic matter [NOM]); and (iii) validate applicability of those findings in selected natural waters. In this study, five amines were carefully selected based on their structure and exposed to varying levels of chloramine with different ratios of dichloramine. Amines that were sensitive to monochloramine (i.e., ranitidine) had relatively fast NDMA formation and reached the maximum yield within 24 hours. On the other hand, amines that were sensitive to dichloramine had relatively slow NDMA formation rates. Due to scarce availability of the dichloramine, these reactions were limited to decomposition of the monochloramine to dichloramine. During the NDMA formation from dichloramine sensitive amines, presence of NOM was found to decrease the NDMA formation rate by creating a competition for the dichloramine; however, presence of sulfate can work against this effect. In addition, pH was also found to influential as both chloramine speciation and protonation state of the amine depends on it. These factors showed distinct changes in dichloramine sensitive amines but less influential for monochloramine sensitive ranitidine. When selected natural waters were investigated some NDMA formation by monochloramine was recorded; however, dichloramine was found to be the dominant species responsible for NDMA formation. These findings indicate that NDMA formation is relatively slow due to chloramines decomposition, and it could be further formed in distribution systems as water age increases.

Chloramines species importance in NDMA formation

248th National Meeting (2014)

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