N-nitrosodimethylamine (NDMA), a probable human carcinogen, is a disinfection by-product that has been detected in chloraminated drinking water systems. To reduce NDMA levels in distribution systems, removing its precursors prior to chloramination is essential. The goals of the present study were to examine the effect of commonly applied pre-oxidants (i.e., chlorine, chlorine dioxide and ozone), relevant to USEPA standards for Giardia control, on deactivation of selected NDMA precursors, and investigate the effect of pH on pre-oxidation reactions. In this study, 15 model compounds as NDMA precursors likely to occur in water sources were examined. Their NDMA yields ranged approximately from 0.1% to 90%. Pre-chlorination reduced NDMA formation from most precursors by 10% to 50% except quaternary amines (polymers). Pre-oxidation with chlorine dioxide and ozone achieved the same or higher deactivation of NDMA precursors (e.g., ranitidine). However, for some precursors (e.g., daminozid) pre-oxidation with chlorine dioxide or ozone led to increases in NDMA formation, indicating that certain types of oxidation products may account for enhanced NDMA formation. For example, the molar conversion of daminozid increased from 0.1% to 2% and 50% due to pre-oxidation with chlorine dioxide and ozone, respectively. Such increases during chlorine dioxide exposure were probably because of releasing oxidation products with dimethylamine (DMA) moiety which may form more NDMA upon chloramination than their parent compound. Furthermore, ozone-triggered increases could be related to direct NDMA formation from the precursor. On the other hand, hydroxyl radicals formed from the decomposition of ozone would be also involved in deactivation of precursors, reducing NDMA levels. During pre-oxidation, pH conditions influenced significantly the effectiveness of deactivation of precursors. The findings of this study indicate that chlorine, chlorine dioxide, and ozone can be used to control NDMA formation; however, their effectiveness depends on the types of precursors and pre-oxidation pH conditions.

Effect of pre-oxidation on NDMA precursors

248th National Meeting (2014)

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