Chloramines in drinking water may form N-nitrosodimethylamine (NDMA). Various primary disinfectants can deactivate NDMA precursors prior to chloramination. However, they promote the formation of other byproducts. This study compared the reduction in NDMA formation due to chlorine, ozone, chlorine dioxide, and UV over oxidant exposures relevant to Giardia control coupled with postchloramination under conditions relevant to drinking water practice. Ten waters impacted by treated wastewater, poly(diallyldimethylammonium chloride) (polyDADMAC) polymer, or anion exchange resin were examined. Ozone reduced NDMA formation by 50% at exposures as low as 0.4 mg×min/L. A similar reduction in NDMA formation by chlorination required ∼60 mg×min/L exposure. However, for some waters, chlorination actually increased NDMA formation at lower exposures. Chlorine dioxide typically had limited efficacy regarding NDMA precursor destruction; moreover, it increased NDMA formation in some cases. UV decreased NDMA formation by ∼30% at fluences >500 mJ/cm(2), levels relevant to advanced oxidation. For the selected pretreatment oxidant exposures, concentrations of regulated trihalomethanes, haloacetic acids, bromate, and chlorite typically remained below current regulatory levels. Chloropicrin and trichloroacetaldehyde formation were increased by preozonation or medium pressure UV followed by postchloramination. Among preoxidants, ozone achieved the greatest reduction in NDMA formation at the lowest oxidant exposure associated with each disinfectant. Accordingly, preozonation may inhibit NDMA formation with minimal risk of promotion of other byproducts. Bromide >500 μg/L generally increased NDMA formation during chloramination. Higher temperatures increased NDMA precursor destruction by preoxidants but also increased NDMA formation during postchloramination. The net effect of these opposing trends on NDMA formation was water-specific.
SwitzerlandBench-scale chloramination under uniform formation conditions was used to examine N-nitrosodimethylamine (NDMA) formation in settled and (bio)filtered drinking water and treated wastewater. In this study, water temperature, pH, postchloramination time, and chlorineto-nitrogen (Cl 2 /N) weight ratio were varied to investigate NDMA formation in various water types. Wastewater and certain polymers were investigated as sources of NDMA precursors. Nitrified biofilters were found to be another precursor source. NDMA formation in nitrified biofilter effluent and polymer-impacted water was the highest at Cl 2 /N of 3-5; NDMA formation was the highest at Cl 2 /N of 5-7 in wastewater-impacted waters. Other tests at Cl 2 /N of 4.75 evaluated the impact of pH and temperature, with a 3 min prechlorination, which can result in some precursor abatement. At pH 7, NDMA formation increased with increasing temperature, whereas at pH 8 and 9, low temperature sometimes yielded higher NDMA formation because of lower precursor abatement during prechlorination and, thus, higher NDMA formation during postchloramination.
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