Preoxidation Effectively Destroys N-Nitrosodimethylamine Precursors in Raw, Lime-Softened, and Recarbonated Surface Waters

Nitrosamines, including N-nitrosodimethylamine (NDMA), are disinfection by-products of concern for drinking water utilities that use chloramines as a disinfectant. In this study, batch experiments were performed to investigate the effectiveness of preoxidation with ozone, free chlorine, or both on removal of NDMA precursors in raw, lime-softened, and recarbonated water from a full-scale water treatment plant treating primarily Mississippi River water and in a tributary water sample before and after bench-scale lime softening. NDMA ‘‘precursor’’ concentrations were assessed indirectly by quantifying NDMA formed after addition of preformed chloramines using the uniform formation conditions (UFC) protocol. NDMA concentrations (NDMA_UFC) in the raw plant water were 8 ng/L in summer and 10 ng/L in winter and increased to 13–26 ng/L in lime-softened and recarbonated waters. NDMA_UFC in the raw tributary water was 33.7 ng/L. Preoxidation by ozone at doses of up to 5 mg/L effectively decreased NDMA_UFC concentrations in all water samples with and without hydrogen peroxide addition (hydrogen peroxide:ozone mass ratio of 0.5:1). The effectiveness of ozone at destroying ambient NDMA precursors was independent of water temperature and ozone dose. NDMA precursor destruction, however, increased with increasing ozone dose for a recarbonated water sample spiked with ranitidine (initial NDMA_UFC = 358 ng/L). Free chlorine at a dose of 5 mg/L as Cl₂ for 20 min of contact time (i.e., Ct *70 mg-min/L) was most effective at destroying ambient NDMA precursors in recarbonated water, and similarly, free chlorine treatment following ozone (2 mg/L) was more effective at destroying NDMA precursors in the ranitidine-spiked water than ozone alone. Preoxidation with ozone, free chlorine, or both proved to be an effective strategy for limiting NDMA formation upon subsequent chloramination for surface waters impacted by agricultural activities and municipal wastewater discharges.