Assessing the Effect of Pre-Oxidation at Different Stages of Water Treatment to Limit N-Nitrosodimethylamine (NDMA) Formation

Abstract

N-Nitrosodimethylamine (NDMA) is a disinfection by-product of emerging concern for drinking water utilities that use chloramines as a disinfectant. Identifying and eliminating the sources of NDMA precursors in watersheds may prove to be too difficult in most cases. Hence, pre-oxidation with ozone, ozone/hydrogen peroxide (so-called advanced oxidation), or free chlorine was explored for precursor destruction to limit subsequent NDMA formation following addition of chloramines.In this study, batch experiments were performed to investigate the effectiveness of pre-oxidation on precursor removal when applied at three stages in a drinking water treatment train: raw water (pH 7.9-8.4), lime-softened water (pH 10.6-10.8), and lime-softened water after re-carbonation (i.e., re-carb) to pH 8.6-8.9. NDMA precursor concentrations were assessed indirectly by quantifying NDMA formed after addition of preformed chloramines and incubating for three days at room temperature using the uniform formation conditions (UFC) protocol. Some water samples were spiked with ranitidine before pre-oxidation to add a well-known NDMA precursor compound to the otherwise unknown ambient precursor pool. Residual ozone and hydroxyl radical formation were monitored during the batch experiments. Total ozone exposure and subsequent hydroxyl radical exposure from ozone decay increased with increasing ozone dose. Ambient NDMA precursor concentrations (NDMAUFC) in the raw water ranged from 8 to 10 ng/L and increased to 13 to 26 ng/L after lime softening. In general, pre-oxidation via ozone, advanced oxidation, and free chlorine effectively limited NDMA formation to ≤ 5 ng/L for both winter and summer water quality conditions. The temperature at which pre-oxidation occurred had little effect on precursor destruction for a given ozone dose. Pre-oxidation with ozone also was able to significantly decrease NDMAUFC for water samples spiked with 500 ng/L ranitidine, from 350 ng/L of NDMAUFC before pre-oxidation to 21 ng/L for an ozone dose of 1 mg/L, with further decreases with increasing ozone dose. This study provides critical information on NDMA precursor destruction via pre-oxidation with three common oxidants over a range of water quality conditions (i.e., pH and temperature).