Deactivation of wastewater-derived N-nitrosodimethylamine precursors with chlorine dioxide oxidation and the effect of pH

In this study, the effect of chlorine dioxide (ClO₂) oxidation on the deactivation of wastewater (WW)-derived N-nitrosodimethylamine (NDMA) precursors was investigated under various conditions (i.e., ClO₂ application pH, dose and contact time). At pH 6.0, decreases in NDMA formation potentials (FPs) or occurrences (under uniform formation conditions [UFC]) were relatively low (<25%) with ClO₂ oxidation regardless of WW-impact. A negative removal was also observed after ClO₂ oxidation in some of the non-impacted waters. However, NDMA FP removals were significant (up to ~85%) under the same oxidation conditions in WW-impacted waters at pH 7.8. This indicates that the majority of WW-derived NDMA precursors can be deactivated with ClO₂ oxidation above neutral pH. This was attributed to the better oxidative reaction of ClO₂ with amines that have lone pair electrons to be shared at higher oxidation pH conditions. In addition, relatively short oxidation periods with ClO₂ (i.e., ≤10 min) or low Ct (concentration × time, ~10 mg ∗ min/L) values were sufficient for the deactivation of WW-derived NDMA precursors. ClO₂ oxidation was effective in freshly WW-impacted waters. Natural attenuation processes (e.g., sorption, biodegradation, etc.) can change the reactivity of WW-derived NDMA precursors for oxidation with ClO₂. The effect of ClO₂ on the removal of THM precursors was low (<25%) and independent of oxidation conditions. Given the low formation of regulated THMs and HAAs, ClO₂ oxidation presents a viable option for the simultaneous control of NDMA and regulated DBP formation during water treatment, especially for utilities treating WW-impacted water sources.