Removal of wastewater and polymer derived N-nitrosodimethylamine precursors with integrated use of chlorine and chlorine dioxide

In this study, the effects of five different pre-oxidation scenarios (i.e., individual, simultaneous, and sequential applications of chlorine dioxide [ClO₂] and chlorine [Cl₂]) on the removal of N-nitrosodimethylamine (NDMA) formation potential (FP) from different water matrices (i.e., non-impacted natural waters, wastewater [WW]-impacted, and polymer-impacted waters) with subsequent chloramination were investigated. Practically relevant doses of ClO₂ and Cl₂ were applied for all scenarios to avoid the formation of disinfection by-products (DBPs) at regulatory levels. The removal efficiency of NDMA FP for all the oxidation scenarios (individual or simultaneous) was <20% in non-impacted natural water samples. In 20% WW-impacted waters, pre-oxidation with ClO₂ at pH 7.8 resulted in a significant reduction in NDMA FP (56–73%), whereas pre-oxidation with Cl₂ showed less removals (40–50%). For the integrated oxidation scenarios (i.e., simultaneous or sequential application), NDMA FP removals further increased (20–45%), especially, at pH 6.0 compared to individual application of oxidants in WW-impacted waters. The formation of NDMA in pre-oxidized water samples also decreased significantly under uniform formation condition (UFC). In polymer-impacted waters, integrated applications of Cl₂ and ClO₂ significantly improved the deactivation of polymer-derived NDMA precursors independent of oxidation time (10 vs. 60 min) and pH (6.0 vs. 7.8) compared to individual application of these oxidants. In addition, chlorite (ClO₂⁻) formation was low and maintained well below 1 mg/L for integrated applications of Cl₂ and ClO₂, while chlorate (ClO₃⁻) formation increased significantly as compared to application of ClO₂ only.