Ozonation in Water Reuse: Formation and Mitigation of N-~nitrosodimethylamine

Formation of N-nitrosodimethylamine (NDMA) is a substantial concern for drinking water and water reuse. NDMA, a probable human carcinogen, is formed when water is disinfected with chloramines and ozone. This research focused on three issues regarding NDMA formation and mitigation. The first issue involved understanding the compounds (i.e., precursors) present in water and wastewater that react with ozone to form NDMA. Model precursors were identified and molar yields for NDMA formation were determined. The model precursors form high amounts of NDMA with ozone, but form very little NDMA with chloramines, which means there are two distinct groups of NDMA precursors: ozone-reactive and chloramine-reactive. An investigation into factors that affect NDMA formation resulted in understanding that bromide enhances NDMA formation for some precursors and elimination of hydroxyl radicals, which are produced during ozonation, leads to higher NDMA formation. Comparison of three oxidants, molecular ozone, hydroxyl radicals and dissolved oxygen, revealed that molecular ozone is the agent responsible for NDMA formation. The second issue addressed the strategic use of disinfection oxidants, alone and in combination, to minimize disinfection byproduct (DBP) formation. This study compared the formation and reduction of NDMA and two groups of regulated drinking water DBPs (trihalomethanes (THMs) and haloacetic acids (HAAs)) in treated wastewaters using seven disinfection treatment schemes. The top two treatment schemes resulting in the lowest total DBP formation, after converting concentrations to an equivalent unit based on drinking water risk, were ozonation and ozonation-chloramination. Both treatment schemes also exhibit several advantages for application in water reuse situations. It was demonstrated that pre-chlorination can reduce NDMA formation by inactivating ozone-reactive NDMA precursors, but DBP trade-offs must always be addressed because chlorination causes THM and HAA formation. The third issue investigated non-optimized biofiltration to mitigate NDMA formation by removing NDMA precursors prior to disinfection with ozone or chloramines. NDMA precursor removal (ranitidine (RNTD), daminozide (DMZD), 2-furaldehyde dimethylhydrazone (2-F-DMH) and 1,1,1\u27,1\u27-tetramethyl-4,4\u27-(methylene-di-p-phenylene)disemicarbazide (TMDS)) and DBP formation potential (NDMA, THMs, HAAs) in treated wastewater were assessed before and after biofiltration using three anthracite-containing columns with different contact times. Precursor removal varied (RNTD: 6-7%; DMZD: 73-85%; 2-F-DMH: 15-27%; TMDS: 11-24%) and was correlated to dissolved oxygen concentration or correlated to contact time for some precursors. The investigated wastewater was phosphorus-limited and had low dissolved oxygen. NDMA, THM, and HAA precursor removal may be increased through optimization of the biofilter media and the nutrients available for bacteria growth