||N-Nitrosamines are a group of emerging nitrogenous disinfection byproducts (N-DBPs) that have been frequently detected in drinking water and recycled wastewater worldwide. The observed concentrations of nitrosamines in many drinking water treatment plants (DWTPs) and wastewater treatment plants (WWTPs) typically fall within the trace levels, increasing the difficulties of their detection and quantification. Nitrosamines have been of great concern in these years due to their potent carcinogenesis. Of these nitrosamine species, N-nitrosodimethylamine (NDMA) is the one of the most substantial concern in preceding researches. It has been reported that potential precursors of nitrosamines include secondary, tertiary, and quaternary amines, pesticides or herbicides (e.g., diuron), amine-based polymer (e.g., poly-diallyldimethylammoniumchloride, PolyDADMAC), pharmaceuticals and personal care products (PPCPs, e.g., a pharmaceutical named ranitidine or nizatidine that inhibiting gastric acid secretion). However, the presence of nitrosamines in DWTPs and WWTPs cannot be fully explained by the currently known precursors and associated formation mechanisms. While it is less likely to minimize the formation of nitrosamine by removing the possible precursors, fully investigating and understanding the formation and fate of nitrosamines through DWTPs and WWTPs may represent another important alternative to control the environmental hazards and human health risks posed by the presence of nitrosamines in treated water.|
Six nitrosamines, including N-nitrosodimethylamine (NDMA), N-nitrosodimethylethylamine (NMEA), N-nitrosodiethylamine (NDEA), N-nitroso-di-n-propylamine (NDPA), N-nitrosodi-n-butylamine (NDBA), and N-nitrosopyrrolidine (NPYR) are of interest and investigated in this study. The objective of this study breaks into four parts: First, to collect and analyze the nitrosamines in raw water and effluents from various drinking water treatment processes; Second, to understand and quantify the formation of six nitrosamines in drinking water treatment processes and to discuss the associated effects of treatment technologies, notably the oxidation technologies, on the fate of nitrosamines in the DWTPs; Third, to conduct lab-scale batch experiments with addition of selected NDMA precursors to simulate preoxidation in the selected DWTPs, providing insights into the complex array of the relationships amongst potential precursors, oxidation technologies, and raw water characteristics; Fourth, to assess the carcinogenic risks posed by the concentrations of nitrosamines observed in the selected DWTPs and simulated preoxidation experiments during the previous three stages of this study, understanding the current and future possible health risks to downstream users with respect to the formation of nitrosamines in drinking water under different circumstances discussed in this study.
Nitrosamines in water were pretreated by solid-phase extraction (SPE), followed by analysis with gas chromatography coupled with mass spectrometry (GC/MS) or high pressure liquid chromatography coupled with triple quadruple mass spectrometry (HPLC/MS/MS). The experimental results observed in this study were categorized as following:
1. Source water and effluent from different treatment technologies, especially oxidation, from three major DWTPs in southern Taiwan were selected for sampling and analysis of nitrosamines. The sampling was conducted in 2013 and 2014 to investigate the fate of six nitrosamines among different technologies including preoxidation, coagulation, sedimentation, filtration, biological activated carbon (BAC) and post-disinfection. In the results, the presence of nitrosamines was observed in the source water. Nitrosamines were limitedly treated through all three DWTPs. While preoxidation is the important process to enhance the formation of nitrosamines, the use of ozone for preoxidation may reduce the formation of nitrosamines in water. However, different strategies such as using chorine may be more applicable to minimize the reactions producing nitrosamines if multiple oxidation processes were employed through the drinking water treatment processes. The effect of BAC varied with different nitrosamines. NDMA, NMEA, NDEA and NDBA represent the more important species to be considered in DWTPs selected in this study.
2. The lab-scale batch experiments using source water from the DWTPs of interest in this study were conducted, with dimethylamine (DMA), diuron, and ranitidine, three known and commonly used NDMA model precursors, being added to simulate the pre-chlorination and pre-ozonation in conventional DWTPs. It was shown in the results that other unknown precursors existed in the source water. More importantly, the effects of supplying these precursors during simulated preoxidation varied with different oxidation technologies and the extents of nitrosamine formation were less than expected. It appeared that the matrix amongst nitrosamine precursors, oxidation technologies, and source water characteristics were complicated and simultaneously determine the formation of six nitrosamines during preoxidation.
3. Finally, the concentrations of six nitrosamines observed in different circumstances in the first two stages of this study, including different drinking water treatment processes and lab-scale batch simulation experiments, were applied to calculate the corresponding human health risks to downstream users by ingestion of drinking water and were discussed in terms of the average incremental cancer risk and annual increasing incidence. In the results, the conclusions drawn from the cancer risk assessment were slightly different from those when the concentrations of nitrosamines formed or reduced were of concern. Preoxidation seemed to decrease the cancer risks posed by nitrosamines in water under certain circumstances, whereas the concentrations of most nitrosamines were increased during the processes. Different carcinogenicities among nitrosamine species was the possible explanation. Even though several nitrosamines were present or formed after preoxidation, the excess risks posed by the presence of these carcinogens may be limited as long as the nitrosamines with potent carcinogenicities (e.g., higher cancer slope factors) were treated or replaced by those less potent species.
As reducing and treating the presence of potential precursors in source water may represent another important but difficult approach to control the contaminations of drinking water with respect to the existence of carcinogenic nitrosamines, the focuses of this study were to investigate the formation of nitrosamines through various treatment technologies in three major DWTPs and to discuss the relationships amongst nitrosamine precursors, preoxidation technologies, and source water characteristics by conducting the simulated preoxidation experiments. The results of this study presented a comprehensive understanding regarding the current pollutions of nitrosamines in source water, associated fate through conventional DWTPs in southern Taiwan, importance of optimizing the preoxidation technologies to minimize the formation of nitrosamines in water and more importantly, possible human health risks posed by the presence of nitrosamines in treated water. These findings are expected to provide substantial information to develop appropriate management strategies, which may effectively control the environmental hazards and public health risks by these emerging oxidation/disinfection byproducts in the future.