Methyl nitrate as a byproduct in advanced water treatment systems: Liquid chromatographic determination method and cause of formation.

Neutral low-molecular-weight organics such as methyl nitrate that can readily pass through reverse osmosis (RO) membranes employed in potable water reuse facilities attract interest owing to public health considerations. In this study, a novel determination method based on high-performance liquid chromatography, online photochemical conversion to peroxynitrite, and luminol chemiluminescence detection was developed for methyl nitrate measurement in treated water. The maximum photochemical conversion efficiency of methyl nitrate to peroxynitrite was found to be 6.5% using a 222-nm excimer lamp. The calibration curve for the developed method was linear between 1.0 × 10-9 and 1.0 × 10-7 M, and the limit of detection was 0.3 nM (0.03 µg/L) given an injection volume of 200 µL. The methyl nitrate concentrations in RO permeate from reclaimed wastewater and product water after subsequent treatment by a UV/H2O2 advanced oxidation process (AOP) were 2.2 and 22.5 nM (0.17 and 1.7 µg/L), respectively. UV irradiation of RO permeate in the laboratory using a low-pressure Hg lamp confirmed the formation of methyl nitrate in the permeate in the absence of H2O2 and residual chloramines. This chemiluminescent detection method for methyl nitrate will promote a greater understanding of the origin and formation of this treatment byproduct in reclaimed wastewater.

NDMA and NDMA precursor attenuation in environmental buffers prior to groundwater recharge for potable reuse.

Natural attenuation of N-nitrosodimethylamine (NDMA) and NDMA precursors was evaluated in infiltration basins, a riverbed filtration system, and constructed wetlands operated as part of a managed aquifer recharge system. Initial NDMA concentrations up to 9.0 ng/L in infiltration basins (advanced purified, recycled water) before sunrise declined to non-detect (<1.5 ng/L) by 10:00 A.M due to natural photolysis (half-life of 33 to 86 min dependent on solar irradiance). NDMA fortified controls adjacent to the infiltration basin showed similar results, while concentrations in dark controls did not change over the basin's hydraulic retention time. NDMA precursor concentrations did not change significantly in the basin containing advanced-treated water from a potable reuse treatment plant, indicating that photolysis did not remove NDMA precursors nor did photolysis produce a significant amount of precursors. For the other environmental buffers evaluated, NDMA removal was variable through laboratory scale soil columns (22 cm height), in full-scale riverbed filtration system that pre-filters water prior to infiltration basin recharge, and in the constructed wetland. Variability in NDMA removal through the wetlands is attributed to high turbidity. In the case of the riverbed filtration system, variability is likely due to short exposure times to sunlight. For the soil columns, limited NDMA removal is attributed to inefficacy of soil aquifer treatment in removing NDMA over short travel times/distances. NDMA precursors were also ineffectively removed in these systems, with effluent concentrations occasionally exceeding influent concentrations. Overall, the removal of NDMA in environmental buffers utilized for planned or de facto indirect potable reuse is dependent on the system's capacity for photolysis, while NDMA precursors are more recalcitrant and unlikely to be removed in such systems without enhancement or sufficient hydraulic residence times.

Influence of reverse osmosis membrane age on rejection of NDMA precursors and formation of NDMA in finished water after full advanced treatment for potable reuse.

The influence of reverse osmosis (RO) membrane age on rejection of N-nitrosodimethylamine (NDMA) precursors was evaluated for a full-scale potable water reuse facility. The rejection of NDMA precursors decreased slightly with increased membrane age in most RO membrane products evaluated, but remained high overall (91% average). Chloride rejection was well-correlated with rejection of NDMA precursors. Precursor removal varied (75-98%) by membrane product, with certain membrane products maintaining better precursor rejection over time. NDMA rejection, however, did not decline significantly over time, while passage of other low molecular weight organics (LMWOs) increased with membrane age. Thus, rejection of NDMA was not highly correlated with rejection of these LMWOs, suggesting that NDMA is not a good surrogate for these compounds. Incomplete removal of NDMA precursors by RO and a UV/advanced oxidation process (UV/AOP) led to NDMA formation in the finished water and miles downstream in the transmission pipelines. An average NDMA formation rate of 0.7 ng/L/hr in the transmission lines was observed, despite typical removal of NDMA by UV/AOP to non-detect levels. The study indicates that RO membranes throughout their lifetime are not an absolute barrier to NDMA precursors, and that while older membranes continue to sufficiently remove NDMA precursors to a high degree, NDMA precursor rejection may decrease slightly as membranes age. Thus, the potential exists for NDMA to form from these precursors in purified, potable reuse water after treatment despite the effective removal of NDMA by UV/AOP.

An inline ion-exchange system in a chemiluminescence-based analyzer for direct analysis of N-nitrosamines in treated wastewater.

A newly developed, ion exchange-based inline pretreatment system was used to mitigate the effect of background constituents in natural water and treated wastewater to achieve rapid, reliable, and sensitive analysis of N-nitrosamines. The pretreatment system (anion exchange module, AEM) was incorporated into a high-performance liquid chromatograph (HPLC) coupled with a photochemical reactor (PR) and chemiluminescence (CL) detector (HPLC-PR-CL), which can analyze four hydrophilic N-nitrosamines at ng/L levels. This system requires no pre-concentration of the water sample nor the use of deuterated surrogates, unlike other conventional N-nitrosamine analytical techniques. The AEM converted anions in the eluent to hydroxide ions after HPLC separation and increased eluent pH, allowing for the subsequent photochemical reactions, which are otherwise achieved by pH conditioning with an additional dosing pump of basic chemical. The AEM also removed anionic interfering compounds (e.g. nitrate) from the samples, allowing for improved N-nitrosamine analysis in treated wastewater. The operating conditions of the AEM and PR were optimized to obtain sensitive and stable analytical performance. As a result, the lowest-concentration minimum reporting levels of N-nitrosodimethylamine, N-nitrosomorpholine, N-nitrosomethylethylamine, and N- nitrosopyrrolidine using the optimized system were 0.42, 0.54, 0.58, and 1.4 ng/L, respectively. The improved analytical method was validated by comparing the results with a conventional method based on gas chromatography coupled with a mass spectrometric ion trap detector. These results indicated that HPLC-PR-CL equipped with an inline AEM can be competitively applied as a rapid analytical technique for the determination of N-nitrosamines in various water matrices.

Veterinary antibiotics used in animal agriculture as NDMA precursors.

The formation of carcinogenic N-nitrosodimethylamine (NDMA) during chloramination at drinking water treatment plants has raised concerns as more plants have switched from chlorine to chloramine disinfection. In this study, a source of NDMA precursors that has yet to be investigated was examined. Veterinary antibiotics are used in large quantities at animal agricultural operations. They may contaminate drinking water sources and may not be removed during wastewater and drinking water treatment. Ten antibiotics used in animal agriculture were shown to produce NDMA or N-nitrosodiethylamine (NDEA) during chloramination. Molar conversions ranged from 0.04 to 4.9 percent, with antibiotics containing more than one dimethylamine (DMA) functional group forming significantly more NDMA. The highest formation for most of the compounds was seen near pH 8.4, in a range of pH 6 to 11 that was investigated. The effect of chlorine-to-ammonia ratio (Cl2/NH3), temperature, and hold time varied for each chemical, suggesting that the effects of these parameters were compound-specific.

The effect of natural organic matter polarity and molecular weight on NDMA formation from two antibiotics containing dimethylamine functional groups.

N-nitrosodimethylamine (NDMA) is a disinfection byproduct preferentially formed in chloraminated water. NDMA may be formed from certain chemicals containing dimethylamine (DMA) functional groups. This reaction may be slowed by the presence of natural organic matter (NOM). In this study, NOM fractionated by size or polarity was tested for its ability to slow or impede the formation of NDMA from two DMA-containing precursors, the antibiotics tetracycline and spiramycin. The high molecular weight NOM fractions (>10KDa) were shown to be the most effective in reducing the amount of NDMA formed from the precursor chemicals. The filtrate of a C-18 non-polar cartridge was also effective at reducing NDMA formation from tetracycline (spyramycin not tested). Therefore, polar and charged NOM components may be responsible for the reduction in NDMA formation. A possible mechanism for the reduction of NDMA formation from tetracycline is complexation due to the hydrogen bonding of the DMA functional group on tetracycline to polar phenolic functional groups in the NOM.

NDMA formation during drinking water treatment: A multivariate analysis of factors influencing formation.

The formation of the carcinogen N-nitrosodimethylamine (NDMA) during drinking water treatment has raised concerns in the drinking water industry. Many bench-scale laboratory tests and pilot plant studies have been completed to try to determine which factors during water treatment increase or decrease the amount of NDMA formed in drinking water. This study used data from over 20 drinking water treatment plants in the United States and Canada to determine which factors are most highly correlated with the NDMA concentration in delivered water using a mixed effects model with a random intercept. This type of analysis has not been used previously with trihalomethane (THM) models due to the fact that those studies did not sample such a large number and range of plants as was done in this NDMA study. Ultraviolet absorbance at 254 nm (UV254) in the plant influent and pre-chlorination time used at the plant were highly correlated in all models with NDMA concentration in finished water as well as the percentage change between NDMA formation potential in the plant influent and actual formation in the finished water. Specifically, an increase in UV254 absorbance in a model was associated with an increase in NDMA and an increase in pre-chlorination time in a model was associated with a decrease in NDMA. Other water quality parameters including sucralose concentration in the plant influent, polyDADMAC polymer dose, pH, and chlorine-to-ammonia weight ratio used in the plant were also correlated with NDMA concentration in the distribution system. Lastly, NDMA precursor loading was correlated with the use of polyDADMAC (where precursors were added) and the use of ozone and granular activated carbon (GAC) treatment (where precursors were removed).