Page 3 - Guidelines for Canadian Drinking Water Quality: Guideline Technical Document: N-Nitrosodimethylamine (NDMA)
Part II. Science and Technical Considerations
4.0 Identity, use, sources and fate in the environment
4.1 Identity, uses and sources in the environment
N-Nitrosodimethylamine (NDMA) is the simplest dialkylnitrosamine, with a molecular formula of C2H6N2O (ATSDR, 1989). NDMA is also known as dimethylnitrosoamine, N, N-dimethylnitrosoamine, N-methyl-N-nitrosomethanamine, N-nitroso-N, N-dimethylamine, DMN and DMNA. NDMA is a volatile, combustible, yellow, oily liquid. Its physicochemical properties are listed in Table 1.
|Property||ValueTable 1 footnote 1|
|Relative molecular mass||74.08|
|Vapour pressure||1080 Pa at 25°C|
|Log n-octanol/water partition coefficient (Kow)||0.57|
|Henry's law constant (Kaw)||3.34 Pa·m³/mol at 25°C|
NDMA is produced as a by-product of industrial processes that use nitrates or nitrites and amines under a range of pH conditions (ATSDR, 1989; IPCS, 2002). It is inadvertently formed when alkylamines, mainly dimethylamine (DMA) and trimethylamine, come into contact and react with nitrogen oxides, nitrous acid or nitrite salts or when transnitrosation via nitro or nitroso compounds occurs (ATSDR, 1989). Therefore, NDMA may be present in the environmental discharges of such industries as rubber manufacturing, leather tanning, pesticide manufacturing, food processing, foundries and dye manufacturing, as well as in effluent from sewage treatment plants. These discharges account for its presence in a number of media, including air, soil and water. Almost all of the releases are to water. NDMA can also occur in drinking water through the degradation of dimethylhydrazine (a component of rocket fuel) (Siddiqui and Atasi, 2001; Mitch et al., 2003b). In addition, NDMA has been detected in diesel vehicle exhaust emissions (Goff et al., 1980).
NDMA may form directly in sewage as a result of the biological and chemical transformation of alkylamines in the presence of nitrate or nitrite (Ayanaba and Alexander, 1974; ATSDR, 1989). It may also be released into the environment as the result of application of sewage sludge to soils rich in nitrate or nitrite. It has been demonstrated that disinfection of wastewater effluent by chlorine produce NDMA (Mitch and Sedlack, 2004). It may also be released into the environment as the result of application of sewage sludge to soils rich in nitrate or nitrite. The NDMA precursor, DMA, may enter surface water streams from agricultural runoff, as it has been detected in the faeces of dairy cattle (van Rheenan, 1962).
NDMA may also be formed during the treatment of drinking water (OME, 1994). Water treatment plants incorporating a chlorination or chloramination process in the presence of nitrogen-containing organic matter form NDMA as a disinfection by-product (Richardson, 2003). NDMA precursors such as DMA and trimethylamine may be present after drinking water treatment with nitrogen-based cationic polyelectrolytes (Wilczak et al., 2003) or ion exchange resins (Kimoto et al., 1980). For further details on NDMA's formation during disinfection processes, see Section 7.0.
NDMA may be released into the environment as a result of the use of certain pesticides contaminated with this compound (Pancholy, 1978). NDMA is present in various technical and commercial pesticides used in agriculture, hospitals and homes as the result of its formation during the manufacturing process and during storage. The following DMA formulation pesticides may contain NDMA as a microcontaminant: benazolin, bromacil, dicamba, 2,4-dichlorophenoxyacetic acid (2,4-D), mecoprop and 4-(2-methyl-4-chlorophenoxy)acetic acid (MCPA) (IPCS, 2002).
In the 1990s, in testing in Canada of over 100 samples of formulated pesticidal products (DMA salt of phenoxy acid herbicides) potentially contaminated by NDMA, the compound was detected in 49% of the samples with an average concentration of 0.44 µg/g; only six samples contained concentrations above 1.0 µg/g (1.02-2.32 µg/g). Concentrations of NDMA in pesticides have decreased over time. In 1994, approximately 1 million kilograms of DMAformulated phenoxy acid herbicides for commercial use were applied to the terrestrial environment in Canada (Moore, 1999). Based on the amount of DMA-formulated phenoxy acid herbicides applied in 1994, the average NDMA concentration of 0.44 µg/g and the 49% detection rate, it has been estimated that approximately 200 g of NDMA may have been released into the environment through the use of these herbicides (IPCS, 2002).
There are no industrial or commercial uses of NDMA in Canada or the United States. NDMA was used in Canada in the past, and may still be used in other countries, in rubber formulations, as a fire retardant and in the organic chemical industry as an intermediate, catalyst, antioxidant, additive for lubricants and softener of copolymers (ATSDR, 1989; Budavari et al., 1989).
4.2 Environmental fate
NDMA has a low vapour pressure (1080 Pa at 25°C). If emitted to, or formed in, air, it is not likely to adsorb to airborne particulate matter and is expected to be almost entirely in the vapour phase. In daylight, it degrades rapidly by direct photolysis to form dimethylnitramine. The photolytic half-life of NDMA vapour exposed to sunlight ranges between 0.5 and 1.0 h (Hanst et al., 1977). Half-lives for the reaction with hydroxyl radicals in air range from 25.4 to 254 h (Atkinson, 1985). Modelling of environmental partitioning used a half-life for NDMA in air of 5 h (Environment Canada and Health Canada, 2001). The short half-life for NDMA in air suggests that it is not persistent in this medium.
Since NDMA is miscible in water and has a low vapour pressure and a low n-octanol/water partition coefficient (log Kow of 0.57), it is not likely to bioaccumulate, adsorb to particulates or volatilize to any significant extent (Thomas, 1982; ATSDR, 1989; OME, 1991). Oxidation, hydrolysis, biotransformation and biodegradation are not significant factors affecting the fate of NDMA in lake water (Tate and Alexander, 1975). Photodegradation is the main process for removing NDMA from the aquatic environment. The efficiency of removal of NDMA depends on the characteristics of the particular water environment. Typically, photodegradation of NDMA is much slower in waters with high concentrations of organic substances and suspended solids than in clear water bodies. The rate of degradation through photolysis may be significantly decreased in the presence of interferences with light transmission, such as ice cover on receiving water bodies (IPCS, 2002; WHO, 2008). This observation is supported in the groundwater compartment, where, in the absence of light, NDMA has the potential to persist (OME, 1991). A half-life for NDMA in surface water at 25°C of 17 h was assumed by DMER and AEL (1996) in their modelling of environmental partitioning for Environment Canada and Health Canada (2001). Howard et al. (1991) reported a half-life range for NDMA in groundwater of 1008-8640 h, based on estimated unacclimated aqueous aerobic biodegradation.
On soil surfaces, photolysis and volatilization rapidly remove NDMA. Oliver (1979) reported that 30-80% of an unreported concentration of NDMA volatilized from the soil within the first few hours of application to the soil surface. As outlined by Haruta et al. (2008), NDMA rapidly disappears from soils receiving reclaimed wastewater, though microbial degradation and volatilization.
Once incorporated into subsurface soil, however, NDMA will be highly mobile, with the potential to migrate into groundwater supplies. Subsurface biodegradation is slightly slower under anaerobic conditions (ATSDR, 1989). Soil type only slightly affects the biodegradation of NDMA. Aeration of soil improved biodegradation compared with waterlogged soil. Pre-exposure of bacteria to NDMA increased biodegradation in soil (Mallik and Tesfai, 1981).
Canadians can be exposed to NDMA through its presence in food, water, and air. In addition, certain segments of the population may be exposed through the use of specific consumer products or in occupational settings.
Releases of NDMA into water in Canada have been measured primarily in Ontario and vary considerably. Evidence for this variation in NDMA levels is provided by a 1996 incident in which a chemical plant released wastewater containing NDMA into the St. Clair River at a concentration of 0.266 µg/L (Environment Canada, 1997). In April 1997, concentrations of NDMA in the wastewater at this same chemical plant varied considerably from the previous year's measurements, since NDMA levels at the point of release to surface water ranged from 0.096 to 0.224 µg/L. NDMA releases are expected to decrease, as the company installed a wastewater treatment plant in 1998 (IPCS, 2002).
In a 1990 survey of sewage treatment plant effluent in Ontario, NDMA was detected in 27 of 39 samples, with a maximum concentration of 0.22 µg/L (OME, 1991). In 390 samples of raw surface water from 101 water treatment plants sampled for NDMA in Ontario from 1990 to July 1998, concentrations were detectable (>0.001 µg/L) in the raw water at 37 plants. The average concentration in raw water was 1.27 × 10-3 µg/L (IPCS, 2002). The highest concentration of NDMA in raw water taken at two water treatment plants in 1996 was 0.008 µg/L (IPCS, 2002).
In 1990, concentrations of NDMA in 24 groundwater samples taken from various locations in Ontario were below detection limits (detection limits ranged from 0.001 to 0.010 µg/L) (OME, 1991). Concentrations of NDMA in the municipal aquifer in Elmira ranged from 1.3 to 2.9 µg/L and were attributed to contamination from a nearby chemical facility (Kornelsen et al., 1989). The municipal wells using this aquifer were closed in 1989 (Ireland, 1989).
In 313 samples of treated water analysed from 100 locations within Ontario between 1994 and 1996, NDMA was detected in at least one sample at 40 of these 100 sites at levels greater than the detection limit of 0.001 µg/L. The mean concentration was 0.0027 µg/L. The highest concentrations were measured in samples from drinking water treatment plants using a specific pre-blended polyamine/alum water treatment coagulant (OME, 1996). These included a concentration of 0.04 µg/L at the water treatment plant in Huntsville, Ontario. NDMA was detected in all 20 samples collected from four water treatment plants using the specific coagulant. The mean concentration of NDMA in these 20 samples was 0.012 µg/L, whereas the mean concentration in the remaining 293 samples for the locations where the specific coagulant was not used was 0.002 µg/L.
More recent data from Ontario's Drinking Water Surveillance Program report levels of NDMA in finished drinking water exceeding the province's standard of 0.009 µg/L in three cases between 1998 and 1999. These exceedances ranged from 0.012 to 0.027 µg/L (OME, 2009).
Recent surveys conducted in Canada at different drinking water treatment plants and distribution systems reported levels of NDMA ranging from < 1 to 12 ng/L, with one exception where levels were reported to exceed 100 ng/L in the distribution system (Charrois et al., 2004, Tugulea et al., 2008). Drinking water and source water samples were tested as part of the validation of the analytical methods developed by Charrois et al. (2004). One series of samples was from an Alberta city that used chloramination and UV disinfection. The source water had no detectable NDMA (detection limits ranged from 0.0004 to 0.0016 µg/L), but the finished water contained NDMA at 0.067 µg/L, and water in the distribution system contained 0.16 µg/L. Further investigation confirmed that distribution system samples had much higher levels of NDMA than the finished water at the treatment plant. There is a need for more studies that measure NDMA levels in the distribution systems of water treatment facilities using chloramine.
Another study by Charrois et al. (2007) sampled 23 locations in 20 distribution systems for NDMA in Alberta in 2004. The authors reported concentrations of NDMA in at least one location in each of 6 systems, ranging from below the method detection limit of 5 ng/L to 100 ng/L. In 5 of the 6 systems, chloramine was used as a secondary disinfectant or there was naturally occurring ammonia present in the source water.
In a 2009 Canadian survey of 33 plants (Tugulea et al., 2010), low concentrations of NDMA were detected at 3 plants. Concentrations of NDMA ranging from 1-2 ng/L were found in both the treated water and the distribution system samples of each plant. All 3 plants where NDMA was detected were using chloramine as a secondary disinfectant.
A survey of NDMA concentrations in drinking water systems in California was conducted by the California Department of Health Services in 2001 (DHS, 2002). In 3 of 20 chloraminated supplies, NDMA concentrations exceeded 0.01 µg/L, whereas all 8 supplies that used only free chlorine had levels below 0.005 µg/L.
The presence of NDMA in drinking water is usually associated with its formation during water treatment rather than with its presence in the source water. Factors affecting this formation include the nature and amount of precursor compounds and the disinfection strategy used. Chloramination is the secondary disinfection process that is most often associated with the formation of NDMA, the predominant species of nitrosamines formed during this process. The nitrosamine formation pathways relevant to drinking water disinfection are discussed in Section 7.0. Nitrosamine formation is expected to continue within chloraminated distribution systems.
There is little information on the presence or concentrations of NDMA in ambient (i.e., outdoor) air in Canada or elsewhere. Available Canadian data are limited to the province of Ontario, where short-term measurements have been taken in the immediate vicinity of potential point sources of discharge to the atmosphere, for comparison with background measurements from other urban locations.
No data on airborne concentrations at rural locations have been identified. At industrial and urban locations in Ontario in 1990, based on seven samples taken in five cities, concentrations of NDMA were all below the detection limit (detection limits ranged from 0.0034 to 0.0046 µg/m³) (OME, 1990). In surveys of ambient air conducted in the vicinity of a chemical production facility in Elmira, Ontario, in 1990, concentrations of NDMA ranged from not detected (detection limits ranged from 0.0029 to 0.0048 µg/m³) to 0.230 µg/m³ in 41 samples; concentrations in 20 of the 41 samples were at or above the detection limit (OME, 1990). The highest concentrations were measured within the perimeter of the production facility, and the maximum concentration measured beyond this perimeter was 0.079 µg/m³. Similar concentrations of NDMA were found in samples taken in the vicinity of an industrial site in Kitchener, Ontario, in 1992 (OME, 1992).
Available data indicate that levels of NDMA were elevated in indoor air contaminated with environmental tobacco smoke in the United States (Brunnemann and Hoffmann, 1978) and Austria (Stehlik et al., 1982; Klus et al., 1992). The maximum concentration of NDMA in environmental tobacco smoke-contaminated indoor air was 0.24 µg/m³, whereas NDMA was not detected (i.e., <0.003 µg/m³) when the indoor air of a residence of a non-smoker was sampled in the same manner (Brunnemann and Hoffmann, 1978). Concentrations of NDMA in environmental tobacco smoke-contaminated indoor air in these countries (United States and Austria) were generally between 0.01 and 0.1 µg/m³ (Health Canada, 1999).
NDMA can be formed during food processing, preservation or preparation from precursor compounds already present in, or added to, the specific food items. The foodstuffs that have been most commonly contaminated with NDMA can be classified into several broad groups (IPCS, 2002):
- foods preserved by the addition of nitrate or nitrite, such as cured meat products (in particular, bacon) and cheeses (as these methods of preservation introduce nitrosating species into the food);
- foods preserved by smoking, such as fish and meat products (as oxides of nitrogen in the smoke act as nitrosating agents);
- foods dried by combustion gases, such as malt, low-fat dried milk products and spices (as combustion gases can contain oxides of nitrogen);
- pickled and salt-preserved foods, particularly pickled vegetables (as microbial reduction of nitrate to nitrite occurs); and
- foods grown or stored under humid conditions, leading to nitrosamine formation by contaminating bacteria.
It should be noted, however, that most data on levels of NDMA in foodstuffs have been derived from studies conducted in the 1970s and 1980s and may not be reliable for estimating current exposure to this substance, because of the limited analytical methodology available at the time. Moreover, efforts have been made to reduce the potential for exposure to NDMA in foodstuffs in Canada and other countries through continued reduction of allowable nitrite levels during preservation, suspension of the use of nitrate for certain food groups and increased use of nitrosation inhibitors, such as ascorbate or erythorbate (Cassens, 1997; Sen and Baddoo, 1997).
Levels in Canadian foods in the late 1970s and early 1980s have been fully reviewed in IPCS (2002). NDMA concentrations in meat ranged from less than the detection limit of 0.1 µg/kg to 17.2 µg/kg; for various fish and seafood, the range was from <0.1 to 4.2 µg/kg. For cheese, the range was <1 µg/kg to a maximum of 68 µg/kg in a sample of wine cheese. NDMA was not detected in milk products, with the exception of skim milk powder, in which it was present at concentrations below 0.7 µg/kg (IPCS, 2002). A 1981 report (Sen and Seaman, 1981) indicated the presence of NDMA in powdered infant formulas containing skim milk powder, with levels ranging from trace amounts to 1 µg/kg in three of eight samples tested. NDMA was not detected in baby food, apple juice, ketchup, sauces, margarine or butter (IPCS, 2002). Cooked bacon was reported to contain as much as 17.2 µg/kg, but controls on the use of nitrate and nitrite are believed to have reduced NDMA levels in bacon. Malt beverages such as beer and whiskey contain NDMA, but, again, levels have been dropping. Canadian beer analysed in 1988-1989 had a mean NDMA concentration of only 0.10 µg/L. Concentrations in imported beer averaged 0.71 µg/L in 1991-1992 and 0.15 µg/L in 1994 (Sen et al., 1996).
In addition to the presence of NDMA in various food sources, NDMA can also form endogenously from the nitrosation of secondary amines found in various foods. This process involves the reaction of nitrite and nitrate in foods with stomach acid to form nitroso groups, which can then react with amines to form NDMA. Although the mechanisms of NDMA formation have been well studied, data are insufficient to assess the quantities formed endogenously in humans (Fristachi and Rice, 2007).
5.4 Consumer products
Exposure to NDMA can result from the use of consumer products. NDMA has been detected in a variety of personal care and cosmetic products (e.g., shampoos, hair conditioners and toners, bath and shower gels, creams and oils, face tonics, cleansers). This is likely due to the reaction of nitrosating agents such as nitrite or nitrogen oxides, which occur frequently in these products (Spiegelhalder and Preussmann, 1984), with amine-containing compounds used extensively in ingredients of personal care products (ECETOC, 1990).
Rubber-containing products that come into contact with human skin are another potential source of exposure to NDMA, since dialkylamines used by rubber manufacturers as accelerators and stabilizers during rubber vulcanization can react with nitrosating agents during processing to form nitrosamines (Biaudet et al., 1997). NDMA has been detected in a diverse selection of workplace, consumer and medical products containing rubber, including rubber bottle nipples (Health Canada, 1999; Fristachi and Rice, 2007).
Lastly, the nitrosation of natural constituents of tobacco during curing and fermentation results in the formation of three major classes of N-nitroso compounds in tobacco and tobacco products: volatile, non-volatile and tobacco-specific N-nitrosamines (Hoffmann et al., 1984; Tricker et al., 1991). In addition, the combustion of cigarette tobacco results in the pyrolytic formation of volatile N-nitrosamines, including NDMA (Tricker and Preussmann, 1992). The yields of these volatile N-nitrosamines in cigarette smoke from combustion of tobacco depend on many chemical and physical parameters, including the amounts of organic nitrogen and nitrate present. Furthermore, nicotine serves as a specific precursor for the formation of NDMA (Hoffmann et al., 1987).
5.5 Contribution of drinking water to total exposure
Canadian data for environmental media, used to estimate population exposure to NDMA, are limited in both spatial and temporal scope. In a worst-case estimation of exposure to NDMA in contaminated air, water and food, the daily intake of a 20- to 59-year-old was approximated to be 0.005-0.016 µg/kg bw per day (Environment Canada and Health Canada, 2001). Daily intake of NDMA from ingestion of drinking water for the same age group was estimated at 0.0003-0.001 µg/kg bw per day (Environment Canada and Health Canada, 2001), based on a mean NDMA concentration of 0.012 µg/L and a maximum concentration of 0.04 µg/L obtained from 20 samples from four water treatment plants using a pre-blended polyamine/alum product during the treatment process (OMEE, 1996). Comparing the low- and high-end values for daily intake of NDMA from ingestion of drinking water the total exposure to NDMA in contaminated air, water and food indicates that human exposure to NDMA via drinking water is very low.
In a detailed study that examined NDMA exposure from a number of different sources, The authors estimated the proportional oral intake of NDMA attributable to drinking water over a 75-year lifetime to be less than 1% (Fristachi and Rice, 2007). Although the concentrations of NDMA in foods are low and hence exposure from foods is relatively low, foods were estimated to be a much more significant source of exposure to NDMA than was drinking water (Fristachi and Rice, 2007). However, it should be noted that higher concentrations, as observed in contaminated groundwater, could result in significantly higher exposures via drinking water (OEHHA, 2006).
5.6 Multi-route exposure through drinking water
In order to assess the potential exposure to NDMA via inhalation and dermal absorption resulting from activities such as showering and bathing, the relative contribution of each exposure route was assessed through a multi-route exposure assessment. This approach was derived from physiologically based pharmacokinetic modelling (Krishnan, 2004). Both the dermal and inhalation routes of exposure for a volatile organic chemical are considered significant if they contribute to at least 10% of the drinking water consumption level.
5.6.1 Dermal exposure
To determine whether dermal exposure represents a significant route of exposure for NDMA, tier 1 of the multi-route exposure assessment determines whether or not this route of exposure contributes a minimum of 10% of the drinking water consumption level (i.e., 10% of 1.5 L = 0.15 L). In order for a chemical to contribute at least 0.15 litre-equivalents (Leq), the skin permeability coefficient (Kp) for the chemical must be greater than 0.024 cm/h. The Kp for NDMA can be calculated using the following formula, described in Krishnan (2004):
- MW is the molecular weight of NDMA (74.08);
- log Kow is the log n-octanol/water partition coefficient (0.57).
Since the skin permeability coefficient for NDMA is greater than 0.024 cm/h, dermal absorption is considered to be significant during showering and bathing. As such, it is necessary to perform a tier 2 calculation to determine what value of L-eq is needed to account for dermal exposure.
The L-eq for dermal exposure to NDMA in drinking water can be calculated using the following formula, described in Krishnan, 2004:
This L-eq should be considered in calculation of the MAC, and the value of 1.5 L/d normally considered for ingestion of drinking water becomes 1.9 L/d for total exposure from drinking water.
5.6.2 Inhalation exposure
A two-tier assessment was also used to evaluate the inhalation route of exposure. Similar to the approach used for dermal exposure, tier 1 of the assessment determines whether the inhalation of NDMA during bathing or showering is likely to contribute at least 10% of the drinking water consumption level. For a tier 1 goal of 0.15 L-eq, the air to water NDMA concentration (Fair:water) value should be greater than 0.000 89. Using Henry's law constant (Kaw), the Fair:water value for NDMA was determined using the following equation (Krishnan, 2004):
- Kaw is the unitless Henry's law constant of 3.3 × 10-5 at 25°C;
- 0.61 is 61% transfer efficiency (McKone and Knezovich, 1991);
- 80.25 is the ratio of the volume of air in an average bathroom (6420 L) to the average volume of water (80 L) used during the showering/bathing event (Krishnan, 2004).
Since the Fair:water value is less than 0.000 89, exposure to NDMA via inhalation from bathing or showering is not considered to be significant. Tier 2 of the assessment, which calculates the volume of water (in L-eq) to account for inhalation exposure, is not needed (Krishnan, 2004).
6.0 Analytical methods
The U.S. Environmental Protection Agency (EPA) has approved one method, EPA Method 521, for the analysis of NDMA and has identified it as the method to use under the Unregulated Contaminants Monitoring Rule 2. EPA Method 521 uses solid-phase extraction with capillary column gas chromatography and chemical ionization tandem mass spectrometry. The method lists a detection limit of 0.28 ng/L and a lowest concentration minimum reporting level of 1.6 ng/L (U.S. EPA, 2004).
Standard Methods for the Examination of Water and Wastewater has one method for the analysis of NDMA under Method 6410B, which uses liquid-liquid extraction with gas chromatography/mass spectrometry (APHA et al., 2005). However, this method does not identify any method detection limit.
Other methods have been published with different extraction and concentration methods, such as liquid-liquid extraction and solid-phase extraction (Cheng et al., 2006). A number of studies are currently being conducted to improve the detection of NDMA and other nitrosamines; however, none is currently approved for routine monitoring.
7.0 Treatment considerations
The presence of NDMA in drinking water is usually associated with its formation during water treatment rather than with its presence in the source water. NDMA formation in water is largely a function of the nature and amount of precursor compounds and the disinfection strategy used.
7.1 NDMA formation during disinfection
The formation of NDMA is primarily associated with chloramination. Research has identified three nitrosamine formation pathways relevant to drinking water disinfection. For the first two pathways, the organic nitrogen precursors (i.e., DMA and trimethylamine) are likely to be similar, although DMA and trimethylamine concentrations in source waters are generally insufficient to account for NDMA formation (Gerecke and Sedlak, 2003; Mitch and Sedlak, 2004; Mitch and Schreiber, 2008).
In the first pathway, chloramination has been associated with the formation of nitrosamine (Mitch et al., 2003a), possibly involving the reaction of dichloramine with organic nitrogen precursors over the course of days (Schreiber and Mitch, 2006a). Although monochloramine predominates in chloraminated systems, low concentrations of dichloramine generally co-occur. Organic nitrogen precursors are typically associated with wastewater effluents, and utilities employing wastewater-impacted source waters may be at particular risk (Schreiber and Mitch, 2006b). Organic nitrogen precursors are also produced as a consequence of the general oxidation of NOM such as humic substances (Chen and Valentine, 2007). Industrial emissions of nitrosamines may be a concern for these water supplies (Sedlak et al., 2005). Other precursors of importance to drinking water systems include quaternary amine-based cationic coagulation polymers (Wilczak et al., 2003) and anion exchange resins (Najm and Trussell, 2001).
In the second pathway, chlorination in the presence of nitrite can rapidly form nitrosamines (Choi and Valentine, 2003; Schreiber and Mitch, 2007). This pathway is likely to be less important, as free chlorine and nitrite do not co-occur in significant concentrations in drinking water plants, but it could be more of a concern in the distribution system.
The third pathway for NDMA formation involves ozonation of the degradation products of the fungicide tolylfluanide, as documented by Schmidt and Brauch (2008).
There is little research investigating NDMA formation within distribution systems. In the case of NDMA formation through the first pathway, nitrosamine formation is expected to continue within chloraminated distribution systems because of the slow reaction time associated with dichloramine. It has been shown that NDMA levels in the distribution system increased with increased distribution residence time (Barrett et al., 2003; Wilczak et al., 2003; Charrois and Hrudey, 2007).
Breakpoint chlorination and free chlorine contact time have significant implications in the formation of NDMA. In a bench-scale experiment, Charrois and Hrudey (2007) reported a 93% reduction of NDMA formation to an effluent level of 3 ng/L when allowed a free chlorine contact time of 2 h prior to chloramination, compared with no free chlorine contact time. In a laboratory study of secondary disinfection, breakpoint chlorination reduced NDMA formation. Chlorination at a Cl:N molar ratio of 0.5 produced concentrations of NDMA greater than 200 ng/L, compared with negligible levels of NDMA when chlorinating at a Cl:N molar ratio of 4 (Schreiber and Mitch, 2005). When ammonia is already present in the source water, precursor deactivation can be achieved with free chlorine by applying a chlorine dose that exceeds the breakpoint (Schreiber and Mitch, 2007).
7.2 Preventing NDMA formation
Existing treatment facilities and processes should be optimized to reduce the formation of disinfection by-products, including NDMA, without compromising the effectiveness of disinfection. Strategies to prevent NDMA formation during disinfection focus on the removal of its most important precursors, the organic nitrogen precursors (i.e., DMA and trimethylamine) and dichloramine.
Application of strong oxidants, including free chlorine (Schreiber and Mitch, 2005; Charrois and Hrudey, 2007), chlorine dioxide or ozone (Lee et al., 2007a), upstream of chloramination can deactivate organic precursors. In a laboratory study with both synthetic and natural water, oxidation by either ozone or chlorine dioxide showed reduction of NDMA formation potential (Lee et al., 2007a).
As stated previously, some treatment processes may result in the formation of NDMA. To minimize NDMA formation, drinking water utilities should pay special attention when selecting polyelectrolyte coagulants and ion exchange resins and should minimize the use of quaternary amine-based coagulation polymers (Wilczak et al., 2003).
7.3 Treatment technology
NDMA is the nitrosamine species predominantly formed during chloramination, but it can also be a by-product of chlorination. In laboratory- and full-scale tests, the levels of other nitrosamines formed (N-nitrosoethylmethylamine, N-nitrosodiethylamine) were one or two orders of magnitude lower than the concentrations of NDMA (Sacher et al., 2008). More research on the toxicity and treatment characteristics of the other nitrosamines is required.
In order to reduce NDMA levels in the finished water, it is important to study the influence of treatment on the formation of NDMA and other disinfection by-products. In particular, the treatment study (including pilot testing) should assess the disinfection strategy for its potential to form disinfection by-products. This assessment will help ensure that the treatment strategy implemented minimizes the formation of all potential disinfection by-products.
7.3.1 Municipal scale
The most commonly used treatment method for the reduction of already formed NDMA is photolysis by ultraviolet radiation (UV) (Mitch et al., 2003a). NDMA can be removed by activated carbon adsorption, reverse osmosis, ozone oxidation or biodegradation or by the advanced oxidation process (AOP) of UV with hydrogen peroxide (UV/H2O2). (Siddiqui and Atasi, 2001; Mitch et al., 2003a), although these methods are not highly efficient. NDMA is biodegradable; however, the rate of degradation is in the order of days, which makes it impractical for drinking water treatment processes. Sharpless and Linden (2003) and Liang et al. (2003) concluded that the addition of hydrogen peroxide as an AOP is of limited economic benefit when it is used only for NDMA removal.
18.104.22.168 UV irradiation and advanced oxidation
The most common process currently used for NDMA reduction is UV irradiation. The UV dose required for 90% reduction of NDMA is about 1000 mJ/cm², approximately 10 times higher than that required for virus inactivation (Mitch et al., 2003a). NDMA reduction using UV irradiation is technically feasible but expensive, and it may be difficult for smaller utilities.
The principal by-products of UV photolysis of NDMA are DMA and nitrite (Bolton and Stefan, 2000; Mitch et al., 2003a). When UV/ H2O2 is applied, nitrate is the major degradation product, and the concentration of DMA is significantly lower than with direct photolysis (Bolton and Stefan, 2000).
A study conducted to compare the ability of low- and medium-pressure mercury UV lamps to degrade NDMA in "synthetic" drinking water spiked at a concentration of 75 µg/L demonstrated that both types of lamps gave effective degradation with similar efficiencies (Sharpless and Linden, 2003). The addition of hydrogen peroxide at 100 mg/L gave a 30% increase in the degradation rate for the low-pressure lamp but did not improve the efficiency of the medium-pressure lamp.
A study by Lee et al. (2005) demonstrated that irradiation with a 13 W low-pressure mercury lamp required approximately 15 min for complete degradation of a 7.5 µg/L NDMA solution at pH 7; a 750 µg/L solution required 5 h. The subsequent analysis showed that the degradation products from this treatment process were methylamine and DMA, and their relative concentrations depended on the reaction conditions.
Liang et al. (2003) conducted a bench-scale study to determine the effectiveness of pulsed UV in oxidizing NDMA, as pulsed UV can deliver higher UV intensities compared with continuous-wave UV technology. High removal rates were observed with pulsed UV, but the viability of such systems needs to be further studied.
AOP studies for degradation of NDMA were very dependent on the hydroxyl scavenging rates of the natural waters. In experiments with ozone and hydrogen peroxide, Lee et al. (2007b) observed NDMA oxidation of 55% and 78% with ozone doses of 160 µM and 320 µM, respectively, with an ozone to H2O2 ratio of 2:1, pH of 7.9 and an initial concentration of 1 µM (74 µg/L) NDMA. Ozone doses of 40 and 160 µM provided NDMA reductions of 10% and 25%, respectively, at pH 7 with an initial NDMA concentration of 1 µM.
A bench scale study (Zhao et al., 2008) suggests that UV degradation or AOP treatment may provide a source of precursors that can form NDMA during subsequent chlorination steps. The study also indicates that because of the different pattern of NDMA formation, the natural organic matter and anthropogenic organic contaminants of different source waters may affect NDMA removal. Although UV and AOP can reduce concentration of NDMA in water, the selection of that treatment option will require pilot study/consideration regarding the potential formation of NDMA subsequent to chlorination steps.
22.214.171.124 Reverse osmosis and adsorption
NDMA is poorly removed by reverse osmosis (RO). A laboratory-scale study using three different membranes resulted in rejection rates of 54%, 61% and 70% (Steinle-Darling et al., 2007). This study also indicated that additional coating and/or foulant accumulation can be significant and, depending on conditions, can increase or decrease rejection. This underlines the importance of assessing membranes individually through pilot testing.
NDMA sorbs poorly to soil, activated carbon or other sorbents. Various laboratory tests conducted to compare the effectiveness of various carbonaceous adsorbents and zeolites for NDMA reduction (Fleming et al., 1996; Zhu et al., 2001; Kommineni et al., 2003) found these adsorbents to be largely ineffective.
7.3.2 Residential scale
Generally, it is not necessary to use drinking water treatment devices with municipally treated water. Where consumers choose to use a device, it is important to note that Health Canada does not recommend specific brands of drinking water treatment devices, but it strongly recommends that consumers look for a mark or label indicating that the device has been certified by an accredited certification body as meeting the appropriate NSF/American National Standards Institute (ANSI) standards. These standards have been designed to safeguard drinking water by helping to ensure the material safety and performance of products that come into contact with drinking water. Meeting these standards ensures that a drinking water treatment device does not introduce additional contaminants into the drinking water.
Certification organizations provide assurance that a product conforms to applicable standards and must be accredited by the Standards Council of Canada (SCC). In Canada, the following organizations have been accredited by the SCC to certify treatment devices and materials as meeting NSF/ANSI standards:
- Canadian Standards Association International;
- NSF International;
- Water Quality Association;
- Underwriters Laboratories Inc.;
- Quality Auditing Institute; and
- International Association of Plumbing and Mechanical Officials.
An up-to-date list of accredited certification organizations can be obtained from the Standards Council of Canada.
NSF International (NSF) has developed several standards for residential water treatment devices designed to reduce the concentrations of various types of contaminants in drinking water, but NDMA is not currently included in any NSF standard. Research is ongoing in the private and public sectors to test and adopt efficient methods for the reduction of NDMA in drinking water. At this time, there are no residential-scale treatment devices specifically for NDMA reduction. Reverse osmosis filtration, although not very efficient, can provide some reduction. Because NDMA reduction is dependent on membrane type, testing of various RO filters should be done to select the most appropriate system for the water being treated. Products that use RO technology can lose removal capacity through usage and time and should be maintained or replaced as per the manufacturer's recommendations. Although UV is a technology that can reduce NDMA levels, residential UV treatment units do not operate at high enough doses to reduce NDMA concentrations in water.
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