Page 8: Guidelines for Canadian Drinking Water Quality: Guideline Technical Document – Vinyl Chloride
Part II. Science and Technical Considerations (continued)
Vinyl chloride is typically found in groundwater in the vicinity of landfills and where there have been spills of vinyl chloride or chlorinated precursor compounds. As such, it is important to characterize these groundwater sources in order to select the appropriate treatment for removal of vinyl chloride. The selection of an appropriate treatment process will depend on many factors, including the characteristics of the raw water supply and the source and concentration of vinyl chloride.
Municipal water filtration plants that rely on conventional treatment techniques (coagulation, sedimentation, filtration and chlorination) are generally ineffective in reducing concentrations of VOCs, such as vinyl chloride, in drinking water (Love and Eilers, 1982; Love et al., 1983; Lykins and Clark, 1994). Two common treatment technologies reported to be effective for the reduction of most VOCs in drinking water are aeration and adsorption.
Vinyl chloride is a gas under ambient environmental conditions. It has a relatively low solubility in water and a low capacity to adsorb to particulate matter and sediment. Volatilization is the most rapid process for the removal of vinyl chloride from surface waters (IPCS, 1999). Its high Henry's Law constant (2.8 kPa·m3/mol at 25°C) suggests that vinyl chloride would be very amenable to removal by air stripping (U.S. EPA, 1985a; Crittenden et al., 1988; Haarhoff and Cleasby, 1990). The U.S. EPA has identified packed tower aeration (PTA) as the best available technology (BAT) for vinyl chloride removal from drinking water and considers a 99.9% reduction to be achievable under all anticipated conditions (U.S. EPA, 1985a, 2009c). WHO (2011) states that vinyl chloride levels of 1 µg/L are achievable using air stripping.
Small system compliance technologies for the removal of vinyl chloride in drinking water include granular activated carbon (GAC), PTA, diffused aeration, multistage bubble aeration, tray aeration and shallow tray aeration (U.S. EPA, 1998, 2009c). The PTA process is effective, but the equipment is relatively expensive to build and maintain, and it may not be appropriate for small water treatment utilities.
Vinyl chloride has been reported as a degradation product of the microbial reductive dechlorination of chlorinated hydrocarbons such as tetrachloroethene and trichloroethene (IPCS, 1999; Bradley, 2000).
Vinyl chloride is primarily of concern as a potential contaminant from some older grades of PVC pipes, mainly manufactured prior to 1977 (Flournoy et al., 1999; Carroll and Eckstein, 2001; Beardsley and Adams, 2003; MDNR, 2006).
Air stripping treatment technology is commonly used to reduce the concentration of VOCs in drinking water (Cummins and Westrick, 1990; U.S. EPA, 1991; Dyksen, 2005). An air stripping process brings water and air into contact, allowing the transfer of volatile contaminant from the water to the air, as the driving force of the process is the contaminant concentration gradient between the two phases.
A variety of configurations exist with respect to air stripping equipment; however, PTA provides an optimum system for the removal of VOCs from water, as it allows for greater air to water ratios compared with other aeration systems. In the PTA column, the contaminated water flows downward by gravity over a bed of packing material, while the air is introduced into the tower below the packed bed and flows countercurrent to the water flow. Several factors affect the rate of stripping of the VOCs from water: the air to water ratio; the available area of mass transfer; the hydraulic loading rate; the temperature of the water and air; and the physical and chemical properties of the contaminant (AWWA, 1991; Crittenden et al., 2005; Dyksen, 2005). As the PTA transfers VOCs from water to air, treatment of the stripping tower off-gas may be necessary to reduce the contaminant concentrations prior to discharge into the atmosphere (Crittenden et al., 1988; Adams and Clark, 1991).
A common operating problem is the scaling and fouling of the packing material of the column. The main causes of fouling are calcium carbonate or calcium sulphate scale, iron oxidation and microbial growth. Methods to prevent the fouling of the column include pH suppression of the PTA influent, the use of scale inhibitors or iron removal prior to the PTA application (ESE, 1984; Dyksen, 2005). Algal growth can also be a problem in locations where light could be introduced into the tower. Post-treatment processes, such as the use of a corrosion inhibitor, may also be required to reduce the corrosive properties of the water as a result of increased dissolved oxygen from the aeration process. Environmental conditions, such as water temperature, may have an impact on the packed tower performance. Contact between the water and the air in PTA column leads the air temperature typically to approach the water temperature. The temperature influences both the Henry's Law constant and the rate of mass transfer. These parameters affect the size of the column and the efficiency of removal of the VOCs (Crittenden et al., 2005).
A study using a full-scale PTA system indicated that an average influent vinyl chloride concentration of 8.4 µg/L can be reduced to below 0.3 µg/L using an air to water ratio of 61, a design hydraulic loading rate of 29.8 gpm/ft2 (20.1 kg/m2/s), an air stripper length of 7.5 meters, and a packed column diameter of 2.4 meters (Hand et al., 1988).
U.S. EPA pilot studies conducted at more than 30 water treatment sites showed that PTA achieved > 99% VOC removals. As vinyl chloride is more easily removed by aeration than are other VOCs included in the studies, the U.S. EPA concluded that PTA systems designed using reasonable engineering practices could achieve 99.9% removal of vinyl chloride (U.S EPA, 2009c). Pilot-scale data have demonstrated removal efficiencies of 99.9% and 99.5% using air to water ratios of 20 and 5, respectively. No information was provided on the influent concentrations (ESE, 1984).
After an evaluation of the cost of PTA technology for the control of selected organic compounds in drinking water, Adams and Clark (1991) indicated that the cost-effective PTA design parameters for the reduction of vinyl chloride include an air to water ratio of 5 and a packing depth of 6.9 m. Under these estimated conditions, a 99% reduction of vinyl chloride could be achieved, from an influent level of 100 µg/L to a level of 1 µg/L in the finished water.
Spray aeration and air-lift pumping are identified as alternative air stripping treatment technologies for the reduction of vinyl chloride in drinking water, achieving > 99.0% and 97.0% reductions, respectively (U.S. EPA, 1985b).
The U.S. EPA identified GAC as an effective technology for the removal of VOCs from drinking water, with the exception of vinyl chloride. Study reports showed that the adsorption of vinyl chloride by GAC is a sporadic and erratic process and that vinyl chloride cannot be effectively removed (U.S. EPA, 1987a, 1987b; Hand et al., 1988 Lykins and Clark, 1994). A carbon adsorption isotherm for vinyl chloride has not been reported in the literature.
A pilot-scale study reported an erratic pattern for the treatment of vinyl chloride by GAC adsorption (U.S. EPA, 1987b). Data from the study demonstrated that a series of four 30-inch (0.76 m) GAC columns was capable of reducing vinyl chloride concentrations of up to 19 µg/L to a concentration below 0.5 µg/L in the finished water, achieving 810, 1250, 2760 and 2050 bed volumes for an empty bed contact time of 6, 12, 19 and 25 minutes, respectively.
Adams and Clark (1991) estimated cost-effective design parameters for liquid-phase GAC treatment of VOCs, including vinyl chloride, in drinking water. The estimated carbon usage rate to reduce an influent vinyl chloride concentration of 100 µg/L to a finished water concentration of 1 µg/L was 2.94 lb/1000 gallons (0.35 kg/m3), with an empty bed contact time of 30 minutes and a bed life of 28 days.
GAC adsorption with a combination of aeration technologies could be extremely effective for producing water with low VOC concentrations in the finished water (Robeck and Love, 1983; McKinnon and Dyksen, 1984). The aeration step reduces the organic load to the adsorbent and may remove compounds competing for adsorption sites (Stenzel and Gupta, 1985). In addition, this step can significantly extend carbon bed life (Hess et al., 1981a; McKinnon and Dyksen, 1984).
The U.S. EPA (1998) identified GAC as a small system compliance technology for the removal of vinyl chloride in drinking water. GAC is the most widely used technology for small systems due to the general ease of use, practicality and affordability. Generally, small systems use closed GAC systems which have a higher treatment efficacy.
Ozonation and advanced oxidation processes (AOPs) have been reported to be effective for the reduction of vinyl chloride concentrations in drinking water, although data from full-scale systems were not available for these treatment methods (Lykins and Clark, 1994; Schwammlein and Leitzke, 1995; Zhong et al., 2002). AOPs refer to the use of appropriate combinations of ultraviolet (UV) light, chemical oxidants and catalysts to generate highly reactive radicals, such as hydroxyl radicals, which are strong oxidants and react rapidly and non-selectively with organic contaminants. Physical and chemical properties of the water to be treated, such as alkalinity, pH, natural organic matter, iron and manganese, and turbidity, have a major impact on AOPs, as they scavenge hydroxyl radicals or absorb UV light used to produce the radicals (Crittenden et al., 2005).
In pilot plant ozone (O3) tests, removal efficiency for vinyl chloride in groundwater in the range of 70-100% was observed with O3 doses of 2-6 mg/L. Concentrations in the influent and in the finished water were not identified (Clark et al., 1988). Other pilot-scale tests conducted with O3/hydrogen peroxide (H2O2 ), UV/ O3 and UV/ H2O2 showed that all three processes achieved greater than 97% removal of vinyl chloride in groundwater under a variety of operating conditions. The results demonstrated reduction of an influent concentration in the range of 4-25 µg/L to a concentration below 0.5 µg/L (Schwammlein and Leitzke, 1995).
The performance of AOPs for the treatment of contaminated groundwater with chlorinated ethenes, including vinyl chloride, has been reported in non-peer-reviewed paper. The pilot-scale application of ozone/hydrogen peroxide treatment was reported to be effective for the treatment of an influent vinyl chloride concentration of 900 µg/L to levels below 0.5 µg/L in the finished water with an ozone dose of 32.2 mg/L (Bowman, 2003).
The formation of by-products, including disinfection by-products, from the oxidation and/or advanced oxidation of vinyl chloride or other inorganic or organic compounds in the source water should be considered when using these processes. The types and concentrations of by-products formed will be dependent on the source water quality, concentrations of the oxidants and the reaction contact time. Dowideit and von Sontag (1998) and von Gunten (2003) identified hydroxymethylhydroperoxide, formyl chloride and formic peracid as the by-products formed during ozonation of vinyl chloride in water. Another drawback of the use of AOPs such as ozone/hydrogen peroxide and ozone/ultraviolet is the formation of bromate in waters containing bromide ion (Fronk et al., 1987; Crittenden et al., 2005). The presence of by-products may require additional treatment following AOPs and/or process optimization to minimize by-product formation.
New experimental technologies have been shown to have potential for removing VOCs, including vinyl chloride, but there is not sufficient information available at present to fully evaluate the technologies:
- Catalytic hydrodehalogenation: Catalytic hydrodehalogenation processes (replacement of halide by hydrogen), using hydrogen gas and palladium catalyst supported on alumina or GAC, have been evaluated for the treatment of chlorinated ethenes in contaminated water (Marques et al., 1994; Shreir and Reinhard, 1995). In a laboratory experiment using 0.5 g of 0.5% palladium on alumina and 0.1 atm H2, vinyl chloride at an initial concentration of 1 mg/L in spiked tap water was completely hydrodehalogenated within 10 minutes at room temperature. The presence of nitrite, nitrate and sulphate ions caused a minor decrease in the rate of the hydrodehalogenation reaction. The presence of sodium bisulphide resulted in deactivation of the palladium catalyst (Shreir and Reinhard, 1995).
- Pervaporation: Although the use of membranes for the pervaporation extraction of VOCs has been applied primarily in wastewater treatment, this technique also has been studied for the removal of VOCs from groundwater (Jian and Pintauro, 1997; Uragami et al., 2001; Peng et al., 2003). Pervaporation is a membrane process in which a liquid stream containing VOCs is placed in contact with one side of an organophilic polymer membrane while a vacuum or gas purge is applied to the other side. The contaminants in the liquid stream adsorb into the membrane, diffuse through it and evaporate into the vapour phase.
In the polymerization process, some of the vinyl chloride monomers are retained in the product matrix as a residue and may be released into the air or water.
The migration of vinyl chloride monomer from PVC pipes, mainly manufactured prior to 1977, was reported to be a possible source of vinyl chloride in drinking water (Flournoy et al., 1999; Carroll and Eckstein, 2001; Beardsley and Adams, 2003; MDNR, 2006). A study examining PVC pipes from the dead-end water distribution lines installed between 1960 and 1977 reported vinyl chloride levels in the range from non-detectable to 9.4 µg/L, the maximum being 25 µg/L (MDL of 0.1 µg/L) (Beardsley and Adams, 2003). As stated in section 5.1.1, plastic pipes and components used for potable water applications must now comply with NSF/ANSI Standard 61 which ensures minimal leaching of vinyl chloride into drinking water (CSA, 2009; NSF, 2012; NRCC, 2010).
Several studies have investigated the factors affecting the rate of leaching and the accumulation of vinyl chloride from PVC and CPVC piping used in drinking water applications (Al-Malack et al., 2000; Al-Malack and Sheikheldin, 2001; Al-Malack, 2004; Richardson and Edwards, 2009; Walter et al., 2011). Static laboratory experiments conducted with PVC reactors (PVC locally manufactured in Saudi Arabia) indicated that vinyl chloride at 2.5 µg/L accumulated in extracted (double-distilled) water after 30 days of exposure to 45°C; vinyl chloride at 2.3 µg/L accumulated in extracted water after 14 days of exposure to direct UV radiation at an intensity of 218 mW/cm2 and a temperature of 25°C (Al-Malack, 2004); vinyl chloride at > 2.5 µg/L was detected in extracted water after 30 days of exposure to direct solar radiation (Al-Malack and Sheikheldin, 2001); and vinyl chloride at concentrations up to 2.1 µg/L was detected in both reactors with groundwater (total dissolved solids 2670 mg/L) and chlorinated drinking water (total dissolved solids 160 mg/L) after 17 days of exposure to 45°C. The MDL for all experiments was 0.6 µg/L.
Laboratory experiments compared the leaching rate and the accumulation of vinyl chloride, in drinking water from static PVC and CPVC fragment/segment reactors and from in-use consumer water lines (Richardson and Edwards, 2009; Walter et al., 2011). The pipes were provided by different U.S. manufacturers and were certified to NSF International/American National Standards Institute (NSF/ANSI) Standard 61 for potable water applications. A 2-year laboratory study using static new schedule 40 PVC segment reactors filled with chlorinated tap water reported vinyl chloride concentrations below the detection limit until day 63 (limit of quantification [LOQ] of 0.095 µg/L). An equilibrium concentration of approximately 0.3 µg/L was reported on day 715 (LOQ of 0.006 µg/L; Walter et al., 2011).
In a short-term (4 days) laboratory study, all new pipes (schedule 40 PVC, schedule 80 CPVC and SDR11 CPVC) provided by different manufacturers showed detectable levels of vinyl chloride in chlorinated tap water in the range of 0.011-0.025 µg/L at day 4 of the experiments (LOQ of 0.006 µg/L). The authors reported no statistically significant differences in vinyl chloride levels between the types of piping (PVC vs. CPVC). However, variations in vinyl chloride levels were seen between different pipe manufacturers (Walter et al., 2011).
In order to investigate the effect of pipe age on leaching rate, Richardson and Edwards (2009) conducted laboratory experiments with new and aged PVC/CPVC segment reactors (schedule 40 PVC, schedule 80 CPVC and schedule 80 PVC). Laboratory experiments were conducted on new PVC/CPVC segment reactors with no biofilm or with biofilm artificially grown for 11 months, both present and subsequently removed. Based on the graphical representation of the experimental data, on day 7 of the experiment, reactors with artificially grown biofilm had estimated vinyl chloride levels of 0.009 µg/L in the PVC reactors and 0.007 µg/L in the CPVC reactors. In the reactors where the biofilm was removed, vinyl chloride levels were detected but not quantifiable in the PVC reactors and were estimated to be 0.009 µg/L in the CPVC reactors. In both new PVC and CPVC reactors, vinyl chloride levels were > 0.014 µg/L and < 0.008 µg/L, respectively (limit of detection [LOD] of 0.0045 µg/L; Richardson and Edwards, 2009).
Richardson and Edwards (2009) concluded that a faster accumulation of vinyl chloride occurred in 15-year-old PVC segment reactors compared with 2-year-old PVC loop segment reactors, regardless of whether biofilm was present or removed. In laboratory experiments conducted with 2-year-old schedule 40 PVC loop segment reactors and chlorinated tap water, vinyl chloride levels were approximately 0.0072 µg/L on day 3. By day 7 of the experiments, vinyl chloride concentrations were in the range of 0.01-0.012 µg/L when the biofilm was intact and 0.01-0.014 µg/L when the biofilm was removed. Experiments conducted with 15-year-old schedule 80 PVC segment reactors filled with chlorinated tap water showed vinyl chloride levels of 0.0057 µg/L (biofilm intact) and 0.0086 µg/L (biofilm removed) by the second hour. By day 7 of the experiments, vinyl chloride concentrations reached levels > 0.10 µg/L for reactors with biofilm intact and > 0.12 µg/L for those with biofilm removed (LOD of 0.0045 µg/L; Richardson and Edwards, 2009). According to the authors, the faster leaching rate for a 15-year-old pipe, whose surface had a rust colour, may be caused by damage to the pipe structure. The authors also noted that PVC pipe segments with natural biofilm removed (2-year-old and 15-year-old PVC) leached vinyl chloride at rates slightly higher than those with biofilm intact.
Long-term (2 years) laboratory experiments using schedule 40 PVC and schedule 80 CPVC fragment reactors (no biofilm) investigated the impact of temperature on the leaching rate of vinyl chloride in the water. Based on the graphical representation of the experimental data, vinyl chloride concentrations were below the detection limit in all PVC and CPVC reactors incubated at 4°C. Vinyl chloride levels were estimated to be 0.1 to < 0.16 µg/L at 22°C, 0.07 to < 0.12 µg/L at 37°C and > 0.1 µg/L at 52°C for PVC reactors; and < 0.05 µg/L at 37°C and 0.04 to < 0.06 µg/L at 52°C for CPVC reactors. Henry's Law constants were applied to calculate the vinyl chloride concentrations in the water at the different temperatures (Richardson and Edwards, 2009).
In order to investigate the effect of mixed-species biofilm coverage on biotic reactors, experiments were conducted with schedule 40 PVC and schedule 80 CPVC fragment reactors with biofilm, with killed biofilm (soaked in formaldehyde), with scraped biofilm and without biofilm. The biofilms were artificially grown on new pipe fragments and stored at 22°C for more than 2 years. Richardson and Edwards (2009) reported quantifiable vinyl chloride levels in the range of 0.124-0.158 µg/L (LOD of 0.045 µg/L) for all PVC reactors, regardless of whether biofilm was present, killed or scraped. Vinyl chloride levels were below the detection limit for all CPVC with the exception of the reactor with biofilm. In this case, the levels were above the detection limit, but not quantifiable.
In summary, the results from static -pipe segment/fragment reactors indicated that the quantity of vinyl chloride leached from newer PVC reactors is higher than that from CPVC reactors in term of leaching rate and equilibrium concentrations (Richardson and Edwards, 2009).
In order to assess the vinyl chloride concentrations in tap water resulting from PVC and CPVC pipes, field samples were collected from 15 consumer homes. Vinyl chloride at concentrations in the range of 0.011-0.023 µg/L (LOQ of 0.006 µg/L) was detected in 3 of the 15 consumers' tap water samples. These homes had small-diameter CPVC pipes (<1") and were connected to chlorinated municipal water supplies (Walter et al., 2011). The age of two of the pipes were established (6 and 21 years), whereas the third pipe's age was unknown. Flushing of the pipes did not result in any differences in vinyl chloride concentrations before or after flushing in these homes. The study suggested that the chlorine residual in the water may contribute to vinyl chloride accumulation in the distribution system via disinfection by-product reactions (Walter et al., 2011). This fact is supported by laboratory experiments conducted with CPVC and copper pipe reactors. CPVC reactors with chlorinated tap water accumulated higher levels of vinyl chloride (levels not reported) than did reactors with dechlorinated water. Copper pipe reactors, which eliminated vinyl chloride accumulation via leaching, showed detectable levels of vinyl chloride in chlorinated water and not detectable levels of vinyl chloride in dechlorinated water by the 101st hour (4 days). After 2 months of the experiments, all reactors with dechlorinated water showed levels below the detectable level, whereas two of the triplicate samples with chlorinated water showed detectable levels up to 0.01 µg/L (LOQ of 0.006 µg/L) (Walter et al., 2011).
Another study reported no detectable levels of vinyl chloride in the raw water (MDL of 0.013 µg/L). However, average (and maximum) vinyl chloride concentrations of 0.04 µg/L (0.48 µg/L) and 0.014 µg/L (0.25 µg/L) were reported in treated and distributed water, respectively (Chung et al., 1997).
Several studies reported the formation of chloroacetaldehyde (CAA) and chloroacetic acid as a result of the reaction between the vinyl chloride migrating from the PVC pipes and the chlorine residual in the water (Ando and Sayato, 1984; Wolf et al., 1987; IPCS, 1999).
It is not generally recommended that drinking water treatment devices be used to provide additional treatment to municipally treated water. However, the migration of vinyl chloride monomer from PVC pipes manufactured mainly prior to 1977 was reported to be a possible source of vinyl chloride in drinking water distribution systems. In such cases, as well as where an individual household obtains its drinking water from a private well, a private residential drinking water treatment device may be an option for reducing vinyl chloride concentrations in drinking water.
There is little information in the published literature regarding the effectiveness of GAC treatment technology at the residential scale and no certified residential treatment devices are currently available for the reduction of vinyl chloride in drinking water. However, one study conducted on treatment devices using activated carbon filters demonstrated that they may be effective for the reduction of vinyl chloride concentrations. The authors reported that countertop-style carbon filters were capable of reducing levels of vinyl chloride monomer of approximately 13 µg/L to levels well below 2 µg/L (Carroll and Eckstein, 2001). Carbon filters may be installed at the faucet (point of use) or at the location where water enters the home (point of entry). Point-of-entry systems are preferred for VOCs, because they provide treated water for bathing and laundry as well as for cooking and drinking. Some point-of-entry or point-of-use treatment technologies such as activated carbon may remove vinyl chloride from the drinking water below the MAC if two or more units are installed in series. No drinking water treatment device is certified specifically for vinyl chloride removal at this time, as vinyl chloride is not currently included in any ANSI/NSF treatment standard.
Before a treatment device is installed, the water should be tested to determine general water chemistry and to verify the concentration of vinyl chloride. Periodic testing by an accredited laboratory should be conducted on both the water entering the treatment device and the finished water to verify that the treatment device is effective. Products that use adsorption technology can lose removal capacity through usage and time and need to be maintained and/or replaced. Consumers should verify the expected longevity of the adsorption media in their treatment device as per the manufacturer's recommendations and service it when required.
Health Canada does not recommend specific brands of drinking water treatment devices, but it strongly recommends that consumers use devices that have been certified by an accredited certification body as meeting the appropriate NSF/ANSI drinking water treatment unit 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. 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 drinking water devices and materials as meeting NSF/ANSI standards (SCC, 2011).
- CSA International (www.csa-international.org);
- NSF International (www.nsf.org);
- Water Quality Association (www.wqa.org);
- Underwriters Laboratories Inc. (www.ul.com);
- Quality Auditing Institute (www.qai.org); and
- International Association of Plumbing & Mechanical Officials (www.iapmo.org).
An up-to-date list of accredited certification organizations can be obtained from the SCC (www.scc.ca).
As noted previously, low levels of vinyl chloride may leach from some PVC/CPVC pipes used in drinking water systems and is best controlled by specification of material quality. The NPC requires that all plastic pipes used for cold water applications for distribution system and premise plumbing pipes and components (i.e., fittings) meet the CSA standard for plastic pipes. The standard requires that PVC and CPVC pipes and components (e.g., tubing and fittings), used for drinking water applications, comply with the requirements of NSF/ANSI Standard 61 (CSA, 2009). NSF/ANSI Standard 61 (Drinking Water System Components—Health Effects) ensures that materials meet health-based leaching requirements and are safe for use in potable water applications. This standard evaluates PVC and CPVC products and materials for the concentration of residual vinyl chloride monomer in the product wall. Products and materials must contain 3.2 mg/kg or less residual vinyl chloride monomer in the product wall (equivalent to 0.2 µg/L or less in the drinking water) in order to be certified to NSF/ANSI Standard 61 (NSF/ANSI, 2012).
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