Page 8: Guidelines for Canadian Drinking Water Quality: Guideline Technical Document – Carbon Tetrachloride
Part II. Science and Technical Considerations - Continued
Municipal water filtration plants that rely on conventional treatment techniques (coagulation, sedimentation, filtration, and chlorination) have generally been found to be ineffective in reducing concentrations of VOCs in drinking water (Love et al., 1983; Robeck and Love, 1983). Two common treatment technologies reported to be effective for the reduction of carbon tetrachloride in drinking water include granular activated carbon (GAC) adsorption and air stripping (Love et al., 1983; U.S. EPA, 1985, 1991a,b; AWWA, 1991; Lykins and Clark, 1994). To a lesser degree, oxidation and reverse osmosis membrane filtration may also be effective in the reduction of VOCs from drinking water.
The selection of an appropriate treatment process for a specific water supply will depend on many factors, including the characteristics of the raw water supply and the operational conditions of the specific treatment method. These factors should be taken into consideration to ensure that the treatment process selected is effective for the reduction of carbon tetrachloride in drinking water.
GAC adsorption is widely used to reduce the concentration of VOCs in drinking water. A removal efficiency of 99% (U.S. EPA, 1985, 2003b; Lykins and Clark, 1994) to achieve effluent concentrations below 1 µg/L is considered feasible for carbon tetrachloride under reasonable operating conditions (O'Brian et al., 1981; Lykins et al., 1984; AWWA, 1991).
Full-scale data demonstrated that the use of GAC operating with a flow rate of 40 gallons per minute (0.22 ML/day), a total empty bed contact time (EBCT) of 130 min, and a carbon usage rate of 11.6 lb/1000 gallons (1.4 kg/m³) was capable of reducing influent carbon tetrachloride concentrations of 72.9 mg/L to an effluent concentration of below 1 µg/L (O'Brian et al., 1981). Another treatment facility operating with a flow rate of 0.23 million gallons per day (0.87 ML/day) and using a downflow GAC adsorber with an EBCT of 35 min and a carbon depth of 9 ft. (2.7 m) reported that influent concentrations of carbon tetrachloride of 6 µg/L could be reduced to below 1 µg/L (AWWA, 1991).
Adams and Clark (1991) estimated the cost-effective design parameters for liquid-phase GAC treatment of carbon tetrachloride in drinking water. The estimated carbon usage rate to reduce an influent carbon tetrachloride concentration of 100 µg/L to an effluent concentration of 5 µg/L was 0.25/1000 gallons (0.03 kg/m³) using an EBCT of 15 min and a bed life of 168 days. Under these conditions, a 95% reduction of carbon tetrachloride in drinking water may be achievable.
The adsorption capacity of activated carbon to remove VOCs is affected by a variety of factors, such as concentration, pH, competition from other contaminants, preloading with natural organic matter, contact time, and the physical/chemical properties of the VOC and carbon (Speth and Miltner, 1990). GAC filtration effectiveness is also a function of the EBCT, flow rate, and filter run time.
Air stripping is commonly used to reduce the concentration of VOCs in drinking water (Cummins and Westrick, 1990; U.S. EPA, 1991b; WHO, 2004b; Dyksen, 2004). Although various air stripping equipment configurations exist, packed tower aeration (PTA) is recognized as the most effective method for the reduction of carbon tetrachloride in drinking water. Removal efficiencies of 99% (U.S. EPA, 1985, 2003b) to obtain effluent concentrations of 1 µg/L are considered to be achievable using PTA.
Design considerations for PTA include the temperature of the air and water, physical and chemical characteristics of the contaminant, air to water ratio, contact time, and available area for mass transfer (Adams and Clark, 1991; U.S. EPA, 1991b; Crittenden et al., 2005; Dyksen, 2004). PTA provides an optimum system for the removal of VOCs from water, as it allows for greater air to water ratios than with traditional diffused aeration systems. As PTA transfers VOCs from water to air, treatment of the stripping tower off-gas to reduce the contaminant concentrations prior to discharge may be necessary (Crittenden et al., 1988; Adams and Clark, 1991).
Typical and model-generated PTA designs for the removal of commonly occurring VOCs have been reported by several authors (Crittenden et al., 1988; Adams and Clark, 1991). According to Crittenden et al. (1988), typical full-scale plant (>8.17 ML/day) air stripping design parameters for reduction of carbon tetrachloride include an air to water ratio of 6.2, an air stripper length of 13.7 m, and a packed column diameter of 1.5 m. Under these conditions, a 99% reduction of an influent carbon tetrachloride concentration of 100 µg/L to an effluent concentration of 1 µg/L in drinking water could be achieved.
After an evaluation of the cost of PTA and GAC adsorption for the control of selected organic compounds in water, Adams and Clark (1991) indicated that the cost-effective PTA design parameters for plants ranging in size from 1 to 100 ML/day include an air to water ratio of 20 and a packing depth of 31.5 ft. (9.6 m) for reduction of carbon tetrachloride. Under these estimated conditions, a 99% reduction of an influent carbon tetrachloride level of 100 µg/L to an effluent level of 1 µg/L could be achieved. According to the authors, the PTA treatment technology appears to be more cost-effective than liquid-phase GAC treatment for the contaminant, even when vapour-phase GAC treatment of the stripping tower off-gas is required.
Alternative air stripping treatment technologies that have been identified as potential methods for the reduction of carbon tetrachloride in drinking water include diffused aeration, multi-stage bubble aerators, tray aeration, and shallow tray aeration. These technologies may be particularly useful for small systems where the installation of GAC or PTA treatment is not feasible (U.S. EPA, 1998).
Combining PTA and GAC into a two-step treatment train has been suggested as the most effective method for achieving low effluent levels of VOCs. In a municipal-scale treatment plant combining these processes, air stripping is used for the bulk reduction of VOCs in the water, and activated carbon is used in the second step to further reduce the residual VOC concentrations (McKinnon and Dyksen, 1984; Stenzel and Gupta, 1985; U.S. EPA, 1991b). In addition, the use of air stripping preceding GAC can significantly extend carbon bed life.
Reverse osmosis has shown some promise for its potential to remove VOCs from drinking water (Clark et al., 1988). Bench-scale investigations demonstrated that selected reverse osmosis membranes were capable of reducing 70-100% of the carbon tetrachloride concentration in water (Fronk et al., 1990; Lykins and Clark, 1994; WHO, 2004b).
The ability of reverse osmosis to remove other synthetic organic chemicals has been found to be dependent on a variety of system components, including type of membrane, flux, recovery, solubility of the organic chemical, charge, and molecular weight (Taylor et al., 2000).
There are several emerging treatment technologies for the removal of carbon tetrachloride from drinking water, including the following:
- High-energy electron beam (E Beam): This technique involves injecting high-energy electrons into an aqueous solution of contaminants following the formation of highly reactive species such as aqueous electrons, hydroxyl radicals, and hydrogen radicals, which mineralize the organic molecules. Pilot-scale experiments were capable of reducing influent concentrations of carbon tetrachloride of 133, 848, and 8490 µg/L to effluent concentrations of 3.38, 6.15, and 174 µg/L, respectively, with respective percentage removals of 97.5, 99.3, and 98.0 (Mak et al., 1997).
- Pervaporation: This technique involves the removal of VOCs by permeating the liquid through a membrane and then evaporating the VOC into the vapour phase. Although it is considered an emerging technology for treatment of water contaminated with VOCs, no information was found on the removal of carbon tetrachloride specifically (Lipski and Cote, 1990; Uragami et al., 2001).
- Membrane air stripping: Air stripping of VOCs with microporous polypropylene hollow fibre membranes has been introduced as an alternative method to PTA (Semmens et al., 1989; Castro and Zander, 1995). Pilot-scale studies demonstrated up to 85% reduction of carbon tetrachloride and greater mass transfer coefficients with than with the use of traditional air stripping towers (Zander et al., 1989).
Generally, it is not recommended that drinking water treatment devices be used to provide additional treatment of municipally treated water. In cases 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 carbon tetrachloride concentrations in drinking water.
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 International (NSF)/American National Standards Institute (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, 2009):
- 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);
- 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).
Treatment devices to remove carbon tetrachloride from untreated water (such as a private well) can be certified for either the removal of carbon tetrachloride alone or the removal of a variety of VOCs, including carbon tetrachloride. Treatment devices are designed to 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 such as carbon tetrachloride because they provide treated water for bathing and laundry as well as for cooking and drinking. Only treatment devices certified for the removal of VOCs can verify that a final concentration of less than 1.8 µg/L of carbon tetrachloride is achieved; treatment devices certified specifically for carbon tetrachloride removal can only verify that they would meet a final concentration of 5 µg/L, which is above the maximum acceptable concentration (MAC) of 0.002 mg/L.
For a drinking water treatment device to be certified to NSF/ANSI Standard 53 (Drinking Water Treatment Units - Health Effects) for the removal of carbon tetrachloride, the device must be capable, in surrogate qualification testing, of reducing the concentration by more than 98% from an influent (challenge) concentration of 0.078 mg/L to a maximum final (effluent) concentration of less than 0.0018 mg/L (NSF/ANSI, 2009a). Treatment devices that are certified to remove VOCs under NSF/ANSI Standard 53 are generally based on activated carbon adsorption technology. Reverse osmosis systems certified to NSF/ANSI Standard 58 (Reverse Osmosis Drinking Water Treatment Systems) may also be certified for the reduction of VOCs to achieve a final concentration of less than 0.0018 mg/L (NSF/ANSI, 2009b). These devices, however, are applicable only for point-of-use treatment.
A number of residential treatment devices from various manufacturers are available that can remove carbon tetrachloride from drinking water to concentrations below 1.8 µg/L. Certified point-of-use treatment devices as well as a limited selection of point-of-entry devices are currently available for the reduction of VOCs, including carbon tetrachloride. In the case where certified point-of-entry treatment devices are not available for purchase, systems can be designed and constructed from certified materials. Periodic testing by an accredited laboratory should be conducted on both the water entering the treatment device and the water it produces to verify that the treatment device is effective. Devices will lose removal capacity through usage and time and need to be maintained and/or replaced. Consumers should read the manufacturer's recommendations to verify the expected longevity of the components in their treatment device.
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