Page 8: Guidelines for Canadian Drinking Water Quality: Guideline Technical Document – Benzene
Part II. Science and Technical Considerations - Continued
Municipal drinking water treatment 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). Coagulation and filtration treatment techniques were reported to achieve benzene reductions ranging from 0 to 29%; however, the observed reductions may be partially attributed to incidental volatilization during the treatment process (Clark et al., 1988; Najm et al., 1991; U.S. EPA, 1991a; Lykins and Clark, 1994).
Two common treatment technologies reported to be effective for the reduction of benzene in drinking water are granular activated carbon (GAC) adsorption and air stripping (Love et al., 1983; U.S. EPA, 1985a, 1991a, 1991b; AWWA, 1991; Lykins and Clark, 1994). These treatment methods are capable of achieving effluent concentrations of benzene below 1 µg/L. To a lesser degree, oxidation and reverse osmosis membrane filtration may also be effective for the removal of benzene from drinking water (Whittaker and Szaplonczay, 1985; Fronk, 1987; Lykins and Clark, 1994).
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 benzene in drinking water.
GAC adsorption is widely used to reduce the concentration of VOCs in drinking water, and a removal efficiency of 99% (U.S. EPA, 1985a, 2003b; Lykins and Clark, 1994) to achieve effluent concentrations below 1 µg/L is considered feasible for benzene under reasonable operating conditions (Koffskey and Brodtmann, 1983; Lykins et al., 1984; AWWA, 1991; Dyksen et al., 1995).
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 (NOM), contact time, and the physical/chemical properties of the VOC and carbon (Speth, 1990). GAC filtration effectiveness is also a function of the empty bed contact time (EBCT), flow rate, and filter run time.
Full-scale studies of fixed-bed GAC adsorbers and GAC sand replacement filters have demonstrated that both methods are capable of reducing influent benzene concentrations of 10 µg/L to below 0.1 µg/L in the finished water. Operating conditions of the GAC filter adsorber included a bed volume of 23.8 m3, a flow rate of 1.5 ML/day, and an EBCT of 23.7 minutes. No breakthrough of benzene was observed during the 180-day study period (Koffskey and Brodtmann, 1983). Other full-scale data demonstrated that three GAC adsorbers operating in parallel with a flow rate of 5 ML/day, an EBCT of 21 minutes, and a bed life of 12 months were capable of reducing benzene concentrations of 20 µg/L to 0.2 µg/L (AWWA, 1991).
Model predictions using equilibrium data (Weber and Pirbazari, 1982; Speth and Miltner, 1990) have been used to predict full-scale GAC performance for the reduction of benzene in drinking water (Clark et al., 1990; Lykins and Clark, 1994). The estimated carbon use rate to reduce an influent benzene concentration of 100 µg/L to an effluent concentration of 5 µg/L is 0.013 kg/m3 using an EBCT of 15 minutes and a bed life of 389 days (Lykins and Clark, 1994). As demonstrated with the full-scale data reported above, effluent benzene concentrations of 1 µg/L or lower should be achievable within reasonable operating conditions and costs.
The use of powdered activated carbon (PAC) adsorption has shown limited success as a treatment for the removal of benzene in drinking water. Pilot-scale studies demonstrated that a combined jet flocculation/PAC system was capable of reducing benzene concentrations from 100 to 5 µg/L using 60 mg/L of PAC, 100 mg/L of silica clay, and a contact time ranging between 2 and 8 minutes (Sobrinho et al., 1997).
Air stripping is commonly used to reduce the concentration of VOCs in drinking water (Cummins and Westrick, 1990; U.S. EPA, 1991a; WHO, 2004; Dyksen, 2005). Although various air stripping equipment configurations exist, packed tower aeration (PTA) is recognized as the most effective method for the reduction of benzene in drinking water. Removal efficiencies of 99% (U.S. EPA, 1985a, 2003b) to obtain effluent concentrations of 1 µg/L are considered to be achievable using PTA (Crittenden et al., 1988; U.S. EPA, 1990; Adams and Clark, 1991).
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, 1991a; Crittenden et al., 2005; Dyksen, 2005). 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).
Data from a full-scale drinking water treatment plant demonstrated that countercurrentflow PTA can reduce average influent levels of benzene of 30 µg/L to 1.5 µg/L in finished water using an air-to-water ratio of 75, an air stripper length of 5.50 m, and a packed column diameter of 1.52 m (Allan, 1988). Other full-scale data demonstrated that PTA using an air-to-water ratio of 100, an air stripper length of 10.05 m, and a packed column diameter of 3.05 m was capable of reducing influent benzene concentrations of 200 µg/L to less than 2 µg/L (AWWA, 1991). Pilot testing data have demonstrated that modification of the air-to-water ratio, air stripper length, or packing material can increase the removal efficiencies to achieve effluent concentrations below 1 µg/L (U.S. EPA, 1990).
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; Clark and Adams, 1991). Typical full-scale plant design (> 8 ML/day) parameters for the reduction of benzene from drinking water include an air-to-water ratio of 32.7, an air stripper length of 11.05 m, and a packed column diameter of 2.55 m. Under these conditions, a 99% reduction of benzene in drinking water from an influent concentration of 100 µg/L to an effluent concentration of 1 µg/L may be achievable (Crittenden et al., 1988). Modelling conducted by Adams and Clark (1991) to determine the cost-effective design criteria for PTA contactors estimated that an air-to-water ratio of 40 and a packing depth of 12.95 m may also be capable of achieving a 99% reduction of benzene to effluent concentrations of 1 µg/L.
Pilot plant studies examining the most effective operating conditions of PTA for the reduction of VOCs in groundwater demonstrated removal efficiencies for benzene ranging from 77% to over 99% and in some cases achieved effluent concentrations below 1 µg/L (Stallings et al., 1985; U.S. EPA, 1985b, 1990; Ball and Edwards, 1992).
Alternative air stripping treatment technologies that have been identified as potential methods for the reduction of benzene in drinking water include diffused aeration, multistage 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, 1998a).
Cost evaluations conducted by Adams and Clark (1991) indicate that in most cases the use of PTA for the reduction of benzene in drinking water is more cost-effective than GAC, even when vapour-phase GAC treatment of the stripping tower off-gas is required (Adams and Clark, 1991). The analysis included evaluation of system sizes ranging from 1 to 100 ML/day.
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, 1991a). In addition, the use of air stripping preceding GAC can significantly extend carbon bed life. However, no performance data were available for demonstrating benzene removal efficiencies using this combined treatment method.
Oxidation and advanced oxidation processes (AOPs) have been reported to be effective for the reduction of benzene in drinking water, although full-scale data were not obtained for these treatment methods.
Pilot-scale treatment tests demonstrated that ozone doses of 6 mg/L achieved an 81% degradation of benzene in distilled water from approximately 50 µg/L to effluent concentrations of 10 µg/L. Ozone doses of 12 mg/L achieved a 94% reduction of benzene in both distilled water and groundwater matrices over a wide range of pH (Fronk, 1987). Additional pilot studies observed greater than 75% degradation of benzene with an ozone dose between 0.8 and 1.5 mg/L (Kang et al., 1997).
The rate of degradation of benzene in natural water is also dependent on the reaction of ozone with NOM, which produces hydroxyl radicals. The reaction rate between hydroxyl radicals and benzene is higher than the reaction rate between benzene and ozone; therefore, the ratio of the concentration of hydroxyl radical to the concentration of ozone is considered to be an important factor in the effectiveness of ozonation for the reduction of benzene in drinking water (Crittenden et al., 2005). Lower effluent concentrations may be achievable depending on the influent concentrations of benzene and NOM in the source water and by varying the ozone dose, contact time, and pH of the water.
A pilot-scale photocatalytic oxidation system was successful at reducing influent benzene concentrations from 123 µg/L to below 0.5 µg/L in the finished water. The oxidation system utilized ultraviolet (UV) light with a titanium dioxide semiconductor combined with the addition of 70 mg/L of hydrogen peroxide and 0.4 mg/L of ozone. To prevent fouling of the photocatalytic reactor, an ion-exchange pretreatment system was used to remove iron and manganese from the groundwater (Topudurti et al., 1998). Similar pilot studies found that greater than 99% removal of benzene could be achieved using a UV/titanium dioxide oxidation process (Al-Bastaki, 2003).
The formation of by-products during the application of ozonation or AOPs for the treatment of benzene in drinking water should be considered in the process selection, optimization, and post-treatment monitoring. By-product formation will depend on several factors, including the source water quality, the type and dose of the oxidant, and the reaction contact time. Smaller, oxygenated compounds such as phenolics, aldehydes, ketones, and carboxylic acids have been suggested as potential by-products of the ozonation of benzene (Fronk, 1987). In addition, by-products such as bromate and nitrite may form as a result of the oxidation of inorganic material present in the source water.
Reverse osmosis has shown some promise for its potential to remove VOCs from drinking water (Clark et al., 1988). Pilot plant investigations demonstrated that selected reverse osmosis membranes were capable of reducing 94% of benzene in water; however, the influent concentrations were 1000 µg/L, and the applicability of this treatment to achieve lower effluent concentrations was not investigated (Whittaker and Szaplonczay, 1985). Other studies, however, have found less than 20% removal of benzene using cellulose, polyamide, and thin film composite membranes (Lykins et al., 1988). 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, chemical solubility, charge, and molecular weight (Taylor et al., 2000).
New drinking water treatment technologies for benzene are being developed but are still primarily in the experimental stage and/or do not have published information on the effectiveness of pilot- or large-scale application. Some of the emerging technologies include the following:
- Other AOPs: Laboratory studies examining the effectiveness of various AOP methods demonstrated the complete degradation of benzene using a UV-assisted photo-Fenton process and titanium dioxide-mediated photocatalysis (Ollis et al., 1991; Tiburtius et al., 2005).
- Alternative adsorbents: Synthetic carbonaceous resins and fibreglass-supported activated carbon filters have been shown to have a higher adsorbent capacity for benzene, toluene, ethylbenzene, and xylenes (BTEX) in water relative to activated carbon (Yue et al., 2001; Shih et al., 2005). In addition, the use of an adsorbent impregnated with a platinum and titanium dioxide catalyst demonstrated high removal efficiencies over a prolonged adsorbent bed life (Crittenden et al., 1997). The use of organoclays to enhance carbon filtration has also been shown to be successful (Alther, 2002).
- Bioreactors: Bioreactors using various materials to support microbial growth have been effective for the biodegradation of benzene in water (De Nardi et al., 2002; Sedran et al., 2003).
- Electron beam radiation: The use of a low-energy electron beam to generate electrons and hydroxyl radicals that oxidize benzene in water has demonstrated moderate effectiveness for the reduction of benzene in water (Lubicki et al., 1997).
- Membrane pervaporation: Although the use of membranes for the pervaporation extraction of benzene has been applied primarily in wastewater treatment, this technique has more recently been studied for the removal of benzene from groundwater (Jian and Pintauro, 1997; Uragami et al., 2001; Peng et al., 2003).
Municipal treatment of drinking water is designed to reduce contaminants to levels at or below their guideline values. As a result, the use of residential-scale treatment devices on municipally treated water is generally not necessary, but is primarily based on individual choice. 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 benzene concentrations in drinking water.
A number of residential treatment devices from various manufacturers are available that can remove benzene from drinking water to concentrations below 1 µg/L. Filtration systems may be installed at the faucet (point-of-use) or at the location where water enters the home (point-ofentry). Point-of-entry systems are preferred for VOCs such as benzene, because they provide treated water for bathing and laundry as well as for cooking and drinking. 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 benzene. 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 can lose removal capacity through usage and time and need to be maintained and/or replaced. Consumers should verify the expected longevity of the components in their treatment device as per the manufacturer's recommendations.
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, 2003):
- Canadian Standards Association 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 benzene from untreated water (such as a private well) can be certified either specifically for benzene removal or for the removal of VOCs as a group. However, only treatment devices certified for the removal of VOCs as a group can verify that a final benzene concentration of less than 0.001 mg/L is achieved. For a drinking water treatment device to be certified to NSF/ANSI Standard 53 (Drinking Water Treatment Units-Health Effects) for the removal of VOCs, the device must be capable of reducing the concentration of benzene by greater than 99% from an influent (challenge) concentration of 0.081 mg/L to a maximum final (effluent) concentration of less than 0.001 mg/L (NSF/ANSI, 2006). 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 0.001 mg/L (NSF/ANSI, 2005). This standard is applicable only for point-of-use reverse osmosis systems.
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