Page 8: Guidelines for Canadian Drinking Water Quality: Guideline Technical Document – Arsenic
In water in the pH range of 4-10, the predominant As(III) species are neutral in charge, while As(V) species are negatively charged. The neutral charge on As(III) makes its removal efficiency poor in comparison with that of As(V) (U.S. EPA, 2001a).
As(III) can be converted to As(V) using a pre-oxidation step. Chlorine, ferric chloride, potassium permanganate, ozone, and hydrogen peroxide are effective at oxidizing As(III) to As(V). However, pre-oxidation with chlorine may create undesirable concentrations of chlorinated disinfection by-products (U.S. EPA, 2000).
The selection of an appropriate treatment process for a specific water supply will depend on the characteristics of the raw water supply and many other factors. It is important to determine what, if any, pretreatment is required. Pretreatment may be necessary to remove competing ions such as iron, fluoride, sulphate, and silicate, as well as total dissolved solids; to adjust the pH; and to oxidize As(III) to As(V). Pretreatment is critical for ensuring arsenic removal efficacy with any subsequent treatment technology. Speciation may be performed to assess the species of arsenic present; however, there appears to be limited benefit with respect to time and costs involved. Oxidation of As(III) to As(V) is the preferred method of removing inorganic arsenic, as it ensures that total arsenic is reduced in an efficient manner. Most treatment technology is used in combination with pretreatment and a polishing step, which typically involves polishing the finished water with ion exchange to remove the resulting negative As(V) ion. In addition, contact time, system maintenance, and cost effectiveness are key considerations when selecting a treatment process for arsenic removal. An in-depth review of the various treatment technologies used to remove arsenic from drinking water is beyond the scope of this document. However, detailed information on the effectiveness and application of the various treatment technologies for arsenic removal is available in a review by Thirunavukkarasu and Viraraghavan (2003).
The most practical municipal-scale technologies for the removal of arsenic from drinking water include coagulation/filtration, lime softening, activated alumina, ion exchange, reverse osmosis, and manganese greensand filtration. The U.S. EPA has also identified electrodialysis reversal as a best available technology for arsenic removal. Removal efficiency can be very good (>90%) for some of these technologies; however, manganese greensand filtration and electrodialysis reversal usually achieve lower removal rates (U.S. EPA, 2001a). Recently, adsorption/filtration has also shown promise for arsenic removal.
Although it is difficult to achieve low levels of arsenic using coagulation/filtration alone, when coagulation/filtration is combined with pretreatment (oxidation to convert arsenic to its pentavalent form) and a polishing step (polishing the finished water with ion exchange), the process can reduce total arsenic levels in finished drinking water to concentrations as low as 0.003-0.005 mg/L (U.S. EPA, 2000).
Lime softening is widely used in large utilities and is effective at reducing total arsenic in drinking water to concentrations of 0.001-0.003 mg/L. However, lime softening is an expensive process and is not recommended unless there is also a need to reduce hardness in the raw water feed (U.S. EPA, 2000). The performance and consistency of lime softening can be improved by pretreating the raw water using oxidation and polishing the finished water with ion exchange.
Currently, the most common arsenic removal process for municipal-scale treatment uses activated alumina adsorption followed by microfiltration. Several studies have demonstrated that activated alumina is an effective treatment for the removal of arsenic (As(V)) from drinking water. Pilot plant studies of arsenic removal using activated alumina achieved effluent arsenic levels of <0.01 mg/L (Simms and Azizian, 1997). The U.S. EPA has identified activated alumina as a best available technology for arsenic removal, with a removal efficiency of 95% (U.S. EPA, 2001a). However, the chemical handling requirements may make this process too complex and potentially dangerous for smaller utilities (U.S. EPA, 2000), and therefore this treatment process is not commonly used for these smaller utilities.
The treatment processes described above are effective, but relatively expensive to build and maintain on a municipal scale, and they may not be appropriate for small water treatment utilities. These systems also create significant quantities of either sludge or brine, which must be disposed of appropriately, thus increasing the cost of these processes (NDWAC, 2001).
Ion exchange processes in combination with an oxidation pretreatment step have been shown to reduce total arsenic in finished drinking water to levels as low as 0.003 mg/L. Laboratory column studies using ion exchange resin achieved effluent concentrations as low as 0.002 mg/L where the influent had an arsenic concentration of 0.021 mg/L (Clifford et al., 1999). These systems are recommended for water supplies with low concentrations of total dissolved solids and sulphate (U.S. EPA, 2000).
Reverse osmosis systems, when combined with a pretreatment step, can remove up to 85% of total arsenic from drinking water. These systems are reliable but require large quantities of influent water to obtain the required volume of drinking water, as they reject a significant portion of the influent water as an arsenic-rich brine; as such, they may not be suitable for use in areas where water resources are scarce (U.S. EPA, 2000).
In manganese greensand filtration, the arsenic contained in the water passing through the filter is oxidized and then trapped in the filter. This technology does not achieve a high removal efficiency and is dependent on the presence of iron in the water to remove arsenic. It may be appropriate where the source water has a high iron level and requires only little arsenic removal (U.S. EPA, 2000).
Adsorption/filtration appears to be a promising technology that is applicable to small water treatment utilities. Adsorption using media such as iron, aluminum, and titanium oxide is effective at removing arsenic. Fixed-bed treatment systems, such as adsorption and ion exchange, are becoming increasingly popular for arsenic removal in small water treatment systems because of their simplicity, ease of operation and handling, and regeneration capacity. Several studies that tested the removal of arsenic from drinking water under both laboratory- and pilot-scale conditions showed that adsorptive materials containing various iron oxides are capable of removing As(III) and As(V). More specifically, iron oxide-coated sand and granular ferric hydroxide can remove As(III) and As(V) present in the water to a concentration below 0.005 mg/L (Pierce and Moore, 1980, 1982; Fuller et al., 1993; Hsia et al., 1994; Wilkie and Hering, 1996; Raven et al., 1998; Driehaus et al., 1998; Ramaswami et al., 2001; Thirunavukkarasu et al., 2001, 2003a,b).
A non-treatment option for delivering water with reduced levels of arsenic is water blending. Water blending consists of combining water from a source that has high levels of arsenic with one that has a much lower concentration of arsenic. This ensures that the water being delivered to the consumer has a final concentration of arsenic that meets the guideline.
Municipal treatment of drinking water is designed to reduce contaminants to levels at or below guideline value. As a result, the use of residential-scale treatment devices on municipally treated water is generally not necessary but 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 (treatment device) can be used for reducing arsenic concentrations in drinking water. Residential treatment devices are affordable and can remove arsenic from drinking water to concentrations below 0.010 mg/L. Periodic testing by an accredited laboratory should be conducted on both the water entering a treatment device and the water it produces to verify that the device is effective.
The most common types of treatment devices available for the removal of arsenic from drinking water in residential systems are reverse osmosis and steam distillation. Other types of systems based on alternative technologies such as adsorption are also becoming more common. Filtration systems may be installed at the faucet (point of use) or where water enters the home (point of entry).
Before a treatment device is installed, the well water should be tested to determine general water chemistry and to verify the concentration of arsenic. The testing should also include assessing the presence and concentration of competing ions (e.g., fluoride, iron, sulphate, silicate) and organic matter in the water, which could interfere with arsenic removal.
Given that most technology cannot effectively remove trivalent arsenic, pretreatment with an oxidation step is recommended to convert trivalent (dissolved) arsenic to pentavalent (filterable) arsenic, to ensure good removal by the treatment device (U.S. EPA, 2001a). Individuals should refer to the manufacturer's claims in its literature to obtain information on the amount of arsenic that the treatment device will remove, as well as operational and maintenance requirements.
Residential reverse osmosis systems have been shown to effectively remove total arsenic from drinking water. The amount of arsenic removed depends on the type of membrane filter employed in the system. Reverse osmosis requires larger quantities of influent (incoming) water to obtain the required volume of drinking water, as reverse osmosis systems reject (waste) part of the influent water. A consumer may need to pretreat the influent water to reduce fouling and extend the service life of the membrane. The major advantage of using reverse osmosis systems is that they are widely available, affordable, and easy to service and can remove up to 98% of other dissolved minerals as well as fine colloidal and coarse suspended matter (U.S. EPA, 2000).
Distillation systems can remove virtually all arsenic in drinking water. These systems are more complex than reverse osmosis systems. Although distillation systems are usually installed in commercial applications, more systems are becoming available for residential applications. It should be noted that while there are no known harmful health effects associated with the long-term ingestion of drinking water from distillation or reverse osmosis systems, no specific studies have been conducted on the effects of ingestion of water from these systems. Since beneficial minerals such as calcium and magnesium are removed by both distillation and reverse osmosis processes, it is important to consume a reasonably well-balanced diet to offset the removal of these minerals.
Adsorption/filtration appears to be a promising technology that is applicable to residential-scale treatment. Adsorption using media such as iron, aluminum, and titanium oxide is effective at removing arsenic. Fixed-bed treatment systems, such as adsorption and ion exchange, are becoming increasingly popular for arsenic removal in small water treatment systems because of their simplicity, ease of operation and handling, and regeneration capacity.
Health Canada does not recommend specific brands of 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 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 treatment devices and materials as meeting NSF/ANSI standards:
- 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); 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).
The NSF/ANSI standards for arsenic removal currently test for removal to a concentration of 0.01 mg/L under specific water quality conditions. This underlines the importance of characterizing the raw water to ensure effective removal of arsenic. Certified devices are frequently designed to remove arsenic to well below the 0.010 mg/L concentration, but certification to the standard verifies only that a final concentration of less than 0.010 mg/L is achieved. A qualified professional can design a system to meet residential needs and achieve arsenic concentrations below 0.005 mg/L. For example, a system designed with two or more filters in series will often result in removal of virtually all arsenic. As stated above, the selection of an appropriate treatment process for a specific water supply will depend on the characteristics of the raw water supply and many other factors. It is important to determine what, if any, pretreatment is required and to have the finished water tested by an accredited laboratory to ensure that any designed system is attaining the desired arsenic removal.
For a drinking water treatment device to be certified to NSF/ANSI Standards 53 (Drinking Water Treatment Units -- Health Effects) or 58 (Reverse Osmosis Drinking Water Treatment Systems), or for distillation systems to be certified to NSF/ANSI Standard 62 (Drinking Water Distillation Systems), the device will have to be able to reduce the concentration of arsenic in water from 0.3 to 0.010 mg/L. Devices that can be certified as reducing the concentration of arsenic from 0.3 to 0.010 mg/L are appropriate for treating well water with high concentrations of arsenic. Devices certified as reducing the concentration of arsenic from 0.05 to 0.010 mg/L are intended for treating water with lower initial concentrations (i.e., less than 0.05 mg/L) of arsenic.
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