The multiple-barrier approach is the best approach to eliminate enteric viruses and other waterborne pathogens from drinking water. The provision of drinking water free of enteric viruses begins with protection of the surface water or groundwater source, to minimize the input of faecal contamination. Raw surface water intakes should be located as far as possible from sewage outfalls. The possible flooding of sewage collection and treatment systems cannot be overlooked, and sudden increases in indicator organisms can give advance warnings of problems. Efficient treatment technologies remain the primary means to ensure high-quality pathogen-free drinking water.
The removal or inactivation of enteric viruses depends basically on two factors - their physical characteristics and their susceptibility to disinfection. The removal and inactivation of some enteric viruses from raw water are complicated by their small size and relative resistance to commonly used disinfectants such as chloramines. Reviews of available treatment options are beyond the scope of this document and are available elsewhere (U.S. EPA 1991; Health and Welfare Canada 1993; AWWA 1999b; Deere et al. 2001).
Barring system-specific exemptions, all public (municipal) supplies should be disinfected. A disinfectant residual should be maintained throughout the distribution system at all times. In addition to disinfection, minimum treatment of all supplies derived from surface water sources and groundwater impacted by surface waters should include coagulation, flocculation, clarification and filtration, or equivalent technologies.
Physical characteristics of the water, especially temperature, pH and turbidity, can have a major impact on disinfection and pathogen removal. For example, inactivation rates increase 2- to 3-fold for every 10°C rise in temperature. When temperature is close to 0°C, as is often the case in winter in Canada, the efficiency of disinfection can be seriously impaired. Some disinfectants are pH-dependent and may be inefficient when water is alkaline. An increase in pH from 6 to 9 reduces the effectiveness of free chlorine by a factor of 3, but pH has little effect on the virucidal action of ozone or chlorine dioxide. An increase in turbidity from 1 to 10 nephelometric turbidity units (NTU) has been shown to result in an 8-fold decrease in disinfection (free chlorine) efficiency (LeChevallier et al. 1981; Hoff 1986). The effect of turbidity on treatment efficiency is further discussed in the supporting document for the turbidity guideline of the Guidelines for Canadian Drinking Water Quality (Health Canada 2003). Disinfectant studies involving several enteric viruses have shown varying levels of resistance to chlorine, highlighting the need for a multi-disinfectant strategy (e.g., UV light and chlorination) (Hoff 1986; Payment and Armon 1989; U.S. EPA 1989; AWWA 1999a, 1999b).
The efficacy of chemical disinfectants can be predicted based on knowledge of the residual concentration of disinfectant, temperature, pH (for chlorine only) and contact time to first customer (AWWA 1999b). This relationship is commonly referred to as the CT concept, where CT is the product of "C" (the residual concentration of disinfectant, measured in mg/L) and "T" (the disinfectant contact time, measured in minutes). CT values for chlorine, chlorine dioxide, chloramine and ozone developed by the U.S. Environmental Protection Agency (EPA) to inactivate enteric viruses are provided in Tables 1 and 2.
|Virus||CT values for 99% (2-log) inactivation|
aND = not determined.
From Tables 1 and 2, it is apparent that ozone, free chlorine and chlorine dioxide are much better disinfectants than chloramine. The latter should not be used as a primary disinfectant. Ozone and chlorine dioxide are more expensive than free chlorine, but all three can form unwanted by-products. Ozone appears most effective, but, as with all disinfectants, it may be unreliable when turbidity is high or variable, because viruses are protected in flocculated particles. The action of chlorine and chlorine dioxide appears to be dependent on the type of virus present; however, enteric viruses (including adenoviruses) are inactivated by commonly used free chlorine concentrations and contact times applied to drinking water treatment (U.S. EPA 1999; Thurston-Enriquez et al. 2003).
UV light disinfection is an emerging (alternative) treatment approach that appears to be highly effective for inactivating protozoans. Several studies have investigated the inactivation of enteric viruses using UV light (Chang et al. 1985; Arnold and Rainbow 1996; Meng and Gerba 1996; AWWA 1999b; US EPA 2000b; Cotton et al. 2001). In general, UV light disinfection is not as efficient at inactivating viruses as the more traditional chlorine-based disinfection processes (U.S. EPA 2003a). Cotton et al. (2001) and Thurston-Enriquez et al. (2003) recently described the UV inactivation kinetics of adenovirus and rotavirus. A UV dose of 226 mJ/cm2 was required for a 4-log (99.99%) inactivation of adenovirus (AD40) in buffered demand-free water. In contrast, a 4-log inactivation of rotavirus was achieved using a dose of 56 mJ/cm2. Similarly, a 4-log inactivation of HAV was achieved using a dose of 16-39 mJ/cm2. Meng and Gerba (1996) have also shown that adenoviruses are extremely resistant to UV disinfection, compared with other enteric viruses. It appears that double-stranded DNA viruses, such as adenoviruses, are more resistant to UV radiation than single-stranded RNA viruses (e.g., HAV) (Meng and Gerba 1996). As a result of its high level of resistance to UV treatment, adenovirus is being considered by the U.S. EPA as the basis for establishing UV light inactivation requirements for enteric viruses. It is important to note, however, that the use of adenovirus to define UV dose requirements may not be appropriate, given that community surface water treatment systems typically apply chlorine as a secondary disinfectant (which should effectively inactivate enteric viruses, including adenoviruses). A multi-disinfectant strategy involving UV light as the primary disinfectant followed by a secondary disinfectant (free chlorine) may prove to be most effective in controlling enteric viruses, as well as other microorganisms, in drinking water.
Complete conventional treatment is the most practical method to achieve high removal and inactivation of enteric viruses. In Canada, Payment and Franco (1993) showed that greater than 99.999% of enteric viruses were removed from heavily polluted water by full conventional treatment (flocculation, settling, pre- and post-disinfection with chlorine dioxide and chlorine and filtration) at three treatment plants in the Montreal area. Slow sand filtration can be highly effective, while rapid sand filtration needs to be aided by appropriate coagulants. Filter backflushing must be carried out regularly, and backflush water should not be recirculated through the treatment plant without treatment. Water types vary, however, and the selection of the most appropriate system must be made by experienced engineers after suitable pilot testing. An effective system of operator training and process control is essential in areas of known contamination where the risk is high (U.S. EPA 1991; Health and Welfare Canada 1993; AWWA 1999b).
In the United States, the U.S. EPA has promulgated the Surface Water Treatment Rule to control the presence of viruses and the protozoan Giardia in public drinking water systems using surface water and groundwater under the influence of surface water (U.S. EPA 1989). Under the rule, a public water system using surface water or groundwater under the influence of surface water must use filtration unless it meets certain water quality, operational and public health standards. It is assumed that filtration removes at least 90% (1 log) of viruses and that disinfection provides a further 99.9% (3-log) inactivation (Table 3). Similar regulations are in place in Alberta (Alberta Environment 1997), Quebec (Ministère de l'environnement du Québec 2001a) and Saskatchewan (Saskatchewan Environment 2002). A guide for the design of facilities has also been prepared by the Ministère de l'environnement du Québec (2001b).
|Treatment||Assumed log removal||Minimum disinfection|
|Slow sand filtration||2||2|
|Diatomaceous earth filtration||1||3|
Recognizing that systems with very poor source water may not be adequately protected by a 4-log reduction in enteric viruses, the U.S. EPA has promulgated an Interim Enhanced Surface Water Treatment Rule (U.S. EPA, 1998). The Long Term 1 Enhanced Surface Water Treatment and Filter Backwash Rule (U.S. EPA 2000a) and the Ground Water Rule (U.S. EPA 2000b) have been proposed to further reduce the risk of waterborne viral and parasitic illnesses.
Drinking water technologies meeting the turbidity limits prescribed in the Guidelines for Canadian Drinking Water Quality (Health Canada 2003) can apply the estimated potential removal credits for Giardia, Cryptosporidium and enteric viruses given in Table 4. These log reduction credits are based on the mean or median removals established by the U.S. EPA as part of the Long Term 2 Enhanced Surface Water Treatment Rule (U.S. EPA 2003b). Facilities that do not meet the requirements, or facilities that believe they can achieve a higher log credit than is automatically given, can be granted a log reduction credit based on a demonstration of performance.
|Technology||Cyst/oocyst creditc||Virus credit|
|Conventional filtrationa||3.0 log||2.0 log|
|Direct filtrationa||2.5 log||1.0 log|
|Slow sand or diatomaceous earth filtrationa||3.0 log||2.0 log|
|Micro- and ultrafiltration, nanofiltration and reverse osmosisb||Removal efficiency demonstrated through challenge testing and verified by direct integrity testing||No credit for micro- and ultrafiltration; for nanofiltration and reverse osmosis, removal efficiency demonstrated through challenge testing and verified by direct integrity testing|
a Conventional/direct/slow sand/diatomaceous earth filtration should be followed by free chlorination to obtain additional virus credit.
b Micro- and ultrafiltration should be followed by free chlorination for the inactivation of viruses.
c Depending on (oo)cyst levels in source water, additional treatment is required using UV light, ozone, chlorine or chlorine dioxide.
Minimum treatment of all semi-public and private supplies derived from surface water sources or groundwater under the influence of surface waters should include adequate filtration (or equivalent technologies) and disinfection. Semi-public and private supplies are considered to be residential-scale for the purposes of this document.
An array of options is available for treating source waters to provide high-quality pathogen-free drinking water. For public systems, these include various filtration methods and disinfection with chlorine-based compounds or alternative technologies, such as UV light or ozonation. Semi-public and private systems can employ many of the same technologies but on a smaller scale, along with others, such as distillation, not used by public systems. These tech-nologies have been incorporated into point-of-entry devices, which treat all water entering the system, or point-of-use devices, which treat water at only a single location, for example, at the kitchen tap. The commonly used drinking water disinfectants are chlorine, chloramine, chlorine dioxide, ozone and UV light. All of the above disinfectants are used in public systems; however, semi-public and private systems using disinfection are more apt to rely on chlorine or UV light.
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) standard. 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 or service conforms to applicable standards. In Canada, the following organizations have been accredited by the Standards Council of Canada (http://www.scc.ca) to certify drinking water devices and materials as meeting the appropriate NSF/ANSI standards: