Page 8: Guidelines for Canadian Drinking Water Quality: Guideline Technical Document – Enteric Viruses

7.0 Treatment technology

The multi-barrier approach is the best approach to reduce enteric viruses and other waterborne pathogens in drinking water. Since available analytical methods make it impractical to routinely monitor for microbial pathogens in treated drinking water, the focus should be on characterizing source water risks and ensuring that effective treatment barriers are in place to achieve safe drinking water. Source water protection measures to minimize faecal contamination, especially control of domestic and sanitary sewage, should be implemented where feasible.

Source water quality should be characterized in terms of the concentrations and variability of waterborne pathogens and faecal indicators. Some means of achieving this are routine analysis for microbial pathogens and/or faecal indicators in source water; and sanitary surveys and/or source tracking, to identify potential sources of human and animal faecal contamination. In order to understand the full range of source water quality, data should be collected during normal conditions as well as during extreme weather or spill/upset events (e.g., spring runoff, storms). For example, the flooding of sewage collection and treatment systems during heavy rainfall events can lead to sudden increases in enteric viruses and other microbial pathogens in the source water.

Generally, minimum treatment of supplies derived from surface water sources or groundwater under the direct influence of surface waters should include adequate filtration (or technologies providing an equivalent log reduction credit) and disinfection. Recent published information has shown the presence of enteric viruses in some groundwaters that were considered to be less vulnerable to faecal contamination (i.e., those not under the direct influence of surface waters) (Abbaszadegan et al., 1998, 1999; Borchardt et al., 2003, 2004; Locas et al., 2007). As a result, it is recommended to ensure adequate treatment of all groundwaters to remove/inactivate enteric viruses, unless exempted by the responsible authority. In the case of small systems, technologies classified as residential scale may be used to achieve a 4-log reduction of enteric viruses, depending on the capacity requirements of the system. Although they may be classified as residential scale, many of them have a rated capacity for volumes greater than that of a single residence. Specific guidance on technologies that can be used in small systems should be obtained from the appropriate drinking water authority in the relevant jurisdiction.

Once the source water quality has been characterized, pathogen removal/inactivation targets and effective treatment barriers can be established in order to achieve safe finished drinking water. To optimize performance for removal of microbial pathogens, the relative importance of each barrier must be understood. Some water systems have multiple redundant barriers such that failure of a given barrier still provides adequate treatment. In other cases, all barriers must be working well to provide the required level of treatment. For these systems, failure of a single treatment barrier could lead to a waterborne disease outbreak.

The removal of enteric viruses from raw water is complicated by their small size and relative ease of passage through filtration barriers. However, viruses are effectively inactivated through the application of various disinfection technologies individually or in combination, at relatively low dosages. In most cases, a well-operated conventional treatment plant should be able to produce water with a negligible risk of disease transmission. Options for treatment and control of viruses are discussed briefly in this document; however, more detailed information is available in other references (U.S. EPA, 1991; Health and Welfare Canada, 1993; AWWA, 1999b; Deere et al., 2001; Hijnen et al., 2004, 2006; LeChevallier and Au, 2004; Medema et al., 2006).

7.1 Municipal scale

In general, all drinking water supplies should be disinfected, and a disinfectant residual should be maintained throughout the distribution system at all times. In addition to primary disinfection, treatment of surface water or groundwater under the direct influence of surface waters should include physical removal methods, such as chemically assisted filtration (coagulation, flocculation, clarification and filtration) or technologies providing an equivalent log reduction credit. It is essential to achieve the physical removal and disinfection targets prior to the first consumer in the distribution system. Adequate process control measures and operator training are also required to ensure the effective operation of treatment barriers at all times (U.S. EPA, 1991; Health and Welfare Canada, 1993; AWWA, 1999b).

Treatment technologies should be in place to achieve a minimum 4-log (99.99%) removal and/or inactivation of enteric viruses. With this level of treatment, a source water concentration of 1 virus/100 L can be reduced to 1 × 10−4 virus/100 L, which meets the population health target of 10−6 disability adjusted life year (DALY)/person per year (see Section 8.0 for a detailed discussion of the DALY). However, raw water could have a much higher virus concentration and therefore require additional treatment for removal/inactivation in order to produce safe drinking water.

7.1.1 Level of treatment necessary

The level of treatment needed is based on the known or estimated concentration of pathogens in the source water. For example, a source water concentration of 1 virus/100 L would be reduced to 1 × 10−4 virus/100 L using a 4-log (99.99%) removal/inactivation process. A source water with higher virus concentrations will require greater removal/inactivation in order to meet an acceptable level of risk in the treated drinking water. Table 1 indicates the overall level of treatment required for a range of source water virus concentrations resulting in an acceptable level of risk of 1 × 10−6 DALY/person per year.

Table 1: Overall treatment requirements for virus log reduction as a function of approximate source water concentration to meet a level of risk of 1 × 10 −6 DALY/person per year)
Source water virus concentration (no./100 L) Overall required treatment reduction for viruses (log10)
1 4
10 5
100 6
1000 7

Where possible, source water virus concentrations should be characterized based on actual water sampling and analysis. Such characterization should take into account normal conditions as well as event-based monitoring, such as spring runoff, storms or spill events. Testing results should also take into account recovery efficiencies for the analytical method and pathogen viability in order to obtain the most accurate assessment of infectious pathogens present in the source water. In many places, source water sampling for enteric viruses may not be feasible and the potential risk from enteric viruses may be estimated using a combination of sanitary surveys, comparison with other source waters, indicator organisms or related research studies. Because it is difficult to analyse for viruses, regular monitoring of indicator organisms and source water characteristics can be a practical solution for assessing the need for treatment adjustments. However, given the uncertainty of such estimates, engineering safety factors or additional treatment reductions should be applied in order to ensure production of microbiologically safe drinking water.

The overall treatment requirements can be achieved through one or more treatment steps involving physical removal and/or primary disinfection. The virus log reductions for each separate treatment barrier can be combined to define the overall reduction for the treatment process.

7.1.2 Physical removal

The physical removal of viruses can be achieved by clarification and filtration processes. Clarification is followed by a filtration process. Some filtration systems, however, are used without clarification (direct filtration). Viruses in the water can be either free flowing or particle associated, and their adsorption depends on a number of factors, such as the isoelectric point and hydrophobicity of both the virus and of the particle (Templeton et al., 2008). The isoelectric point is the pH at which the virus has no net electrical charge and varies among different viral species. The association of virus with particles plays a role in both the physical removal and the disinfection/inactivation of the virus.

The addition of a chemical coagulant to the raw water produces flocs that adsorb the particle-associated viruses. These flocs are then removed from the water using gravity sedimentation, a sludge blanket or dissolved air flotation. Studies have shown virus removal ranging from 1.1 to 3.4 log for the clarification process only (coagulation, flocculation and sedimentation steps) (LeChevallier, 1999; Hijnen et al., 2004).

The granular media filter is traditionally the most common type used. Treatment systems using coagulation, flocculation, clarification and rapid sand filtration are often referred to as conventional treatment. Studies have shown virus removal of 0.1-3.8 log for the filtration only. Combining these process steps together, conventional filtration is given a credit of 2.0-log physical removal of enteric viruses (Table 2). A greater log removal of enteric viruses is possible using conventional filtration with optimization of the treatment for turbidity and particle removal (Xagoraraki et al., 2004).

Table 2: Virus physical removal credits for various treatment technologies meeting the turbidity limits established in the Guidelines for Canadian Drinking Water Quality
Treatment barrier Virus removal creditTable 2 - Footnote 1 (log10)
Conventional filtration 2.0
Direct filtration 1.0
Slow sand filtration 2.0
Diatomaceous earth filtration 1.0
Microfiltration No creditTable 2 - Footnote 2
Ultrafiltration Demonstration and challenge testing; verified by direct integrity testing
Nanofiltration and reverse osmosis Demonstration and challenge testing; verified by integrity testing

Membrane filtration by microfiltration does not provide an ultimate physical barrier to viruses because of the size of the pores, which range from 0.1 to 10 µm. However, several studies have demonstrated that viruses can be removed to a 4-log level when a coagulation process precedes microfiltration (Zhu et al., 2005a,b; Fiksdal and Leiknes, 2006). Ultrafiltration membranes have pore sizes ranging from 0.01 to 0.1 µm and can reject viruses. A review of several studies indicates that ultrafiltration membranes typically remove viruses to greater than a 3-log level (AWWA, 2005). Nanofiltration and reverse osmosis membranes are typically considered to be non-porous and represent a physical barrier to viruses. Because any breach in the integrity of the membranes would allow viruses to pass through the filter, direct integrity testing should be conducted during the filter operation for ultrafiltration membranes. Currently, it is not possible to conduct direct integrity testing for nanofiltration and reverse osmosis membranes without disrupting production for an extended period of time. However, indirect integrity testing is required on a continuous basis, and direct integrity testing should be performed on a regular basis.

It is important to note that many treatment processes are interdependent and rely on optimal conditions upstream in the treatment process for efficient operation of subsequent treatment steps. For example, coagulation and flocculation should be optimized for particles to be effectively removed by filtration. Filters must be carefully controlled, monitored and backwashed such that particle breakthrough does not occur (Huck et al., 2001), and filter backwash water should not be recirculated through the treatment plant without additional treatment subsequent to coagulation, flocculation and clarification (Medema et al., 2006).

Slow sand filtration can also be effective, with physical removals in the range of 0.9-3.5 log for viruses (Hijnen et al., 2004). Several factors can negatively affect the removal of viruses by slow sand filtration, such as cold water, higher hydraulic loading and decreased sand depth. A 1-year monitoring study of three full-scale riverbank filtration facilities reported an average male-specific and somatic bacteriophage reduction of 2.1 log and 3.2 log, respectively (Weiss et al., 2005).

7.1.2.1 Physical log removal credits for treatment barriers

Drinking water treatment plants that meet the turbidity limits established in the Guidelines for Canadian Drinking Water Quality (Health Canada, 2003b) can apply the estimated physical removal credits for enteric viruses given in Table 2. These log removal credits are based on the mean or median removals established by the U.S. EPA (1999) as part of the Disinfection Profiling and Benchmarking Guidance Manual and the Long Term 2 Enhanced Surface Water Treatment Rule (LT2ESWTR) (U.S. EPA, 2006b). Alternatively, log removal rates can be established on the basis of demonstrated performance or pilot studies. The physical log removal credits can be combined with the disinfection credits to meet overall treatment goals. For example, if an overall 4-log (99.99%) virus removal is required in a given water supply and conventional filtration provides 2-log removal, then the remaining 2-log reduction must be achieved through another barrier, such as disinfection.

7.1.3 Chemical disinfection

Chemical disinfectants commonly used for treating drinking water include chlorine, chloramine, chlorine dioxide and ozone. Disinfection is typically applied after treatment processes that remove particles and organic matter. This strategy helps to ensure efficient inactivation of pathogens and minimizes the formation of disinfection by-products (DBPs). It is important to note that when describing microbial disinfection of drinking water, the term "inactivation" is used to indicate that the pathogen is no longer able to multiply within its host and is therefore non-infectious, although it may still be present.

7.1.3.1 Water quality characteristics

Physical characteristics of the water, such as temperature, pH and turbidity, can have a major impact on inactivation and removal of pathogens. For example, inactivation rates increase 2- to 3-fold for every 10°C rise in temperature. When temperatures are near 0°C, as is often the case in winter in Canada, the efficacy of disinfection is reduced, and an increased disinfectant concentration or contact time, or a combination of both, is required to achieve the same level of inactivation.

The effectiveness of some disinfectants is also dependent on pH. When using free chlorine, increasing the pH from 6 to 10 reduces the level of virus inactivation by a factor of 8-10 times (see CT tables in Appendix B). However, a recent study by Thurston-Enriquez et al.
(2005a) reported that chlorine dioxide was 1.9 and 19.3 times more effective at pH 8 than at pH 6 for adenovirus type 40 and feline calicivirus (used as a surrogate for norovirus), respectively. Similar findings have been reported for other enteric viruses using chlorine dioxide (Alvarez and O'Brien, 1982; Moss and Olivieri, 1985). pH has been shown to have little effect on virus inactivation efficiency of ozone, although a higher pH will impact ozone stability and therefore increase ozone demand.

Reducing turbidity is an important step in the inactivation of viruses and other microorganisms. Chemical disinfection may be inhibited because protection of viruses and other microorganisms can occur within the associated particles. Negative impacts of particle-associated viruses on disinfection processes have been demonstrated in several studies (Templeton et al., 2008). The effect of turbidity on treatment efficiency is further discussed in the Guideline Technical Document on turbidity (Health Canada, 2003b).

7.1.3.2 CT concept for disinfection

The efficacy of chemical disinfectants can be predicted based on knowledge of the residual concentration of disinfectant, temperature, pH and contact time (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). Generally, CT objectives are determined in controlled laboratory studies. In treatment facilities, the residual concentration is usually determined at the exit of the contact chamber rather than using the applied dose or initial concentration, to account for disinfectant decay. To account for the mixing hydraulic of the contact chamber, the contact time "T" is typically calculated using a T10 value, such that 90% of the water meets or exceeds the required contact time. The T10 values can be estimated based on the geometry and flow conditions of the disinfection chamber or basin. Hydraulic tracer tests, however, are the most accurate method to determine the contact time under actual plant flow conditions.

Complete CT tables for 2-log, 3-log and 4-log inactivation of viruses can be found in Appendix B. Some selected CT values are presented in Table 3 for 4-log (99.99%) inactivation of enteric viruses using chlorine, chloramine, chlorine dioxide and ozone. The CT values illustrate the fact that chloramine is a much weaker disinfectant than free chlorine, chlorine dioxide or ozone, since much higher concentrations and/or contact times are required to achieve the same degree of virus inactivation. Consequently, chloramine is not recommended as a primary disinfectant. Free chlorine is the most common chemical used for primary disinfection because it is widely available, is relatively inexpensive and provides a residual that can be used for maintaining water quality in the distribution system. For example, a moderate chlorine concentration of 0.5 mg/L with 15-min contact time can achieve greater than 4-log virus inactivation at 20°C (Table 3). Ozone is another strong disinfectant for virus inactivation, as noted by the low CT values required for 4-log inactivation. However, ozone decays rapidly after being applied during treatment and cannot be used to provide a secondary disinfectant residual. Although ozone and chlorine dioxide are effective disinfectants, they are typically more expensive and complicated to implement, particularly in small treatment systems.

Table 3: CT values for 99.99% (4-log) inactivation of enteric viruses Table 3 - Footnote a by various disinfectants at 5°C and 20°C (pH 6-9)
Temperature (°C) T values for 99.99% (4-log) inactivation
Free chlorine
(Cl2)
Chloramine
(NH2Cl)
Chlorine dioxide
(ClO2)
Ozone
(O3)
5 8 1988 33.4 1.2
20 3 746 12.5 0.5
7.1.3.3 Chemical resistance of different viruses

Research studies involving several enteric viruses have shown varying levels of resistance to chemical disinfectants (Engelbrecht et al., 1980; Payment et al., 1985; Hoff, 1986; Sobsey et al., 1988; Payment and Armon, 1989; U.S. EPA, 1989; AWWA, 1999a,b; Thurston-Enriquez et al., 2003a, 2005a,b). Table 4 presents CT values from various research studies for 2-log (99%) inactivation of several viruses using various chemical disinfectants. In these studies, HAV was found to be more resistant to chemical inactivation using chlorine dioxide and ozone than other types of viruses. For free chlorine disinfection, HAV was shown to be consistently more resistant than rotavirus and adenovirus 40; however, the susceptibility of coxsackievirus B5 and poliovirus 1 varied significantly between studies. Further research on the inactivation of these viruses is needed. As a result, virus disinfection targets and guidance tables of CT values have been based on HAV (U.S. EPA, 1991).

Table 4: Comparison of CT values from research studies for 99% (2-log) inactivation of selected viruses by various disinfectants at 5-15°C
Virus CT values for 99% (2-log) inactivation
Free chlorine
(Cl2)
pH 6-7
Chloramine
(NH2Cl)
pH 8-9
Chlorine dioxide
(ClO2)
pH 6-7
Ozone
(O3)
pH 6-7
Poliovirus 1Table 4 - Footnote a 1.1-6 768-3740 0.2-6.7 0.1-0.2
RotavirusTable 4 - Footnote b 0.01-0.05 3806-6476 0.2-2.1 0.006-0.06
Hepatitis A virusTable 4 - Footnote c 0.7-1.18 428-857 <0.17 - 2.8 0.5
Coxsackievirus B5Table 4 - Footnote d,Table 4 - Footnote e 1.7-12 550 n.a. n.a.
Adenovirus 40Table 4 - Footnote e,Table 4 - Footnote f 0.02-2.4 360 0.25 0.027
7.1.3.4 Disinfection by-products

In addition to microbial inactivation, chemical disinfection can result in the formation of DBPs, some of which may pose a human health risk. The most commonly used disinfectant, chlorine, reacts with naturally occurring organic matter to form trihalomethanes and haloacetic acids, along with many other halogenated organic compounds (Krasner et al., 2006). The use of ozone and chlorine dioxide can also result in the formation of inorganic DBPs, such as bromate and chlorite/chlorate, respectively. When selecting a chemical disinfectant, the potential impact of DBPs should be considered. Where possible, efforts should be made to minimize the formation of these DBPs without compromising the effectiveness of disinfection. The issues of DBPs are further discussed in the Guideline Technical Documents on trihalomethanes (Health Canada, 2006c), haloacetic acids (Health Canada, 2008), bromate (Health Canada, 1998) and chlorite and chlorate (Health Canada, 2008).

7.1.4 Ultraviolet light disinfection

UV light disinfection is considered an alternative method of disinfection. UV dose, usually called fluence, is expressed in millijoules per square centimetre (mJ/cm2), which is equivalent to milliwatt seconds per square centimetre (mW·s/cm2). UV light is usually applied after particle removal barriers such as filtration in order to prevent shielding by suspended particles and allow better light penetration through to the target pathogens. Several recent studies have examined the effect of particles on UV disinfection efficacy, and most have concluded that the UV dose-response of microorganisms is not affected by variations in turbidity up to 10 nephelometric turbidity units (Christensen and Linden, 2002; Oppenheimer et al., 2002; Mamane-Gravetz and Linden, 2004; Passantino et al., 2004). However, the presence of humic acid particles and coagulants has been show to significantly affect UV disinfection efficacy, with lower inactivation levels being achieved (Templeton et al., 2005). Further research is needed to better understand the effect of particles and coagulants on microbial inactivation by UV light.

7.1.4.1 UV resistance of viruses

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; U.S. EPA, 2000; Cotton et al., 2001). Studies have shown that adenoviruses are much more resistant to UV disinfection compared with other enteric viruses (Cotton et al., 2001; Thurston-Enriquez et al., 2003b; Nwachuku et al., 2005). A relatively high UV dose of 152 mJ/cm2 was required for a 4-log (99.99%) inactivation of adenovirus 40 in buffered demand-free water. In contrast, a recent study (Linden et al., 2007) obtained 3-log inactivation of adenovirus 40 by use of a polychromatic source at fluence of approximately 30 mJ/cm2 and wavelength of 220 nm and 228 nm. A 4-log inactivation of rotavirus was achieved using a mean UV dose of 40 mJ/cm2. Table 5 lists typical UV dosages required to achieve a 4-log (99.99%) inactivation for various types of enteric viruses. A more detailed table of UV doses for multiple log reductions of various viruses is presented in a study by Chevrefils et al. (2006).

Table 5: Comparison of typical UV dose requirements (mJ/cm 2) for 1-log (90%), 2-log (99%), 3-log (99.9%) and 4-log (99.99%) inactivation of various enteric viruses Table 5 - Footnote a
Virus 1-log 2-log 3-log 4-log
Hepatitis A virus 4.1-5.5 8.2-13.7 12.3-22 16.4-29.6
Coxsackievirus B5 6.9-9.5 13.7-18 20.6-27 36
Poliovirus type 1 4.0-8 8.7-15.5 14.2-23 20.6-31
Rotavirus SA-11Table 5 - Footnote b 7.1-10 14.8-26 23-44 36-61
Adenovirus 58 100 143 186

It appears that double-stranded DNA viruses, such as adenoviruses, are more resistant to UV radiation than single-stranded RNA viruses, such as HAV (Meng and Gerba, 1996). The mechanisms for this higher resistance are not totally understood. The resistance may occur as the result of the physical or chemical properties of the viruses or repair of the UV-induced damage either by the virus or with the help of host cell enzymes (Shin et al., 2005). Because of their high level of resistance to UV treatment and because adenoviruses cause illness in children and immunocompromised adults, adenoviruses have been used by the U.S. EPA as the basis for establishing UV light inactivation requirements for enteric viruses in the LT2ESWTR. Accordingly, the LT2ESWTR requires a UV dose of 186 mJ/cm2 to receive a 4.0-log credit for viral inactivation (U.S. EPA, 2006b).

For water supply systems in Canada, a UV dose of 40 mJ/cm2 is commonly applied, often in combination with chlorine disinfection or other physical removal barriers (MOE, 2006). This dose is considered to be protective of human health because most enteric viruses are inactivated at a UV dose of 40 mJ/cm2. However, a UV dose of 40 mJ/cm2 would provide only a 0.5-log inactivation of adenovirus but the addition of free chlorine can provide additional log removal credit. In a laboratory study, Baxter et al. (2007) found that a concentration of 0.22 mg/L of chlorine with 1 minute of contact time in a demand-free water, provided a 4-log inactivation of adenovirus.

For drinking water sources considered to be less vulnerable to human faecal contamination, the responsible authority may choose an enteric virus such as rotavirus as the target organism (i.e., as found in Table 5) to determine the required UV dose. Where a system relies solely on UV disinfection for pathogen control and the source water is known or suspected to be contaminated with human sewage, either a higher UV dose such as that stated in the LT2ESWTR or a multi-disinfectant strategy should be considered.

7.1.5 Multi-disinfectant strategy

A multiple disinfectant strategy involves using two or more primary disinfection steps to meet treatment objectives. For example, UV light and free chlorine are complementary disinfection processes, which can inactivate protozoa, viruses and bacteria. UV light is highly effective for inactivating protozoa and bacteria (but less effective for some viruses), whereas chlorine is highly effective for inactivating bacteria and many viruses (but less effective for protozoa). In some treatment plants, ozone may be applied for taste and odour control, followed by chlorine disinfection. In such cases, both the ozone and chlorine disinfection could potentially be credited towards meeting the minimum of 4-log reduction for enteric viruses while meeting taste and odour treatment objectives.

Site-specific assessments of drinking water supplies should be carried out in order to determine the most appropriate treatment strategy based on the source water quality, including the organisms of concern. For example, utilities using surface water or groundwater under the direct influence of surface water will need to treat source waters for all three types of organisms (protozoa, viruses and bacteria) and therefore may need to consider the use of a multi-disinfectant strategy. Groundwater from sources less vulnerable to faecal contamination, on the other hand, may need to be treated only for the presence of enteric viruses, and therefore a multi-disinfectant strategy would not be necessary. When determining whether a multiple disinfectant strategy is required to meet overall treatment objectives, the contribution from any physical removal treatment process also needs to be considered. Specific guidance on disinfection requirements should be obtained from the appropriate drinking water authority in the relevant jurisdiction.

7.2 Residential scale

Various options are available for treating source waters to provide high-quality pathogen-free drinking water. These include filtration and disinfection with chlorine-based compounds or alternative technologies, such as UV light. These technologies are similar to the municipal treatment barriers, but on a smaller scale. In addition, there are other treatment processes, such as distillation, that can be practically applied only to small or individual water supplies. Most of these technologies 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 use of UV light has increased owing to its availability and relative ease of operation. However, scaling or fouling of the UV lamp surface is a common problem when applying UV light to raw water with moderate or high levels of hardness, such as groundwater. UV light systems are often preceded by a pretreatment filter to reduce scaling or fouling. A pretreatment filter may also be needed to achieve the water quality that is required for the UV system to operate as specified by the manufacturer. In addition, the regular cleaning and replacement of the lamp, according to manufacturer's instructions, are critical in ensuring the proper functioning of the unit. Alternatively, special UV lamp-cleaning mechanisms or water softeners can be used to overcome this scaling problem.

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 International (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.

For example, NSF/ANSI Standard 55 (Ultraviolet Disinfection Systems) provides performance criteria for two categories of certified systems, Class A and Class B. UV systems certified to NSF/ANSI Standard 55 Class A are designed to deliver a UV dose at least equivalent to 40 mJ/cm2 in order to inactivate microorganisms, including bacteria, viruses, Cryptosporidium oocysts and Giardia cysts, from contaminated water. As such, UV systems certified to NSF/ANSI Standard 55 Class A can provide 4-log reduction for most viruses (Table 5) and are suitable for this use. However, it must be noted that they are not designed to treat wastewater or water contaminated with raw sewage and should be installed in visually clear water. It is important to note that systems certified to NSF Standard 55 Class B are designed to deliver a UV dose at least equivalent to 16 mJ/cm2 and cannot provide 4-log reduction for most viruses (Table 5). Class B systems are intended for a drinking water supply that is already disinfected, tested and deemed acceptable for human consumption.

Reverse osmosis membranes have a pore size smaller than the viruses and could provide a physical barrier to them. Currently, the NSF/ANSI standard for reverse osmosis systems does not include a claim for virus reduction; as a result, reverse osmosis units cannot be certified to this standard for virus reduction.

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 to certify drinking water devices and materials as meeting the appropriate NSF/ANSI standards:

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