Page 6: Guidelines for Canadian Drinking Water Quality: Guideline Technical Document - Enteric Protozoa: Giardia and Cryptosporidium
Partie II. Science et considérations techniques
The multi-barrier approach, including watershed or wellhead protection, appropriate treatment, optimized filtration for effective fine particle removal and disinfection, a well-maintained distribution system and monitoring the effectiveness of treatment (e.g., turbidity, disinfectant residuals), is the best approach to reduce protozoa and other waterborne pathogens in drinking water. In general, all water supplies should be disinfected, and an adequate concentration of disinfectant residual should be maintained throughout the distribution system at all times.
Where events leading to protozoan impacts on the source water are well characterized, it may be possible to implement other barriers/risk management measures in addition to those mentioned above. These may include limiting capture of raw water during high-risk events, selectively operating an additional barrier during high-risk events, use of alternative sources or blending of varying sources (groundwater and surface water).
Source water quality should be characterized. The best means of achieving this is to conduct routine monitoring for Giardia and Cryptosporidium in order to establish a baseline, followed by long-term targeted monitoring. Monitoring of source water for protozoa can be targeted by using information about sources of faecal contamination from a sanitary survey, together with historical data on rainfall, snowmelt, river flow and turbidity, to help to identify the conditions that are likely to lead to peak events. Sanitary surveys are not a substitute for routine or targeted monitoring. 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 protozoa and other microbial pathogens in the source water.
Once the source water quality has been initially characterized, a health-based treatment goal can be established for the specific source water, and effective pathogen removal and/or inactivation strategies can be put in place in order to achieve safe levels in the finished drinking water. To optimize performance for removal and/or inactivation 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 outbreak.
The inactivation of protozoa from raw water is complicated by their resistance to commonly used disinfectants such as chlorine. Treatment systems that rely solely on chlorine as the treatment barrier will not be able to inactivate Giardia and Cryptosporidium that may be present in the source water. The combination of physical removal and disinfection barriers is the most effective way to reduce protozoa in drinking water. In most cases, a well-operated water treatment plant using conventional treatment (i.e., filtration preceded by coagulation, flocculation and clarification) should be able to produce water with a negligible risk of infection from protozoan pathogens (Ireland Environmental Protection Agency, 1995). Options for treatment and control of protozoa are discussed briefly in this document; however, more detailed information is available in other publications (U.S. EPA, 1991; Health and Welfare Canada, 1993;Deere et al., 2001; Hijnen et al., 2004a; LeChevallier and Au, 2004; MWH, 2005; Smeets et al., 2006; AWWA, 2011). These treatment and control options also need to take into account other treatment requirements, such as turbidity, disinfection by-product (DBP) formation and distribution system maintenance.
Treatment of surface water or surface water-impacted groundwater systems should include physical removal methods, such as chemically assisted filtration (coagulation, flocculation, clarification and filtration), and disinfection, or equivalent technologies. It is essential that the physical removal and disinfection targets are achieved before drinking water reaches the first consumer in the distribution system. Adequate process control measures, such as turbidity removal, 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; MWH, 2005; AWWA, 2011).
The level of treatment needed is based on the source water pathogen concentration and the required drinking water quality. Most source waters are subject to faecal contamination, as such, treatment technologies should be in place to achieve a minimum 3-log (99.9%) removal and/or inactivation of Cryptosporidium and Giardia. With this level of treatment, a source water concentration of 34 cysts/100 L can be reduced to 3.4 × 10−2 cysts/100 L, which meets the population health target of 10−6 disability-adjusted life year (DALY)/person per year (see Section 9.0). Similarly, a source water concentration of 13 oocysts/100 L can be reduced to 1.3 × 10−2 oocysts/100 L. However, many surface waters in Canada have much higher (oo)cyst concentrations (see Sections 5.1.1 and 5.2.1) and therefore require additional removal/inactivation in order to meet the same concentration in the treated drinking water (see Section 9.3.4).
Source water Giardia and Cryptosporidium concentrations should be determined 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. Where source water sampling and analysis for Giardia and Cryptosporidium are not feasible (e.g., small supplies), (oo)cyst concentrations can be estimated. Estimates should be based on a source water assessment along with other water quality parameters that can provide information on the risk and/or level of faecal contamination in the source water. Because these estimates will have a high level of uncertainty, engineering safety factors or additional treatment reductions should be applied in order to ensure production of microbiologically safe drinking water.
The health-based treatment goal can be achieved through one or more treatment steps involving physical removal and/or primary disinfection. The (oo)cyst log reductions for each separate treatment barrier can be combined to define the overall reduction for the treatment process.
Conventional filtration is a practical method to achieve high removal/inactivation rates of (oo)cysts. A recent review of pilot- and full-scale study data concluded that coagulation, flocculation and sedimentation processes were associated with a 1.6 log Cryptosporidium removal credit (range of 0.4-3.7 log) and a 1.5 log Giardia removal credit (range of 0-3.3 log) (Hijnen et al., 2004a). Another review (Emelko et al., 2005) found that granular media filtration can achieve a 3 log removal, or better, of Cryptosporidium when filters are operated at or near optimal conditions. Coagulation and flocculation should be optimized for particles to be effectively removed by filtration. The end of a filter run is a vulnerable period for filter operation. The deterioration in oocyst removal by several log units was observed in the early stages of breakthrough when filter effluent particle counts had just begun to rise and turbidity had not always increased (Huck et al., 2002). Filters must be carefully controlled, monitored and backwashed such that particle breakthrough does not occur (Huck et al., 2001; Emelko et al., 2005), and filter backwash water should not be recirculated through the treatment plant without additional treatment. Slow sand and diatomaceous earth filtration can also be highly effective, with physical removals in the range of > 4 log and 3.3 log for Cryptosporidium and Giardia, respectively (Hijnen et al., 2004b). As there is wide variability in the characteristics of source waters, the selection of the most appropriate system must be made by experienced engineers after suitable analysis and/or pilot testing.
Many treatment processes are interdependent and rely on optimal conditions upstream in the treatment process for efficient operation of subsequent treatment steps. Thus, in order to effectively remove Cryptosporidium and Giardia through filtration barriers, it is important that the preceding coagulation and flocculation steps be optimized.
Membrane filtration has become an increasingly important component of drinking water treatment systems (Betancourt and Rose, 2004; Goh et al., 2005). Microfiltration membranes have the largest pore size (0.1 µm; Taylor and Weisner, 1999). Whereas nanofiltration and reverse osmosis processes are effective in removing protozoan (oo)cysts, microfiltration and ultrafiltration are the most commonly applied/used technologies for microbial removal because of their cost-effectiveness. Jacangelo et al. (1995) evaluated the removal of G. muris and C. parvum from three source waters of varying quality using a variety of microfiltration and ultrafiltration membranes. Microfiltration membranes of 0.1 µm and 0.2 µm and ultrafiltration membranes of 100, 300 and 500 kilodaltons were assessed. Both microfiltration and ultrafiltration were capable of absolute removal of G. muris and C. parvum. The concentration of protozoa in the different raw waters tested varied from 104 to 105/L, and log removals of 4.7-7.0 for G. muris and 4.4-7.0 for C. parvum were achieved. More recently, States et al. (1999) reported absolute removal of Cryptosporidium (challenge concentration of 108 oocysts) and Giardia (challenge concentration of 107 cysts) by microfiltration. Parker et al. (1999) also reported an absolute removal of C. parvum from an influent concentration of approximately 2 × 105/100 L to an effluent concentration of less than 1/100 L(5.3 log removal) using microfiltration membranes (0.2 µm).
Although membrane filtration is highly effective for removal of protozoan (oo)cysts, system integrity (breaks, O-rings, connectors, glue), membrane fouling and degradation must be considered. Membrane fouling is usually caused by accumulation of particles, chemicals and biological growth on membrane surfaces. Membrane degradation is typically the result of hydrolysis and oxidation. There can be very significant differences in pathogen removal, because the physical characteristics of a membrane can vary during the manufacturing process by different manufacturers and because polymeric membranes, regardless of their nominal classification, have a range of pore sizes. The (oo)cyst removal efficiency for a specific membrane must be demonstrated through challenge testing conducted by the manufacturer and subsequently verified by direct integrity testing at the treatment plant. This process involves measuring pressure loss across the membrane or assessing removal of spiked particulates using a marker-based approach. More detailed information on filtration techniques can be found in the guideline technical document on turbidity (Health Canada, 2012b).
Drinking water treatment plants that meet the turbidity limits established in the Guideline Technical Document on turbidity (Health Canada, 2012b) can apply the estimated potential removal credits for Giardia and Cryptosporidium given in Table 7. These log removals are adapted from the removal credits established by the U.S. EPA as part of the "Long Term 2 Enhanced Surface Water Treatment Rule" (LT2ESWTR) (U.S. EPA, 2006b) and the "Long Term 1 Enhanced Surface Water Treatment Rule" (LT1ESWTR) Disinfection Profiling and Benchmarking Guidance Manual (U.S. EPA, 2003). 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 5 log (99.999%) Cryptosporidium removal is required for a given system and conventional filtration provides 3 log removal, then the remaining 2 log reduction must be achieved through another barrier, such as primary disinfection.
|Treatment barrier||Cryptosporidium removal creditTable 1 Footnote b||Giardia removal creditTable 1 Footnote c|
Table 7 Footnotes
|Conventional filtration||3 log||3 log|
|Direct filtration||2.5 log||2.5 log|
|Slow sand filtration||3 log||3 log|
|Diatomaceous earth filtration||3 log||3 log|
|Microfiltration and ultrafiltration||Demonstration and challenge testingTable 7 Footnote d||Demonstration and challenge testingTable 7 Footnote d|
|Nanofiltration and reverse osmosis||Demonstration and challenge testingTable 7 Footnote d||Demonstration and challenge testingTable 7 Footnote d|
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 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.
Physical characteristics of the water, such as temperature, pH and turbidity, can have a major impact on the inactivation and removal of pathogens. For example, inactivation rates for Cryptosporidium and Giardia increase 2- to 3-fold for every 10°C rise in temperature (see Section 220.127.116.11 and CT tables in Appendices A and B). When water temperatures are close to 0°C, as is often the case in winter in Canada, the efficacy of disinfection is reduced, and an increased disinfectant concentration and/or contact time are 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 9 reduces the level of Giardia inactivation by a factor of 3 (see CT tables in Appendix A). On the other hand, pH has been shown to have little effect on Giardia inactivation when using ozone or chlorine dioxide.
Reducing turbidity is an important step in the inactivation of Cryptosporidium and Giardia and other microorganisms. Chemical disinfection may be inhibited by particles that can protect Cryptosporidium and Giardia and other microorganisms. Additionally, turbidity will consume disinfectant and reduce the effectiveness of chemical disinfection. An increase in turbidity from 1 to 10 nephelometric turbidity units (NTU) resulted in an 8-fold decrease in free chlorine disinfection efficiency (Hoff, 1986). The effect of turbidity on treatment efficiency is further discussed in the guideline technical document onturbidity (Health Canada, 2012b).
The efficacy of chemical disinfectants can be predicted based on knowledge of the residual concentration of a specific disinfectant and of factors that influence its performance, mainly temperature, pH, contact time and the level of disinfection required (AWWA 1991). 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) for specific disinfectants at pH values and temperatures encountered during water treatment. To account for disinfectant decay, the residual concentration is usually determined at the exit of the contact chamber rather than using the applied dose or initial concentration. Also, the contact time T is often calculated using a T10 value, which is defined as "the detention time at which 90% of the water passing through the unit is retained within the basin" (AWWA, 1991) (i.e., 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 (AWWA 1991). Hydraulic tracer tests, however, are the most accurate method to determine the contact time under actual plant flow conditions. The T value is dependent on the hydraulics related to the construction of the treatment installation. For this reason, it is less easily adjustable than the disinfectant dosage during the treatment plant operation. However, changing the hydraulics can be achieved through physical modifications such as the addition of baffles to the contact chamber or basin.
Complete CT tables for 0.5 log to 3 log inactivation of Giardia and Cryptosporidium can be found in Appendix A and Appendix B, respectively. Some selected CT values are presented in Table 8 for 3 log (99.9%) inactivation of Giardia 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, as much higher concentrations and/or contact times are required to achieve the same degree of (oo)cyst inactivation. Consequently, chloramine is not recommended as a primary disinfectant for protozoa.
|Temperature (°C)||CT values|
|Free chlorine (Cl2)Table 1 Footnote c||Chloramine (NH2Cl)||Chlorine dioxide (ClO2)||Ozone
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. However, inactivation of Giardia using free chlorine requires relatively high concentrations and/or contact times, particularly in cold waters. Chlorination is also not effective for the inactivation of Cryptosporidium. Ozone and chlorine dioxide are effective disinfectants against Cryptosporidium and Giardia. Ozone is a very strong oxidant capable of effectively inactivating Cryptosporidium and Giardia. Whereas both ozone and chlorine dioxide are effective disinfectants, they are typically more expensive and complicated to implement, particularly in small treatment systems. Also, ozone decays rapidly after being applied during treatment and cannot be used to provide a secondary disinfectant residual. Chlorine dioxide is also not recommended for secondary disinfection because of its relatively rapid decay (Health Canada, 2008). In addition, operational conditions, such as temperature and pH, should be considered when selecting a disinfectant, as its effectiveness can be impacted by these factors. In general, disinfectants are less effective at colder water temperatures (Table 8).
Although protozoa can be inactivated through chemical disinfection, they are much more resistant than bacteria or viruses. In general, Cryptosporidium is much more resistant to chemical disinfection than Giardia. This is, in part, due to the thick protective wall surrounding the oocyst, which is difficult to penetrate. CT values required to inactivate Cryptosporidium are approximately 5-200 times higher than those for Giardia, most notably for chlorine-based disinfection (Korich et al., 1990; U.S. EPA, 1991; Finch et al., 1994, 1997). Therefore, the concentration of free chlorine necessary for inactivation of Cryptosporidium is not feasible, because it would conflict with other water quality requirements (i.e., DBP formation, taste and odour, etc.). As such, treatment systems that use free chlorine as the primary disinfectant must remove or inactivate Cryptosporidium by an additional treatment barrier, such as granular media filtration or UV disinfection. Watershed protection and an intact distribution system are also key to reducing Cryptosporidium and other waterborne pathogens in drinking water produced by treatment plants relying upon chlorination.
In addition to differences in disinfectant susceptibility between Giardia and Cryptosporidium, varying levels of resistance to disinfectants among strains must be considered. Chauret et al. (2001) observed that a 2 log (99%) inactivation required CT values of 70, 530 and 1000 mg·min/L for three different strains of Cryptosporidium parvum. Differential susceptibilities to disinfection have also been reported between environmental and laboratory strains (Maya et al., 2003). These findings highlight the importance of considering strain variability when reviewing treatment removals and potential health risks.
In addition to microbial inactivation, chemical disinfection can result in the formation of DBPs, some of which pose a human health risk. The most commonly used disinfectant, chlorine, reacts with naturally occurring organic matter to form trihalomethanes (THMs) and haloacetic acids (HAAs), 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. It is critical to ensure that efforts made to minimize the formation of these DBPs do not have a negative impact on the effectiveness of disinfection.
UV light disinfection is highly effective for inactivating protozoa. 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. Studies have shown that relatively low UV doses can achieve substantial inactivation of protozoa (Clancy et al., 1998; Bukhari et al., 1999; Craik et al., 2000, 2001; Belosevic et al., 2001; Drescher et al., 2001; Linden et al., 2001, 2002; Shin et al., 2001; Campbell and Wallis, 2002; Mofidi et al., 2002; Rochelle et al., 2002). Based on these and other studies, the U.S. EPA developed UV light inactivation requirements for Giardia and Cryptosporidium in the LT2ESWTR (U.S. EPA, 2006a). The LT2ESWTR requires UV doses of 12 and 11 mJ/cm2 to receive a 3 log credit for Cryptosporidium and Giardia removal, respectively (see Table 9). For water supply systems in Canada, a UV dose of 40 mJ/cm2 is commonly applied (MOE, 2006); thus, protozoa should be effectively inactivated.
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 NTU (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 shown to have a significant impact on UV disinfection efficacy for two viral surrogates (MS2 coliphage and bacteriophage T4), with lower inactivation levels being achieved (Templeton et al., 2005). Further research is needed to better understand their relevance to protozoa inactivation as well as the effect of particles and coagulants on microbial inactivation by UV light. The hydraulic design of a UV reactor influences the UV dose delivered to the microorganisms passing through the reactors. The reactor hydraulics should be such that they allow for all microorganisms to receive the minimum required dose of UV radiation (U.S. EPA, 2006c).
|Log inactivation||UV dose (mJ/cm2) requirements for inactivation|
A multiple disinfectant strategy involving two or more primary disinfection steps (i.e., sequential combination of disinfectants ) is effective for inactivating protozoa, along with other microorganisms, in drinking water. For example, the use of UV light and free chlorine are complementary disinfection processes that can inactivate protozoa, viruses and bacteria. As UV light is highly effective for inactivating protozoa (but less effective for viruses) and chlorine is highly effective for inactivating bacteria and viruses (but less effective for protozoa), the multi-disinfectant strategy allows for the use of lower doses of chlorine. Consequently, there is decreased formation of DBPs. In some treatment plants, ozone is applied for the removal of taste and odour compounds, followed by chlorine disinfection. In such cases, both the ozone and chlorine disinfection may potentially be credited towards meeting the overall disinfection, depending on factors such as the hydraulics of the ozone contactor and the presence of an ozone residual at the point of contactor effluent collection.
In an effort to better understand and evaluate treatment systems, surrogates have been used as indicators of microbial inactivation and removal. Both non-biological and biological surrogates have been used, including polystyrene microspheres and bacterial spores ,respectively. Microspheres represent a feasible approach to evaluate oocyst removal through filtration (Emelko et al., 2003; Baeza and Ducoste, 2004; Emelko and Huck, 2004; Amburgey et al., 2005; Tang et al., 2005). Bacterial spores are not appropriate surrogates of oocyst inactivation, as they are inactivated more readily than Cryptosporidium and are typically more sensitive to certain disinfectants (e.g., chlorine dioxide) (Driedger et al., 2001; Larson and Mariñas, 2003; Verhille et al., 2003). Yeast cells have also been used (Rochelle et al., 2005) for assessing oocyst inactivation, but additional research on their feasibility is needed.
Residential-scale treatment is also applicable to small drinking water systems. This would include both privately owned systems and systems with minimal or no distribution system that provide water to the public from a facility not connected to a public supply (also known as semi-public systems). Minimum treatment of all supplies derived from surface water sources or groundwater under the direct influence of surface waters should include adequate filtration (or equivalent technologies) and disinfection.
An array of options is available for treating source waters to provide high-quality drinking water. These include various filtration methods, such as reverse osmosis, and disinfection with chlorine-based compounds or alternative technologies, such as UV light or ozonation. 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 water systems. 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.
Semi-public and private systems that apply disinfection typically rely on chlorine or UV light because of their availability and relative ease of operation. It is important to note that inactivation of Giardia using free chlorine requires relatively high concentrations and/or contact times. Chlorination is not effective for inactivation of Cryptosporidium. When applying UV light in systems with moderate or high levels of hardness, such as groundwater, scaling or fouling of the UV lamp surface is a common problem 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 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, treatment units meeting NSF Standard 55 for Ultraviolet Disinfection Systems (Class A) are designed to inactivate microorganisms, including bacteria, viruses, Cryptosporidium oocysts and Giardia cysts, from contaminated water. They are not designed to treat wastewater or water contaminated with raw sewage and should be installed in visually clear water.
There are also NSF standards for cyst reduction claims; these include NSF Standard 58 for Reverse Osmosis, NSF Standard 53 for Drinking Water Treatment Units and NSF Standard 62 for Drinking Water Distillation Systems. These standards require a removal of 3 logs or better in order to be certified to a cyst reduction claim. However, they cannot be certified for inactivation claims, as the certification is only for mechanical filtration.
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 (SCC) to certify drinking water devices and materials as meeting the appropriate NSF/ANSI standards:
- Canadian Standards Association International;
- NSF International;
- Water Quality Association;
- Underwriters Laboratories Inc.;
- Quality Auditing Institute; and
- International Association of Plumbing & Mechanical Officials.
An up-to-date list of accredited certification organizations can be obtained from the Standards Council of Canada.
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