Page 9: Guidelines for Canadian Drinking Water Quality: Guideline Technical Document - Enteric Protozoa: Giardia and Cryptosporidium

10.0 Rationale

Several species and genotypes of Giardia and Cryptosporidium are known to infect humans. These pathogens are excreted in the faeces of infected persons and animals and can potentially be found in source water. Their occurrence in source water varies over time and can be significantly affected by extreme weather or spill/upset events (i.e., increases in (oo)cyst levels associated with these events). The best way to safeguard against the presence of hazardous levels of Cryptosporidium and Giardia in drinking water is based on the application of the multi-barrier approach, including source water protection and adequate treatment, as demonstrated using appropriate physicochemical parameters, followed by the verification of the absence of faecal indicator organisms in the finished water. The protection of public health is accomplished by setting health-based treatment goals. To set health-based treatment goals, the level of risk deemed tolerable or acceptable needs to be determined. The Federal-Provincial-Territorial Committee on Drinking Water has chosen this acceptable level of risk as 10−6 DALY/person per year, which is consistent with the reference level adopted by WHO. This is a risk management decision that balances the estimated disease burden from Cryptosporidium and Giardia with the lack of information on the prevalence of these pathogens in source waters, limitations in disease surveillance and the variations in performance within different types of water treatment technologies.

Although all enteric protozoa of concern need to be identified, risk assessments do not usually consider each individual enteric protozoan. Instead, the risk assessment includes only specific enteric protozoa (reference pathogens or, in this case, reference protozoa) whose characteristics make them a good representative of all similar pathogenic protozoa. It is assumed that if the reference protozoa are controlled, this will ensure control of all other similar protozoa of concern. Cryptosporidium parvum and Giardia lamblia have been selected as the reference protozoa for this risk assessment because of their high prevalence rates, potential to cause widespread disease, resistance to chlorine disinfection and the availability of a dose-response model for each organism.

In Canada, many surface water sources will have Cryptosporidium and Giardia concentrations in the range of 1-200 (oo)cysts/100 L of water. The QMRA approach used in this guideline technical document demonstrates that if a source water has a concentration of (oo)cysts at the lower end of this range--for example, approximately 13 oocysts/100 L and/or 34 cysts/100 L--a water treatment plant would need to consistently achieve at least a 3 log reduction in (oo)cyst concentration in order to meet the reference level of 10−6 DALY/person per year. Thus, a minimum 3 log reduction and/or inactivation of Cryptosporidium and Giardia has been established as a health-based treatment goal. Many source waters in Canada may require more than the minimum treatment goal to meet the acceptable level of risk.

QMRA can be used on a site-specific basis to evaluate how variations in source water quality may contribute to microbiological risk and to assess the adequacy of existing control measures or the requirement for additional treatment barriers or optimization. In most cases, a well-operated treatment plant employing effective coagulation, flocculation, clarification, filtration and disinfection achieving a sufficient CT value should produce water with a negligible risk of infection from enteric protozoa. Where possible, watersheds or aquifers that are used as sources of drinking water should be protected from faecal waste.

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