Page 10: Guidelines for Canadian Drinking Water Quality: Guideline Technical Document – Enteric Viruses

9.0 Rationale

There are more than 140 types of enteric viruses known to infect humans that are excreted in the faeces and sometimes urine of infected persons and animals and are potentially found in both surface water and groundwater sources. Enteric viruses are responsible mainly for acute illnesses, although some links to chronic illnesses have been made. Many of these viruses cannot be cultured, and their occurrence in water varies in time and space. At any given time, one virus type may be more prevalent than another in the sewage from a community and affect the source water quality in communities downstream. The best means of safeguarding against the presence of hazardous levels of enteric viruses in drinking water are 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 large number of enteric viruses, the temporal and spatial variations in enteric virus occurrence and methodological limitations make it impractical to routinely monitor for these organisms, and therefore a water quality target, such as a maximum acceptable concentration, for enteric viruses in drinking water is not established. Instead, 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 (2004). This is a risk management decision that balances the estimated disease burden from enteric viruses 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 viruses of concern need to be identified, risk assessments do not usually consider each individual enteric virus. Instead, the risk assessment includes only specific enteric viruses (reference pathogens or, in this case, reference viruses) whose characteristics make them a good representative of all similar pathogenic viruses. It is assumed that if the reference virus is controlled, this would ensure control of all other similar viruses of concern. Rotavirus has been selected as the reference virus for this risk assessment because of the prevalence of infection in children, the possibility of severe outcomes and the availability of a dose-response model.

A source water concentration of 1 rotavirus/100 L of water generally represents groundwater sources and relatively pristine surface water sources. In Canada, many surface water sources will have virus concentrations on the range of 1-100 viruses/L of water (100-10 000/100 L). The QMRA approach used in this guideline demonstrates that if a source water has a mean concentration of approximately 1 rotavirus/100 L, a water treatment plant would need to consistently achieve at least a 4-log reduction in virus concentration in order to meet the reference level of 10−6 DALY/person per year. Thus, a minimum 4-log reduction and/or inactivation of viruses has been established as a health-based treatment goal. A jurisdiction may allow a groundwater source considered less vulnerable to faecal contamiantion to have less than the recommended minimum 4-log reduction if the assessment of the drinking water system has confirmed that the risk of enteric virus presence is minimal. 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 viruses. Where possible, watersheds or aquifers that are used as sources of drinking water should be protected from faecal waste.

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