Page 4: Guidelines for Canadian Drinking Water Quality: Guideline Technical Document – Enteric Viruses
Part I. Overview and Application (continued)
Part I. Overview and Application
3.0 Application of the guideline
Note: Specific guidance related to the implementation of the drinking water guidelines should be obtained from the appropriate drinking water authority in the affected jurisdiction.
Exposure to viruses should be limited by implementing a "source-to-tap" approach to protect the quality of drinking water. This approach includes assessing the entire drinking water system, from the source water through the treatment and distribution systems to the consumer.
Source water assessments should be part of routine vulnerability assessments and/or sanitary surveys. They should include identifying potential sources of faecal contamination in the watershed/aquifer that may impact the quality of the water. Groundwater sources should also be assessed for their vulnerability to contamination. Risk factors may include the type of overlying soil, the land uses surrounding the well and the condition/construction of the well. Sources of human faecal matter, such as sewage treatment plant effluents, sewage lagoon discharges and improperly maintained septic systems, have the potential to be significant sources of human enteric viruses. Faecal matter from wildlife and other animals are not considered a significant source of enteric viruses capable of causing illness in humans, since viruses are generally host specific.
Assessments of water quality need to consider the "worst-case" scenario for that source water. For example, there may be a short period of poor source water quality following a storm. This short-term degradation in water quality may in fact embody most of the risk in a drinking water system. Although routine monitoring of drinking water for enteric viruses is not practical, collecting and analysing source water samples for enteric viruses can provide useful information to help determine the level of treatment that should be in place to reduce the risk of illness from enteric viruses. Source water samples are generally collected at a location that is representative of the quality of the water supplying the drinking water system, such as at the intake of the water treatment plant or, in the case of groundwater, close to the well. In many places, source water sampling for enteric viruses may not be feasible. The potential risk of enteric viruses can be estimated using information from the source water assessment along with the results of other water quality parameters, such as indicator organisms, to provide information on the risk and/or level of faecal contamination in the source water. Because all water quality assessments will have a level of uncertainty associated with them, additional factors of safety during engineering and design of the treatment plant or a greater log reduction than calculated using a QMRA approach should be applied in order to ensure production of drinking water of an acceptable microbiological quality.
The information obtained from source water assessments is a key component to carrying out site-specific risk assessments. This information should be used along with treatment and distribution system information to help assess risks from source to tap. This document suggests the use of QMRA as a tool that can help provide a better understanding of the water system by evaluating the impacts of variations in source water quality and treatment process performance on the overall risk, including the potential impact of hazardous events, such as storms, contamination events or the failure of a treatment barrier. The resulting analysis can be used to assess the adequacy of existing control measures, to determine the need for additional treatment barriers or for optimization and to help establish limits for critical control points.
A minimum 4-log reduction of enteric viruses is recommended for all water sources, including groundwater sources. Recent published information has shown the presence of enteric viruses in some groundwaters sources that were considered to be less vulnerable to faecal contamination. A jurisdiction may allow a groundwater source 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. In many source waters, particularly surface water sources, a greater than 4-log reduction is necessary.
Reductions can be achieved through physical removal processes, such as filtration, and/or by inactivation processes, such as disinfection. 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. For groundwater sources (i.e., those not under the direct influence of surface waters), it is recommended to ensure adequate treatment to remove/inactivate enteric viruses, unless exempted by the responsible authority. The appropriate type and level of treatment should take into account the potential fluctuations in water quality, including short-term water quality degradation, and variability in treatment performance. Pilot testing or other optimization processes may be useful for determining treatment variability. In systems with a distribution system, a disinfectant residual should be maintained at all times.
As part of the multi-barrier approach, indicators that can be routinely monitored, including turbidity, chlorine residual and organisms such as E. coli and total coliforms, should be used to verify that the water has been adequately treated and is therefore of an acceptable microbiological quality (see Guideline Technical Documents on E. coli, total coliforms, and turbidity). These indicators can also be used for assessing the distribution system and to verify that the microbiological quality of the water is being maintained through the distribution system to the consumer's tap.
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