Page 11: Guidelines for Canadian Drinking Water Quality: Guideline Technical Document – Turbidity

Part II. Science and Technical Considerations (continued)

10.0 Rationale

The most important health-related function of turbidity is its use as an indicator of the effectiveness of drinking water treatment processes, particularly filtration, in the removal of potential microbial pathogens. Effective removal or inactivation of microbial pathogens is best achieved when water of low turbidity is produced. High turbidity values or measurement fluctuations can be indicative of inadequate water treatment or disturbances in the distribution system. Reducing turbidity to as low as reasonably achievable and minimizing fluctuations are most protective of public health.

Turbidity is neither a direct indicator of the presence or absence of pathogens in treated water nor a direct indicator of potential health risks from consumption. It is recognized as a readily measurable parameter to indicate the effectiveness of filtration in removing pathogens. Turbidity reduction, particle removal and pathogen removal are each largely dependent upon the source water quality and the selection and operation of the treatment technology. Although turbidity levels should be kept as low as reasonably achievable, treatment limitations are a key consideration in establishing guideline values for turbidity.

The HBTL are intended to apply to representative samples of individual filter effluent turbidity. Filtration is a dynamic process; as such, brief fluctuations in turbidimeter measurements or other circumstances that cannot reasonably be prevented through process optimization may not allow measured turbidity values to be below the applicable HBTL 100% of the time. These turbidity limits are expected to be achievable by most filtration plants and, in combination with disinfection, should provide an acceptable level of protection from pathogens (i.e., bacteria, protozoa, viruses) in the finished water.

Systems whose source is either surface water or groundwater under the direct influence of surface water (GUDI) should strive to achieve a treated water turbidity target from individual filters or units of less than 0.1 NTU at all times. This applies to drinking water produced by systems that use conventional, direct, slow sand, diatomaceous earth or membrane filtration technologies. Where this is not achievable, the treated water turbidity levels from individual filters or units should be less than or equal to 0.3 NTU for conventional and direct filtration; less than or equal to 1.0 NTU for slow sand or diatomaceous earth filtration; and less than or equal to 0.1 NTU for filtration systems that use membrane filtration. These HBTLs are expected to be met in at least a certain proportion of measurements. Assessing whether a system's performance satisfies the HBTL in at least 95% or 99% of turbidity measurements requires the collection of data over a period of time. Analysis needs to be conducted to determine whether further actions are needed to improve filter effluent turbidity. Actions will be dependent on site-specific considerations. Never to exceed values are also specified for each treatment process, because readings above this value suggest a significant problem with filter performance, and should be investigated and addressed immediately.

For systems using a groundwater source, turbidity should generally be below 1.0 NTU. Best practice for these systems includes appropriate well siting, construction and maintenance, as well as monitoring source water turbidity and ensuring that turbidity levels do not interfere with the disinfection and distribution of the water supply. In some cases, a less stringent value for turbidity may be acceptable if it is demonstrated that the system has a history of acceptable microbiological quality and that a higher turbidity value will not compromise disinfection.

Turbidity can impact disinfection and the effective operation of distribution systems. Achieving low levels of turbidity immediately prior to where disinfection is applied is the best method to minimize potential interference with disinfection as well as to reduce sediment loading to the distribution system. Therefore, it is considered best practice for turbidity to be below 1.0 NTU in water entering the distribution system.

All drinking water systems should monitor and control turbidity throughout the entire distribution system including areas with long retention times, decreased disinfectant residual, or that have demonstrated deteriorating water quality. Increases in distribution system turbidity can be indicative of deteriorating water quality. If an unusual, rapid, or unexpected increase in turbidity levels does occur, the system should be inspected, the cause determined and appropriate corrective actions taken.

Health Canada will continue to monitor new research in this area and recommend any changes to the guideline technical document that it deems necessary.

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