Page 3: Guidelines for Canadian Drinking Water Quality: Guideline Technical Document – Turbidity

Part I: Overview and Application (continued)

2.0 Executive summary

Turbidity is a measure of the relative clarity or cloudiness of water. It is not a direct measure of suspended particles, but rather a general measure of the scattering and absorbing effect that suspended particles have on light.

This Guideline Technical Document reviews and assesses all identified health risks associated with turbidity in drinking water. It assesses new studies and approaches, and takes into consideration the availability of appropriate treatment technology. From this review, several guidelines for turbidity in drinking water are established, depending on the source water type and treatment processes used for filtration.

2.1 Health effects

The types of suspended particles that are most frequently encountered in natural water are not considered to be significant chemical hazards. 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. There is no precise relationship between the magnitude of turbidity reduction and the removal of 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.

Turbidity also has different implications for water quality and treatment depending on the nature of the particles involved and the location of the turbidity within the drinking water system. High turbidity measurements or measurement fluctuations can indicate a decline in source water quality, inadequate water treatment or disturbances in the distribution system.

2.2 Treatment and distribution

Generally, minimum treatment of supplies derived from surface water sources or groundwater under the direct influence of surface water (GUDI) should include adequate filtration (or technologies providing an equivalent log reduction credit) and disinfection. In the production of safe drinking water, filtration is an important barrier for removing particles that cause turbidity. Microorganisms, in addition to being particles themselves, can become attached to soil and waste particles in the environment and can aggregate or attach to inorganic or other particles during treatment. Effective removal of microbial pathogens is best achieved when water of low turbidity is produced and effective inactivation of microbial pathogens is best achieved when low-turbidity water is disinfected.

The most important consideration when dealing with turbidity is the need to reduce it to a level as low as reasonably achievable and to minimize fluctuation. Optimizing treatment performance for turbidity reduction and particle removal also generally optimizes pathogen removal and subsequent disinfection while reducing the potential formation of undesirable disinfection by-products. To maximize protection of public health from microbial contamination, filtration systems should strive to achieve the turbidity target of 0.1 NTU. Systems will have a higher risk for the passage of pathogens into the filtered water if they are not optimized to: (1) reduce filtered water turbidity levels to as low as possible; and (2) reduce the magnitude and likelihood of increases in turbidity levels.

The health-based treatment limits (HBTL) for the different filtration technologies have been established to help ensure that systems are meeting the minimum levels of pathogen removal (log removal credits) provided in the enteric protozoa guideline technical document. The HBTL are achievable by most filtration systems. However, filtration systems should be designed, operated and appropriately optimized to decrease turbidity levels as low as reasonably achievable and strive to achieve a treated water turbidity target from individual filters of less than 0.1 NTU at all times.

Where filtration is not required to meet pathogen removal goals, it is best practice to keep turbidity levels below 1.0 NTU to minimize the potential for interference with disinfection. In addition, to minimize particulate loading and effectively operate the distribution system, it is also good practice to ensure that water entering the distribution system has turbidity levels below 1.0 NTU.

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