Page 2: Guidelines for Canadian Drinking Water Quality: Guideline Technical Document - Chlorine

Part I. Overview and Application

1.0 Guideline

It is not considered necessary to establish a guideline for chlorine in drinking water, based on its low toxicity at concentrations found in drinking water as a result of treatment. Any measures taken to limit the concentration of chlorine or its by-products in drinking water supplies must not compromise the effectiveness of disinfection.

2.0 Executive summary

Most drinking water treatment plants in Canada use chlorine as a disinfectant. The use of chlorine in the treatment of drinking water has virtually eliminated waterborne diseases, because chlorine can kill or inactivate most microorganisms commonly found in water. All drinking water supplies should be disinfected, unless specifically exempted by the responsible authority. The majority of drinking water treatment plants in Canada use some form of chlorine to disinfect drinking water: to treat the water directly in the treatment plant and/or to maintain a chlorine residual in the distribution system to prevent bacterial regrowth. Disinfection is an essential component of public drinking water treatment; the health risks associated with disinfection byproducts are much less than the risks from consuming water that has not been adequately disinfected. Free chlorine concentrations in most Canadian drinking water distribution systems range from 0.04 to 2.0 mg/L.

This Guideline Technical Document focuses on the health effects related to exposure to chlorine in drinking water supplies. It does not review the benefits or the processes of chlorination, nor does it assess the health risks related to exposure to by-products formed as a result of the chlorination process.

Health Canada recently completed its review of the health effects associated with chlorine in drinking water, as well as taste and odour considerations. Based on this review, the Federal-Provincial-Territorial Committee on Drinking Water does not consider it necessary to establish a guideline for chlorine in drinking water.

2.1 Health effects

Health Canada has classified chlorine as unlikely to be carcinogenic to humans. Studies in laboratory animals and humans indicate that chlorine exhibits low toxicity, regardless of the route of exposure (i.e., ingestion, inhalation, dermal). Studies in animals have not been able to identify a concentration of chlorine associated with adverse health effects, in part because of aversion to its taste and odour. No adverse health effects have been observed in humans from consuming water with high chlorine levels (up to 50 mg/L) over a short period of time.

2.2 Exposure

Human exposure to chlorine primarily results from the ingestion of free chlorine present in treated drinking water. However, because of chlorine's reactivity and because of the low concentrations applied at the treatment plant, only low levels of chlorine would reach the consumer's tap. Levels of chlorine in ambient air are estimated to be low. The use of chlorine is permitted in food processing, but the chlorine must be rinsed out or otherwise removed from contact with food. Chlorine and hypochlorite salts are not expected to accumulate or bioconcentrate in the food chain.

2.3 Treatment

As chlorine is added to drinking water as a disinfectant and to maintain a residual concentration in the distribution system, treatment of the water for chlorine removal is generally not required. However, for water treatment facilities that include a "superchlorination" treatment step, chlorine will need to be reduced to an appropriate level before distribution to the consumer. This is usually done by the addition of sodium bisulphite or sulphur dioxide to the water.

A private residential drinking water treatment device may be an option for reducing chlorine concentrations in drinking water if the consumers find the taste objectionable. Because of the low toxicity of chlorine at concentrations found in drinking water, the applicable NSF International /American National Standards Institute standard for the reduction of chlorine is based on aesthetic effects.

3.0 Application of the guideline

Note: Specific guidance related to the implementation of drinking water guidelines should be obtained from the appropriate drinking water authority in the affected jurisdiction.

Chlorine can be used in drinking water systems for both primary and secondary disinfection. Studies in laboratory animals and humans show that free chlorine exhibits low toxicity. Health effects observed are generally attributed to the by-products of reactions between chlorine and the organic precursors naturally present in the raw water. As a result, it is important that the use of chlorine include strategies that reduce the formation of chlorinated disinfection by-products (CDBPs), without compromising the effectiveness of disinfection. To that end, it is recommended that chlorine used for primary disinfection be applied following the removal of organic precursors.

Chlorine used as a secondary disinfectant should be applied so as to maintain a sufficient residual concentration throughout the distribution system. Maintenance of an adequate free chlorine residual will minimize bacterial regrowth in the distribution system and provide a measurable level of chlorine; therefore, a rapid drop in free chlorine concentrations suggesting unexpected changes in water quality can be more quickly detected. Specific requirements for chlorine residual concentrations are set by the regulatory authority and may vary among jurisdictions. A suggested operational range for free chlorine residual is between a detectable level and 5 mg/L.

Most Canadian drinking water supplies maintain free chlorine residuals in the 0.04- 2.0 mg/L range in the distribution system. At these concentrations, taste and odour related to chlorine or its by-products are generally within the range of acceptability for most consumers. However, individual sensitivities in the population are widely variable; sensitive individuals may detect it at levels as low as 0.6 mg/L. Consequently, taste and odour concerns should be taken into account during the selection of operational and management strategies for the water treatment and distribution systems. However, even when taste and odour thresholds are exceeded, it does not mean the water is not safe to consume. Aesthetics of drinking water should be addressed on a system-specific basis given the variability in acceptance of taste and odour in the population and the multitude of factors, besides chlorine itself, that can contribute to aesthetic concerns. Utilities should maintain sufficiently low levels of free chlorine at the tap, ensuring that both adequate disinfection has occurred and that a residual in the distribution system can be maintained, while keeping in mind consumer acceptability of drinking water.

3.1 Monitoring

Where chlorine is used as a primary disinfectant, disinfection equipment should be operated in such a manner as to prevent the distribution of inadequately disinfected water, and to alert staff when the disinfection process is not operating properly. For secondary disinfection using chlorine, it is recommended that a continuous analyser be used at the point of entry and throughout the distribution system or that grab samples be frequently tested. This ensures that the target chlorine level is being applied at all times and provides a comparison against residual levels observed throughout the distribution system. In addition, sample locations should be chosen to represent all areas of the distribution system, taking into account geographical location, age and materials of the water main, structural integrity of the distribution system, water storage, and retention times.

It is recommended that water leaving a treatment plant be tested at least daily for both chlorine residual and turbidity, and at least weekly for total coliforms and Escherichia coli to confirm the microbiological safety of the supply. In the distribution system, the presence of adequate chlorine residuals should be confirmed when sampling for total coliforms and E. coli.

All field measurements of chlorine in the distribution system should be carried out at a free-flowing or flushed sampling location according to appropriate sampling procedures, in order to achieve a fresh sample that is reflective of the system water quality. Several portable field instruments are available for this purpose, typically using a colorimetric method such as the N,N-diethyl-p-phenylenediamine (DPD) method for the determination of chlorine concentrations. Chlorine concentrations may decrease during transportation to the laboratory and while in storage.

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