Page 2: Guidelines for Canadian Drinking Water Quality: Guideline Technical Document – Chlorite and Chlorate
Chlorite and chlorate in drinking waterFootnote 1
Part I. Overview and Application
The maximum acceptable concentration (MAC) for chlorite in drinking water is 1 mg/L. The MAC for chlorate in drinking water is 1 mg/L. A guideline for chlorine dioxide is not required because of its rapid reduction to chlorite in drinking water.
Utilities should make every effort to meet the guidelines, however, any method of control employed must not compromise the effectiveness of water disinfection.
2.0 Executive summary
The use of disinfectants in the treatment of drinking water has virtually eliminated waterborne diseases. 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 residual in the distribution system to prevent bacterial regrowth. Chlorine dioxide is a chlorinated disinfectant that can be used as an alternative to chlorine at the treatment plant (as primary disinfectant). Disinfection is essential to safeguard drinking water; the health risks from disinfectants and disinfection by-products are much less than the risks from consuming water that has not been disinfected.
Chlorine dioxide is an effective drinking water disinfectant at the treatment plant, but it is very reactive and must be produced on site. Treatment plants using chlorine dioxide as primary disinfectant should not exceed a maximum feed dose of 1.2 mg/L, which will ensure that the chlorite and chlorate guidelines can be met, and that consumers are not exposed to concentrations of chlorine dioxide that could pose health risks. Chlorine dioxide is not effective to maintain a disinfectant residual in the distribution system.
Chlorite and chlorate are disinfectant by-products that are found in drinking water when chlorine dioxide is used for disinfection. Chlorite and chlorate ions can also be formed during the generation process of chlorine dioxide, where the generation technology and the generator "tuning" will affect the levels of chlorite and chlorate fed into the drinking water. Subsequently, the majority of chlorine dioxide added to drinking water will eventually form chlorite. Chlorate can also be formed when hypochlorite solutions do not meet quality specifications and are not stored and/or used appropriately.
Health Canada and the Federal-Provincial-Territorial Committee on Drinking Water recently completed their review of the health risks associated with chlorite, chlorate and chlorine dioxide in drinking water. The Committee concluded, based on the scientific data available, that a guideline for chlorine dioxide was not necessary, but that the inclusion of a maximum feed dose would ensure that consumers are not exposed to concentrations of chlorine dioxide or its disinfectant by-products that could pose health risks. Based on this review, the drinking water guideline for chlorite is a maximum acceptable concentration of 1 mg/L; the drinking water guideline for chlorate is a maximum acceptable concentration of 1 mg/L; and no guideline is established for chlorine dioxide.
2.1 Health effects
Studies on chlorite, chlorate and chlorine dioxide do not provide sufficient information to assess their potential as carcinogens. The guideline for chlorite is based on a two-generation study in rats in which the effects of concern were lower startle amplitude (reaction to sudden noise), decreased brain weight and altered liver weights in two generations. As sodium chlorate is used as a herbicide, several cases of chlorate poisoning in humans have been reported. Animal studies on chlorate suggest an increase in the utilization or metabolism of thyroid hormones.
Chlorine dioxide can affect the neurobehavioural and neurological development of rats exposed before birth to levels significantly higher than those that could exist in drinking water.
Chlorine dioxide reacts quickly in water to form chlorite and chlorate. Because of this rapid reaction, the concentrations of chlorine dioxide in drinking water are expected to be much lower than levels of concern, and no guideline is proposed for chlorine dioxide. However, to ensure that consumers are not exposed to concentrations of chlorine dioxide that could pose health risks, a maximum feed dose is recommended.
Canadians can be exposed to chlorite and chlorate from drinking water that has been treated with chlorine dioxide either as a disinfectant or to help control taste and odour. As few drinking water treatment plants in Canada currently use chlorine dioxide, drinking water is not expected to be a significant source of exposure for the average Canadian. Exposure to chlorate may also be linked to the use of hypochlorite solutions as a source of chlorine in municipal treatment plants. This exposure can be reduced through appropriate storage/use of hypochlorite solutions at the treatment plant.
If chlorine dioxide and chlorite ion are not removed prior to secondary disinfection with chlorine, they will react with free chlorine to form chlorate ion. Once chlorate ion is present in water, it is very persistent and very difficult to remove. It is therefore recommended that municipal treatment plants control the production of chlorate ion. In the case of treatment plants using hypochlorite solutions, operators must ensure that the solution they use meets quality specifications and is stored and used appropriately. In the case of treatment plants using chlorine dioxide generators, the formation of chlorate can be reduced by tuning the chlorine dioxide generator, ensuring maximum efficiency of chlorine dioxide production and removing any chlorite ion with activated carbon, iron-reducing agents or sulphur-reducing agents before adding a chlorine residual.
It is not generally recommended that drinking water treatment devices be used to provide additional treatment to municipally treated water. Nevertheless, some residential-scale treatment devices using a granular activated carbon filter may remove chlorite, although none is currently certified for this use.
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.
There is no scientific evidence to suggest that chlorite, chlorate or chlorine dioxide are human carcinogens. The guidelines for chlorite and chlorate are based on a lifetime exposure from drinking water.
No guideline for chlorine dioxide in drinking water is established because it is rapidly reduced to chlorite and the guideline for chlorite is protective against health effects from chlorine doxide. However, to ensure that the chlorite and chlorate guidelines can be met, and that consumers are not exposed to concentrations of chlorine dioxide that could pose health risks, treatment plants using chlorine dioxide as primary disinfectant should not exceed a maximum feed dose of 1.2 mg/L. In addition, because of its high reactivity, it is recommended that chlorine dioxide be used as a primary disinfectant only, to be added in the treatment plant to kill or inactivate microorganisms present in the raw water. Chlorine dioxide is not generally considered to be a good option as a secondary disinfectant, as it reacts quickly (i.e., the level of chlorine dioxide is quickly reduced in the distribution system), failing to provide the required health protection against microorganisms.
It is also recommended that the on-site generation process of chlorine dioxide be optimized to prevent the contamination of the chlorine dioxide solution with unreacted chlorite, and the formation of chlorate in the generator. Exposure to chlorate may also occur when hypochlorite solutions do not meet quality specifications; an appropriate storage and use can greatly reduce this potential source of exposure.
Short-term exceedances above the guideline value are unlikely to have an effect on health. However, in the event that monitoring data show elevated levels on a yearly basis, it is suggested that a plan be developed and implemented to address these situations.
The maximum levels of chlorite and chlorate in the distribution system usually occur in the mid-system and end locations, respectively. A minimum quarterly monitoring of chlorite and chlorate is recommended, ideally at representative locations for chlorite and chlorate in the distribution system. For systems using hypochlorite solutions, levels of chlorate should be monitored in the treated water at the plant.
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