Page 9: Guidelines for Canadian Drinking Water Quality: Guideline Technical Document – Trihalomethanes

7.0 Treatment technology

THMs are formed in drinking water primarily as a result of chlorination of organic matter present in raw water supplies. It is therefore important, in assessing the risks associated with the ingestion of THMs in drinking water, to recognize the substantial benefits to health associated with disinfection by chlorination. The use of chlorine has virtually eliminated waterborne microbial diseases because of its ability to kill or inactivate essentially all enteric pathogenic microorganisms, including viruses and bacteria from the human intestinal tract. Chlorine is the most convenient and easily controlled disinfectant; it is a strong oxidant for which a residual can be maintained in the distribution system to prevent bacterial regrowth.

7.1 Municipal-scale

Existing treatment facilities and processes should be optimized to reduce the formation of THMs to levels as low as reasonably achievable without compromising disinfection.

At the municipal level, there are three approaches for reduction of THM concentrations in treated drinking water:

  • removal of THM precursors prior to disinfection;
  • modification of disinfection strategies and use of alternative disinfectants; and
  • use of alternative water supply.

7.1.1 Removal of precursors prior to municipal disinfection

At the municipal level, control technologies for reduction of THM concentrations include optimization of precursor removal using conventional treatment, such as coagulation and sedimentation (Reid Crowther & Partners Ltd., 2000). In some situations, membrane filtration such as nanofiltration and ultrafiltration may be more suitable than conventional treatment, for treatment and economic reasons.

7.1.2 Alternative municipal disinfection strategies

Modification of chlorination practices, such as optimizing the chlorine dosage and changing the point of contact for chlorine, can help reduce THM concentrations in finished drinking water.

Alternative disinfectants to chlorine include chloramines, ozone and ultraviolet (UV) irradiation. Chloramines are a much weaker disinfectant than chlorine and are not recommended as primary disinfectants, especially where virus or parasite cyst contamination may be present (NAS, 1987). Moreover, although chloramines do not form significant levels of THMs, they are capable of inducing halogen substitution in organic compounds and thus may produce significant quantities of total organic halogen. Little is known about these oxidant residuals. The nature and toxicity of products formed from the organic base precursor fractions, particularly the organic chloramine portion of the chlorine residual, have not been characterized.

Ozone has been used as a primary disinfectant in water treatment plants in some parts of Canada and Europe. Ozone is an excellent disinfectant and does not form CDBPs; however, it must be used in combination with a secondary disinfectant to maintain a residual in the distribution system. Ozonation by-products include bromate, acids, and aldehydes, and chlorination of ozonated drinking water will result in increased levels of chloral hydrate as a result of the chlorination of acetaldehyde. Chloral hydrate may subsequently degrade to chloroform depending on pH, temperature, and maturity (e.g., age) of the water (LeBel and Benoit, 2000).

UV disinfection is a physical process that uses photochemical energy to effectively prevent cellular proteins and nucleic acids (i.e., DNA and RNA) from replicating. As a result, the microorganism cannot infect its host. UV disinfection does not induce any disinfectant residual in the water, requiring a secondary chemical disinfectant to maintain a residual in the distribution system. Since UV disinfection is dependent on light transmission to the microbes, the water quality characteristics affecting the UV transmittance need to be considered in the design of the system. UV irradiation under typical disinfection doses (less than 500 mJ/cm2) does not form significant levels of DBPs, nor does it affect the formation of CDBPs (especially THMs and HAAs) in the subsequent chlorination or chloramination processes (Reid Crowther & Partners Ltd., 2000).

The most effective approach for reduction of THMs in drinking water is the improvement of specific conventional water treatment processes and/or membrane filtration to remove organic compounds prior to disinfection, and the addition of special processes such as carbon adsorption and pre-oxidation. Initial removal of organic precursors precludes the need for reducing contact time, thus improving the efficiency of the disinfection process while still minimizing the formation of chlorinated organic by-products. The formation of THMs can be reduced with the use of granular activated carbon filtration. The level of reduction will be a function of the type and adsorbability of organic matter in the water as well as the process design criteria.

It is recommended that any change made to the treatment process, particularly when changing the disinfectant, be accompanied by close monitoring of lead levels in the distributed water. A change of disinfectant has been found to affect the levels of lead at the tap, for example in Washington, DC, where a change from chlorine to chloramines resulted in significantly increased levels of lead in the distributed drinking water. When chlorine, a powerful oxidant, is used as the disinfectant, lead dioxide scales formed in distribution system pipes have reached a dynamic equilibrium in the distribution system. In Washington, DC, switching from chlorine to chloramines decreased the oxidation-reduction potential of the distributed water and destabilized the lead dioxide scales, which resulted in increased lead leaching (Schock and Giani, 2004; Lytle and Schock, 2005). Subsequent laboratory experiments by Edwards and Dudi (2004) and Lytle and Schock (2005) confirmed that lead dioxide deposits could be readily formed and subsequently destabilized in weeks to months under realistic conditions of distribution system pH, oxidation-reduction potential and alkalinity.

7.2 Residential scale

Municipal treatment of drinking water is designed to reduce contaminants to levels at or below guideline value. As a result, the use of residential-scale treatment devices on municipally treated water is generally not necessary but primarily based on individual choice. For households that obtain their drinking water from a municipal system or a private well that chlorinates the water, treatment devices may be installed at the faucet (point-of-use) or where water enters the home (point-of-entry) to reduce THM levels. Certified point-of-use treatment devices are currently available for the reduction of THM levels.

Alternatively, for households which obtain their water from a private source, an ultraviolet (UV) disinfection system may be used to disinfect the water supply instead of chlorination. Certified UV disinfection systems are currently available for residential use.

Before a treatment device is installed, the water should be tested to determine general water chemistry. Pretreatment may be required to address water quality issues and to ensure that the treatment device will be effective. Consumers must refer to the manufacturer's literature to obtain information on the effectiveness of any treatment device being considered, as well as its operational and maintenance requirements and life span.

Health Canada does not recommend specific brands of drinking water treatment devices, but it strongly recommends that consumers look for a mark or label indicating that the device has been certified by an accredited certification body as meeting the appropriate NSF International (NSF)/American National Standards Institute (ANSI) standards. These standards have been designed to safeguard drinking water by helping to ensure the material safety and performance of products that come into contact with drinking water. Certification organizations provide assurance that a product conforms to applicable standards and must be accredited by the Standards Council of Canada (SCC). In Canada, the following organizations have been accredited by the SCC to certify treatment devices and materials as meeting NSF/ANSI standards:

An up-to-date list of accredited certification organizations can be obtained from the Standards Council of Canada.

7.2.1 Filtration devices

Point-of-use and point-of-entry filtration systems, as well as some pour-through filters that use activated carbon filters, can be effective at removing chlorine and its by-products. It is important that the equipment be monitored and maintained according to the manufacturers' recommendations, in particular the regular replacement of the filter media. The performance of filters intended for CDBP removal is dependent on a number of factors, including filter type, media type, CDBP group, flow rate, water quality and age of the filter. The use of filters in areas of high turbidity may cause filters to clog up very quickly without pretreatment.

For a drinking water treatment device to be certified to NSF/ANSI Standard 53 (Drinking Water Treatment Units -- Health Effects), the device must reduce the concentration of THMs in water, using chloroform as a surrogate chemical, from an influent challenge concentration of 0.300 mg/L (300 µg/L) to less than 0.015 mg/L (15 µg/L), representing a chemical reduction of more than 95% (NSF International, 1999).

7.2.2 Alternative residential disinfection strategies

As with the municipal scale, UV irradiation is an alternative disinfection technology which can be installed for residential-scale treatment. UV disinfection is dependent on light transmission to the microbes through the raw water. For this reason, some pre-treatment of the raw water may be required to ensure the effectiveness of the UV disinfection.

The NSF/ANSI Standard 55 covers the certification requirements for UV disinfection systems. In particular, it addresses the Class A systems which are designed to inactivate and/or remove microorganisms, including bacteria, viruses, Cryptosporidium oocysts and Giardia cysts, from contaminated water. The Class A systems are not designed to treat wastewater or water contaminated with raw sewage, and should be installed in visually clear water (NSF International, 2002).

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