Page 7: Guidance on Waterborne Bacterial Pathogens

Part B. Supporting information

B.4 Residential-scale treatment

B.4.1 Residential-scale and private drinking water systems

Residential-scaleFootnote 2 treatment is also applicable to small drinking water systems. This would include both privately owned systems and systems with a minimal or no distribution system that provide water to the public from a facility not connected to a municipal supply (previously referred to as semi-public systems).

The presence of E. coli in a residential-scale or private drinking water system indicates that the source or the system has likely been affected by recent faecal contamination; as a result, the water should be deemed as unsafe to drink. The absence of E. coli during routine verification should be an adequate indication of the sufficient removal and inactivation of enteric bacterial pathogens. Where applicable, testing frequencies for residential-scale systems will be determined by the responsible authority and should include times when the risk of contamination is greatest--for example, in early spring after the thaw, after an extended dry spell or following heavy rains. For owners of private supplies, existing wells should be tested two to three times per year and during these same periods. New or rehabilitated wells should also be tested before use to confirm microbiological safety.

Non-faecal bacterial pathogens that occur naturally in the water environment can be found in groundwater, although typically at a lower frequency and in lower numbers than in surface waters. The levels of organisms necessary to cause disease in healthy individuals are uncertain, although limited study has suggested that reasonably elevated numbers beyond those typically found in source waters are required. These organisms are most likely to be found in distribution system biofilms and can survive and grow there to reach significant populations. In smaller systems, distribution system biofilms are less of a concern than in municipal systems, because distribution systems are smaller or non-existent and the retention time for the finished water is shorter.

Various options are available for treating source waters to provide high-quality pathogen-free drinking water. These include filtration and disinfection with chlorine-based compounds or alternative technologies, such as UV light. These technologies are similar to the municipal treatment barriers, but on a smaller scale.

Private homeowners should also be aware that domestic hot water systems can be contaminated with Legionella; as a result, water heaters should be kept at a suitable temperature (at least 60ºC) to protect against the potential growth of this organism. The National Plumbing Code of Canada includes requirements for a minimum water temperature of 60°C in hot water storage tanks to address the growth of Legionella (NRCC, 2010). Homeowners should also take appropriate safety measures to reduce the risk of scalding at the tap. These measures include installing thermostatic or pressure-balanced mixing valves to control the water temperature at the tap (Bartram et al., 2007; Bentham et al., 2007). This strategy may also be useful in reducing other microorganisms in hot water heaters, as many of them do not survive at this higher temperature (LeChevallier and Au, 2004; AWWA, 2006).

Larger plumbing systems could use additional control measures. These measures include temperature control; control of water system design and construction to prevent the accumulation of biofilms, sediments or deposits; and nutrient control strategies (Bartram et al., 2007; Bentham et al., 2007).

B.4.2 Use of residential-scale treatment devices

The information on treatment, disinfection and inactivation of microorganisms in this document is relevant primarily to municipal-scale systems. Municipal treatment of drinking water is designed to reduce microbial contaminants to levels below those typically shown to be associated with disease. The use of residential-scale treatment devices on municipally treated water is generally not necessary, but is primarily based on individual choice. In cases where small systems or individual households obtain drinking water from private wells or surface water supplies such as lakes, treatment devices can be used as an additional barrier for reducing pathogen concentrations in drinking water.

Health Canada does not recommend specific brands of drinking water treatment devices, but it strongly recommends that consumers use devices that have been certified by an accredited certification body as meeting the appropriate NSF International/American National Standards Institute drinking water treatment unit 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.

Point-of-use systems (installed at the faucet) and point-of-entry systems (installed where water enters the home) are of interest for use in treatment and disinfection of drinking water in small, rural or remote communities, particularly those using a groundwater source. The most common types of treatment device that are generally able to inactivate waterborne pathogens (including bacteria) use UV disinfection. Although membrane filtration (reverse osmosis) may be able to reduce pathogens, certified devices are generally intended for use on water supplies deemed microbiologically safe. Before a treatment device is installed, the water should be tested to determine its general water chemistry. Periodic testing by an accredited laboratory should be conducted on both the water entering the treatment device and the finished water to verify that the treatment device is effective. Devices can lose removal capacity through usage and time and need to be maintained and/or replaced. Consumers should verify the expected longevity of the components (e.g., UV lamp, membrane) in their treatment device as per the manufacturer's recommendations and service the device when required. Homeowners should ensure that the selection and installation of treatment devices comply with applicable local regulations.

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