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Bacteriological Quality of Drinking Water


To reduce the risk of enteric illnesses that may be attributed to public water supplies, the following standards are applicable in the Province of Newfoundland and Labrador. If any of the criteria are exceeded, corrective action should be taken in the affected area immediately.

Standards - Bacteriological:

Bacteriological standards are based on Guidelines for Canadian Drinking Water Quality 6th Edition, 1996 as revised, and shall be considered generally applicable to the Province of Newfoundland and Labrador. The Guideline notes that the maximum acceptable concentration (MAC) for the bacteriological quality of public, semi-public and private drinking water systems is no coliforms detectable per 100 mL. However, because coliforms are not uniformly distributed in water and are subject to considerable variation in public health significance, drinking water that fulfils the following conditions is considered to conform to this MAC:

  1. No sample should contain Escherichia coli (E.coli).
  2. No consecutive samples from the same site or no more than 10% of the samples from each distribution system in a given sample set should show the presence of total coliforms.

Standards - Sampling:

The number of bacteriological samples to be taken is based on the following:

No distribution system, PWDUs or very small system serving less than 100 people. 1 sample per month
Distribution systems serving:
Less than 5,000 population 4 samples per month
5,000 to 90,000 1 sample per 1,000 population per month
more than 90,000 90 plus one sample per additional 10,000 population per month

Standards - Disinfection:

Continuous disinfection is required for community and public facility water systems. Chlorine is the most common chemical used for disinfection, and where used:

  • All water entering the distribution system or public facility, after a minimum 20 minute contact time, shall contain a residual disinfectant concentration of free chlorine of at least 0.3 mg/L, or equivalent CT value;
  • A detectable free chlorine residual must be maintained in all areas in the distribution system. For potable water dispensing units (PWDUs) where the finished water is continuously disinfected immediately before being dispensed to the public using ultra-violet radiation (UV) or any other approved method for that unit, no disinfection residual is required.

Water systems which are primarily disinfected by means other than chlorination, must be provided with a sufficient residual disinfectant as determined to be appropriate for that system.

Rev: July 2, 2008

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