Minimum Treatment Requirements for Small Water Systems


Health Canada requires that small water systems ensure drinking water meets the water quality criteria set out under the Guidelines for Canadian Drinking Water Quality (GCDWQ). The minimum level of treatment required to make drinking water microbiologically and chemically safe depends on the quality and type of the water source, as well as the size and type of the population served.

Glossary of Terms

Qualities that are attractive to the senses (e.g., the pleasing appearance, taste, and smell of potable water).


Microscopic living organisms that usually consist of a single cell. Most bacteria use organic matter for their food and produce waste products as a result of their life processes.


A group of bacteria found in humans, plants, soil, air, and water. Fecal coliforms are a specific class of bacteria that only inhabit the intestines of warm-blooded animals. The presence of coliform bacteria is an indication that the water is polluted and may contain pathogenic (disease-causing) organisms.


A waterborne intestinal parasite that causes a disease called cryptosporidiosis in infected humans. Symptoms of the disease include diarrhea, cramps, and weight loss. Cryptosporidium contamination is found in most surface waters and some groundwaters. Commonly referred to as "crypto."


The process designed to kill or inactivate most microorganisms in water, including essentially all pathogenic (disease-causing) bacteria. There are several ways to disinfect, with chlorination being the most frequently used in water treatment.

free chlorine residual

That portion of the total available residual chlorine composed of dissolved chlorine gas (Cl2), hypochlorous acid (HOCl), and/or hypochlorite ion (OCl-) remaining in water after chlorination. This does not include chlorine that has combined with ammonia, nitrogen, or other compounds.

free chlorine

Chlorine (Cl2) in a liquid or gaseous form. Free chlorine combines with water to form hypochlorous (HOCl) and hydrochloric (HCl) acids. Wastewater-free chlorine usually combines with an amine (ammonia or nitrogen) or other organic compounds to form combined chlorine compounds.


Ground Water that is at Risk of containing Pathogens


Ground Water Under the Direct Influence of surface water


A waterborne intestinal parasite that causes a disease called giardiasis (GEE-are-DIE-uh-sis) in infected humans. Symptoms of the disease include diarrhea, cramps, and weight loss. Giardia contamination is found in most surface waters and some ground waters.


Nephelometric Turbidity Units: a measure of clarity of water.


A means of measuring turbidity in a sample by using an instrument called a nephelometer. A nephelometer passes light through a sample, and the amount of light deflected (usually at a 90-degree angle) is then measured.

potable water

Water that does not contain objectionable pollution, contamination, minerals, or infective agents and is considered satisfactory for drinking.


Single-celled organisms with a more complex physiology than viruses and bacteria; average diameter of 1/100 mm.

residual chlorine

The amount of free and/or available chlorine remaining after a given contact time under specified conditions.

surface water

All water naturally open to the atmosphere (rivers, lakes, reservoirs, streams, impoundments, seas, estuaries, etc.); also refers to springs, wells, or other collectors that are directly influenced by surface water.


The cloudy appearance of water caused by the presence of suspended and colloidal matter. In the waterworks field, a turbidity measurement is used to indicate the clarity of water. Technically, turbidity is an optical property of the water based on the amount of light reflected by suspended particles.

Typically, two or more forms of treatment are required, in a multi-barrier approach, as no one type of treatment system is effective in treating all hazards.

For a specific water source, it is recommended the owner or operator of a small water system consult with the local Drinking Water Officer and Public Health Engineer to confirm the necessary treatment objectives and specific types of treatment needed.

Treatment for Pathogens

The most common health risks associated with consuming drinking water are pathogens, which include disease-causing bacteria, viruses and protozoa. Under B.C.'s Drinking Water Protection Regulation, disinfections (primary and/or secondary) are required if the water originates from (a) surface water or (b) Ground Water that is at Risk of containing Pathogens (GARP).

The B.C. Ministry of Health has developed the following documents to outline the minimum treatment requirements for reducing pathogens in the water supply:

As outlined in the two documents above, water treatment systems are expected to achieve, at a minimum, the following treatment objectives to reduce the risk of waterborne illness:

  • 4-log (99.99%) reduction or inactivation of enteric viruses
  • 3-log (99.9%) reduction or inactivation of protozoa (Giardia and Cryptosporidium)
  • Two treatment processes for surface water
  • Less than 1 NTU of turbidity, with a target of 0.1 NTU
  • No detectable E. Coli, fecal coliform and total coliform

4-log (99.99%) reduction or inactivation of enteric viruses

This risk is most likely to occur in surface water or ground water at risk of contacting pathogens (GARP). While enteric viruses are easily inactivated by using chlorine, requirements for chlorine residual concentrations are determined by the responsible authority. Most jurisdictions specify a minimum level of free chlorine residual that should be applied at the treatment plant and/or detectable within the distribution system.

In most provinces, First Nations, and territories, higher chlorine residuals can be permitted by the regulatory authority, as deemed necessary on a case-by-case basis. Operators should aim to maintain a chlorine residual of 0.2 to 0.5 mg/L throughout the distribution system to minimize taste and odour issues at the consumer's tap.. 

3-log (99.9%) reduction or inactivation of protozoa (Giardia and Cryptosporidium)

The risk of protozoa is most likely to occur in surface water, GARP or GWUDI sources. Giardia can be inactivated by large doses of chlorine with a long exposure time, while Cryptosporidium is notably resistant to chlorine.

However, UV irradiation is highly effective for inactivating both. For small water treatment systems, commercially available UV systems, certified to NSF/ANSI Standard 55 Class A, can be used to achieve 3-log inactivation of protozoa by delivering a minimum dose of 40 mJ/cm2.

Two treatment processes for all surface drinking water systems

Some pathogens are more resistant to certain forms of treatment than others. As no single type of treatment system is effective in addressing all hazards, the treatment objectives achieved through a multi-barrier approach typically includes two or more forms of treatment, filtration and disinfection.

Less than 1 NTU of turbidity, with a target of 0.1 NTU

Turbidity can impact the disinfection process. The Guidelines for Canadian Drinking Water Quality specify that the treated water turbidity should be maintained less than one nephelometric turbidity unit (NTU) at all times, with a target of 0.1 NTU.

No detectable fecal coliform and E. Coli

Under the Drinking Water Protection Regulation, no detectable E. coli or fecal coliform shall be observed per 100 ml water sample, where total coliforms shall be also zero, based on sampling frequency established by the Drinking Water Officer. In general, the E. coli. and coliforms can be easily controlled through disinfection process (i.e. chlorine and/or UV light) and reduced by filtration.


Treatment for Chemical and Physical Parameters

To be considered potable, drinking water must also meet the health-based water quality criteria set out in the Guidelines for Canadian Drinking Water Quality, which requires water to be chemically safe. 

To provide chemically safe drinking water, a water treatment system must provide treatment to ensure that any toxic chemicals does not exceed the maximum allowable concentrations (MAC) as stated in the Guidelines for Canadian Drinking Water Quality.

Further, a number of chemical and physical parameters in water do not directly pose a health threat but may cause aesthetically objectionable effects or render water unsuitable for domestic use (e.g., taste, odour, and/or appearance). Aesthetic objectives (AOs), under the Guidelines for Canadian Drinking Water Quality, reflect the aesthetic quality of drinking water. Water suppliers are encouraged to treat water to ensure that an AO for a particular chemical does not unreasonably exceed the level set in the guidelines.