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.
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.
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:
- Drinking Water Treatment Objectives (Microbiological) for Surface Water Supplies
- Drinking Water Treatment Objectives (Microbiological) for Ground Water Supplies
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.
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.