Providing safe drinking water is a core public health service. Health Canada, in collaboration with the Federal-Provincial-Territorial Committee on Drinking Water, developed Canadian Drinking Water Guidelines. Each guideline was established based on published scientific research related to health effects, aesthetic effects, and operational considerations. This research forms the basis of Health Canada’s Guidelines for Canadian Drinking Water Quality (GCDWQ). The GCDWQ has specific standards for the microbiological, chemical, physical and radiological quality of drinking water in Canada.
In BC, water suppliers (including small water systems) are expected to provide potable water (defined as “safe to drink and fit for domestic purposes without further treatment”) that meets the water quality criteria set out in the GCDWQ. The minimum level of treatment to make drinking water safe (microbiologically and chemically) will depend on the type of source water and its quality.
Treatment for Pathogens
Pathogens are disease-causing microorganisms (or microbes), and the most frequent cause of water-borne illness. They include bacteria, viruses and protozoa. These waterborne pathogens can cause diarrhea, vomiting, cramps, nausea, headaches, fever, fatigue, and sometimes even death. Infants, children, elderly people, and people with weakened immune systems are more likely to get sick, suffer, and possibly die from pathogens in drinking water.
Under BC's Drinking Water Protection Regulation (DWPR), disinfection is required if the water originates from (a) surface water or (b) groundwater at risk of containing pathogens (GARP). Under the DWPR, “surface water” is defined as “water from a source which is open to the atmosphere and includes streams, lakes, rivers, creeks and springs.”
Treating the source water to reduce or inactivate pathogens is referred to as Primary Disinfection. Secondary Disinfection is discussed under the section Designing Treatment for Small Water Systems and relates to the maintenance of a disinfectant residual throughout the distribution system.
No single treatment method is effective at removing all microbiological hazards. It is important to utilize a multi-barrier approach, which involves the use of two or more forms of treatment. Water treatment is only one part of the multi-barrier approach to providing safe drinking water. Choosing an appropriate water source, protecting that source and reducing distribution system risks, together with treatment, are essential steps in reducing microbiological threats to drinking water.
Minimum Treatment Objectives
The BC Ministry of Health’s drinking water treatment objectives provide a minimum performance target for water suppliers to produce microbiologically safe drinking water. The actual amount of treatment required will depend on the risks you’ve identified for your water source. Your particular situation may require greater levels of treatment. Your local Drinking Water Officer (DWO) may require additional treatment requirements in your operating permit, and will need adequate data to address the public health risks based on the microbiological and chemical quality of your source water. In all cases, the DWO must be contacted to confirm the necessary treatment objectives for microbiological parameters when planning or upgrading water supply systems. The regional health authority’s Public Health Engineer (PHE) should also be contacted as they can provide technical information regarding treatment processes.
To reduce the risk of waterborne illness, the minimum treatment objectives for surface water and GARP sources are:
- 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 or equal to (≤) one (1) nephelometric turbidity unit (NTU) of turbidity
- No detectable E. coli, Fecal Coliform and Total Coliform.
These treatment objectives are set based on an assumed level of pathogen in the raw water and a target level of pathogen in the treated water and they are considered a minimum requirement. Water with really high levels of pathogens may require a greater log reduction (such as a 7-log reduction of viruses).
Log reduction is the method used to measure how much you are reducing pathogens. It uses the the logarithmic scale which is a mathematical process based on factors of 10 (in this case, it refers to 10 fold increases or decreases in a specific microbe).
For drinking water treatment, log reduction expresses the relative number or percent of viable pathogens that are reduced/inactivated (killed or unable to replicate) by the treatment process. The log reduction method shows how effective your treatment system is as it measures the difference between the number of pathogens in your treated water compared to your raw water (source water). There is more information on this in BC Ministry of Health’s Small Water System Guidebook in section 6.4.2.
For every log, there is a 10-fold reduction in pathogens:
- 1-log reduction = number of pathogens in raw water ÷ 10
- 2-log reduction = number of pathogens in raw water ÷ 100
- 3-log reduction = number of pathogens in raw water ÷ 1000
- 4-log reduction = number of pathogens in raw water ÷ 10,000
The log inactivation of microbes can also be expressed as a percent reduction of the target microbe:
- 1-log reduction = 9 out of 10 = 90% reduction
- 2-log reduction = 99 out of 100 = 99% reduction
- 3-log reduction = 999 out of 1,000 = 99.9% reduction
- 4-log reduction = 9,999 out of 10,000 = 99.99% reduction
Log reduction also calculates the number of microbes left behind once disinfection has taken place. For a water sample containing 1,000 viruses, a 1-log or 90% kill rate will leave 100 viruses behind.
Water supply systems should provide, as a minimum, 4-log reduction of viruses for all surface water systems and groundwater at risk of containing pathogens (GARP). Depending on the surface water source, especially those subject to human fecal contamination, a greater than 4-log reduction may be necessary. It should be noted that a 4-log reduction or inactivation of enteric viruses will also provide effective treatment for pathogenic bacteria such as E. coli.
The physical removal of viruses is challenging due to their small size but by using certain types of filtration systems, you can reduce them. Effective filtration for this use are conventional, direct, slow sand and diatomaceous earth filtration systems. Enteric (intestinal) viruses are inactivated by chemical disinfection such as chlorine. The regional health authority (typically, the PHE) will provide guidance regarding the required chlorine concentration and contact time prior to the first water service connection.
Ultraviolet light radiation will also provide some reduction of viruses.
The risk of protozoa is most likely to occur in surface water; however, groundwater at risk of containing pathogens (GARP) and groundwater under the direct influence of surface water (GWUDI) sources are also at risk of contamination from protozoa. Giardia can be inactivated by large doses of chlorine with a long exposure time, while Cryptosporidium is notably resistant to chlorine.
UV light radiation differs from chlorination in that it is highly effective at inactivating both Giardia cysts and Cryptosporidium oocysts.
Some pathogens are more resistant to certain forms of treatment than others and no single type of treatment system is effective in addressing all hazards. Therefore, as part of the multi-barrier approach, two or more forms of treatment are necessary for systems with surface water sources. As most disinfection systems require clear water for effective protection, the GCDWQ recommends that filtration and one form of disinfection be used to meet the treatment objectives.
GARP sources also need two forms of treatment; however, ‘subsurface filtration credits’ may apply if certain criteria are met (with reference to the Drinking Water Treatment Objectives (Microbiological) for Ground Water Supplies in B.C.).
Turbidity is a measure of the relative clarity or cloudiness of water. The turbidity of filtered water is usually measured in nephelometric turbidity units (NTU), using a device called a turbidimeter. Turbidity is not a direct measure of suspended particles, but rather a general measure of the scattering and absorbing effect that suspended particles have on light. Turbidity includes both:
- Inorganic particles (clay, silt and metal precipitates)
- Organic particles (decomposed plant and animal debris and microorganisms)
Turbidity is generally acceptable when less than 1 NTU and it becomes visible when above 5 NTU. Turbidity interferes with the disinfection process as the particles can harbour microorganisms and protect them from disinfection.
Turbidity is effectively reduced through filtration. The goal of treating water for turbidity is to reduce its level to as low as possible and minimize fluctuation. Ideally, a target of 0.1 NTU in treated water should be maintained at all times. Where this is not possible, treated water from filters or units will depend on the method used:
- conventional and direct filtration - less than or equal to 0.3 NTU
- slow sand or diatomaceous earth filtration - less than or equal to 1.0 NTU
- membrane filtration - less than or equal to 0.1 NTU
An inability to achieve these levels indicates a system that is not working well enough to avoid potential health impacts for their users. As stated in the GCDWQ, the treated surface water entering the distribution system should have a turbidity level less than or equal to (≤) 1 NTU.
There must be no detectable Escherichia coli (E. coli) and other fecal coliforms and total coliforms in drinking water based on a 100mL sample with the sampling frequency established by the DWPR or through agreement with the Drinking Water Officer (DWO).
E. coli and other fecal coliforms are members of the total coliform group of bacteria, which are found naturally in water, soil, vegetation and feces. E. coli is the only member found exclusively in the feces of humans and other animals. The presence of E. coli and other fecal coliforms in water indicates recent fecal contamination which means there is a greater risk that pathogens (disease-causing bacteria, viruses and protozoa) are present. The ability to detect fecal contamination in drinking water is a necessity, as pathogenic microorganisms from human and animal feces in drinking water pose the greatest danger to public health.
The absence of E. coli, fecal coliform and total coliform is used as an indicator that treated water is free from intestinal disease-causing bacteria. Their presence in drinking water distributed from a treatment plant indicates a serious failure requiring immediate corrective action. The presence of total coliform bacteria in the water distribution system indicates that the distribution system may be vulnerable to contamination (e.g., due to inadequate chlorine residuals or pipe leaks) or experiencing bacterial regrowth.
E. coli, fecal coliform and total coliform are easily controlled with disinfection processes such as chlorine or UV light and can also be reduced by filtration.
Design and construction guidelines for BC waterworks
Guidelines for the Construction of Waterworks, Interior Health, BC, 2014
For more information on viruses, refer to Health Canada’s Guidelines for Canadian Drinking Water Quality: Guideline Technical Document -- Enteric Viruses
BC Ministry of Health’s Small Water System Guidebook
Guidelines for the Construction of Waterworks, Interior Health
Small water systems design guidelines (groundwater source), Northern Health
Small water systems design guidelines (surface water source), Northern Health
Lists of certified water treatment devices and systems are available on NSF’s Certified Products and Systems web page under “Water and Wastewater” then open the link to “NSF Certified Drinking Water Treatment Units, Water Filters