Minimum Treatment Requirements

 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.