Primary & Secondary Disinfection

Surface water and ground water at risk of containing pathogens (GARP) sources require two methods of treatment, or dual treatment. Filtration alone is not an effective method of treatment against all pathogens. Using the multi-barrier approach, filtration is usually followed by inactivation processes (disinfection) either individually or in combination. Disinfection removes, inactivates or kills pathogenic microorganisms (bacteria, viruses and protozoa) by destroying the cell wall or interfering with its metabolic processes. Because no single disinfectant is effective against all pathogens equally, a multi-barrier approach employing multiple disinfection technologies is recommended. The most common disinfectants are chlorine, chloramine, chlorine dioxide, ozone and ultraviolet light radiation.

Each type of disinfection process has advantages and disadvantages. Ultimately, no single disinfectant is effective against all waterborne pathogens; a multi-barrier approach is the most effective strategy to treat drinking water.

Potable (drinkable) water is required from all parts of the water supply system, including the distribution system. The distribution system is that part of the water supply system used to convey water from the treatment plant or water source to the water users. As potability must be maintained throughout the distribution system, Health Canada recommends a combination of primary and secondary disinfection for surface water and GARP sources.

Primary Disinfection vs Secondary Disinfection

There is a clear distinction between primary disinfection and secondary disinfection. They are often completely separate treatment processes that provide different outcomes. 

Primary Disinfection (Primary Treatment) is intended to provide disinfection before the water is delivered to the first consumer. It is intended to kill or inactivate pathogenic microorganisms that may be present in the source water. Section 5(2) of the Drinking Water Protection Regulation (DWPR), requires a water supplier to disinfect water originating from surface water or GARP sources. 

Some disinfectants are effective as a primary disinfectant but they are not maintained throughout the distribution system. For example, while ultraviolet light radiation (UV) is effective as a primary disinfectant, it does not prevent pathogen regrowth in the distribution system as it does not produce a chemical residual. The chemicals ozone and chlorine dioxide are effective primary disinfectants but they degrade too quickly and do not maintain a residual.

Secondary disinfection (also referred to as ‘residual disinfection’ or ‘distribution disinfection’) is the maintenance of a disinfectant residual throughout the distribution system in order to protect the water quality. Secondary disinfection differs from primary disinfection in that the concentration of disinfectant used will not necessarily be strong enough, or have the necessary contact time, to deactivate pathogens to the extent needed to meet the requirements of section 6 of the DWPA. 

A distribution system has many components (reservoirs, water mains, pump stations, pumps, hydrants, valves, etc.) which may be vulnerable to contamination during an emergency event or if system maintenance is inadequate. Leaks in pipes or improper connections may allow the intrusion of pathogens into the distribution system. This contamination is often of fecal origin (sewage, animals, surface water, etc.) which presents an increased risk of waterborne illness. 

Once pathogens are introduced to the distribution system, favourable conditions (e.g. temperature and stagnant water) may provide a suitable environment for bacterial growth and pathogen survival in biofilms. Biofilm is a slime layer formed from particles, nutrients and microorganisms. Biofilm attached to the walls of pipes in the distribution system provides an environment that can harbour pathogens (primarily, bacteria). If the biofilm is disturbed, these pathogens can be released into the drinking water in high concentrations. Systems with many dead ends or systems that do not have a high rate of water turnover are at risk for having stagnant zones in the distribution system that contain “old water,” particularly if they do not practice regular flushing.

Secondary disinfection protects the distribution system by:

  • protecting the water from microbiological recontamination (due to pipe leaks, cross connections, etc.)
  • reducing bacterial re-growth
  • controlling biofilm formation
  • serving as an indicator of the microbiological integrity of the distribution system (through bacteriological sampling)

Currently, chlorine and chloramine are considered the most effective secondary disinfectants but most water systems use chlorine as a secondary disinfectant. Chlorine is an effective primary disinfectant and secondary disinfectant but it requires a minimum concentration and contact time before pathogens can be reduced or eliminated. The regional health authority (typically, the Public Health Engineer) can provide guidance regarding the determination of the required chlorine concentration and contact time prior to the first water service connection. This is also discussed under Calculating CT Disinfection.

At the end points of the distribution system, operators should aim to maintain a free chlorine residual of 0.2 to 0.8 mg/L, or if chloramine is used, a minimum residual of 0.5 mg/L to ensure safe drinking water and to minimize taste and odour issues at the consumer's tap. On a case-by-case basis, most provinces and territories may permit higher chlorine residuals.

After reviewing system records, source-to-tap risk assessments or reports from qualified professionals; a Drinking Water Officer (DWO) may request or order a water supplier to have secondary disinfection. The DWO may also add this requirement as a condition on the operating permit.