MYTH vs TRUTH: “If my disinfectant residual is always high enough, I don’t have to worry about microbes”

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Courtesy of LuminUltra Technologies Ltd.

Monitoring disinfectant residuals (i.e. Free and Total Chlorine and/or Chloramine concentration) is one of the most essential practices in drinking water management programs.  This ensures that sufficient protection is maintained at all points in the distribution system.  The absence of a disinfectant residual means that suppression of microbiological growth is much more difficult and the rate of regrowth can be significantly accelerated.  However, does maintaining an adequate disinfectant residual provide enough protection?

Here are 3 important facts to keep in mind about disinfectant residual and biological growth:

Microbes can and do survive in numerous ways – Microorganisms tend to form attached communities in distribution systems called “biofilm” on the walls of pipes and tanks. Disinfectants in the bulk water phase will therefore only contact the outer layers of the biofilm while the communities within the biofilm are protected.
Established biofilms in water piping are notoriously difficult to remove in terms of time, effort, and overall cost. As such, it is much more cost-effective to prevent significant biofilm problems from occurring rather than trying to remove them after the fact. Most water distribution systems experience challenges associated with biofilms to some degree, so the realistic goal should be to manage the risk rather than to expect to totally eliminate it.
Also, there are several classes of microorganisms that are resistant to disinfectants, making them especially difficult to deal with. For example, Cryptosporidium is highly resistant to chlorine, therefore alternative disinfectants such as chlorine dioxide, ozone, or UV treatment should be used.
In general, it’s not surprising to see elevated microbiological content at times when disinfectant residual is considered sufficiently high (see Figure 1).

  • Disinfection efficacy is highly pH-dependent – A fundamental aspect of water chemistry is the effectiveness of chemicals in various pH ranges. For example, while chlorine is effective at a neutral pH and below, it is only one third as effective at a pH of 8:

Figure 2: Chlorine Activity Curve (courtesy of Hach Company)

  • Disinfectants like chlorine, are not selective to microorganisms – Chlorine is a broad-spectrum oxidizer, meaning that it reacts with pretty much anything it touches.  This includes microorganisms but it will also react with organics, sediment, and pre-existing pipe build-up.  Loss of residual could be due to several chemical and physical factors, not just microbiological growth, therefore a loss of residual is not necessarily an indicator of biological contamination.

Effectively handling microbiological regrowth in your distribution system is best achieved using a holistic approach that includes a combination of both treatment methods and monitoring tools that allow you to quantify the complete microbiological risk.

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