Alternating Biocides: Myth or Fact

Controlling microbiological growth in cooling tower systems is critical for several reasons. One factor is that microbiological fouling can have an insulating effect and reduces heat transfer. The resulting energy inefficiency may cost thousands of dollars. Another vital concern is controlling Legionella bacteria. Controlling this hazardous microbiological waterborne pathogen represents a serious concern for health and safety in and around a cooling tower.

Industry Recommendations

Typically, both oxidizing and non-oxidizing biocides are used to control microbiological growth. The CDC, ASHRAE, and OHSA all refer to incorporating an oxidizing biocide along with a non-oxidizing biocide for the most effective control of Legionella. However, there are no defined laws for application of these biocides.

Here’s a statement taken directly from the OHSA website:

Periodic biocide treatment in water systems is another way to prevent or reduce Legionella growth. Although there is not consistent data about biocide effectiveness in all types of water systems, existing evidence suggests that halogen oxidizers (including certain chlorine and bromine compounds), ozone, peroxides, and non-oxidizing biocides help control Legionella when properly used.

This is not law or even a directive, it is a suggestion. Similar “suggestions” are made by other professional organizations for minimizing risk from Legionella. But lets look at some reasonable options for controlling both Legionella and microbiological growth.

Chlorine, an oxidizing biocide, cannot penetrate biofilm. When/if biofilm is present, the use non-oxidizing biocide can complement the oxidizers. Or, in another example, if 90% of the bacteria in a system are controlled by chlorine or bromine, what about the other 10%? They can continue to grow unchecked, generate more biofilm, and cause more problems.

Here at Chemtex, we recommend that for controlling all types of microbiological growth you complement an oxidizing biocide with a non-oxidizing biocide. And, your selection of non-oxidizing biocide(s) should be based on the results of on-site field testing and/or toxicant evaluations.

Legionella Control

Prescribed methods for controlling Legionella should be employed, and that means incorporating an oxidizing biocide. Oxidizing biocides are specifically mentioned in ASHRAE 188 as control measures for Legionella, so we believe all biocide programs should incorporate them.

If a plant were not using an oxidizing biocide and Legionella resulted, would that be considered on the edge of negligence? We think it could be, by not following suggested control measures the door is left open for greater liability and maybe even negligence.

Remember, it’s important to note that total bacterial counts (TBC) are not a good means to determine whether Legionella is under control. Actual Legionella testing will be the true validation of an applied biocide program.

Ultimately, controlling bacteria and microbiological growth can and should be done economically and efficiently. Complementing the oxidizer(s) with a proven non-oxidizing biocide will improve performance, and can help provide protection, efficiency, and peace of mind.

Our Recommendation

In summary, here’s what Chemtex recommends:

• Use an oxidizing biocide.
• Create targets for the feed rates
• Document test results of the residual using approved testing methods
• Compliment it with a non-oxidizing biocide.
• Prove it is effective with bacteria testing.
• ATP, plate, counts or dipslides
• Document test results
• Follow recommendations from industry associations to minimize risk.

Please reach out to Chemtex if you have specific questions. We’re your partner in safe, economical and efficient water treatment programs.