Cooling Water

Do Current Antimicrobial Programs for Cooling Systems Control Legionella?

By Paul Puckorious


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Because of the recent outbreaks of Legionnaire’s Disease such as in New York City (Summer 2015), it was decided to investigate how and why this could have occurred by cooling tower water systems that were chemically treated. Cooling tower water systems are normally treated by the traditional method of applying microbicides and often monitored with traditional total bacteria monitoring methods.

This has prompted a detailed review of the traditional microbiological treatment applications in numerous cooling tower water systems in all types of industrial systems to see if there are any reasons that Legionella bacteria could still be present.

The study showed that traditional microbiological control techniques might be incorrect and misleading in controlling Legionella bacteria due to the way the microbicides are being applied. Examples are identified that was found in numerous cooling tower water systems and in various industries.

These examples show some very interesting and revealing information in how the microbicides are applied that may not be an indication of effective control of Legionella bacteria. It also shows that the traditional monitoring levels of total bacteria levels that have indicated good microbiological control as reported by culture or “dip” slides may be incorrect and that the guidelines by the Cooling Technology Institute and others, which indicate that 104 or less as “good” microbiological control as being incorrect. This study and the results suggest new applications and monitoring methods should be adopted.

Cooling tower water systems have been associated with Legionnaire’s Disease for many years, even though they are the not the primary cause. It has been reported that cooling towers relate to about 20% of the Legionnaire’s Disease outbreaks, while potable water accounts for 80%.


The original outbreak of the disease was in Philadelphia in 1976. That was at the Legionnaire’s Convention and was due to the drift from a Legionella bacteria contaminated cooling tower; thus, the name of the disease.

Cooling tower water systems do have the perfect environment for the growth of all types of microbiological organisms, including Legionella bacteria. This includes water, nutrients, and warmth. They also discharge water in the form of fine water droplets called “drift” to the surrounding area. This is characteristic to their operation. The drift is how cooling tower water systems can spread the bacteria. This drift as very fine water droplets were suspected to carry as far as one mile from the cooling tower as reported in a Legionnaire’s Disease outbreak in Wisconsin.

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