Controlling Legionella in Cooling Towers

( The following contains excerpts from the ASHRAE IAQ Applications, Spring 2000 )

It’s a fact! Cooling towers regularly contain Legionella bacteria. Growth of Legionella can occur at temperatures between 68 F and 122 F, though optimal growth occurs between 95 F and 115 F. The warm waters in the sump and piping of cooling towers are rich in dissolved nutrients providing an ideal habitat for the bacteria. Many studies have found Legionella to be a normal occurrence in cooling towers everywhere.

In one study, more than half of the 80 samples collected from cooling towers contained Legionella. Not surprisingly, cooling towers are indicted as a principle source of Legionella infections. Heated spas, misters and fountains also are indicted as sources of Legionella outbreaks, and many investigators indict plumbing systems as the greatest risk of Legionella infection.

Legionella is difficult to kill with normal chemical biocides. They do not live suspended in the water itself, but thrive in the backwaters of the system, on submerged equipment and particle surfaces. Here they form part of the bacterial slime coating these surfaces. The mucus in which they are embedded protects these Legionella from exposure to biocides in the water.

Further, Legionella are protozoonotic, that is, they live and reproduce within the body of other microbes, especially amoebae and paramecia. These protozoans are not very sensitive to the biocides used to control bacteria, and therefore represent a safe haven for populations of Legionella. One study reported that bacteria living in amoebae are 30 to 120 times less susceptible to common bactericides than they would be if living in water. Even after as system has been cleaned, Legionella from surviving protozoa can easily reestablish a population within weeks.

Legionella also survive in low numbers in many potable water systems, and can contaminate a clean cooling tower with the addition of makeup water. For the reason, ASHRAE and other groups recommend preventive maintenance, regular cleaning of the tower basin and a well-designed and monitored water treatment program as the best way to minimize the potential for disease transmission.

Because many commonly used biocides are ineffective at eliminating Legionella, care must be taken to use those that are considered effective. Some concern exists that Legionella, through constant exposure, may become resistant to even these effective agents. Consequently some authors recommend rotating biocides. Thermal shock, 140 F for 8 hours, has been suggested as a Legionella disinfection procedure for some systems, but is not practical for cooling tower treatment.

Oxidizing biocides are often recommended as the best mode of chemical control. However, this type of biocide can be aggressive towards metal surfaces and therefore is not appropriate for every system. For this reason, non-oxidizing biocides are in common use on many cooling tower systems.

Chlorine-based oxidants can be effective, though they seem to work best if used as periodic shock treatment in large slugs. Protozoa are not as susceptible to chlorine as are bacteria, and it must be used at high levels to ensure these Legionella-harboring organisms are eliminated.

Ozone also proves effective, and can be used continually. Ozone will oxidize the organic constituents of slime and if concentrations are correct, will deny this protective habitat to Legionella. Also, ozone effectively kills protozoa, reducing the availability of this Legionella sanctuary.

Although a regular biocidal treatment program cannot guarantee that Legionella are not present in your system, it will ensure that populations in the reservoir are at the minimum levels that can be attained. This will reduce the potential for transmission of the disease.

The above paragraphs were excerpted from ASHRAE IAQ Applications, Spring 2000 Volume 1, No.2. The entire article can be requested from us and a copy can be sent to your attention.