Controlling & Monitoring Legionella in Cooling Towers

What Cooling Tower Operators Need to Know

One of the larger considerations for teams that monitor and maintain cooling towers at industrial facilities is the risk that comes from Legionella – a bacteria that can cause a serious lung infection or pneumonia known as Legionnaires’ disease, as well as a milder illness called Pontiac fever. There is no known safe level of Legionella in any water system. And because Legionnaires’ disease can be acquired when an individual breathes in water droplets containing Legionella bacteria, facilities that don’t monitor or check for Legionella could be creating hazardous conditions for both workers at the facility and those who live in surrounding areas.

Why Are Cooling Towers at Risk for Legionella

Cooling tower systems present perfect breeding grounds for Legionella bacteria because they contain pools of warm water that are open to the atmosphere. Legionella bacteria will generally thrive in such an environment, if the water is not routinely tested, cleaned, and/or disinfected through regular maintenance.

That said, inevitably, most cooling towers are likely to become contaminated with Legionella at some point in their serviceable life and no water treatment or maintenance system is guaranteed to fully or permanently eliminate the bacteria. What you can do, however, is routinely monitor and test for it, keeping Legionella at bay through regular maintenance and water treatment using microbial control.

Factors That Contribute to Legionella Development in Cooling Towers

Typically, several factors can contribute to a Legionella problem at industrial or manufacturing facilities.

No. 1: Source Water Quality. The source of your makeup water – in this case, the water that replaces that which was lost from drift, evaporation and the blowdown during the cooling system process – will certainly matter. Sometimes, this makeup water comes from a municipal water source or a well, or it may come from a holding tank, which can contain rust, sediment, or even sludge. In addition, it can also come from surface water (from lakes, rivers or reservoirs) that may be full of microorganisms to start with. 

No. 2: Biofilms. Biofilms are basically microorganisms that can grow on many different surfaces. Microorganisms that form biofilms include bacteria, fungi and protists. Source water can be filled with microorganisms as mentioned above, but also dirt, dust, and other particles can carry biofilms as well, which enter the cooling tower water during the cooling process. And, depending on where the cooling tower is located, the amount of material can be substantial in some cases. Through its nutrient-rich nature, biofilms provide the foundation for Legionella bacteria to survive. Reducing the presence of these microorganisms should be part of any Legionella control efforts, including using environmentally acceptable dispersants or detergents to penetrate biofilm and sediments.

No. 3: The Water Treatment/Disinfection Process. Your water treatment process obviously helps to keep Legionella at bay, but it’s important to remember that temperature, the flow velocities of the cooling tower water and other environmental factors can affect treatment performance and efficacy. One rule here is the operation of your HVAC system design (which allows for water to circulate through the cooling tower process) should be coordinated with any fuller chemical treatment of the water, including steps to keep the system clean of all dirt and debris. Typically, even if you do apply chemicals or other disinfectant agents to the water, it can rapidly become recolonized with microorganisms as cooling towers are open to external environment. If your cooling tower needs to be shut down for more than 3 days, be sure to fully drain the entire system.

No. 4: Material Used in Cooling Towers / Cooling Tower Design. Some metal cooling tower systems might suffer corrosion or degradation that can lead to a proliferation of microorganisms. That’s why it’s probably worth considering an engineered plastic or other anti-microbial material that won’t degrade in any environment.

Also, the design of some cooling towers can certainly increase the risks of Legionella bacteria. For example, a cooling tower design that inadvertently creates areas of stagnant water can prevent proper chemical treatment of the cooling towers during any treatment process. In the ideal world, any cooling tower system should be designed for easy cleaning in and maintenance to avoid the buildup of deposits, sludge or other sediments. Poor piping design can also create the environment where Legionella can thrive. Make sure the design of any cooling tower system can recirculate water during intermittent operation.

No. 5: Warm (Seasonal) Weather. Because Legionella thrives in warmer water (between 20 and 45 degrees Celsius), any summer heatwave, in particular, can raise the ambient temperature of stored cold water. Hence, it’s important to monitor water temperatures during spells of hot weather, potentially investing in improved insulation on pipes and tanks. This will help to regulate and reduce the temperature even on very hot days.

​Risks From Cooling Tower Spray Drifts

Cooling tower drift are small water droplets that are emitted from cooling towers. This drift water has the same chemical composition as the circulating water in the tower, can potentially have high salt concentrations, particulate matter and bacteria, making it harmful and potentially toxic. Cooling towers generally use drift eliminators to address this issue but even under normal operations, water droplets can leave the drift eliminator. If Legionella is present, the aerosolized water can spread the bacteria over miles. For this reason, cooling tower systems should not be installed near air intakes or windows that might be opened. Certainly, any facility should also be sure there are no gaps within the drift eliminator that allow a free flow of dirt or other foreign matter.

Monitoring and Testing for Legionella

The safe operation and regular maintenance of cooling towers will protect employees, visitors, and the nearby community from exposure to Legionella. The experts at rqmicro and organizations such as the CDC recommend setting up a water management program specific for your facility that includes taking into account regular testing and monitoring water parameters such as disinfectant residual and pH, on a regular basis. Measurement frequency will be dictated by the general performance of your particular water management program and any Legionella indicators that appear during the monitoring process. 

At rqmicro, we have developed tests that enable cooling tower operators to monitor the presence of Legionella with reliable results generated on-site or in labs within 2 hours, using an easy, four-step process of:

  • Filtration
  • Reaction
  • Analysis
  • Results/Hygiene assessment

Learn more about the rqmicro.COUNT tests that we provide for cooling tower operators or contact us today. You can also view our video that discusses how you can implement testing to detect Legionella or other bacteria quickly and easily.


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rqmicro at IWA World Water Congress & Exhibition in Denmark
11 - 15 September 2022 | Copenhagen, Denmark