Legionnaires’ disease is a dangerous type of pneumonia. People can get it by breathing in mist or vapor from water that’s contaminated with Legionella bacteria, and cooling towers are a common source. If these systems are not cleaned and maintained regularly, they can quickly fill with harmful bacteria, putting everyone nearby at risk of illness.
Testing cooling towers for Legionella is important for the safety of your building. Failure to do so can lead to serious problems, ranging from public health outbreaks to legal action and hefty fines.
This post explains why cooling towers are at risk for Legionella, how to collect and test samples, who should handle testing, and what to do if your results come back positive.
How Cooling Towers Become a Source of Legionella Bacteria
What Makes Cooling Towers Susceptible to Legionella Growth
Cooling towers are an ideal place for Legionella to grow. They keep water warm (between 68–122°F) and moist, exactly what this bacteria likes. If you add stagnant water, biofilm buildup, and skip regular maintenance, Legionella can multiply quickly.
Common Outbreak Cases from Cooling Systems
In 2024, a Legionnaires’ disease outbreak at the RiverWalk Resort in Lincoln, NH, made seven people sick after bacteria were found in a cooling tower. It’s a clear warning: regular cooling tower maintenance and Legionella testing are what keep people safe from these kinds of illnesses.
How to Test for Legionella in Cooling Towers
Recommended Legionella Sampling Procedures
- Surface Swabbing – Use a cotton swab to wipe different surfaces inside the cooling tower, such as drift eliminators, fill material, and plastic or metal parts. This lets you find any slimy biofilm buildup that isn’t easy to see during a normal inspection.
- Bulk Water Sampling – Collect samples of the water from different areas of your system, like basins, sumps, and return lines. Testing these samples can show if there are bacteria or chemical problems in your cooling tower’s water.
Common Testing Methods
- Culture Method (ISO 11731) – The gold standard of Legionella testing in cooling towers. It involves placing water samples on a growth medium to see if Legionella bacteria develop. While results take 7–10 days, this method confirms if live bacteria are present.
- PCR (Polymerase Chain Reaction) – This test uses genetic analysis to spot Legionella DNA in a sample. Because it doesn’t depend on bacterial growth, you typically get results within 24 to 48 hours. However, it won’t tell you if the bacteria are alive or dead.
- Legiolert® Test – With this method, a liquid culture detects the presence and amount of Legionella in your water sample. It’s simple to use, requires limited training, and provides clear positive or negative results within 24 to 48 hours. It’s a good option if you need to make a fast decision about system safety.
Testing Frequency Guidelines
- Monthly Testing – If your cooling tower is in a hospital, care facility, or any place that serves people with higher health risks, monthly testing is the safest bet. This way, you’re more likely to catch problems quickly and take care of them before they get worse.
- Quarterly Testing – For office buildings, schools, or commercial properties with no prior issues, quarterly testing supports your cooling tower maintenance plan. It helps keep your water safe, makes it easier to pass health inspections, and lets you find small problems early, before they become expensive to fix.
Who Should Conduct Legionella Testing in Cooling Towers?
Make sure you use an accredited lab or a certified Legionella specialist for testing. When choosing who to work with, check that they have the right lab certifications, can tell you how quickly you’ll get results, and will provide easy-to-read reports. A good specialist will run the tests, explain what the results mean, and help you figure out what to do if Legionella is found.
Legionella Cooling Tower Water Treatment After Testing
Steps to Take If Legionella Is Detected
- Start with a Shock Disinfection – Use a high-strength biocide, like chlorine dioxide or bromine, to kill any active Legionella bacteria in the system.
- Apply Heat Treatment – Raise the water temperature in the cooling tower to at least 160°F (71°C) for 30 minutes to help get rid of bacteria in hard-to-reach spots.
- Flush and Clean the Whole System – Drain and refill the cooling tower, remove any leftover debris or biofilm, and give the basins, fill, and nozzles a thorough cleaning.
- Test Again – After all the cleaning, collect new water samples and check that Legionella is no longer in the system.
Long-Term Water Treatment Strategy
- Use either a continuous or scheduled biocide feed to prevent Legionella and other bacteria from taking hold.
- Using a high-efficiency filter that fits your cooling tower helps catch dirt, debris, and organic matter before they cause trouble.
- Match your cleaning schedule and Legionella checks with your Water Management Plan to prevent bacterial outbreaks or equipment problems.
Long-Term Prevention of Legionella Disease in Cooling Towers
- Set a Schedule for Inspections and Cleaning – Clean every part, check chemical levels, and look over all the system components to make sure everything’s doing its job.
- Check for Risks Twice a Year – Look for spots where water sits still or where bacteria might have a chance to take hold.
- Write Everything Down – Water test results, chemical treatments, and any repair or cleaning work. Notes make it easy to stay compliant and help you see if anything’s changing over time.
- Stick to your Water Management Plan – If you keep up with routine care and regular checks, you make it much less likely that Legionella bacteria will get a foothold in your cooling tower.
Frequently Asked Questions
Q1: Is Legionella testing in cooling towers required by law?
It depends on the jurisdiction. However, ASHRAE 188 and OSHA strongly recommend routine testing. In many states (e.g., New York City, New Jersey), it’s a legal requirement.
Q2: How long does Legionella testing take?
Culture-based methods typically take 7–10 days, whereas rapid options like PCR or Legiolert® can return results in 24–48 hours.
Q3: What happens if Legionella is found in a cooling tower?
You must administer immediate disinfection, notify appropriate authorities if required, and perform follow-up testing to confirm system safety.
The Bottom Line
Testing your cooling towers for Legionella is required if you want to keep people safe and follow the law. With reliable testing tools, the help of a certified professional, and a clear step-by-step plan, you can keep Legionella from becoming a problem at your site.
Want to make sure your cooling towers stay safe and bacteria-free? Get in touch with our certified water safety experts for a full testing and treatment plan. We’ll handle every step so you have nothing to worry about. Call us now or contact our experts for reliable cooling tower water testing and compliance solutions.
