Cooling towers require consistent water treatment, monitoring, maintenance, and microbiological control to operate safely and efficiently. Because these systems continuously recirculate water and discharge aerosols into the surrounding environment, they can present conditions favorable to Legionella growth when treatment or maintenance practices are inadequate.
Routine Legionella testing is an important component of a comprehensive cooling tower management program. Testing provides valuable information regarding microbiological conditions within the system and can help verify whether established treatment and control measures are performing as intended.
New York City, for example, enacted recent changes to cooling tower regulations requiring increased frequency of routine Legionella culture testing. These requirements reinforce the importance of maintaining a coordinated program that integrates water treatment, monitoring, sampling, corrective actions, and documentation.
Effective Legionella Control Begins With Proper Water Treatment
Cooling towers operate under conditions that naturally support microbiological activity. Warm water temperatures, aeration, sunlight, airborne debris, and contaminants introduced through makeup water can contribute to bacterial growth.
The presence of sediment, scale, corrosion products, and biofilm can further complicate microbiological control. Biofilm is particularly significant because it can provide an environment where microorganisms are protected from normal treatment conditions.
An effective cooling tower water treatment program should address these conditions through a combination of chemical treatment, monitoring, inspection, and preventive maintenance.
Typical program components include:
- Oxidizing and/or non-oxidizing biocide treatment
- Routine monitoring of disinfectant residuals
- pH and conductivity control
- Scale and corrosion inhibition
- Microbiological monitoring
- Proper bleed and makeup water control
- Routine inspection of chemical feed and control equipment
- Evaluation of tower cleanliness
- Identification of stagnant or low-flow conditions
- Scheduled cleaning and disinfection
- Documentation of operating conditions and corrective actions
Legionella culture testing complements these activities but does not replace them. A satisfactory laboratory result represents conditions at the time and location of sampling. Maintaining control between sampling events depends on the continued performance of the treatment and maintenance program.
New York City: A Prime Example of Legionella Testing Frequency
Effective May 8, 2026, New York City requires Legionella culture testing at least once every calendar month, with no more than 31 days between sampling events while the cooling tower is operating.
Testing is also required within 3 to 14 days following startup.
Samples must be analyzed by a laboratory certified by the New York State Department of Health Environmental Laboratory Approval Program (ELAP) for the applicable method.
The change from the previous 90-day interval to monthly sampling requires closer coordination between facility personnel, water treatment providers, Qualified Persons, and laboratories.
Example: Establishing a Sampling Schedule
Consider a commercial property operating its cooling tower from May through October. If a routine sample is collected on June 1, a practical sampling schedule may include:
- June 1
- July 1
- July 31
- August 30
- September 29
Sampling should generally be scheduled with sufficient allowance for weekends, holidays, site access, laboratory schedules, and other operational considerations.
Waiting until the end of the allowable sampling interval leaves little margin for unexpected delays and increases the potential for a compliance issue.
Reference: New York City Department of Health and Mental Hygiene, Cooling Towers: Information for Owners and Operators.
The Importance of Trending
Historical data can provide valuable information regarding changes in cooling tower performance. With the increased frequency of testing, more trend data can be collected to assist in identifying problems and resolving water treatment issues.
Consider two towers with the same current Legionella result of 8 CFU/mL. Tower A:
- Not detected
- 2 CFU/mL
- 4 CFU/mL
- 8 CFU/mL
Tower B:
- 18 CFU/mL
- 13 CFU/mL
- 10 CFU/mL
- 8 CFU/mL
Although the current results are identical, the trends are different.
Tower A demonstrates an increasing trend, while Tower B demonstrates a decreasing trend. These results should be considered along with treatment history, microbiological data, and operating conditions when evaluating overall system performance.
Maintaining organized historical records allows gradual changes to be identified and investigated before they become more significant.
Conclusion
Routine Legionella testing is an important component of cooling tower management. For cooling towers operating in New York City, Legionella culture testing is currently required at least once every calendar month, with no more than 31 days between sampling events while the cooling tower is operating, in addition to required startup testing. However, the frequency of sampling for locations outside of New York may vary and should be based on cooling tower conditions and historical records. The best practice is to test once every 90 days to verify the water treatment system is working as intended.
