Controlling Legionella in Cooling Towers

Dec. 30, 2016
Treatment method trial helps hospital maintain water quality in cooling system

About the author: Mary Wolter Glass is president of Mexel USA LLC. Glass can be reached at [email protected].


According to the Centers for Disease Control and Prevention (CDC), an increasing number of people across the U.S. are being diagnosed with Legionnaires’ disease. Annually, approximately 5,000 people are diagnosed and at least 20 outbreaks are reported. The outbreaks have fostered tighter regulation and increased concern for cooling tower owners and operators.

In the summer of 2015, 138 residents of the South Bronx contracted legionellosis and 16 died. The source of the outbreak was tracked back to a cooling tower infected with Legionella bacteria. In response, on July 19, 2016, the state of New York issued comprehensive regulations aimed at protecting against similar outbreaks in the future. Other states and localities are adopting similar regulations to require registration, inspection and testing of cooling towers at hospitals, residential health care facilities and other sensitive buildings.

The New York regulations reflect the kinds of requirements tower owners and operators may face:

  • Registration of the tower with the designated agency and periodic updates on changes;
  • Inspection of the tower for deficiencies or problems at startup, after maintenance, and every 90 days during use;
  • Monitoring programs with routine bacteriological and Legionella sampling and analysis;
  • Immediate sampling and appropriate response to elevated Legionella levels; and
  • Reporting to regulatory and public health agencies on routine and non-routine maintenance.

Griffin Hospital in Connecticut conducted a trial to determine the best treatment option to manage Legionella in its cooling system. 

Cooling Tower Treatment Options

CDC has issued guidebooks on how to evaluate and treat a variety of water systems for Legionella, available at This general guidance is useful in gaining an understanding of the nature of the problem, but focused, pragmatic approaches for the average tower owner are needed. For some, there is a high level of urgency and uncertainty, leading to a rush to respond. Simple, effective and economical cooling tower water treatment programs are available.

Experienced water treatment professionals can recognize the unique demands of each system, large or small. “In addition to controlling Legionella, a well-executed program improves the operational and financial performance of the tower,” said Dennis Tomsheck, a certified water technologist for Jamestown Technologies, located in the New York City area. “This requires site-specific analysis of chemicals, equipment, cost, training and regulatory requirements.” It may sound complicated, but an experienced expert can quickly analyze a system and devise a practical, workable solution.

Tomsheck believes optimal programs should include:

  • Consistent testing of key cooling tower water quality parameters—conductivity, pH, temperature and oxidation-reduction potential—three times a week at two-day intervals;
  • Weekly biologic testing using dip slides;
  • Visual inspection and Legionella testing at least every 90 days;
  • Thorough tower cleaning and disinfection twice a year, including at startup or after being idle; and
  • Maintenance of written records of all of the above actions as reference for program consistency and proof of responsible tower management.

One of CDC’s key findings was that biofilm presents a significant challenge to maintaining a clean system. Bacteria attached to a surface in the system extrude an exopolymer that creates an inviting environment ideal for the rapid growth of Legionella under the right conditions. Many traditional biocides cannot penetrate this biofilm layer to eliminate the source of contamination.

Legionella Control Trial

To address this problem, Tomsheck performed a trial comparing a traditional water treatment program to a new program aimed specifically at preventing biofilm accretion using filming amine cooling water treatment with biocidal properties. The trial was conducted on a 1,000-ton rooftop cooling tower connected to a chiller plant in the basement of Griffin Hospital in Derby, Conn. The tower typically was run at five to six cycles using makeup water with moderate to high hardness and alkalinity.

During the first three months, the trial was conducted using chlorine, phosphonates and molybdate to treat the tower. This treatment method achieved excellent control of bacteria counts (less than 10 colonies per cu cm). The tower then was switched to only the filming amine, Mexel 432/0, with small bromine supplements three times a week. The filming amine’s mode of action is to protect surfaces with a microscopic layer that prevents adhesion of microbes and other materials to wetted surfaces. Small doses were injected for 30 minutes daily. The amine emulsion addressed the potential for sessile bacteria to thrive in the biofilm on the surfaces while the bromine provided additional protection from planktonic bacteria in the water column.

During the subsequent four-month trial of the new program, the bacteria counts remained at the previous low levels, and other operating parameters maintained levels consistent with expected performance. Two Legionella tests showed no detectable levels of the bacteria. Field service requirements were simplified due to use of fewer chemicals requiring less space and monitoring. The amine emulsion was viewed as less hazardous to handle for service representatives.

This is just one example of the many simple solutions that can be made available to tower owners. The combination of experienced professionals and effective technology make it possible to significantly reduce the public health risk of Legionella at sites like Griffin Hospital. 

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