Protect Against Legionella By Cleaning Your Cooling Towers

Cooling Tower21 photo (commercial hvac maintenance and efficiency )Since 1976, when it was first discovered, Legionnaire’s Disease has proven to be a problem for operators of cooling towers. Not too long ago, we wrote about the trouble caused when guests at a Florida hotel died because they were exposed to Legionella in the water supply.

While in that case, the hotel’s cooling towers were not the culprit, there have been many times when an infection can be traced back to them. The hotel was shut down for days, causing them to lose millions of dollars in revenue and future bookings.

Cooling towers are an ideal breeding ground for Legionella Pneumophila, the bacteria that causes Legionnaire’s Disease. According to an article in Wikipedia, the bacteria is present in almost all sources of water, so small amounts should not be a cause for alarm.

The European Working Group for Legionella Infections (EWGLI) recommends testing according to ISO 6222(21). A count of 1,000 or less means your system is in good shape. A count of 10,000-100,000 means you should retest and then evaluate control measures, and anything higher means you should treat your system immediately to reduce the count.

Legionella grows best in warm water, such as cooling tower water when it is heated by the sun. While UV radiation will kill it, the bacteria is safe it grows inside sludge or dirt.

As described in a recent article in the American Journal of Infection Control, the chances of a cooling tower being infected with deadly strains of Legionella are close to 50 percent. The authors of the study looked at 96 cooling towers and found that 47 were colonized, some so much so that they needed to be immediately treated with a biocide.

The researchers took 69 samples and cultured them, and discovered that 55 of them were comprised of Legionella Pneumophila.  Further research concluded that some operating practices encouraged the growth of the bacteria, and there were some that discouraged it.

Training, testing, sunlight protection, the use of chlorine and implementation of a risk assessment and management plan all contributed to lower Legionella counts, whereas the absence of testing and planning, treatment or sunlight protection all contributed to a greater risk of Legionella contamination.

Further, sediment, sludge and scale, according to the Cooling Technology Institute, can not only harbor but actually promote the growth of Legionella, so keep your cooling towers clean.  If you discover that Legionella is a problem, according to the CTI, treat your water with chlorine or bromine, both of which kill legionella bacteria.

Legionnaire’s disease spreads when infected water is spread through the air as an aerosol, for example, when a gust of wind speeds through your cooling tower water and turns it into a mist. Research has shown that the drops can travel for miles, causing infections as it travels.  If there is an outbreak, one of the first places investigators will look is cooling towers upwind of the infections. They can – and will — test your water and determine if the strain of Legionella making someone ill is the same as that present in your cooling tower.

The good news in all this is that while the risks are definitely present, it is a fairly simple matter to keep them under control by keeping your system clean and well-maintained – good advise no matter what the levels of bacteria — and testing to make sure things stay that way.

Rich Silverman
Goodway Blogging Team

Image by Frobles courtesy of Wikimedia Commons under a GNU Free Documentation License


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2 comments


  • a very informative and well written article, wherever possible i would recommend trying greener alternatives instead of harsher chemicals such as sulphuric acid which leave behind greater contaminants.

    May 20, 2010
  • [...] A couple of months ago marked the 35th anniversary of an outbreak of pneumonia among people attending a convention of the American Legion at the Bellevue-Stratford Hotel in Philadelphia.  This was the first reported incident of Legionnaires Disease.  It was determined that the source was the hotel cooling tower via the air conditioning system. [...]

    October 19, 2011

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