Legionella in Cooling Towers: How It’s Formed, Distributed and Eradicated

There is a lot of focus being placed on testing and inspecting cooling towers for Legionella after it has already formed inside the cooling tower, as well as research on how to curb Legionnaires’ disease from cooling towers. Recently, one senator called for nationwide expansion of New York’s recent legislation involving cooling towers and $250 million in additional funding for the CDC. This post examines how Legionella forms in cooling towers, how it’s distributed to people and what technology already exists for curbing Legionnaires’ disease from cooling towers.

How Does Legionella Form Inside a Cooling Tower?

A cooling tower is a great air scrubber because it sucks in whatever is in the air (dirt, sand, brake dust, debris). That debris often settles in the bottom of the cold-water basin and builds up because the water velocity in the cold-water basin is quite low, moving at no more than 1 to 2 feet per second when operational. This buildup makes it difficult for biocides to get to and kill the bacteria. Furthermore, sunlight enters the air-inlet louvers and can create algae in the cold-water basin. Algae is a primary food source for bacteria, like Legionella. Additionally, scale buildup sometimes happens in the system and is another primary food source for bacteria, like Legionella. When you couple these food sources with the stagnant water in the cold-water basin, it becomes a perfect breeding ground for Legionella to grow, especially during warmer weather.

How Does Someone Get Infected from a Cooling Tower?

Because there are fans on top of the cooling tower, there is a great deal of water that gets sucked up and thrown into the air above the cooling tower in the form of mist or “drift”. The mist eliminators are in place to catch most of those water droplets, but they can’t catch all of them. A typical 1,000-ton refrigeration system with standard mist eliminators will spray an average of 6.3 gallons of water into the air above the cooling tower every hour. If there’s Legionella in the cold-water basin of the cooling tower, there will be Legionella in those 6.3 gallons of mist. Some of that mist may land on people and some may enter the fresh air intake of a building. When someone with a reduced immune system breathes air that has that mist, they are susceptible to Legionnaires’ disease.

New York’s Legislation

NYC is now registering every cooling tower so they are easily identified. The city’s representatives are also requiring quarterly inspections for the existence of Legionella and annual certification that the cooling towers are free from Legionella. These are all good things, but testing and inspection doesn’t cure the root cause. A poignant example is The New York Times article highlighting how Legionella grew back within two months causing many additional illnesses and another death in the Bronx. Fortunately, temperatures dropped in October making Legionella less likely to significantly breed again until next summer and the number of cases in the media have dropped off again. It seems apparent this new local legislation that’s already failed is not quite ready to be expanded to a national level. The intent is right and heading in a good direction, but there needs to be a component of prevention to go with the inspection and reporting.

Reducing the Breeding of Legionella within a Cooling Tower

Most major conventional cooling tower manufacturers and after-market providers can provide a basin sweeper system that’s used with a filtration system. The basin sweeper system keeps the water in the basin agitated, so most of the sediment stays suspended in the water instead of settling and building up in the basin. That sediment is then automatically removed with a filtration system. This keeps the cold-water basin clean and makes it nearly impossible for bacteria to grow into high colony-forming units. This means even if someone breathes in mist from the cooling tower, they are less likely to become infected with Legionnaires’ disease because the bacteria count is so low.

When a new cooling tower or complete HVAC system is needed for a building, the building’s owner will typically hire an engineering firm to design the system. Very few engineers specify basin sweeper systems today because they are not required to do so by law, they are not accustomed to doing so and they are always trying to keep cost down. If the engineer doesn’t specify the basin sweeper system, the winning contractor who bid for the project is highly unlikely to spend the extra money to install one.

Reducing Mist Exiting Cooling Towers

Most cooling towers come from the factory with mist eliminators that contain mist to just 0.002 to 0.005 percent of the water flow through the cooling tower (the average of which is 6.3 gallons per hour for a 1,000-ton system). However, for a very small additional cost of the cooling tower, most major manufacturers can provide mist eliminators that contain mist to less than 0.001 percent of the water flow. That’s just 1.8 gallons per hour for the same system, or a 71 percent reduction in the amount of mist that’s traveling to potentially susceptible hosts.

Because these slightly more expensive mist eliminators are not required by building code, they are not in most base specifications. Very few engineers currently specify the cooling tower to have 0.001 percent or better-rated mist eliminators.

Saving Lives and Money

Not only should the entire country replicate New York’s cooling tower Legionella legislation, building owners also should require technology that the major manufacturers can already provide to significantly eliminate the root cause of Legionnaires’ disease from cooling towers. Specifically, require all cooling towers have a basin sweeper system or totally enclosed basin that eliminates algae combined with a filtration system and mist eliminators that contain mist to no more than 0.001 percent of the water flow. These simple guidelines can be a requirement for all specifications on new projects, and this equipment can be easily retrofitted into existing cooling towers. The industry should partner with ASHRAE on a national standard for testing and methodology and help businesses pay for the cost of retrofits and/or replacements to come into compliance with the two root causes mentioned above (similar to the cash for clunkers program that got so many environmentally unfriendly cars off the road).

About the Author

Dan Coday
Dan Coday is sales manager, international, for Tower Tech Inc.

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