Updated on 6/24/19

Cooling Tower CleaningNew York City building owners should consider six key safety precautions in complying with health commissioner Dr. Mary T. Bassett’s August 6th order to disinfect their cooling towers:

1. Perform “online” rather than “offline” disinfection unless full physical cleaning is absolutely necessary at the time. Cooling towers can be disinfected “online” or “offline.” The online procedure involves only chemical disinfection while the cooling tower is operating. The offline procedure involves shutting the cooling tower down and scrubbing it physically in addition to disinfecting it with chemicals. A full offline disinfection and cleaning is more thorough but is expensive and presents some serious risks, as discussed below.

Although the online method does not address the physical cleaning of the cooling tower, it is effective in killing Legionella and is much safer, cheaper, and quicker. It is also less corrosive. A typical online disinfection basically amounts to maintaining a free chlorine concentration of approximately 5 parts per million (ppm) for at least 6 hours. More information on the method can be found in the Cooling Technology Institute’s publication Legionellosis Guideline: Best Practices for Control of Legionella, which can be downloaded free at Cooling Technology Org.

If you use the offline method:

2. Perform online disinfection of the cooling tower prior to cleaning it physically. Otherwise, the Legionella risk could get worse before it gets better. Physically cleaning a cooling tower without disinfecting it chemically beforehand is like aggressively sweeping a dirty garage floor–you might eventually get it clean but in the short term you’ll inhale a lot of dust. Hence precaution #4 below.

3. Provide personal protective equipment for the workers. Respirators will reduce the risk of inhaling Legionella bacteria. Boots, protective clothing, and goggles will help protect their eyes and skin from chemicals.

4. Close building outdoor air intakes within the vicinity of the cooling tower. The Centers for Disease Control and Prevention (CDC) has recommended closing intakes within 30 meters.

5. Do it properly. Check with organizations such as the Cooling Tower Institute and the American Society of Heating, Refrigerating and Air-Conditioning Engineers for their latest recommendations. Follow instructions on chemical product labels. Keep records of the procedure, including the chemicals used, the time the chemicals were added to the system, and chlorine and pH test results. Be sure to follow all EPA and other applicable regulations and observe safety precautions. Utilizing a highly qualified and experienced water treatment specialist is the best way to ensure proper protocol is followed.

6. With either method – online or offline – understand the long-term limitations of the procedure. As pointed out in “Five Steps New York City Building Owners can take to Reduce the Risk of Legionnaires’ Disease,” studies have shown that Legionella bacteria may reemerge within only days after fully disinfecting a cooling tower. A program of regular water treatment, maintenance, and inspections provides better protection than periodic disinfection. The same applies to plumbing systems, hot tubs, and other water systems that can harbor and transmit Legionella — the best protection is to implement a water management plan per ANSI/ASHRAE Standard 188.

Update: In 2016, both New York City and New York State established permanent laws for registering and maintaining cooling towers to reduce the risk of Legionella. New York State requires hospitals and nursing homes to maintain all water systems, not just cooling towers, to minimize Legionella.

Do you have additional safety recommendations? Please comment below.


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