Cooling towers and evaporative condensers have the potential to develop infectious concentrations of legionellae, the bacteria responsible for causing Legionnaires' Disease. Prudent water treatment is necessary to control the growth and amplification of Legionella.

Legionellae can be found in certain free-living amoebae and ciliated protozoa.

Cooling towers are used in many process cooling applications. While an effective means of cooling, like any water-based cooling system, cooling towers can present an ideal environment for Legionella, a genus of bacteria known to cause disease. Proper care and maintenance and a good water treatment program can help you protect your workers.

Around since the 1920s, Legionella is the named genus of gram-negative, rod-shaped, aerobic bacteria that are common to warm-water environments. The plural, referring to more than one Legionella microorganism, is ligionellae. At least 40 species of Legionella have been identified, and more than half are linked to human diseases.

When selecting the location of a cooling tower, operators should take into consideration prevailing winds and proximity to building air intakes. Cooling towers have the potential to develop infectious concentrations of the bacteria responsible for causing Legionnaires' Disease.

Legionnaires' Disease acquired its name from a media reference to a pneumonia-like illness that befell attendees of an American Legion convention in Philadelphia in 1976. Pennsylvania Department of Public Health recorded 221 cases of the illness with symptoms including high fever, chills, headache, muscle pain, dry cough and difficulty breathing. More than two-thirds of the patients required hospitalization and 34 died. Matched by Centers of Disease Control (CDC), Atlanta, to unsolved, archived illness tissue samples, it was finally recognized and named in 1976.

Legionellosis is any illness caused by exposure to Legionella; Legionella Pneumo-phila is the named species of Legionella that causes more than 90% of legionellosis cases. It is found in lakes, rivers, spas, vegetable misters, shower heads and many standing waters, including cooling towers. Legionellosis is so common in the environment that CDC acknowledges there is no way to eliminate it.

Legionnaires' Disease and Pontiac fever are the two most common types of legionellosis. Legionnaires' Disease is a potentially fatal, multisystem respiratory illness accompanied by pneumonia that has a mortality rate of 15 to 20%. CDC estimates that the disease infects 10,000 to 15,000 people annually within the United States; Occu-pational Safety and Health Administration estimates the figure to be more than 25,000.

Although sources of biological material are everywhere, risk can be minimized:

  • When microorganisms do not multiply.
  • If no means exists for them to become airborne.
  • If bioaerosols do not reach susceptible people.

The most obvious indication that a biological problem is present within a system is by observing bacterial growth. Scale, sediment and biofouling within these systems all support bacterial growth, including Legionella. A maintenance program designed to minimize scale, sediment and biofouling buildup is essential. A biocide program is first on the list of a good maintenance program.

A good biocide program includes the use of alternating biocides at regular intervals and at the proper legal dosage. A typical program uses an oxidizing biocide (halogen donor) alternated with a nonoxidizing biocide such as isothiazolone or glutaraldehyde. Cooling tower operators should maintain written records detailing the type of biocide used, frequency of biocide additions and amount of biocide added.

Biocide treatment plays an important part in microbiological control programs, but it does not target specific microbiological organisms. Thus, it is not 100% efficacious. The efficacy of any specific biocide only can be determined by testing for the presence of legionellae.

Drums used to ship and store biocides should include an Occupational Safety and Health Administration material safety data sheet (MSDS). Biocide treatment plays an important part in microbiological control programs; however, efficacy of any specific biocide can only be determined by testing for the presence of legionellae.

Biodispersants play an important role in microbiological control programs, particularly against Legion-ella. These chemicals act to loosen microbial deposits and promote system cleanliness. They also enhance a biocide's effectiveness by penetrating the biofilm, which is a nutrient-rich matrix layer of microorganisms often discernible as slime on surfaces in contact with water. Biofilms are where Legionella flourish within cooling towers. In addition to living within biofilms, legionellae can be found in certain free-living amoebae and ciliated protozoa, which allows them to survive typical potable water chlorination.

A regular cleaning schedule is essential to a good maintenance program, and cooling towers should be shut down and cleaned manually at least twice a year. If the cooling tower receives a high organic load, more frequent cleaning may be required. Perform manual cleaning just before the cooling season starts in the spring and at the end of the cooling season in the fall. Any system that has been offline for an extended period of time must be cleaned before it is brought back online. New systems should be cleaned prior to being brought online to remove construction debris. In addition, periodic system disinfection is essential and normally is done concurrently with manual cleaning or when visible signs of biological fouling are present. Operators should keep records with dates of all manual cleanings.

Drift eliminators minimize aerosols created when a system is in operation. If the system does not already have them, drift eliminators should be installed. In addition, side-stream filters fitted to a system and run continuously will help keep particulate matter from entering the system.

Visual inspection can detect bacterial growth, an obvious indication that a biological problem exists within a system.

Because Legionella infects a potential host via inhalation from aerosolized, colonized water, a cooling tower's location should be selected only after taking into consideration prevailing winds and proximity to building air intakes. The cold-water basin, distribution deck and other wet surfaces should be shielded or covered from sunlight to prevent algae growth, and materials of construction should be smooth and nonporous. Piping should avoid dead-end branch lines that are difficult to drain, and the cooling tower system should be designed to be drained completely or pumped out. If piping system dead-end branch lines cannot be avoided, blow them down regularly, particularly after biocide treatments and cleanings.

As with any disease, the mere presence of the organism does not necessarily constitute a threat. Catching any disease requires a susceptible person, a viable host environment, a virulent strain of the organism and a high enough dose of the disease getting into the body via an infectious route. By following simple guidelines, the danger of Legionella infection can be averted. Having a proper tower treatment program in place is the first step; however, making certain that personnel follow it is just as important.

CDC Guidelines Protect Tower Workers

In 1997, Centers for Disease Control, Atlanta, outlined its recommended guidelines for Legionella control in cooling towers. An abridged statement of the guidelines was issued by Association of Water Technologies in July 2000. To minimize workers' Legionella risk when working near cooling towers, owner/operators should follow these steps when treating the tower:

1. Shut off cooling tower fans.
2. Keep makeup water valves open and circulation pumps operating.
3. Close outdoor air-intake vents located within 100' (30 m) of cooling tower.
4. Achieve an initial free residual chlorine (FRC) of at least 50 ppm (50 mg/l).
5. Add a dispersant to tower water within 15 min of chlorine addition.
6. Maintain 10 ppm (10 mg/l) FRC for 24 hr.
7. Drain and refill the system.
8. Repeat steps 4, 5 and 6 at least once to remove all visible algae-like film.
9. Using a brush and water hose, thoroughly clean all water-contact areas, including basin, sump, fill, spray nozzles and fittings.
10. Circulate 10 ppm (10 mg/l) FRC for 1 hr, then flush the system until free of all sediment.
11. Refill the system with clean water and return to service.