Water and wastewater consultant Paul Puckorius explains the causes of Legionnaires' Disease and concerns associated with cooling towers.

Figure 1. Conditions for Contracting Legionnaires' Disease
Four conditions must exist for an individual to contract Legionnaires' Disease: susceptibility, inhalation, concentration and distribution. The disease cannot be contracted by drinking or washing with contaminated water, but it can be spread by cooling tower mist.

Legionellosis, the technical name for Legionnaires' Disease, is caused by a bacteria known as Legionella Pneumophila. To contract the disease, the bacteria must be inhaled deeply into a susceptible person's lungs, where infection can occur. The infection has essentially all of the symptoms of pneumonia.

Legionella bacteria is a class of a large number of variations. Some variations cause the dreaded disease Legionella Pneumophila; others only cause a bad cold or flu such as Pontiac fever. The Legionella Pneumophila species has a number of subgroups known as sero-groups. Sero-Group One is the major cause of the disease though other sero-groups also are known causes.

The bacteria most frequently is found in water. Normal disinfection of drinking water by chlorine products does not kill Legionella bacteria. You cannot get the disease by drinking, swimming or washing with Legionella-contaminated water. The bacteria most frequently is spread by fine water droplets as small as 5 to 10 Km. Any water, aerosol or misting such as the drift from a cooling tower, evaporative condenser or decorative fountain can carry the Legionella bacteria. Likewise, fine dust, soil and dirt also can carry the bacteria.

An outbreak of a pneumonia-type human disease in 1976 was found to have originated from a hotel cooling tower at the American Legion Convention in Philadelphia. Thus, the name was established as the Legionnaires' Disease. The cause of this outbreak was identified by the Centers for Disease Control (CDC), Atlanta, as a bacteria.

In the past, many cooling systems used to use chromate as a corrosion inhibitor. Chromate is somewhat toxic to microorganisms - certainly not a nutrient. However, during the past decade, chromate programs have been replaced with phosphorous- and nitrogen-based treatment chemicals. These treatment chemicals are an excellent nutrient for microorganisms.

Four specific conditions are needed to contract Legionnaires' Disease:

  • The person must be susceptible.
  • The person must inhale water droplets containing a specific type of the Legionella bacteria.
  • The system water must contain sufficiently high levels of the bacteria.
  • The cooling operation must distribute a contaminated mist of the system's water.

Table 1. Guidelines for Legionella Remedial Action

Although no formal standards have been published by CDC, Legionella bacteria should be absent or at low levels in the system water at all times. A microbiological control program must be applied and maintained.

Pathcon Corp., Norcross, GA, and other laboratories have done extensive testing of cooling water samples. These laboratories report the presence and levels of Legionella bacteria as colony-forming units (cfu) per milliliter (ml) of water. The suggested action levels recommended by Pathcon are listed in table 1.

Continuous maintenance of chlorine (or bromine if water pH is above 7.5) at 1.0 to 2.0 mg/l of "free" chlorine residual is accepted as an effective means of killing Legionella bacteria. Lower levels also may be adequate. It is important that the entire cooling water piping, chillers and cooling tower be contacted by this level of chlorine. Untreated piping dead legs, warmup loops and stagnant areas can quickly recontaminate the entire water circuit.

Not all microbiocides used in cooling systems are effective for Legionella control. Oxidants and some nonoxidents are effective. Dibromonitriloproprionamide (DBNPA), glutaraldehyde, chloroisothiazole, ozone and chlorine dioxide can provide a good microbiological control program. It is critical that the proper dosage, frequency of addition and persistence be maintained. Testing for Legionella is the only method of determining if a microbiological program is effective. If high levels of Legionella are found, the accepted State of Wisconsin disinfection procedure should be used.

It is important that drift eliminators be well maintained and free of biological deposits. Fans should operate at design specifications, and water sprays must be kept clean and in good operating condition. Use of a dust mask is advisable when in the presence of water or dust potentially contaminated with Legionella.

Smokers, persons over 60 years old, persons with little resistance to disease and those with respiratory ailments are most susceptible. Men are more prone than women. Can Legionnaires' Disease come from a cooling system? Yes, it can. A water treatment service company can advise clients if its program is designed to control Legionella and what testing may be needed.