Legionnaires' disease bacteria (LDB) cannot survive without water, and a properly operated, well-maintained HVAC system is unlikely to be a source of problems unless water contaminated with the bacteria enters the system.
Air conditioning units without humidifiers have not been identified as sources of LDB. For a Legionnaires' disease outbreak to be linked directly with the HVAC system, LDB-contaminated water must enter the system, be aerosolized, and be delivered to building occupants.
To effectively control contamination, be aware of the internal and external conditions that may promote growth and distribute LDB:
External sources may emit contaminated aerosolized water that is drawn into a system's fresh-air intake. Consider the following:
- Fresh-air intake airways, typically concrete plenums located at grade level, supplying fresh air to air handlers in the basement or lower levels of buildings can collect organic material (such as leaves and dirt).
- Aerosols from spray irrigation.
- Open windows.
Internal sources may provide contaminated aerosolized water that is then disseminated by the air-distribution system. Consider the following:
- HVAC system humidifiers are potential sources of aerosol exposure if contaminated with LDB.
- Direct evaporative air coolers with sprays or misters used as humidifiers include sumps, which may stagnate when not in use.
- Indirect evaporative air cooling systems using water coils may develop a leak that may inject cooling tower water directly into the supply air stream.
- Air-to-air heat exchangers may develop leaks, which may allow the wet air stream to mix with supply air and cause problems if the wet air stream is contaminated with LDB
- Wet evaporative coolers, slinger air coolers, and rotary air coolers with improperly operated and maintained systems that use warm, stagnant sump water may be potential sources of LDB.
- Residential humidifiers, such as free-standing or portable units, often contain sumps that are frequently contaminated with LDB.
- Computer room air conditioners may contain a humidifier sump filled with contaminated water.
- Improperly drained condenser pans may produce tepid conditions that can encourage microbial and fungal growth.
A Legionella management plan for a facility’s cooling towers should include these eight key measures:
- Locate cooling towers to minimize exposure to people. Place the towers as far away as reasonably possible (preferably at least 100′) from operable windows, outdoor air intakes, parking lots, roads, driveways, sidewalks, and outdoor areas frequented by people. Furthermore, make ground level cooling towers inaccessible to visitors and passers-by (e.g., with a fence or wall).
- Treat cooling water for control of Legionella and other microbes. Fms should communicate to their water treatment vendor that their treatment program should be effective against a wide range of microbes, including Legionella, in biofilms as well as in the circulating water.
- Fms should follow ASHRAE Guideline 12-2000, “Minimizing the Risk of Legionellosis Associated with Building Water Systems,” regarding cooling tower shutdown and startup.
- Check water treatment chemical levels and chemical pumps daily. Legionella levels can skyrocket in a short time if a biocide drum is empty or a chemical pump is broken.
- Install and maintain high efficiency drift eliminators. Even the most efficient drift eliminators will not block all Legionella bacteria but they will minimize the mist that escapes the tower.
- Control the buildup of dirt and debris. Clean basins and sumps as needed and the entire system twice a year. The cleaning alone has little effect on Legionella concentrations, but it is crucial for effective water treatment. Fms should also consider side stream filtration to reduce suspended solids and the demand on the biocide further. For most systems, filtering 3% to 5% of the circulated water to 20 to 50 microns is sufficient. A filter vendor can recommend a media based on a particle size analysis.
- Routinely purge stagnant lines. Stagnant water is conducive to Legionella growth and prevents the biocide from circulating throughout the system.
- Test the water routinely to evaluate the water treatment program and the overall risk reduction plan. For most cooling towers, tests for pH, total dissolved solids, and disinfectant levels (if applicable) should be run daily. Check total bacteria counts (TBC) or Adenosine Triphosphate (ATP) at least once a week to evaluate the water treatment—a significant rise indicates a problem.
The only way to estimate Legionella concentrations is to test for Legionella, so that should be done once every month or two during the operating season. If Legionella is not found in the tower, Facility Managers should not relax the water treatment or preventive maintenance. If Legionella is found, particularly at levels greater than 10 colony forming units per milliliter (cfu/ml), then the treatment regimen should be adjusted appropriately.
The information above has been adapted from the following reference: