The HVAC Factor: Preventing Legionella

Posted on:

By Matt Freije
Originally published in the June 2011 issue of Today’s Facility Manager

The buildup of slime on this cooling tower indicates inadequate water treatment and a need for cleaning. (Photo: HC Info)

By the end of this year, the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) plans to finalize Standard 188, “Prevention of Legionellosis Associated with Building Water Systems.” The standard addresses plumbing systems, cooling towers, and several other types of water systems or devices that can harbor Legionella bacteria and release water droplets into the air. As ASHRAE states, the purpose of this upcoming standard “is to present practices for the prevention of legionellosis associated with building water systems.”

The new standard won’t be the first document to discuss the risk of Legionnaires’ disease associated with cooling towers and ways to minimize that risk. Government agencies and industry associations issued guidelines years ago, including ASHRAE in 2000, and articles on Legionella have appeared in industry publications for decades. What makes ASHRAE Standard 188 different is that it requires a HACCP (Hazard Analysis and Critical Control Point) plan.

Facility managers (fms) should bear in mind that a HACCP Legionella plan involves, not just cooling towers, but plumbing systems, decorative fountains, whirlpool spas, and other water systems that can harbor and transmit Legionella.

Creating a HACCP boils down to four steps that fms need to take:

  • assess the hazard;
  • establish control measures and monitoring;
  • verify that the control measures are implemented; and
  • validate that the plan is working.

A Legionella management plan for a facility’s cooling towers should include these eight key measures:

  1. 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).
  2. 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.
  3. Fms should follow ASHRAE Guideline 12-2000, “Minimizing the Risk of Legionellosis Associated with Building Water Systems,” regarding cooling tower shutdown and startup.
  4. 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.
  5. 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.
  6. 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.
  7. Routinely purge stagnant lines. Stagnant water is conducive to Legionella growth and prevents the biocide from circulating throughout the system.
  8. 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, fms 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.

Fms should also consider monitoring biofilm. Slime on any test surface indicates there’s slime in less accessible areas (e.g., heat exchangers).

Other Actions Required

In addition to control measures, the HACCP system requires the following:

  • Identify and train the people responsible for the cooling tower maintenance and water treatment.
  • Set specific control limits (e.g., acceptable bacteria counts) and determine remedial steps to take if those limits are exceeded.
  • Establish a reporting system that verifies the plan was implemented.
  • Document all inspections and maintenance procedures.

Writing and implementing the plan may seem like a pain in the neck at first, but most of the procedures are simply good maintenance. Moreover, it is not a waste of time or money because Legionella preventive measures really work—the steps fms take may prevent a serious illness, keep them out of court, or even save a life.

Freije is a consultant, author, and course instructor specializing in Legionella and other waterborne pathogens. His consulting firm, HC Info, located in San Diego, CA, provides HACCP Legionella plans and training for hospitals, hotels, and office buildings. Freije’s book Legionellae Control in Health Care Facilities: A Guide for Minimizing Risk has sold in more than 30 countries. His new book, Protect Yourself from Legionnaires’ Disease: The waterborne illness that continues to kill and harm, was published this year.

Other posts by

Leave a Comment

» Comments RSS Feed