Controlling Legionella in Cooling Towers

Key Points

  • Scale, corrosion, sediment controls, and system cleaning are critical for cooling tower operations and Legionnaires’ disease prevention.
  • Disinfectant residual should be monitored and adjusted by an automated system.
  • Legionella risks are similar for open and closed-circuit cooling tower systems.

Purpose

Use this document to:

  1. Help evaluate hazardous conditions associated with all types of cooling towers and evaporative condensers
  2. Implement Legionella control measures for cooling towers per ASHRAE Guideline 12-2020external icon
  3. Complement existing resources for water management programs
  4. Support environmental assessments conducted during public health investigations

Sediment and biofilm, temperature, water age, and disinfectant residual are the key factors that affect Legionella growth in potable water systems.

Design

Open- vs. Closed-Circuit Cooling Towers

All cooling towers use the evaporation of water to remove heat and release it into the atmosphere. Cooling towers use circulating water to cool chillers, heat pumps, compressors, condensers, heat exchangers, and other process devices. Both kinds of cooling towers, open- and closed-circuit, require the same basic operation and maintenance protocols. Both types of cooling towers can release aerosolized water to the atmosphere. If Legionella is present, the aerosolized water can spread the bacteria over miles. However, closed-circuit cooling towers have an additional, closed loop that can keep the fluid used in the cooling processes from being exposed to the atmosphere. Closed-circuit cooling towers can operate in cool temperatures in a “dry” mode that does not use water or generate aerosols.

Design Recommendations

Understanding cooling tower design components is critical for Legionella control.

  • Use high-efficiency drift eliminators.
  • Locate cooling towers at least 25 feet from building air intakes to ensure that the cooling tower’s drift plume is not drawn into a ventilation system.
  • Ensure system piping is designed to avoid stagnation or dead legs.
  • Recirculate water during intermittent operation.
  • Design and install an automated water treatment system.

Operation, Maintenance, and Control Limits

Safe operation and regular cooling tower maintenance protect building operators, staff, visitors, and the adjacent community from exposure to Legionella. The necessary frequency of these activities depends on the cooling load, the environmental conditions present in the area where the cooling tower is located, and the cooling tower’s design. Use a water management program to establish, track, and improve operation and maintenance activities. Operate and maintain cooling towers with the following guidelines in mind:

  • Follow manufacturer recommendations for cleaning and disinfection prior to commissioning, before startup, when idling, and after shutdown.
  • Operate cooling tower systems at the lowest possible water temperature, and below the most favorable Legionella growth range (77–113°F, 25–45°C), if possible.
  • Automate anti-corrosion, anti-scale, and disinfectant addition and monitoring.
  • Monitor water parameters, like disinfectant residual and pH, on a regular basis. Measurement frequency should be based on performance of the water management program or Legionella performance indicators for control. Adjust frequency according to the stability of performance indicator values. For example, the measurement frequency should be increased if there is a high degree of measurement variability.
  • Flush low-flow pipe runs and dead legs at least weekly.
  • Balance operating times among cooling towers to prevent stagnation when multiple cooling towers or cells exist.
  • Implement automated blowdown (intentional discharge of system water and replacement with supply water) to maintain system water quality.
  • Consider filtration to reduce the level of suspended solids in the cooling water based on system factors (e.g. cooling tower location, particle load).
  • Perform an off-line disinfection and cleaning at least annually.
  • Monitor cooling towers for water service disruptions and develop plans to respond accordingly.
  • Consider testing for Legionella in accordance with the routine testing module of this toolkit.
  • Maintain site-specific log sheets, test procedures, service reports, and test results on-site.

Remediation

If an outbreak or illness is suspected, test in conjunction with public health in order to:

  • Confirm the presence of Legionella before performing remediation.
  • Confirm elimination of Legionella after remediation activities.

Cleaning, disinfecting, and remediating cooling towers involves a hierarchy of protocols. Determine how the following response protocols fit into your water management program. The protocols are listed in order of increasing intensity from routine treatment to offline emergency disinfection. Consult ASHRAE Guideline 12-2020 for instructions for each response. These steps may require customization based on system components, operating conditions, or other factors.

  • Online remedial treatment
  • Online disinfection
  • Offline cleaning and disinfection
  • Offline emergency cleaning and disinfection

If an associated outbreak or illness is suspected by the public health authority having jurisdiction (AHJ), perform an offline emergency cleaning and disinfection using the procedures below. Note: The public health AHJ determines whether there are associated illness(es) or an outbreak. Consult a water treatment professional for guidance on applying these procedures.

  1. Review the current water treatment program (e.g., cleanliness, maintenance, disinfectant program).
  2. Remove heat load from the cooling system. Shut off fans associated with the cooling tower. Disengage all automated chemical feed and control equipment.
  3. Shut off system blowdown and keep make-up water valves open and operating.
  4. Close building air intake vents near the cooling tower, especially those downwind, until after the cleaning procedure is complete.
  5. Circulate water through all system equipment, including any bypass or standby components.
  6. Add an oxidizing disinfectant sufficient to achieve a disinfectant residual of at least 20 ppm as free available oxidant.
  7. Add an appropriate dispersant and apply antifoam, if needed. Apply appropriate corrosion inhibitors.
  8. Reduce the cycles of concentration (if necessary) to achieve and maintain a pH of less than 8.0 for chlorine-based disinfectants or less than 8.5 for bromine-based disinfectants.
  9. Maintain a free available oxidant residual of 10 ppm for a minimum of 24 hours. Shorter contact times can be effective at higher concentrations.
  10. Drain the system after the disinfection period to the sanitary sewer, following all applicable rules, regulations, and permits that may be required.
  11. Physically clean all accessible system equipment. Consideration should be given to all cooling tower equipment, including fill pack, drift eliminators, equalizer lines, remote sumps, basins, strainers, chillers, free cooling heat exchangers, and any bypass or standby components.
  12. Refill the system and circulate water through all system equipment including any bypass or standby components.
  13. Add an oxidizing disinfectant and maintain a free available oxidant residual of at least 10 ppm for one hour.
  14. Drain the system after the disinfection period to the sanitary sewer following all applicable rules, regulations, and permits that may be required.
  15. Refill the system and return all chemical feed and control equipment to normal operation.
Table 1. Legionella Control Measures for Cooling Towers
Water Parameter Control Measure Recommendations
Sediment and Biofilm Cleaning frequency, scale and corrosion inhibitors
  • Cleaning frequency varies based on operational factors.
  • Remove from service, clean, and disinfect at least annually.
  • Monitor scale and corrosion inhibitor levels frequently as indicated by water quality measurements.
Temperature Control limits
  • Operate at the lowest possible water temperature outside Legionella’s favorable growth range (77–113°F, 25–45°C).
Water Age Make-up water quality and turnover frequency
  • Flush low-flow pipe runs and dead legs at least weekly.
  • During wet system standby (water remains in system and shutdown for less than 5 days), maintain water treatment program and circulate water 3 times a week through the open loop of a closed-circuit cooling tower and entire open-circuit cooling system. Ensure system water quality is managed through automated system blow down.
  • Use potable water for system make-up water or ensure reclaimed or condensate sources are appropriately managed.
Disinfectant Residual Control limits
  • pH: Maintain based on type of disinfectant used and manufacturer recommendations to prevent corrosion.
  • Oxidizing disinfectants (e.g., chlorine & bromine): Maintain measurable residuals throughout each day. Consult manufacturer recommendations.
  • Non-oxidizing disinfectants: Maintain based on product label concentration and contact time.
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