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 can grow and spread in both open- and closed-circuit cooling tower systems.
Purpose
Use this document to:
- Help evaluate hazardous conditions associated with all types of cooling towers and evaporative condensers
- Implement Legionella control measures for cooling towers per ASHRAE Guideline 12
- Complement existing resources for water management programs
- Support environmental assessments conducted during public health investigations
Key factors affecting Legionella growth
Design
Open- vs. closed-circuit cooling towers
Similarities
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
- Compressors
- Condensers
- Heat exchangers
- Heat pumps
- Other process devices
Both open- and closed-circuit cooling towers:
- Require the same basic operation and maintenance protocols
- Release aerosolized water to the atmosphere
If Legionella is present, the aerosolized water can spread the bacteria over miles.
Differences
Closed-circuit cooling towers have an additional, closed loop. That loop 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 drift eliminators: Use high-efficiency drift eliminators.
Consider placement: Locate cooling towers at least 25 feet from building air intakes. This will help prevent the cooling tower's drift plume from being drawn into a ventilation system.
Avoid stagnation: Ensure system piping is designed to avoid stagnation or dead legs. Recirculate water during intermittent operation.
Automate disinfectant use: Design and install an automated water treatment system.
Water management program
Use a water management program to establish, track, and improve operation and maintenance activities.
Operation, maintenance, and control limits
The necessary frequency of operation and maintenance activities depends on the
- Cooling load
- Cooling tower’s design
- Environmental conditions in the area of the cooling tower's location
Guidance
Clean and disinfect as directed: Follow manufacturer recommendations
- Prior to commissioning
- Before startup
- When idling
- After shutdown
Perform an offline disinfection and cleaning at least annually.
Monitor water parameters on a regular basis. Base measurement frequency 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, increase the measurement frequency if there's a high degree of measurement variability.
Lower water temperature: Operate cooling tower systems at the lowest possible water temperature. If possible, operate below the most favorable Legionella growth range (77–113°F, 25–45°C).
Use automation: Automate anti-corrosion, anti-scale, and disinfectant addition and monitoring.
Avoid stagnation: Flush low-flow pipe runs and dead legs at least weekly. Balance operating times among cooling towers 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. Base the decision on system factors like cooling tower location and particle load.
Monitor for water service disruptions and develop plans to respond accordingly.
Maintain paperwork: Keep site-specific log sheets, test procedures, service reports, and test results on site.
Consider testing for Legionella in accordance with the routine testing module of this toolkit (linked below).
Remediation
Protocols
Cleaning, disinfecting, and remediating cooling towers involves a hierarchy of protocols. Determine how the following response protocols fit into the water management program.
The protocols are listed in order of increasing intensity from routine treatment to offline emergency disinfection:
- Online remedial treatment
- Online disinfection
- Offline cleaning and disinfection
- Offline emergency cleaning and disinfection
Consult ASHRAE Guideline 12 for instructions for each response. These steps may require customization based on system components, operating conditions, or other factors.
Protocol for suspected outbreak or illness
If the public health authority having jurisdiction suspects an associated outbreak or illness, then perform offline emergency cleaning and disinfection. See specific guidance below.
Note: The public health authority having jurisdiction determines whether there are associated illness(es) or an outbreak.
Outside help may be needed
Role of testing during 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.
Steps for offline emergency cleaning and disinfection
Review the current water treatment program (e.g., cleanliness, maintenance, disinfectant program).
Remove heat load from the cooling system. Shut off any associated fans. Disengage all automated chemical feed and control equipment.
Shut off system blowdown. Keep make-up water valves open and operating.
Close building air intake vents near the cooling tower, especially those downwind, until cleaning procedure is complete.
Circulate water through all system equipment, including any bypass or standby components.
Add an oxidizing disinfectant. Achieve a disinfectant residual of at least 20 ppm as free available oxidant.
Add an appropriate dispersant. Apply antifoam, if needed. Apply appropriate corrosion inhibitors.
Reduce the cycles of concentration (if necessary) to achieve and maintain a pH of less than:
- 8.0 for chlorine-based disinfectants
- 8.5 for bromine-based disinfectants
Maintain a free available oxidant residual of 10 ppm for at least 24 hours. Shorter contact times can be effective at higher concentrations.
Drain the system to the sanitary sewer. Follow all applicable rules, regulations, and permits.
Physically clean all accessible system equipment, including:
- Bypass or standby components
- Chillers
- Drift eliminators
- Fill pack
- Strainers
- Equalizer lines
- Free cooling heat exchangers
- Remote sumps or basins
Refill the system and circulate water through all system equipment, including bypass or standby components.
Add an oxidizing disinfectant. Maintain a free available oxidant residual of at least 10 ppm for one hour.
Drain the system to the sanitary sewer. Follow all applicable rules, regulations, and permits.
Refill the system. Return all chemical feed and control equipment to normal operation.
Legionella control measures
Described below are control measures and recommendations for each water parameter.
Sediment and biofilm
Control measures: Cleaning frequency and scale and corrosion inhibitors
Recommendations: 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 measures: Control limits
Recommendations: Operate at the lowest possible water temperature outside Legionella's favorable growth range (77–113°F, 25–45°C).
Water age
Control measures: Make-up water quality and turnover frequency
Recommendations: 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. 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 measures: Control limits
Recommendations:
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.
Resources
Form: Legionella environmental assessment
Illustration: Cooling tower
Toolkit: Developing a water management program to reduce Legionella growth and spread in buildings
Training: Preventing Legionnaires' disease: A training on Legionella water management programs