Cooling tower maintenance and Legionella control

Cooling towers reject heat from chillers and other equipment by evaporating a small portion of the water circulating through them. The same conditions that make them effective, warm water, large wetted surfaces, constant exposure to outdoor air and nutrients, also make them a potential breeding ground for Legionella bacteria. When contaminated water is carried out of the tower as drift, people nearby can inhale droplets and develop Legionnaires' disease, a serious form of pneumonia.

Cooling towers have been linked to community outbreaks in Canada and elsewhere, including the 2012 outbreak in Quebec City that led Quebec to introduce mandatory cooling tower regulation. This guide covers the maintenance practices, water treatment and management plans that control the risk, along with the standards and regulations owners should know.

Why Legionella grows in cooling towers

Legionella bacteria occur naturally in fresh water at low levels. They multiply when conditions favour them:

• Water temperatures in the range of roughly 25 to 45 C (77 to 113 F), which matches typical condenser water temperatures.
• Stagnant water, such as in idle towers, dead legs and bypass piping.
• Biofilm, sediment, scale and corrosion products that provide shelter and nutrients.
• Inadequate or inconsistent biocide treatment.

Biofilm is the critical factor. Bacteria in biofilm are shielded from biocides, so a tower with clean water chemistry but heavy biofilm can still harbour Legionella.

Standards and regulations

Quebec has the most prescriptive regime in Canada, with requirements administered by the Régie du bâtiment du Québec, including routine Legionella sampling and actions when results exceed set thresholds. Elsewhere in Canada, requirements are less uniform. Public health authorities have the power to investigate and order action during an outbreak, and some municipalities maintain cooling tower registries. Owners should check current rules with their provincial and local authorities, and many follow ASHRAE 188 as good practice regardless.

The water management program

ASHRAE 188 centres on a written water management program developed by a team that understands the building. For cooling towers, the program typically includes:

1. System description and flow diagram. Show the tower, basin, pumps, chillers, bypasses, makeup water, blowdown and any dead legs.
2. Hazard analysis. Identify where conditions could allow Legionella growth or spread, such as stagnation, warm temperatures, poor treatment, or drift near air intakes and public areas.
3. Control measures. Define how each hazard is controlled, for example biocide dosing, drift eliminators, regular cleaning and circulation of idle towers.
4. Control limits and monitoring. Set acceptable ranges for parameters such as biocide residual, pH, conductivity and microbial counts, and how often each is checked.
5. Corrective actions. Specify what happens when a parameter is out of range, including enhanced treatment, cleaning and disinfection.
6. Verification and validation. Confirm the program is followed and that it actually controls risk, often through Legionella sampling.
7. Documentation. Keep records of monitoring, treatment, cleaning and corrective actions.

Water treatment

A competent water treatment program is the foundation of cooling tower hygiene. It addresses four problems at once:

• Microbial control with oxidizing biocides such as chlorine or bromine, often supplemented with non-oxidizing biocides on a rotating schedule, and biodispersants that help penetrate biofilm.
• Scale control with inhibitors and blowdown to keep dissolved solids within limits, measured by conductivity.
• Corrosion control with inhibitors suited to the metals in the system.
• Suspended solids control with filtration, often side-stream filters, and basin cleaning.

Treatment should be automated where possible, with chemical feed and blowdown controlled from conductivity and other measurements. Manual slug dosing alone tends to produce inconsistent residuals. Our guide to water treatment for closed and open loops covers the chemistry in more detail.

Physical maintenance

Water treatment cannot overcome a dirty or poorly maintained tower. Regular physical tasks include:

• Drift eliminators. Inspect for damage, gaps and fouling. Effective drift eliminators greatly reduce the amount of water leaving the tower as droplets.
• Fill. Inspect for scale, biofilm, sediment and damage. Fouled fill also reduces thermal performance.
• Basin and sumps. Remove sludge and debris. Sediment provides nutrients and shelter for bacteria.
• Distribution system. Check nozzles and distribution basins for blockage and even water distribution.
• Makeup and float valves. Confirm correct operation and level.
• Mechanical components. Inspect fans, gearboxes, motors and bearings, and check vibration.

Most guidance calls for cleaning and disinfection at least once or twice a year, typically before seasonal start-up and after shutdown, and whenever monitoring indicates a problem. Cleaning should follow a defined procedure: pre-disinfection to protect workers, physical cleaning, then disinfection and flushing before return to service.

Seasonal and idle operation

Many Canadian towers shut down for winter or run only intermittently. Stagnant water in an idle tower is a major risk. Practical rules:

• Drain and clean towers that will be idle for an extended period, or keep water circulating with treatment maintained.
• Clean and disinfect before restart after a seasonal shutdown.
• Avoid dead legs in piping, and flush bypass lines and standby equipment regularly.
• For towers operated in winter, maintain treatment despite low loads, and manage freeze protection with basin heaters and controls. See cooling tower selection and winter operation.

Worker safety

Maintenance staff working in or near towers can be exposed to aerosols. Provincial occupational health and safety requirements apply, and good practice includes respiratory protection rated for aerosols during cleaning, pre-disinfection before entry, and confined space procedures where applicable.

Frequently asked questions

How often should cooling tower water be tested for Legionella?

In Quebec, sampling frequency is set by regulation. Elsewhere, the water management program should define it, often monthly or quarterly, with more frequent testing after a high result or a system upset. Routine chemistry monitoring happens much more often.

Do closed-circuit fluid coolers carry the same risk?

They still have an open spray water circuit that can grow Legionella, so they need a water management program, though the risk profile differs from open towers.

Who should prepare the water management plan?

A team that includes the owner or facility manager, the water treatment provider, operators and, where needed, an engineer or consultant familiar with ASHRAE 188.

Key takeaways

• Cooling towers can spread Legionella through drift, making them a public health concern as well as a maintenance item.
• ASHRAE Standard 188 and Guideline 12 define a water management program approach, and Quebec mandates cooling tower maintenance and sampling.
• Biofilm control through consistent treatment and physical cleaning is essential.
• Idle and seasonal towers need specific procedures to avoid stagnation.
• Keep records of treatment, cleaning and sampling, and verify the program is working. Accurate condenser water flow, confirmed through water balancing, supports both thermal performance and even distribution over the fill.