How is legionella controlled in a cooling tower?

A cooling tower is controlled by treating its water continuously. That means constant biocide dosing to stop legionella multiplying, scale and corrosion control to protect the treatment, routine water-quality and legionella monitoring, and a documented clean and disinfect carried out at least every six months by a competent person.

None of that is optional and none of it can be replaced by heat. A cooling tower works by evaporating water to shed heat, so the recirculating water sits warm, aerated and nutrient-rich, squarely inside the range legionella grows best in. There is no 60°C store and no cold feed doing quiet background work. If the chemical dosing fails, nothing else is holding the bacteria back.

Why cooling towers are the highest-risk source

Two features make a cooling tower one of the most dangerous water systems there is. First, the water is perpetually in the growth window, roughly 20°C to 45°C, and full of the oxygen and organic matter legionella feeds on. Second, and more seriously, the tower's job is to throw water into the air. It draws air through a spray of warm water to cool it, and in doing so it generates a fine aerosol, the exact particle size that carries legionella deep into the lungs.

That aerosol does not stay on site. A cooling tower on a roof can drift contaminated droplets hundreds of metres downwind, over people who never entered the building and have no idea it is there. This is why cooling towers have been responsible for the largest community outbreaks of Legionnaires' disease on record, where members of the public simply walking past a premises fell ill. A domestic hot water system can infect the person using a shower. A cooling tower can infect a neighbourhood.

The duty nobody else has: notification

Owning a wet cooling tower or an evaporative condenser carries a legal duty that no other water system does. Under the Notification of Cooling Towers and Evaporative Condensers Regulations 1992, you must tell your local authority, in writing, that the device is on the premises. You give its location and the responsible person's details, and you must also notify the authority when the device is taken permanently out of use.

This is a standing register held by environmental health, and it exists precisely because these systems are so hazardous: it lets the authority trace suspect sources quickly when a cluster of cases appears in an area. Failing to notify is an offence in its own right, separate from any failure to control the water. If you have inherited a building and are not certain whether a rooftop unit is a notifiable wet tower or a dry air-cooled unit, find out, because the duty rests with you regardless of whether you knew.

Which guidance applies: HSG274 Part 1

The technical detail for controlling a cooling tower is in HSG274 Part 1, which covers evaporative cooling systems. This matters because most legionella material online, and most of the search results, either come from the United States and quote temperatures in Fahrenheit, or cover Part 2, the hot and cold water guidance that applies to taps, showers and calorifiers. Part 2 does not tell you how to run a tower. For a UK cooling system the reference is Part 1, sitting beneath the duty set by the Approved Code of Practice L8.

The control regime: continuous chemical treatment

Because temperature offers no protection, control of a cooling tower rests entirely on its water treatment programme. A typical regime has three strands working together.

• Biocide dosing. This is the core control. Most systems run a continuous oxidising biocide, such as a controlled bromine or chlorine residual, backed by a non-oxidising biocide dosed periodically. It is common practice to alternate between two different non-oxidising biocides so the microbial population cannot build resistance to a single chemistry. Dosing is usually automatic and controlled to a measured level rather than done by hand.
• Scale and corrosion inhibitors. Evaporation concentrates dissolved solids, which form scale and drive corrosion. Both create rough surfaces and sludge that shelter biofilm from the biocide, so inhibitors are dosed to keep the metal clean and protect the effectiveness of the whole programme.
• Dispersants and bleed control. Dispersants (anti-foulants) keep suspended matter and biofilm mobile so the biocide can reach it, while automatic bleed-off dumps concentrated water and tops up with fresh to hold the water chemistry in range.

Monitoring: weekly, monthly and quarterly checks

A treatment programme is only as good as the evidence that it is working, so a cooling tower is checked far more often than a domestic system.

• Weekly. Biocide and inhibitor levels, conductivity, and a general microbiological check using dip slides to read the aerobic bacterial count. The physical condition of the tower, the pack, the drift eliminators and the water is inspected at the same time.
• Monthly. A fuller water chemistry review, confirming that inhibitor and biocide programmes are performing and that the automatic dosing and bleed are in calibration.
• Legionella sampling. Samples are sent to a laboratory for legionella culture, at least quarterly under stable control, and more often where the system is new, unstable, or the treatment has been in doubt. This is the direct test, distinct from the weekly dip slide, and it is worth understanding how a legionella test differs from a general bacterial count.

Cleaning and disinfection at least every six months

On top of routine dosing, a cooling tower must be physically cleaned and disinfected at least twice a year, that is at least every six months, by a competent person. The tower is drained, the pack, sump and distribution system are cleaned of scale, sludge and biofilm, and the system is disinfected before being returned to service. A clean is also carried out before commissioning a new or refurbished tower and after any shutdown, since standing water in a dormant tower is a classic seeding event. Given the hazard, this work is normally carried out by a contractor holding Legionella Control Association registration, and every clean is recorded.

What a positive legionella result triggers

The response to a legionella detection in a cooling tower is stricter and faster than for a domestic hot or cold water system, because the consequence of getting it wrong is a public outbreak rather than a single exposure. The action levels used for cooling systems mirror the familiar figures.

• Up to 100 CFU/litre. The system is regarded as under control. Continue the programme and keep sampling.
• Over 100 and up to 1,000 CFU/litre. Review the treatment programme and resample. Repeated counts in this band mean control is slipping and the system needs corrective action, which may include disinfection.
• Over 1,000 CFU/litre. Take immediate action. Resample, review the whole regime, and carry out remedial disinfection and cleaning without delay.

Where control cannot be regained, or an outbreak is suspected, the tower may need to be shut down, and the local authority or HSE can act on the notification register to trace and stop a source. The full sequence for any detection is set out in our guide on what to do if legionella is detected. None of this substitutes for the tower's own risk assessment: it decides your sampling frequency and your action thresholds.

Cooling towers and the guidance

Controlling a cooling tower sits inside the same legal framework as any other water system, but with heavier obligations at every point. ACOP L8 sets the duty to identify and assess the risk, appoint a competent person, and keep records. HSG274 Part 1 provides the technical scheme for evaporative cooling. The Notification Regulations add the register your local authority holds. Together they treat a cooling tower as what it is: a system that can make people ill without them ever setting foot on your premises, and one that must therefore be documented, dosed and monitored continuously. Record every reading, every dose and every clean in your written scheme of control, because with a cooling tower the evidence that it is under control is as important as the control itself.