Positive Legionella result in a cooling tower: the first 48 hours

The email from the lab arrives, and there it is: Legionella detected in the cooling tower sample. Your stomach drops a bit. That's normal. The good news is that a positive result is something the guidance plans for, and if you act quickly and sensibly, it's very manageable.

Late summer is when you'd expect more of these. Warm water, high heat loads and heavy evaporation all put a treatment programme under pressure. Here's what to do in the first couple of days, in roughly the order you'll need it.

Hour one: read the result properly

Before anything else, look at what the certificate actually says:

• The count. Results are reported in cfu/litre (colony forming units per litre). The number matters, because HSG274 Part 1 sets different responses for different levels.
• The sample point and date. Which system, which tower, and when it was taken. On multi-tower sites, make sure you know exactly which one it is.
• Any typing information. Some labs report whether it's Legionella pneumophila serogroup 1 or another type. Note it down, as it helps your adviser judge the risk.

Then check it against your written scheme, which should set out exactly what happens at each level. As a plain-English summary of HSG274 Part 1 for cooling systems:

• Above 100 and up to 1,000 cfu/litre: resample. If the result is repeated, or most samples come back positive, review the control measures and risk assessment and carry out the remedial actions identified, which may include disinfection.
• Above 1,000 cfu/litre: resample, and immediately review the control measures and risk assessment, with remedial action that may include disinfecting the system.

Our positive result response page sets these out in a table alongside the TVC levels. Your written scheme and the guidance itself have the full wording.

The first few hours: tell the right people

A positive result shouldn't sit in one person's inbox. Let your responsible person for Legionella control know straight away, along with your water treatment contractor and whoever runs the plant. If your scheme names other people, such as a site health and safety lead, include them too.

A positive sample on its own isn't generally something you have to report to anyone. What you must do is act on it and record what you did. Things change if there's a suspected case of Legionnaires' disease linked to the site. The local authority or UKHSA may then get in touch, and if the person affected is a worker whose illness is linked to their work, there may be reporting duties under RIDDOR. If you find yourself anywhere near that situation, get competent advice quickly.

Day one: find the likely cause

A count doesn't go up for no reason. Fixing the number without finding the cause just means you'll be back here in three months. The usual suspects:

• Dosing. Is the biocide pump running and primed? Is the drum empty, or the wrong product connected? Is the inhibitor going in?
• Bleed and chemistry. Has the conductivity controller or bleed valve failed, letting the water concentrate?
• Recent dip slides. Have TVCs been creeping up over the last few weeks?
• Cleanliness. When was the last clean and disinfection? Is the pond visibly dirty?
• Operating pattern. Has the tower been idle, on low load, or running much harder than usual in hot weather?

An engineer on site, looking at the tower and the dosing kit with the records in hand, will usually spot the likely cause quickly.

Day one to two: corrective action

What you do depends on the count, the history and what the investigation turns up. Typical actions include:

• Fixing the dosing fault or bleed problem straight away
• An online disinfection of the system, at the level and contact time set out in your written scheme or method statement
• Bringing forward a full clean and disinfection if the system is dirty
• Stepping up dip slides and chemistry checks for a while

Should you shut the tower down? Not automatically. Many results are dealt with by resampling, fixing the cause and disinfecting while the system keeps running. In some situations, particularly at higher counts or where vulnerable people are nearby, isolating or shutting down the tower is the right call. Bear in mind that switching off and leaving water stagnant can make things worse, so if you do shut down, have a plan for what happens next. Make the decision with your adviser, and write down why.

Resample, and time it sensibly

A resample tells you whether control has been restored. Timing matters. Sample too soon after a disinfection and the result may look clean while the underlying problem is still there. Your written scheme or adviser should set the timing, and samples should go to a UKAS-accredited laboratory, the same as routine ones. Keep resampling as the scheme requires until results show the system is back under control.

Record everything

This is the part that gets forgotten once the adrenaline wears off. Your log book should show the original result, who was told and when, what the investigation found, what was done (with chemical readings and dates), the resample results and any changes to the risk assessment or written scheme. If an inspector or environmental health officer ever asks, that record is your evidence that you took it seriously and acted promptly.

In short

Read the result, check it against your scheme, tell the right people, find the cause, fix it, resample and record. It's a process, not a crisis. If a dirty system turns out to be the cause, our post on cleaning frequency is worth a read, and if the tower had recently been idle, see restarting after a shutdown.

Got a result in front of you right now and not sure what to do next? Give us a ring and we'll talk it through.