Legionella in drinking water: what to do now

Legionella in drinking water: what the result means

The number in the report is not a school grade, it is a fork in the road. Below 100 CFU per 100 millilitres you are under the technical action level, and nothing further is legally required of you. From 100 upwards, duties hang on that number, and they hang on exactly 100, not on 101.

How much you have to do depends on how high the value is.

The action level of 100 CFU/100 ml is set by the Trinkwasserverordnung (Annex 3 Part II), Germany's drinking water ordinance, and the duties that follow from it by § 51 TrinkwV. The values of 1,000 and 10,000 CFU/100 ml come from DVGW W 551 and are brought together in the Umweltbundesamt recommendation of 14 January 2021. Chart by Aqvior.

A result in the water and an illness are two separate questions. The action level is a verdict on your installation. How an infection progresses, and how to recognise it, is covered under Legionella: symptoms and infection. If there is a concrete suspicion, the route leads to a doctor, and that is where it belongs.

Step 1: notifying the Gesundheitsamt

The notification comes at the start of this chain, not at its end. Anyone who waits until the specialist firm has drafted a remediation plan has already breached the duty, even if everything is cleanly remediated in the end. Establishing the cause and producing the risk assessment are due without delay, whoever filed the notification. The law requires notification without delay, as soon as the value is reached (§ 51 TrinkwV).

Two reports run in parallel here. The approved testing laboratory reports an exceedance to the Gesundheitsamt on its own initiative (§ 53 TrinkwV). The authority may well have the result in front of it at the same moment you do.

Your own duty to notify falls away only if you hold proof that this report was made (§ 51(1) no. 1 TrinkwV). Assuming it happened is not enough. Until that proof is in your hands you notify yourself, and it makes a real difference whether the authority hears it from you or has to call you. Note down when you reported the exceedance, too: at a later inspection, that date is your only evidence that "without delay" really was without delay.

Step 2: establishing the cause and the risk assessment

The Gesundheitsamt does not only want to know that you have a positive result. It wants to know why your installation produced one, and that is answered by a qualified expert whom you have to appoint.

It cannot be done from a desk. Establishing the cause includes a site inspection: somebody walks the building and looks at the installation, above all at the risers and outlets where nobody has showered for weeks. Why anything grows there in the first place is covered in the overview of why an installation lets Legionella grow. What comes out of it is the risk assessment (Risikoabschätzung), in writing, and it is the basis for everything the authority orders afterwards.

Step 3: measures, and when the shower gets closed

What has to happen now follows from the risk assessment, and it aims at the cause. Point-of-use filters on the showers can lower the risk for guests and residents while remediation is under way, but they do not take the place of removing the cause.

The question that really occupies operators at this point is a different one: does the shower have to close? Between 1,000 and 10,000 CFU/100 ml a restriction on use must be considered, and nobody else will make that call for you. Whether showering can continue in a given case depends on the installation and the type of building. The reasoning behind it is set out under the shower as the critical point.

None of this is decided at reception. It is decided with the Gesundheitsamt. Anyone who notified early sits in that conversation in a far better position than an operator whose installation the authority knew about first.

Step 4: informing the people affected

Almost everyone overlooks this step, and it is the only one aimed at the people in the building.

It does not kick in automatically with the result. The duty arises once the Gesundheitsamt orders hazard-response measures, and then it applies without delay, after discussing it with the authority (§ 52 TrinkwV).

"There was a positive result" is not enough. The people affected have to learn what the risk is and where it comes from, which measures are running, and above all what they must not use right now. Then comes the practical part, and the most important line in it is: do not drink standing water. People at particular risk you address separately, and you tell them how to protect themselves.

And there is a second message: once the danger is demonstrably over, the same people have to be told that too, and that normal operation is running again. A notice that stays up for months does not do the job. It only prolongs the damage.

Step 5: the follow-up testing

Remediated and proven are two different states. After an exceedance and after remediation the Gesundheitsamt orders follow-up testing, and only that result shows whether the measure worked.

This sample is taken by an approved testing laboratory too, exactly like the first. When and how widely you retest is set by the authority, and it can also shorten the regular testing interval for your building. How often testing is due without a positive result, and who is subject to it at all, is set out in the overview of Legionella testing.

The follow-up result belongs in the same file as the original one. Together the two numbers make up the story you will have to tell at the next inspection, and it is an easier story to tell when it is complete.

So a positive result does not disappear because something was repaired. It disappears when a fresh sample says so.

Frequently Asked Questions

How long do I have to report the result?

There is no deadline in hours or days. The drinking water ordinance requires notification without delay, and that is stricter than a fixed date: you have to act as soon as you know the value, without letting anything sit on a desk. If you find a figure like 72 hours online, it is worth knowing that neither § 51 nor § 52 TrinkwV contains any such deadline.

The laboratory has already reported it to the Gesundheitsamt. Do I still have to notify?

Only for as long as you do not hold proof that the report was made. The law words your duty with an express condition: you notify "unless proof is available to him that the report … has already been made by the approved testing laboratory" (§ 51(1) no. 1 TrinkwV). An assumption does not count as proof. Ask the laboratory for it, and until it arrives, notify yourself.

The value comes from a single sample. Do I still have to act?

Yes. The duties hang on the action level being reached, not on how many outlets showed it. Whether only one riser is affected or the whole installation is what the investigation into the cause then clarifies, and the scope of the remediation follows from that.

Does a Legionella result mean somebody has fallen ill?

No. The action level is a technical figure about the installation, not a statement about any individual. It shows that the installation favours Legionella growth, and that is precisely why you act before it turns into a health problem.