Positive Legionella result: what operators must do now
If the Legionella result is 100 CFU per 100 ml or higher, the operator must act without delay: inform the health authority, establish the cause with a site inspection, draw up a written risk assessment, carry out the necessary measures and inform the affected consumers (Sections 51 and 52 TrinkwV). Notification is only unnecessary if there is proof that the laboratory has already notified the result.
Result guide: assess your measured value
Enter the Legionella result from your test report. The guide shows which obligations and levels apply. It does not replace advice; all the levels are also shown in the table below.
What levels are there?
| Result in CFU per 100 ml | What applies | Source |
|---|---|---|
| below 100 | Action value not reached; continue testing at the prescribed interval | Section 31 TrinkwV |
| from 100 | notify, establish the cause, risk assessment, measures, inform consumers | Sections 51, 52 TrinkwV |
| 100 to 1,000 | if the risk assessment shows a need for action, further remedial measures | LGL Bavaria |
| above 1,000 | short-term remediation measures | DVGW W 551, as reported by LGL Bavaria |
| above 10,000 | additional immediate measures, for example a ban on showering or point-of-use filters | LGL Bavaria |
| High-risk areas | any detection leads to a hygiene assessment and restrictions on use | UBA recommendation of 13 October 2025 |
The steps after a positive result
- Read the result. Check the value, the sampling point and the date in the laboratory's report. The value shows whether the technical action value of 100 CFU per 100 ml has been reached.
- Inform the health authority. The operator notifies the result without delay. This is only unnecessary if they hold proof that the accredited laboratory has already notified the result; the laboratory itself is obliged to do so under Section 53.
- Establish the cause. The investigation must include a site inspection and a check of whether the generally recognised engineering standards have been met in the drinking water installation.
- Draw up a risk assessment. The operator has a written risk assessment drawn up, following the recommendation of the German Environment Agency (UBA). Section 51 still refers to the December 2012 version; the new version of 13 October 2025 becomes binding once the Ordinance refers to it.
- Carry out measures. The operator carries out the measures needed under recognised engineering standards to protect consumers, and reports them to the health authority without delay. For very high results, immediate measures are among them.
- Inform consumers. The affected consumers are informed without delay as soon as the risk assessment and details of any restrictions on use are available.
Which measures are possible?
Which measures are needed follows from establishing the cause and from the risk assessment. In practice, it comes down to the cause within the system: hot water temperatures, which under DVGW W 551 should reach at least 60 °C at the outlet of the drinking water heater and at least 55 °C in the circulation, pipes with stagnant water, and removing the contamination.
- Thermal disinfection. At least 70 °C for at least 3 minutes at every outlet, under DVGW W 551-3.
- Chemical system disinfection. Carried out by a specialist contractor under DVGW W 551-3; the code names chlorine dioxide, sodium hypochlorite and hydrogen peroxide as proven chemicals.
- Point-of-use filters. At the outlets, as an immediate measure for high results.
How chlorine dioxide works against Legionella in the biofilm is set out on the page Fighting Legionella in drinking water. The process of a system disinfection is described on the page Flushing and disinfection.
Frequently asked questions about a positive result
How quickly must I inform the health authority?
Without delay. The Ordinance does not specify a deadline in days. If the accredited laboratory has already notified the result under Section 53 and you hold proof of this, your own notification is unnecessary.
What is the difference between a hazard analysis and a risk assessment?
The 2023 Drinking Water Ordinance refers in Section 51 to a written risk assessment. The earlier recommendation from the German Environment Agency (UBA), to which the section refers, was titled "Empfehlungen für die Durchführung einer Gefährdungsanalyse" (Recommendations for conducting a hazard analysis). In both cases, what is meant is the written assessment of cause, risk and the necessary measures.
Do I have to ban showering?
For results above 10,000 CFU per 100 ml, the Bavarian State Office for Health and Food Safety (LGL) says immediate hazard-prevention measures are needed, for example a ban on showering or point-of-use filters. Which restrictions on use apply in an individual case follows from the risk assessment and from consultation with the health authority.
Does thermal disinfection help?
It works if at least 70 °C is present at every outlet for at least 3 minutes. If it does not remove the cause, for example temperatures that are too low in operation or pipes with stagnant water, Legionella can return. This is why establishing the cause is always part of it.
Do I have to inform tenants?
Yes. Under Section 52(3) TrinkwV, the operator informs the affected consumers without delay as soon as they have received the risk assessment and details of any restrictions on use.
Sources
- Section 51 TrinkwV, Section 52 TrinkwV, Section 53 TrinkwV
- German Environment Agency (UBA): recommendation on risk assessment, dated 13 October 2025
- German Environment Agency (UBA): systemic testing for Legionella, dated 13 October 2025 (high-risk areas: Table 1, footnote d)
- Bavarian State Office for Health and Food Safety: Legionella
- DVGW W 551-3
This page and the result guide do not replace legal advice or information from the health authority. Your test report, the risk assessment and the version of the Drinking Water Ordinance in force at the time are authoritative.