Fighting Legionella in drinking water
Legionella are bacteria that multiply strongly in hot water at around 25 to 45 °C and can cause a severe form of pneumonia when inhaled as a fine water mist. From 100 CFU per 100 ml, the operator must act under the Drinking Water Ordinance. They disappear for good only if the biofilm in which they live is also tackled.
Legionella in the building installation is an application of biostream® ZERO in which Legionella and its biofilm in the drinking water installation of a building are controlled with chlorine dioxide.
Key facts
- Product
- biostream® ZERO
- Technical action value
- 100 CFU per 100 ml (Drinking Water Ordinance)
- Testing obligation
- Section 31 TrinkwV, single and two-family houses exempt
- Obligations on a positive result
- Sections 51 and 52 TrinkwV
- Code of practice
- DVGW W 551
- Standard
- GEOhybrid grounding
- Last updated
What is Legionella, and when does it become dangerous?
Legionella are bacteria that occur naturally in water. According to the DVGW, they multiply very well at water temperatures between about 25 and 45 °C; from 50 °C their growth is inhibited, and above 60 °C they die off quickly. They become dangerous when they are inhaled as a fine water mist, for example while showering. They can then trigger Legionnaires' disease, a severe form of pneumonia, or the milder Pontiac fever.
Limit value: the technical action value is 100 CFU per 100 ml
The German Drinking Water Ordinance (TrinkwV) sets a technical action value of 100 colony-forming units (CFU) per 100 ml in Annex 3 Part II. Once it is reached, the operator must immediately inform the health authority, establish the cause, draw up a written risk assessment and take measures (Section 51). For higher results, the Bavarian State Office for Health and Food Safety names further levels, partly citing DVGW code of practice W 551, up to immediate measures such as a ban on showering above 10,000 CFU per 100 ml. The table is under Action value and levels.
Who must have their drinking water tested for Legionella?
The operator, if the system has a drinking water heater with more than 400 litres or more than 3 litres in a hot water pipe, there are showers or other aerosol-generating fixtures, and the water is supplied commercially or publicly. Single and two-family houses are exempt (Section 31 TrinkwV). The intervals:
- Public activity (for example schools, clinics, sports halls): at least once a year
- Commercial activity (for example letting): at least every three years
- Mobile and temporary systems: the frequency is set by the health authority
Details on the criteria, sampling and deadlines: Legionella testing.
Which methods help against Legionella?
Several approaches are used in practice, each with different strengths and limits:
| Method | How it works | Limits |
|---|---|---|
| Thermal disinfection | At least 70 °C for at least three minutes at every outlet. | Energy-intensive; not every system reaches the temperature at every outlet; risk of scalding; has little effect in dead legs. |
| UV irradiation | Inactivates germs in the water flowing past at the point of installation. | No residual concentration in the pipe network; it does not reach the biofilm in the pipes further downstream. |
| Chemical disinfection with chlorine dioxide | Oxidises the cell components of the germs and penetrates the slime layer of the biofilm in which Legionella lives. | Only substances from the list under Section 20 TrinkwV; if the installation is the cause of the deficiencies, it may only be disinfected there on the instruction of the health authority (Section 23(3)). System disinfection carried out by specialist contractors. |
| Point-of-use filters | Hold back Legionella at the outlet. | Immediate measure for high results; does not remove the cause within the system. |
Sources: Drinking Water Ordinance (TrinkwV); Bavarian State Office for Health and Food Safety (LGL); DVGW. This information does not replace legal advice. The version of the TrinkwV in force at the time is authoritative.
All pages on Legionella
Where does Legionella live in the drinking water installation?
Legionella grows wherever hot water is too cool or stagnant. Typical locations are the drinking water heater if it is permanently run below 60 °C, a circulation where the temperature falls below 55 °C, rarely used outlets and dead legs, and cold water pipes that warm up in a warm environment. A biofilm that protects the germs easily forms in all of these places.
This is why the German Environment Agency (UBA) recommendation for testing names sampling points at the outlet of the drinking water heater, at the point where the circulation re-enters, and at the ends of the risers. Which temperatures the rules require is set out under Hot water and temperature.
Why Legionella survives in the biofilm
The biofilm protects Legionella from the effect of conventional disinfectants. This allows it to keep multiplying. How biofilms form and are structured is explained in detail under Fighting biofilms.
Legionella represents a water hygiene problem that is often underestimated. It settles in what are known as biofilms, which, in our experience, hydrogen-peroxide-based disinfectants can barely combat. Chlorine, too, has less disinfecting power than the active substance chlorine dioxide used in our products.
To fight the biofilm along with the Legionella, Pseudomonas and other pathogenic germs it contains, biostream® ZERO relies on the active substance chlorine dioxide.
Chlorine dioxide penetrates the biofilm
As a gas dissolved in water, chlorine dioxide is able to penetrate the protective layers of slime-forming micro-organisms to kill Legionella, Pseudomonas and other pathogenic germs. This inhibits the growth of the biofilm and, in our experience, also removes biofilms that have already formed. It is also effective in the gas space.
Efficacy tested. In 1,620 individual tests at the Institute of Food Science and Human Nutrition of Leibniz University Hannover (led by Dr. rer. nat. H.-D. Werlein), biostream® ZERO was shown to have an efficacy of 99.9 % against germs occurring in drinking water, at a contact time of 5 minutes.
The cause: biofilm
Permanently controlling Legionella and Pseudomonas in drinking water systems is only possible together with controlling the biofilm in the pipes. Only once the biofilm has been removed as a breeding ground can pathogenic germs and bacteria actively be eliminated.
Where a biofilm is already present, we recommend a thorough decontamination of the pipe system by shock or static disinfection. This involves introducing the active substance chlorine dioxide into the pipe system at a higher dose. After a contact time that depends on the pipework, the entire pipe system is flushed through to remove the dissolved biofilm. In buildings, this system disinfection is carried out by a specialist contractor under DVGW W 551-3.
After that, what matters is preventing regrowth: with the correct temperatures, regular water exchange and, where necessary, disinfection with chlorine dioxide. If the condition of the installation is the cause of the deficiencies, the drinking water there may only be disinfected on the instruction of the health authority under Section 23(3) TrinkwV, and the operator must remediate the installation. Get in touch, we will advise you on your system.
Who is responsible for Legionella in the building?
Responsibility lies with the operator of the water supply system, which under Section 2 No. 3 TrinkwV is the business operator or other holder. In a rented building this is usually the landlord; a property management company acts on their behalf. They arrange testing, notify results of 100 CFU per 100 ml or more, have the cause established and the measures implemented, and inform the residents. The details are summarised on the page Obligations under the Drinking Water Ordinance.
The Robert Koch Institute recorded more than 2,200 notified cases of legionellosis for 2024. More on this in our article Legionella in Germany: more than 2,200 cases a year.
How to proceed as an operator
- Check the obligation. Establish whether your system falls under Section 31 TrinkwV: storage capacity over 400 litres or more than 3 litres in a hot water pipe, showers, commercial or public supply.
- Arrange testing. Commission an accredited laboratory; it takes the samples at the drinking water heater, at the circulation and at the ends of the risers.
- Assess the result. From 100 CFU per 100 ml, the obligations under Section 51 apply. The result guide shows what to do for your value.
- Remove the cause. Temperatures, stagnant water, dead legs and biofilm are the usual causes. Remediation, for example a thermal or chemical system disinfection, is carried out by a specialist contractor.
- Prevent recurrence. Keep to the temperatures required by DVGW W 551, ensure water exchange at all outlets, and have testing repeated at the prescribed interval.
The legal basis for this is summarised on the page Obligations under the Drinking Water Ordinance; all disinfection methods are compared on the page Methods compared.
Efficacy, rules and alternatives
Permitted in drinking water. Chlorine dioxide is included in the list of permitted treatment substances and disinfection methods kept by the German Environment Agency (UBA) under Section 20 of the Drinking Water Ordinance (TrinkwV). Its production and use are governed, among others, by DIN EN 12671 and DVGW code of practice W 224; for the disinfection of installations, DVGW W 291 and W 551-3 name chlorine dioxide as a proven chemical.
Hydrogen peroxide is not a disinfectant in the drinking water list. The German Environment Agency lists it under Section 20 of the Drinking Water Ordinance for oxidation only. Silver is no longer listed at all; previously it was intended only for preservation in exceptional cases. Chlorine dioxide is listed there for disinfection. For the disinfection of installations whose disinfection water is discarded afterwards, DVGW codes of practice W 291 and W 551-3 name hydrogen peroxide and hypochlorite in addition to chlorine dioxide.
Frequently asked questions about Legionella
Does chlorine dioxide work against Legionella?
Yes. In 1,620 individual tests by Leibniz University Hannover (Institute of Food Science and Human Nutrition, led by Dr. rer. nat. H.-D. Werlein), biostream® ZERO was shown to be 99.9 % effective against germs occurring in drinking water, with a contact time of only 5 minutes, including against Legionella pneumophila.
Why is it not enough to disinfect just the water?
Because Legionella lives in the biofilm, a slime layer on pipe walls, in hot water storage tanks and in dead legs. There it is protected from many disinfectants and keeps multiplying. Chlorine dioxide penetrates this slime layer and fights the germs at their source, instead of only treating the water flowing through.
What do I have to do if the result is positive?
From 100 CFU per 100 ml, inform the health authority without delay, establish the cause, draw up a written risk assessment, carry out measures and inform consumers. Notification is only unnecessary if you hold proof that the laboratory has already notified the result. You will find the result guide and the steps under Positive Legionella result: what to do now.
Which hot water temperature protects against Legionella?
In systems with a central drinking water heater and circulation, at least 60 °C at the outlet of the heater and at least 55 °C throughout the circulation. For small systems in single-family houses, the rules give 50 °C as the lower limit; 55 to 60 °C is recommended.
Does a UV system help against Legionella?
A UV system only disinfects the water flowing past the unit and does not build up any residual concentration in the pipe network. It does not reach Legionella living in the biofilm in the pipes further downstream.
Use biocides safely. Always read the label and product information before use. All statements on efficacy are based on biostream® GmbH's many years of experience, on information from the specialist literature, laws and technical rules, and on positive customer references.
What it is not
Legionella in the building installation is an application, not a separate product: biostream® ZERO is used; quantities and contact times are given in the dosing instructions, not on this page. This page is not legal advice; the test report, the risk assessment and the health authority are authoritative.