When does a drinking water installation become a risk? Whenever Legionella bacteria are detected in drinking water through laboratory analysis, it highlights just how closely responsibilities, prevention and technical planning are interlinked.
For buildings with centralised drinking water heating systems, what happens within the installation is crucial: temperatures fluctuate, water stagnates, biofilms form, and rarely used taps become weak points. In most cases, Legionella in tap water is the undesirable result of an interplay between the installation, temperature control, operation, and the operator’s responsibilities.
The crucial question is therefore: how can planning, operating and the pipework system be coordinated to prevent hygiene risks, such as Legionella in tap water, from arising in the first place?
Legionella are bacteria that occur naturally in fresh water. In low concentrations, their presence is not considered unusual. They become a concern when technical water systems create conditions in which the bacteria can multiply. This applies primarily to hot water systems in buildings, but also to other systems in which water is heated, stored, distributed, or atomised.
From a health perspective, Legionella pneumophila is particularly significant. It is considered to be the most important species in relation to legionellosis. Infection usually does not result from drinking water, but from inhaling finely atomised water droplets. Such aerosols can form, for example, whilst showering, in whirlpools, via humidifiers or in technical systems where water is released into the air.
The potential illnesses differ significantly. Pontiac fever usually presents with flu-like symptoms and does not involve pneumonia. Legionnaires’ disease, on the other hand, is a severe form of pneumonia and can be particularly dangerous for older people, those with weakened immune systems, or people with underlying health conditions.
Legionella bacteria multiply under certain conditions: these can include poorly designed, inadequately maintained, or modified plumbing systems. The relationship between water temperature and Legionella plays a key role here. Temperature ranges in which Legionella can multiply are particularly critical. However, the technical assessments and guidelines that result from this vary from country to country. The USA and Germany provide prime examples of this.
In Germany, the Drinking Water Ordinance and generally accepted technical rules, govern planning and operation. The DVGW regulations stipulate that, for large-scale technical systems for heating drinking water, a temperature of at least 60 °C must be reached at the outlet of the water heater. In the circulation system, the temperature must not fall below 55 °C. The DVGW (German Technical and Scientific Association for Gas and Water) and the Federal Environment Agency state that a temperature of 55 °C throughout the entire circulation system provides reliable protection against the proliferation of Legionella.
The US Centres for Disease Control and Prevention (CDC) describe 25 °C to 45 °C as a particularly favourable growth range. At the same time, they point out that Legionella can also grow at lower temperatures under certain conditions. In their guidance on cold water, they cite temperatures from around 20 °C. In practice, this means that hot water which cools down too much and cold water which heats up too much, fall into a range that is unfavourable from a hygiene perspective. Therefore, the clear separation of cold and hot temperature zones is one of the most important tasks in planning and operation.
Further risk factors:
This makes Legionella prevention a key aspect of drinking water hygiene. The overall system is crucial: sizing, pipework layout, insulation, temperature control, usage, and regular water replacement must all be properly coordinated.
The requirement for Legionella testing depends on the type of system and its use. In Germany, the Drinking Water Ordinance stipulates which water supply systems must be regularly tested for Legionella spec. – that is, for Legionella species as defined by the Drinking Water Ordinance.
Testing is mandatory if there is a large-scale drinking water heating system and drinking water is supplied as part of a commercial or public activity. Typical examples include larger residential buildings, hotels, care homes, hospitals, schools, sports facilities, commercial properties, or public institutions. Detached and semi-detached houses are not usually covered by this requirement.
A drinking water heating system is considered a large-scale system if:
The inspection intervals depend on the system’s usage. In the case of public supply, testing for Legionella is typically required annually; for exclusively commercial use, at least every three years. For newly commissioned water supply systems, subject to testing, the first Legionella test must be conducted no earlier than three months and no later than twelve months after commissioning, in accordance with Section 31 of the Drinking Water Ordinance (TrinkwV).
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Use of the system |
Typical testing interval |
|
Supply to the public |
Annually |
|
Exclusively commercial use |
Every three years |
|
New installations |
Initial inspection no earlier than 3 months and no later than 12 months after commissioning |
For a Legionella test to yield meaningful results, a single sample from any random tap is not sufficient to assess the condition of the entire drinking water system. Rather, the aim of the systematic test is to determine whether Legionella is present in relevant concentrations in the circulating hot drinking water.
For operators, this means that Legionella testing forms part of their legal responsibility. To this end, sampling points must be selected in such a way that both central and peripheral areas of the system can be assessed. These typically include areas near the drinking water heater, the circulation system and representative sampling points within the pipework network.
This has practical implications for both planning and operation. Sampling points must be accessible, documentable, and properly integrated. Forward-thinking pipework planning takes these requirements into account at an early stage, rather than having to retrofit them later at considerable expense. This facilitates routine testing, improves the interpretability of the results, and reduces operational uncertainty.
In everyday language, people often refer to Legionella limit values in drinking water. The more technically precise term is ‘technical action value.’ For Legionella spec, this value in Germany is 100 CFU per 100 millilitres of drinking water. CFU stands for colony-forming units and describes a microbiological measure of bacteria capable of reproduction.
It is important to note that the technical action level is not a value at which there is automatically an acute health risk to all users. However, it marks a threshold above which the operator must act. If the value is reached, further steps are then required to assess the quality of the drinking water, identify the causes, and implement appropriate measures.
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Findings |
Classification |
What does this mean? |
|
< 100 CFU/100 ml |
Technical action level not reached |
No action required based on the action level; regular monitoring as specified |
|
≥ 100 CFU/100 ml |
Technical action level reached |
Reporting to the Public Health Authority, investigation of the cause, risk assessment, and measures in accordance with the Drinking Water Regulations |
A positive test result for Legionella calls for a structured response. The first step is to evaluate the laboratory result. The key factors are whether the technical action threshold has been reached, which sampling points are affected, and whether the result indicates localised or systemic contamination.
Depending on the situation, several steps then follow:
Possible response measures range from flushing procedures and temperature management, through the removal of sections of pipework with no water flow, to technical refurbishments. In sensitive areas, temporary restrictions on use or end-of-line filters may be required. Which measures are appropriate must be decided on a professional and site-specific basis.
It is important to set realistic objectives: in complex existing systems, the immediate, complete, and permanently safe elimination of Legionella from drinking water is not always achievable. The aim is to minimise the risk in accordance with accepted technical standards and to restore hygienically safe operation.
The most effective strategy against Legionella in drinking water is prevention. Planning, installation, and operation must prevent Legionella from finding favourable conditions. To achieve this, it is crucial to take all factors into account.
These requirements must be considered right from the planning stage. An isometric drawing of the pipework makes the spatial layout of the pipework network clear: pipe lengths, branches, height differences, fittings, and connection points become visible. It therefore provides an important basis for aligning the pipework layout, circulation, planned sampling points, and maintenance areas with the building’s future use at an early stage.
A suitable pipework system helps to reliably meet the planned requirements. aquatherm green is a prime example of PP pipework systems for drinking water applications. The corrosion-free material and the permanently leak-tight joints achieved through the material-bonding PP welding process help to ensure that installations are hygienic and dependable in operation. The result is a pipework system that combines planning reliability, high-quality installation, and long-term operational performance.
Legionella in drinking water is a planning and operational issue. Ultimately, laboratory results merely reveal what the installation and its operation have previously made possible.
Planning, tendering, and ordering of pipework systems must therefore consider at an early stage how temperature control, the prevention of stagnation, sampling points, maintainability, and operators’ responsibilities interact. This is because drinking water hygiene is achieved through the interplay of professional planning, suitable materials, clean installation, and operation in accordance with the intended purpose.
aquatherm supports planners, specialist contractors and operators in the design and implementation of suitable pipework systems for drinking water applications: technically sound, project-specific and with a view to ensuring the long-term safe operation of buildings.
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