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▶Article Evaporative cooling systems and cooling towers
Legionella Sampling Strategy for Drinking Water Installations
A positive Legionella result describes a sample taken at one place and one time. It signals that action is needed, but it does not explain the cause or the extent of the contamination. The question behind it is larger: does the drinking water installation as a whole carry a risk, or is the finding confined to one location? Whether the data can answer that question at all depends on how the sampling was planned.
One device, one sample or one operating instruction cannot resolve a positive result on its own.
This article sets out what a robust sampling strategy should contain before the next sample is taken. It covers the questions sampling has to answer, how sampling points are chosen from the structure of the installation, and why operating state and use must be recorded with every result. It does not set sampling points or intervals. Those depend on a review of the specific installation, which a general article cannot replace.
Start with the question the sampling has to answer
Sampling, testing and reporting must follow a clear question and be documented, so that the course of events can be reconstructed later. “Is this outlet positive?” is a different question from “Is the problem central, or confined to one branch?” The first can be answered by one sample. The second needs a set of samples taken with a purpose, and a record of how each one was taken.
The recommendation of the German Environment Agency (Umweltbundesamt, UBA) of 13 October 2025 describes systemic investigation as a standardised way to detect possible contamination in the system. If a systemic investigation finds a problem, further investigation may be needed to locate its extent and possible causes. The two types of investigation serve different purposes, and our white paper on Legionella in drinking water advises against conflating them.
One sample can reveal a local problem, but it cannot explain the whole installation. A short-term fall in the count after a corrective step is not enough either, because disinfection may influence a result in the short term while the operating conditions that favour growth remain. Durable remediation addresses the cause and demonstrates that the intervention worked.
Sampling points, the system map and the operating state
Convenient locations are not automatically informative. Points are selected according to how the installation is built, with locations at the water heater, in the circulation and on the supply branches that matter. An acceptable temperature at the heater outlet says little about a distant or hydraulically weak branch.
The basis for choosing points is a current system map. It connects water paths, operating states, sampling points and actual use, covering water heating, storage, circulation, distribution, risers, outlets and rarely used areas. Drawings often show the original installation, while alterations, changes of use and temporary connections may have changed the real hydraulics. An on-site inspection compares plans, visible pipework and operating data, so that a later result can be placed in the installation instead of being read as an isolated number.
The map should show the relevant water paths, operating conditions and sampling locations, and it is updated when findings, modifications or operating deviations become known. The right plan is as detailed as the technical decision requires, neither maximal nor minimal. A follow-up plan then tests hypotheses, and every additional sample should answer one of them: does the finding change along the water path, and are rarely used or temperature-critical areas affected?
Results are interpreted together with the date and time, the sampling location and procedure, the temperature, the use and the relevant operating state. Flushing, interruptions of use or other interventions shortly before sampling may affect the result and should be recorded. Every abnormal sampling point should be traceable to a water path, an operating state and documented use.
Read together, the structure of the installation, the operating data, the patterns of use and the sampling results show whether an abnormal result is locally confined or points to a systemic risk. A single abnormal outlet may have a local cause. Repeated findings across several water paths require a wider investigation.
From a positive result to protection and verification
One device, one sample or one operating instruction cannot resolve a positive result on its own. Protection, engineering work and verification belong to one documented process. A measure is defensible when its intended effect on a risk condition and the method for measuring that effect are clear.
Verification is planned before the measure begins. The intended condition, target values, sampling times and release criteria are defined in a way that fits the measure, and normal operation is recorded before follow-up results are interpreted. A context-free sample may capture a favourable moment without representing the stability of the system.
In Germany, the operator’s duties regarding Legionella are set out in the Drinking Water Ordinance (Trinkwasserverordnung, TrinkwV). Section 31 contains the examination duties for Legionella species. Section 51 sets out the operator’s duties to act when the technical action value for Legionella species is reached. These provisions are named here only to show where the duties sit, so check the current text before relying on it. This article does not describe the rules that apply outside Germany.
For the next sampling round, start from a current system map. Map the water paths, identify how and how often each area is used, and record the operating states the installation goes through. The sampling plan then follows from that map, and every sample answers a defined question. Where the technical action value has already been reached, section 51 TrinkwV requires the operator to act without delay, so short-term protection and the technical investigation begin in parallel.
A few direct questions show whether a programme is ready. Can you name the question each sampling point is meant to answer? Are the operating state and use recorded with each result? Who decides what protective action follows a positive finding, and how will its effectiveness be verified? This checklist is a starting point for internal review rather than a compliance assessment.
Have a water related question? Use Hydreatio Smart Match to find the assessment most relevant to your operation. The result provides directional guidance and does not replace laboratory testing, site inspection or expert assessment.
SourcesTrinkwasserverordnung (TrinkwV), official text (gesetze-im-internet.de). Hydreatio technical report "Legionella in Drinking Water". Technical documentation from Voigtländer and Liquid Engineering. Anonymised reference projects.