Drinking water analysis: criteria for the sampling of drinking water

Drinking water is a perishable resource. Yet its deterioration cannot be detected without technical analysis. Accordingly, routine drinking water testing is prescribed by law for many types of buildings. While the residential sector follows a three-year cycle, properties used in healthcare or as hotels must be tested at least annually or even more frequently. To ensure robust results from the drinking water analysis, representative tapping points are to be defined and sampling must be carried out to professional standards.

Just like any other foodstuff, drinking water is a perishable good. If allowed to stagnate, bacteria such as Legionella can propagate to levels that are harmful to health. Depending on how the water is used, there are two separate ‘use-by dates’ for drinking water. If the drinking water is intended for direct consumption or use in preparing food, it should not have been present in the fitting and drinking water installation for more than four hours. When used for other purposes such as handwashing or showering, this period is extended to a maximum of 72 h without use. The four-hour period also applies when testing for material parameters, which can leach into drinking water comparatively quickly. The 72-hour period is primarily used for microbiological parameters, as bacteria require a certain amount of time to reach harmful levels. When commissioning water testing work, it is therefore important to specify the exact purpose of the testing, so that the laboratory can prepare the correct sampling vessels and take appropriate samples of water when on site. Water samples to test for material contamination must be taken (as S-0, S-1 and S-2) after water has rested in the pipes for four hours, while testing for Legionella is carried out during normal operation.

Sampling drinking water: the right places to take samples for drinking water analysis

For Legionella testing, suitable and representative tapping points must be provided in drinking water installations. Pursuant to section 41 of the 2023 German Drinking Water Regulation, these must be set up by the building operator. A prudent strategy here is to seek advice from design planners, tradespersons or an expert appraiser. Only in the case of a systemic investigation for Legionella can samples be taken from special sampling valves – such as an angle valve – but for no other types of analysis. To comply with section 10 of the Regulation, the tapping point outlet is the site for sampling. Here – and not at flush valves, for example – is where drinking water sampling needs to take place. Terminal flush valves or dedicated piping layouts do not constitute exemptions to this requirement.

Group	Type of fittings	Suitable as sampling points? 1	Wash basins, shower and bath filling fittings, etc. with integrated thermostat	No 2	Fittings with upstream angle valve thermostats or a central mixer	No 3	Electronic fittings with mixer shaft	Yes, if the respective isolating valves are closed before sampling 4	Single-lever mixers and self-closing fittings with mechanical temperature limitation	Yes, if the respective isolating valves are closed before sampling

Duties of drinking water installation operators: temperature limits

Building operators are also responsible for ensuring that all areas of the drinking water installation are in a technical state that can guarantee the supply of safe drinking water to every tapping point as part of routine usage. There are no exemptions provided for legacy installations in cases where deviations could result in a hazard to human health. In the case of a large-scale system for heating drinking water (water tank larger than 400 l and/or the longest flow path to the most remote tapping point has a volume of more than 3 l), for example, a minimum temperature of 60 °C must be present at the drinking water heater outlet. No maximum value here has been specified to date in the technical codes. However, it is advisable not to set temperatures above 65 °C, in order to protect cold drinking water (PWC) from excessive heating and to minimise energy costs. The hot water (PWH) temperature must also be 55 °C at each tapping point after 3 l have been drained off (VDI 6023 Part 1, table 1). The volume for analysis here is 250 ml. The outdated 30-second rule from DIN 1988-200 no longer applies, as a result of vaguely defined general conditions such as the ‘litre output’ of the fitting as well as the sampling volume.

Untersuchungsstellen_Legionellen_Sampling_Legionella

The same applies for the PWC line: within the overall system and at each tapping point, this must not exceed the familiar value of 25 °C after a maximum of 3 l have been drawn off, which is also measured in a volume of 250 ml (VDI 6023 Part 1). If there are indications that this 25 °C mark has been exceeded in PWC, then, for legal compliance, even this ‘cold water’ must undergo formal Legionella in drinking water testing (DVGW Worksheet W 551). A minimum number of sampling points must also be observed (see figure).

Top 3 criteria for the legally compliant sampling of drinking water

Correct selection of representative sampling points Sampling using suitable equipment – so without flow regulators, hand showers or shower hoses for testing according to table 1, purpose b Samples only taken in areas of the installation that are used and during ‘normal operation’