Draft of the new DVGW W 551-1 (A): a new milestone in Legionella prophylaxis

DVGW W 551-1 (A) is viewed as the authoritative technical code for the prevention and appraisal of Legionella in drinking water installations. Unsurprisingly, the new draft of DVGW W 551-1 (A) also maintains the three core goals of avoiding Legionella, detecting contamination and ensuring successful decontamination. Some 21 years after its predecessor appeared, the worksheet has now been restructured, with revisions and additions to the text, precise definitions for key terms, and a renewed emphasis on the topics of cold water systems and correct sampling. 

Instead, it makes “statements on the appraisal of drinking water installations from a technical and hygienic perspective, and potential contamination with Legionella in the drinking water installation itself. The probability of a Legionella infection is not considered as part of this appraisal.” This fact results from the ‘Legionella paradox’, whereby the harmfulness of Legionella in an installation will vary. Sometimes even high levels of the bacteria fail to cause any disease, while a low-level contamination does cause people to fall sick. Some people – such as men aged 55 and over – are also more at risk.

Specified normal operation (section 3.2) is given a detailed definition as the “…regular use of all tapping points…” while “…complying with temperature requirements, carrying out regular functional tests and performing the necessary maintenance tasks…”.

The existing definitions of large and small systems are retained, so as to simplify communications that make use of these terms. Regrettably, these terms were dropped from the German Drinking Water Regulation. The draft also spends some time clarifying that an installation qualifies as a large system – and is subject to mandatory inspection – if the building has more than two apartments, and the volume in the longest path of flow between the drinking water heater and the tapping point is more than 3 litres. This means that hot water circulation will always be needed – also downstream of apartment systems for hot water heating. Under these conditions, this kind of drinking water installation will also be subject to mandatory inspections. 

Section 6.2 also clarifies how the scope of systemic inspections varies considerably, depending on whether the building is a healthcare facility or some other kind of property. According to recommendations made by the German Federal Environment Agency in 2006, healthcare facilities always require comprehensive inspections. This has been confirmed by the DVGW W 551-1 (A) draft. In practice, however, this difference is not so well-known – even among professionals.

The new draft is the first technical code to describe the exact sampling point selection and sample-taking procedure for Legionella testing, as well as the measurement of the sampling point and system temperatures. With mixed-water fittings, sampling angle valves even make it possible to locate Legionella in the hot or cold water system and identify the actual system temperature in the absence of intermixing.

Important: In all electronic fittings, the hot and cold water line are directly connected to one another. In this situation, you will therefore need reliable type EB backflow preventers to guard against overflow (from hot to cold and vice versa). One of the angle valves must always be shut off before taking any samples, so as to avoid sampling mixed water.

Another new requirement: In the future, the temperature of the cold water line (PWC) must also be determined at each selected sampling point for the hot water line (PWH). If, after drawing off 3 litres, the temperature of another 250 ml sample exceeds 25 °C, the cold water line (PWC) must be immediately sampled and tested by a lab for Legionella. 

The practice to date has been retained. Changes to systemic inspections are possible if the Federal Environment Agency considers these necessary after consulting the Drinking Water Commission. This would make sense in the social housing sector, where tenants are responsible for water exchanges in their apartments and not the landlord, who can only ensure the central installation is working properly. There are therefore discussions as to whether the longest path of flow should remain definitive in the future – but sampled at the first-possible tapping point instead of the last one. This would ensure a clear separation of spheres of responsibility.

If samples taken during the systemic inspection are not taken from tapping points used as intended, then this “…prevents an assessment of the hygienic circumstances in the overall drinking water installation. This can lead to the entire inspection being rejected by the public health authority.” This is a clear condemnation of the substandard sampling methods often encountered in practice that have very expensive consequences.

The draft worksheet specifies two types of temperature measurements: as part of systemic and comprehensive investigations, and as part of risk assessments conducted by appraisers. Requirements vary between the two types. Practitioners should be aware that there is no requirement for regular temperature logging, even when this is advisable for the early detection of risks. Nor are data loggers required for systemic measurements – the temperature is simply measured in the sampled water. Comprehensive measurement and recording to higher standards of accuracy is only required in the event of contamination or reduced temperatures. Assessments are based solely on average temperatures, not short-term deviations, which is important to avoid unnecessary decontamination work.

This measurement has been redefined. Instead of waiting for a constant temperature as before, this measurement is now made in a 250 ml volume after drawing off slightly more than three litres of water. This saves time, cuts costs and provides immediate information about the installation quality. If 55 °C is measured for hot water and no more than 25 °C for cold, the installation complies with the 3-litre rule.

The (draft) DVGW W 551-1 (A) now includes an option to run the circulation at 55 °C/50 °C, so as to optimise heat pumps. A number of preconditions apply, including planning according to applicable technical codes, electronic control valves with temperature logging, precise temperature measurement at multiple locations and 5-year archiving of results, three Legionella inspections in the first year, no Legionella found before reduction (<2 CFU/100 ml), notification of consumers and the public health authority.

Almost all energy savings are made when heating – rather than consuming – hot water. Reduced cold water consumption is problematic for hygiene: as a TU Dresden project has shown, this can promote Legionella in cold water. In new properties, cold water piping should therefore have smaller dimensions – and this applies to all systems with reduced temperatures.

The worksheet deals with the familiar three categories of prevention: primary (avoidance), secondary (detection) and tertiary (rehabilitation). These come from medicine and provide a useful structure for the various measures. Information is deliberately repeated at several junctures to avoid unnecessary page-turning.

Drinking water should be heated in the hot water line, storage must be inspection-ready and heated to 60 °C at least on a daily basis.
The worksheet is very clear about ‘Legionella protection’ settings: regular thermal disinfection not advisable on grounds of hygiene or saving energy and should be deactivated whenever encountered. This does not reach the critical stub lines but simply stresses components for no good reason.

Risk assessment terms used in the event of Legionella contamination

Risk: A combination of the probability of occurrence and the resulting severity for a hazard in a drinking water installation; is always to be weighted in relation to the drinking water installation affected

Probability of occurrence: The likelihood that the deviation in question has caused or will cause Legionella to proliferate (= concentration reaches the technical action value)

Severity: Effects (potential or actual) of Legionella contamination due to deviation from applicable technical codes This procedure is not capable of estimating the risk to human health.

The DVGW W 551-1 (A) draft introduces clear structures, greater terminological precision and new technical rules for Legionella prophylaxis. Particular attention is devoted to the topics of cold water hygiene, easier temperature measurement and a more nuanced set of preventive measures. The original principles of avoidance, detection and rehabilitation are maintained – but brought up to date.