Legionella in shower water – The right measures to take in case of contamination

A guide for building managers according to the current regulations (as of March 2025) of the Canton of Valais (SCAV/DVSV).

Drinking and shower water systems are sensitive systems. Even with the best maintenance, Legionella bacteria can colonize the pipes. These bacteria can cause serious respiratory illnesses (Legionnaires' disease). The responsible Office for Consumer Protection and Veterinary Affairs (SCAV) has issued clear guidelines on how to proceed in the event of a confirmed Legionella contamination.

As a certified analytical laboratory (Swissmedic/GMP) in Steg, Valmonas Analytik AG supports you not only with preventive microbiological analysis, but also with the required follow-up samples after contamination. In this article, we summarize the most important steps for combating Legionella.

Basic information on dealing with Legionella

Completely eliminating Legionella spp . from an already contaminated pipe network is rarely possible in practice. Often, the goal is to reduce the proliferation to a harmless level and keep the situation under control permanently. Legionella can repeatedly recolonize pipes – whether through dead legs, niches such as limescale deposits, or damage to the supply lines.

The measures taken depend on the degree of contamination detected.

Level 1: Moderate contamination (1,000 – 10,000 CFU/l)

This value indicates a need for action to prevent worse outcomes.

Measures to be taken:

• Cleaning: Disassemble, clean, and descale fixtures (hand showers, shower heads, etc.). Fixtures that are visibly heavily worn should be replaced.

• Heat shock (thermal shock): Perform thermal disinfection in the pipe network (> 70 °C for at least 3 minutes at the outlets).

• Optimize temperatures: The temperature at the outlet of the hot water storage tank must be set to a constant minimum of 60 °C. The return temperature of the circulation line must be 55 °C.

• Operating conditions: Examine sustainable improvements to system operation.

Microbiological follow-up samples:

Immediately after the heat shock treatment has ended, an effectiveness check must be carried out. For this purpose, a water sample must be taken directly at the contaminated sampling point and analyzed in the laboratory.

Level 2: Severe to massive contamination (> 10,000 CFU/l)

Important: Reporting requirement and prohibition of use!

If the shower water poses a direct health hazard (> 10,000 CFU/l), the responsible person must immediately notify the SCAV . Furthermore, users must be informed of the temporary ban on use (showers, baths, etc.). All subsequent measures must be carried out in close cooperation with the SCAV (LGV Art. 84 para. 4).

Measures to be taken:

• Replacement: Dismantle, clean, and descale the fittings. All outdated fittings must be replaced.

• Double disinfection: Perform thermal shock (> 70 °C, at least 3 minutes). Additionally, chemical disinfection must be carried out with the assistance of a qualified professional.

• Temperatures: Here too, a minimum boiler outlet temperature of 60 °C and a circulation return temperature of 55 °C are mandatory.

Microbiological follow-up samples – stricter requirements:

1. Immediate check: Samples must be taken immediately after completion of the treatment (heat or chemicals), specifically:

   1. Directly at the contaminated sampling points

   2. At the outlet of the hot water storage tank

   3. At the hot water return

2. Long-term monitoring: For follow-up purposes, further samples must be taken within two months and again after six months .

Note: Recommissioning of infrastructure affected by massive contamination is only permitted after explicit approval by the SCAV / DVSV!

What to do if the measures are ineffective?

If the Legionella concentration does not decrease despite adherence to the above steps, further measures must be taken:

1. Installation of filters: Install terminal anti-legionella filters at all showers (as immediate protection).

2. Refurbishment: A complete refurbishment of the entire system (including hot water preparation and preheating system) must be carried out by a specialist company.

(Further information can also be found in the documents of the BAG/BLV as well as in the SVGW guidelines W3/E3 and W3/E4).

Valmonas Analytik AG – Your local partner for microbiological water analysis

The canton's regulations require fast, accurate, and standards-compliant analyses. This is precisely where we come in. As a modern, GMP-regulated laboratory in Steg, we offer you the complete package:

• On-site sampling: Professional and standards-compliant sampling (including temperature monitoring) directly at your building.

• Rapid microbiology: Quick evaluation of the samples taken thanks to our experienced team in the QC microbiology laboratory.

• Precise reporting: We provide you with all the documents and analysis reports you need for verification with the SCAV.

Don't leave water safety to chance. If you need assistance with preventative checks or emergency follow-up inspections, we are always there for you.

Do you have any questions or need immediate support with follow-up samples?

Contact us. We will be happy to advise you on sampling plans and validated analyses.