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Follow-up study on legionellae shows seasonal effect: 20% less contaminations detected during the summer

On January 31st, Professor Kistemann from the Institute for Hygiene and Public Health of University of Bonn (ihph) presented first results of the follow-up study on legionellae occurence and growth in domestic drinking water installations (warm water systems) for the sponsors of the study being all members of the work group water analytics of figawa, the German Associations of firms in the gas and water sector.

Compared with the initial study the quality of the data provided and assessed was significantly better. Almost every single drinking water data set could be assigned to the different sampling points: drinking water heater outlet, re-entry in the drinking water heating system and peripheral tapping points (often in apartments).

The rate of samples that exceeded the threshold value of 100 colony forming units per 100 ml drinking water was at 6% a little bit higher than the rate of the former study (5,6%).

What was scientifically interesting is the fact that a seasonal effect was proven: In the summer the occurence of legionella exceeding the 100 CFU-limit was 20% higher than in the winter. Referring to Prof. Kistemann the difference is statistically significant since the data base consisted of 305,000 water samples and test results.

Providing all households microbial safe drinking water from source to tap is a major task of water supply infrastructure. This is why the monitoring of the entire supply chain is an essential step to ensure high drinking water quality. A systematic and nationwide collection of drinking water quality data in buildings (90% residential buildings) should show the extent of contaminations of drinking water with Legionella in plumbing systems in Germany.

For this purpose, elicitations were conducted with laboratories and drinking water monitoring contractors (n > 1 Million sampling results). Aims of the intial project were the development of a systematic, hygienic-microbiological status analysis for the presence of Legionella in drinking water plumbing systems, explicitly including commercially used drinking water plumbing systems, the inductive and explorative examination of risk factors for Legionella and the estimation of the hazard potential. The technical threshold level for Legionella of 100 colony forming units (100 CFU/ 100 ml) was exceeded in 5.6% of all samples (n=1,020,328). In 16.5% of all buildings (n=100,213) at least at one tap an exceedance of the technical threshold level was detected between 2012 and 2015. The water temperature was observed to be an important risk factor for culturally detectable Legionella (rs=-0.22; p=<0.001; n=502,009).

The first study was criticized by the German water expert community for its limitations: No data on date and time of the sample drawing was available. More than 90% of all water samples could not be assigned to a specific sampling point. By German laws it is mandatory to take samples from at least three sampling points in the entire installation: one at the outlet of the heater, one at the re-entry of the circulation in the storage tank and at least one in the apartment that is farthest away from the heater (usually one water sample from a top level apartment if the heater is located in the basement).

By means of a second systematic assessment of drinking water data mainly coming from residential buildings the extent of contamination of drinking water in house installations with legionella should be examinated in a more differentiated way, since the data that was now provided allowed the assigment of water samples to the source of tapping: drinking water heater outlet, re-entry in the drinking water heating system and peripheral tapping points (often in apartments). Also, the vast majority of drinking water data sets included the date and time of sample taking. Therefore, research on a possible influence of the season (summer, winter) to the CFU-units detected was part of the follow-up study.

The data provided for the second study came from 4 environmental laboratories and one sampling specialist. 300,000 drinking water data sets contained all relevant informations for the additional research goals and were assessed.

A comparing risk analysis (ROC) showed that the risk of exceeding critical concentrations of legionellae depends on the temperature regime the installations is operated by. An optimal cut-off temperature for reduction of legionellae growth was identified: for outlet of the heater 60°C (140°F). Below this temperature the risk for critical legionelle concentrations is 18.7 times higher. The temperature at re-entry should not fall below 55°C (131°F), since the legionella risk is 7.5 times higher. If the temperature falls under 48°C (118,4°F) at the peripherical sampling points the risk is "only" 2.5 times higher. 

Temperature has been proven as a significant factor for legionella concentration in the water system. The thermal requirements as stipulated in the German technical rule W 551 - Drinking water heating and drinking water piping systems; technical measures to reduce L egionella growth; design, construction, operation and rehabilitation of drinking water installations issued by the DVGW, German Technical and Scientific Association for Gas and Water, that a drinking water heating system should be operated in a temperature rang of 60°C at the outlet and 55°C at the circulation re-entry in the boiler were precisely confirmed by the follow-up study.

Still, the study showed that the water temperature is one relevant factor, but not the only factor that is accountable for the variation of legionella populations in drinking water installations. There to be more, yet unidentified factors that contribute to legionella growth in warm water systems that the research work could not reveal. It also has to be taken in consideration that buildings and installations were very heterogenic ranging from 4-party houses to huge condominiums with 300 residents.

Although temperatures show little variation during the year, a seasonal difference in legionella concentrations was proven. In the summer the risk is 20% lower than in the winter. Other factors than the temperature must cause this effect.

Further information about the follow-up study on legionella in drinking water installations is available at figawa ( You can also address our COO Marcus Pikarek via e-mail (


Drinking water check-ups for legionella offered to enterprises re-opening their offices

Due to covid 19 many industrial, commercial and public buildings have not been used for several months. Since the numbers of new infections with the corona virus have been going down lately companies take a shot at re-opening their businesses. If in the facilities re-opening now drinking water installations have not been in operation or have not been flushed in certain intervals water stagnation may have led to exponential growth of biofilms and - along with that - pathogenic bacteria such as pseudomonas aeruginosa and legionella in the pipes. 

We offer all firms (before re-opening) a thorough check-up of the drinking water for legionella and other pathogenic germs to attractive conditions. Do not hesitate to ask us. We work for German industries as well as for Anglo-American companies from commercial and industrial branches that run subsidiaries in Germany.



Although coming from the housing sector we have seen an increase in business relations with industrial  enterprises. Before the German drinking water directive passed the Bundesrat firms of the producing and processing industry have always been very sensitive for safety issues. Labor protection laws ordered them to have the water used or processed tested for potentially pathogenic bacteria.

Gas leak detection with GAS-Control

WATERcontrol has evolved from the "big sister" GAS-Control GmbH, a specialist for preventive gas safety checks within builidngs. GAS-Control has worked for the German housing industry for 25 years.

To detect gas leakages, even on concealed lines, the firm operates with portable gas leak detectors that pinpoint tracers of natural gas where they escape from the pipe.