Warm summer months increase the risk of legionella
Due to increased temperatures, stagnant water and organic deposits
The risk of legionella and other pathogens accumulating and multiplying in health-threatening concentrations in drinking water pipes increases, especially in the summer months.
Elevated temperatures, stagnant water and organic deposits, such as biofilm, are an ideal breeding ground for legionella, which multiply rapidly between 25 °C and 50 °C and pose a significant health risk. In the 2021 annual report, the RKI assumes that 18 - 35 illnesses per 100,000 inhabitants in Germany can be attributed to legionella. Legionella pneumonia is characterized by a severe course of the disease with a mortality rate (lethality) of up to 5 %.
Date: 04.10.2023 | Reading time: 12. Min.
Causes of legionella contamination in the water installation
A large number of known pathogens such as legionella, salmonella, coliform germs, noroviruses and less dangerous bacteria can be found in drinking water. This natural occurrence of bacteria is considered harmless. The Drinking Water Ordinance stipulates a threshold value of 100 colony-forming units per milliliter (CFU/ml), which must not be exceeded.
In addition to increased water temperatures, drinking water installations in buildings are often the cause of increased bacterial contamination. VDI/DVGW 6023 recommends that the water should be flushed every 72 hours to avoid stagnant water. This means that there must be no excessive pipe diameters, unused installation sections or dead pipes. In stagnant water, as occurs in dead pipes, pathogens can accumulate unhindered and contaminate the entire drinking water installation. Biofilm in pipes is also a risk factor.
Maximum water temperature of 25 °C recommended
As the growth of legionella bacteria only stagnates at a temperature of approx. 55 °C and they are only killed at 70 °C and above, a hot water installation that is not operated at sufficiently high temperatures can also be considered a risk. Energy-saving measures, such as permanently lowering the water temperature, must therefore be regarded as hazardous to health. The periodic heating of hot water to over 60 °C - the so-called legionella protection circuit - should be observed.
In order to minimize the risk of pathogens in drinking water, a maximum temperature of 25 °C is prescribed at all water tapping points. The recommendation of the World Health Organization is 20 °C. To ensure that the 25 °C for cold water points is not exceeded, an average temperature of 10 °C at the house connection is assumed as a planning premise. However, research results show that the average temperature at the house connection is over 14 °C, which means that the 25 °C cannot be achieved across the board in the summer months.
Thermal or chemical disinfection
Advantages of disinfection methods for the prevention of legionella
The thermal disinfection process is often used to disinfect affected buildings from pathogens. This involves passing hot water at over 70 °C through the cold water installation to remove existing germs.
However, particularly in large installations, it is not guaranteed that sufficient hot water will reach all water tapping points and that the contamination will be completely removed. In addition, the material of the drinking water installation is subject to considerable stress. Welded seams, seals or cold water fittings are not designed for these high temperatures and are severely affected.
Disinfection with a sodium hypochlorite-based agent is carried out using the dynamic method as part of a flush, in which the disinfectant is introduced into the drinking water installation as a 3 % solution (30 ml/liter) using a dosing pump. The disinfectant kills pathogens such as bacteria, enveloped viruses and fungi. The biofilm is also dissolved and removed.
The disinfecting effect can be measured via the redox value of the rinse water. The redox voltage [mV] is a measure of the germicidal and oxidative effect of disinfectants in water. The voltage depends on the concentration ratio of oxidizing agent to reducing agent (e.g. organic impurities). The higher the redox value of the water, the lower the contamination. During the disinfection process, the drinking water installation is rinsed until the water has a redox value of 650mV - 700mV. At this value, the water and the water installation are demonstrably germ-free.
When using sodium hypochlorite-based agents, germs cannot develop resistance as the agent takes effect immediately.
Practical report: Legionella in an apartment building - 35 residential units affected
Since the end of 2012, it has been mandatory to carry out a legionella test of drinking water installations in apartment buildings every three years. During a routine inspection in 2020, a greatly increased concentration of legionella was detected in an apartment building with 35 residential units.
A thermal disinfection measure of the non-insulated and only partially modernized installation was carried out to eliminate the legionella. This was only crowned with short-term success, so that in 2021 legionella was again detected in a greatly increased concentration of over 600 colony-forming units per milliliter.
In the course of chemical disinfection based on sodium hypochlorite, BEULCO Service was able to disinfect the entire residential complex and record and document this by measuring the redox value.
A prompt inspection of the 35 apartments revealed a recontamination of legionella. This recontamination affected three apartments, so that a more detailed investigation revealed that there were structural defects here. In these three apartments, there were dead pipes in which legionella had accumulated and from there contaminated the entire apartment building. By specifically locating the apartments, it was possible to take targeted structural measures to permanently remove the legionella bacteria.