Struggling with Legionella

Precautions to Reduce Legionella Proliferation

In Germany, for the installation and operation of drinking water heating and distribution systems, the “Technical Measures to Reduce Legionella Proliferation” outlined in the DVGW-Arbeitsblatt W 551 (Worksheet W 551) dated April 2014 are applicable. According to this, in order to combat Legionella, the temperature of hot water at the outlets of hot water production facilities should be maintained at a minimum of 60 °C. In circulation systems, the temperature of the water in the system cannot be more than 5 °C lower than the outlet temperature. Therefore, the backflow temperature in the circulation of the hot water preparation unit should be at least 55 °C.

Drinking Water Precautions

In addition, drinking water (cold water) should be kept as cool as possible or protected against unwanted heating, for example, from sunlight or proximity to heating equipment lines. This poses a technical challenge in the use of geothermal, solar thermal, and heat pump systems for water heating purposes.

Storing hot water below 60 °C in a hot water tank can result in increased Legionella growth. The following measures can be taken as precautions:

Special “Legionella circuits” that periodically raise the temperature of the tank contents to higher levels. Pipe systems passing through the tank where cold drinking water is heated. Heating the stored hot water in special “freshwater stations” located outside the tank using a robust plate heat exchanger, allowing the added cold water to be kept below 55 °C (providing sufficient temperature for bathing and showering, e.g., 40 °C).

An internal content of drinking water with a value of 100 CFU (colony-forming units)/100 ml is considered contaminated (low infection risk, “technical precautionary value”), while the need for immediate action arises with contaminations exceeding 10,000 CFU/100 ml. The Worksheet W 551 addresses cases of “excessively high contamination” and requires urgent measures such as disinfection of pipe systems or issuing a shower ban.