Legionella in factory water systems – a regulated future

Legionella are pathogenic fresh water bacteria in natural and manmade water systems with a unique water-to-air transmission route. The water temperature is an important factor influencing the colonisation of water systems. Potential critical areas are cooling water, wet scrubbers, sanitary water systems, and aerobic part of wastewater treatment plants. The most recent legislation in Europe, 42nd German Federal Immission Control Act, regulates the technical and organizational requirements for the operators of evaporative cooling, cooling towers, and wet scrubber systems. The regulation specifies two test and one action limit for Legionella and the required measures when exceeding each limit. The reference method, ISO 11731:2017, has disadvantages e.g. analysis time of 7–10d and interfering non-Legionella bacteria that can give false negative results. Alternative methods includes quantitative PCR and fluorescence in situ hybridisation (FISH). Both methods are suitable for in-house evaluation of Legionella. The operation of Legionella positive system is a challenge mainly due to significant resistance to biocides when the bacteria are associated with biofilms. Preventative and corrective actions with focus on cooling water systems are summarised.