Monitoring for Legionella – The Bacterial Threat


In July 1976, 130 attendees at a convention hosted by the American Legion war veterans’ associations at the Bellevue-Stratford Hotel in Philadelphia contracted a severe case of pneumonia, from which 25 of them sadly died.

The US public health authorities investigated  the unknown cause of the mass disease, which was given the name Legionnaires’ disease. Two years later, Legionella Pneumophila was identified as the cause. The Legionella bacterium was finally identified and isolated, it was found to be breeding in the cooling tower of the hotel’s air conditioning system, which then subsequently spread throughout the building. This prompted new regulations worldwide for climate control systems.

Symptoms of Legionnaires’ disease


Reducing the risk
Legionella outbreaks have been linked to hot tubs, cooling towers, decorative fountains and water used for drinking and showering. Hotspots for legionella include any installation where untreated or under-treated water is warm and where it is possible for it to become an aerosol/misted in the environment. The bacteria multiplies between 22 °C and 45 °C with the optimum growth temperature between 35 °C and 40 °C. Effective and consistent monitoring of water quality is essential to maintain control.

One preventative measure is thermal disinfection for full efficiency, the water heaters and water storage tanks as well as the complete hot-water pipe system including the taps must be heated to above 70 °C. This is not readily possible or practical for most piping systems. Fundamentally, a complete drinking water distribution system should not contain sections with stagnant water, i.e. water that does not move for longer than four hours, in which water temperatures of between 25 °C and 55 °C can occur.

Chemical disinfection                                                                                                                         In this process, chemicals such as chlorine dioxide, ozone, hydrogen peroxide, hypochlorous acid, etc. are added to the drinking water to kill off any legionella present. Due to the partly harmful residues in the lines, chemical disinfection should only be carried out by trained personnel.

UV Irradiation                                                                                                                                   This is a disinfection method that uses short wavelength ultraviolet light to kill or inactivate microorganisms by destroying the nucleic acids and disrupting their DNA.

Summary                                                                                                                                                Air containing aerosols/mists contaminated with legionella can cause severe pneumonia. The development  of legionella in drinking water lines can be prevented by avoiding stagnation in water pipes. Importantly HSE guidelines also dictate cold water must be < 20 °C and heated water is stored above 60 °C. Hence monitoring is critical. Achieving these steps will help prevent the formation of biofilms on the insides of the pipes. Finally even with these controls in place public and commercial properties must have water samples tested regularly by an authorised laboratory.

Rotronic Solutions – The continuous monitoring RMS software makes all measured values and data available in a concise and neat form, exactly as required. It is conveniently available anywhere, on any device

Warning of potential risks can be obtained by logging temperature and flow velocity at critical points in the distribution system and evaluating the data. Rotronic offers a starter kit for this comprising components, hardware and software.

Please contact Rotronic for full product information on 01293 571000 or email



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