Control of Legionellae in Domestic Water Systems

Legionella bacteria may enter the potable water system in a building in small numbers in the incoming mains or borehole water. If storage tanks are inadequately maintained, have an excess capacity which facilitates stagnation, and hold water at more than 20°C, legionellae may proliferate. From the supply tank legionellae can enter calorifiers in the make-up water. Many calorifiers are prone to temperature stratification, and legionellae can often be found at their base where optimum temperatures (25–45°C) and the presence of organic nutrients within the sediment, encourage the active multiplication of many micro-organisms, including legionellae.

In the environment, legionellae will survive at temperatures below 20°C but are unlikely to multiply. As temperature rises above 50°C legionellae are less able to survive, and at 60°C the majority will be killed within minutes. Maintaining water temperature below 20°C and above 50°C is therefore the main method of controlling legionellae in domestic water systems as recommended by the DoH (HTM 2040) and HSE (ACoP - L8).

It is recommended that calorifiers in a recirculating hot water system should distribute hot water at 60°C+ and receive the return hot water supply at not less than 50°C. This will eliminate most legionellae in the recirculating water. However, legionellae that enter the warm stagnant water trapped between the recirculating supply and underused outlets, after surviving a calorifier when the water temperature has fallen (eg during peak demand), are likely to proliferate. The inability of a hot water regime to eradicate legionellae from a model domestic water system was confirmed in two BSRIA studies evaluating the efficacy of copper and silver ions (Pavey 1996), and chlorine dioxide (Pavey and Roper 1997) in controlling legionellae in hot and cold water services.

Alternative methods for controlling legionellae in potable water systems were investigated at the Royal Liverpool and Broadgreen University Hospital (RLBUH). These include:

• Ultraviolet Light
• Trace Heating Elements
• Regular Weekly Flushing of Showers (increase frequency for highly susceptible occupants)
• Self-purging Shower
• Chlorine Dioxide

In most cases, control of legionellae in domestic water systems can be achieved by ensuring that the systems are kept in regular use, kept clean and that temperatures in the range 20–45°C are avoided. However, as it has been shown that adopting a hot water regime alone may not be sufficient to control legionellae in some hot water systems, other effective control measures discussed here may be required. This approach is now supported by the DoH and HSE in their guidance on controlling legionellae in water systems.

Micro-organisms, however, often develop resistance to many of the methods intended to eradicate them. Resistance to UV light and the ability to repair UV-damaged DNA is evident in some micro-organisms. Tolerance to temperatures above 60°C and resistance to some potent biocides have been described in others. Legionellae retained by point of use filtration at one outlet may emerge in larger numbers at other outlets in the system where filters have not been fitted.

As legionellae already have a proven track record in adapting to new environments, it would be prudent to remain vigilant for developing resistance to whichever control mechanism is applied, and to never underestimate the value of the “low-tech” approach of simple engineering solutions.

Article by:
Dr T Makin
Department of Medical Microbiology
Royal Liverpool and Broadgreen University Hospitals