Many jurisdictions require a disinfection process if sampling returns positive Legionella test results. Usually the two methods health departments prefer are hyperchlorination and pasteurisation. Each process has it’s merits and pitfalls. In this blog we look at pasteurisation. Hyperchlorination is covered in another post.

The History

Pasteurisation is named after the great microbiologist of the 19th Century Louis Pasteur. He used the technique to prevent wine from going off and to preserve meat for the French army. The process avoids boiling which can damage biological products like milk and wine. These days it is recognised as one of the major means of preventing the spread of tuberculosis and other diseases (Listeria, Brucellosis) from dairy produce. It is also a modern mainstay of safe food production.

In the usual process the product heats to around 70°C for short time. Today pasteurisation of milk and fruit juices usually involves heating to 72°C for 16 seconds. This heating selectively kills susceptible microbes. Since most disease causing bacteria (pathogens) are acclimatised to human body temperature (35°C) this kills them very rapidly. Other more temperature tolerant and usually less harmful species survive. This is why the process disinfects but does not sterilise.

Louis Pasteur 1822-1895

Source: Wikipedia

Potable Water Systems

In potable water systems, pasteurisation can be used to disinfect hot and warm water supplies. There is no need for chemical additions. The heating process means the disinfection effects can get into areas of the water system that chemicals might not reach, for example the components of sensor taps. Providing the water system has sufficient capacity it is relatively easy to heat the water and flush through the system. Of course it is necessary to ensure all the system heats up and requires flushing at every outlet for a minimum of 5 minutes. Naturally people cannot use the system during this process. Warning signs need to be in place at every outlet.

Source: SA Government, Guidelines for the Control of Legionella 2013

Practical Pitfalls

As it says above, if the system is unable to reach the temperature at every outlet it isn’t going to work. Unfortunately, the lower the temperature the longer the flush and the more expensive energy and water goes down the drain. It is an absolute must to have the system capacity to reach the temperatures. The ability to maintain the water above the target temperature throughout the process is also a must.

Pasteurisation of warm water systems requires ‘by-passing’ thermostatic mixing valves. These valves reduce water temperatures to reduce risks of scalds. Servicing and disinfecting the devices as soon as the process is complete is essential before they go back into action. This will need the services of a licenced plumber. Of course this can be labour intensive and a costly exercise in a large building.


Biological Pitfalls

Legionella, Mycobacteria and several other opportunist pathogens will grow between 20 and 45°C. They will also survive for quite long periods at higher temperatures. Here in Australia cold water supplies are often above 20°C in the growth range for these bacteria. Pasteurising a cold water system is impossible so it can’t reach bacteria in the cold water supply. It is pretty obvious that the Legionella came from the mains cold water supply. If it is in the warm water why shouldn’t it also be in the cold water if it is within their growth temperature range? This may explain why thermal disinfection may be less effective than chemical disinfection (UK HSE Legionella Guidance 274 part 2).

Cold water Problems

It is a common ‘myth’ that Legionella ‘are not in the cold water’. However the new enHealth guidelines clearly identify cold water systems as a Legionella risk. So it makes sense to disinfect the entire system if you detect Legionella from any outlet. A South Australian Government fact sheet points this out:

Legionella colonisation of cold water pipes
Cold water systems which are poorly designed, installed and maintained may be at risk of Legionella colonisation. This is predominantly caused by the heating of water within the cold water pipework through inadequate system design, insulation and backflow prevention. Temperature profiling (i.e. regular systematic testing of temperature from water outlets) of the cold water pipework or water sampling will determine if this is an issue. If Legionella colonisation of the cold water is suspected or confirmed, the water should not be used for drinking by people at risk of aspiration pneumonia. If cold water pipework is found to be contaminated, cold and warm pipework and all outlets will require decontamination. This process should be performed by a person or company with specialised knowledge, experience and equipment to ensure the process is successful. SA Health Public Health Fact Sheet #3-13

Thermal Resistance

Unfortunately both Legionella and Mycobacteria are temperature tolerant. Mostly they prefer the 25 to 35°C range – like many of us. However, for short periods they can withstand much higher temperatures. Some reports demonstrate strains capable of survival well above 50°C. Even more disturbing are some recent reports on the failure of pasteurisation. A recent report shows that repeated pasteurisations of a hospital system became less and less effective each time. In this report the authors show shifts in the Legionella populations with each pasteurisation event. They also report a reduction in the kill rate with each event. In low nutrient environments like (hopefully) potable water systems this effect can magnify by cells producing ‘heat shock proteins’ that assist they’re survival.


Pasteurisation is an effective temporary means of making a contaminated warm or hot water system safe. On their web site Queensland Health Department describes it as a short term only disinfection method. Importantly, it relies entirely on the capacity of the heating system. So it is probably not a good option for routine disinfection as it may become less and less effective over time. When preparing your water safety and risk management plan it is wise to consider all the options.

Additionally it is also wise to decide upon a carefully chosen routine disinfection program that will fit your system capacity. This method may be entirely different to your emergency disinfection plan. If your cold water supply exceeds 20°C for long periods (more than a month) then pasteurisation is unlikely to give you the control you need. This is because the (not so) cold water is contributing to your problem and this method of disinfection affect it.

Further Reading

Allegra et al (2011) Longitudinal Evaluation of the Efficacy of Heat Treatment Procedures against Legionella spp. in Hospital Water Systems by Using a Flow Cytometric Assay. Appl. Environ. Microbiol. 77(4):1268–1275

Chang, C.W. et al (2007) Effects of chlorination and heat disinfection on long-term starved Legionella pneumophila in warm water. J Appl Microbiol. 2007 Jun;102(6):1636-44.

SA Health Public Health Fact Sheet #3-13

SA Government (2013) Guidelines for Legionella Control

UK HSE (2014) Legionella Guidance 274 part 2