What is Thermal Disinfection?

In this article the legionella and water safety specialists at Water Treatment Services take a look at the process of thermal disinfection, highlighting how it can be used to control dangerous bacteria and viruses that can sometimes contaminate building water systems.

The article explains what thermal disinfection is, and how it can be used to kill Legionella bacteria and other waterborne pathogens that can cause serious ill health.

Using high temperatures to kill bacteria

If the phrase “thermal disinfection” sounds to you like something from the latest Hollywood sci-fi blockbuster, then you’re not alone. It is one of those phrases which really does lead us to picture an invading alien fleet trying to disinfect Earth with their thermal weapons. Although that’s not really what we mean by thermal disinfection in this case, it is certainly true that it’s a powerful technique which can be used to deal with any number of threats which we may be facing.

From a water safety perspective, thermal disinfection involves using high water temperatures to kill bacteria and viruses, especially Legionella bacteria and other waterborne pathogens that can be found in engineered water systems. If left unchecked, pathogens such as legionella bacteria could cause an outbreak of the potentially fatal Legionnaires’ disease.

Thermal disinfection involves high heat, and the thorough cleaning of hot water vessels, calorifiers and distribution networks. It’s usually the final stage of any water system treatment, only used in situations where standard cleaning methods using chlorine cannot be used or they have shown to be ineffective.

Thermal disinfection relies on very hot water temperatures as a way of dealing with pathogens found within water. By exposing bacteria and viruses to water heated to a specific (high) temperature for a specified period of time, the bacteria can be deactivated and destroyed. The heat targets the proteins found within the bacteria and in most cases kills them entirely or deactivates them so they become harmless.

Killing legionella and other waterborne bacteria

Legionella isn’t the only bacteria which is found in water which can potentially be damaging to human health. Pathogens such as Pseudomonas bacteria, MRSA and Norovirus to name but a few affect thousands of people across the globe every year. Water which comes out of a range of sources such as drinking water, showers or other taps should always be risk assessed and measures put in place to deal with any potential threat to human health.

History of thermal disinfection

The idea of making things clean by heating them up to a very high temperature isn’t anything new. This idea has been applied for centuries. Even before people truly understood that there were microscopic organisms living in water which they couldn’t see, they knew that boiling water before drinking it meant that you were less likely to get sick after drinking it. The reason we’re still using it as a disinfection method today is simply because it’s highly effective.

How does thermal disinfection work to kill bacteria?

As far as legionella is concerned, the bacteria cannot survive for long periods in water temperatures above 60°C. If we can raise the temperature of the water flowing through a pipe or out of a shower head to above this temperature for 5 minutes, then we can be sure that any Legionella bacteria present will not have survived the process. The benefit of using heat in this way is that heated water can get to parts of the plumbing or heating system which chemical disinfectants can’t get to. Heat is also far more effective than many chemicals at penetrating into any protective layer of biofilm, and destroying the bacteria which make it up.

Thermal disinfection is a powerful tool, but there is always a downside to any sanitation method. The disadvantage to thermal disinfection is that the effects are typically short-lived. Just a few weeks after you have treated your water system using thermal disinfection, the bacteria might have started to recolonise the system again. The rate of recolonisation will depend on what other measures you have put in place to manage water quality. In many cases, thermal disinfection will have to be repeated every 2 to 4 weeks to maintain the benefits it brings.

Do I need a disinfection specialist?

It’s always better to get an experienced water treatment company or fully qualified plumber in to carry out any thermal treatment on your water system. This is because during this sort of treatment you will be using water at temperatures much higher than normal which could heighten the risks from scalding. You may also need to by-pass the thermostatic mixing valves, which is a job for someone who knows what they are doing.

What’s involved in the thermal disinfection process?

Thermal disinfection techniques are only ever suitable for use on warm or hot water systems, not on systems designed to supply cold water only.

Usually, the need for thermal disinfection is triggered when traces of Legionella are picked up somewhere in the water storage or distribution system. The most common procedure is for the thermal process to be carried out in close proximity to whichever part of the water system is thought to be responsible for the source of the bacteria, such as taps or shower heads. Depending on the individual circumstances, the entire system may need to be treated, or just a few of the water outlets which have been assessed as higher risk “flash points”. These decisions will be based on the legionella risk assessment, the design and layout of the water system, and should be made by an expert, someone who fully understands the risks involved and how to resolve them.

If you are dealing with a large facility with complex water systems such as a hospital, then it’s even more important to carry out a full risk assessment before starting work. It may be that the best course of action is to carry out a general disinfection process to a schedule. The downside to doing this is that occupants or residents of the building will experience disruption to their water supply on a regular basis while the disinfection is taking place.

What are the risks when using very hot water to kill legionella?

There’s nothing fundamentally risky about thermal disinfection, but there are a large number of issues which you should be aware of if you are thinking about carrying out thermal disinfection to deal with legionella or other water pathogens. These are:

Thermal disinfection can cause wear and tear, or corrosion, on water storage equipment over time. Usually though this is only a problem when the temperatures used are higher than the recommended maximums for the materials used to construct the equipment.
As the temperatures required for thermal disinfection are high, there is always the risks of scalding injury to people managing the process or using the system. Everyone should be made aware of the risks, and know which steps to take to minimise them.
You may have to move people around within a building, to give them access to other sources or warm/hot water while the disinfection process is carried out to minimise the risk of scalding, and also to avoid them being exposed to the Legionella bacteria as part of the process.
PPE – anyone working on thermal disinfection should be supplied with PPE suitable for the task, to minimise danger from both the bacteria and the hot water.
High water temperatures can also cause issues with plasticised or rubberised seals in the water system, which can in turn lead to issues with pumps and other components using such seals. Always use approved products which have been specially designed to resist higher temperatures.

Is Legionella bacteria becoming more temperature resistant?

Just as we have a growing problem with bacteria evolving to avoid antibiotic drugs, there is a school of thought that disinfecting at temperatures, above 60°C but below 70°C could be causing a similar issue with Legionella bacteria which are becoming more thermally resistant. It’s not something which is a huge cause for concern at present, but definitely something to bear in mind if you have undergone repeated disinfections and still have an issue with legionella contamination.

Legionella and water safety risk management solutions

Water Treatment Services offer a range of specialist legionella and water safety risk management solutions to support the business owners and those responsible for the safety of water systems.

With offices in London serving the South and South East England, Manchester (North West), Birmingham (Midlands), Bristol (South East England and Wales), Leeds (North and North East) and Glasgow (Scotland), supported by regional teams of specially trained technicians and engineers we can offer specialist legionella risk assessments, training, water testing and other risk management solutions throughout the UK and Internationally.

Contact us today to learn how our water safety experts can help you manage your water systems, maintain regulatory compliance and so keep people safe.