Legionella and Water Temperature : A Comprehensive Overview

How does temperature affect legionella bacteria?

Temperature is one of the key factors in the increase or reduction of the presence of legionella bacteria in a water system.

The correct water temperature can provide a hostile or dormancy inducing environment to negate the threat of legionella bacteria, however incorrect water temperatures can be in a range that allows legionella bacteria to proliferate and thrive, particularly if biofilms are also present providing shelter and nutrients.

What Temperature Range Promotes Legionella Growth?

A water temperature between 20 and 45°C provides the ideal conditions for rapid growth of legionella bacteria.

Of course, other factors like the presence of a nutrient food source also have a part to play but the following legionella temperature chart displays a range of temperatures and the effect they have:

Legionella Temperature Chart

Temp °C	Temp °F	Bacterial Activity
Above 70°C	Above 158°F	A completely hostile temperature in which legionella bacteria will die instantly.
Above 60°C	Above 140°F	Most legionella bacteria will die quickly, within a matter of minutes.
50 to 60°C	122 to 140°F	Legionella bacteria can survive but do not multiply or grow.
42 to 50°C	109 to 122°F	Higher end of optimal growth range. Legionella bacteria will grow and multiply.
25 to 42°C	77 to 108°F	Peak growth range. Legionella bacteria multiplies and grows most rapidly.
20 to 25°C	68 to 77°F	Start of the optimal growth range. Legionella bacteria will grow and multiply.
Below 20°C	Below 68°F	Legionella bacteria is not killed but growth slows and enters a dormant state.

What Temperature Kills Legionella Bacteria?

As the legionella temperature chart above shows, legionella bacteria stops multiplying above 50°C (122°F) and the bacterium is killed at 60°C (140°F) and above.

This means that the water in a water system needs to be kept consistently above 50°C to prevent legionella bacteria activity and above 60°C to ensure legionella bacteria is killed.

This is an effective method of controlling legionella bacteria and the HSE specify that water services should be operated at temperatures that prevent Legionella growth, specifically:

• Hot water storage cylinders (calorifiers) should store water at 60°C or higher.
• Hot water should be distributed at 50°C or higher (thermostatic mixer valves need to be fitted as close as possible to outlets, where a scald risk is identified).
• Cold water should be stored and distributed below 20°C.

A competent person must routinely check, inspect and clean the system, in accordance with the risk assessment.

How Do You Test Water System Temperature?

Hot and cold-water temperatures should be monitored on a monthly basis. Regular checks at various points throughout the system should be measured and recorded to assist with the management of legionella risks.

This process should be carried out by a responsible person who is trained and competent to carry out the tasks and must understand the importance and significance of the findings in accordance with Legionnaires’ disease. The control of legionella bacteria in water systems – Approved Code of Practice and guidance

Cold Water System	Hot Water System	Calorifiers / Water Heaters
Use an accurate and correctly calibrated thermometer. Hold the thermometer in the cold water flow for two minutes. Record the temperature after two minutes. The temperature needs to be a maximum of 20 degrees Celsius or below. Also make note of which taps/tanks where checked, when, by who and if there were any issues.	Use an accurate and correctly calibrated thermometer. Test the tap furthest away from the calorifier/water heater (sentinel tap), then further selected outlets. Hold the thermometer in the hot water flow for one minutes. Record the temperature after one minutes. The temperature needs to be a minimum of 50 degrees Celsius or below. If thermostatic mixing valves (TMVs) are fitted the hot water inlet to the TMV should be checked instead of the hot taps. Also make note of which taps/tanks where checked, when, by who and if there were any issues.	Use an accurate and correctly calibrated surface temperature probe. Check the water flow and return temperatures at water heaters (calorifiers) on a monthly basis. The outgoing water temperature should be at least 60 °C in order to kill Legionella bacteria thermally.

Are Water Temperatures The Only Way To Stop Legionella?

The approach of thermal disinfection of water systems does face a number of challenges despite it being the most popular method of legionella control. Put simply these are:

• Rising energy costs meaning the cost of controlling legionella by temperature is constantly rising.
• Ensuring that water temperature is raised to required temperature throughout the whole water system, for example making sure water is stored at 50+°C in large storage tanks.
• Any areas of the water system where incoming cold water meets stored hot water and mixes, particularly if this happens at the bottom of vessels or storage tanks where a sediment or debris could be present.
• Plumbing alterations that alter the system from its original design can introduce dead legs or low flow areas. This reduces the chances for correct temperature water for disinfection being distributed throughout the system.
• Scald risks. Water needs to be stored at the disinfection temperatures but delivered at point of use at a temperature that doesn’t pose a hazard.
• If there is any slip in system performance or maintenance breakdowns of component parts can not function correctly to ensure correct temperatures are reached.

These challenges present scenarios where temperature control is no longer suitable as a sole means of legionella control. In these cases, the use of biocides should be assessed as an effective preventative measure in the system water treatment regime.

Water Treatment With a Biocide To Control Legionella Bacteria

A strong alternative or addition to temperature controlled disinfection of a water system are chemical disinfectants called biocides which can be introduced to a water system as a way of controlling bacterial presence.

In this situation it is important to select a biocide that effectively controls micro-organisms, is legal for sale in line with BPR, and is compliant with water regulations.

In the UK the DWI also requires private water supplies where disinfection forms part of the preparation and distribution of water to design, operate and maintain the disinfection process so as to keep disinfection by-products as low as possible without compromising the effectiveness of the disinfection.

As previously covered in our comparison of legionella control options both Chlorine based disinfectants and Copper & Silver Ionisation treatments effectiveness can be reduced due to water temperature.

Chlorine for example degrades very quickly and becomes ineffective at temperatures above 45°C. In recent testing SureClear Stabilised Hydrogen Peroxide has been shown to remain active, providing residual disinfection protection for much longer than chlorine based products at a range of temperatures:

The results showed SureClear stabilised hydrogen peroxide has a very slow degradation rate, with a half-life of eight weeks across the temperature range.

In comparison the results show that free chlorine degrades consistently at ambient temperatures, with a half-life of days not weeks, and even faster at 50 and 70°C.

It can be concluded that in cold water circuits, although free chlorine is acceptable for use in most scenarios, the extended stability of SureClear stabilised hydrogen peroxide gives many advantages for systems with periods of dormancy or reduced use.

In hot water circuits free chlorine is not suitable for any period of residual protection against micro-organisms, whereas SureClear is proven to be stable and effective at high temperatures.

SureClear is an NSF Certified Water Disinfectant that is legal for sale in the UK & EU markets as a multi-use biocide including water treatment. For more information about SureClear and water treatment, please call us on 1300 4 787 325 327, email [email protected] or fill in our contact form and a member of the team will be in touch.