Water temperatures above 60°C will kill Legionella (free-living) bacteria in less than 5 minutes. Thermal treatment (also known as pasteurisation) can be used to disinfect outlets via heat transfer to areas of fittings that may be less readily accessible to chemical disinfectants (as heat is better able to penetrate biofilms compared to chemicals).
Thermal treatment provides only a short-term sanitation measure as recolonization of the water system can occur within weeks to months of treatment, depending on the implementation of other controls.
Who undertakes a thermal treatment
A specialised, qualified and competent professional, such as a licensed plumber should undertake a thermal treatment. This is due to processes such as, but not limited to, the requirement to by-pass thermostatic mixing valves (TMVs) and tempering valves during a thermal treatment.
The water risk management team and a workplace health and safety officer should be consulted before carrying out a thermal treatment.
When to undertake a thermal treatment
It should be undertaken in accordance with your water risk management plan (WRMP).
It may be required:
- immediately following a Legionella detection (and/or case of legionellosis) from a hot or warm water outlet
- based on the complexity of the plumbing infrastructure of your facility, in some areas on a scheduled basis (e.g. every 3 months).
Where to undertake a thermal treatment
Thermal disinfection can only be used in hot or warm water systems. It cannot be performed for cold water systems.
Following a Legionella detection, (and/or a case of legionellosis), a thermal treatment could be undertaken in the location and source (tap/shower) of the detection. This may affect an entire facility or only a limited number of outlets in a localised area (e.g. single wing, ward or cluster of rooms). This is likely to depend on the configuration of the hot/warm water system of your facility.
Larger facilities can have very complex plumbing infrastructure and if the water risk management team considers an area (e.g. single wing, ward or cluster of rooms) a high risk for Legionella, a thermal treatment may be undertaken in that area on a scheduled basis.
Main steps for undertaking thermal treatment should meet the following criteria:
- Check the capacity of your hot water system to deliver sufficient hot water and if the thermostat allows, turn up to maximum temperature.
- Ensure the hot/warm water supply is not accessible by facility occupants (staff/patients/residents) during the thermal treatment process through the use of signage and appropriate communication. For maximum occupancy safety thermal treatments should be performed overnight or late at night.
- Disconnect or isolate any heat sensitive equipment which may be damaged by the thermal treatment, for example, chlorine dosing units.
- All TMVs, tempering devices and 3-way mixing valves must be by-passed (to ensure pasteurisation between the valves and outlets).
- The target temperature at distal outlets in the system being treated should be no less than 70⁰C.
- Clean and sanitise the TMV in a chlorine solution (1%) during the thermal treatment.
- In a large facility, in order to achieve a target temperature of 70⁰C at the most distal outlet in the system, it may be necessary to set the water temperature at the water heater as high as 80⁰C. Lower temperatures will mean flushing will take longer. If 70⁰C cannot be achieved at distal outlets, an alternative disinfection strategy should be considered.
- After the target temperature has been reached at a designated sample outlet, and is stable, each other outlet should be turned on sequentially.
- Run water at 70⁰C for 5-10 minutes, or if water temperature cannot reach 70⁰C a water heater booster may be required at the outlet.
- The water should remain at or above the target temperature throughout the flushing process.
- After the flush, turn off all taps, reinstate TMVs, tempering valves, 3-way mixing valves and cool to operating temperature.
- Hot water at outlets must be returned to not more than 50⁰C for tempered outlets and 45⁰C for outlets served by TMVs before patients/residents can be allowed to access the supply. After any thermal treatment TMV’s should be tested by a plumber and a certificate issued.
Effects of a thermal treatment on your water distribution system
- Duration of the effect of a thermal treatment varies and may last between 2 - 4 weeks.
- In large facilities, it may cause disruption to normal service for several days.
- Provided temperatures do not exceed the specifications of the construction materials, the corrosion/degradation effects from a thermal treatment should be minimal.
- The thermal treatment process increases water temperatures to above 60⁰C, and as a result:
- the risk of scalding to vulnerable patients/residents must be controlled and
- hot/warm water supply should not be accessible by facility occupants and vulnerable groups during the thermal treatment process
- The thermal treatment process may require the isolation of the affected room(s), relocation of patients/residents (if the risk of moving them is less than the risk of scalding/legionellosis) or isolation of the affected water supply.
- Personnel conducting a thermal treatment procedure should wear adequate personal protection (scald protection, microbial protection (if being undertaken after a Legionella detection)).
- High temperatures may cause damage to elastomeric seals causing pump failures unless a WaterMark approved product is used that will tolerate the increased temperatures. These products should be isolated before treatment.
- It should be noted that there is an emerging body of evidence that suggests repeated thermal treatments at less than 70⁰C may, over time, result in more thermally resistant Legionella species and so become progressively less effective.
- SAA/SNZ HB32:1995 Handbook – Control of microbial growth in air-handling and water systems in buildings.
- National enHealth guidelines for Legionella control (PDF, 577KB) – sections 4.1 (pages 34 to 35)
- Australian Drinking Water Quality Guidelines (PDF, 5MB)
- United States EPA Office of Water: Technologies for Legionella Control: Scientific Literature Review (PDF, 1MB)