Water treatment options for Legionella control
Installation of water treatment into the water systems of hospitals and residential aged care can involve significant up front and ongoing costs for facility managers. Indeed, water treatment can also introduce new risks into the water system, if the choice of treatment is incorrect or if problems occur in the operation of the treatment process.
Therefore a health or aged care facility should only consider treatment of its water if no other controls prove capable of maintaining Legionella at acceptable levels. Non-treatment controls could include:
- maintenance of water temperature (keeping cold water cold – preferably below 20 degrees C and hot water hot – i.e. above 60 degrees C, up to the point where it enters a tempering device)
- keeping water flow throughout the facility sufficient to ensure chlorine residuals from the town water supply remain adequate through to the most distal points of the facility plumbing system
- identification and removal of dead legs and other high risk plumbing features
- communication with your drinking water service provider (usually local government) to ensure that they do everything they can to deliver drinking water with an effective disinfection residual
- management of patients or residents to minimise exposure of the most vulnerable individuals to aerosols that might contain Legionella.
If a healthcare facility has considered all of the above and implemented the most appropriate, cost effective measures but still has been unable to control Legionella growth within its plumbing system, then water treatment may be considered.
Despite the installation of many different water treatment systems for Legionella control in healthcare facilities worldwide over the past 30 years, there is no scientific consensus on which treatment system or process is best for all facilities. The discussion below is intended to assist facility managers to be aware of the pluses and minuses of the main treatment options so that when they engage with technology vendors or service providers they can have some assurance that the treatment system they choose will be appropriate and cost-effective. The following advice is based on the enHealth (2015) Guidelines for Legionella control in the operation and maintenance of water distribution systems in health and aged care facilities.
It should also be noted that some treatment options may be used together, especially when attempts to control Legionella have not proved successful. This approach is in keeping with the concept in the Australian Drinking Water guidelines of having “multiple barriers” to contamination, so that if one barrier were to fail, others can still provide some protection.
Heat disinfection or pasteurisation involves periodic heating of hot water to a temperature sufficient to achieve 70°Cat all outlets and then flushing heated water through all heated ring mains, heated water pipework and heated water outlets. This can be achieved centrally or using a mobile unit, only in specific locations.
- Does not require addition of chemicals
- Can be performed on entire plumbing system or isolated sections using just a hot water booster unit, as required
- Scalding hazards from the super-heated water must be managed
- Requires considerable hours of labour, usually out of hours
- When used systemically uses a large amount of water as well as energy to heat the water
- Many facilities do not have sufficient hot water capacity to implement this method systemically
- Does not achieve long-term control, and therefore has to be performed regularly to maintain control
- May unintentionally lead to significant heat transfer to cold water pipes
- Cannot be used to disinfect cold water pipework
Chlorination is where sodium hypochlorite or chlorine gas is added to the water. After dosing into water hypochlorous acid is formed which is a very effective disinfectant.
- Relatively easy to implement and monitor
- Relatively cost effective depending on dosing equipment required and volume of chlorine needed
- Easily installed in existing systems, without major modifications, using a licensed plumber
- Residual effect for downstream decontamination
- Potential corrosion of pipework and other plumbing infrastructure
- pH must be maintained at ≤7.6 in order to be effective
- Free residual chlorine and chloramines decay rapidly at hot water temperatures (≥60 degrees C)
- Different concentrations are required for residual disinfection versus superchlorination.
- Generation of undesirable disinfection byproducts such as trihalomethanes
- May be incompatible with reverse osmosis membranes used in dialysis units
- Removed by activated carbon filtration and UV light
- Undesirable interaction with chloramines if dosed into chloraminated water. This is a crucial disadvantage where drinking water supplied from the treatment plant is chloraminated.
Chloramines are chemical disinfectants that form when chlorine is dosed into water with ammonia. In Australia chloramination is mainly practiced by large, metropolitan drinking water service providers like Seqwater, Sydney Water and Melbourne Water. Some hospitals in Europe and the United States have found chloramines to offer more effective control of Legionella, due to its capacity to penetrate biofilms. However, advice from Queensland water treatment experts suggests that it would be substantially more difficult for healthcare facilities to implement chloramination compared with chlorination.
- Penetrate biofilm more effectively than chlorine
- Being less reactive, chloramines persist longer in water supporting better residual effect for downstream decontamination
- Chloramines are less powerful as disinfectants than chlorine
- May generate undesirable disinfection byproducts, such as nitrosamines
- Unsuitable for use in water with high oxidant demand (i.e. poorer quality water with high organic loads)
- Hazardous to dialysis patients, requiring activated carbon filtration to effectively remove
- Not generally practised in Australian healthcare water systems
- pH control is critical to ensure optimal formation of monochloroamine, to minimise the production of dichloramine and trichloramine (which produce unpleasant odours), and to minimise the amount of free ammonia in the water supplied throughout the facility
Chlorine dioxide can be dosed as a highly water soluble gas or as stabilised liquid.
- Less corrosive than chlorine
- A more effective disinfectant than chlorine but less effective than ozone
- Some residual effect for downstream decontamination
- Inactivation efficiency not pH dependent between pH 5-10
- Produces byproducts chlorite and chlorate which must be monitored (along with chlorine dioxide) and prevented from exceeding health guideline concentrations
- Unsuitable for use in water with high oxidant demand
- Removed by activated carbon filtration and UV light
Copper–silver ionisation occurs where copper and silver ions are released into the water.
- Does not corrode piping or plumbing fixtures
- Remains effective at all water temperatures
- Easily installed in existing systems without major modifications
- Residual effect for downstream decontamination
- Difficult to optimise the correct dosing for each system if the unit is not installed and operated appropriately
- Water hardness and pH can affect the efficiency of the system
- Monitoring levels of silver and copper in the system is difficult
- Not commonly used in Australia or New Zealand therefore minimal local experience is available
Ultraviolet light (UV)
Ultraviolet light disinfection of water usually uses UV light with a wavelength of 250-265 nano metres. UV disinfection is most effective when dosed after a filtration stage in order to remove particles that could shield pathogens from the light.
- Useful for small areas that may need additional or special attention, such as a high-risk unit
- Relatively easy to install
- No adverse effect on water or plumbing
- Leaves no taste or chemicals in the water
- Has no disinfection byproducts
- Limited application (point-of-use or supplementary disinfection tool only)
- Affected by turbidity and particulates, which ‘shield‘ bacteria from the UV, rendering it less effective
- No residual effect (i.e. has an effect at dosing point but not throughout the system)
- Performance depends on the system being designed to be suitable for the flows
- No effect on biofilm that has already formed in water pipes
Ozone gas is generated on site using an ozone generator, using just air or pure oxygen, and added immediately into the water.
- Effective over a wide pH and temperature range compared to chlorine
- Very strong oxidizer
- Effective at low concentration
- No residual effects
- High concentrations may damage piping, fittings and seals
- Minimal impact on Legionella embedded within pipe biofilm
- Unstable so must be produced on site and used immediately
- Can produce harmful by-products in drinking water, but less than chlorination
Point of entry filtration
Point of entry (POE) filtration refers to a water filtration unit located at or close to the point of entry of the town drinking water supply to the healthcare facility. There are various different types of filtration unit that could be used, depending on the quality of the water being supplied, and the desired outcome from the filtration process.
- Can ensure a more consistent quality of incoming water in situations where the quality of the town supply is known to fluctuate
- By reducing suspended particles in the incoming water POE filtration can make subsequent chemical or UV disinfection more effective
- Reduces overall microbial load coming into the facility from the town supply
- No residual effect, other than by reducing overall microbial load
- For maximum effect must include subsequent disinfection step
Point of use filtration - membranes
Point of use microfiltration involves installation of membrane filters, with pore size of ≤0.2µm, at or near individual outlets, which prevent most bacteria from passing through while allowing normal water flow.
- May assist where other methods are unable to control Legionella or in areas where disinfection residuals are to be avoided (eg dialysis)
- Can be a useful supplement to another form of disinfection for high-risk areas
- Can allow continued use of Legionella contaminated systems in high risk areas whilst investigations are undertaken and engineering solutions sought.
- No disinfection by products
- Must be periodically replaced or sterilised
- Relatively high maintenance burden
- Can be very expensive for large facilities
Point of use filtration – activated carbon
Point of use filtration using activated carbon filters is commonly practiced in commercial water filters where drinking water or ice is provided for non-clinical uses. In Queensland activated carbon filters should not be used for drinking water or ice that is served to severely immunocompromised patients where there is a risk of micro-aspiration.
- Commonly used and readily available
- Can improve taste and reduce “undesirable” chlorine odours
- Can remove some disinfection by-products where these have been assessed as presenting a risk to patients, residents or staff
- May have a role as pre-treatment for specific hospital water uses such as dialysis
- Can accumulate suspended particles and support biofilm thus becoming a source for Legionella and other bacteria
- Can remove residual levels of chlorine disinfectant, permitting biofilm and associated Legionella to proliferate in attached plumbing fittings or water chillers/ice machines
- Need to be changed regularly