During the normal operation of a cooling tower, aerosols are formed and then carried into the environment through the tower exhaust. If Legionella bacteria are present in the water of the cooling tower system, breathing these aerosols can result in infection.
Types of cooling towers
Cooling tower systems are normally associated with air-conditioning systems, refrigeration systems and industrial processes. The basic function of the system is to remove heat (see Figure 4). Cooling tower systems temporarily store water, which is usually recirculated, in a basin. The water is sprayed or dripped into a large chamber. Air is forced through this chamber by a thermostatically controlled fan.
Discharges from cooling towers are normally warm and humid; sometimes steam can be observed as condensation.
Figure 4: Mechanism of heat exchange in a cooling tower
The typical layout of air-conditioning systems that use cooling towers is shown in Figure 5. These cooling towers contain fill material inside the tower. Usually made of plastic, this material allows the falling water to spread over a greater area, which increases the surface area of the water to be cooled and allows more effective cooling.
Figure 5: Typical layout of an air-conditioning system
Industrial processes often have a device called an evaporative condenser to eject heat from the process. These units work in a similar manner to cooling towers. The cooled water is distributed over a series of pipes that contain circulating refrigerants or other fluids. Unlike cooling towers, evaporative condensers do not contain any fill material. These systems also present risks for Legionnaires’ disease and fall within the definition of ‘cooling tower’ used in the Public Health and Wellbeing Act 2008 and the Public Health and Wellbeing Regulations 2009.
The design of a typical evaporative condenser is shown in Figure 6.
Figure 6: Typical layout of an evaporative condenser
Cooling towers are often confused with evaporative coolers. An evaporative cooler uses the same general principle of recycling water. The main difference is that cooling towers use air to cool the water, whereas evaporative coolers use water to cool the air. The definition of a cooling tower in the Public Health and Wellbeing Regulations 2009 clearly states that evaporative air coolers and evaporative air-conditioners are not cooling towers.
Evaporative cooler: These units have not been linked to cases of Legionnaires’ disease
There has been no evidence linking evaporative coolers or evaporative air-conditioners to cases of Legionnaires’ disease.
Cooling towers may be found on rooftops; in plant rooms, basements and mezzanines; and at ground level. There are four types of cooling tower, which are described in the following sections.
Induced draught counter-flow
Induced draught counter-flow towers are very common. They can be identified by the fan at the top of the tower. The fan pulls air up through the tower in the opposite direction to which the water is falling. The air usually enters the tower through inlet louvres on the sides of the tower. Water is usually delivered by means of fixed or rotating spray arms. Drift eliminators are usually placed above the sprays to prevent loss of water through drift.
Figure 7 shows a schematic of these types of cooling towers.
Figure 7: Induced draught counter-flow cooling tower
Induced draught cross-flow
In an induced draught cross-flow cooling tower, the fan is also mounted on the top. However, the fan draws (induces) the air across the water falling from the top of the tower to the basin.
Figure 8 shows a schematic of these types of cooling towers.
Figure 8: Induced draught cross-flow cooling tower
Forced draught counter-flow
In a forced draught counter-flow cooling tower, the fan is located at the air inlet just above the basin. Air is forced vertically through the tower fill in the opposite direction to the water flow. The air is forced out through the top of the tower.
Figure 9 shows a schematic of these types of cooling towers.
Figure 9: Forced draught counter-flow cooling tower
Forced draught cross-flow
In a forced draught cross-flow cooling tower, the fan is mounted on one side and pushes the air in a cross-flow manner past the falling water.
Figure 10 shows a schematic of these types of cooling towers.
Figure 10: Forced draught cross-flow cooling tower
Legionella outbreaks – why they happen
Cases of Legionnaires’ disease associated with a cooling tower system usually occur when a number of conditions are met. First, Legionella enters the cooling tower system, presumably from the water supply. The bacteria then multiply as a result of one or more of the following scenarios:
- failure to treat the water to an adequate standard, which can be due to
- a lack or breakdown of a regular treatment schedule or system equipment
- human error
- environmental contamination of the cooling tower water – for example, by airborne dust from nearby construction works
- poor design or location of the cooling tower system
- inadequate or non-existent maintenance (including plans for replacement of ageing cooling tower systems).
The final step in the outbreak pathway is exposure of susceptible people to the Legionella-contaminated droplets generated by the cooling tower system. This is often associated with favourable weather conditions, such as warm and windless days that typically occur in autumn in Victoria.
Critical risks for cooling towers
The development of an RMP that considers all these factors can be very complex, so we have identified the following five most critical risks associated with outbreaks of Legionnaires’ disease from cooling tower systems:
- stagnant water
- nutrient growth
- poor water quality
- deficiencies in the cooling tower system
- location of, and public access to, cooling tower systems.
The Public Health and Wellbeing Act 2008 and the Public Health and Wellbeing Regulations 2009 require each of these critical risks to be addressed in the RMP. Failure to do so will result in the independent accredited auditor being forced to fail the RMP and advise the department of the issue. Similarly, if the RMP does address the critical risks but is not implemented, or the RMP has not been reviewed in the 12 months before the audit, the auditor will also have no choice but to fail the RMP and advise the department. Addressing these risks will significantly reduce the likelihood of the cooling tower system contributing to an outbreak of Legionnaires’ disease.
The Regulations describe the risks associated with stagnant water as the lack of water recirculation in the system, and the presence of dead-end pipework and other fittings in a cooling tower system. Dead-end pipework is sometimes known as ‘dead legs’.
Stagnant water is a risk because:
- a lack of circulation will allow solids in the water system to settle out as sludge –this sludge is implicated in the growth of Legionella (as discussed in ‘Nutrient growth’) and also causes corrosion
- any biocide added to the system will not reach all parts of the system in sufficient concentration to kill the bacteria. A reservoir of Legionella can develop in the biofilm (which is a combination of bacteria, algae, protozoa – including amoebae – and other microorganisms). This Legionella can then reinfect the entire system when the biocide levels drop.
Stagnant water often occurs:
- if a cooling tower system is not used for periods of more than a month
- where the system has disused or superfluous pipes (dead legs) full of water
- where the system has pipes full of water with little or no flow or turbulence.
The way that a cooling tower system is used is significant. The start-up time for a cooling tower is a critical period and must be handled well to prevent problems occurring. Well-maintained cooling tower systems that are in use for most of the year are generally of lower risk than those that remain idle for more than a month. This is because the biofilm is readily disturbed when operations stop and start.
If the system’s circulation is shut down for a month or more, the water may become stagnant. The risk of problems when the system is next turned on increases significantly because Legionella may have grown in the stagnant conditions, if the biocide has not reached all parts of the system.
The lack of a recirculating pump controlled by a timer to circulate water through the system at times when the tower is not in use can be a key contributor to stagnant water.
Similarly, if a tower system has dead legs, even with a high-quality maintenance program it may not be possible to consistently meet the desired standards. This is often because a biocide may not reach all extremities of the system, allowing Legionella to grow and potentially regularly reinfect the system.
According to the Regulations, nutrient growth risks include:
- the presence of algae, biofilm and protozoa
- water temperature within a range that will support rapid growth of microorganisms
- the exposure of the water to direct sunlight.
The amount of nutrients in the water needs to be controlled because it has a significant effect on the ability of bacteria to grow rapidly. The more nutrients are in the water, the more ‘food’ there is for bacteria.
Hiding out: Legionella bacterium being engulfed by an amoeba
Environmental contamination can cause nutrients to enter a cooling tower system. Dust generated on-site or off-site may enter the cooling tower system and provide a steady source of nutrients for bacteria and other organisms. Building demolition or construction, major roads, dirt roads and car parks may all generate dust. Other sources of nutrients include leaf litter from overhanging trees, bird droppings falling into the cooling tower and kitchen exhausts.
Algae, biofilm, protozoa and corrosion all have the ability to conceal and protect Legionella from biocides in the water, increasing the risk posed by the cooling tower system.
Algae can grow rapidly if the cooling tower water is exposed to sunlight. This most commonly happens when the tower basin or other wetted areas, such as the top wet deck of some types of cooling towers, are exposed to sunlight. Other types of cooling towers often have no sunlight protection for the tower basin. Inspection openings may be missing and therefore expose the fill to sunlight. Any algal growth will provide a food source for bacteria, including Legionella.
The control of biofilm is fundamental to minimising risks from Legionella in a cooling tower system. Biofilm can form on any of the wetted surfaces of the cooling tower system. Legionella bacteria are relatively easily killed by moderate concentrations of many biocides, provided that the bacteria are free-floating in the water and exposed to the biocide. However, Legionella has adapted to survive under adverse conditions, and has the ability to live and multiply within protozoans. These engulf the Legionella bacteria, which continue to grow and multiply inside the larger organism. Protozoa can resist much higher concentrations of biocides than Legionella, and the Legionella can survive inside the protozoa, particularly when the larger organism has become part of the biofilm typically found on the inside of pipes and other wetted surfaces. The biofilm may peel away from the pipe surface for a range of reasons, including physical disturbance. This can result in release of Legionella bacteria into the recirculating water and their discharge out of the tower within water aerosols before they can be killed by biocide.
Biodispersants, which are low-foaming detergents, are used to break down biofilm. Systems in which biodispersants are not present are at significantly higher risk of nutrient growth and biofilm formation.
Corrosion is also a risk factor, because any corrosion in the system may release iron, which is a growth factor for Legionella. Internal surfaces of a cooling tower system may become heavily corroded unless anticorrosion chemicals are used and corrosion levels are monitored carefully.
The temperature of the recirculating water can effect nutrient growth. It is impossible to eliminate bacteria from a cooling tower system, and water temperature will be a factor in bacterial growth rates.
Poor water quality
Poor water quality covers seven risk factors (5):
- presence of Legionella
- presence of other heterotrophic bacteria
- water quality and properties
- presence of corrosion products
- presence of scale and fouling
- conductivity/total dissolved salts
- control limits out of range
- suspended solids (e.g. from nearby construction work)
- control of water treatment chemicals
- control of bleed
- presence of protozoa and algae
- characteristics of make-up water
- microbial control program.
- external contamination of the water with dust or soil
- accumulation of solids in the system
- the choice and levels of biocides and anticorrosives
- the presence of high levels of bacteria, including Legionella
- the presence of nutrients supporting microbiological growth.
Poor water quality is a risk because it has a direct effect on the likelihood of Legionella multiplying in a cooling tower system. Among other things, water quality is affected by:
Systems that do not have a comprehensive water treatment program or are not monitored for bacterial levels are significantly more likely to have poor water quality.
Deficiencies in the cooling tower system
Deficiencies in a cooling tower system cover five risk factors (6):
- system size
- system design (surface area available for biofilm development compared with water volume)
- physical condition of the system
- open systems
- aerosol generation
- drift elimination.
A cooling tower system that is poorly designed or maintained is a risk because:
- high water temperature allows rapid bacterial growth
- aerosols that may be contaminated with Legionella can more easily leave the tower
- unsafe conditions – such as non-existent, unstable or rusted climbing ladders – pose a risk to people who need to access the tower to clean and maintain it; a safe working environment will promote better cleaning and reduce the risk of Legionella growth.
The physical design, maintenance and operating performance of the tower and related system can have a significant impact on the potential risk of Legionella transmission. If the system is undersized and water temperature is too high, the potential for rapid bacterial growth is greater. System size is important because towers with low water volume will have a high water turnover, and the biocide is less likely to be effective. The choice and concentration of biocide need to be matched to the water volume.
The risk of aerosol distribution is much greater without design modifications such as fitting of an effective drift eliminator.
Cooling towers – location and public access
The location of, and public access to, cooling towers cover two risk factors:
- system location
- aerosol dispersion.
A poorly located tower can be subject to environmental contamination – for example, from building sites; this can increase the level of nutrients and thus the number of bacteria, including Legionella. In addition, if a cooling tower system is located in an area where large numbers of people have access, this can be a problem if the system becomes contaminated with Legionella because all of these people will be potentially exposed; if the people exposed to the tower are from a susceptible group, the risk will be higher.
The extent to which people are exposed to aerosols is an important factor when assessing the risks associated with a cooling tower system. Steps involved in this assessment are as follows:
- Consider whether the tower is located in or near an acute health or aged residential care facility. This is important because the residents of these types of facilities are highly susceptible and most at risk of serious health consequences from an outbreak of Legionnaires’ disease.
- Estimate the number of people who are in close proximity to the tower during a day. The number of people who may be exposed to the tower aerosols will affect the size of an outbreak and is therefore a significant consideration in a risk assessment. Look closely around the immediate area of the cooling towers; they are sometimes located close to heavily trafficked areas, such as footpaths or roads. Some workplaces allow smokers to leave the building to smoke. Monitor the area around each cooling tower to ensure that it is not an area where smokers congregate. This is a high-risk situation because of the evidence that smoking is a risk factor for Legionnaires’ disease.
5. AS/NZS 3666.3:2011 (Air-handling and water systems of buildings – Microbial control – Performance-based maintenance of cooling water systems)
6. AS/NZS 3666.3:2011 (Air-handling and water systems of buildings – Microbial control – Performance-based maintenance of cooling water systems)
7. AS/NZS 3666.3:2011 (Air-handling and water systems of buildings – Microbial control – Performance-based maintenance of cooling water systems)