Legionella – evaluating the critical risks in cooling towers: Section 7

Section 6 identified and analysed the critical risks. In this section, on this page we evaluate these risks.

Risk classification criteria

The critical risks described in Section 6, if worked through carefully, allow an accurate judgement to be made about the quality of a cooling tower system. A further process is needed to turn that judgement into an estimate of the overall risk.

To simplify an otherwise complex task requiring significant knowledge of risk management, some critical questions are suggested that relate directly to the critical risks. These questions should be answered for every cooling tower system, to help evaluate the overall risks. This approach is particularly suitable for small installations, where access to risk management specialists is not readily available.

The result of this risk evaluation is a recommendation on how to classify the cooling tower system. In Section 6, this recommendation is used to help owners and managers treat these risks and develop an operational program.

Cooling tower with basin exposed to sunlight: This cooling tower does not have sunlight protection to the side and basin of the tower

Stagnant water

Is the cooling tower system (or part of the system) idle for more than a month?

The way that the tower system is used is important. Lack of water circulation is likely to result in solids in the system settling out as sludge. This may encourage the formation of biofilm. Lack of circulation will also almost certainly mean that any biocides or other chemicals being added will not reach all parts of the system. Well-maintained systems that are in use for most of the year are generally of lower risk than systems that are intermittently used. Cooling tower systems used in conjunction with air-conditioning systems are commonly shut down over winter, creating potential dead legs.

Where the system (or part of the system) is idle for more than a month, is a recirculating pump with a timer fitted to automatically circulate the water at regular intervals, to prevent it becoming stagnant?

Fitting a recirculation pump to move the water around all parts of the system is an effective risk reduction strategy in these situations.

Are dead legs present?

Dead legs in a cooling tower system are pipes that are full of water but have little or no flow through them. A potential dead leg is any pipe that branches off from another main pipe and has a length longer than the internal diameter of the main pipe. Other components of a cooling tower system, such as off-line chillers or stand-by pumps, may also potentially become dead legs.

Dead legs have been linked to consistent problems with maintaining water quality and with the presence of Legionella, because of the difficulty in killing Legionella in such areas. Biocide added to one part of the system is unlikely to reach all parts of the system to control bacterial growth. Also, a lack of flow through the system will allow solids in the water to settle out in the pipe as sludge.

Nutrient growth

Are there factors in and around the site that may lead to environmental contamination and an increase in the level of nutrients in the cooling tower system?

Environmental contamination provides nutrients that can encourage more rapid bacterial growth. The introduction of high levels of solids will also reduce the effect of biocides. The site should be inspected to identify potential nutrient sources.

Nutrients may be introduced through dust from building demolition or construction sites, heavy traffic, unsealed roads or carparks, trees or other vegetation, and birds or other animals. Once identified, this can be taken into consideration in developing the RMP.

Is there a corrosion control program?

Without adequate corrosion control, iron may be released as a product of corrosion, encouraging Legionella to grow.

A good corrosion control program will include both the continuous addition of anticorrosion chemicals and close monitoring of the impact of the recirculating water on the metal surfaces of the tower system. This is generally done by regular inspection (at least quarterly) of test plates, called corrosion coupons, which are made of identical metals to those used in the system. Under some circumstances, chemical testing to measure the concentration of copper and iron in solution is used as a supplement to the use of corrosion coupons. It is also important to regularly (annually) inspect components such as condensers for corrosion.

Are any of the wetted surfaces exposed to sunlight?

A physical check of the cooling tower should confirm whether any of the wetted surfaces, including the water in the basin, wet deck (if present) and fill, are exposed to sunlight.

Is a biodispersant used?

Biodspersants should be applied to continually break down biofilm as it forms. Biodispersants need to be compatible with the other chemicals that are used.

Poor water quality

Has an automated biocide dosing device been fitted?

Siphon devices intended to deliver biocide into the cooling tower water are not recommended because they regularly block up – as a result, biocide may not be delivered to the cooling tower water. Manual dosing is also not recommended because it is totally reliant on operator reliability and quality. The department recommends use of a modern automated biocide dosing device to deliver a pre-set amount of biocide (and other chemicals) into the recirculating water at regular intervals, ensuring that the water is continuously treated. Various types of automated devices are available; the department recommends a type that monitors chemical parameters and adds varying amounts of biocide, depending on the water quality. Modern dosing systems can also detect some problems with the system and alert system managers through audible or visual alarms, which can be incorporated into building management systems and personal electronic devices.

Automated biocide dosing: This device has a timer which controls a pump to inject a pre-set volume of biocide into the water

As with any method of biocide dosing, the total water volume of the system must be calculated to ensure that the correct amount of biocide is used to obtain the concentration required to kill Legionella (as recommended by the manufacturer). This concentration will vary, depending on the biocide. Automated biocide dosing devices with poorly calculated dosing will not be effective.

Is a comprehensive water treatment program in place?

A comprehensive water treatment program usually includes the use of:

• two or more biocides in combination, to reduce the likelihood of Legionella becoming resistant to a particular biocide – they must be used in the appropriate concentrations, and at least one must be proven to be effective in controlling Legionella
• a biodispersant compatible with the chemicals in use (including chlorine)
• chemicals or other agents to effectively minimise scale formation, corrosion and fouling
• performance indicators relevant to the water treatment process involved, monitored very frequently, which collectively inspire confidence that the chemistry of the cooling tower system water is under effective control – the measures may address parameters such as the concentration of biocides, levels of solids, conductivity, pH and water clarity
• effective biocide dosing to maximise the impact of the biocides.

Deficiencies in the cooling tower system

Is a modern, high-efficiency drift eliminator fitted to all cooling towers in the system?

Cooling towers that are not fitted with an effective drift eliminator present a higher risk of an outbreak of Legionnaires’ disease if the water treatment regime fails. If the water treatment fails or is ineffective, aerosols leaving the cooling tower can contain Legionella. A drift eliminator fitted and installed to comply with Australian Standard AS/NZS 3666.1 (Air-handling and water systems of buildings – Microbial control – Design, installation and commissioning) can significantly reduce the number of aerosols leaving the tower. For these reasons, it is strongly recommended that any cooling tower system that is not fitted with a drift eliminator is replaced or upgraded. AS/NZS 3666.1 establishes a performance standard for drift eliminators. The performance of a drift eliminator is very difficult to verify in practice, so the manufacturer should be consulted to ensure that the drift eliminator is of modern, high-efficiency design. If the drift eliminator does meet the requirements of AS/NZS 3666.1, its condition and position should be checked to ensure that it has not been bypassed.

Drift eliminator: This shows a typical modern drift eliminator

Has the system design been reviewed?

A review of the system design may highlight issues that affect the overall risk associated with the system. For example, automated valves that shut off part of the system for lengthy periods may create stagnant water.

Detailed operational manuals will greatly assist this process. If these are not available, the review should ensure that there is a detailed understanding of how the system works and of water flows. Mechanical services contractors may be required to assist with a review of more complex systems. Where a detailed understanding of the system design already exists, additional work may not be required.

The review should also establish whether the system complies with AS/NZS 3666.1. It is likely that only relatively new towers will comply in all respects. The key features of AS/NZS 3666.1 include:

• easy and safe access for maintenance
• automatically controlled water treatment systems
• materials used in construction
• tower fill
• ease of cleaning, including drainage of basins
• drift eliminators
• splash prevention
• location
• bleed-off
• sunlight protection.

‘As constructed’ plans may assist with this review.

As a minimum, an assessment should be made to check that:

• there is easy and safe access to the towers to allow cleaning and maintenance; without such access, it may not be possible to adequately clean or maintain the system
• the tower fill and drift eliminator are installed correctly and are in good condition
• wetted surfaces are protected from sunlight
• the towers discharge exhaust away from occupied areas, pedestrian thoroughfares, air intakes, building openings and trafficable areas
• the towers avoid contamination by the exhaust discharges of air-handling systems such as kitchen exhausts or other cooling tower systems.

Modern fill: Fill made of materials such as polypropylene is now available for retrofitting to most types of cooling towers.

Has the operation and performance of the system been reviewed?

A review of the operation of the system can detect practices or procedures that increase the risk of Legionella growing in the system. Such as review should confirm how the system is used, including any manual or automated operation controlling water flow or water temperature.

Location and access

Is the tower system located in, or near, an acute health or aged residential care facility?

In acute health or aged residential care facilities, highly susceptible individuals could potentially be exposed to the tower aerosols. Typically, occupants of such facilities are at greater risk of infection than other members of the community. Cooling tower systems located in these facilities are always classified as the highest risk, regardless of the condition of the tower or operational program. A cooling tower system located near such a facility is also regarded as high risk. Where an RMP is being developed for a cooling tower system located near an acute health or aged residential care facility, it is good practice to discuss the development of the plan with the facility’s management.

How many people are near the tower during a day?

People who are in close proximity to the tower may be exposed in the event that the system becomes contaminated and allows Legionella to escape in aerosols. However, there is no exact or defined distance beyond which a tower is regarded as safe. Clearly, anyone working, visiting or living on or near the site of the tower is at a higher risk than someone who does not pass anywhere near the tower. The department recommends that people within a radius of 500 m should be considered as being potentially exposed to droplets from a cooling tower. We use the terms ‘very high’, ‘high’, ‘moderate’ and ‘low’ to describe the numbers of people that could potentially be exposed to a tower system. Table 1 gives examples of sites that fit these descriptions. Table 1: Sites associated with exposure of different numbers of people to a cooling tower system

Of those people exposed to the aerosols from a tower, not all will be susceptible to Legionnaires’ disease, but it is difficult to estimate the proportion who are at risk. For this reason, unless special circumstances mean that significant numbers of people at risk come into close proximity with the tower, the overall number of people can be used as a guide to the level of risk.

Special local circumstances may need to be taken into consideration in the risk assessment. For example, if the cooling tower is located next to a senior citizens club, a higher risk classification should be used. Where the number of potentially exposed people fluctuates – for example, with higher numbers once or twice a year at a special event – the highest number should be used to categorise the risk of the system.

Evaluating the risk associated with a cooling tower system

The first step in evaluating the risk associated with a particular cooling tower system is to understand and de-scribe the existing situation. Table 2 lists the questions that should be considered for each critical risk.

Table 2: Questions to be considered for each critical risk

Risk classification table

Responses to these questions will enable the overall risk associated with a cooling tower system to be established using the cooling tower risk classification table (Table 3).

We have evaluated possible responses to the questions. For the various combinations of responses, we have evaluated the combined risk and developed a logical grouping of cooling tower systems with similar overall risks. There are four risk categories: A, B, C and D. A is the highest risk, and D is the lowest risk.

These risk categories are used in ‘Legionella – critical risk treatment’ to help owners and managers select an appropriate maintenance or operational program.

Using the risk classification table

Table 3 lists each of the critical risks (in the left-hand column) and, for each risk, the possible combinations of responses to the questions in Table 2.

If a cooling tower system matches any of the combinations of responses in a particular row (for example, the row associated with the stagnant water critical risk), the risk classification (A, B, C or D) is at the base of the column in which the combined response is located.

The overall risk associated with a particular system is the highest classification obtained for any of the critical risks.

For example:

• If a system does match a response to any critical risk in column A, the overall risk classification is category A.
• If a system does not match a response to any critical risk in column A but does match a scenario in column B, the overall risk classification is category B.
• If a system does not match a response to any critical risk in column A or column B but does match a scenario in column C, the overall risk classification is category C.
• If a system does not match a response to any critical risk in column A, column B or column C but does match a response to any critical risk in column D, the overall risk classification is category D.

This process of categorising the cooling tower system should be:

• completed before developing a maintenance or operational plan
• repeated for every cooling tower system on the site
• repeated whenever the cooling tower system or environmental conditions are changed (for example, by completion of a works program).

Chapter 8 discusses how to treat each of the critical risks, and strategies for reducing the overall risk classification.

Table 3: Cooling tower system risk classification