Key messages

  • All cooling tower system owners should aim to lower the overall risk associated with their system, if possible. For example, so that the overall risk classification is reduced from A to B.
  • A basic principle of risk management is to first see if it is possible to eliminate the risk altogether – that is, can the cooling tower be removed altogether.
  • This webpage explains how to use the risk classification process from Section 7 to identify an operational program for the cooling tower system, to reduce the risk.

All cooling tower system owners should aim to lower the overall risk associated with their system, if possible – for example, so that the overall risk classification is reduced from A to B. In most cases, the only way this can be done is via capital investment, such as fitting drift eliminators, automated dosing devices and recirculating pump timers. Without capital investment, the maintenance or operational program for the system will need to increase considerably.

As discussed in Section 7, a number of critical questions need to be considered in relation to the existing condition of the cooling tower system.

This webpage explains how to use the risk classification process from Section 7 to identify an operational program for the cooling tower system.

It highlights the importance of ensuring that the operational program is consistently implemented.

Is a cooling tower system really needed?

A basic principle of risk management is to first see if it is possible to eliminate the risk altogether. Whenever a cooling tower system exists on a site, it is possible to reduce and manage the risks, but not eliminate them.

At an early stage of a review of the risks associated with a cooling tower system, it should be established whether the original purpose for the cooling tower system still exists. For example, for industrial processes, is the cooling tower system still crucial to the process or has it become redundant?

Viable alternatives to the cooling tower system should also be considered. Owners of land and businesses with smaller cooling tower systems should consider a move to air-cooled systems, which are not associated with Legionnaires’ disease because there is no reservoir of recirculating water. Air-cooled systems also eliminate the ongoing costs of water treatment and testing.

If no viable alternative currently exists to the cooling tower system, it is time to begin the risk management process.

Strategies to address the critical risks

Possible responses to the critical risks are described below. Some relate to improvements to the cooling tower system itself, whereas others concern maintenance or operational aspects of the system.

Risk control strategies for stagnant water

Cooling tower system improvements

Improvements to the cooling tower system to minimise the risks associated with stagnant water include the following.

Installing a timer connected to a recirculating pump, set to operate to circulate biocide and other chemicals when the system is idle

If the tower system, or part of the system, is idle for more than a month, a simple strategy to minimise the risk of stagnant water is to install a timer to the recirculating pump. This ensures that water circulates through the system. It will also allow the biocide to treat the water and reduce the likelihood of bacterial growth. This is relatively easy to achieve and is suited to tower systems that are not used for long periods.

Checking whether there are dead legs, and removing or activating them

The first step is to locate any potential dead legs. As a rule of thumb, a dead leg is a pipe that branches off the main pipe and is longer than the internal diameter of the main pipe.

A visual examination for potential dead legs is a vital part of the risk assessment because of their importance in Legionella control. The entire pipe network needs to be followed and inspected to identify potential dead legs.

On small sites with simple systems, a visual inspection may be sufficient. On larger sites or sites with more complex systems, the process of checking for dead legs should include reviewing information from ‘as constructed’ plans of the tower system, considering anecdotal information from staff and contractors, and visually inspecting the system. Some consultants offer services that involve measuring heat losses through the pipe system as a proxy for determining low-flow areas.

Where potential dead legs are identified, it may be possible to confirm their status by draining them. This may require liaison with a mechanical services contractor to avoid damage to the system. Where a pipe can be drained, the presence of sludge in the water confirms that there has been little or no circulation through the pipe, and action must be taken to deal with it. If there is no sludge and the water is clear, the pipe is probably not a dead leg, but a conservative approach will minimise risks. Those involved in draining the potential dead leg should use personal protective equipment to prevent inhalation of aerosols.

Once dead legs have been identified, the risk needs to be addressed – for example, by removing the dead legs. Removing dead legs can be a relatively straightforward task on small sites. On large and complex sites, it may be appropriate to develop a program for the progressive removal of the pipes over a period of years, depending on the current performance of the tower system and the overall risk assessment.

In some cases, removal is not feasible, and conversion of the pipe into active or live use may be an alternative. This process is called ‘activation’. However, the preference is to remove the pipe, wherever possible.

Dead legs may be activated by:

  • installing a pipe connected to a pump, drawing water from the dead leg and injecting it into another part of the system – this achieves circulation in the pipe and reduces deposition of sludge in the pipes, allowing biocides to reach all parts of the system
  • draining or flushing the pipe at regular intervals (say, twice per month) to remove the stagnant water.

Where dead legs are located and cannot be removed or activated for a period, this information should be provided to the water treatment provider. It can then be considered in the development of an appropriate operational program. A higher level of maintenance and testing is used to compensate for the higher risk that the dead legs represent.

Risk control strategies for nutrient growth

Cooling tower system operation

Key strategies to minimise the risks associated with nutrient growth include the following.

Identifying sources of environmental contamination and attempting to reduce the amount

All possible sources of environmental contamination should be identified – for example, dust from demolition or construction sites, dirt car parks or roads, heavily used roads or birds nesting. Where possible, the level of contamination should be reduced. For example, during periods of construction or demolition, water might be used to reduce the amount of dust generated. If this is not possible, other strategies will be needed to reduce the impact of the contamination.

Using a biodispersant

A biodispersant will help break down the biofilm on the wetted surfaces in the tower system.

Controlling corrosion

Control of corrosion is best achieved by a well-considered water treatment program, including use of anticorrosive additives and close monitoring of the impact of the water on the metal surfaces of the tower system.

Corrosion control is critical to some business operations. In these cases, independent specialist advice should be sought on the appropriate control and monitoring techniques.

Instituting a more frequent cleaning program

The Public Health and Wellbeing Regulations 2009 require cooling tower systems to be disinfected, cleaned and re-disinfected at least every 6 months. This needs to include the cleaning of all wetted surfaces in the system. More frequent cleaning will help to control nutrient growth.

Cooling tower system improvements

Improvements to the cooling tower system to minimise the risks associated with nutrient growth include the following.

Protecting the cooling tower basin from sunlight

It is important to protect the cooling tower basin (and the top deck of larger cooling towers) from sunlight. In many cooling towers, the sides are open, allowing sunlight to reach the cooling tower basin and encouraging algae to grow. The risk may be reduced by installing or refitting (where they have been removed) sides to the tower structure. The material used to protect the sides must be durable and easily cleaned. Material such as ultraviolet-stabilised polypropylene is commonly used and is appropriate for this purpose. Other materials that can be used include reinforced fibreglass.

Reducing the water temperature of the system, where possible

The temperature of the water in the system has a direct impact on the rate of bacterial growth. It may be possible, after discussion with equipment suppliers and mechanical service contractors, to lower the temperature by adjusting the thermostats, with little or no detriment to the operating efficiency of the overall cooling tower system.

Risk control strategies for poor water quality

Cooling tower system operation

Key strategies to minimise the risks associated with poor water quality include the following.

Undertaking a comprehensive water treatment program

The Public Health and Wellbeing Regulations 2009 require that the cooling tower system be continuously treated with:

  • one or more biocides to effectively control the growth of microorganisms, including Legionella
  • chemicals or other agents to minimise scale formation, corrosion and fouling
  • a biodispersant.

The water treatment program must therefore involve the use of a biodispersant, anticorrosives and one or more biocides.

The choice of biocides is important. They must be proven to be effective under local conditions in killing Legionella and other bacteria. Material safety data sheets should be reviewed to ensure that such evidence is available, and to indicate any occupational health or environmental issues that are associated with the product. The biocide must be administered in such a way that the recommended concentration is maintained at all times. This requires an accurate calculation of the total water volume and the volume of the biocide required to reach the recommended concentration, taking into account water loss due to evaporation and bleed-off.

The Regulations permit the use of chemical or physical agents as biocides, provided that they are capable of killing microorganisms, including Legionella.

Chemical biocides are the most commonly used in cooling tower systems and are of two types: oxidising and non-oxidising.

Oxidising biocides include commonly used chemicals such as chlorine and bromine. These chemicals kill bacteria relatively quickly, and concentrations in water can be monitored relatively easily. Modern automated dosing units continually test to ensure that the required parameters are met. Oxidising biocides tend to be associated with corrosion, so corrosion needs to be monitored and corrosion control measures put in place.

Non-oxidising biocides include isothiazolone, which is relatively commonly used for water treatment in cooling towers. These chemicals kill bacteria more slowly, and concentrations cannot easily be monitored in the field; a laboratory test is required to determine the concentration.

Best practice usually involves the use of multiple biocides (both non-oxidising and oxidising simultaneously) that are rotated periodically to avoid problems with the bacteria developing tolerance to a particular biocide. Modern automated dosing units are able to dispense a number of different chemicals into a system.

Very few systems use nonchemical biocidal devices, such as devices that generate ultraviolet light, ozone or electromagnetic fields. Solid biocides also exist, including mineral crystals. The department recommends use of nonchemical biocides only as a secondary biocide in conjunction with conventional chemical biocides.

Regularly monitoring the chemical parameters as a measure of water quality

Establishment and frequent monitoring of performance indicators to determine whether a cooling tower system is under control is an important aspect of risk management. Once a performance indicator has been identified, a target range should be established beyond which corrective action is required.

Chemical parameters such as the concentration of biocides (8), pH, conductivity (to measure the build-up of solids) and water temperature are good performance indicators. Table 4 lists the more commonly used indicators and the indicative ranges for each. More precise levels than the indicative ranges shown in the table may be required for particular systems. These can be determined in conjunction with the water treatment provider.

As a minimum, performance indicators should be monitored at least monthly. Monitoring more regularly can reduce the risk of the water chemistry and the system moving out of control without warning to operators, well before a scheduled bacterial test might indicate a problem.

 

Photo 1

pH meter: An example of an automated pH meter linked to a system that treats the water to maintain a predetermined pH

Automation is available for many of these monitoring tasks. Devices to monitor chemical parameters continuously can be linked to building automation systems or to more conventional alarms, with preset levels for each parameter to alert operators of problems requiring action. In higher-risk locations, the use of high levels of automation is strongly recommended to minimise the risks.

Table 4: Indicative water quality values


Indicator

Indicative target range (a)

Bacteria

Legionella

Not detected (<10 cfu/ml)="" (b)="">

HCC

Less than 200,000 CFU/mL (c)

Solids

Total dissolved solids

Less than 1,000 mg/L

Conductivity

Less than 1,500 µS/cm

Suspended solids

Less than 150 mg/L

Calcium hardness

Less than 180 mg/L

pH

pH (for bromine-based compounds)

7–8.5

pH (for chlorine-based compounds)

7–7.6

Total alkalinity

80–300 mg/L

Other additives

Biodispersant

Follow the manufacturers’ specifications

Corrosion inhibitor

Follow the manufacturers’ specifications

CFU = colony forming unit; HCC = heterotrophic colony count

a Water conservation should also be considered. For more information, refer to Australian Institute of Refrigeration, Air Conditioning and Heating 2009, Best practice guidelines: water conservation in cooling towers.

b The Public Health and Wellbeing Regulations 2009 prescribe a series of actions that must be taken following the detection of Legionella in a cooling tower system sample.

c The Public Health and Wellbeing Regulations 2009 prescribe a series of actions that must be taken when a cooling tower system sample has an HCC of more than 200,000 CFU/mL.

Testing frequently for heterotrophic colony count levels

Testing of bacterial levels in the recirculating water of the cooling tower system must be a part of every cooling tower system’s RMP.

Heterotrophic colony count (HCC) is used as an indicator of water quality in cooling tower systems. The test measures the total bacterial load in the sample of water, in colony forming units per millilitre (CFU/mL).

A high HCC (which is regarded as any count of more than 200,000 CFU/mL) indicates that the system is moving out of control and may support Legionella growth, unless corrective action is taken. However, there is no direct correlation between HCC and Legionella concentration. It is possible to have a very low HCC and still detect Legionella and, conversely, a high HCC level but not detect Legionella.

Samples of recirculating water to be tested for HCC should be:

  • taken as described in AS 2031 (Water quality – Sampling for microbiological analysis), which specifies selection of a suitable sampling container and preservation of the sample for later testing
  • collected as described in AS/NZS 3666.3 (Air-handling and water systems of buildings – Microbial control – Performance-based maintenance of cooling water systems). The sample should be stored at 2–10 °C before analysis, and analysis should begin within 24 hours of the sample being taken
  • analysed in accordance with the relevant method in AS 4276.3 (Water microbiology – Heterotrophic colony count methods).

The Public Health and Wellbeing Regulations 2009 require monthly HCC testing. If the HCC is above 200,000 CFU/mL, the Regulations require that action is taken as described in Section 4.5. This includes resampling.

Although testing must occur at least monthly, the frequency should be proportionate to the risk posed by the system – that is, higher-risk systems may need to be tested more frequently.

As part of a risk assessment, it is important to look at past HCC results. HCC levels over time can be graphed. If the action level of 200,000 CFU/mL is marked on the graph, it is readily seen when HCC levels approach it.

Testing for Legionella

Legionella testing is the ultimate performance test of a cooling tower system. The Public Health and Wellbeing Regulations 2009 require testing for Legionella at least once every 3 months. Action must be taken within 24 hours following detection of Legionella in any water sample taken from a cooling tower system. The method of laboratory testing for Legionella is such that an acceptable result is generally reported as ‘less than 10 CFU/mL’.

Although testing for Legionella is required at least every 3 months, the department strongly recommends that the frequency of testing be based on the risk assessment for the system and proportionate to the risks posed by the system. Testing frequencies for Legionella are discussed in Section 8. The use and frequency of Legionella testing should be based on the risk of potential growth of Legionella, combined with the potential for exposure of people to aerosols from the system. The results of testing should be seen as an indicator of system performance. However, because of the inherent difficulties associated with Legionella testing (for example, Legionella lives in the biofilm but may not be picked up in the sample), the absence of Legionella in an isolated test cannot be seen as definitive proof that the system is operating well at another time.

Another important consideration is the impact of a positive Legionella result. This is discussed further in Section 10.

Testing for Legionella requires samples to be:

  • taken in containers as described in AS 2031
  • collected as described in AS/NZS 3666.3
  • stored and transported as described in AS/NZS 3896 (Waters – Examination for Legionella spp. including Legionella pneumophila). This standard requires that the samples be transported to the testing laboratory as soon as possible and then analysed in accordance with AS/NZS 3896. The testing is much more sophisticated than for HCC, and results can take up to 10 days.

When selecting a testing laboratory to perform these tests, it is important to ensure that the organisation follows the relevant Australian Standards in their testing processes. The Regulations require that the laboratory is accredited by the National Association of Testing Authorities.

Using appropriate bleed-off rates suited to the system in use

To overcome build-up of solids, a small percentage of the total water volume should be discharged to waste at regular intervals. This operation is known as bleed-off. The water is drained from the system to the sewer and replaced with fresh water. Automated devices are available to assist in this process – for example, a flow-controlled device that drains a preset volume of water at regular intervals. Modern automated dosing systems also automatically control bleed-off. Conductivity, which is related to the levels of solids in the water, is measured by the units and used to initiate bleed-off at appropriate times, also taking into consideration the biocide dosing interval.

Cooling tower system improvements

Improvements to the cooling tower system to minimise the risks from poor water quality include the following.

 

photo 1: pH meter: An example of an automated pH meter linked to a system that treats the water to maintain a predetermined pHphoto 2: pH meter: An example of an automated pH meter linked to a system that treats the water to maintain a predetermined pHphoto 3: Sand filterPhoto 4: Air-cooled system: Three air cooled systems forming part of an air-conditioning systemphoto 5: Tower ladder: Tower safety is important. Proper decking and ladders must be  providedphoto 6: Wooden tower: An aged tower constructed largely of woodphoto 7: Changing risks: The re-development of an adjacent building to residential use has increased the numbers of people who live close to the cooling  towerphoto 8: Environmental contamination: An unmade car park in the area adjacent to a cooling tower may increase the levels of solids in the water and must be addressed in the RMP

pH meter: An example of an automated pH meter linked to a system that treats the water to maintain a predetermined pH

Installing automated dosing devices

The method of adding chemicals such as biocides, anticorrosives and biodispersants to the water can significantly affect the overall risk. Manual dosing (which relies totally on the operator), and drip-feed or siphon devices are not recommended by the department.

An automated dosing device is more reliable, because a preset volume of biocide (and other chemicals such as biodispersants and anticorrosives) can be injected into the recirculating water at regular intervals. Many of these systems have alarms fitted to warn of problems such as pump failure. Automatic dosing has become the industry standard in Victoria and is highly recommended by the department.

Several types of automated devices are available for chemical dosing:

  • timer-controlled dosing pumps
  • feedback-controlled dosing devices that use oxidation–reduction potential probes
  • feedback-controlled dosing devices, including those that directly measure chlorine and bromine concentrations.

Timer-controlled dosing pumps rely on a pump and timer that are connected to a drum containing the chemical to be dosed. This requires manual setting, based on an operator’s calculation of the volume and time interval required to achieve the target concentration. Alarms are available to warn of pump failure. One pump is required for each chemical to be dosed.

Feedback control is only available for administering oxidising chemicals such as chlorine and bromine. It can be used to keep these biocide concentrations in the target range at all times. The equipment can be connected to building automation systems and alarms to advise of problems or to track the dosing performance.

Feedback-controlled dosing using oxidation–reduction potential probes measures a parameter that has a relationship to the concentration of the oxidising chemicals in the water. Devices are also now available that directly measure either chlorine or bromine concentration.

In large installations comprising multiple cooling towers connected in series (cells), some cells may be shut down in rotation for lengthy periods. The automated dosing device is sometimes only connected to one cell, and it may be necessary to have multiple dosing points in such situations.

It is also important to have a bunded area to contain any spillage or leaks from chemical drums, to prevent discharge to stormwater systems or a safety hazard to workers.

Solid biocide materials that dissolve to release biocides into the circulating water may be regarded as an auto-mated dosing device for the purposes of risk classification.

Selecting an appropriate point for chemical dosing

Selecting an appropriate point for the dosing of chemicals can have a dramatic impact on water quality (as measured by bacterial testing). As a general rule, dosing needs to occur well away from the point where the water quality is monitored by bacterial testing, to ensure that the testing occurs at a point that is representative of the water in the system. If the water is tested immediately after the chemicals have been applied, levels of bacteria in the water immediately around the dosing point may be low, but not truly representative of the bacterial load further down the system, where biocide concentrations are much lower.

Generally, unless there are clear local reasons for dosing at a different point, the department recommends that dosing of chemicals occurs immediately, or soon after, the cooled water leaves the cooling tower. This means that a lower volume of chemicals would be lost from splashing in the cooling tower.

Providing a dedicated water sampling point

The selection of a bacterial sampling point is important. It should be well away from the dosing point. Ideally, if dosing occurs soon after the cooled water leaves the tower, testing should occur just before the warmed water enters the tower. This is obviously only possible where a sampling tap has been fitted. A sampling tap should not have excessively long pipe lengths and should be as close to the main pipe as possible. The tap should be run for at least 30 seconds before sampling. A sampling tap can create a potential dead leg, so the tap should be flushed at least once a month.

Where a sampling tap is not available, sampling is usually only possible from the tower basin, or from water as it falls from the fill into the basin.

In either case, the sampling point should be clearly marked on the tower, along with the department-assigned cooling tower system (CTS) number. Its location should be described in the RMP.

Installing side-stream water filtration in dirty environments

An appropriately installed side-stream filter can be a very effective component in a cooling tower system that is subject to environmental contamination. However, if the filter is not properly maintained with regular backwashing, it can become a site for microbial growth and contaminate the water in the system. These filters use either sand, cartridges or a centrifugal design to filter the water.

 

Photo 3

Sand filter

Risk control strategies for deficiencies in the cooling tower system

Cooling tower system improvements

Key strategies to minimise the risks associated with deficiencies in the cooling tower system include the following.

Undertaking a comprehensive review of the system design to confirm that it complies with AS/NZ 3666

A comprehensive review of the system design should be the first step in a capital works program. The review can be performed by contacting the original supplier, or by full or partial comparison with AS/NZS 3666.

It is very difficult to confirm that drift eliminators comply with the standards after the cooling tower has been installed.

Undertaking a comprehensive review of the current operation and performance of the system

A review of the current operation and performance of the system could include a check of the water temperature in the basin as the water leaves the tower. Ideally, this is compared with the operating design specifications to ensure that the system is not working at an excessively high temperature or above its original design capacity. If the design specifications are not available, all equipment should be checked to ensure that it is operating effectively.

Developing operating and maintenance manuals

AS/NZS 3666.2 states that operating and maintenance manuals shall be provided for a cooling tower system. The standard describes these manuals as including:

  • physical details, including drawings, of the plant, equipment, systems and pre-treatment carried out
  • recommendations on maintenance, including water treatment maintenance and management
  • recommended cleaning methods and dismantling instructions
  • start-up, operating and shut-down procedures
  • particulars of the maintenance management program.

For older systems, much of this information may not be available, but some information may be collectable during the risk assessment process. It is critical to understand the basic design of the system, including the water flow. This may require discussion with maintenance or mechanical services contractors, who may be able to explain the basic functioning of the system as part of a risk assessment. Any information such as schematic or concept drawings should be included in an operational manual for these older systems.

New systems should not be commissioned until such information is available. The recommended shut-down and start-up procedures, in particular, should be documented to minimise risks.

Assessing useful system life

Like other mechanical assets, cooling tower systems have a limited useful life. Beyond a certain point, further maintenance becomes uneconomical, and complete replacement of the tower must be considered. An assessment should be made of the useful life of the tower system and how well the system is meeting business needs. This information, combined with risk considerations, will allow owners of cooling tower systems to make decisions about when the system should be replaced or upgraded.

There may be alternatives to the cooling tower system because of new technology – for example, for small air-conditioning-related or refrigeration-related cooling towers. Air-cooled systems generally have higher capital costs and higher energy consumption, occupy more floor area and create higher noise levels, but they do eliminate the risk of Legionella and the cost of maintaining a water system. They could particularly be considered where the required heat rejection is below 750 kW. However, equipment size and hours of operation per year also need to be considered. Any cost–benefit analysis associated with the possible replacement of a cooling tower with an air-cooled system should consider the potential costs associated with an outbreak of Legionnaires’ disease as well as energy consumption.

 

Photo 4

Air-cooled system: Three air-cooled systems forming part of an air-conditioning system

Installing an effective drift eliminator to comply with AS/NZ 3666.1

Cooling towers not fitted with effective, modern drift eliminators present a greater risk of an outbreak of Legionnaires’ disease in the event of failure of the water treatment regime. A drift eliminator constructed and fitted to comply with AS/NZS 3666.1 can significantly reduce the amount of aerosols leaving a tower. However, no simple field test can confirm that a drift eliminator is working effectively, so an assessment needs to be made of its condition. For example, the supplier can be asked to confirm that the drift eliminator met the standard at the time of installation. Drift eliminators are generally constructed of modern materials such as propylene. Where possible, the drift eliminator should be checked to ensure that it is still in good condition and has not become dislodged from its installation position.

Reviewing and monitoring tower safety

Tower safety (for example, ladders, rails and platforms) is critical to those who work on the tower. The integrity and physical condition of all components must be reviewed and regularly monitored to prevent breakage or other failure, as this may lead to a serious accident. Note that the department has a policy of notifying WorkSafe where there are concerns about tower safety.

Using suitable materials for external components

Wood is not regarded as a suitable material for use in cooling towers because it deteriorates rapidly in a warm and moist environment. However, in some large industrial cooling towers, it may be the only suitable material. If it is used, it will require careful and regular maintenance.

Using suitable materials for internal components

Many older tower systems use inappropriate materials inside the cooling tower – for example, wood for drift eliminators or fill. These should be replaced with durable modern materials such as ultraviolet-stabilised polypropylene.

 

Photo 5

Tower ladder: Tower safety is important. Proper decking and ladders must be provided

 

Photo 6

Wooden tower: An aged tower constructed largely of wood



Risk control strategies for location and access

Cooling tower system operation

Key strategies to reduce the risks associated with location and access include the following.

Restricting access to the tower and its surrounds to staff and contractors with a direct need to access the area

Restricting access to essential staff and contractors reduces the number of people who may be exposed to aerosols. This is best achieved through clarity about individual roles. Identifying those people who require access to the area and establishing a security system is one method of restricting access.

Using high standards of maintenance for towers located in high-risk locations

The highest standards of maintenance (including frequency of inspection and service) and bacterial testing are needed in high-risk locations – that is, where the tower system is located in, or near, an acute health or aged residential care facility, or where large numbers of people would be exposed to aerosols from the system.

Undertaking more frequent cleaning for tower systems exposed to significant environmental contamination

For towers that are exposed to environmental contamination, such as soil or dust from demolition or construction sites, the cleaning frequency may need to be increased to address the risk that the level of solids in the system will increase and encourage bacterial growth.

Cooling tower system improvements

Improvements to the cooling tower system to reduce the risks associated with location and access include the following.

Displaying warning signs to advise staff or contractors that the area has restricted access

All staff and contractors should be discouraged from gathering near the area. A sign should be placed advising of ‘Authorised access only.’

Preventing the area around cooling tower system being used as a gathering place for staff or others

On some sites, smokers use the area around the cooling tower as a place to congregate outside the building. Since smokers are at higher risk of contracting Legionnaires’ disease if exposed to Legionella, such a practice should be discouraged.

 

Photo 7

Changing risks: The re-development of an adjacent building to residential use has increased the numbers of people who live close to the cooling tower


Best practice is to clearly mark or label each cooling tower as a ‘Cooling tower’. It is strongly recommended that all cooling tower systems are marked with the department’s CTS number and a tower reference number, for ease of identification by contractors – for example, ‘Cooling tower 1 – CTS 1234’.

Restricting access to the tower

Access to the tower can be restricted by methods such as locking access points (where access cannot otherwise be restricted) and erecting fencing with locked gate access.

Relocating the tower to a more remote site or a less-contaminated environment

Relocation of the tower is relevant for large sites where a cooling tower system is located close to either high numbers of people or highly vulnerable groups, such as those present in a hospital, nursing home or aged per-sons’ facility. Such a decision would need to consider not just the engineering issues involved, but the potential impact on highly vulnerable people.

Ensuring a safe and stable area for maintenance workers to access the tower system

People who have to access the cooling tower system for maintenance or inspection must be able to do so safely. This includes having safe access to the area near the cooling tower, including ladders, ramps or platforms. The access area around the platform needs to be sufficiently large to facilitate all of the major works that need to be done on the cooling tower system, including access to, and removal of, key components for cleaning.

Installing a side-stream filter

Where a tower is exposed to significant environmental contamination, the use of side-stream filtration (see the section 'Risk control strategies for poor water quality') can reduce the level of solids and improve water quality.

 

Photo 8

Environmental contamination: An unmade car park in the area adjacent to a cooling tower may increase the levels of solids in the water and must be addressed in the RMP


Operational programs

Once a risk assessment has been completed, the risks posed by the cooling tower system can be classified (see 'Evaluating the risk associated with a cooling tower system' in Section 7). The next task is to develop an operational or maintenance program that is proportionate to the risks.

Section 6 identified risks associated with cooling tower systems. Several relate to the treatment of water and the standard of maintenance (including cleaning) of the cooling tower system.

The way that an operational program is implemented will have a dramatic impact on the overall risk associated with a cooling tower system. A well-considered and well-written operational program that is not well implemented can still lead to significant problems. For example, something as simple as the supply of biocide being cut because the container is empty can lead to rapid growth of Legionella. The section 'Maintenance contractors' describes considerations for selecting and monitoring a maintenance contractor.

The first element to consider in risk treatment relating to operational programs is the standard and frequency of maintenance and cleaning programs to address the following critical risks:

  • stagnant water
  • nutrient growth
  • poor water quality.

A well-structured operational program will include:

  • competent personnel who are trained for their tasks
  • inspection
  • servicing
  • HCC testing
  • Legionella testing
  • cleaning
  • performance measures
  • record keeping.

Training of personnel

Personnel with appropriate skills and experience are required to operate and maintain a cooling tower. They should have a skill level appropriate to the task they are required to perform.

Skills can be obtained by practical instruction and/or formal training.

Competencies required to fulfil the tasks described in the following sections include:

  • knowledge of occupational health and safety
  • handling of chemicals used in the process
  • use of cleaning tools
  • understanding of the components of a cooling tower system, including pumps
  • use of water quality testing apparatus
  • sample collection, storage and transport.

Inspection

Inspection means simple monitoring of key components, such as:

  • an observation of water clarity
  • a check that the chemical dosing devices are operating – for example, by monitoring the levels of chemicals within the tanks to confirm that they have decreased since the last inspection.

A more complete list of items to be inspected is included in Appendix 5. A nontechnical person with minimal training can do the inspections. Inspections should be frequent. Where problems are noted, they need to be reported to the responsible person, who can then authorise remedial works.

Servicing

Servicing must be performed by personnel with a much higher degree of knowledge than is required for an inspection. Typically, a service would include:

  • a check of the water quality, including parameters such as pH, conductivity and biocide levels
  • refilling of chemical dosing tanks
  • removal of empty tanks
  • a check of all dosing and control equipment, including timers, pumps and tubing; this should involve a calibration check on the pumps and resetting, if necessary, against desired parameters
  • inspection of the wetted components and general integrity of the system
  • corrosion checks.

Action should immediately be taken to remedy any problems.

HCC testing

HCC testing is described in the section 'Risk control strategies for poor water quality'.

Legionella testing

Testing for Legionella is described in the section 'Risk control strategies for poor water quality'.

Cleaning

A cooling tower system should only be cleaned by a competent person who is trained for that task.

Performance indicators

Another critical element of the operational program is the use of performance indicators, such as those listed in the section 'Risk control strategies for poor water quality'. If operational programs are outsourced, performance indicators should ideally be clarified before the program is defined in a contract.

Record keeping

A written record must be kept of all work associated with the system. Regulation 60 of the Public Health and Wellbeing Regulations 2009 requires that records are kept of all maintenance and corrective activities, such as repairs, and of all microbiological test results, for the preceding 12 months. Appendix 6 gives some guidance on the types of information that should be kept, as a minimum.

These records are usually kept on-site; however, in some circumstances, they may be stored off-site – for example, a property manager may hold the records on behalf of a building owner.

Regulation 60(2) requires reports of all maintenance and corrective actions for the preceding 12 months to be produced for inspection when requested by an authorised officer. However records are stored for the cooling tower system, it is important that staff know where they are kept and how to access them quickly.

Selecting an appropriate operational program

To help with decisions on an appropriate operational program (that is, the standard of maintenance), we have developed a series of standard operational programs, together with a means of selecting the appropriate one for any system. These programs represent the department’s view on what is reasonable practice to maintain a cooling tower system.

A risk classification for the cooling tower can be determined from Table 3 (in Section 7). Table 5 shows the recommended operational program based on that risk classification. For example, for a system that is classified as risk category A, the recommended operational program is program A, and so on.

Table 6 describes the details of the recommended operational programs. Each program meets the ongoing maintenance requirements of the Public Health and Wellbeing Regulations 2009.

Table 5: Selection of an operational program

Risk classification

Recommended operational program

A

A

B

B

C

C

D

D

Table 6: Recommended operational programs

Program A

Program B

Program C

Program D

Weekly inspection

Monthly inspection (2 weeks after service)

Monthly inspection (2 weeks after service)

Monthly service

Fortnightly service

Monthly service

Monthly service

HCC and Legionella tested at a minimum of once each month

HCC and Legionella tested monthly

HCC tested monthly

Legionella tested every 2 months

HCC tested monthly

Legionella tested every 3 months

Six-monthly cleaning, or more frequently where environmental contamination (e.g. dust, soil, building works) is a problem

HCC = heterotrophic colony count

Consideration should be given to increasing the frequency of bacterial testing and monitoring of chemical parameters whenever major changes are made to the system. For example, even if upgrades have been made to the system by installing increased automation, it is important to monitor the system closely to confirm that it is under control before reverting to a lower testing frequency. As well, seasonal variations may increase the risk of Legionella growth; as a result, it may be appropriate to increase the service or testing frequencies during seasons of higher risk.

Model operational program

Appendix 4 is a model operational program that can be completed after the risk assessment has been undertaken. It will form part of the RMP.

Appendix 6 is a model service report that is provided as a guide to the detail that is required at each service.

Appendix 7 contains the key elements of a model service contract, which can be completed and tailored to suit specific needs.

Maintenance contractors

The risk of problems with a cooling tower system can be reduced by using appropriately skilled people or organisations to maintain it. In most cases, owners of cooling tower systems will seek outside assistance to maintain the system. Typically, such services are supplied by specialised water treatment companies. It is good practice to be clear about the standard of maintenance required and for this to be specified in writing.

The qualifications and experience of companies should be carefully considered before they are engaged for these types of services. The outsourcing of a service such as cooling tower system maintenance does not mean that a company has eliminated its legal responsibility – the owner of the land and the owner of the cooling tower system still have the legal responsibilities described in Section 4 of this guide.

Contract management and supervision are critical to the success of an outsourcing arrangement. Regular reports, feedback between the parties and performance monitoring are essential components of contract management.

Some key questions to consider when employing a water treatment provider include the following:

  • Is the organisation a member of relevant industry bodies – for example, the Australian Institute of Refrigeration, Air Conditioning and Heating, or the Plastics and Chemicals Industries Association?
  • What is the formal training level of the personnel (for example, water science, chemistry, mechanical engineering)?
  • What are the competencies, skills and experience of the personnel who would be involved with your site?
  • Is the company experienced with your particular type of system?
  • Can they produce references from other companies that can be substantiated by you?
  • Can they demonstrate to you how they calculate the required dosage rates for the biocides that they propose to use and that the biocide is proven to be effective under local conditions in killing Legionella?
  • What, if any, formal quality assurance systems are used by the company? Are they regularly externally audited?

When evaluating tenders or proposals from companies interested in providing these types of services, the lowest price is not always the best service provider for a particular system.

Many contractors have developed their own service reports. The details provided in their reports should meet or exceed the details in Appendix 6.

Maintenance contractors should be monitored closely to ensure that the service is being delivered consistently and in the required manner. Regular reporting arrangements and meetings at which the performance indicators are discussed should be a standard practice.

As with any contract, it is important to be clear about the arrangements in the event that the service contract terminates for some reason. It is critical to maintain continuity of maintenance of the cooling tower system.

The type of contract entered into should be considered carefully. For example, it may appear cheaper to request a fixed-price all-inclusive contract because the cost can be spread equally across the year. As well, the contractor has an incentive to manage the cooling tower system to a high standard to reduce the likelihood of costly ‘call backs’ to deal with problems, such as adverse microbiological results. However, these types of contracts can introduce other problems – for example, if the agreed price does not adequately cover the actual cost of the required service, contractors may cut corners, affecting the standard of maintenance and increasing the potential risks associated with the system.

Footnotes

8. Currently, technology exists to monitor only bromine or chlorine levels on a continuous basis.