Guidance on the Control of Legionella in Water Systems
||The following guidance on the design and management of cooling towers, evaporative condensers and hot and cold water systems is based on, and replaces, the 1993 guidance The control of Legionella (including Legionnaires’ Disease) HSG70 and the 1998 supplement The Control of Legionellosis in Hot and Cold Water Systems MISC150.This does not form part of the ACOP, but rather it gives practical guidance on how to comply with the requirements of the ACOP.
There is a range of evaporative cooling systems available which vary considerably in size and type. These systems are designed to dissipate heat, using water as a heat exchange medium, from industrial processes and air-conditioning systems. However, such systems can provide an environment for the growth of many micro-organisms, including legionella, which can be spread widely by aerosol into the area around the cooling tower.
Alternative Methods of Cooling
In some circumstances it may be possible to use alternative methods of cooling. Dry cooling (for example, using air blast coolers or air-cooled condensers), will avoid the risks presented by a wet cooling tower or evaporative condenser. The option of dry cooling should therefore be considered, particularly when cooling towers are due to be replaced or when new cooling systems are planned. Large dry cooling systems have some disadvantages as they are generally larger and heavier than cooling towers, so they may be impractical where space and load limitations are limited. They may also be noisier and, while running costs and energy use are comparable for small units, cooling towers are generally cheaper to run for larger systems. These drawbacks will be partially offset by reduced maintenance requirements and savings in the use of water treatment chemicals, cleaning and disinfection costs, regular monitoring and management costs. Adiabatic cooling systems are used increasingly but, if used intermittently, they may pose problems associated with water stagnation; this may result in microbiological proliferation. In practice, each case should be considered on its individual merits.
The ACOP says that plant or water systems should be designed and constructed to be safe and without risk to health when used at work. The following section on design and construction offers guidance on how to do this in cooling systems:
Cooling systems should be designed and constructed so as to control the release of drift, to aid safe operation, cleaning and disinfection (see BS4485: Part 4: 1996). In particular, the following points should be considered:
Drift eliminators, usually made of plastic or metal, should be installed in all towers. In spite of the name, the function of a drift eliminator is to ‘reduce’ rather than actually ‘eliminate’ aerosol drift. Although some types are more effective than others, there is no industry standard. However, they should be well fitted and selected on the basis of their ability to reduce the release of small water droplets. There should be no visible drift released from the tower. Wooden slats do not control the small droplets and should be replaced. Operating conditions, especially the discharge air velocity, affect the efficiency of drift eliminators, for example, if the fan is not running. They are not always fitted on natural draught cooling towers because they may be ineffective.
The area above the cooling tower pond should be as well enclosed as possible to reduce the effects of windage. Wind movements around the tower may cause spray to escape through the sides, especially if it is poorly enclosed. This is particularly significant when the tower runs with its fan off. It may also be necessary to screen the tower or its pond to prevent the entry of birds, vermin, leaves or other debris or contaminants and to reduce solar heat gain.
The water distribution system within the cooling tower should be designed to create as little aerosol (i.e. small water droplets) as possible. The water circuitry should be as simple as is practicable, with the avoidance of deadlegs and ‘difficult to drain’ loops and bends. Easily understood and accurate schematics of the various water circuits should be available, with any deadlegs or dead-ends highlighted and redundant pipework removed. The absence of water circulation means that any microbial population can be left undisturbed for long periods, allowing growth and multiplication. Any subsequent disruption of the deadleg/dead-end could lead to a rapid colonisation of the water system.
Those parts of the tower which become wet should be accessible for cleaning. Packs should be readily removable and easily dismantled. The wetted areas of the tower should, where possible, be shaded from direct sunlight to discourage the growth of algae. The pond should have a sloping bottom with a drain connection at the lowest point which is large enough to carry away water and slurry quickly and easily. A suitably-sized drain-down valve should be located at the lowest point of the system so that it can be conveniently and completely drained, including all pipework and items of equipment. It may be necessary to fit supplementary drain valves to the bottom of individual items of equipment.
The tower should be constructed of materials which can be readily disinfected and which do not support microbial growth. Preserved (see BS5589:1989) timber may be used for the manufacture of cooling towers and packs but it needs to be impervious and easy to clean and disinfect.
Make-up water may not necessarily be mains-supplied (or from another treated water supply). It may come from rivers, lakes, bore holes and other sources. It may, therefore, need pre-treatment to be equivalent quality to the mains supply. If it does not come from a treated water supply, then the quality of water entering the make-up system may show considerable variation in both chemical composition and microbial activity. This may contribute to potential risk and a strategy is required to overcome any identified problems. Inclusion of a water meter in the tower supply pipeline both for the measurement of make-up rates and for the proportional dosage of treatment chemicals is recommended.
A full water treatment programme should be integrated into the system design, with provision made for sample, injection, bleed and drain points and for the incorporation of dosing and bleed equipment; ideally this should be automated.
Cooling towers should be positioned as far away as possible from air-conditioning and ventilation inlets, opening windows and occupied areas, taking note of the prevailing wind direction and the wind distribution over neighbouring buildings. This should also be considered when replacing old cooling towers as it may be possible to reposition them to a more suitable location.