Fighting the foreign LEGIONNAIRES

Legionnaire's disease - the pneumonia-type disease resulting from water-borne bacteria in air conditioning and cooling systems - is relatively rare, but still a major concern for anyone operating such systems. The Health and Safety Executive estimates that there are on average 200 cases of Legionnaire's in the UK every year, of which about half are contracted abroad. This is probably an underestimate, it adds. There were six outbreaks of Legionnaire's associated with industrial cooling systems in 1996 alone, it says, resulting in 34 cases of the disease and three deaths.

The Legionella pneumophillia bacterium requires water temperatures between 20 degrees -50 degrees C to grow, and thrives in the 35 degrees -46 degrees C range. Stagnant water, containing sediments that support the growth of microbes, is also required, as is a pH in the range of 2.0 to 8.5. Cool water, such as that in evaporative condensers, cooling coils and tapwater systems, will support dormant legionella colonies; in showers and spray humidifiers, the bacteria will be active. The bacteria cannot survive in hot water. Cooling towers, however, provide an ideal breeding ground.

Cooling towers tend to harbour biofilms - slimes of algae and other microorganisms - which both provide an ideal environment for Legionella and also make it rather difficult to kill. The best way to keep cooling towers free from Legionella is to keep them clean and scale-free, but biofilm build-up is often difficult to avoid and therefore must be treated chemically. The biocides generally used to kill microorganisms in cooling water systems won't penetrate a slime deposit, so biodespersants must be used as well, leading to large amounts of chemical dosing.

The most popular method of controlling Legionella is to maintain a constant low concentration of chlorine in the water supply. Although it may suppess rather than kill legionella, and cannot penetrate the biofilm harbouring the bacteria, it is very effective at killing off the Legionella's food supply. Suppression of Legionella requires chlorine levels around 3-6mg/litre, generally achieved by dosing with chlorine dioxide, sodium or calcium hypochlorite or, more rarely, by gas chlorination.

Engineering methods to prevent Legionnaire's disease centre around reducing the risk factors. Operating temperatures of systems also have to be carefully calculated, and the potential for generation of aerosols evaluated and, where possible, avoided. If aerosols are formed, they should be dispersed away from occupied areas. As Legionella will only multiply in warm water, cold water pipes should be insulated if necessary; and humidification should be carried out with cold, rather than warm water wherever possible. Water storage should be kept to a minimum; sumps, reservoirs and drain pans should only be used where absolutely necessary, and should be a type that can be drained completely.

According to a set of recommendations drawn up by Pennsylvania State University's architectural engineering department, cooling towers should always be sited at least 100m away from the inlets of air-conditioning units, so that aerosols blown from the cooling tower won't be pulled indoors. The guidelines also include designing in small ozonation units, which will generate ozone - a biocide as powerful as chlorine - in situ, and adding dry convective heat exchangers to the system.

British cooling tower operators will soon have to face a new approved code of practice (ACoP) for controlling Legionella in their installations. Following investigations of recent outbreaks of Legionnaire's from industrial premises and a national inspection programme, the Health and Safety Executive has decided that awareness of Legionella - and, more importantly, standards of control measures - are both poor.

Moreover, the recent developments in Legionella control techniques, such as use of chlorine dioxide and ionisation, are not covered in the current ACoP, which was last revised in 1995. This requires employers to identify and assess sources of risk; to prepare and maintain schemes to prevent and control the risk; to implement and manage precautions; and to keep records of precautions implemented.

The new ACoP will apply to all water systems in workplaces, and will make it clear that `competent help' is required to carry out risk assessment. It will also place more emphasis on the duties of product and service suppliers. The HSE is also proposing that routine monitoring of cooling towers should be mandatory. PE