Controlling Legionella and Legionella Pneumophila in water systems

Legionella bacteria, the fundamental agent of Legionnaire''s Disease, is a water-based organism, which causes infection when inhaled in an aerosol form.

Although normally associated with cooling towers and evaporative condensers, it also can be found in showers, and has been known to affect workers engaged in the manufacture of plastic parts using injection moulding equipment where the organisms have been found in the water used to cool the metal moulds and the process equipment used during the manufacture of plastic parts.

The advent of the Climate Change Levy means that the maintenance of hot water systems to the recommended temperature has now become a major cost burden to most organisations. This is not only energy inefficient it is an ineffectual way of controlling bacteria.

LEGIONNAIRE’S DISEASE

Legionnaire’s disease is a severe, progressive form of pneumonia, which is fatal in up to 15 percent of cases. The cause of the disease was first identified in 1978 after an epidemic outbreak of pneumonia in 1976 amongst American Legion conventioneers in Philadelphia. The cause was identified as a rod shaped bacteria later named Legionella Pneumophila (LP).

Several natural water systems and many man-made systems have been shown to harbour the Legionella bacteria. These include mains water supplies, cooling towers, potable hot and cold water systems, re-circulating water humidifiers and whirl pools spas and Jacuzzis.

PREVENTION

Between 100 and 200 cases of Legionnaire’s Disease are reported each year in England and Wales. One of the major changes in the UK prevention of Legionnaire’s Disease has been the approach to water management programmes after the advent of The Health & Safety Executive's guidance notes L8 (previously HS (G) 70) and the Health & Safety Commission's Approved Code of Practice for the Prevention or Control of Legionellosis.

Some of the key elements in this legislation are the requirements for a responsible person to be nominated to manage the water treatment programme.

Conditions which affect the proliferation of Legionella include:

1. The presence of scale deposits or algae growth in the water.
2. Dead-legs in the pipework or stagnation due to very low use of outlets.
3. Low temperature in potable hot water heaters and distribution systems.
4. Stratification of water in water heaters.
5. Inappropriate water treatment.

LP may be able to colonise certain types of water fitting, pipework and materials used in the construction of water systems. The presence of such materials and of large quantities of sediment may provide nutrients for Legionella and can make eradication difficult. In practice, LP is found in many re-circulating hot and cold water systems particularly in larger, complex systems such as those found in hospitals, hotels, office block and factories.
Legionella is transmitted exclusively by inhalation of contaminated water droplets. The organisms must be present in sufficiently high concentration and must be suspended in aerosol between 3 and 5 microns in diameter. There is no evidence of obtaining the disease from ingestion or contaminated water or absorption through the skin. High-risk areas are therefore primarily associated with showers, sprays, spray taps, taps with high water pressure and refrigeration and air conditioning cooling towers.

SCALE

Scale is a major problem in both hot and cold water systems. Dripping taps can deposit scale in and around the tap and with high ambient room temperatures, provide an ideal growth medium for LP. In hot systems, scale can trap Legionella and biofilms. This provides a perfect growth medium, which disinfectants cannot penetrate. Scale deposits colonised by Legionella can continuously re-contaminate a system, even after disinfection. Biofilms trapped are a source of nutrients for LP and can lead to taste and odour problems from the products of their metabolism. Scale is a major cause of inefficiency in hot water systems. The scale on heat exchange surfaces dramatically reduces the heat transfer efficiency and promotes corrosion in the calorifiers and pipework. Descaling of a hot water system is time consuming and expensive. Water softeners can reduce scale, but there is growing concern over the increase to sometimes-high levels of sodium in the water.

AIR CONDITIONING AND REFRIGERATION WATER SYSTEMS

Many air conditioning and refrigeration plant systems are water-cooled. The heat generated by cooling coils is removed by water, which is passed through a water-cooling tower. These are re-circulating systems, which operate at temperatures ideal for bacterial and algae growth and have plentiful supplies of nutrients. They have been highlighted as a major possible source of Legionnaire’s disease mainly because of the large number of people that can be affected.

However, in a tower that is well-designed and maintained, chances of problems with LP are greatly reduced. Most cases of outbreaks have occurred in towers, which were badly designed and had little or no maintenance. In cooling towers, temperature, water hardness, pH, scale and corrosion are all factors which increase the chance of biofilm, algae and Legionella colonisation. Many agents are used to control these factors, including scale and corrosion inhibitors, dispersants and biocides. Water softeners are sometimes used but soft water can cause a problem with foaming.

Biofilms are a major problem in cooling towers. Biofilms and scale can reduce the efficiency of cooling systems to the point where the system no longer functions with regard to heat transfer. The UK Health & Safety Executive recommends the periodic chlorination and descaling of cooling towers. However, chlorine is not always compatible with other treatment chemicals like corrosion inhibitors, is not effective in alkaline water and can itself cause corrosion. Some biocides are effective against LP if used in sufficient concentration. Strains of LP and other bacteria may become resistant to particular biocides, hence dual or alternating biocides are used.

ALTERNATIVES

What is required in all the systems, cold, hot and process, is a method of continuously controlling scale deposition and a water treatment regime which prevents the growth of biofilms, bacteria and in particular, Legionella Pneumophila. This method is now available in the form of electronic scale treatment of water to prevent scale deposition and the chlorination of all water supplied to a building or factory both hot and cold

ELECTRONIC WATER TREATMENT

This involves the fitting of electronic water descaling equipment, such as the patented ENiGMA at strategic points in the water system. Water treated by such systems will prevent scale from forming in pipework and on heat transfer surfaces and will also, over a period of time, remove existing scale deposits.

There are many advantages to this nonintrusive engineering solution:

• Energy use is greatly reduced due to heat exchange surfaces remaining free of scale deposits (just a ¼ inch of scale increases energy costs by around 40%).
• Corrosion caused by scale deposits is eliminated.
• Extensive downtime and labour cost involved in descaling systems is eliminated.
• A source of colonisation by biofilms and LP is removed.
• Water distribution efficiency and pressure is increased by removal of scale deposits which can reduce pipe diameters considerably.