Legionella in water systems

The Legionella bacteria is common in wet and humid environments. The optimal temperature range at which the bacteria form is 35-46 C, and thus they typically thrive in water that has a temperature of 30-50 C. There are many different species of Legionella bacterium, and far from all of them cause sickness in humans. Legionella bacteria can be the cause of two illnesses in humans:

• Legionnaires’ disease, an infection that produces pneumonia
• Pontiac fever, which resembles acute influenza

When designing air conditioning systems, how to reduce the risk of bacteria growth must be considered. All hot and cold-water systems should be provided with an effective water disinfection system which is able to remove both biofilm and kill free bacteria and other micro-organisms without affecting taste and smell of the water. National building codes, legislation and other national guidelines concerning hot water systems have to be observed as well. 

There are various hygienic methods of minimising the risk of the bacteria growth and of killing them:

• Disinfection, e.g. chlorine dioxide
• Heat treatment by circulating hot water
• Filter system

Cold water systems:

When designing cold water systems, considering water temperature, retention time, pipe material and regular system maintenance is of great importance to prevent micro-bacterial growth.

Especially in large and tall buildings cold water often heats up to a level where a wide selection of bacteria can breed. Where the water main enters the building, the cold water has a temperature of 8-15 °C. After that point the water temperature starts to increase.

Depending on the consumption, the water temperature often reaches almost the same temperature as the surrounding air, and the cold water is likely to contain bacteria. Like other micro-organisms, legionellae live and feed in biofilm which is found inside pipes and tanks.

Many cold-water systems are at risk of getting infected but there is an increased risk of growth in systems where:

• Pipe and tank insulation are missing or in poor condition
• Cold and hot water pipes are co-insulated
• There are dead-ends where there is no water flow
• Roof top tanks and break tanks are used. Tanks should be located inside the building and should be sized with low retention time
• Water tank in organic material. The tank itself will serve as food source for bacteria
• Pipes are oversized. Stagnant water increases risk of bacteria growth
• Pipe material can rust. Rust is a good food source for bacteria

Cold water systems in buildings with a risk of scaling during low consumption periods should be provided with an effective water disinfection system. 

Cooling towers:

Cooling water in cooling towers for air-conditioning purposes are often subject to micro-bacterial growth. A disinfection system should always be installed. Cooling towers and evaporative condensers are used to dissipate unwanted heat to the atmosphere through water evaporation. Water is sprayed into the cooling tower through spray nozzles and tiny airborne droplets are formed.

While falling through the tower, some of the water evaporates but some droplets, known as drift, are carried out of the tower by the air stream produced by the fans. The presence of drift has been detected as far as 6 km away from the cooling tower.

Legionella bacteria grow often in the water and are easily dispersed together with the drift. This water mist can be breathed into the respiratory system, causing risk of Legionella disease and Pontiac fever. Cases in which hundreds of persons are affected by one cooling tower have been reported.

Hot water systems:

When designing hot water systems, the water temperature, water retention time, pipe material and ensuring regular system maintenance are of great importance in order to prevent micro-bacterial growth.

All hot water systems are at risk getting infected but there is an increased risk of growth in systems where:

• Warm water remains more or less stagnant due to low consumption
• Biofilm inside tanks and pipes have been allowed to build up
• Water temperature is between 25 °C and 46 °C which is ideal for legionella
• There are dead-ends without flow
• There are sediment, rust, scale and sludge which provide good food sources for the bacteria
• Pipe and tank insulation are missing or in poor condition
• The system is not maintained properly    

However, water temperatures in hot water tanks should always be kept at 60 °C. Temperatures at the tap pipes should be no less than 55 °C. If the water temperature exceeds 60 °C, undesirable scaling will occur in tanks and pipes. 

When designing a hot water system, it should always be considered if a hot water exchanger can be used instead of a hot water tank. A hot water exchanger is often a plate exchanger and is characterised by having no water volume where bacteria can grow.

Other sources:

Micro-bacterial growth is a problem in hot and cold-water systems and cooling towers, and also in many other applications in commercial buildings such as water fountains, spas, swimming pools and fruit and vegetable moisturising systems.

Common for systems below are that legionella often grows in the water and aerosols easily are dispersed to the surroundings. All the systems should be provided with effective water disinfection systems, which are able to remove both biofilm and kill free bacteria and other micro-organisms.

• Water fountains:

Water fountains in shopping malls, airports, hotels, fun parks are subject to bacteria growth. Water is sprayed into the air and airborne droplets are formed and are easily inhaled into the lungs of the guest. Fountain water is the same temperature as the surrounding air 25 - 35 °C. At that temperature legionella and other bacteria grow easily in the water and biofilm. 

• Spas:

Legionella are a particular problem in spa baths because the water is at an optimum temperature for the legionella bacteria to grow and because dirt, dead skin cells etc. from the people using the bath provide food for the bacteria. Furthermore, the piping for the air and water circulation provides a large surface area for the bacteria to grow on. The agitated water in spas forms aerosols in which legionella bacteria can be contained and inhaled.

• Fruit and vegetable moisturising:

In order to maintain fruit and vegetables fresh as long as possible, water is sprayed into the air in many groceries and supermarkets. This procedure is not only able to reduce moisture and weight loss of fruit and vegetables also promote re-hydration. Re-hydration enables fresh produce to regain the moisture already lost since harvest and therefore extend fruit and vegetable life dramatically.

Use the Grundfos sizing tool to ensure correct sizing of your disinfection system to combat Legionella in the water system.