Legionella; How it becomes a problem

Whether your building is a new build or 100+ years old, chances are, sooner or later, it can become colonized with opportunistic premise plumbing pathogens (OPPP’s) such as Legionella. There are over 40 different species of legionella bacteria, however, legionella pneumophila is considered the most dangerous as it causes about 90% of the cases of infections.

Legionella bacteria live naturally in a wide variety of natural and artificial water systems, such as lakes, rivers and soils and so and are very likely to enter a building’s water system, no matter what precautions are in place.

However, they will enter a building’s water systems in very low numbers, it is when they start to grow and multiply do they become a health risk.

If the legionella gets the chance to grow it needs a way to spread and any source that generates aerosol or a fine mist of water has the potential to transmit Legionella. This is why you must manage your water systems effectively; to minimise the risks and prevent growth. The three key elements contribute to Legionella risk:

  • Intrusion introduces Legionella into the system
  • Growth occurs when Legionella increases within the system
  • Transmission happens when aerosols, or small water droplets, containing Legionella are inhaled

Getting into your system

Peer-reviewed studies indicate that drinking water systems are the primary source of Legionella bacteria and Legionnaires’ disease. The bacteria are found naturally in freshwater environments, and so there is very little that can be done to stop them entering your water system. The bacteria can become a health concern when they grow and spread in human-made building water systems like;

  • Showerheads and sink faucets
  • Cooling towers (structures that contain water and a fan as part of centralized air cooling systems for buildings or industrial processes)
  • Hot tubs
  • Decorative fountains and water features
  • Hot water tanks and heaters
  • Large, complex plumbing systems

Legionella bacteria lives in the biofilm and corrosion that coats the interior walls of the pipes that carry the water. Under normal conditions, the bacteria remains in the biofilm, however, public water systems, especially those that are older, are disrupted routinely by water main breaks, fire hydrant use, flooding, source water changes, new chemical treatments, and regular maintenance.

When this happens, the biofilm where the bacteria lives is often released into the public water system where it can multiply.

Legionella contamination can occur when water supply systems are improperly maintained, leading to an environment that feeds Legionella growth.

Growing in your system

The key to preventing Legionnaires’ disease is to prevent Legionella growth. Legionella can colonize in water delivery lines and building plumbing systems, contaminating water supplies after the water has been centrally treated at a public water facility.

One of the best ways to reduce the risk of Legionella growth is to design, implement and regularly update an overall water safety plan for an entire system, taking into account any potential hazardous conditions for a particular system and including industry best practices for prevention.

Parts of a water system with insufficient circulation or lukewarm temperature can provide the ideal environment for Legionella growth.

Legionella bacteria will grow best between 20°C and 45°C, it will rapidly be killed at temperatures above 60°C, and do not multiply at temperatures above 50°C. They will not multiply at temperatures below 20°C either but can remain alive until the temperature rises to a level allowing multiplication to occur. In addition to appropriate temperatures, Legionella bacteria require nutrients to enable them to multiply. These nutrients are found in the water systems and include other common water organisms, sludge, scale and sediment. Legionella bacteria also require iron to grow, which is often provided by corrosion.

There are several key elements that can promote the growth of Legionella bacteria in a water supply system, including:

  • Excessive water age /stagnant water

The longer water sits in a system or piping in a system, the greater likelihood the water disinfectant will dissipate over time, leading to pathogen growth.

  • Biofilm

When biofilm, a sticky substance created by bacteria, forms on the inside wall of water supply piping, it protects Legionella from heat and disinfectant.

  • Lukewarm water

Legionella growth is enabled by lukewarm water temperatures, usually in the range of 77 degrees Fahrenheit to 108 degrees Fahrenheit.

  • Dead legs

A “dead leg” is caused by no flow or rare flow of water in pipes.

  • Insufficient disinfectant

Effective water disinfectant strategies are necessary to control Legionella in a water system. For example, chlorination is one method used by water districts to disinfect drinking water that provides a lasting residual disinfectant.¹

  • Inadequate corrosion control

Corrosion can occur in system pipes, depending on several water quality variables, including disinfectants used, water temperature and pH levels. Improper corrosion control can create the ideal environment for Legionella growth.¹¹

  • Cross connections 

Cross connections between potable and non-potable water can introduce Legionella into the potable water supply system.

Control Methods

To help control the growth of Legionella bacteria the following can be implemented;

  • Cold Water – If you can manage the cold water temperatures throughout the system to ensure that cold water is stored  below 20°C and distributed to all outlets within two minutes of opening the tap below 20°C then the cold water circuit will not encourage bacterial growth including legionella growth.
  • Hot Water – Hot water should be stored at 60°C and distributed and supplied to all outlets above 50°C within 1 minute of operation.
    • Stagnation: Stagnation can be prevented by introducing routine flushing programmes and reducing the volumes of stored water.
    • Chemical control: We always use chemicals as the last line of defence, and we always explore the fundamental measures for control and management before embarking upon or recommending any chemical treatment programme for domestic systems.

What if it is in your supply?

If you find a small amount of Legionella bacteria in one of your building water system, it   does not mean it is the source of an outbreak.

It means a more thorough investigation is needed to figure out how the bacteria got there and where else in the building and surrounding area it could still potentially be found.

The water used in virtually all building water systems comes from the drinking water supply, in some systems it is refreshed regularly and is sometimes treated by water management professionals on behalf of building owners. If incoming water contains bacteria, it can enter a sub-system and in most cases the water management practices consistent with “Best Practices” will neutralize it.

Significant disruptions to the public water system can release large volumes of contaminants into a building and consume the chemical treatments—increasing the risk of bacterial growth.


Everyone wants safe, clean water when they turn on the tap. However, keeping potable and non-potable water supply systems safe can be challenging, as underground water infrastructure ages or becomes outdated and as newer types of water supply systems and lower flow rates keep water in pipes longer.

Legionella bacteria is a specific form of bacteria which can be very dangerous if an outbreak occurs, unfortunately not much can be done to stop this entering your water system, BUT a lot can be done to prevent the growth and transmission of it.

You should speak to water treatment specialists to ensure the water you are supplying is safe. At Aquachem we have an inhouse microbiologist who provide precise recommendations and advice, contact us to find out more.