When to test for Legionella bacteria in water systems

By: James Greenwood on Oct 6, 2021

In 2020, there were 254 confirmed cases of Legionnaires’ disease in the U.K., including an outbreak in West Bromwich during September.

While the disease is potentially fatal, the good news is that harmful bacteria causing it can be removed by correctly maintaining water systems. Also, employers and landlords have a legal duty to do so and protect the health of people in their buildings.

When and where do you find Legionella bacteria?

Legionella bacteria are most commonly found in water that meets one, or all, of the following three conditions:

  1. When the water is maintained between 20–45°C
  2. When the water contains nutrients 
  3. When the water is stagnant 

Legionnaires’ disease is contracted when people breathe in tiny droplets of water in the air that contain the bacteria. It is a type of pneumonia that can prove fatal, especially for at-risk populations, including the elderly, people with compromised or weakened immune systems, smokers, and alcoholics. 

Legionella bacteria are most likely to be found in: 

  • Atomisers
  • Hot water heaters (calorifiers)
  • Cooling towers
  • Wet air conditioning plants
  • Whirlpool or hydrotherapy baths
  • Showers
  • Domestic Hot and Cold water systems
  • Dead legs or low use areas 

What are the first steps?

The first step to combatting Legionnaires’ disease is to appoint a competent person to assess the risk of legionella bacteria growth in their building’s hot and cold water systems. Each business and building is different; therefore, the person responsible needs to understand how their unique systems work. 

This person must carry out a Legionella Risk Assessment that considers the risks of Legionnaires’ disease on their site. If no one in-house is sufficiently qualified, the Legionella Control Association has a list of all specialist companies on their website that are trained to carry out assessments on their behalf.

How often should you test water systems for Legionella?

The Legionella Risk Assessment should highlight the risk in your building. However, if there is doubt over its findings—or if any procedures employed are ineffective—legionella testing should be considered to be carried out weekly until the system is back under control.

When the results are evident or the procedures show effective results, it is safe to reduce or remove testing.

Cooling towers are also at risk; therefore, HSG274 Part 1 demands they are tested regularly: every quarter unless there are problems or the risk assessment advises otherwise, in which case testing should be more frequent. 

How can you test for Legionella?

When inspecting water systems, there are several visual warning signs indicating a higher risk of the presence of Legionella bacteria, including corrosion, scale, and sediment.

There are also several ways of testing for Legionella bacteria, and the technology is rapidly changing. Each method has its own positives and negatives, and the merits of each one should be explained in depth by a  professional who can offer advice based on the requirements of your building and the type of water system in it.

The culture method

Traditionally, water samples are collected from water systems using the bottle bulk water sampling method. This allows water to be collected from potable water fixtures or from warm-water-containing mechanical equipment. 

The water sample is then sent to a UKAS accredited laboratory, which will identify the presence or absence of Legionella in the water sample collected, the bacterial count and Legionella species present. It is essential to collect the water samples at the right points within the water system and with the right methodology for accurate results. It is also important that samples are transported in the correct way and in the correct timescales. 

However, these traditional lab-based culture methods are time-consuming, and it can take up to two weeks before the building owner or manager is aware of the result.

The PCR method

The PCR (or polymerase chain reaction) method is a technique that only takes a few hours to complete. The method consists of several steps: 

  1. Viable cells are concentrated through membrane filtration. 
  2. They are then subjected to sonication and heating procedures, which lyse the cells and free the DNA. 
  3. Finally, the DNA is purified for the Polymerase Chain Reaction. 

PCR is a fast and accurate way of detecting Legionella bacteria, often succeeding in detecting Legionella types when traditional methods fail; however, this method is traditionally more expensive, requires specialist equipment and results can be difficult to interpret. PCR’s are very useful in managing outbreak situations.

The rapid swab method

Rapid swab-testing kits that enable the detection of Legionella in just 25 minutes are now widely available. They are incredibly accurate at identifying the fatal Sero group 1 strain of Legionella.

Detecting Legionella so quickly allows rapid remedial action, which can significantly reduce the possibility of human infection. However, while being a helpful self-test tool in the toolkit, the rapid-swab method is not a substitute for a robust water testing programme and UKAS laboratory certification and can only currently detect Sero Group 1.

From where should you take Legionella samples?

Legionella samples should be taken from the locations identified in the risk assessment or from areas where there is a potential concern. It is often not suitable just to take one test as this will not provide the full picture as to the system’s health.  

As well as the water that regularly flows through the system, special consideration should be given to high-risk areas:

  • Where water temperatures drop-out of specification
  • Where stagnant water may be sitting in pipes or tanks

Using temperature control to monitor and control Legionella 

Another way to detect places that potentially contain Legionella bacteria is to test water temperature. While physically testing is necessary to ensure public safety, as always, prevention is the best cure. The best way to do this is to control water temperature, as the bacteria only grow to levels that can become dangerous between 20–45°C.

  • Hot water storage cylinders (calorifiers) should store water at 60°C or higher
  • Hot water should be distributed at 50°C or higher (thermostatic mixer valves must be fitted as close as possible to outlets, where a scald risk is identified)
  • Cold water should be stored and distributed below 20°C

The temperature for water storage cylinders should be checked in accordance with the risk assessment, but this is likely to be every month, and cold water tank temperatures should be checked at least every six months. Also, ‘sentinel’ outlets, which are the closest and furthest from each tank or calorifier, should be checked for their monthly distribution temperatures. 

Why rapid detection is important

Managing the risks of Legionella bacteria is key to protecting the people’s lives inside your buildings, and it is also a legal requirement.

Making sure that your building has a risk assessment and has regular controls in place will help to avoid outbreaks of Legionnaires’ disease.

Legionella and water system site survey

Topics: Legionella

James Greenwood

Written by James Greenwood

James Greenwood as been working in the Water Treatment and Water Hygiene Industry for over 20 years. He is currently the Sales and Marketing Director for WCS Group the largest water hygiene and treatment Company in the UK. James has been instrumental in bringing significant innovations to the UK market over the years always focusing on enhancing client’s compliance and delivering true return on investment projects offering monetary and environmental savings.