loader image

Legionella Risk in Water Recycling 

What is legionella?

Legionella is the name of the genus of bacteria that cause, among many others, the Legionosis infection and legionnaires’ disease, a severe pneumonia with a relatively high fatality rate. At least 50 species of Legionella have been described and twenty have been associated with disease in humans, but the predominant cause of legionnaires’ disease is L. pneumophila.

How is it spread?

Legionella spp. are opportunistic pathogens of humans and normally inhabit warm moist or aquatic environments where they grow in association with other organisms. In particular, they are known to grow in a range of protozoa. Their fondness for warm water means that they are capable of colonizing artificial water systems and equipment containing water. 

Legionnaires’ disease is not transmitted from person to person, but is of environmental origin and usually contracted by inhaling the organism in an aerosol produced from water contaminated with the organism. 

There is a chain of events leading to an individual becoming infected with legionnaires’ disease: 

  • the water system needs to become contaminated with the bacteria; 
  • conditions have to exist within the system for the amplification of the bacteria to sufficient concentrations to cause infection; 
  • the contaminated water usually needs to be dispersed into droplets fine enough to form an aerosol for transmission to the victim(s); 
  • inhalation of contaminated aerosols or, in rare cases, aspiration of contaminated drinking water; and 
  • the exposed individual has to be susceptible to succumb to infection. 

It is not a guarantee that a water system will become infected with Legionella, particularly if the turnover of water is high enough to prevent stagnation. However, if left for a period of 5-7 days, the water is highly likely to contain legionellae. 

Spa pools and hot tubs can expose many users and anyone in the immediate vicinity, while showers and taps are most likely to lead only to the exposure of individual users. Finally, for an individual to become infected following exposure they have to be susceptible, usually having predisposing conditions. Only a very small proportion of those exposed develop the disease, but increasing age, particularly 50 years and over, smoking, being male and being immunosuppressed through disease or treatment increases susceptibility.

The Legionella Risk Assessment

A risk assessment can be carried out to the following considerations:

  • Contamination – The chances of legionellae being introduced into the water or moist environment of the equipment/system are higher if the water entering is derived from a natural source, such as a river, lake or spring, or a private water supply, rather than a treated and disinfected mains water supply. 
  • Amplification – In order to grow, legionellae require nutrients and specific physical and chemical conditions. One of the most important elements of the environment is temperature. Legionellae can survive below 20°C but generally are not able to replicate.  They are considered to be capable of growth between 20°C and 45°C but rapid growth occurs between 32 and 42°C. 


  • Transmission – Legionella is transmitted via droplets in aerosolised water. Aspiration occurs when water is drunk but, instead of going down the throat into the stomach, goes down the wrong way into the lungs. Aerosols can also be produced by running a tap, flushing a toilet, water splashing onto hard surfaces or even small bubbles bursting on the surface of water 
  • Exposure –  The closer a person is to the source, the more likely they are to inhale the aerosol before it has become disseminated and the bacteria in it have died. This is what causes the high inherent risk in spa pools – the proximity of the water surface to the airways of the user allows for easy inhalation and aspirated. 
  • Host susceptibility – Some individuals are much more likely to become infected than others. Susceptibility increases with age, and males are more likely to become infected than females (ratio of 3:1). Smoking is a significant risk factor. Disease or therapy that reduces immunity, such as organ transplantation, cancer, blood disease and diabetes, also significantly increases the risk of infection.

Legionella risk in Greywater systems

Greywater is collected from hand wash basins/sinks, showers and baths which will therefore contain a lot of surfactants (soaps). These surfactants provide nutrients to bacteria and allow prolific growth. The temperature of the water, if between 20-45°C, also provides the perfect conditions for growth.

Despite the increased risk of Legionella being present in Greywater, the uses of Greywater generally do not create aerosols and therefore the risk of inhaling the contaminated water is minimal. If you were to use Greywater for irrigation, you would have to use drip-feed irrigation or would have to have a UV installed.

Legionella risk in Rainwater systems

Contamination: The rainwater, though pure when falling, can become contaminated by making contact with bacteria on the surface that the water is being collected from (an example of this could be bird excrement on the roof). 

Temperature: If the temperature of the water reaches 20°C or higher, the bacteria can grow. It is common during the summer months for roofs to reach temperatures upwards of 20 degrees so this is something to consider. This is a benefit of having a below-ground rainwater tank – they generally stay cooler and darker which also prevents growth of bacteria. 

Irrigation: If you’re planning on using rainwater for irrigation, it is important to consider what kind of irrigation you will be using e.g. either drip-feed or spray. Legionnaires disease is contracted through the aspiration of contaminated droplets so using a drip-feed irrigation system will pose less risk compared to spray or sprinklers. It is also worth reconsidering using a spray hose attachment for the same reason. If you decide to use a spray irrigation system or even a pressurised vehicle washing system (i.e. a pressure washer), then you will require the addition of a UV light which acts as a biocide, and kills any lurking bacteria.

Stagnation: It is also extremely important to note that if you are using your Rainwater harvesting system purely for irrigation purposes, you should not leave water to sit for any more than 7 days without draining/flushing the system and replenishing with new water. Stagnant water with no UV light will result in a contaminated water supply. However, if you are using rainwater for WC flushing, laundry and irrigation, the turnover of water will be much higher so stagnation is highly unlikely to occur. 

How is contamination prevented?

Having little or ideally no stagnation in any water tank will help prevent Legionella from materialising. This also goes for slow running water. Having one or more biocides e.g. UV light, bromine/chlorine tablets included in the system will help to sterilise the water before entering the tank or once inside.

 Legionella prevention in Aquaco’s systems

Aquaco’s systems have been developed by our in-house engineers using guidance British Standard 8580:2010 regarding Water Quality – Risk Assessments for Legionella control – Code of practice. 

If you place an order for one of our systems, we will recommend a system that is suitable for your requirements in terms of water turnover and capacity. Having a system that is disproportionate to the amount of water you will be using on a regular basis increases the risk of stagnation in your tank and therefore increases the chance of Legionellae being present. We can also supply additional equipment such as UV lights to help prevent your systems from being in contact with Legionella and bromine tablets to sterilise the water before it reaches the tank. 

Aquaco’s Managing Director James, specialises in Legionella risk assessment and is a City and Guilds accredited LCI Legionella Risk Assessor. You are in safe hands with Aquaco and can use our systems, safe in the knowledge that they have been designed with your health and safety in mind. 

Please contact us if you would like further advice or guidance on our systems.