A key part of any legionella risk assessment will focus on the efficiency of cold water storage tanks and calorifiers within the wider water system. This is where hot and cold water is stored, ready to be distributed to the necessary outlets for use.

However, the nature of these tanks and their use can directly affect legionella bacterial growth and so the risks need to be considered as part of any control scheme.

The importance of legionella control

Legionella bacteria is widely found in natural water sources, such as rivers and lakes, but if man-made water systems are not properly controlled they can also encourage growth of the bacteria – which leads to a greater risk of contracting Legionnaires’ disease. This is a potentially fatal form of pneumonia which is especially dangerous for the elderly and those with health conditions, and has in the past led to some tragic incidents and heavy financial penalties for those involved.

When it comes to managing water systems on premises, growth of legionella bacteria is identified as a huge risk in two main areas:

• Higher than normal water temperatures of between 20° C and 45°
• Presence of nutrients in the system, such as rust, sludge, and scale.

How the water tanks fit into the system

A traditional gravity fed system involves the water entering the building through a rising main into a cold water storage tank. The cold water will simply circulate directly to the appropriate outlets, while the rest of the water is directed into the hot water tank (or calorifier) where it is heated.

In order to ensure hot water is available close to the necessary outlets, this heated water is then continuously pumped out of the calorifier and around a circuit of pipework near the outlets, before returning to the calorifier to be reheated.

The legionella risks in stored water

Given the nature of this storing, heating, and reheating of water, there are several legionella risks associated with water tanks. The most important to consider are:

Ensuring the water storage maintains appropriate temperatures

The HSE has confirmed that legionella bacteria are dormant below 20° C and above 60° C, which means organisations should ensure their stored water maintains the appropriate temperatures. Testing done as part of a legionella risk assessment (and importantly as ongoing maintenance) can be done at both the source and at the outlets.

Indeed, testing both areas will help confirm the temperatures are being sufficiently controlled both within the tanks themselves and the outlet pipework – especially in the case of the hot water, which relies on a circulating pump to keep temperatures above 50° C on the distribution circuit.

Cleanliness of water tanks

The build-up of dirt within a system can lead to bacterial growth. It is therefore imperative that hot and cold water tanks are inspected at least annually for signs of sludge and scale.

Water tanks can be cleaned through water chlorination where needed, or can be repaired or improved (through the fitting of air vents or rodent screens, for example). And, where the results require significantly more work to address, they can even be replaced.

Size of tanks and frequency of use

The storage of cold water should be kept to a level appropriate for its use on premises – advised by the HSE to be about one day’s use. To check this, a typical day’s water consumption should be recorded and matched against the tank capacity/water being stored. This will make sure that any stored water is not being left to stagnate and will help maintain the cleanliness of the tank. 

In the case of multiple tanks, make sure they are linked together in the system. This allows the water to flow through and, again, avoids stagnation.

Who is responsible for mitigating these risks?

Every organisation with premises in the UK has a legal obligation to ensure the health and safety of their staff and visitors. When it comes to legionella control, this means appointing someone with the appropriate training, skills and experience.

While organisations are allowed to manage this work internally, facilities managers often have many other responsibilities that can impact their ability to ensure compliance. Which is why it’s safer to seek out a water treatment partner who can shoulder the burden of both work and risk, and prioritise the health and safety of those in your care.

If you need more information about legionella control and identifying the risks in your system, download our comprehensive guide, The facilities manager’s guide to legionella risk assessments, today.