Minimising the risk of Legionella in domestic water services

Good engineering design and building management are key to the successful minimisation of Legionella bacteria. Building services engineer Dr Richard Beattie advises owners and occupiers on the best ways to reduce the risk of bacterial growth and the development of the disease from domestic water services.

While outbreaks of Legionnaires’ disease are infrequent, they are potentially life threatening. Contributory factors such as the building water usage and turnover (water profile), occupancy times, and sanitary specifications (low-water-use appliances) may contribute to bacterial growth which can harbour and provide a source of nutrients for Legionella. The reason is that the use of low-flow devices and mixing valves makes it increasingly difficult to size components of hot and cold water systems accurately.

Legionella is an aquatic bacteria that comprises of many species that can infect the human lungs. The bacteria is widespread in nature, in rivers, lakes and so on, however it is rarely contracted from these sources. In building services the bacteria is typically associated with air conditioning, wet cooling towers and domestic water systems.

However, there are steps that can help to reduce the bacterial risk through engineering design and operational-side maintenance and management. The successful control of Legionella bacteria requires a combination of good technical engineering design and client-side control management. One of the key considerations should be to keep in mind the sizing guides/methodologies to account for modern practices when at the design stage. This will require more research, data analysis and sharing of raw, live, consumption data from actual buildings.

Identifying the problem

Where water is not moving through a system it will become stagnant, which subsequently could lead to public health issues, including bacteria growth. It is therefore possible for biofilm to form that may protect or support bacterial growth. Keep in mind that water flowing or pumped through pipework only causes shearing of bioflm, it does not remove the entire biofilm once it is established. Therefore, regular draw off and water movement through the system by, automated flushing or via manual intervention by janitorial or facilities management staff are important to help minimise the risk. From a design point of view, consideration in the sizing of the components of the domestic water systems should have included an accurate estimation of the water consumption (water profile) anticipated by the end users. Thus the control of biofilm formation within water systems is of paramount importance for the control of Legionellae, as all bacterium need a substrate – or food source.

For Legionella to proliferate, certain conditions are required, including:¹

• Water temperature in the system of between 20°C and 45°C
• Potential for water aerosols to be formed and become airborne
• Water being stored and/or recirculated
• Presence of deposits in the system, such as sludge, organic matter, rust, scale and nutrient
• Amoebae, which are known to cause retrograde contamination of Legionella bacteria.

Bacteria can proliferate in showers, taps, spray heads, spa baths, storage tanks, calorifiers, pipework and plant, filters, thermostatic mixer valves and particular types of fittings and materials. Other risk systems include humidifiers and air washers, car-wash lances, and horticultural misting systems.²

The ideal temperature for the bacterial growth is 37^oC. HSE L8¹ requires a need to maintain temperatures above 50^oC – 60^oC for domestic hot water systems. HSE L8 also mentions to ensure stored hot water is generally not less than 60^oC with a recommended distribution temperature of not less than 50^oC, with a safe maximum flow temperature at the appliance outlet not exceeding 60^oC. Temperatures in excess of approximately 45^oC could potentially result in burns to the skin. Hot water temperatures therefore need to be controlled either via thermostatic control devices such as thermostatic mixer valves or suitable warning signs at hot water outlets, or by using a thermostatic mixing tap directly on the WHB, thus minimising pipe lengths from lowered blended temperatures from a TMV to the outlets.

Water that is contaminated can pose a risk of infection if it can become an aerosol and airborne. Small particles can remain suspended in air for long periods and travel over considerable distances.² They are dry and contain no free moisture. Only bound water is present, which represents a small percentage of the total mass. When contaminated air is inhaled into the lungs, particles that are 5µm in diameter or less will be retained in the lungs because these sizes are difficult to expel. The risk increases with duration of exposure, respiratory rate and number of Legionellae in the air². Also, certain groups of people are known to be more susceptible, for example; over 45 year olds, smokers, alcoholics, diabetics and the immune compromised¹.

Controlling the risk

Grounded with the HSE L8¹, we suggest the following 11 measures to reduce the risk of Legionella proliferation, by ways which may minimise growth and also by reducing exposure to water droplets.

These measures include:

1. Consider the source of system water supply;
2. By avoiding water temperatures 20^oC to 45^oC;
3. Control water spray;
4. Avoid water stagnation;
5. Do not use materials that can harbour bacteria or nutrients;
6. Maintain cleanliness of the systems;
7. Use water treatment techniques;
8. Ensure correct and safe operation and maintenance;
9. If applicable prepare a written scheme;
10. Maintain records;
11. Employ a competent person (CP).

References:

1 – UK Health and Safety Executive, Approved Code of Practice and Guidance, The control of legionella bacteria in water systems, L8, fourth edition (Health and Safety Executive Books, 2013).

2 – CIBSE, TM13, 2013, Minimising the risk of Legionnarie’s disease, The Chartered Institution of Building Services Engineers, London. See www.cibse.org/knowledge

 A version of this article was first published in the CIBSE Journal, May 2018.

Also by this author –

‘Taking the heat out of cold water design ‘ – CIBSE SoPHE Journal

‘Potential issues with domestic cold water storage tanks’ – CIBSE SoPHE Journal