Environmental health Control of legionella bacteria
This page is not exhaustive in providing advice on the control of legionella bacteria in water systems.
For further information and practical advice on how to comply with the law, the HSE published a guidance book on 'The Control of legionella bacteria in water systems (L8)'.
Legionnaires disease
This is a potentially fatal form of pneumonia which can affect anybody, but which principally affects those who are susceptible because of age, illness, immunosuppression, smoking etc. It is caused by the bacterium Legionella pneumophila and related bacteria that can be found naturally in environmental water sources such as rivers, lakes and reservoirs, usually in low numbers.
As they are commonly found in environmental sources they may also be found in purpose built water systems such as cooling towers, evaporative condensers, whirlpool spas and hot and cold water systems. If conditions are favourable the bacterium may grow creating conditions in which the risk from legionnaires' disease is increased.
Legionella control - Hot and cold water systems
- One way to minimise the growth of legionella is to store hot water above 60C and distribute it at above 50C. However, care is needed where water runs hot. The risks of scalding should be assessed and appropriate measures taken to prevent burns, eg warning notices and/or thermostatic mixing valves (TMVs) on taps
- Where TMVs are fitted these need to be placed as close to the point of use as possible. Ideally a single TMV should not serve multiple tap outlets but, if they are used, the mixer water pipe work should be kept as short as possible
- Water systems should be designed to avoid conditions that favour the growth of legionella by ensuring adequate insulation of storage tanks and pipes, using materials that do not encourage growth of legionella and protecting against contamination by fitting storage tanks with lids
- Water systems need to be routinely checked and inspected by a competent person. For instance an annual visual inspection of the cold-water storage tank should check that the tank/lid is in good condition, that there is good thermal insulation to protect it from extremes of temperature and that the insect screen is intact on the water overflow pipe. The water should not contain any debris or contamination and if considered necessary then the tank should be cleaned and disinfected and any faults rectified. If any debris or vermin is found then inspection will need to be more frequent. Also the calorifiers should be drained and checked for debris in the base
- Risk assessments should be reviewed regularly
- Water stagnation can encourage conditions that favour growth of legionella. It is therefore advisable to remove dead runs in pipe work from the system, flush out seldom used shower heads, taps and remaining dead legs periodically (weekly), and to remove any dirt or limescale. Showerheads should be dismantled, cleaned and descaled at least quarterly
- Water temperatures in the sentinel taps (not taps controlled by TMVs) should be checked monthly to see if: hot water has reached 50C after one minute and the cold water is below 20C after two minutes
- Routine microbiological monitoring of hot and cold water systems is usually not necessary however taste, odour and clarity of water might identify that a problem exists and then the system should be fully investigated including microbiological sampling
- Other water treatment methods include copper/silver ionisation and chlorine dioxide
Legionnaires control – Spa pools
Spa-pool systems are a recognised source of diseases caused by infectious agents including the organism that causes legionnaires' disease, primarily Legionella pneumophila.
Legionella control - Notification of cooling towers and evaporative condensers
- A business or premises that has wet cooling towers and/or evaporative condensers within the districts of Shropshire Council needs to register with this department
- It is the duty of each person who has, to any extent, control of premises to ensure that no notifiable device is situated on those premises unless the information set out in the regulations has been notified in writing on the approved form
- These regulations require local authorities (in whose area the equipment is situated), to be notified of wet cooling towers and/or evaporative condensers, regardless of whether or not it is the local authority or Health and Safety Executive (HSE) that is the enforcing authority for the premises concerned
- We will make information from the notifications available to the HSE concerning equipment on premises for which the HSE has enforcement responsibility
- The primary purpose of cooling tower and condenser notification is ensure that the relevant enforcing authority can be satisfied that suitable control measures are in place to protect employees and the general public from the risk of infection by organisms such as legionella
- If a cooling tower and/or evaporative condenser becomes redundant and is decommissioned or dismantled, this should be notified to the local authority so that the register can be altered accordingly
How do I register a cooling tower or an evaporative condenser?
Information and guidance on how to register can be found on our cooling towers webpage.
Duties under the law
You must identify and assess the sources of risk. It is also important to prepare a scheme (course of action) on how you intend to control the risk from legionella. This can be done by completing the following...
- Provide an up-to-date plan or schematic diagram of your system including associated pipe work, valves and pumps etc
- Appoint a person to be managerially responsible (responsible person) for the prevention and control of risks from legionella
- Describe the safe and correct operation of the system
- Describe what control measures and other precautions will be used
- Detail what checks will be carried out on the control scheme and how often these checks will be carried out
Monitoring and inspection activities should include (not exhaustive)
- Daily checks to ensure that the biocide dispensing equipment is operational
- Weekly monitoring of microbiological activity (aerobic bacteria) by use of dip slides - '7' below for action levels
- Identifying changes in water chemistry by checking suspended solids and pH etc., weekly
- The inspection of the condition of sprays/troughs, eliminators, pack, pond, immersion heater, fans and sound attenuators at least on a monthly to three monthly basis, according to risk
- Periodic sampling in addition to routine sampling of aerobic bacteria, for the presence of legionella by UKAS accredited laboratories. (at least quarterly) - '7' below for action levels
- Disinfecting, cleaning and desludging of cooling towers, which should be undertaken at least twice a year or more frequently dependent upon operating conditions or risk assessment conclusions
Action levels following microbial monitoring for cooling towers
Aerobic count: 10,000 or less
Legionella bacteria cfu/litre: 100 or less
Action required: System under control
Aerobic count: (greater than) 10,000 - (less than) 100,000
Legionella bacteria cfu/litre: (greater than) 100 - (less than) 1,000
Action required: Review control measures and risk assessment to identify remedial actions. Resample.
Aerobic count: (greater than) 100,000
Legionella bacteria cfu/litre: (greater than) 1,000
Action required: Implement corrective action. Resample. Shot dose system with appropriate biocide. Review control measures and risk assessment
Water treatment contracts
It has also frequently been found that operators of water cooling towers/systems are unclear as to the precise terms of their contracts with their water treatment company. Some water treatment companies only have a remit limited to provide biocidal treatment of the water, with no-one responsible for implementing the other equally important areas of control (such as the checking and cleaning of drift eliminators, the maintenance of tower condition etc).