Legionella & Legionnaires' Disease
Legionnaires' disease is caused by the bacterium Legionella pneumophila, which lives and breeds in natural water sources such as rivers, lakes and reservoirs and also in domestic water systems. It is a pulmonary infection which targets the lungs and is transmitted to humans via inhalation of aerosolized water contaminated with the bacteria. Any domestic water system which creates an aerosol droplet (water suspended in air) can potentially be at risk of harbouring the bacteria and spreading the disease.
Legionnaires' disease acquired its name in July 1976, when an outbreak of pneumonia occurred among the delegates of a convention of the American Legion at the Bellevue-Stratford Hotel in Philadelphia.
In Europe it is believed 6,000 cases of Legionnaires’ disease are diagnosed every year, while in 2010 more than 350 cases were identified in the UK with approximately 10% of these resulting in fatalities.*
High Risk Systems
Areas of high risk due to the generation of aerosol droplets of water suspended in air include cooling towers, domestic hot water services (DHWS), showers, taps and whirlpool spa baths. Legionella is also found in untreated surface waters, is not always removed by conventional water treatment processes and can easily colonise environments such as hot and cold water distribution systems. In these systems Legionella is able to flourish at temperatures between 20°C and 45°C, especially where dirt, scale or biofilms are present. The bacteria can survive under a wide variety of environmental conditions. The organisms are dormant below 20°C and will not survive above 60°C, but grow most prolifically at about 37°C.
The majority of reported outbreaks of Legionnaire's Disease are from Domestic Hot Water Systems. The following applications are high risk as they create aerosol droplets.
- Cooling towers
- Whirlpool spas
- Domestic cold water systems
- Shower units
- Outdoor watering systems
The disease is particularly associated with hotels, fountains, cruise ships and hospitals with complex potable water and cooling systems.
The main factors affecting bacterial growth are:
- Water Temperature
- Below 20ºC - dormant
- Above 70ºC - instant total kill
- Bacterial growth most prolific at 37ºC
- Stagnation - this allows biofilms & bacteria to attach onto surface of pipework
- Nutrients within fresh water - these allow biofilms and legionella bacteria to develop - feeding the bacterial growth
- Existence of biofilms - are required by the Legionella bacteria to establish colonies
L8 compliance and legal implications
The Health & Safety at Work Act (HSWA) 1974 and Control of Substances Hazardous to Health Regulations (COSHH) 1999 have identified the risk of legionella. More information can be found here http://www.hse.gov.uk/
L8 - HSE ACOP (Approved Code of Practice) gives practical advice on the requirements of HSWA & COSHH L8 has a 'special' legal status; following L8 advice demonstrates compliance with the law.
Legionella can be controlled by either physical (Thermally or by UV/Ozone) or by chemical (Chlorine Dioxide and ionisation) means.
Chemical Control Methods
Ionisation - electrolytic generation of copper & silver ions
- Silver ions difficult to maintain in hard water areas is pH sensitive may require additional water treatments: scale control / filtration / pH control / water softening the Water Supply Regulations prescribe maximum levels for copper & silver ions, care should be taken not to exceed these requirement
Chlorine Dioxide - an oxidising biocide
- 0.5mg.l (ppm) can, if properly managed, be affective levels of 0.5mg. l may be difficult to maintain in areas of low turnover of water
- The Drinking Water Inspectorate prescribe a maximum level for the total oxidants (combined chloride dioxide, chlorite and chlorate concentration) in wholesome water should not exceed 0.5mg.l as chlorine dioxide.
Physical control methods
UV or ozone
- These systems are not intended to be dispersive ie. spread throughout system
- Have NO residual effect
- Usually installed at or close to point of application
- Sampling each outlet required to verify efficiency
Hattersley's Thermal Circulation Valves
Thermal Circulation Valves are designed to be installed in Domestic Hot Water systems to help to keep hot water hot so that the Legionella bacteria cannot survive.
So How Does Thermal Disinfection Work?
The key is to keep water hot. Thermal disinfection raises water temperature to a level at which Legionella will not survive. This can be carried out by raising the temperature of the whole contents of the calorifier to 70°C then circulating this water throughout the system for at least an hour. To be effective, the temperature at the calorifier should be high enough to ensure that the temperature at the taps and appliances does not fall below 60°C.
*Health Protection Report Vol. 6 No. 9, March 2012