Incidence of Legionnaires Disease
There are more than 500 cases of Legionnaires Disease a year in the UK - figures for 2006 show 551 reported cases of Legionnaires' disease in England and Wales, 160 of which were picked up while the affected person was travelling abroad and 340 contracted within the UK. In about 1 in 10 cases the infection proves fatal.
Symptoms
These first appear between 2 and 10 days after exposure to the bacteria. Legionnaires' initially produces a flu-like illness with tiredness, high fever (often 39.5?C or above), headache, muscle aches and a dry cough. As the pneumonia develops there may be chest pain, shortness of breath, abdominal pain, vomiting and diarrhoea, and hallucinations.
Causes
Legionnaires' is caused by a bacteria known as Legionella pneumophila. The disease and the bacterium were discovered following an outbreak at an American Legion convention in Philadelphia in 1976, hence the name.
The bacteria is found widely in low concentrations throughout natural water systems such as rivers and ponds but temperature and stagnancy are critical to its growth and it is in the tepid and warm static water of artificial water systems that it can really thrive, forming a biofilm or layer of living bacteria over artificial structures. Legionella will grow in water at temperatures from 20 to 50oC (68 to 122oF) with the optimum at 35oC. This temperature is readily found in above ground water tanks during the summer months. As the bacteria reproduce at the greatest rate in stagnant water, such as found in rainwater storage tanks, above ground tanks should be avoided if at all possible or at least sited in the shade out of direct sunlight.
How the infection is caught
Legionellosis infection normally occurs after inhaling an aerosol (suspension of fine particles in air) containing Legionella bacteria. Such particles could originate from any infected water source. When mechanical action, such a toilet flushing or garden sprinklers, breaks the surface of water, small water droplets are formed, which evaporate very quickly. If these droplets contain bacteria, the bacteria cells remain suspended in the air, invisible to the naked eye but small enough to be inhaled into the lungs. This often occurs in poorly ventilated areas such as unventilated bathrooms and cloakrooms where the flushing action of the toilet can spread it throughout the entire room, infecting anyone not immune to the strain of bacteria. Potential sources of such contaminated water include central air conditioning systems, hot water systems, showers, whirlpool spas and poorly designed rainwater harvesting systems.
Which Rainwater Harvesting Systems are Most at Risk?
In order to avoid the risk of Legionnaires Disease, you must avoid the two factors that contribute most to the breeding of the bacteria:
- Stagnant water
- Tepid or warm water
Rainwater harvesting systems that switch to mains water directly in the house when the water in the tank is low leave a small amount of water in the tank which stagnates, increasing the risks when the tank supply recommences. The best and safest systems keep the water in the tank fresh by adding more water to the tank, not by-passing it by feeding mains water straight to the appliances.
If the tank is too large for the rate of water use then stagnation can also occur due to the increased retention time.
Underground rainwater harvesting tanks keep the water much cooler than above ground tanks, particularly in summer. In a sunny position, it is not unusual to find water in an above ground tank with a temperature of 35 degrees C - the OPTIMUM breeding temperature for Legionella pneumophila. AVOID ABOVE GROUND TANKS if at all possible.
As the risks are so serious and oversized tanks were being sold by some 'Rainwater Harvesting' companies, the British Standards Institute brought out a new standard for rainwater harvesting, the BS 8515 2009. This stipulates the maximum size for the tank, depending on use, but many companies are ignoring it. Insist that the tank is sized according to BS 8515 2009.
